BONE
METABOLISM
These lecture notes accompany my lectures on pathophysiology in the study module "Musculoskeletal System" at the Medical University of Innsbruck. 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 1.7 e ©Arno Helmberg 2009-2020
Terms of use
1. BONE FORMATION AND BONE RESORPTION
Living bone is constantly being remodeled. The state
of our bones is always close to an equilibrium between bone formation
and bone resorption. In childhood and during the teens, bone formation
is slightly predominant. We reach peak bone mass in the twenties, and from
then onwards, resorption has the upper hand. There are two reasons for
the constant remodeling process. Firstly, it allows our bones to adapt
to changes in load. For example, consider how easily skilled orthodontists
maneuver teeth in the jaw bone by applying modest targeted strain. Secondly,
continuous remodeling is necessary to repair the damage caused by recurrent
microtraumas. At a typical
remodeling site, termed basic multicellular unit, specialized osteoclasts
first remove bone over a period of approximately three weeks. The resulting
resorption lacuna is subsequently filled by osteoblasts, a process lasting
about three months. Bone tissue is found in two forms: substantia compacta and substantia spongiosa. As much weight is
saved as possible: only the outer contour, the cortical compact bone,
is massive. The inner part is made up of trabecular, or cancellous bone,
a three-dimensional scaffold of pillars and beams that is constantly
modified to accommodate load. Prominent examples of cancellous bone
are found in vertebral bodies or at the ends of long bones.
The fundamental unit of compact bone is the osteon or
Havers system. A central vascular canal is surrounded by massive concentric
lamellae of mineralized fibers. In consecutive lamellae, matrix fibers
are arranged in spirals with alternating sense of rotation, contributing
to mechanical strength. Encased bone cells, osteocytes, are interspersed
between lamellae.
In essence, bone metabolism is due to only two types
of cells: osteoblasts and osteoclasts. Osteocytes are simply osteoblasts
that have encased themselves in bone. Individual osteocytes remain connected
by long cellular processes, forming a network connected by gap junctions.
Osteocytes are able to sense mechanical strain, which they report to
the bone construction units via this network.
Osteoblasts
differentiate from stromal marrow cells. They produce the organic part
of the bone matrix, an array of proteins collectively
termed osteoid. Out of a much larger number,
let's take a look at three functionally important proteins:
- Collagen
type I represents the bulk of osteoid. It consists of triple helix
units containing two α1-chains and one α2 chain, which
already form in the endoplasmic reticulum of the osteoblast after
the individual chains have been posttranslationally hydroxylated
on lysines and prolines. This procollagen unit is secreted, followed
by proteolytic removal of C- and N-terminal peptides. The resulting
collagen monomers spontaneously aggregate in a staggered fashion,
forming long fibrils that are subsequently covalently cross linked
via their hydroxylated lysines. A cofactor required for lysine and
proline hydroxylation is vitamin C. Lack of vitamin C results in
scurvy, characterized by collagen that is instable due to insufficient
cross linking.
- Osteocalcin
is a small protein that is carboxylated on glutamic acid residues
with the help of vitamin K. As glutamate already contains one COO−‑group, carboxylation of the γ-C-Atom
creates a second one right next to the first. The two adjacent negative
charges are ideal docking sites for double positive Ca2+
ions. Osteocalcin binds hydroxyapatite Ca5(PO4)3(OH),
but is not required for its formation, as osteocalcin null mice have
increased bone mineralization. The hydroxyapatite nucleation process,
which makes sure that Ca2+ and phosphate precipitate in the
bone and not in other tissues of the body, therefore must rely on other
osteoblast products. But fracture toughness in these mice is substantially
reduced: osteocalcin seems to prevent crack growth by stretching and
dissipating energy. Osteocalcin may thus function as a shock-absorber
between organic and inorganic matrix components.Vitamin K is also
necessary to carboxylate clotting factors II, VII, IX, X, providing them
with functionally essential Ca2+ binding sites. Therefore, deficiency
of vitamin K results in bleeding disorder long before effects on bone
might cause problems. A second vitamin is important for osteocalcin: the transcription
of its gene is induced by activated vitamin D receptor. Osteocalcin itself
has a second function, too. A proportion of non-carboxylated osteocalcin
enters the blood stream and functions as a metabolic hormone enhancing
insulin activity. Via a G
protein-coupled receptor, it stimulates proliferation of pancreatic
β-cells, and sensitizes fat cells to insulin by stimulating them to
secrete adiponectin. Via this mechanism, bone metabolism influences energy
metabolism.
- Osteonectin
is an osteoid component that makes contact to collagen type I as
well as to hydroxyapatite, forming a link between organic and inorganic
bone matrix.
In addition, osteoblasts engage in targeted export of
Ca2+ and phosphate, inducing local super saturation
conditions to mineralize the freshly produced osteoid. For this process,
alkaline phosphatase tethered to the outside of the osteoblast plasma
membrane seems to be important, although the enzyme's role remains insufficiently
understood. It may increase extracellular phosphate concentration by
dephosphorylating organic molecules or cleaving pyrophosphate.
Bone statics may be compared to the statics of
reinforced concrete. Hydroxyapatite is highly resistant to compressive
stresses, while the built-in collagen fibers provide the combined matrix
with high strength in tension. The comparison with armored concrete
illustrates why bone formation and bone resorption have to go hand in
hand. Mechanical strain constantly results in micro fissures in the
bone matrix. There, collagen ("steel") fibers are torn. Repair
involves a large resorption lacuna, allowing to embed new fissure-spanning
fibers in fresh mineral matrix ("concrete"). Pure mineralization
of ("plastering over") the fissure would not restore the structure
to original strength. This is convincingly demonstrated in rare genetic
diseases with defective bone resorption (osteopetrosis). The result
is bone tissue that is extremely dense but at the same time fragile,
as it is permeated by insufficiently repaired micro fissures that have
been merely plastered over.
Once osteoblasts
have encased themselves in bone, they change their expression pattern, becoming osteocytes. Osteocytes secrete
sclerostin, which inhibits further bone formation in nearby osteoblasts by
binding and antagonizing LRP5/6 receptors, thereby inhibiting Wnt-signaling. Thus,
sclerostin promotes "sclerosis", rigidification of bone. Factors
promoting remodeling, e.g., mechanical
loading, parathyroid hormone or prostaglandin E inhibit production of sclerostin.
Pharmacology cross reference: Romosozumab is a monoclonal antibody binding to and inhibiting
sclerostin. In phase III testing, it showed good efficacy against osteoporotic
fractures, but there were more adverse cardiovascular events compared with
controls. In 2019, romosozumab was approved subject to conditions.
