-Immunology
CONTENTS: Introduction
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HEPATIC PATHOPHYSIOLOGYThese lecture notes accompany my lectures on
function and dysfunction of the liver in the study module "Nutrition
and Digestion" at
Innsbruck Medical University. The English version serves two purposes:
as a learning aid for international students and to encourage German-speaking
students to familiarize themselves with medical English; the lectures
are delivered in German. The translation from the original German
version is my own; I am afraid it will occasionally sound
appalling to native English speakers, but it should at least be intelligible.
There is also a printable pdf-version.
Version
8.2 e İArno Helmberg 2000-2023 To maintain its integrity, our organism carefully guards its borders at all times. Yet, we could not survive without massive exchange with the outside world. Exchange comes with considerable risks: carefully maintained balances might be disturbed, and toxic substances might wreak havoc in the body. The "port authority" dealing with these complex challenges is the liver. A huge resorptive area –the intestinal
epithelia—is required for taking up sufficient amounts of nutrients. Intestinal
epithelial cells ("dock workers") are good at moving stuff. Making
them also check and metabolize all this stuff probably would negatively affect
their efficiency. Accordingly, these functions are performed later, after all
the blood coming from this huge surface area has been collected and funneled via
the portal vein into the liver. This implies two consecutive capillary exchange
systems: one in the intestinal wall, the second, with low remaining blood pressure,
in the liver. The exchange of huge amounts of molecules between hepatocytes and
blood is facilitated by fenestration of the endothelial cells lining wide
sinusoids, allowing plasma to enter the perisinusoidal space, also known as
space of Disse. In this slow-flow compartment, blood plasma comes into direct
contact with the hepatocytes' microvilli, which are packed with transport
proteins. The perisinusoidal space is also home to specialized pericytes, hepatic
stellate cells or Ito cells, which store vitamin A and are able to synthesize
components of the extracellular matrix. In healthy liver tissue, extracellular
matrix is very subtle to minimize diffusion distances. Two additional cell
types, both part of the innate immune system, are found within the sinusoids,
in direct contact with the blood: a large number of macrophages, called Kupffer
cells, and Pit cells, a type of natural killer (NK) cells.
The importance of liver functioning is
illustrated by situations where it suddenly stops, such as death cap (Amanita phalloides) poisoning. Amanitin,
the main poison, is a cyclical octapeptide with a structure stable enough to
withstand cooking. It binds and inactivates the 140 kDa subunit of RNA
polymerase II, blocking all gene expression leading to proteins. This results
in cessation of liver function, hepatocyte necrosis and in most cases, death of
the patient after 48 to 72 hours.
In the following, individual liver functions
will be discussed separately, making the different symptoms of liver
dysfunction easily understandable.
FUNCTION: Homeostasis of energy metabolism
DYSFUNCTION: Fatigue, weakness, disorders of lipid
metabolism, non-alcoholic fatty liver disease, insulin resistance, metabolic
syndrome
Meals, after which lots of nutrients are
resorbed by the small intestine, alternate with times between meals or even fasting
phases. Yet, most cells need energy all the time. The liver has important
buffering functions to maintain a continuous supply of energy sources in the
blood.
Following meals, large amounts of glucose are taken up by the small intestine, and gluconeogenesis
is therefore inhibited: in hepatocytes, insulin causes phosphorylation and
breakdown of the transcription factor Foxo1. Otherwise, Foxo1 drives the transcription
of enzymes involved in gluconeogenesis (PEPCK, glucose-6-phosphatase). Part of the glucose
absorbed in the intestine is transported into the hepatocytes with the help of
the insulin-independent transporter GLUT2 (KM 15-20 mM). Glucose as such cannot be stored, but it can be
polymerized to glycogen in the liver and in skeletal muscle. The liver is able
to store up to 10% of its weight in glycogen (100-120g). However, the amount of
energy that can be stored in this way is limited: for their hydroxyl groups, glucose units are very hydrophilic: 1 g of
glycogen binds 2.7 g of water. Hence, this form of energy storage carries too
much dead weight and volume to be efficient. Surplus glucose is thus
metabolized to fatty acids via acetyl-CoA. Fatty acids are combined with
glycerol, and the resulting triglicerides for the most part are released into
the blood in the form of VLDL (very low density lipoprotein). Yet, too frequent or high intake of
nutritional carbohydrates over time causes fat to accumulate in hepatocytes. Increased
concentrations of free fatty acids and complex fats result in functional
impairment of the cells, starting with insulin resistance, a condition termed
non-alcoholic fatty liver disease (NAFLD). The third task of the liver during
this phase is to take up remnants of chylomicrons, sort out the complex mix of
lipids contained (e. g., lipid-soluble vitamins or xenobiotics) and to
recombine and release the remaining lipids as VLDL. Remember that lipids are
the only segment of nutrients that go past the liver via lymphatic vessels.
From there, enterocyte-produced chylomicrons enter the blood stream via
thoracic duct and venous angle. Lipoprotein lipase (LPL), which is anchored to
endothelial plasma membranes, helps to move triglyceride components from
chylomicrons into fat and muscle cells; only chylomicron remnants are taken up
by the liver.
Between meals,
or more precisely after completion
of intestinal nutrient uptake, energy stores are tapped under control of
glucagon and sympathetic activity (plus growth hormone during the night) to
maintain levels of energy sources in the blood. Fatty acids are available in virtually
unlimited quantities from fat depots, but not all cells are able to handle
fatty acids. For some tissues (CNS, erythrocytes, renal medulla), glucose is a sine qua non. Muscle glycogen is not
available for these purposes, as myocytes use it themselves, metabolizing part
of it to lactate. A continuous level of blood glucose is maintained by first
breaking down liver glycogen.
At the same time, gluconeogenesis, the synthesis of new glucose
from lactate, glycerol or amino acids, increases. This is regulated via two pathways:
1.
Falling insulin removes its previous inhibition of the Foxo1-dependent
transcription of enzymes of gluconeogenesis in the hepatocytes.
2.
Falling blood sugar and insulin levels inhibit the release of leptin by fat
cells. The CNS reacts to this drop in leptin with an activation of the
"stress axis" CRH-ACTH-cortisol (typical high level of cortisol in
the early morning hours after overnight fast). Cortisol, in cooperation with
the now low insulin levels, triggers lipolysis in adipose tissue. Free fatty
acids (non-esterified fatty acids, NEFA) and glycerol mobilized from fat cells
rise in the blood and reach the liver. Hepatocytes break down these fatty acids
into acetyl-CoA; Acetyl-CoA allosterically activates the enzyme pyruvate
carboxylase, which catalyses the first reaction of gluconeogenesis (to
oxaloacetate). Lipolysis in adipose tissue thus causes increased
gluconeogenesis via increased turnover of liver pyruvate carboxylase.
