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Journal
of Clinical
Endocrinology
and Metabolism
Copyright
0 1996 by The Endocrine
Society
Vol. 81, No. 3
Printed
m U.S.A.
Invited
Review of a Workshop:
Anabolic
Bone: Basic Research
and Therapeutic
RONALD
N. MARGOLIS,
ERNEST0
CANALIS,
NICOLA
AND
Hormones
Potential
in
C. PARTRIDGE
Endocrinology
Section, National
Institute of Diabetes and Digestive and Kidney Diseases, National
Institutes of Health (R.N.M.), Bethesda, Maryland
20892; the Department
of Research, St. Francis
Hospital and Medical Center (E.C.), Hartford,
Connecticut
06105; the University of Connecticut
School of Medicine
(E.C.), Farmington,
Connecticut
06030; and the Departments
of Pharmacology
and
Physiological
Sciences, St. Louis University School of Medicine
(N.C.P.), St. Louis, Missouri
63104
ABSTRACT
Age-, postmenopause-,
and disease-related
conditions
that result
in low bone mass represent
important
public health issues. Maintenance of bone mass is a balance between
bone resorption
and formation and is influenced
by diet, body composition,
activity
level, and the
interactions
between
and among a large number
of hormones,
growth
factors,
and cytokines.
Recent research
has emphasized
establishing
a more complete
understanding
of the hormonal
regulation
of bone
and developing
anabolic
agents with therapeutic
potential
for the
treatment
of low bone mass. The NIDDK
at the NIH recently
spon-
B
sored a Workshop,
entitled
Anabolic
Hormones
in Bone: Basic Research and Therapeutic
Potential,
that attempted
to define the current state of the art knowledge
of hormones,
growth
factors,
and
cytokines
that affect bone mass, with particular
emphasis
on those
that could potentially
have a role as anabolic
agents in bone. This
review presents
a condensed
proceedings
of that workshop
along with
a summary
of the optimal
requisites
for the development
of anabolic
agents with therapeutic
potential
in bone. (J Clin Endocrinol
Metab
81:872-877,
1996)
ONE PROVIDES a skeletal framework for the body and
acts as a dynamic reservoir of mineral, particularly
calcium and phosphorus. Bone turnover, the product of bone
resorption and formation, is a tightly coupled process, with
the net balance between the two determining the bone mass
and the serum calcium level (1). Regulation of bone turnover
requires the input of a large number of hormones, growth
factors, and cytokines (1). In this context, an anabolic agent
is one that can shift the net balance in the direction of increasedbone mass.Additionally, an antiresorptive agent can
increase bone massif it shifts the regulatory balance toward
net formation by suppressing resorption. Postmenopausal
and aging-related osteoporosis, with attendant loss of bone
mineral, results in widespread health problems (2, 3). Consequently, there is a great need to understand how to precisely regulate net homeostatic balancesin bone. Much of the
current knowledge of bone development and growth has
been built upon fundamental insights derived from animal
and tissue culture studies aswell asfrom recent observations
of the effects of mutations in bone-related proteins on bone
structure and function. New knowledge derived from mutations in genes that cause skeletal abnormalities has provided needed information on both normal bone function and
the effects of disease on these processes(see Ref. 4 for re-
view). Still more understanding has been developed from
resultant phenotypes derived from mouse models of hormone non- or overexpression, either from targeted disruption of a gene by homologous recombination (null allele) or
from targeted overexpression in transgenic animals (4). Dissecting the role(s) of individual systemic hormones, growth
factors, and cytokines in the regulation of bone turnover is
a necessaryfirst step in identifying the appropriate stagesat
which therapeutic interventions with anabolic agents can be
developed to affect net bone formation. With this in mind, an
NIDDK’ workshop entitled Anabolic Hormones in Bone:
Basic Researchand Therapeutic Potential was held May l-2,
1995, at the NIH. The initial objective of the workshop was
to focus on hormones, growth factors, and cytokines with
demonstrated anabolic effects on bone. It gradually became
clear that a large number of factors interact at the level of the
osteoblast, osteoclast, and other cells to regulate the balance
between net resorption and formation. Thus, the workshop
was broadened to include agents that affect both sides of the
regulatory equation and could be recognized as potential
focal points for interventions directed at the sameoutcome:
increased bone mass.