Osteoclasts
are giant, multinucleated cells that derive from hematopoietic stem
cells in the bone marrow, branching from the lineage leading to macrophages
and neutrophils. A series of cytokines induces precursor cells to differentiate
to osteoclasts. The basic mix combines M-CSF
(macrophage colony stimulating factor) with RANKL (explained in the
following section on parathyroid
hormone), two cytokines produced by osteoblasts. In addition, mediators
produced by macrophages and other cells during inflammatory responses
enhance osteoclast differentiation: IL‑1, IL‑6 , TNFα
and prostaglandin E. Osteoclasts break down bone tissue much like macrophages
break down phagocytosed material; only the process is shifted to the
extracellular space. Employing normal lysosomal chemistry, it involves
acidification and activation of acid hydrolases. Osteoclasts seal off
a certain matrix area, which they acidify with the help of a proton
pump. To maintain intracellular pH, they release HCO3- at their back side. Hydroxyapatite
dissolves in the acidic environment, setting free Ca2+. Thus,
on the scale of the entire body, an orchestrated activation of osteoclasts
is a means to increase extracellular Ca2+-concentration.
After the mineral has melted away, acid proteases like Cathepsin K hydrolyze the remaining matrix proteins.
Growth of long bones is not possible in bone tissue itself, but happens in epiphyseal cartilage, the growth plate. Three zones of chondrocytes at different stages of differention can be observed. In all three zones, chondrocytes secrete poteins and proteoglycans that form the cartilage extracellular matrix, like collagen and aggrecan. Closest to the epiphysis is the resting zone, containing chondrocytes that serve as progenitor cells. Next is the proliferative zone, where spatial arrangement of cell division leads to long columns of chondrocytes parallel to the long axis of the bone. These cells produce collagen type II, which is characteristic for hyaline cartilage. Near the metaphysis, chondrocytes in the hypertrophic zone undergo terminal differentiation, grow in volume and secrete collagen type X and VEGF (vascular endothelial growth factor). At the border zone, hypertrophic chondrocytes undergo cell death. Attracted by VEGF, new capillaries sprout into the zone. The cartilaginous tissue is first simply mineralized (enchondral ossification), but soon remodeled to osteon structure by immigrating osteoclasts and osteoblasts. So, growth in cartilage results in elongation of the bone. The regulation of this process is complex. Genome-wide association studies identified about 200 genetic loci that influence height. Proliferation of chondrocytes is regulated by a multitide of paracrine factors, such as a bone morphogenetic protein gradient or C-type natriuretic factor, as well as endocrine factors, such as growth hormone, IGF-1, sex steroids and leptin. The endocrine factors link rapid growth to the availability of sufficient nutrients.
A second
ossification mechanism, intramembranous ossification, is the direct
transformation of fibrous mesenchymal tissue to bone. This type of ossification
is found in the development of large parts of the skull, as well as
in healing of bone fractures.
2.
REGULATION OF BONE METABOLISM
2.1 Calcium and phosphate balance
We looked at calcium and phosphate before, when we studied their renal handling. Here, our goal is to understand the interdependence between calcium/ phosphate balance and our bones.
2.1.1 Soluble Ca2+, hydroxyapatite and
calcitonin
As calcium (Ca2+)
is one of the main components of our bones, large amounts are present
in our body. At the same time, comparatively low extracellular concentrations
of Ca2+ are fine-tuned to regulate important functions, not
to speak of even far lower intracellular concentrations. This dichotomy
is possible due to the low solubility product of Ca2+ and
phosphate (PO43−): if one ion is added to a solution
of the other, most of it precipitates as calcium phosphate.
In the bone, the two ions combine with hydroxide (OH-)
to form hydroxyapatite Ca5(PO4)3(OH),
a hard mineral forming hexagonal crystals. Up to 70% of the weight of
bone is due to hydroxyapatite. Dental enamel consists almost exclusively
of the mineral, accounting for its mechanical resistance. The disadvantage
to this solution is that hydroxyapatite is sensitive to acidity. Low
pH attacks enamel via the same mechanism that osteoclasts use to resorb
bone. Citric acid from an orange, or lactic acid produced by bacteria
metabolizing sugar in dental plaque make protons come into contact with
the enamel surface. A proton H+ pulls out the hydroxide ion
OH− from Ca5(PO4)3(OH)
to form a H2O water molecule, with the rest disintegrating
into 5 Ca2+ und 3 PO43− ions. The hydroxyapatite complex
dissolves, ultimately leading to caries. If OH-
is replaced by a fluoride ion F- to form fluorapatite Ca5(PO4)3F,
the mineral is much more stable at low pH. Fluorapatite forms spontaneously
if enough fluoride ions are present, a condition that can be promoted
by addition of fluoride to toothpaste, salt or, in some countries, drinking
water.
Plasma Ca2+ concentration is physiologically
maintained in a small window between 2.2 and 2.7 mM. This measured Ca2+
is the sum of three forms: Ca2+ bound to plasma proteins
(about 45%), Ca2+ complexed with small organic anions (10%)
and free ionized Ca2+ (about 45%). Hence, total Ca2+
depends on plasma protein concentration. The biologically relevant,
regulated parameter is free Ca2+.
Ca2+ balance is basically maintained by two
hormones: parathyroid hormone (PTH) and calcitriol (1,25-dihydroxyvitamin
D). PTH regulates short-term plasma Ca2+ concentrations by
dipping into bone reserves. Vitamin D strategically maintains the total
Ca2+ pool of the body. A third hormone, fibroblast growth factor
23 (FGF23), regulates elimination of phosphate via the kidneys, which directly impacts
on the calcium balance.
A fourth Ca2+ regulating
hormone, calcitonin, is of minor importance in
humans. It is secreted by parafollicular C cells in the thyroid gland
and lowers plasma Ca2+ levels for a short time by directly
inhibiting osteoclast activity, with the system quickly swinging back
to a neutral position. Neither a total loss of calcitonin-producing
cells (e. g., by thyroidectomy), nor massive overproduction by rare tumors lastingly interfere with Ca2+
balance. Probably, calcitonin is a remnant from evolution. Animals
such as salmon, which switch from fresh water to calcium-rich sea water,
seem to rely on calcitonin to cope with massive differences in Ca2+
intake.
Pharmacology cross-reference: Salmon calcitonin is actually used to treat patients,
although it is nowadays produced recombinantly or by peptide synthesis.
Why don't we use the human version? On a molar basis, salmon calcitonin
is about 10 times as potent as the human peptide. Although it differs
from the human version in 14 of the 32 amino acids, immunological complications
are surprisingly rare. Calcitonin is used to lower Ca2+ in
acute hypercalcemic situations. In addition, it is used in diseases
with high bone resorption to intermittently inhibit osteoclast activity,
e. g., in osteoporosis, Paget's disease and bone metastasis, where it may have the added
benefit of alleviating pain.