In addition to a limited amount of
glycerol and lactate, the starting material required for gluconeogenesis is
primarily amino acids, which cortisol mobilizes from muscles and bones. Of
muscles and bones? But don't we need to protect those? Pfff... we have such
vast amounts of protein in our muscles and bones that we can easily
"borrow" this tiny fraction for a short period of time for
gluconeogenesis. Through ingenious amino acid metabolism in the muscle cell,
the muscles supply mainly alanine to the liver as the standard energy carrier.
Keep in mind that both pathways
require very low levels of insulin. If the insulin level rises slightly, the rate
of gluconeogenesis decreases. Let's also remember that insulin blocks
lipolysis. So if we nibble on a sweet in the evening in front of the telly or head
hungry for the fridge at night, we prevent the lipolysis planned for this time or head hungrily for the fridge at
night, we prevent the lipolysis planned for this time; we reactivate the
absorption phase and synthesize fat instead of breaking it down.
Pharmacology cross-reference: Pharmacological doses of glucocorticoids stimulate
gluconeogenesis, causing a sharp increase in blood sugar. With prolonged use,
they have a peripherally catabolic effect – we use amino acids, which we take
from bones and muscles, as material for gluconeogenesis – so that osteoporosis
and loss of muscle mass occurs.
Fasting phase. If the phase without food intake or with insufficient food intake lasts longer
than a day, the liver glycogen is used up. In addition, the delivery of alanine
from the muscle and thus the rate of gluconeogenesis decreases: blood sugar
drops from around 90 to around 70 mg/dl (5 to 4 mM). It is not yet sufficiently
clear how this regulation takes place, it is in any case organized by the CNS. Leptin
level is halved, TSH decreases even more, with the resulting reduction in
thyroid hormone, the overall energy consumption is also reduced: e.g., mitochondrial
energy production in the liver is put on the back burner. You start to fell
cold, blood circulation in the skin decreases, in particular hands and feet
feel icy. This reduces heat losses to the outside and thus saves on ATP heating
costs. When resting, cardiovascular enery ist saved, too: the heart beats a
little more slowly, blood pressure drops a little. While the body was able to take
out an amino acid loan at the skeletal muscles in the post-absorption phase for
a short time without any inhibition or danger, it now has to change its
strategy: it cannot cannibalize the muscles in the long term, as they continue
to be needed for searching for food, etc.
At the same time, the liver also
produces so-called ketone bodies, β-hydroxybutyrate and acetoacetate, from
the ever-increasing flood of fatty acids. This happens because the liver via
β-oxidation breaks down more fatty acids to acetyl-CoA than it can feed
into the citric acid cycle. To do that, it would require more oxaloacetate
("fats burn in the flame of carbohydrates"), which is now also
consumed for gluconeogenesis and thus becomes limiting. From the pool of
actyl-CoAs lined up in this traffic jam, every two are fused, detached from CoA and released
into the blood; mainly in the form of
β-hydroxybutyrate, with a smaller portion as acetoacetate. Both are medium-strong acids, so they release
protons, resulting in a ketoacidotic metabolic state. The renal cortex now gets
involved into gluconeogenesis, contributing up to a third of the now reduced synthesis
rate: Recall that the proximal tubule combines the excretion of acid in the
form of NH4+ with gluconeogenesis. The heart and the cortex of the
kidney prefer ketone bodies to glucose anyway; heavy energy consumer CNS, which
cannot do anything with fatty acids, after a while gets the hang of ketone
bodies, too, over time preferentially metabolizing ketone bodies (up to 75% of
the energy requirement) over glucose. That way, a lot of glucose can be saved by
our energy-hungry brain during longer periods of caloric deficiency. In turn, that
saves amino acids: protein consumption drops from around 75g per day to 20g. Without
this adaptation, the muscles would be broken down far too quickly during
prolonged periods of hunger. So,
basically, ketosis is a trick to feed the CNS from fat stores: the adipose
tissue releases fatty acids, which are converted by the liver into ketone
bodies, which in turn become the staple food of the CNS. In addition, increased
fatty acid concentrations cause resistance to the already very low insulin in
the typical insulin-controlled tissues muscle, adipose tissue and liver, so
that a larger proportion of glucose, which is only sparsely produced anyway,
remains for the CNS, erys and renal medulla.
It is possible, then, that insulin resistance induced by elevated
free fatty acid concentrations, which we have long been regarding as an
exclusively pathological phenomenon, is a physiological component of an
emergency program that enabled our ancestors to survive long periods of
starvation with very low levels of blood sugar. Sustained food abundance has
never occurred in human evolution. Today this mechanism is our undoing. When we
are severely overweight with metabolic syndrome, we have the opposite
nutritional situation - plentiful instead of meager - but also an increased
concentration of free fatty acids, which now leads to insulin resistance in the
presense of very high blood sugar levels.
Ketogenic diet. The percentage of protein in most people's diets ranges from 10-35%. A proportion beyond 35% is not advisable, for several reasons. For one, the more protein we eat, the more stress we put on our kidneys. We
can split the rest between carbohydrates and fat. A common recommendation is
45-65% carbohydrates, 20-35% fat. A ketogenic low-carb diet reduces the
carbohydrate content extremely in favor of the fat content. In some aspects, this
imitates the fasting phase. Due to greatly reduced insulin release, this type
of diet, in combination with caloric restriction, can help to reduce weight.
After a transition period characterized by the loss of considerable amounts of
water (as glycogen stores are depleted; looks great on the scale: "Wow,
finally a diet that works, I've lost 2 kg already!"), the body adapts
surprisingly well to this change in food
supply. Some populations, such as the Inuit, systematically nourished
themselves this way due to a lack of carbohydrates. For a short to medium term,
such a diet can indeed help to reduce weight. What about the long term?
Long-term ketogenic diet? How healthy or unhealthy are different distributions of our three
energy carriers, carbohydrates, fat and protein? Unfortunately, the data on this
question is very insufficient. Ideally, we would like to see randomized
controlled trials over thirty years and more with tens of thousands of
participants, in which people constantly, with precise documentation and
control, adhere to a specified distribution of macronutrients, of course while
complying with all requirements for micronutrients (minerals, vitamins, trace
elements). All this with meticulous documentation how their health develops.
Apart from the fact that this is not possible: Would you take part in such a
study? No sweets for thirty years because you were randomized to the keto
group? For these reasons, there are very few studies that offer reasonably
reliable data on our question. In these studies, the participants were asked by
questionnaire at the beginning, at most once again later, about their eating
habits. This type of self-reporting already introduces uncertainty. Self-reported
food items were then converted into a macronutrient distribution using standard
conversions – how many calories from carbohydrates, fat and protein are in a
serving of leeks? a bread with jam? a pizza? It was then assumed that these
dietary habits did not change significantly during study years. As you can see,
these data contain numerous levels of uncertainty. In my opinion, the most
meaningful data come from three prospective studies: PREDIMED (Spain), PURE (18
states with a focus on Asia and South America) and ARIC (4 communities in the
USA):
1.