PTHJPTH-related
peptide
(PTHrP)
It haslong been known that chronic elevation of PTH results
in bone resorption, asPTH is the most important hormone that
regulates bone calcium levels (1). In addition to its resorptive
actions, intermittent administration of PTH produces anabolic
effects (3). The resorptive effects of PTH are well documented
in patients with asymptomatic primary hyperparathyroidism
(seeRef. 5 for review), in which chronic mild hypercalcemia
reflects gradual erosion of bone mineral asa consequenceof the
elevated levels of I’TH. Further indications of the resorptive
Received
August
8, 1995. Revision
received
September
27, 1994.
Accepted
November
3, 1995.
Address
all correspondence
and requests for reprints
to: Ronald N.
Margolis,
Ph.D., Endocrinology
Section, National
Institute
of Diabetes
and Digestive
and Kidney Diseases, National
Institutes
of Health, Building 45, Room 5AN.l2J,
45 Center Drive, MSC 6600, Bethesda, Maryland
20892-6600.
E-mail: margolisrQep.niddk.nih.gov.
’ Cosponsored
by the National
Institute
of Arthritis
and Musculoskeletal and Skin Diseases and the NIH Office of Research on Women’s
Health.
872
INVITED
effects of PTH were obtained when one of the factors linked to
the hypercalcemia
of malignancy was discovered to be PTHrP,
a peptide with sequence and structural similarities
in the Nterminal regions to PTH (5). Although
PTH is restricted to
production
by the parathyroid
gland, acting primarily
as an
endocrine factor, PTHrP is produced by a variety of tissues and
cells, where it acts as a paracrine and/or autocrine factor (4-8).
Gene knockout by homologous
recombination
has set the stage
for major advances in our understanding
of the precise role of
PTH/PTHrP
in skeletal and nonskeletal
tissue development
(see Ref. 4 for review). Despite lack of the PTHrP gene, mice
survived to birth, although numerous
developmental
defects
occurred, with major alterations in endochondral
bone formation. These included
premature
ossification
of the ribs and
death during the perinatal period. Conversely, when overexpressed in transgenic mice, PTHrP was implicated
in the development of numerous other tissues, including
pancreatic islets, where it had an anabolic effect (9). These data confirm the
importance
of PTHrP as a locally active factor, mediating
terminal differentiation
and function of bone and other tissues. A
major implication
of these findings is the putative role of PTH
and I’THrP as anabolic agents in bone.
Although
it appears that PTH and PTHrP have distinct roles
in the regulation of calcium homeostasis, their actions converge
on a common PTH/PTHrP
receptor (10). This member of the
seven-transmembrane
G-protein-coupled
receptor superfamily
initiates complex cellular responses through two different signal transduction
pathways coupled through increased CAMP
production
and/or stimulation
of phospholipase
C. The mechanism of the anabolic action of PTH and the signal transduction
pathways involved are not known. Nevertheless, regulation
of
the PTH/PTHrP
receptor becomes a point at which control of
bone turnover can occur. The amount of receptor in the osteoblast is crucial to the ability of this cell to respond to PTH and/or
PTHrP. In the osteosarcoma cell line ROS 17/2.8, a number of
factors up-regulate
PTH/PTHrP
receptor levels, including
glucocorticoids and transforming
growth factor-p, (TGFP,) (11).
These effects appear to be at the level of gene transcription.
Human mutations of the PTH/PTHrP
receptor result in Jansentype metaphyseal
chondrodysplasia,
a form of short limb
dwarfism (12). In this rare disorder, patients have hypocalcemia
and hypophosphatemia,
with normal or near-normal
serum
levels of PTH and PTHrP. The mutation causes a chronic activation of the receptor, resulting in a gain of function, signaling
the cell constantly through increased production
of CAMP. Defects are seen in bone and kidney, target organs of PTH action.
Among the several putative hormonal
anabolic agents for
bone only intermittent
administration
of PTH has demonstrated efficacy (1, 5, 13, 14). In a recent small scale clinical
trial, intermittent
PTH prevented
bone loss in premenopausal women
with endometriosis
treated with gonadotropin release analogs known to cause osteopenia
(14).
The interplay between physiological
levels of PTH/PTHrP
and the common receptor in homeostatic
control of serum
calcium levels and bone mineral density is essential to long
term maintenance
of bone structure and function.
The disruption of this normal balance, either during development
or
in the adult, can have profound
effects on the skeleton (and
other tissues). Clearly, further research is required
to define
the precise mechanism(s)
of action of PTH/PTHrP
in elicit-
REVIEW
873
ing anabolic
effects in bone. An understanding
of when,
where, and how to intervene must then follow to develop a
better understanding
of the potential
therapeutic
use(s) of
PTH, PTHrP, and the appropriate
analogs.
