骨生物学:当前与未来骨质疏松治疗的基础
BONE BIOLOGY AS THE BASIS FOR CURRENT AND PROSPECTIVE TREATMENTS OF OSTEOPOROSIS
T.J.Martin 

[主要职位]
St.Vincent医药研究所主任。
澳大利亚健康技术委员会DNA实验学组主席。
康辰骨质疏松医药研究奖评委会名誉顾问。
[主要研究领域]
主要致力于骨代谢疾病的诊断、钙调节与骨代谢研究。

摘  要
 摘  要
 Bone modelling and remodelling
 Osteoclast formation and bone resorption
 Bone formation

 Diseases of Bone - osteoporosis
 Osteoclast inhibition as a therapeutic approach
 New therapeutic approaches based on mechanistic understanding
 Conclusion
 REFERENCES
   骨通过吸收与形成过程以维持骨骼正常的生理状态。这有赖于骨组织中成骨细胞与破骨细胞之间的联系。这是由于循环中激素的作用,但起更主要作用的是在骨局部产生的一系列细胞因子和生长激素。在一段时间内,骨的净吸收过程超过骨的形成,这是形成骨质疏松的基础。破骨细胞的形成被一些激素和细胞因子紧密地控制着,这些激素和细胞因子作用于成骨细胞系从而促进RANKL(receptor active NF-KB ligand)的合成,也就是肿瘤坏死因子α配体家族中的一员。RANKL与M-CSF共同作用促进破骨细胞的形成。OPG(osteoprotegerin)也是成骨性基质细胞的产物,它是RANKL的假受体,可以阻滞破骨细胞的形成以及活化。骨吸收的抑制剂(如治疗骨质疏松的药物),在不同的方面影响破骨细胞的形成与活化的过程。然而,在治疗骨质疏松的药物中,我们还需要能够促进骨形成的药物。骨形成的过程也就是保持分化的成骨细胞表现型。最近的研究发现,转录因子cbfal控制这一过程。这一因子调整许多成骨细胞的表达--特异性基因,这对于骨的形成是必需的。关于如何通过促进骨形成治疗骨质疏松,我们已经取得了一些进展。在一项可信的研究中发现,间断的注射甲状旁腺素(PTH)将会增加骨量并减少骨折的发生。另一项研究提示了有趣的结果,用于降低胆固醇的HMG CoA诱导酶抑制剂,在体外和动物研究中能够促进骨的形成。治疗骨质疏松的新方法可能还包括骨形成促进剂和骨吸收抑制剂的合理合并使用。

   The tightly regulated processes of bone formation and resorption are essential for the achievement and maintenance of skeletal strength and form. Circulating hormones are important controlling factors, but the key influences are locally generated cytokines, which influence bone cell function and communication in complex ways, and often are themselves regulated in turn by the hormones. For the normal amount of bone to be maintained, bone formation and resorption need to be equal. Concepts of the pathogenesis of osteoporosis have developed that focus on changes in the number of bone cells, either over-production of osteoclasts or under-production of osteoblasts, in the latter case either through inadequacy of precursors or failure in differentiation.

Bone modelling and remodelling.

 摘  要
 Bone modelling and remodelling
 Osteoclast formation and bone resorption
 Bone formation

 Diseases of Bone - osteoporosis
 Osteoclast inhibition as a therapeutic approach
 New therapeutic approaches based on mechanistic understanding
 Conclusion
 REFERENCES