PREDIMED
is the only randomized interventional study of the three. In Spain, three
groups of subjects at cardiovascular risk were compared. All three were on a
Mediterranean diet. The first group was encouraged to supplement there
nutrition with ample extra virgin olive oil; the second, to supplement it
instead with a daily allocation of nuts (almonds, hazelnuts and walnuts, rich
in polyunsaturated fatty acids; oil or nuts, respectively, were provided). The
third group was asked to try to reduce fat – which automatically means
increased carbohydrates – and received small non-food gifts instead of
oil/nuts. The composite endpoint included myocardial infarction, stroke, death
from cardiovascular causes. Result: the fat-reduced group had a clearly
increased risk compared to the two groups that had consumed more vegetable
fats.
2.
The
main message of PURE: a higher proportion of fat in the diet was not associated
with increased, as had long been assumed, but rather with reduced mortality.
Importantly, however, this statement only applied to carbohydrate percentages
between "medium" (lowest quintile: 46% of energy requirements) to
high (highest quintile: 77%). The result for this part of the spectrum was: the
more carbohydrates, the higher the mortality.
3.
Only
the ARIC study included a proportion of subjects consuming less than 46%
carbohydrates. A U-shaped mortality curve emerged with a minimum at a
carbohydrate content of 50-55%; above and below, mortality increased. The
subjects with the lowest carbohydrate intake therefore satisfied their energy
requirements mainly with fat. For the majority of probands in this group, this
meant high intakes of saturated animal fat and a significant increase in
mortality. Only a small part of the probands consumed primarily vegetable -
unsaturated - fats and proteins. This part showed reduced mortality, but the
proportion was too small to draw firm conclusions.
What do we learn from this? All
three studies argue against a low-fat diet, which has been heralded as ideal
for decades. PREDIMED and PURE do not provide any information about a
carbohydrate-reduced diet; ARIC finds a minimum of mortality at 50-55%
carbohydrate content, below which mortality increased again. So far we do not
have reliable data on ketogenic nutrition. In the ARIC study, carbohydrate
intake did not reach the low levels required for a ketogenic diet. However, one
can extrapolate from the ARIC data that in the long run, a ketogenic diet is
probably all the less healthy if energy intake is mainly based on animal fats
and proteins. Uncertainty remains about a plant-based ketogenic diet. We cannot
exclude that a plant-based ketogenic diet MIGHT be healthy, but so far we lack
the data. A plant-based ketogenic diet is not easy to implement in daily practice
(pure peanut butter, anyone? Tofu dumplings in olive oil?).
Isn't that a contradiction to the
expectation that long-term food restriction will have a life-prolonging effect in
humans, as it does in many animal species? Isn't it also a ketogenic diet if you
live in a hypocaloric state for years? No: a hypocaloric diet is not fasting.
If you eat a hypocaloric diet that is balanced in terms of macronutrients,
ketosis does not develop, since you consume carbohydrates regularly and the
body does not have to switch to predominantly burning fat.
Type 1 diabetes mellitus. Before insulin became available as a drug, DM1 was a death sentence.
The absence of insulin induces the ketoacidotic state of starvation. In front
of full pots and with sky-high blood sugar, the children died of "internal
starvation". They shed muscle and substance until it was no longer
compatible with life.
Control of gluconeogenesis: We tend to view the hormone insulin only from the perspective of the resorption
phase following nutrient intake: the high glucose levels characteristic of this
phase enhance insulin secretion, and insulin in turn helps to move all that
glucose into skeletal muscle and fat cells by inserting units of GLUT4 (KM 5 mM) into the membrane. Yet, insulin remains important between meals, e. g.,
over night.
At the low post-absorption glucose levels
(80-100 mg/dl, equivalent to 4.5-5.5 mM), critical tissues like the central
nervous system are continuously supplied with glucose via GLUT1 and GLUT3. With
their KM of 1 mM, these transporters always work in the saturation
range. In contrast, little glucose enters β-cells of the pancreatic
islets, due to the much higher threshold of GLUT2 (KM 15-20 mM). If
the blood sugar level is slightly increased by gluconeogenesis, proportionally
a little more glucose enters β-cells, causing release of a little more
insulin. Insulin inhibits glucagon secretion in the neighboring α cells;
less glucagon and more insulin enter the venules draining into the portal vein.
In this situation, systemic insulin levels remain too low to promote uptake of
glucose into muscle or adipose tissue. However, since insulin is secreted into
the portal blood, its concentration in the liver is higher than in the rest of
the body; this insulin in combination with reduced glucagon limits
gluconeogenesis. Gluconeogenesis is thus constantly throttled by an insulin
feedback loop.In the
absence of insulin-dependent throttling, the liver would generate glucose at much
higher rates. In people with metabolic
syndrome, due to insulin resistance of the liver, gluconeogenesis is
insufficiently restricted. The result is glucose overproduction between meals,
as illustrated by elevated levels of fasting blood sugar in the morning. This
explains a fact that is otherwise counter-intuitive: why should glucose be
elevated in a person who is hungry after many hours without food? Bottom line:
between meals, it's not like the liver laboriously scrapes together some sugar.
Rather, gluconeogenesis is a bountiful source of glucose in need of constant
throttling by insulin.
Pharmacology cross reference: Metformin inhibits
gluconeogenesis, making it the premier drug in type 2 diabetes mellitus. Over
time, several mechanisms have been proposed to explain this effect. Among those,
most convincing seems a metformin effect on a mitochondrial redox enzyme, which
leads to increased NADH and diminished NAD+ concentrations in the cytosol of the
hepatocyte. As a result, less lactate can be transformed into pyruvate which is
required for gluconeogenesis. This also explains a tendency to lactic acidosis.
As we will see shortly, we observe a similar effect when metabolizing alcohol, making the metformin + alcohol combination particularly
problematic.
FUNCTION: Amino acid metabolism, nitrogen excretion (urea
synthesis)
DYSFUNCTION: Hepatic
encephalopathy, acid-base instability
Amino acids, derived from muscle protein, are the
main source of material for gluconeogenesis. This solves the problem of glucose
supply in fasting, but there are snags. For starters, if you transform amino
acids to glucose, you are left with the amino groups. A hydrolyzed amino group
is ammonia (NH3) or rather, at physiological pH, the
ionized ammonium ion (NH4+). Surplus systemic ammonium is
only inefficiently eliminated via the kidneys. Elevated levels are toxic,
especially for cells of the CNS.
Disposal of nitrogen also affects acid-base balance, which we addressed
in renal pathophysiology. There, we anticipated a critical
decision made by the liver: the percentages of nitrogen disposed of as urea and NH4+, respectively. We now take a closer look at this regulation.