Vitamin
D
Vitamin
D (1,25-dihydroxyvitamin
Ds), a member of the
steroid/thyroid/retinoid
supergene
family of hormones,
is
the key hormone that regulates intestinal calcium absorption
(1, 15). In addition
to this role for vitamin
D in calcium
absorption,
it has direct effects on bone cells. In small scale
trials, vitamin D administration
and dietary calcium supplementation
increased
bone mineral
density at the femoral
neck of elderly postmenopausal
women (see Ref. 16 for review). In other studies, vitamin D alone decreased the vertebral fracture rate in patients with mild to moderate osteoporosis (16). Despite such promising
clinical data, the risk of
hypercalcemia
and the lack of knowledge
of the precise
mechanism(s)
of action of vitamin D have limited the putative anabolic uses of this hormone.
Vitamin
D is a primary
resorbing
factor in bone through
its ability to stimulate the
differentiation
of undifferentiated
bone cell precursors
(17).
Vitamin D has effects on the osteoblast, including
regulation
of the expression of extracellular
matrix (ECM) proteins such
as osteocalcin (18,19). The molecular
mechanism
of action of
vitamin
D appears to be through
hormone
binding
to the
receptor, leading to its heterodimerization
with other accessory factors, specifically
members of the retinoid subfamily
(15). Binding
of heterodimers
of the VDR and a retinoid
receptor (RXR) to specific consensus sequences in the promoter regions of vitamin D-dependent
genes (e.g. osteocaltin) regulates
gene expression
in target cells. The study of
human diseases such as vitamin
D-dependent
rickets has
provided
new insights by demonstrating
that a mutation
in
the vitamin D receptor causes resistance to vitamin D (20).
Work is underway
to design and test vitamin D analogs that
can provide therapeutic
potential by increasing
bone formation without concomitantly
increasing
serum calcium levels.
Androgens
Although
the central role of estrogen (E2) deficiency
in
bone loss in postmenopausal
women
has received widespread attention,
the role of anabolic
steroids is not well
established.
Androgens
have been used (and abused) for the
enhancement
of body composition,
including
muscle mass
(21). During puberty in boys, administration
of androgens
accelerates the accumulation
of calcium in bone, consistent
with enhanced
mineralization
(22), and in elderly or hypogonadal
men, administration
of androgens
has a positive
effect on bone mass (23). The effects of androgens
on body
composition
may involve
indirect
effects on the skeleton
through
increases in lean body mass (i.e. muscle) or direct
effects on bone (24, 25). Indeed, human osteoblast-like
cells
express endogenous
receptors and respond to adrenal androgens such as dehyroepiandrosterone
sulfate with changes
in gene expression
consistent
with a stimulation
of bone
formation.
Anabolic
steroids represent
a potential
area of
future research, although
their use has been hampered by the
absence of definitive
knowledge
about their effects on bone
874
MARGOLIS,
CANALIS,
and concern over significant and undesired effects in other
tissues. Development of analogs that can exploit the positive
effects of androgen on bone without expressing less desirable
secondary effects should be pursued.
E, /progesterone
(P)
The role of sex steroids in the maintenance of bone mineral
density is well established (see Ref. 16 for review), with the
loss of cycling hormones at the menopause resulting in rapid
and significant loss of bone mineral. Our understanding
of
the role of sex steroids in bone biology was enhanced after
the discovery of functional E, and P receptors in osteoblasts
and osteoclasts (16,26). Indeed, the presence of E, receptors
(ER) in both osteoblasts and osteoclasts has fueled speculation on the potential effects of E,. Although E, appears to act
on osteoclast-like cells to inhibit bone resorption (16), major
attention has focused on the role of E2 in regulating the
behavior of the osteoblast. E, has an antiresorptive effect on
osteoclasts and alters the release of cytokines from osteoblasts and/or mononuclear cells involved in remodeling (reviewed in Refs. 4, 27, and 28). Cytokines, such as interleukin-l (IL-l) or IL-6, which recruit osteoclasts, are suppressed,
whereas possible anabolic growth factors, such as insulinlike growth factor I (IGF-I) and TGFP, are stimulated by E,
(see Refs. 27 for review). E, also has been implicated as a
hormone responsible for increasing the levels of ECM-bound
growth factors (29). Indeed, the role of the ECM in cellular
adhesion and differentiation is an area of potential importance in the regulation of bone turnover. A specific example
is the demonstration
that PTH induces the production of
fibronectin (FN) in osteoblasts. FN is a key element of the
ECM potentially important in the recruitment, differentiation, and subsequent function of preosteoblasts. E, deficiency
also interferes with FN production, with the absence of E,
reducing the effects of PTH on FN production (30). Thus,
interactions between E, and other hormones may play a
major role in defining the net effect of hormonal action on
bone turnover. Clearly the role(s) of steroid hormones acting
directly and/or indirectly on bone requires further effort to
achieve a mechanistic understanding.