2.1.2
Parathyroid hormone
Parathyroid hormone (PTH) is named for the four parathyroid
glands producing it, tiny epitheloid bodies located right behind the
thyroid. An increase in the concentration of free Ca2+ activates
the calcium-sensing receptor (CaSR)
located at the membrane of their chief cells. The cells react by decreasing
PTH production. A second means to lower PTH secretion is a high concentration
of 1,25 dihydroxyvitamin D. The message of Vitamin D seems to be: "Stop
cannibalizing our bones, I'll organize more Ca2+ from outside
in a minute!" PTH is a small protein of 84 amino acids and has
an extremely short half-life of about four minutes. PTH increases Ca2+
concentration via two main mechanisms: by liberating it from bone and
by influencing the kidneys.
PTH's net effect in bone is an increase in resorption
by activation of osteoclasts. This is achieved via a detour, as osteoclasts
do not express PTH receptors. PTH is sensed by osteoblasts, which react
by producing IL‑1, IL‑6 and other cytokines to activate
osteoclasts. In addition, PTH increases osteoblast production of the
two molecules that induce differentiation and proliferation of more
osteoclasts: M-CSF (macrophage colony-stimulating factor) and RANKL.
RANK-ligand (RANKL)
is a molecule from the TNF-superfamily. It acts as a trimer, either
on the surface of osteoblasts, or, "cut off", as a soluble
signaling molecule. In the bone marrow, M-CSF and RANKL encounter precursor
cells of the hematopoietic lineage leading to macrophages and neutrophil
granulocytes. These precursor cells express RANK (receptor-activator
of NFκB),
a transmembrane protein of the TNF receptor superfamily. RANK functions
as receptor for RANKL. As precursor cell RANK is trimerized by osteoblast-emitted
RANKL, the precursor cells are activated to differentiate first to mononucleated
osteoclast precursors that subsequently fuse to mature polynucleated
osteoclasts. Osteoblasts secrete a further protein, osteoprotegerin
(OPG), that looks like a soluble receptor for RANKL. This is called
a decoy receptor; by neutralizing RANKL, it acts as its inhibitor. Thus,
the formation rate of osteoclasts depends on the relative amounts of
RANKL and OPG produced by osteoblasts. While PTH induces expression
of M-CSF and RANKL, it inhibits production of OPG, cranking up the generation
of osteoclasts.
If PTH just mobilized Ca2+, not much would
be gained: due to the low solubility product with phosphate, it would
soon reprecipitate. Therefore, PTH simultaneously lowers phosphate levels
by inhibiting renal reabsorption in both the proximal and distal tubule.
This is achieved by removing the Na/phosphate cotransporter from the
luminal membrane and parking it in vesicles below. Apart from inducing
phosphaturia, PTH increases reabsorption of Ca2+ in the distal
tubule, further reducing the already minimal loss of Ca2+
in the urine. The third renal function of PTH is to stimulate hydroxylation
of carbon atom 1 of vitamin D: this is the last and rate-limiting step
in its activation. From there, 1,25 dihydroxyvitamin D sets out to refill
the Ca2+ pool.
Pharmacology cross-reference: Cinacalcet (Mimpara®, Sensipar®) is a small molecule binding
to another site of the calcium-sensing receptor (CaSR), allosterically
sensitizing the receptor to free Ca2+. Its main use is in
treating secondary hyperparathyroidism in patients with chronic renal
failure on dialysis. Failing kidneys excrete too little phosphate and
activate too little vitamin D, resulting in high plasma phosphate and
low Ca2+, a dysequilibrium that is ultimately taken out on
the bones via PTH.
2.1.3
Vitamin D
Vitamin D3 is
actually a hormone produced in our own skin from 7-dehydrocholesterol. This
requires sunlight to open the second ring of the cholesterol backbone. This UV B-dependent
synthesis is probably the cause of Caucasians' pale complexion. Until the first
wave of homo sapiens left Africa
about 60,000 years ago, probably all modern humans had dark skin. The further
north the people migrated, the less ultraviolet light they absorbed. Those with
lighter complexions obtained a selective advantage, as they were better able to
synthesize vitamin D.
(The selective
advantage of individuals with a lighter complexion in northern regions was
probably not only due to better bone stability. UV B-generated Vitamin D
also seems to play a role in combating infectious disease. Low vitamin D levels
during wintertime may contribute to susceptibility to infections of the
respiratory tract. It seems that vitamin D is important for macrophage
function. Remarkably, macrophages are able to activate vitamin D by themselves
by expressing 1α-hydroxylase, which is otherwise only expressed in the
kidneys. On vitamin D stimulation, macrophages increase synthesis of the
antibacterial peptide cathelicidin. Largely for empirical knowledge, patients
of the mountain-based tuberculosis clinics in Europe during the 19th and early 20th century were seated outside in the sun every day,
even in winter. At that altitude, this approach increased UV exposure.
The ability of
macrophages to activate vitamin D is of particular importance in sarcoidosis.
Via this mechanism, sarcoidosis may be accompanied by hypercalcemia. Expression
of 1α-hydroxylase in macrophages is not under control of PTH.
Occasionally, hypercalcemia has also been described in other granulomatous diseases,
e.g. tuberculosis or leprosy.)
Ethnic groups using
the sea as their primary food source, like the Inuit, took up enough vitamin D3
with their food and were thus able to retain a higher level of pigmentation
than people living off the land. So, in the absence of sufficient de novo
synthesis, fat-soluble vitamin D3 can as well be taken up with animal source food
(especially abundant, for example, in fatty fish such as cod -think of cod
liver oil!- mackerel, salmon). The causal relationship between a lack of
sunlight and rickets was only recognized in the late 19th century.
As an alternative to synthesis or intake of cholecalciferol
(vitamin D3), a very similar molecule, ergocalciferol (vitamin D2, with a slightly different side chain, formed by
breakdown of the fungal sterol ergosterol) is present, e. g., in UV-irradiated
mushrooms, but its concentration
is usually too low to make up for the deficit.
Two successive hydroxylation steps are required to metabolize
D3 and D2, which already contain one hydroxyl group, to their active
form, calcitriol. The first hydroxyl group is added at position 25,
the end of the side chain, in the liver. The second hydroxylation occurs
in the kidney, at position 1 of the first ring of the erstwhile cholesterol
structure. This decisive, last activation step is performed in the proximal
tubule under tightly regulated conditions. PTH stimulates hydroxylation,
while the end product calcitriol as well as increased levels of FGF23 and/or phosphate
act inhibitory. 1,25-dihydroxyvitamin D (calcitriol) equilibrates over
the entire body and binds to the vitamin D receptor (VDR), a member
of the nuclear receptor superfamily. As a ligand-dependent transcription
factor, one of its functions is the induction of genes that are necessary
to maintain Ca2+ reserves.