As usual, while having breakfast, we muse about
pathophysiology. Apart from the nitrogen problem, metabolizing proteins for
energy also has more complex acid-base implications. "Burning" carbohydrates
and fats (bread and butter) results in CO2 and water. CO2 is a potential acid, but gets eliminated via the lungs. "What about
proteins?", we wonder, while enjoying our eggs.
Most amino acids are neutral: they contain two ionized
groups of opposite charge, a carboxy- and an amino-group. When metabolized,
they give rise to the same amount of HCO3− as NH4+ (net, as actual metabolization is
much more complicated). Consumption of 100 g/d of protein results in production
of approximately 1000 mmol HCO3− and 1000 mmol NH4+ per day. From an acid-base
perspective, NH4+ could spare a proton while HCO3− could take one up. Yet, with a pKa of 9.2, NH4+ is quite unwilling to release its
proton at the physiological pH of 7.4. HCO3−, on
the other hand, readily accepts protons, followed by elimination as CO2,
so that the net process would result in alkalization.
In alarm, we almost choke on our scrambled eggs- NH4+ is toxic, HCO3− is alkaline: something must
be done! The simplest and most logical idea would be to fuse the two into some
garbage molecule. Bingo! That's exactly what the urea cycle is about. Compared
to the two rioters, urea is extraordinarily good-natured: nonreactive,
nontoxic, unsuspicious from an acid-base perspective, nitrogen-condensing. A little
hard to excrete, maybe, but we trust our kidneys will come up with something.
Nitrogen-condensing? Yes, because urea contains
two NH2-groups per C=O unit. Um, then how does that
square with our acid-base balance? If we transmogrify NH4+ into a NH2-group, that leaves an H+
on the table; OK, HCO3− is missing one, so those
two may cancel each other out, but what about the second NH4+? When incorporating that one, a
proton is most certainly left over! (A caveat: If we don't intend to become
biochemists, we better leave it at that. Technically, as always, it is more
complicated: the second amino group has its origin not directly in NH4+, but is donated by aspartic acid.
We could try to follow stoichiometry from reaction to reaction, but ultimately,
the fact remains that protons are left over. At this point, we become aware of
another spine-crawling sensation:) Alarm! Have we been putting out the fire
with gasoline? One minute ago, we were whirling into alkalization, now we are
sinking into an acid swamp! What are we to do, renounce proteins completely?
Limit ourselves to sugar and fat? Pure chocolate diet?
Then –phew!- tension falls off: we remember
that our kidneys are quite proficient in excreting acid- what is more, they
even excrete it preferentially in the
form of NH4+! Now, the only thing we need to see to is that
all these acid equivalents are not sent from the liver to the kidneys as NH4+; that would be far too toxic. We
need a secure acid/ammonium tanker: glutamine. From this tanker, the kidney
retrieves NH4+ and excretes it. Each NH4+ excreted by the kidney needs not be
neutralized by HCO3− and thus saves HCO3− (one or one-half, depending on how one looks at it).
Consequently, in the liver we have two options
to deal with NH4+:
1.
We
use it to neutralize HCO3− generated by breaking
down amino acids and put it into urea
2.
We
put it into the glutamine tanker to ship it to the kidneys, where it is
excreted
Option 1 consumes HCO3−,
option 2 saves HCO3−. Now, if we succeed in
controlling the ratio of these two options intelligently, we escape both forms
of potential acid-base catastrophe originated by burning amino acids for
energy.
Lo and behold, the ratio between the two
options turns out to be controlled by pH- that's as intelligent as it gets! A
small dip in pH reduces urea production, yet enhances glutamine synthesis. With
a lower part consumed and a higher proportion saved, the remaining HCO3− counters the upcoming tendency to acidosis. Conversely, an increase in pH has
mirror-inverted results.
[No learning content- Exclusively for our
biochemistry aficionados:
The reduction in urea synthesis rate by dipping
pH is mediated by the enzyme glutaminase. In mitochondria of periportal cells,
glutaminase provides NH4+, which is then fused with HCO3− and an ATP-derived phosphate to carbamoylphosphate. Carbamoylphosphate is fed
into the urea cycle. Activity of liver glutaminase depends directly on pH:
lower pH → less carbamoylphosphate → less urea per unit time.
Increased loading of the glutamine tanker is
simply and directly mediated by glutamine synthetase, which is controlled by pH
in the reverse mode: lower pH → more glutamine that is dispatched
to the kidney. It makes sense that this enzyme is predominantly expressed near
the center of the liver lobule: NH4+ generated in the lobule's periphery
and not incorporated into urea drifts downstream and needs to be loaded onto
the glutamine tanker in its non-toxic form.]
FUNCTION: "Filtering" particulate matter from portal
blood
DYSFUNCTION: Increased
susceptibility to infections
Kupffer cells constitute more than 80% of the
body's resident macrophages. They are very efficient in phagocytosing
particulate matter from portal blood. "Particulate matter" includes
aging red blood cells, but also bacteria that are swept in from the intestines.
Kupffer cells express a large range of receptors for pathogen-associated
molecular patterns (PAMPs). By recognizing, phagocytosing and inactivating
pathogens, Kupffer cells are an important component of the inborn defense
system against infections (please see section on macrophages in immunology lecture notes).
FUNCTION: Elimination of
unwanted or questionable lipophilic molecules (biotransformation,
cytochrome P450 oxidases)
DYSFUNCTION: Toxicity,
depending on specific molecule
Intestinal epithelia are not picky. They absorb
many substances that are potentially noxious or at least of questionable value.
Consequently, it is up to the liver to get rid of them; not an easy task,
especially for lipophilic molecules, as everything that leaves our body is more
or less aqueous (as opposed to lipophilic, anyway). The hepatocyte's solution
is biotransformation, a mechanism consisting of two steps. In a first step, a
reactive group (a "handle"), like –OH, is introduced into the
molecule. Most frequently, this is accomplished by the cytochrome P450 enzyme
system. In a second step, a hydrophilic molecule (e. g., glucuronic acid,
sulfate, glutathione) is conjugated to the handle. Usually, the entire
conjugate is then hydrophilic enough to be excreted via the bile, sometimes
even via the kidneys.
Cytochrome P450 enzymes contain heme as a prosthetic group, with a central coordinated Fe atom
that makes redox reactions possible. (The name P450 is derived from
"pigment with an absorption maximum at 450 nm", from the original
method to measure these enzymes following saturation with CO). The human genome
contains about 50 genes for this type of enzyme, the majority of which are
expressed in hepatocytes. They are designated by enzyme family (numbers),
subfamily (letters) and individual gene, e. g., CYP3A4, CYP2D6,
CYP2C19, CYP2E1. Human individuals differ with respect to the range of their
cytochrome P450 activities for several reasons:
Enzyme induction. The
detoxification system is not rigid but capable of adapting to varying demands.