   Bone formation and resorption proceed throughout life. The processes are more rapid during skeletal growth, at which stage the term modelling is used. Modelling takes place from the beginning of skeletogenesis during fetal life until the end of the second decade when the longitudinal growth of the skeleton is completed. In the modelling process , bone is formed at a location different from the sites of resorption, leading to a change in the shape or macroarchitecture of the bone. It is responsible for determining the size and shape of bone, such as the simultaneous widening of long bone and development of medullary cavity by bone formation at the periosteal surface and resorption at the endosteal surface, respectively. The remodelling process, which continues throughout adult life, is necessary for the maintenance of normal bone structure and requires that bone formation and resorption should be balanced. The remodelling concept owes much to the ideas of Frost (1 ), and can be outlined briefly as follows. Both bone formation and resorption occur at the same place so that there is no change in the shape of the bone. After a certain amount of bone is removed as a result of osteoclastic resorption and the osteoclasts have moved away from the site, a reversal phase takes place in which a cement line is laid down. Osteoblasts then synthesize matrix, which becomes mineralized (2). Remodelling thus maintains the mechanical integrity of the skeleton by replacing old bone with new bone.
   The fact that resorption is followed by an equal amount of formation is crucial, and has come to be known as "coupling", with the uncoupling of resorption from formation resulting in osteoporosis, the commonest metabolic bone disease. In the adult human skeleton, approximately 5 to 10% of the existing bone is replaced every year. The characteristic feature of bone remodelling is that the process does not occur uniformly throughout the skeleton. Remodelling of bone occurs in focal or discrete packets known as bone remodelling units (BRU) or basic multicellular units of bone turnover. The cellular sequence is always initiated by osteoclastic bone resorption to be followed by osteoblastic new bone formation. This sequence of events is initiated at asynchronous sites throughout the skeleton, which are geographically and chronologically separated from each other. Bone remodelling is an integral part of the calcium homeostatic system. It also provides a mechanism for self-repair and adaptation to physical stress. The processes of bone resorption and formation are controlled by systemic hormones and cytokines.
  The maintenance of a normal, healthy skeletal mass depends on interactions between osteoblasts, osteoclasts and constituents of the bone matrix to keep the processes of bone resorption and formation in balance. One of the intriguing issues of bone cell biology has been to determine how osteoclast precursors are recruited and induced to differentiate into mature osteoclasts and, in turn, how osteoblasts are instructed to replace just exactly that amount of bone which has been resorbed.

Osteoclast formation and bone resorption

 摘  要
 Bone modelling and remodelling
 Osteoclast formation and bone resorption
 Bone formation

 Diseases of Bone - osteoporosis
 Osteoclast inhibition as a therapeutic approach
 New therapeutic approaches based on mechanistic understanding
 Conclusion
 REFERENCES

   Osteoclast formation is controlled by several circulating hormones such as parathyroid hormone, estrogen and 1,25 dihydroxyvitamin D3 (calcitriol) (3). The bone marrow microenvironment also plays an essential role as a source of cytokines such as tumour necrosis factors and interleukins. These systemic and local factors regulate osteoclast formation and function. However, because the receptors for these systemic and local factors are expressed in cells of the osteoblast lineage they need to rely on secondary signals generated by osteoblasts to mediate their effects (3,4). For many years it has been recognised that bone-resorbing factors, in order to produce their effects, must first act on cells of the osteoblast lineage. These cells were considered to possess a cell surface molecule, known as osteoclast differentiation factor (ODF), which acted upon hemopoietic precursors to promote osteoclast formation (3-5).
   The discovery of osteoprotegerin (OPG), a soluble member of the TNF receptor superfamily, revealed it as a very effective inhibitor of osteoclast formation (6). This provided the means of identifying and cloning the elusive ODF, known now as RANK ligand (RANKL), and the common factor mediating osteoclast formation in response to all known stimuli (7). Osteoblasts/stromal cells are also the source of M-CSF, which plays a crucial role in osteoclast formation by promoting the proliferation of precursors.
   When hemopoietic cells are treated with M-CSF and RANKL, osteoclasts are formed without any participation of osteoblasts/stromal cells (8). The communication with the hemopoietic lineage results from RANKL binding to its receptor on the osteoclast lineage, known as RANK.
   All of these discoveries have been validated by studies in genetically altered mice, as follows:
(i) Overexpression of OPG results in mice with osteopetrosis because of failure to form osteoclasts (6). Genetic ablation of OPG, on the other hand, leads to severe osteoporosis (9).
(ii) Genetic ablation of RANKL results in osteopetrosis because RANKL is necessary for normal osteoclast formation (10).
(iii) Genetic ablation of RANK leads to osteopetrosis also because it is the receptor for RANK (11). Because this signalling pathway is functional also in immune cells, RANK-null mice have severe abnormalities in that system, with failure of lymph node development and impaired immune responses.
   Osteoclastic resorption takes place in a sealed -off microenvironment (12,13). The most prominent ultrastructural feature of osteoclasts is the deep folding of the plasma memrane, the so-called ruffled border, in the area facing the bone matrix. This structure is surrounded by a peripheral ring tightly adherent to the bone matrix, thus sealing off the sub-osteoclastic resorbing compartment. The mechanism of bone resorption requires acidification of the resorption space by the H+ ions produced by the cells, ressulting ion dissolution of the bone mineral, thereby exposing the organic matrix to proteolytic enzymes (12,13). These enzymes, which include collagenases and cathepsins, are responsible for the degradation of the organic matrix. This process explains how bone resorption contributes to the maintenance of extracellular fluid calcium and phosphate. It also explains the presence of biochemical markers of collagen degradation, such as hydroxyproline and pyridinoline crosslinks, in plasma and urine, thereby providing an estimate of the bone resorption rate (14,15).
   The discoveries in osteoclast biology of the recent few years have identified several new targets for development of drugs that inhibit bone resorption. Of the existing drugs, calcitonin inhibits bone resorption by inhibiting osteoclast activity. The bisphosphonates do so by inhibiting osteoclast activity, and probably also by enhancing osteoclast apoptosis (cell death). The mechanisms by which estrogen inhibits bone resorption are still not certain. Estrogen withdrawal results in enhanced production of certain bone-resorbing cytokines (IL-1, TNFa and IL-6), and enhanced responsiveness to M-CSF. Any or all of these effects could contribute to estrogen action (16).

