The Actions and Interactions of Sex Steroids and Growth Factors/Cytokines on the Skeleton

Thomas C. Spelsberg, M. Subramaniam, B. Lawrence Riggs and Sundeep Khosla

Department of Biochemistry and Molecular Biology (T.C.S., M.S.) Division of Endocrinology, Department of Internal Medicine (B.L.R., S.K.) Mayo Foundation Rochester, Minnesota 55905

INTRODUCTION

The discoveries that sex steroid receptors for estrogen [estrogen receptor (ER)], androgen [androgen receptor (AR)], and progesterone [progesterone receptor (PR)] are present in the bone-forming osteoblasts (OB) and bone-resorbing osteoclasts (OCL), and that these cells are targets for sex steroid action, have generated much excitement in the field of bone biology. The subsequent discoveries that the sex steroids regulate the production of growth factors and cytokines in these OB and OCL cells and that these factors mediate much of the steroid action on the skeleton have added to the excitement and complexity of the system. Because of the required brevity of this minireview, we will discuss only selected aspects of this field using a limited number of references and review articles. For more information in this field, readers are referred to several recent books and chapters that review bone biology, bone diseases, and the role of sex steroids in bone biology and disease (1, 2, 3, 4, 5, 6, 7, 8).

As a brief background to this unique system, the skeletal maintenance involves a continuous remodeling at discrete sites termed bone-remodeling units (BRU). As outlined in Fig. 1Go, the multinucleated bone-resorbing OCLs first dissolve bone at these BRU sites, resulting in resorption cavities. While the mechanism(s)/signal(s) that initiates resorption at a given site is unknown, there appears to be a marked regulation of overall bone resorption via the regulation of the OCL differentiation and activity (5, 6, 7, 8). These processes are discussed in more detail below. The bone-forming OBs are then recruited by unknown signals, to replace the previously resorbed bone. The identities of the primary factors that couple these processes of resorption to formation to maintain bone mass are also addressed later in this review and are still the subject of much debate. During periods of skeletal growth in children, the increase in bone formation largely involves bone modeling (accretion of bone at surfaces in the absence of bone resorption). The response to mechanical stresses also utilizes mostly modeling. During early adulthood (20–40 yr), bone resorption and formation are in balance and bone mass is maintained. The increased serum estrogen (E) level, which occurs at the onset of puberty in girls, is accompanied by an increase in growth velocity and, ultimately, in the closure of the epiphyseal growth plate and the cessation of linear growth (9). E is the primary hormone responsible for maintaining bone mass in adult women and may serve a similar role along with androgen, in adult men. Finally, during periods of bone loss, such as in postmenopause in women and aging in both genders, bone resorption outpaces bone formation and, in some cases, the bone loss is sufficient to cause osteoporosis.



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Figure 1. Schematic of the Steroid Actions and Interactions with Growth Factors/Cytokines in Bone Cells at Bone Resorption and Formation Site

This model is self-explanatory. The solid arrows represent pathways with significant support in the literature. The broken lines represent potential pathways not currently well identified or characterized (4 5 6 7 8 ).

 
It is currently accepted that in postmenopausal females, a deficiency of E results in osteoporosis, whereas in aging males, the deficiency of E and androgens (A) may lead to bone loss (1, 2, 3). Ovariectomized (OVX) animals and postmenopausal women experience increased bone turnover, increased bone resorption, and an overall decrease in bone mass. On the other hand, E replacement therapy inhibits this bone loss by inhibiting bone turnover and bone resorption, which prevents the development of osteoporosis (1, 3). Although, the complete mechanisms of action of E in this protection against bone loss are still unclear, new studies are beginning to elucidate the molecular processes involved. The following review summarizes some of the important molecular events in the actions of sex steroids, primarily estrogens, which have been recently reported on the skeleton and bone cells, as well as the role that growth factors and cytokines play in this action. For more details, the readers are referred to several recent reviews that emphasize various aspects of steroid action on bone biology (4, 5, 6, 7, 8).

E ACTION ON OB DIFFERENTIATION and ACTIVITIES

Action of E on Mature OB Activities
That bone cells, especially OB cells, contain ER is fairly well established. Using sensitive techniques, such as immunofluorescence, semiquantitative RT-PCR, in situ hybridization, and in situ RT-PCR, both the ER mRNA and ER protein have been identified in periosteal cells, OBs, osteocytes, and OCLs, at levels less than the reproductive organs, but equivalent to those in liver (9, 10, 11, 12, 13). Moreover, Bodine et al. (14), using RT-PCR and primary cultures of rat calvarial OBs, found that ER concentrations increased markedly during OB differentiation.