We often absorb small amounts of all sorts of toxins as we navigate a world
where everyone is grappling to defend themselves and their ecological niche
against others. Consider two examples that we don't usually take up via natural
pathways but that are of medical interest:
·
The Pacific yew (Taxus brevifolia) protects itself from being misused as chow by
synthesizing a substance that interferes with the function of microtubules and,
with that, the cell division spindle. We call this poison paclitaxel and use it
as a drug in cancer therapy, e.g. against breast cancer.
·
The soil-borne bacterium Amycolatopsis rifamycinica keeps
competitors at bay by synthesizing a compound that interferes with the function
of DNA-dependent RNA polymerase from other bacteria. We call this substance
rifampicin and use it as an antibiotic against bacterial infections, e.g.
against tuberculosis.
The two substances are complex and lipophilic. In hepatocytes, they both
bind to a member of the family of nuclear receptors, the pregnane-X- receptor
(PXR). Nuclear receptors are our sensors for lipophilic molecules from both
inside the body and outside, sensors that help us to adapt gene expression to
changed needs. The PXR then binds to response elements in the promoter of
CYP3A4 and other CYPs, increasing their expression. In addition, PXR induces conjugating
enzymes for the second step, e.g. glutathion S transferase, as well as membrane
transporters for discharge of the conjugate into the bile canaliculus. Both
substances are metabolised by CYP3A4. Via this mechanism, paclitaxel and
rifampicin soon accelerate their own degradation while also affecting the
breakdown of other drugs. Progesterone, too, induces the proteins of this
detoxification mechanism via binding to the pregnane X receptor, so that
pregnant women metabolize drugs differently; something that we obviously can
not or do not want to test. An analogous way to react against foreign
("xenobiotics") or endogenous lipophilic substances works via another
nuclear receptor, the constitutive androstane receptor (CAR). CAR binds other
ligands than PXR and activates a different range of cytochrome P450 oxidases.
Parenthesis: Xenohormones
(endocrine disruptors). All
of us are currently participating in a large-scale food experiment: We are
taking in substances that mankind has never encountered before. According to
Paracelsus' dosis facit venenum, we
hope that nothing will happen as long as we keep the dose small. Substances
that work in low concentrations by binding to high-affinity receptors could
spit us into the soup. In particular, lipophilic substances that are easily
absorbed, often accumulate in adipose tissue and bind to nuclear receptors are
under discussion. We express 48 different nuclear receptors; for the majority
of them, the natural ligand is still unknown. Since many nuclear receptors are
expressed in almost all body cells and each affect the expression of hundreds
of genes, a theoretical impact assessment is impossible. Temporal windows of
particular sensitivity (e.g. embryogenesis) and epigenetic effects on later
generations are conceivable. Some examples from a long list of substances
discussed:
·
Bisphenol A (BPA) has
weak estrogenic effects and other poorly understood effects. For example, an
increase in T helper cells in the spleen was reported, which could have an
impact on the incidence of allergies. BPA is found in plastic soda bottles, the
liners of food and beverage cans, plastic toys and was used in baby bottles
until 2011, and on thermal paper from receipts until 2020. In 2023, based on
new data, the European Food Safety Administration recommended reducing the
BPA-TDI (tolerable daily intake) by a factor of 20,000 (from 4 micrograms/kg
body weight and day to 0.2 nanograms).
·
Phthalates have an
estrogen effect. They are used as plasticizers in PVC, also in medical cables
and hoses, and in cosmetics.
·
Atrazine alters
ovarian hormone production. It is widely used as a herbicide in the US but has
been banned in the EU since 2003.
·
Perfluorooctanoic acid (PFOA) binds to estrogen receptor and PPARα. Because of its oil- and
water-repellent properties, PFOA is used in the manufacture of many plastics,
e.g. non-stick coatings for frying pans and outdoor clothing. It is practically
non-degradable, accumulates in fatty tissue and can be detected in almost every
individual today. There are hints that it may promote the development of kidney
and testicular cancer. It has been banned in the EU since 2020.
It is possible, albeit difficult, to reduce potential risks without understanding
them in detail: the principle is to minimize contact between food and plastics.
This is especially true when they are heated (microwave, frying pans, fast food
containers,…). No canned foods. In addition, buying organic reduces potential
risks from herbicides and pesticides.
*
While
biotransformation is all about reducing risks, it inherently also entails
risks: a substance that is entirely harmless by itself may inadvertently be
converted to something more dangerous. A classical example is aflatoxin B1.
Aflatoxin is produced by Aspergillus
flavus, a fungus contaminating peanuts, pistachios, corn etc., if stored at
other than cool and dry conditions. The fungus-produced molecule by itself is inactive
when taken up with food. Yet, the hepatic cytochrome P450 enzymes oxidize it to
a highly reactive metabolite. Aflatoxin-epoxide forms DNA adducts, promoting
mutations and over time, hepatocellular carcinoma.
Let's take a closer
look at the effects of these systems on pharmaceuticals. On the one hand, many
drugs lose activity due to metabolization. With many oral drugs, a so-called
first pass effect is observed: a large fraction is extracted from portal blood
and metabolized so efficiently that it is sometimes hard to reach useful blood
levels.
On the
other hand, also in drugs, metabolization may result in toxicity, a process
that may be enhanced by metabolism of other molecules. A medically relevant case
in point for this type of interactions is the metabolization of alcohol and acetaminophen.
1. Alcohol: Ethanol is mainly
oxidized to acetaldehyde by the enzyme alcohol dehydrogenase (ADH). In
addition, chronic intake of alcohol induces CYP2E1, which uses O2 and NADPH to produce acetaldehyde. This second pathway generates free radicals
and interferes with NADPH-dependent regeneration of glutathion. Even if induced
strongly, the capacity of CYP2E1 remains small compared to that of ADH, leaving
intact the cap on metabolization rate at 0.11‑0.12 g/kg body weight per
hour (in round terms, 0.1 ‰ per hour). Produced at this rate, acetaldehyde
already acts slightly cytotoxic; it is further oxidized to acetate by the
enzyme aldehyde dehydrogenase. Acetate is then activated to Acetyl-CoA. In both
oxidation steps, NADH + H+ is produced. Metabolization of alcohol
thus yields Acetyl-CoA plus NADH, both of which can be used to produce ATP via
citric acid cycle and respiratory chain. Surplus material is simply used for
fatty acid synthesis. By drinking alcohol, we synthesize fat instead of breaking
it down. The pathway explains the two main forms of pathology directly caused
by alcohol: alcoholic hepatitis and fatty liver. With ongoing alcohol
consumption, both of these may over time result in cirrhosis. NADH not only
inhibits fatty acid oxidation. As we have seen with metformin, high
concentrations of NADH also inhibit gluconeogenesis by preventing the oxidation
of lactate to pyruvate. In fact, NADH will cause the reaction to reverse. This
way, too much alcohol may lead to hypoglycemia and lactic acidosis. Type 2
diabetics who consumed alcohol the night before often marvel about their
"excellent" blood sugar levels the following morning.