The recent use of gene knockout methodologies has resulted in a mouse with a null allele for the ER (31). Viable
offspring provided a unique model to investigate the effects
of the absence of ER on bone development. Perhaps equally
important has been the discovery of a human mutation in the
ER resulting in a man with E, resistance (see Ref. 4 for
review). Initial study has revealed that the most striking
skeletal effect was osteopenia and a failure of closure of the
epiphyses, hinting at a role for E, in bone metabolism in both
sexes. The administration
of E2 was not effective in this
individual, confirming the E, resistance. In addition, a man
with a mutation causing a deficiency in aromatase, an enzyme that converts androgens to E,, also had marked osteopenia and failure of epiphyseal closure (32). Together,
these data suggest a convergence in the role of E, regulation
of bone turnover in women and men.
In postmenopausal women, administration of E2 in a regimen of hormonal replacement therapy prevents the loss of bone
mineral (16). However, in addition to effects on bone turnover,
AND PARTRIDGE
JCE & M . 1996
Vol81.No3
E2 has effects on a variety of other tissues, including breast and
uterus. The use of E, alone in a regimen of hormonal replacement therapy to reduce osteopenia in postmenopausal women
has been compromised by the small, but real, increase in the risk
of developing breast and uterine tumors (16,33), with progestins now added to partly offset these effects. As P receptors (PR)
are also present in bone cells, it is possible that P could have
direct effects on bone (16). The effects of P at the molecular level
are complicated by the recent demonstration that two isoforms
of the PR exist, a full-length B form and a truncated A form (see
Ref. 34 for review), each with cell- and promoter-specific actions. When both receptors are present, the A form acts as a
dominant suppressor of the B form. Although this has immediate implications for antihormone therapy directed at PR-positive tumors of the breast, the corresponding effects on bone
cells are entirely unknown. A promising approach to treating
receptor-positive breast cancer has been the use of antihormone
therapy. Tamoxifen (TAM) is an anti-E, in the breast, with
partial agonist effects on bone, but it can also be a partial agonist
in uterus (34). Preliminary data on E,-like analogs, such as
raloxifene (LY 139481 HCI), a benzothiophene derivative that
binds to the ER, have shown positive effects in bone without
effects on the uterus (16). Clearly, more research is needed to
develop analogs that express the agonist effects of E, in bone,
but act as antagonists in other tissues (35).
GHI IGF
Although GH has general anabolic effects, including those
in bone (36,37), questions regarding its efficacy and mode of
action have limited its usefulness. GH stimulates the hepatic
production of IGF-I, with IGFs also produced locally in various tissues, including bone, alone and under the control of
other hormones (see Ref. 38 for review). IGF-I released by
liver in response to GH circulates as part of a larger complex
consisting of IGF-I and IGF-II, IGF-binding proteins (IGFBP),
and an acid-labile subunit (see Ref. 13 for review). To date,
six IGFBPs have been cloned, each of which binds IGFs at
sites distinct from that on the cell surface receptor(s) (13,39).
Tissue-specific proteases degrade binding proteins, thus affecting their total availability at local sites (13, 28, 40). The
production of IGF-I by osteoblasts has led to speculation that
IGF-I has both systemic and autocrine actions in bone (13).
The effects of IGF-I on bone are anabolic, stimulating longitudinal growth and possibly bone mass (13, 41). The importance of IGF-I to normal development and growth has
been demonstrated through gene knockout techniques (38).
Mice homozygous for a null IGF-I allele are small and die at
birth. The most noticeable deficiency in these mice is in
muscle mass, although calcification of bones is diminished,
suggesting a defect in the rate of ossification.