The central target organ in this respect is the duodenum.
Here, calcitriol induces several proteins that in concert enhance absorption
of Ca2+ from food. While Ca2+ concentrations in
the lumen of the gut and in blood are in the nanomolar range, they are
much lower inside the cell; too much free Ca2+ in the cytosol
would be dangerous. At the luminal side of the duodenal epithelial cell,
vitamin D induces a channel, allowing Ca2+ to trickle in
passively. In the cytosol, the Ca2+-affine protein calbindin
is increased to neutralize passaging Ca2+. At the basolateral
membrane, an ATP-driven Ca2+‑H+-antiporter
as well as a Na+-driven Ca2+‑Na+-antiporter
are induced to pump Ca2+ into the blood against a steep concentration
gradient. Calcitriol also enhances phosphate absorption in the small
intestine.
In the kidney, the action of vitamin D parallels that
of PTH by increasing reabsorption of Ca2+ in the distal tubule,
although its effect is much weaker. Contrary to PTH, vitamin D also
enhances reabsorption of phosphate: both ions are required to promote
bone mineralization.
Together, these
effects of vitamin D raise Ca2+ and phosphate concentrations above
their solubility product, inducing their precipitation in osteoid. This predominant, indirect effect outweighs an
opposite direct, receptor-mediated activation of osteoblast and osteoclast
precursors that would enhance bone turnover and Ca2+ mobilization.
In addition,
vitamin D-stimulated transcription of the osteocalcin gene in osteoblasts helps
to build fracture resistance.
2.1.4. Fibroblast growth factor-23 (FGF23)
FGF23 is produced by
osteocytes and osteoblasts in response to 1,25-dihydroxyvitamin D and dietary
phospate loading. It increases renal phosphate excretion by reducing the number
of Na/phosphate cotransporters in the apical membrane of the proximal tubule.
In this function, it acts similar to PTH. Yet, it counters PTH by inhibiting 1α-hydroxylation
of vitamin D. It thus lowers active vitamin D, which in turn reduces uptake of Ca2+ via the intestines
CKD-MBD (chronic kidney disease- mineral and bone disorder): We take up all
available phosphate from the intestines and balance that by eliminating the
surplus via the kidneys. In many old people, a problem arises from the decline
in glomerular filtration rate. As filtrated volume comes down, a progressively
higher percentage of filtrated phoshate needs to be excreted, so FGF23 levels
steadily rise higher and higher. FGF23 keeps calcitriol down, meaning plasma Ca2+ concentration can only be
maintained by parathyroid hormone. Over time, secondary hyperparathyroidism
results in bone disorder.
X-linked hypophosphatemia: The X-chromosomal PHEX gene (phosphate-regulating neutral
endopeptidase, X-linked) encodes a peptidase which inhibits FGF23. Deficiency
of this peptidase causes FGF23 hyperactivity resulting in renal phosphate
losses. In affected children, the disease is reminiscent of rickets, yet does
not respond well to vitamin D. Instead,
patients may benefit from a monoclonal antibody against FGF23, Burosumab.
2.2 Growth hormone and IGF‑1
Growth hormone (GH) is essential for longitudinal bone
growth. It is a 191 amino acid protein produced by somatotrophs in the
anterior pituitary under control of the hypothalamus: GH secretion is
stimulated by GH releasing hormone (GHRH), inhibited by somatostatin.
GH is secreted in short bursts of pulses only during sleep or during
exercise (it makes no sense to assay GH levels at daytime in a child
at rest). GH has a few fast, direct effects that are almost directly
opposed to the more important slow, indirect effects via insulin-like
growth factors. The receptor-mediated fast effects are antipodal to
insulin action and include lipolysis in fat cells, gluconeogenesis in
liver and an inhibition of glucose uptake by muscles. Many other effects,
including those promoting growth in cartilage and bone, are mediated
indirectly. GH stimulates hepatocytes to secrete insulin-like growth
factor‑1 (IGF‑1) into the blood. Likewise, many other cells,
including chondrocytes and osteoblasts, are induced to produce IGF‑1
that acts in a paracrine fashion.
IGF‑1 is closely related to insulin, with slightly
less than 50% identical amino acids. Like insulin, it binds to a heterotetrameric receptor consisting of two
extracellular α chains and two transmembrane β chains containing
a tyrosine kinase domain. "Mixed-chain" insulin/IGF‑1
receptors exist that can be activated by either hormone. IGF‑1
is protected from proteolysis by IGF‑1-binding proteins and integrated
into the organic cartilage and bone matrix. In bone, part of it is embedded
by mineralization, together with other growth factors like TGFβ
and PDGF (transforming growth factor β and platelet-derived growth
factor). This creates a reservoir of growth factors that is only activated
in case of bone resorption (one reason why metastasizing tumor cells
frequently find a fertile soil in bone). IGF‑1 acts back on the
cells in a paracrine fashion, stimulating chondrocytes in the epiphyseal
plate and osteoblasts to divide. IGF‑1 levels directly depend
on GH, but their effect is much more stable due to these buffering mechanisms.
Both a deficiency of GH and a deficiency of IGF‑1 lead to stunted
growth, while overproduction of GH results in gigantism.
Pharmacology cross-reference: Following
insulin, GH was licensed for Genentech as the second protein drug produced
by recombinant DNA technology in 1985. Before, human GH was purified
from the pituitaries of deceased individuals. Indications are GH deficiency
as well as small stature as a result of Turner's syndrome or renal insufficiency,
where GH is applied symptomatically to increase height. In some countries,
pediatricians are exposed to pressure from parents to administer GH
to healthy children to increase height, as this is viewed as a social
advantage. IGF‑1 is used in patients who suffer from GH receptor
defects, a condition known as Laron dwarfism.
[GH and IGF‑1 have more functions than just promoting
growth. In some countries, e. g. the USA, bovine GH is used to increase
milk yield in cows. Unfortunately, this only works when cows are fed
optimally, meaning it is of little help to the countries needing it
most. The EU, after intense discussions, decided not to license bGH
treatment of dairy cows.]
2.3 Thyroid hormone
GH and IGF‑1 are necessary, but not sufficient
for bone growth and maintenance of bone mass. Also necessary are thyroid
hormone and, depending on gender, estrogens or androgens. Like IGF‑1,
thyroid hormone and sex steroids are under indirect control of the CNS.