Two alleles leading
to more rapid accumulation of acetaldehyde are frequent in people from
Hepatic
metabolization of alcohol in relation to body weight is equal in females and males.
Still, the female organism is more sensitive: intake of the same quantity of
alcohol results in higher blood alcohol concentrations. This is not only due to
females' lower average weight: alcohol mainly distributes in the aqueous phase
of the body, a fraction that is smaller in women than in men. A second
difference is an ADH-isoenzyme expressed in gastric mucosa, which is less
active in women. Since at least 20% of alcohol is taken up via the gastric
mucosa, a larger fraction of this percentage reaches the blood in women. Statistically,
the probability of liver cirrhosis in women increases from a daily alcohol
consumption of 20g; in men, this threshold is in the region of 40‑50
g/day. In men, daily intake of 70-80 g/day usually results in cirrhosis.
Alcohol content of
beverages is not given in g, but in % by volume: beer around 5%, meaning 50 ml
per liter; wine around 12%, that is 120 ml/l. To calculate g from ml, the
density of alcohol (about 0.8 g/ml) has to be taken into account, which is
lower than that of water (1 g/ml). Thus, the number of ml times 0.8 equals
the amount of alcohol in g: 0.5 l of beer contain 20 g of alcohol,
0.25 l of wine, about 24 g. Daily intake of the equivalent of 4 (Continental)
beers or one bottle of wine (0.75 l) is beyond the cirrhosis threshold,
even in men.
Blood alcohol level
after a number of drinks can be roughly estimated according to Widmark: blood
alcohol concentration equals the quantity of ingested alcohol (in g), divided
by the person's body weight (in kg) times the estimated fraction representing
its aqueous phase (about 0.6 for women and 0.7 for men). The formula yields g
alcohol per kg distribution volume, a part of which is blood, and therefore
blood alcohol concentration in per mill (g alcohol per 1000 g of blood; 1 per
mill is a tenth of one percent). Usually, this estimate exceeds measured values
by 10-30%, as part of the ingested alcohol is already metabolized during
mucosal passage, and part is excreted (urine, respiration) or metabolized in
the liver while drinking and resorption are still going on. For more accurate
results, alternative methods for estimating blood alcohol content factor in
additional variables such as individual height/weight relations or age. For a
rough estimate of blood alcohol content a few hours after alcohol consumption,
0.11 to 0.12 ‰ (0.011 to 0.012 %) per hour are subtracted from the
starting value.
2. Acetaminophen (called Paracetamol
in German): At recommended dosage, virtually all of acetaminophen is sulfated
and glucuronidated in the liver; only negligible amounts are metabolized by
CYP2E1 to a highly reactive intermediate, NAPQI
(N-acetyl-p-benzo-quinone-imine). NAPQI is toxic, as it has the ability to
covalently bind to cellular macromolecules. To detox NAPQI and other
intermediates, cells produce a certain amount of glutathione, a small, amino
acid-based molecule containing an –SH group. With its many electron pairs, the
sulfur atom reacts with NAPQI and similar molecules very efficiently, making
glutathione a protective shield for cellular macromolecules.
At toxic dosage,
acetaminophen first saturates sulfation and glucuronidation pathways, leaving
more acetaminophen to be metabolized to NAPQI by CYP2E1. Over time, all
available glutathione is consumed, after which NAPQI acts directly toxic. In case
of CYP2E1 induction by chronic alcohol consumption, the fraction of
acetaminophen metabolized to NAPQI is much larger to begin with; in other words,
the threshold for acetaminophen toxicity is much lower (under these
circumstances, cases of liver toxicity by 5‑6 tablets à 500 mg per day
have been described).
These considerations
illustrate the problem caused by cytochrome P450 enzymes in pharmacotherapy. If
effective dose and toxicity of many drugs are influenced by CYPs, but CYP
configurations vary between individuals, many drugs are bound to act
differently in different individuals. A few examples: CYP2D6 is instrumental in
breaking down antidepressants, antipsychotics and some beta blockers, but
required to activate the opioid tramadol (Tramal®-drops) to its
active form. CYP2D6 comes in many allelic variants: some individuals carry
defective alleles, others, especially from
In principle,
critical allelic CYP variants may be diagnosed to identify persons at risk. One
diagnostic test relying on oligonucleotide DNA array technology was introduced
in 2004 (AmpliChip CYP450®). It analyzes CYP2D6 and CYP2C19 genes
for known polymorphisms so that enzyme activities may be extrapolated. Over
time, improvements in high throughput-sequencing should further facilitate
determination of an individual's cytochrome P450 status.
To appreciate the
full potential for complications, however, it has to be kept in mind that these
genetic differences are superseded by differences in expression levels due to
enzyme induction, gender and age, as well as by competitive inhibition if drugs
or food constituents are metabolized by the same cytochrome P450 enzyme. E. g.,
naringenin, a constituent of grape fruit juice, inhibits CYP3A4 and other CYPs,
thereby increasing bioavailability of many drugs, including statins.
FUNCTION: Inactivation
of steroid hormones
DYSFUNCTION: Gynecomastia,
testicular atrophy, changes in body hair pattern
The method of
conjugating lipophilic substances with hydrophilic moieties to facilitate
elimination is also used for endogenous molecules, e. g., for steroid hormones
or bilirubin. This way, steroid hormones are inactivated and excreted. Chronic
liver insufficiency in men causes estrogen, which is produced at low rates, to
accumulate, causing the above-mentioned symptoms.
FUNCTION: Elimination
of bilirubin
DYSFUNCTION: Jaundice
Bilirubin is a
porphyrin metabolite. Its primary source is the heme group of hemoglobin; a
small fraction stems from coenzymes in respiratory chain and cytochrome P450
enzymes. Bilirubin has to be efficiently eliminated, as it is toxic at moderately
elevated concentrations. Several transport systems, organic anion transporter
proteins (OATPs), facilitate bilirubin uptake into hepatocytes, making
problems at this step unlikely. Once within the cell, bilirubin is conjugated with
glucuronic acid by UDP-glucuronyl transferase (UGT). Conjugated bilirubin is then
pumped against a steep concentration gradient into the canaliculus by
canalicular multispecific organic anion transporter (cMOAT, also known as MRP2=
mdr related protein 2, systematic designation ABCC2); this step requires ATP.
In case a transport problem arises at this latter step, both conjugated and
unconjugated bilirubin levels rise, making bilirubin detectable (and visible)
in urine.
Genetic deficiencies
concerning bilirubin elimination include Gilbert-Meulengracht syndrome (very
frequent and harmless), Crigler-Najjar syndromes type I and II (all three of them UGT dysfunctions of
different intensities), Dubin-Johnson syndrome (defective cMOAT) and Rotor
syndrome (extremely rare, combined deficiency of OATPs required for uptake of
bilirubin from sinusoidal blood).