IGF-I interacts with other bone-active hormones, including
PTH and E,, with the former increasing the synthesis of IGF-I
(13,28), perhaps accounting for some of the anabolic effects of
PTH on bone (13). The ability of bone cells to respond to a
variety of signaling molecules by increasing IGF production
suggests that the convergence of a number of independent
signals occurs at the level of this key effector. Thus, osteogenic
protein-l [also known as bone morphogenetic protein-7 (BMP7)l and BMP-2, both members of the TGFf3 superfamily, also
INVITED
increasethe production of IGF-II and selectedIGFBPs (28).This
effect may involve PTH aswell, asBMPs increasePTH binding
to its bone cell receptor, resulting in increasedbone cell activity
(42).Prostaglandins(e.g.PGE,) up-regulate transcription of the
IGF-I genein osteoblast-likecells(43). Regulation of IGF-I gene
expressionis complex, with several transcripts observed in cells
reflecting a number of putative start sites for transcription,
perhapsthe result of different signalsinteracting with different
regions of the IGF-I promoter (43).
IGFs affect primarily the differentiated function of the osteoblastwith modest mitogenic effects. In vitro coculturing experiments have suggested that IGFs also stimulate bone resorption by increasing osteoclastrecruitment (44). In addition
to control at the level of gene expressionthe role of the IGFBPs
and IGF receptors must be considered. There are two cell surface receptors: the type I, resembling the insulin receptor, a
member of the receptor tyrosine kinase superfamily, and the
type II or mannose-6-phosphatereceptor (13, 45). IGF-I and
IGF-II bind to and appear to signal their anabolic activity
through the type I receptor (13). IGFBPs govern IGF tissue
availability, locally and systemically (13,40,42). IGFBP-3 is the
predominant binding protein in serum and is a marker of GH
action, as its synthesis by liver is sensitive to GH (46). Other
IGFBPs, such asIGF’BP-4and IGFBP-5, may regulate the availability of IGF at the level of bone (13). At the tissue level,
elements of the ECM, including glycosaminoglycans, play a
role in the availability of IGFs (47). The releaseof IGFs from
IGEBPs and the ECM is mediated by a family of proteases
shown to degradeIGEBPs,releasingthe growth factor and thus
increasing immediate tissue availability. Stability and selectivity may be governed by the ECM, which in some instances
confers a degree of protection over proteolysis of IGFBPs (47).
In addition, the synthesis of IGFBPs is regulated by IGFs and
other growth factors (42). These revealing experimental data
have served as the focal point for the design of studies to test
the efficacy of IGFs for use in human patients. IGF-I administration to patients with GH resistancedue to a mutation in the
GH receptor hasresulted in increasedlean body massand bone
mineral (48). Short term treatment of postmenopausalwomen
with IGF-I results in increased bone turnover (seeRef. 3 for
review). Additional trials are required to establisha potential
role for the IGFs astherapeutic agentsfor the treatment of low
bone mass.The ability of IGFs to regulate bone turnover and
cause a net anabolic effect on bone is governed by a host of
tissue- and cell-specific interactions, including interactions
among other growth factors, systemic hormones, ECM, and
IGEBPs.If IGFs were to be used assystemic therapeutic agents
in disordersof bone metabolism,questionsconcerning the optimal route of administration and potential side-effectsmust be
answered.
Other growth
factors
The multiplicity of responses of skeletal cells to systemic
and local factors is nowhere more apparent than when
considering the effect(s) of growth factors on bone. A
prime example is the BMP group (4,28). This large family
of growth factors, which acts through serine/threonine
kinase receptors, regulates numerous cellular events, both
during development and in the adult (28). Although some
growth factors, such as tumor necrosis factor-a, appear to
have resorptive effects on bone, others, including TGFP,
REVIEW
875
have anabolic effects (28), often interacting with the signals generated by other systemic and local factors (4, 25,
28). Recent advances in our understanding of the structure
and function of the receptors for the TGFP superfamily
revealed a two-subunit model (49), with ligand binding to
the type I receptor resulting in recruitment and serine/
threonine phosphorylation of the type II receptor. Among
the key downstream events is stimulation of a2(1) collagen
gene expression (see Ref. 4 for review), which is important
to a variety of tissues and critical to bone function. TGFp
also may regulate the differentiation of precursors to mature osteoblasts. Mechanical loading, which is important
in defining skeletal growth, increases tissue levels of TGFP
(50). TGFP is present in a latent form, with many factors,
including binding to elements of the ECM, regulating this
latency (28). Increased mechanical loading may activate
the latent form of TGFP (50).