It is fairly well established that E has major effects on OB cells. As summarized in Table 1Go, E acts directly on OB cell proliferation and expression of genes coding for enzymes, bone matrix proteins, hormone receptors, transcription factors, as well as growth factors/cytokines (4, 5, 6, 7, 8, 15, 16). However, results on the specific genetic and metabolic alterations induced by E in these in vitro systems have been conflicting. As outlined in Table 2Go, these discrepancies could be caused by differences in the receptor concentrations, animal species, skeletal locale of the OB cells, the stages of OB differentiation, and the concentrations of the two ER species (ER-{alpha} and ER-ß) in the OB cells. Although E inhibits the proliferation of mature human OB cells in culture (15, 16), the possibility that E may stimulate OB proliferation under some circumstances has not been excluded. In mature OB cells in culture, E has been shown to induce the synthesis of transforming growth factor-ß (TGF-ß), insulin-like growth factor-I (IGF-I), and IGF-binding proteins, and to inhibit the synthesis of interleukin (IL)-1, IL-6, and IL-11 (5, 6, 7, 8). E deficiency increases the production of IL-1 and IL-6, as well as macrophage-colony stimulating factor (M-CSF) in bone marrow stromal cells. Further, E has also been reported to inhibit the IL-1 or tumor necrosis factor-{alpha} (TNF-{alpha})-induced IL-6 production in human OB and rat bone marrow stromal cells (5, 6, 7, 8, 17, 18). More recently, E has been shown to regulate the synthesis of bone matrix proteins, bone morphogenetic protein-6 (BMP-6), osteoprotegerin (OPG), as well as transcription factors, such as TIEG (TGF-ß inducible early gene), nuclear factor-{kappa}B and c/EBP-ß, and c-fos in human OBs (5, 8, 19, 20, 21, 22). E has also been shown to regulate the activities of the estrogen response element (ERE)-reporter genes and the IGF-I gene promoter-reporter genes transfected into primary rat or human OB cells in culture, as well as the inositol phosphate receptor gene promoter-reporter in normal and transformed OB cells (5, 6, 7, 8).


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Table 1. Reported Actions of Estrogen on Gene Expression in Osteoblast Cells in Culture

 

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Table 2. Suspected Explanations of Differential Actions of E on Osteoblasts

 
Action of E on OB Differentiation
The E regulation of bone mass and remodeling may be initially dependent on mature bone cell activity or on the regulation of bone cell populations (i.e. osteoblastogenesis and osteoclastogenesis). As suggested by Jilka et al. (6, 23), it is possible that changes in osteoblastogenesis induced by E deficiency, paracrine factor production, or cell populations may, in turn, regulate osteoclastogenesis. Indeed, SAMP-6 and OSF2 (-/-) mice, which have decreased or no osteoblastogenesis, respectively, also show decreased osteoclastogenesis (for review see Ref. 6). These results, combined with the finding that E deficiency results in increased osteoblastogenesis as well as osteoclastogenesis in mouse bone, suggests that osteoclastogenesis and increased bone remodeling caused by E deficiency may occur in response to the initial osteoblastogenesis (6, 23). Whether this phenomena is due to changes in bone marrow stromal cell activity or simply populations or to changes in the mature OB activities and cell populations is currently unknown.

Role of ER-{alpha} and ER-ß in OBs
At least some of the variable actions of E on OB cells may be attributed to different concentrations of ER in the OB lines or to different ratios of ER-{alpha} and ß. In humans, Smith et al. (24) described a man with an inactive mutated ER-{alpha} who had osteopenia and open epiphyses, thereby demonstrating the importance of E in maintaining bone mass in men. In contrast, ER-{alpha} knock-out mice displayed only a 10% reduction in mineralization, but otherwise had normal skeletal development (25), suggesting that another ER species or nonreceptor mechanisms may be playing major roles in the E action in skeletal tissues. Significantly, a separate species, ER-ß, has been identified in humans and in rat tissues, including OB cells, and found to have partial homology to the extensively studied ER-{alpha} (26, 27). The ligand-binding affinities and DNA-binding domains of ER-{alpha} and ER-ß are similar, but their transactivation domains, tissue distribution, and molecular sizes differ significantly. In some cell systems, using transfection studies with reporter genes, the ER-ß was shown to be a weaker regulator of gene transcription than is the well known ER-{alpha} (26, 27, 28). Further, the ER-{alpha} was shown to activate, whereas ER-ß inhibits, AP-1 regulatory elements (28). While both ER-{alpha} and ER-ß are expressed in rat and human OB cells, the two species are differentially expressed during OB differentiation (13, 29). Although the exact functions of ER-ß and ER-{alpha} in the human skeleton must await further studies, their differential expression in bone cells may help explain the different E responses among the various OB preparations, as well as among the different actions of the selective ER modulators (SERMs), described below.