At present, the exact molecular mechanisms of their actions can be insufficiently
described. Virtually all tissues express thyroid hormone receptors,
and many tissues express receptors for estrogens and androgens. The
three receptor types are related. Along with vitamin D receptor and
glucocorticoid receptor, they are members of the superfamily of nuclear
receptors. All nuclear receptors are ligand activated transcription
factors, each regulating large numbers of genes. In the presence of
ligand, many genes are transcriptionally activated and even more genes
are silenced. At present, we know too little about which of these genes
are important for bone growth and maintenance.
While other members of the nuclear receptor superfamily
are primarily cytoplasmic, switching to the nucleus only following ligand
binding, the thyroid hormone receptors (α and β) are bound
to the DNA no matter whether ligand is present. In the absence of ligand,
they frequently repress transcription from the respective genes. Triiodothyronine
(T3), or to a lesser extent thyroxine binding to the receptor causes
it to actively contribute to transcription initiation. Thyroid hormone
receptors are expressed in chondrocytes, bone marrow stromal cells,
osteoblasts and osteoclast precursors. It is unclear whether T3 has
direct actions in osteoclasts.
Lack of thyroid hormone in children causes dwarfism.
On the other hand, hyperthyroidism causes secondary osteoporosis.
2.4 Estrogens, progesterone and androgens
Likewise, the importance of sex steroids for bone metabolism
became obvious from clinical observations. In
various forms of hypogonadism, lack of these hormones results in osteoporosis. Overproduction
of androgens or estrogens during childhood initially accelerates growth
(as normally seen around puberty) but results in early epiphyseal closure
with reduced final height. Postmenopausal osteoporosis starts with a
decrease in estrogen concentrations.
Both estrogen and androgen receptors are expressed in
either sex. While there is only one androgen receptor, two forms of
DNA-binding estrogen receptors (ER) exist, ERα and ERβ. ERα
is expressed predominantly in ovary, uterus, and breast, while ERβ
is expressed in numerous additional tissues, but both types are expressed
in bone cells. In addition to these receptors, which shuttle
between nucleus and cytoplasm, a totally unrelated G protein coupled
estrogen-binding protein exists in the membrane of the endoplasmic reticulum,
with no known involvement in bone metabolism. Numerous mechanisms have
been proposed to account for the anabolic action of estradiol and related
estrogens. Regarding bone resorption, estrogens reduce the number and
activity of osteoclasts. Part of this effect is mediated via the RANK
system. Activated estrogen receptors do not directly regulate the promoter
of RANKL or related genes. Rather, they regulate the system indirectly
via several different points of contact. For example, estrogens stimulate
OPG production by osteoblasts and inhibit production of M-CSF, IL-1,
IL-6 and TNFα. The result is a decrease in the rate of osteoclast
generation. By the same and other mechanisms, activity levels and lifespan
of existing osteoclasts are reduced. All in all, it is firmly established
that estrogens reduce bone resorption. There are ample indications that
they also directly stimulate bone formation, but there is no consensus
regarding the involved mechanisms.
In contrast, progesterone drives RANKL expression and osteoclast formation. Progesterone peaks during pregnancy. This helps to release Calcium from the mother's bones to help calcify the bones of the fetus. In addition, progesterone-driven RANKL expression is important to spur mammary epithelial cell division, helping to develop new breast tissue for lactation.
Follicle stimulating hormone (FSH), the hormone stimulating
estrogen production, has also been found to have a direct effect on
bone. It was reported to stimulate osteoclast activity, an effect that
would counter that of thyrotropin. Before menopause, this effect is
more than compensated for by the anabolic actions of estrogen, but following
menopause, it might be responsible for the phase of high-turnover bone
loss.
In males, androgen levels come down only at a later age.
As a result, male osteoporosis manifests itself at least ten years later
then in females. Androgen-dependent mechanisms stimulating bone mass
probably overlap to a large extent with those due to estrogens. Yet,
there is a second possibility: also in males, androgens are being converted
to estrogens by the enzyme aromatase, expressed in fatty tissue. Consequently,
it is possible that some of the bone-protective effect of androgens
might in fact be due to estrogens.
2.5 Cortisol and related glucocorticoids
Glucocorticoids affect bone formation as well as bone
resorption. Glucocorticoids inhibit osteoblast function, e. g., by inhibiting
transcription of collagen and osteocalcin genes (this also happens in
other tissues: an impairment in collagen production can sometimes lead
to visible striae in skin). Glucocorticoids also reduce
the life span of osteoblasts. On the resorption front, glucocorticoids
simultaneously induce RANKL and inhibit OPG production in osteoblasts.
Combined, these two effects increase the number and activity of osteoclasts.
All in all, glucocorticoid effects on both sides of the coin therefore
strongly promote osteoporosis.
2.6 Mechanical strain
Being subject to mechanical load is essential to maintain
bone mass. The trabeculae of cancellous bone are constantly being remodeled
to adapt the bones to mechanical strain. Inactivity leads to a rapid
loss of bone mass, as can be observed in bedridden patients. A reduction
of load due to conditions of zero gravity, as for astronauts manning
the space station, has the same results. Osteocytes, the
bone cells positioned between osteon lamellae, sense mechanical strain and
react by reducing sclerostin secretion and modifying other signals addressing
osteoblasts. Osteocytes are able to relay such signals via their gap
junction-connected network of long cellular processes. Unfortunately, the molecular details of the
osteocytes' load sensor remain insufficiently understood. One of the
hypotheses under discussion holds that mechanical strain results in
a fluid wave permeating porous bone and deforming osteocytes (like laundry
on the line). This might open mechanosensitive ion channels, leading
to a signal that may be relayed through gap junctions to sites near
active bone construction units (basic multicellular units).
2.7 Food
situation: leptin
Leptin was first identified in a mouse strain that had
been inbred by selecting its obese phenotype. In Ob/Ob mice, a gene
encoding an extracellular signaling molecule was found to be defective.
Accordingly, the signaling molecule was termed "leptin" (the
Greek word leptos means "thin"). In addition
to being obese, homozygous mice were infertile. Remarkably, their bone
mass was increased, while hypogonadism is otherwise associated with
osteoporosis. For experimental reasons, the molecular mechanisms of
leptin biology have been elucidated in the mouse. From what we know
from data points verifiable in humans, these findings likely apply to
humans as well.
The signal protein leptin is almost exclusively secreted
by adipocytes (an adipokine).
Its long term plasma level is proportional to the size of an individual's
fat storage. Around this level, leptin levels oscillate diurnally dependent
on food intake, with a minimum at breakfast and a maximum late in the
evening. In addition, changes in food situation cause temporary divergence.