Mutations in the mouse give rise to a condition known as
short ear (seeRef. 4 for review). BMP-5 appearsto be the product of the short ear locus, which is altered in mice expressing
short ear. The resulting phenotype includes a number of alterations, involving somethat develop both early and late in skeletal formation (4). BMPs and TGFP play key roles in the development of the skeleton and its maintenance and repair.
Understanding their precisemolecular mechanismsof action is
essential to consider their potential uses for the repair and
treatment of bone disease,including osteoporosis.
Fibroblast growth factors (FGFs) are members of a large
family of heparin-binding growth factors with actions in a
number of tissues (4, 28, 51). There are four FGF receptors
(FGFRl-4), members of the protein tyrosine kinase superfamily of receptors (52). Originally studied for their role in
angiogenesis,it is now clear that FGFs are involved in neural
development, wound healing, and chondrogenesis (51). A
seriesof human mutations in the receptors for FGF have been
particularly revealing for defining the roles played by FGFs
in skeletal development. Mutations in the FGFR3 cause
achondroplasia, the most common form of inherited dwarfism (seeRef. 4 for review), and mutations in the FGFR2 gene
cause Crouzon syndrome, characterized by premature fusion of the bonesof the skull (4). Administration of basic FGF
by miniosmotic pumps near the tibia1 growth plate stimulated long bone growth and angiogenesis, suggesting the
potential for therapeutic use (53). Further research defining
the mechanisms of growth factor action may allow us to
exploit those functions associated with anabolic effects in
bone.
Cytokines
The precursors of osteoclastsand osteoblastsare present as
relatively undifferentiated stem cells of the hematopoietic and
mesenchymal lineage, respectively (4, 27). Consequently,
agents that regulate the recruitment, replication, and/or differentiation of thesestem cellscan have a role in influencing the
balance between formation and resorption (27,28). Cytokines
represent a group of such factors (27). Included are the ILs,
especially IL-l, IL-4, IL-6, and IL-11, which have been implicated in bone turnover (27,28,54). IL-11 and IL-6 are produced
by osteoblasts(27,28). IL-6 is up-regulated in E, deficiency and
may be responsiblefor the enhanced bone resorption in the
E,-deficient state (27). Overproduction of IL-4 in transgenic
876
MARGOLIS,
CANALIS,
mice results in osteoporosis
by decreasing the activity of osteoblasts (54). Administration
of an antagonist to the IL-1 receptor in ovariectomized
rats decreases bone loss by decreasing
osteoclast-dependent
bone resorption (27). One potential common point of convergence for these cytokines may be the signal
transduction
machinery,
which operates through a common
class of receptors (27, 55). The cytokine (or erythropoietin)
receptor has an extracellular
ligand-binding
domain, but no apparent intracellular
signal transduction
machinery. Rather, the
receptor is tightly bound to a glycoprotein
of -130 kDa, known
as gp130 (27,55), and it is this protein that appears to be essential
for signal transduction
through cytokine receptors in many
cells, including
osteoclasts. It is this receptor complex that is
up-regulated
in E, deficiency (27, 28). As communication
between osteoblasts and osteoclasts is important
in determining
the coupling or uncoupling
of formation and resorption, crosstalk fostered by levels and timing of IL release could be an
important
regulatory
link. This may be the case with IL-11,
which is released by osteoblasts and stimulates osteoclastogenesis (27). An important
area for potential pharmacological
intervention could be to alter the balance between osteoclastogenesis and osteoblastogenesis
in favor of net bone formation.
Colony-stimulating
factor-l (CSF-1) has an important
function in bone (4,28,56), and disruptions
in the CSF-1 gene result
in osteopetrosis (57), a disorder characterized
by an absence of
osteoclastic bone resorption.
In some mouse models of osteopetrosis (e.g. the op/op mouse), administration
of CSF-1 results
in restoration
of osteoclastogenesis,
indicating
a defect at the
level of production
of this cytokine and revealing a role in
normal osteoclast function (57). In another model of osteopetrosis, mutations of a signaling protein tyrosine kinase called
C-SK resulted in failure of development
of mature osteoclasts,
suggesting a defect in cytokine receptor signal transduction
(4,
28). The similarity in phenotype that results from perturbation
of either CSF-1 or C-SK suggests a convergence on the signal
transduction
pathway through
C-SK in osteoclasts. CSF-1 is
released by the osteoblast in response to PTH or PTHrP, and
thus probably represents part of the overall bone remodeling
sequence of regulatory
events and another potential
site for
pharmacological
intervention.