Bone formation

 摘  要
 Bone modelling and remodelling
 Osteoclast formation and bone resorption
 Bone formation

 Diseases of Bone - osteoporosis
 Osteoclast inhibition as a therapeutic approach
 New therapeutic approaches based on mechanistic understanding
 Conclusion
 REFERENCES

   Bone formation results from a complex cascade of events that involves proliferation of primitive mesenchymal cells, differentiation into osteoblast precursor cells (osteoprogenitor, preosteoblast), maturation of osteoblasts, formation of matrix, and finally mineralization (2). Although in its common usage the term osteoblast is used to describe those cells responsible for the synthesis of bone matrix, it is clear that the osteoblast family also includes the osteocyte and the bone lining cell. The latter are also called surface osteocytes, resting osteoblasts, inactive osteoblasts, endosteal lining cells, and flattened mesenchymal cells. It seems likely that at the end of the remodelling sequence when matrix synthesis is no longer required, osteoblasts lose their synthetic capacity and become bone lining cells or can become trapped behind the advancing calcification front, becoming embedded in bone as osteocytes. The osteocytes in their lacunae communicate with each other and with surface osteoblasts or lining cells by a complex system of cell microprocesses within canaliculi.
   Much less is known of the factors which promote bone formation. Osteoblasts produce powerful growth factors, TGFb, IGF1 and 2, and FGF and store these in bone in large amounts. It is considered likely that production and activation of these bone growth factors is a vital step in stimulating bone formation in response to hormones and to physical processes and drugs (2).
   A discovery which is very important for our understanding of the bone formation process is that of Cbfa1 (osf2), a transcription factor which appears to be essential for the progression of primitive mesenchymal precursor cells through to osteoblasts(17). Replenishment of osteoblasts after bone loss is a key requirement in restoring bone, and Cbfa1 is central to this. Cbfa1 also plays an important role in maintaining the osteoblast in a differentiated state (18). Its regulation is important, and the pharmaceutical industry rightly views it as a target through which bone-forming drugs might be developed.

Diseases of Bone - osteoporosis.