Leptin levels decrease following a few hungry days, and increase after
a period of feasting. Leptin acts on numerous tissues, but its main
target is thought to be the brain. Leptin is able to cross the blood
brain barrier and affects the autonomic nervous system via hypothalamic
centers. A fall in leptin causes the sensation of hunger, an increase
a feeling of satiety. For some time, leptin was hoped to be the answer
to the obesity epidemic. However, obese individuals were found to be
leptin resistant, much like type 2 diabetics are insulin resistant,
meaning increased leptin does not reduce their appetite.
The leptin effect on bone metabolism, too, is mediated
via the autonomic nervous system. Leptin-induced hypothalamic impulses
are conveyed to the bone via sympathetic neurons, directly affecting
osteoblasts by stimulating their β-adrenergic receptors
with norepinephrine. In the osteoblast, these signals are modulated
by the cellular molecular clock. Depending on the phase of
this clock, incoming signals either accelerate or delay osteoblast cell
division and function. Osteoclast function, too, is affected by this
adrenergic pathway. This mechanism explains a fact that has been known
for a long time: markers of bone metabolism, like plasma osteocalcin,
follow a circadian rhythm. It seems plausible, for example, that bone
remodeling is easier to accomplish during nighttime.
From what we
presently know, leptin acts as an input into the central nervous system
reflecting the prevailing food situation. In response, the CNS reacts with
adaptations concerning, e.g., eating behavior, reproductive functions but also bone
metabolism. With respect to the latter, computed output is additionally
embedded in a useful circadian rhythm. We are far from fully understanding all
of leptin's effects on bone, but the complete lack of leptin in ob/ob mice obviously
results in increased bone mass.
Pharmacology cross
reference: If the net effect of leptin is decreased bone formation
via a β-adrenergic mechanism, β-blockers should positively
affect osteoporosis. Retrospective data analyses seem to strengthen
this case. Conclusive prospective studies remain to be conducted; a first small prospective trial suggested a positive effect of
β1-blockers.
3.
DISORDERS OF BONE METABOLISM
3.1 Osteoporosis
Primary or idiopathic osteoporosis, by far the most common
form of the disease, affects people in the second half of their lives.
Though mechanisms in females and males are probably similar, the disease
starts earlier in women, as estrogens in women decrease earlier than
androgens in men. In women, the disease is termed postmenopausal osteoporosis.
The main symptoms of osteoporosis are bone fractures.
Typically, these affect the femoral neck or the vertebral bodies (impression
fractures). Of course, fractures occur at peak loads as happen in falls,
but at comparable loads, the probability of fracture increases with
decreasing bone mass. When a bone is weakened to an extent that it breaks
from a minor stress, we speak of a pathological fracture. We reach our
peak bone mass in our twenties. From then on, the net effect of the
many factors affecting bone metabolism is slightly negative. In women,
net resorption accelerates with menopause due to the fall-off in estrogens.
During the first 5-10 years following menopause, accelerated, so-called
high turnover bone loss is observed, characterized by increased osteoclast
activity. Later, the system swings back to normalized osteoclast activity
with a slight deficit in osteoblast function (low turnover bone loss).
In primary osteoporosis, several factors contribute to
the negative net effect:
- Decrease
in estrogen and androgen concentrations
- Reduced
physical activity
- Insufficient
vitamin D and calcium intake
- Reduced
UV exposure, resulting in lower endogenous production of vitamin
D
- Reduced
renal function secondary to diabetes, arteriosclerosis, or analgesics
abuse, resulting (also via FGF23) in insufficient 1-hydroxylation of
vitamin D
With bone resorption outbalancing bone formation, plasma
Ca2+ levels increase. Lower PTH levels mean less renal Ca2+
reabsorption, or more urinary Ca2+ loss. In addition, PTH
dependent 1-hydroxylation of vitamin D is decreased in the kidneys,
lowering intestinal Ca2+ uptake. All in all, Ca2+
balance follows bone mass balance (anything else would make little sense:
where to put all that Ca2+?) and both are negative.
Interestingly, an excess of weight to a certain degree
protects from osteoporosis. Whether this is due to increased load, or
to enhanced residual estrogen synthesis from androgens by fatty tissue
aromatase remains to be elucidated.
Diagnostics
The most relevant property of bone would be its resistance
against fractures. Of course, this cannot be tested. Measurable surrogates
are bone density and biochemical markers of bone formation and bone
resorption.
Bone density is
usually determined by the DXA method (dual energy X-ray absorptiometry).
DXA is based on the fact that the density of tissue affects its differential
absorption properties for low energy versus high energy X rays. Put
differently, from two images taken at different X ray energies, the
density of the X-rayed tissue can be inferred; with a few assumptions
and a lot of computing even the density of bone. The result is expressed
as a multiple of the standard deviation from mean bone density of
30 year-olds of the same sex. This dimensionless value is termed
T-score. Osteoporosis is defined by a T-score smaller than -2.5 (a bone
density more than two and a half standard deviations below the mean
bone density of individuals at peak bone mass). Another method to assess
bone density is quantitative computed tomography (QCT). It is technically
more complex and thus more expensive, but yields more information, e.
g., allowing individual assessment of compact and cancellous bone.
Bone
formation can be assessed
by measuring plasma osteocalcin, as osteocalcin is produced exclusively
by osteoblasts. In addition, peptides released during collagen polymerization
can serve as proxies for bone formation, as most of collagen type I
synthesis occurs in bone. Specifically, these are procollagen I C‑terminal
propeptide (PICP) and procollagen I N‑terminal propeptide (PINP).
An additional marker of osteoblast activity is bone specific alkaline
phosphatase (ostase).
Bone resorption by osteoclasts
in turn involves cleavage of cross-linked collagen. The C-terminal cross-linked
("x") fragments of collagen I (CTx-I, also designated "crosslaps")
can be measured in plasma, serving as a marker for bone resorption.
When these collagen fragments are further degraded in the body, what
remains is the specific chemical structure formed by cross-linking hydroxylysines,
pyridinoline. Pyridinoline (Pyr) and strictly bone specific deoxypyridinolin
(Dpyr) can be measured in urine (sometimes, these are termed "crosslinks").
For the circadian rhythm of bone metabolism, it is important
to take blood samples for each check‑up at the same time of day.
Therapeutic
options:
Estrogen
replacement?
For many years, estrogen replacement therapy was very
popular to mitigate unwelcome effects of menopause. People were unconcerned
about potential long term side effects. When results of the necessary
large scale randomized double blind studies finally came in (Women's
Health Initiative Study in the USA,
Million Women Study in the UK),
they were sobering. There was a reduction in femoral neck fractures,
but this was more than outweighed by increases in endometrial carcinoma,
breast cancer, myocardial infarction, stroke and pulmonary embolism.