Another factor involved in cytokine action is PGE,, which is
produced and released by osteoblasts and participates
in the
regulation
of bone resorption
(4, 28, 58). PGE, production
requires up-regulation
of PGH synthase (4). This mitogen-inducible enzyme may be a potential point of intervention
when
considering
how to generate anabolic effects in bone.
Therapeutic
potential
When taken in the context of regulation
of the dynamic
counterbalanced
process of bone turnover,
the results of this
workshop
pointed toward anabolic agents primarily
in the
context of those that increase bone formation and secondarily
those that decrease bone resorption
and thus alter the balance
of bone turnover toward net increase in bone mass. A number of inert agents that have positive effects on net accumulation of bone mineral, such as fluoride and bisphosphonates,
have been used (3). However,
novel agents acting by modulating the behavior
of the cells responsible
for regulating
bone turnover
may have a greater potential.
Agents that
increase the recruitment
and subsequent
differentiation/
proliferation
of bone cell precursors
or that act on the mature
AND PARTRIDGE
JCE & M . 1996
Vol81 . No 3
osteoblast would have the most direct effect on bone formation (45). Factors with antiresorptive
effects or actions on the
osteoclast
or osteoblast
should positively
influence
bone
mass, but perhaps
to a lesser extent than those that are
primarily
anabolic (59). Combinations
of both types of agents
could offer the most potential
as therapeutic
agents. Even
more novel approaches
may be required,
perhaps using synthetic gene constructs that can modify specific signal transduction events in defined target cells (60). Other approaches
that warrant
consideration
include a focus on agents that
affect cell to cell and/or
cell to matrix interactions,
as the
context of the cellular microenvironment
can be as important
as the presence/absence
of a given hormone,
growth factor,
or cytokine in the overall regulation
of bone turnover.
Another consideration
is whether a particular
agent can be used
systemically
or must be present locally in the bone marrow
or bone matrix to be effective. Regardless of the level of the
regulatory
process, several parameters
must be met to obtain
maximal
therapeutic
potential:
1) bone mass must be increased without sacrificing bone quality; the latter is essential
to provide for maximal bone strength and resistance to fracture; 2) the magnitude
of the effect must be clinically
significant; only agents that improve bone strength and quality
will improve quality of life; 3) the effects must be progressive
and sustained; short term effects that are not sustainable will
provide only transitory
improvement;
4) the actions of such
agents must be selective for bone, with no side-effects; agents
that lack specificity
or express unwanted
secondary actions
will not be effective or desired; and 5) ideally, such agents
should be effective when taken orally once a day; ease of
administration
will encourage
long term continued
use.
Although
a number of promising
agents are being tested
in experimental
animals, in tissue culture models, and in
limited clinical trials, much basic research is yet required
to
fully understand
the regulation
of bone turnover.
Major research questions focused on the roles of systemic and local
factors in the regulation
of bone cell development,
differentiation, proliferation,
and action need to be answered.
Only
then can the more practical
questions
about agents with
therapeutic
potential
be resolved.
Acknowledgments
This NIDDK
Workshop
was cosponsored
by the NIAMS
and the
NIH Office of Research
on Women’s
Health.
The contributions
to this
workshop
of all speakers
and discussants
is gratefully
acknowledged:
L. Attisano,
L. Avioli,
A. Baird,
R. Baron,
D. Baylink,
L. PowellBraxton,
D. Clemmons,
C. Conover,
J. Harrison,
J. Hock, K. Horwitz,
A. Kahn, A. Karaplis,
S. Krane,
D. LeRoith,
J. Lian, R. Lindsay,
S. Manolagas,
R. Marcus,
L. McCauley,
D. McDonnell,
G. Mundy,
R. Nissenson,
E. Orwoll,
R. Pacifici,
J. Pike, L. Raisz, F. Ramirez,
G. Rodan, G. Roodman,
I’. Rotwein,
G. Segre, E. Smith, T. Spelsberg,
A. Stewart,
L. Suva, S. Teitelbaum,
J. Termine,
J. Wasmuth,
and
E. Weir. Special thanks go to Dr. Paula Stern for her efforts in serving
to germinate
this workshop.
The secretarial
assistance
of Mrs. Terri
Jacobs is acknowledged.
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