 摘  要
 Bone modelling and remodelling
 Osteoclast formation and bone resorption
 Bone formation

 Diseases of Bone - osteoporosis
 Osteoclast inhibition as a therapeutic approach
 New therapeutic approaches based on mechanistic understanding
 Conclusion
 REFERENCES

   The involvement of the skeleton in disease results from disordered bone resorption and formation, with an excess of one process over the other, either as localised or generalised effects on the skeleton. Loss of ovarian function increases the rate of bone remodelling and leads to an imbalance between bone resorption and formation, resulting in net bone loss. As much as 4-8% of cancellous bone volume may be lost in women after the menopause, and up to 50% in the first 3 months following ovariectomy in rodents..
   There are a number of disorders that can impair optimal bone mass acquisition during childhood and adolescence (19). In some disorders, such as Turner's syndrome, Klinefelter's syndrome, glucocorticoid excess, hyperthyroidism or growth hormone deficiency, low bone mass has been attributed to abnormalities in a single hormone. In diseases such as anorexia nervosa and exercise-associated amenorrhea, malnutrition, sex steroid deficiency and other factors combine to increase the risk of osteopenia or low bone mass.). This is probably also the case of various chronic diseases, which in addition may require therapies that can affect bone metabolism.
   The onset of substantial bone loss occurs at the age of 50 and 65 years in females and males, respectively (see for review: 20). Female sex hormones appear to be mandatory not only to the maximal acquisition of bone mass in both males and females (21-23), but also to the maintenance of this mass by controlling bone remodelling during reproductive life in females (24) and in aging men (25,26). Even a shortening of the luteal phase could be associated with abnormal bone loss (64). Other pathological conditions associated with premature estrogen deficiency, such as anorexia nervosa, secondary amenorrhea due to strenuous exercise, or the use of inhibitors of gonadotropin secretion (19.27,28), support the concept of a causal link between estrogen deficiency and accelerated bone loss. By accelerating bone turnover and uncoupling bone formation from resorption, estrogen deficiency appears to be a main cause of osteoporosis observed in women after the fifth decade, and possibly in men, and thus is directly implicated in the age-related increase in the incidence of fragility fractures (24). It is now clearly established that bone loss does not attenuate with age, but continues throughout the whole life, at least in peripheral skeletal sites (29).
  Apart from gonadal deficiency, which is an important cause of osteoporosis in men, a number of other endocrine diseases can also lead to bone loss.. The effect of primary hyperparathyroidism on bone is to increase the activation frequency of bone remodelling. This increase in bone turnover is associated with a reduction in cancellous bone volume as observed by histomorphometric technique. Osteodensitometry indicates a decrease in a BMD at both axial and appendicular sites (30). An excess of thyroid hormones also increases the rate of bone remodelling. Thus, bone loss can occur in hyperthyroidism and in patients under long-term thyroid replacement therapy (69). The major net effect of glucocorticoid excess is the reduction of bone formation. In addition, there is some evidence that the administration of glucocorticoids in pharmacological excess decreases the intestinal absorption of calcium and perhaps the tubular reabsorption of calcium. These latter two effects would lead to a negative calcium balance and consecutive increased bone resorption through a mechanism which may involve secondary hyperparathyroidism (37).
  Among nutritional factors, deficiencies in calcium, vitamin D (33-35) - to the extent its cutaneous production is insufficient to cover the needs -, and more recently proteins (36) have been shown to be associated with deficient skeletal growth or accelerated bone loss. Vitamin K deficiency has also been shown to be a predictor of hip fractures (35). . There is solid evidence sustaining the notion that calcium contributes to the preservation of the bony tissue during adulthood, particularly in the elderly. It is also clear that without an appropriate supply of vitamin D, from cutaneous and/or exogenous source, the bioavailability and metabolism of calcium is disturbed. This results in accelerated bone loss during adult life. In young adults and middle-aged premenopausal women there is evidence for a positive association between calcium intake and bone mass (35).
  In the elderly, several alterations contribute towards a negative calcium balance. Indeed, with ageing there is a decrease in: the calcium intake by reduction in dairy product consumption; the intestinal absorption of calcium; the absorptive capacity of the intestinal epithelium to adapt to a low calcium intake; the exposure to sunlight; the capacity of the skin to produce vitamin D; the renal reabsorption of calcium, as well as the tubular calcium reabsorptive capacity to respond to the stimulatory effect of parathyroid hormone (PTH). Furthermore, the mild renal insufficiency regularly observed in the elderly can contribute to a state of chronic hyperparathyroidism that favors negative bone mineral balance and thereby osteoporosis. Increasing calcium intake is certainly an important strategy which is relatively easier to implement than other possible preventive measures