Especially for myocardial infarction, the opposite had been assumed,
as heart attacks are less frequent in premenopausal women than in men
of the same age. With these studies, general hormone replacement therapy
is no longer an issue; newer studies propose beneficial effects of replacement
for a few years immediately following menopause.
Raloxifen
is an alternative.
Raloxifen is a selective estrogen receptor modulator (SERM) like tamoxifen.
These lipophilic ligands bind to the estrogen receptor and induce a
change in conformation, leading to a complex that is slightly different
from the original estradiol-receptor complex in three-dimensional structure.
In some tissues, SERMs have estrogen-like effects, while in others,
effects are estrogen-antagonistic. This depends on the individual cells'
mix of transcription coactivators and corepressors, some of which bind
the slightly modified complex better than the original, while others
bind it less well. Raloxifen has estrogen-like effects in bone, slowing
progression of osteoporosis. In the mammary gland, raloxifen acts antagonistically
and actually reduces the risk of breast cancer. According to presently
available data, Raloxifen seems to be neutral with respect to the risks
of endometrial carcinoma and vascular disease, with exception of a slight
increase in the risk of venous thrombosis.
Bisphosphonates
In phosphonate, the central phosphorus atom is surrounded
by three oxygen atoms, instead of four as in phosphate. In bisphosphonate,
two such groups are attached to a carbon atom: the phosphorus atoms
are directly bound to the carbon, not via oxygen as in a phosphate.
While phosphates are easily removed by phosphatases hydrolytically cleaving
the O-C bond, this is not the case for the P‑C‑P bonds of
bisphosphonates; bisphosphonates are thus very stable in the body. Once
settled in the bone, they have a half-life of several years. In many
respects, they behave just like phosphate: they form insoluble complexes
with Ca2+, making it difficult to take them up from the intestines
(they are frequently administered parenterally). They preferentially
adhere to hydroxyapatite. From there, they are ingested by "nibbling"
osteoclasts and interfere with their function via several mechanisms;
over time, many osteoclasts enter apoptosis. In summary, bisphosphonates
like Alendronate (e. g., Fosamax®) inhibit bone resorption,
reestablishing some balance between resorption and formation in osteoporosis.
There are also indications that bisphosphonates help to prevent establishment
of bone metastases in cancer. In addition, some data suggest that bisphosphonates
counteract "hibernation" of micrometastases in bone marrow
(please see section on metastasis below). Problematic side effects result
when bisphosphonate toxicity extends to other bone cell types, in rare
cases causing the especially worrying osteonecrosis of the jaw.
Denosumab
As a natural RANKL-neutralizing protein, osteoprotegerin
was a logical candidate to treat osteoporosis. It was introduced into
early clinical trials by biotech
company Amgen. However, a few worrying aspects instigated a search for
better solutions. Apart from RANKL, OPG binds to additional members
of the TNF superfamily. OPG is also expressed by endothelial cells and
natural OPG levels correlate with coronary heart disease.
As an alternative solution with a presumably higher margin
of safety, Amgen developed a monoclonal antibody, denosumab (Prolia®),
mimicking the function of OPG. Advantages are an increased specificity
for RANKL and a reduced risk of causing neutralizing antibodies against
OPG. The human monoclonal IgG2 antibody (IgG2 is far less able to activate
complement than IgG1 or IgG3) is injected subcutaneously twice per year.
In clinical studies, it proved to be effective against postmenopausal
osteoporosis as well as against the osteolytic effects of breast cancer
metastases. In May 2010, denosumab was approved for the treatment of
osteoporosis in postmenopausal women at increased risk of fractures,
and for the treatment of bone loss associated with hormone ablation
in men with prostate cancer. As RANKL knockout mice also show immunological
problems (although these are induced early in development), potential
long-term side effects will have to be monitored carefully.
Parathyroid hormone analogues (teriparatide, abaloparatide)
In view of the physiological
bone-demineralizing effect of PTH, an effect actually enhanced in hyperparathyroidism,
it seems surprising that therapeutically administered PTH analogs may be used
in the treatment of osteoporosis. It seems that the temporal pattern of
administration makes all the difference: the intermittent, short-term peak of
the PTH analog administered once daily activates osteoblasts more than
osteoclasts, leading to the opposite effect compared to chronically elevated
PTH. Parathyroid hormone analogues are only being used for a limited duration,
especially when fractures have already occurred as a result of postmenopausal
osteoporosis.
Vitamin
D and Ca2+
To counteract the multiple potential causes for deficiency,
it makes sense to orally substitute vitamin D. As it can only be effective
with an adequate supply of nutritional Ca2+, Ca2+
is substituted as well.
Physical
activity and sunlight
Weight bearing physical activity (this includes, e. g.,
walking, jogging, dancing, but not swimming) markedly counters progression
of osteoporosis, not to speak of numerous additional beneficial effects
on metabolism and mind. Resistance/ impact
training of high intensity conferring strong mechanical strain on bone works
best. Unfortunately for most older people this is not realistic. Yet, any form of activity is
better than inactivity. As an additional
benefit, concomitant UV exposure stimulates vitamin D synthesis.
Secondary osteoporosis
Apart from its primary or idiopathic occurrence, osteoporosis
may be caused by a number of diseases, for example:
- Hypercortisolism
- Hyperparathyreoidism
- Hyperthyreosis
- Anorexia
- some
forms of neoplastic disease like multiple myeloma, which in many
cases is diagnosed only following a pathologic fracture
Pharmacology cross reference: Prolonged
glucocorticoid therapy significantly increases the risk of fractures. To a
small extent, fracture risk may also be raised by SGLT2 inhibitors, which are
taken by diabetes patients for a long time: Since SGLT2 inhibitors inhibit the
reabsorption of glucose AND Na+ in the proximal tubule of the nephron,
a higher concentration of tubular Na+ remains available to drive
other transport processes. This way, phosphate re-absorption via the Na-Pi cotransporter can be enhanced. Secondary effects include a decrease in Ca2+ concentration, increasing the release of parathyroid hormone, and an increase in
FGF23, thereby lowering calcitriol.
3.2 Rickets and osteomalacia
Lack of vitamin D causes rickets in children, osteomalacia
in adults. In both cases, organic matrix is produced in sufficient amounts,
but is insufficiently mineralized. The difference in symptoms result
from the fact that children's bones, which are still growing, are easily
deformed if too soft. Consequently, affected children show deformities
of the skull, thorax and legs (craniotabes, Harrison's
groove caused by the pull of the diaphragm, bow legs) and
hyperplastic epiphysial cartilage and costochondral joints
(widened wrists and rachitic rosary). Logically, dental enamel is defective.