FUNCTION: Elimination
of cholesterol
DYSFUNCTION: Hypercholesterolemia,
dyslipoproteinemia
Our organism is able to synthesize cholesterol,
but by and large unable to break its core structure down- hence the need for
excretion. This is done in two ways: either directly or by conversion to bile
acids. 30-60% of secreted biliary cholesterol is reabsorbed, the rest is
eliminated. In cholestasis, an atypical cholesterol-rich lipoprotein appears in
the blood, lipoprotein‑X.
FUNCTION: (Bile
secretion- in parentheses, as this is no value in itself)
DYSFUNCTION: Cholestasis,
cholelithiasis
Irrespective of its
cause, a stoppage or marked reduction of bile flow is referred to as cholestasis. As there are different
transport systems for different bile components, the term cholestasis is used
for a range of situations, from an impairment concerning all bile components
–as in mechanical obstruction-- to one restricted to bile acids. Cardinal
symptom is itch, caused by a systemic increase in bile acids.
Starting
from cholesterol, hepatocytes synthesize bile
acids by adding hydroxyl groups at positions 7 and 12 and shortening and
oxidizing the side chain to a COOH group. Production is subject to negative feedback:
expression of the rate-limiting enzyme, 7α-hydroxylase, is reduced when
enough bile acids are present. Bile acids in the hepatocyte bind to the
farnesoid X receptor (FXR), another member of the nuclear receptor family that
suppresses the expression of the 7α-hydroxylase gene (CYP7A1). De novo
synthesis yields the primary bile acids cholic acid and chenodeoxycholic acid,
which are frequently conjugated with either taurine, glycine, sulfate or
glucuronate. In the intestine, bacteria partially remove the 7α-hydroxyl
group, forming the secondary acids deoxycholic and lithocholic acid.
Unconjugated bile "acids" are very weak acids, while conjugated forms,
which have lower pKa, are mostly ionized and are therefore referred to as bile
salts. Bile salts and -acids are subject to enterohepatic recirculation: the
entire pool is recirculated 5-10 times a day. This requires efficient transport
proteins at both sides of the hepatocyte, which have to accommodate all these
various forms and thus cannot be terribly specific. Consequently, they are also
able to transport other molecules such as certain drugs or complex toxins.
Transport from portal blood into the hepatocyte is facilitated by the Na+-powered Na‑taurocholate cotransporting
polypeptide (NTCP, SLC10A1). In addition, the family of organic anion transport proteins
(OATPs, encoded by genes SLCO1A2, SLCO1B1 and SLCO1B3) transport ionized bile
salts into the cell (OATPs have also been shown to transport the death cap's poison, amanitin, into
hepatocytes). Protonated bile acids can enter the hepatocyte by non-ionic
diffusion. In hepatocytes, bile salts are buffered by binding proteins. They
are then actively secreted into the canaliculus against a 100- to 1000-fold concentration
gradient by the ATP-driven bile salt export pump (BSEP, ABCB11). For sulfated
and glucuronidated forms, cMOAT is used, too. If the bile acid concentration in the hepatocyte
increases, the farnesoid X receptor induces BSEP, so that more bile acids can
be secreted. Defective
BSEP alleles cause familial cholestasis syndromes of varying intensities.
Pharmacology cross reference: OATP1B1 (gene: SLCO1B1) also transports
statins from the blood into the hepatocytes; their main site of action but also
where they are metabolized and excreted. A common allele – 18-28% heterozygous,
2-3% homozygous in a European genetic background –, T521C with the amino acid
exchange valine 174 to alanine, shows a greatly reduced transport rate for
statins, so that statin plasma concentrations are increased in these
individuals, with a concomitant increase in the risk of myopathy. This effect
is not equal for all statins; it is more pronounced for simvastatin than for
others. Other drugs taken up via OATP1B1 amplify this effect; these include
amiodarone and cyclosporine.
Apart from problems
caused elsewhere in the body –generalized pruritus, fat maldigestion, increase
in cholesterol levels, jaundice etc.--, cholestasis also feeds back onto the
performance of hepatocytes themselves. Bile salts are pretty toxic molecules to
begin with. Increasing their cellular levels leads to atypical, fetal bile
salts with additional hydroxylations in wrong positions. These are even more
toxic, further increasing cholestasis. Generally, many different conditions are
able to cause cholestasis: cholestasis is a logical result of acute hepatitis;
it may be a relatively isolated adverse reaction to some drugs (e. g.,
competitive BSEP inhibition by steroids, ciclosporin A, rifampicin) or it may
be induced mechanically by gallstones or tumors.
The
frequent occurrence of gallstones is
not surprising seen the small window of solubility for lipophilic molecules in
a largely aqueous transport medium. Typical ranges for biliary molecules other
than water would be around 67% bile
salts and 22% phospholipids, both of which are required to keep 4% of
cholesterol and fractions of a percent of bilirubin-diglucuronide in solution. As
soon as cholesterol or bilirubin exceed certain thresholds, bile salts and
phospholipids fail to keep them soluble in the form of micelles and stones
start to nucleate. Small crystals can still be transported into the intestine; slightly
larger conglomerates are dangerous, as they can get stuck at bottlenecks of the
biliary duct system. Large stones may completely fill the gallbladder without
causing symptoms. Cholesterol "stones" are the most frequent type; if
dark Ca-bilirubinate predominates, we speak of pigment stones.
DYSFUNCTION: Steatorrhea,
vitamin deficiencies ADEK
Bile is a form of
liquid soap required to emulgate nutritional fat. Active substances are bile
acids and phospholipids. Triglycerides constitute more than 90% of fat in food,
forming fairly large liquid droplets at the body temperature around 37°C.
Lipases, protein enzymes better soluble in the aqueous phase than in lipids,
can only be active at the surface of these droplets. To digest fat efficiently,
it is thus necessary to massively increase this boundary surface, which can
only be done by addition of large amounts of surface-active bile acids and
phospholipids. In the intestine, these biliary fats make up two to four times
the amount of fats from food. Lipases cleave larger, more neutral lipid
molecules like triglycerides, cholesterol ester or lecithin into smaller,
relatively more water-soluble fragments such as fatty acids, monoglycerides, cholesterol
or lysolecithin, which in turn line up at the boundary layer and contribute to
increasing total surface area. By this continuous redistribution in favor of
surface-active fragments, fat droplets shrink over time from large,
multilamellar vesicles to small vesicles with a single double membrane and
further to tiny mixed micelles, with a single layer of surface-active lipids
surrounding fewer and fewer neutral lipids. These tiny structures are able to
diffuse into the mucus layer at the enterocyte surface that is continuously
acidified by the Na+-driven Na+-H+-antiporter,
until the micelles practically bump into the apical brush border of the cells (otherwise,
contents of the small intestine are alkaline). At this low pH, fatty acids are
protonated, facilitating non-ionic diffusion into and through the cell
membrane. Other lipids enter the cell by diffusion, too; in addition, uptake of
some of them is probably facilitated by membrane transport proteins. Within the
cell, lipids are reassembled, combined to chylomicrons and set free at the
basolateral side of the enterocytes. From the extracellular fluid, they reach
the blood via the lymph stream at the venous angle, bypassing the liver. In
case too little bile reaches the intestine, fat from food is insufficiently
digested and absorbed; most of it is eliminated in a light-colored, voluminous
form of diarrhea termed steatorrhea.