Osteoclast inhibition as a therapeutic approach

 摘  要
 Bone modelling and remodelling
 Osteoclast formation and bone resorption
 Bone formation

 Diseases of Bone - osteoporosis
 Osteoclast inhibition as a therapeutic approach
 New therapeutic approaches based on mechanistic understanding
 Conclusion
 REFERENCES

  A number of therapeutic strategies have been developed or are being explored to inhibit the formation on activity of osteoclasts in osteoporosis, cancer - related bone disease, Paget's disease and inflammatory bone disease of rheumatoid arthritis and periodontal disease. Inhibition of bone resorption can be accomplished by reducing either osteoclast generation (for example with estrogens) or osteoclast activity (with bisphosphonates (37).  The molecular mechanism of action of estrogens on bone, as well as on other tissues, is not fully understood. Two estrogen receptors (ER's), a and b, have been identified, but their relative contributions to the various effects of estrogens are still under investigation. Broadly, ERa seems to be responsible for most of estrogen's effects on reproduction and reproductive organs, which are fully compromised in its absence in mice. No unique function has yet been assigned to ERb. The discovery that agents (historically referred to as antiestrogens) were able to exert full or partial estrogen agonist effects on various tissues initiated the development of a new class of drugs known as SERMs. The first SERM identified was tamoxifene, a triphenylethylene compound that was found to prevent bone loss (38). Raloxifene, considered in the early 1980s to be a possible treatment for breast cancer, was found to prevent bone loss induced by estrogen deficiency in rats and monkeys. In clinical studies of raloxifene in post-menopausal women, a 40% reduction in relative risk of vertebral fractures was achieved, despite the fact that there was only a 3 to 4% increase in bone density (39). The mechanism by which SERMs inhibit bone resorption is likely to be the same as estrogen's mechanism, that is by blocking production of cytokines that promote osteoclast differentiation (16). Although estrogen and the SERM's are effective at inhibiting bone resorption to produce a therapeutic benefit in osteoporosis, they are clearly not sufficiently powerful to inhibit the greatly increased osteoclast formation that occurs with the skeletal complications of cancer. The same almost certainly applies to Paget's disease and inflammatory bone diseases.Bisphosphonates are analogs of pyrophosphate (P-O-P) in which the oxygen in P-O-P has been replaced by a carbon with various side chains (40). They concentrate in bone and are, to date, the most effective inhibitors of bone resorption, a property discovered empirically during studies of bone mineralization. Nitrogen-containing BPs are taken up by osteoclasts, where they inhibit farnesyl diphosphate synthase, an enzyme in the mevalonate pathway of cholesterol synthesis (41). This leads to reduction in the levels of geranylgeranyl diphosphate, which is required for prenylation of guanosine triphosphate (GTP)-binding proteins (such as Rho, Rab and Cdc42) that are essential for osteoclast activity and survival. Consequently, BPs inactivate osteoclasts, which then undergo apoptosis, resulting in reduced bone resorption, lower bone turnover, and a positive bone balance.
   Bisphosphonates have been successful in preventing the osteoclast-mediated bone loss of osteoporosis, and have reduced fracture incidence as a result (42). The greatly increased bone resorption throughout the skeleton in the HHM syndrome, as well as the increased osteoclast formation necessary for bone metastasis formation, can also be treated by bisphosphonates (36). These drugs have reduced the incidence of bone metastasis formation in a number of trials, but so far there are no reports of enhanced patient survival. Nevertheless bisphosphonates are the first line of therapy for hypercalcemia of cancer, metastatic bone disease and Paget's disease, as well as being used in treatment of a large proportion of patients with osteoporosis.