Additional symptoms are caused by low serum calcium (agitation, sweating,
muscle weakness, pot belly, constipation, tetany). In adults, osteomalacia
causes bone pain and insidious pathological fractures. Rickets and osteomalacia
are prevented or treated by supplementation of vitamin D and Ca2+.
Infants who are breast-fed should be supplemented with vitamin D drops
on a regular basis.
3.3 Bone resorption in the context of inflammatory
disease
In chronic forms of arthritis, for example in rheumatoid
arthritis, activated macrophages and other cells produce IL‑1,
IL‑6 , TNFα and prostaglandin E. In surrounding cells like
synoviocytes, these induce matrix metalloproteases that break down the
organic matrix of cartilage and bone. In addition, the inflammatory
cytokines induce osteoclast differentiation in adjacent bone. This process
is enhanced by RANKL, which is expressed by several cell types in inflamed
tissue, including synoviocytes and T cells. Together, these mechanisms
can lead to massive destruction of bone next to inflammatory joints.
The same mechanism is responsible for painful dental
necks and ultimate loss of teeth due to parodontitis. Bacteria in plaque at the gingival margin cause
chronic low-key inflammation of the gums. Inflammatory cytokines induce
osteoclasts which progressively break down the thin layer of bone around
the neck of teeth.
3.4 Paget's diesease of bone (osteitis deformans)
Paget's disease remains an enigmatic disease characterized
by focal areas of increased but disorganized bone turnover. Bone in
affected areas may be dense, but unable to withstand normal load. In
many cases, Paget's disease is only diagnosed by chance on an X-ray
for unrelated causes, and remains limited to a single focus in a single
bone. Yet, depending on location and intensity, it may cause pain, pathological
fractures, scoliosis or neurological
symptoms (e. g., hearing loss or radiculopathy) by nerve compression
in case of skull or spine involvement. Altogether, it is quite frequent
(percentage in the single digits), affecting mainly people in the second
halves of their lives. Both genetic and environmental factors are thought
to contribute to the disease. In an affected bone area, the process
seems to start with hyperactive osteoclasts. Several disease-associated
alleles have been identified that may explain this hyperactivity: polymorphisms
in RANKL, for example, or in Sequestosome 1 (SQSTM1), which is important
for RANK-dependent activation of transcription factor NFκB. In
addition, environmental factors such as nutritional deficits in Ca2+ and paramyxoviral infections
have been discussed. Unfortunately, neither of these hypotheses is able
to explain the focal characteristics of Paget's disease, suggesting
an important etiological factor remains out of sight. For screening
and therapy monitoring, bone-specific alkaline phosphatase is determined.
Diagnosis involves conventional radiology and bone scans, including
scintigraphy. Bisphosphonates are the mainstay of therapy, combined
with supplementation of Ca2+ and vitamin
D, exercise and sunlight.
3.5 Bone metastasis
Certain types of cancer preferentially metastasize into
bone, including mammary, prostate, lung and thyroid carcinoma. Obviously,
conditions in the bone promote settling of metastasizing tumor cells.
Migrating cells are frequently directed by chemokine
gradients. For example, breast cancer cells express CXCR4, the receptor
for CXCL12, which is secreted in bone. Another relevant factor is RANKL,
which is strongly expressed by osteoblasts. Mammary carcinoma, prostate
carcinoma and melanoma cells frequently express its receptor RANK. RANK,
trimerized by RANKL, induces migration and other metastasis-promoting
changes in these cells. Injections of OPG strongly inhibited bone metastasis
formation in an experimental mouse model. Consequently, this might be
another future application of the
OPG-mimicking monoclonal antibody denosumab.
Bone tropism may have additional causes. The molecular
changes at the root of the malignant tumor may activate a master switch
in osteoblast differentiation, e. g., the transcription factor Runx2. Tumor cells originating from mammary
or prostate tissue may thus secrete osteoblast-specific proteins like
osteocalcin and osteonectin. In other words, they assume properties
from bone cells, a behavior termed osteomimicry. These changes are likely
to facilitate settling in bone.
Once established, metastases may grow osteolytically
or osteoblastically (osteosclerotically). This depends on whether signaling
by mestastasizing cells preferentially activates osteoclasts or osteoblasts.
Prostate carcinoma cells frequently express endothelin‑1. Endothelin‑1
stimulates osteoblasts and inhibits bone resorption, leading to osteoblastic
metastases.
It is common for metastases of breast cancer to cause
osteolysis. All molecules mentioned as stimulators of osteoclast differentiation
(M-CSF, RANKL, TNFα, IL-1β, IL-6) have been found overexpressed
in some osteolytic metastases. In addition, some tumors such as mammary
carcinoma, bronchial carcinoma, melanoma or hematologic neoplasms tend
to overexpress PTHrP (parathyroid-hormone-related protein). PTHrP binds
to the PTH receptor, causing bone resorption. Systemic action can lead
to tumor hypercalcemia, a life-threatening condition for the effect
of Ca2+ on membrane potential. Local action may contribute
to osteolysis, making room for further tumor growth. Osteolysis induces
a vicious cycle: as a considerable amount of growth factors like IGF‑1 and TGFβ is embedded
in bone matrix, these in turn stimulate tumor cell growth. Metastasizing
cells break down bone, and resorption of bone stimulates growth of metastasizing
cells.
Metastases sometimes appear years after removal of the
primary tumor. Micrometastases of single or very few cells somehow are
able to survive for a long time in some backwaters of the body. There
are indications that bone marrow may be such an area of retreat. Hematopoietic
stem cells at the highest hierarchical level spend most of their lives
in a Rip Van Winkle-sleep (a state called dormancy or quiescence). From
this G0 phase, they are only woken up in case of a sudden
requirement for a lot of hematopoietic cells. This may be indicated
by a sudden surge of G‑CSF (incidentally, recombinant G-CSF is
indicated to mobilize stem cells in stem cells donors). Following a
few rapid divisions, the stem cells sink back into their deep sleep.
The rationale for this behavior is to minimize chances for mutations,
e. g., by misincorporation. What keeps a very small number of stem cells
in dormancy? There are indications that the surrounding cells somehow
form a functional stem cell "bedroom", a niche promoting dormancy.
These niches seem to be located at the border zone between bone and
marrow, the so-called endosteum, and are made up of a specific form
of osteoblasts, spindle-shaped, N-cadherin positive osteoblasts
(SNO). One hypothesis to explain the observed dormancy of micrometastases
holds that single metastatic tumor cells may get trapped in one of these
stem cell bedroom niches and be put to sleep. After a long Rip Van Winkle-sleep,
they may still awaken years later.