Over time,
this condition may cause a deficiency of lipid-soluble
vitamins. Most prominently, impaired coagulation, a common occurrence in
liver insufficiency, is aggravated by vitamin K deficiency (explained in the
next section). Compared to vitamin K, deficiencies of other lipid-soluble vitamins
are of minor importance. Vitamin A is stored in hepatic stellate (Ito) cells.
In the blood, it is transported via retinol-binding protein that is itself
synthesized in the liver. Low levels of vitamin A may cause impaired vision in
the dark ("night-blindness"). Vitamin D is either taken up with food,
or produced within the body from 7-dehydrocholesterol with the help of
ultraviolet light (UV). In both cases, the inactive precursor has to be
activated by two sequential hydroxylation steps. The first is performed in
hepatocytes by cytochrome P450 hydroxylation at position 25. The second is done
in the kidney under tight regulation by parathyroid hormone. Lack of vitamin D
over time lowers Ca2+ reserves of the body, leading to insufficient
mineralization of bone. Finally, vitamin E has antioxidant function. No defined
symptoms of a vitamin E deficiency are known.
Pharmacology cross reference: The
transmembrane protein NPC1L1 (Niemann-Pick C1-Like 1) is involved in the
transport of cholesterol from the intestinal lumen into the enterocytes. Ezetimibe can block this transport,
preventing uptake of dietary cholesterol as well as re-uptake of cholesterol
secreted via the bile. It thus reduces LDL cholesterol levels, but this effect
is not on par with that of statins.
Orlistat covalently and
irreversibly blocks lipases in the gastrointestinal lumen. It is approved in
the EU for weight loss. Fats cannot be absorbed unless they are broken down by
lipases. This kind of treatment enforces dietary discipline, since even a relatively
modest dietary fat content leads to explosive diarrhea with fatty stools.
FUNCTION: Synthesis
of plasma proteins (albumin, clotting factors, acute phase proteins,
transferrin, etc.)
DYSFUNCTION: -Hypoproteinemia/ edema/ ascites
-Clotting
problems (coagulopathy)
The majority of plasma proteins is
synthesized and secreted by the liver. Therefore, chronic liver dysfunction
results in reduced plasma protein concentrations. Albumin, accounting for 60% of total plasma protein, is
instrumental in maintaining oncotic pressure, necessary to reabsorb interstitial fluid into the venous
leg of capillary vessels. Low albumin levels cause fluid to accumulate
interstitially, while blood volume tends to be low. As chronic liver
dysfunction is frequently associated with cirrhosis and portal hypertension,
the combination of increased portal filtration pressure with reduced oncotic
pressure frequently results in pronounced ascites.
Acute phase proteins like C‑reactive
protein (CRP) or mannan-binding lectin (MBL) contribute to defense against
infections. More information may be found in the lecture notes on immunology.
Clotting factors are adversely affected by hepatic insufficiency via two mechanisms. In
addition to a general shortfall in protein synthesis, specific factors lose
biological activity due to vitamin K deficiency. Vitamin K is required to add
additional carboxyl groups to the second to last-carbon atom of glutamic acid
residues of clotting factors II, VII, IX and X, producing two adjacent COO--groups. Via binding to Ca2+,
these double COO--groups
anchor the respective factor to the phospholipid membranes of aggregating
thrombocytes (remember that one way to prevent clotting of a blood sample is to
remove Ca2+ by citrate or EDTA). In the absence of the second COO-,
the factors remain soluble and never meet, further impairing blood coagulation.
Vitamin K is also required for Ca2+-binding proteins in bone, e. g.,
osteocalcin. Of course, in vitamin K deficiency, reduced blood clotting causes
symptoms far earlier than problems with bone mineralization.
Pharmacology cross reference: Derivatives of coumarin (acenocoumarol, phenprocoumon) are vitamin
K antagonists, inducing artificial vitamin K deficiency with the purpose to
inhibit clotting activity (e. g., following pulmonary embolism). In case these
drugs are discontinued, e. g., to allow dental work or surgery, it takes
considerable time until sufficient amounts of biologically active factors are
resynthesized.
FUNCTION: Monitoring
intestinal import in a low-pressure capillary system
DYSFUNCTION: Portal hypertension
In events causing pronounced loss of hepatocytes
(e.g., viral hepatitis, alcohol, sustained cholestasis) the liver's attempts to
regenerate lead to secondary remodeling, coarsening the organ's delicate
architecture. In addition, hepatic stellate (Ito) cells are activated to increase
production of extracellular matrix, e. g., collagen and proteoglykans, causing
fibrosis. The increase in diffusion distances, combined with reduced
endothelial fenestration, impair the exchange of material between hepatocytes
and blood plasma. Destruction of normal liver architecture reduces the total
cross section of all portal blood vessels, resulting in portal hypertension
(imagine a clogged filter).
In turn, portal hypertension causes
hypersplenism (sequestration and breakdown of blood cells in the spleen),
portocaval anastomoses (esophageal and gastric varices, hemorrhoids, caput
medusae), and ascites.
Via portocaval anastomoses, blood coming from
the intestine is shunted directly into the systemic circulation, avoiding the
liver with its filtering and detox mechanisms. Esophageal varices may rupture,
leading to life-threatening bleeding episodes that are extremely hard to stop.
Hepatorenal syndrome is renal failure due to underperfusion, secondary to liver dysfunction. Two
mechanisms are thought to contribute to renal underfilling: the reduction in
effective blood volume by reduced oncotic pressure and the dilation of blood
vessels in the splanchnic circulation "stealing" blood from the
systemic circulation. The decrease in effective blood volume sensed by the
juxtaglomerular apparatus permanently activates the renin-angiotensin-aldosterone
system, with secondary hyperaldosteronism in turn aggravating ascites by
retention of sodium and water.
***
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TEXTBOOK SOURCES AND FURTHER READING: Kumar V. et al. (eds.): Robbins and Cotran Pathologic Basis of Disease, 10th Edition, Elsevier, Philadelphia, 2021 Boron W. F. and Boulpaep E. L. (eds.): Medical Physiology, 3rd Edition, Elsevier, Philadelphia, 2017 in German: Höfler G. et al. (eds.) Pathologie, 6th Edition, Urban und Fischer, 2019 Schwarz et al. (Hrsg.): Pathophysiologie, Maudrich, Wien, 2007 Siegenthaler W. und Blum H. E.(Hrsg.): |