New therapeutic approaches based on mechanistic understanding
 摘  要
 Bone modelling and remodelling
 Osteoclast formation and bone resorption
 Bone formation

 Diseases of Bone - osteoporosis
 Osteoclast inhibition as a therapeutic approach
 New therapeutic approaches based on mechanistic understanding
 Conclusion
 REFERENCES

   There are a number of approaches that are being used in the search for new inhibitors of bone resorption (37) - although the question might be asked - we have bisphosphonates, why do we need new resorption inhibitors? Arguments in favour of a continued search for such drugs are that we need a potent, safe, orally active drug that rapidly reduces bone resorption in a dose-dependent manner and is no longer effective after treatment is stopped. Such a drug could even be a candidate for the prevention of skeletal complications of cancer, given long-term as an adjuvant in appropriate patients with breast cancer, for example. The bisphosphonates are useful drugs, but are poorly and erratically absorbed from the intestine, and are extremely stable, becoming stored in bone so that they exert activity long after treatment is stopped. Some of the bisphosphonates, notably those containing amino groups, have significant gastrointestinal side effects.
   New drugs may come from the greater understanding of osteoclast biology of the last few years. The discovery of RANKL, a key factor in osteoclast formation and activity, RANK, its receptor, and OPG ( the inhibitory "decoy"receptor ) provides a number of new therapeutic targets for osteoclast inhibition (6-8, 37). These members of the tumor necrosis factor ( TNF ) receptor and ligand families are crucial for osteoclast control and mediate the regulation of osteoclast formation and activity by cytokines and hormones. OPG injected into rats decreases blood calcium in cancer-induced hypercalcemia (43), and prevents bone loss following removal of the ovaries (44). Furthermore, OPG blocks the periarticular bone destruction in adjuvant-induced arthritis in mice, without influencing the inflammation in and around the joint (45), as well as reducing cancer-induced bone destruction and pain in mice (46).
   These observations identify OPG's interaction with RANKL as a target for therapeutic intervention. Should the protein itself be used for therapy? Although apparently effective, it is a large protein, needs to be given in substantial doses, and may induce immune responses and act in organs other than bone. Physiologically, OPG may accumulate to some extent in the bone matrix (47) and be able to block osteoclast formation from there. If the physiological process requires tight local regulation of OPG production, then perhaps a logical therapeutic approach would be to search for ways to modulate OPG production by bone cells. The possibility of OPG gene therapy might be considered in the future. The only study at the time of writing is one in which ovariectomized mice were injected with a recombinant adenoviral vector carrying the cDNA either of full-length OPG or a fusion protein combining the OPG ligand-binding domain with the human immunoglobulin constant domain. Sustained elevation of plasma OPG levels and prevention of ovariectomy-induced bone loss was achieved only in mice given the vector containing OPG-Fc. (48). It is interesting that efficacy of OPG in the other animal models referred to above also required injection of OPG-Fc, thereby complicating this as a definitive therapy.
   The pathway to resorption inhibition through the TNF ligand and receptor families would mainly be aimed at inhibition of osteoclast formation, even though RANKL does promote osteoclast activity also and OPG inhibits it. It may be that osteoclast inhibition will be more effective by inhibiting their formation than inhibiting their activity because inactivated cells can so readily be replaced. There are nevertheless considerable efforts being made by academic and commercial groups to develop inhibitors of osteoclast activation. These targets include avb3 integrin, vacuolar H+-ATPase, p60-c-src kinase, p38 kinase and cathepsin K (reviewed in 37).
   A recent surprising finding was that "statin" drugs (simvastatin, lovastatin etc) are able to promote bone formation in mice, and prevent the bone loss following ovariectomy (49). Some early clinical evidence supports this also. These cholesterol-lowering drugs mainly target the liver, and their bioavailability to bone is likely very limited. The important aspect of this is that as HMGCoA reductase inhibitors, they draw attention to that pathway as a target for bone anabolic drugs.
   An exciting example of the great power of mouse genetics in investigation is the discovery that leptin deficient (ob/ob) and leptin signalling deficient (db/db) mice have greatly increased bone mass, despite their hypogonad and hypercortisol state (50). This work has raised the intriguing possibility that leptin, acting in the brain, is a central regulator of bone formation by reducing the release into the circulation of a promoter of bone formation.
   Finally, the first apparently effective stimulator of bone formation in osteoporosis is reaching the end of its first trials. PTH, long known to be a powerful stimulator of bone formation in animal experiments, has been studied in about 1600 patients for 21 months, with a 65% reduction in vertebral fractures and 54% reduction in non-traumatic, non-spine fractures (51). This leads to optimism that replacement of bone in severely osteoporotic subjects is a real prospect.
   The ability of PTH to promote bone formation is dependent upon the hormone being administered intermittently in a way that yields a peak blood level, which is not maintained. In that circumstance, processes are initiated in bone which result in anabolic effects, presumably as a result of activation of genes responding specifically to a rapid increase in PTH or PTHrP. On the other hand, if PTH or PTHrP is infused, or administered in such a way that elevated plasma levels are maintained, the dominant effect is stimulation of osteoclast formation and bone resorption, to the extent that these over-ride any anabolic response. There are recent in vivo studies in the rat that support this view. Infusion of PTH into rats caused a robust and sustained increase in RANKL and decrease in OPG production in bone, as well as rapid depletion of matrix stores of OPG, all of which preceded hypercalcemia and enhanced osteoclast formation. In these conditions also, sustained elevated levels of PTH resulted in decreased expression of genes associated with the bone formation phenotype of the osteoblast (52). These included cbfa1, osteocalcin, bone sialoprotein and type 1 collagen. These observations were in contrast to the findings with repeated single injections of PTH, which although they triggered a rapid but transient increase in the RANKL/OPG ratio, resulted in increased bone formation and enhanced expression of the genes associated with bone formation (47).
   Much remains to be learned of the cellular and molecular events that determine whether the actions of PTH and PTHrP are resorptive or anabolic. We need to know whether the anabolic response is predominantly the result of enhanced differentiation of existing osteoblast precursors, of of inhibition of apoptosis, or a combination of the two, or whether specific genes, such as cbfa1, mediate the PTH/PTHrP effect. Important as that is, there may be some relatively simple lessons and hypotheses to develop from the existing information.
   A central role for PTHrP in bone development and growth is suggested by the results of the mouse genetic experiments, including the important observation that PTHrP haplosufficient mice are osteopenic. What, then, are the ways in which PTHrP as a paracrine/autocrine factor in bone, can contribute? The pharmacologic effects of intermittent versus sustained PTH/PTHrP treatment are striking and very different. If the behaviour of osteoblasts in response to stimulation through PTHR1 requires this type of variation in delivery of the relevant ligand, can PTH, as a circulating peptide hormone, achieve this? That is doubtful. On the other hand, the regulated, local production of PTHrP could fulfil this role, with its regulation the result of hormonal, cytokine or neural control. In the case of PTHrP there is an added possibility, that biological activities within the remainder of the molecule could influence local events, either through independent processes or by modifying actions through the PTH1R.

Conclusion
 摘  要
 Bone modelling and remodelling
 Osteoclast formation and bone resorption
 Bone formation

 Diseases of Bone - osteoporosis
 Osteoclast inhibition as a therapeutic approach
 New therapeutic approaches based on mechanistic understanding
 Conclusion
 REFERENCES

   The new insights of the last few years into the control of bone cell function have greatly improved our understanding of the pathophysiology of the bone diseases, as well as giving new leads to the development of interventions to prevent and treat those diseases. The factors regulating osteoclast formation and activity, and therefore bone resorption, are understood to a greater extent than those influencing bone formation. The latter is proceeding apace however, and especially with the excitement surrounding the first effective anabolic treatment - PTH - and the discoveries surrounding osteoblast development and differentiation, we can expect rapid advances in that field also.

REFERENCES
 摘  要
 Bone modelling and remodelling
 Osteoclast formation and bone resorption
 Bone formation

 Diseases of Bone - osteoporosis
 Osteoclast inhibition as a therapeutic approach
 New therapeutic approaches based on mechanistic understanding
 Conclusion
 REFERENCES

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首届康辰骨质疏松医药研究奖颁奖大会暨2001年国际骨质疏松高级研讨会
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