ACTION OF E ON OCL DIFFERENTIATION AND ACTIVITY

Direct Action of E on OCL Activity
The major action of E on the skeleton in vivo is believed to be an inhibition of bone resorption, mostly by indirect actions of E. This indirect E action involves regulation of growth factor and cytokine production in OBs and their precursor cells, which, in turn, regulate OCL differentiation and activity (discussed further below). However, some direct actions of E on mature OCL activities have been reported. E must have some direct actions on OCLs since avian OCL cells have been shown to contain E receptors, and E has been shown to regulate bone-resorbing activity and specific gene expression by these cells (10, 30). Table 3Go summarizes the results from several laboratories, which demonstrated that E directly inhibits bone-resorbing activity in avian, rabbit, and human OCL via the regulation of specific gene activities (5, 10, 30, 31, 32). E was shown to antagonize the induction of a novel 150-kDa superoxide dismutase-related membrane glycoprotein by OCL differentiating agents (31). E was also shown by that laboratory to induce an IL-1 decoy receptor and to decrease the expression of an IL-1-signaling gene (32). Other studies using murine and rabbit OCL have substantiated the above avian OCL studies (33, 34). Not only was ER found in rabbit OCL but E inhibited (and anti-E blocked) effects on OCL-resorptive activity, including the production of bone resorbing enzymes (33). E-induced decreases in bone resorption may also be caused by the reduced OCL numbers that are due to E-induced apoptosis (33, 34). This direct E induction of apoptosis may also explain the E inhibition of bone resorption by reducing the OCL life span and population. The E induction of apoptosis may be further enhanced in vivo since TGF-ß, whose production by OB is enhanced by E, was also shown to induce OCL apoptosis in these cultures (34). It should be mentioned, however, that some laboratories have not been able to detect ERs in, nor E effects on, OCL cells (5, 35). As with the OB cells (Table 2Go), these contradictory results may potentially be explained by the age of the animal (embryonic, neonate, or adult) or differences in the OCL cell preparations used (5).


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Table 3. Reported Actions of Estrogens on Gene Expression in Osteoclast Cells

 
Actions of Estrogens on OCL Differentiation: Role of Cytokines
Recently, it has become clear that many actions of E on bone cells are mediated by paracrine factors. Only a few examples of this mechanism are described here, with references to more extensive reviews (2, 4, 5, 6, 7, 8).

The IL-1, IL-6, TNF-{alpha} Story
As mentioned at the beginning of this minireview, the coupling between the OB and OCL at the BRU sites infers a cross-talk by chemical signals (factors) between the OB and OCL cells and their precursors (see Fig. 1Go and reviews in Refs. 4, 5, 6, 7, 8). As outlined in Fig. 2Go, recent studies suggest that OB production of bone-resorbing cytokines, such as interleukin (IL)-1, TNF-{alpha}, M-CSF, IL-6, and PGE2 may mediate bone loss after E deficiency via acting on OCL cell differentiation and activity (6, 7, 8). Previous studies have shown that the production of IL-1 and a specific competitive inhibitor of IL-1 action, the IL-1 receptor antagonist, IL-1ra, by peripheral blood monocytes is increased in oophorectomized and postmenopausal women and is suppressed by estrogen replacement therapy (36). Moreover, the administration of the IL-1ra to OVX rats decreased bone resorption and bone loss (37). Also, the overexpression of the soluble TNF receptor (TNF-R) in transgenic rats, or treatment of the animals with soluble TNF-R , prevented the increase in bone resorption and bone loss after ovariectomy in rats (37, 38). Thus, both IL-1 and TNF-{alpha} and their receptors play important roles in mediating the bone loss after E deficiency. Recently, IL-1{alpha} was reported to induce the expression in osteosarcoma cells of OPG, an inhibitor of OCL differentiation (39). Another IL family member, IL-11, as well as PGs and PTH, have been shown to increase the production of the OPG-ligand (OPG-L), an inducer of OCL differentiation, in murine OB cells (40). The important roles of OPG and OPG-L are discussed in the next section.



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Figure 2. Model of the Interaction and Coupling between OB and OCL via OPG, OPG-L, and Other Growth Factors/Cytokines (8 )

 
Still other studies indicate that IL-6 plays a key role in mediating bone loss after E deficiency (Fig. 2Go). Some studies have shown that E can suppress IL-6 production by murine bone marrow stromal and osteoblastic cells. Moreover, an IL-6 neutralizing antibody prevented the increase in osteoclastogenesis induced by ovariectomy in rats (18). In support of a central/critical role for this pathway, IL-6-knockout mice were reported to be protected against ovariectomy-induced (i.e. E-deficient) bone loss (6, 7, 8). Considering only the positive data, Fig. 2Go outlines a plausible hypothesis that several bone-resorbing cytokines, such as IL1-{alpha}, TNF-{alpha}, IL-6 (and probably others, e.g. IL-11 and PGs) (40, 41), act cooperatively in enhancing osteoclastogenesis and OCL activity, and inducing bone resorption after E deficiency. E is now believed to suppress the production of these cytokines in OBs and other neighboring cells which, in turn, act to inhibit OCL activity or osteoclastogenesis. It should be remembered, however, that studies from other laboratories have not shown a consistent regulation of these factors by E (2, 4, 8). This has raised the issue that additional mediators may be involved, novel candidates of which (OPG/OPG-L) are described below. It should be mentioned that IL-1, IL-6, and TNF-{alpha} display both interactive inductions (as well as auto-amplification) of their gene expressions in OBs (6). All three have been shown to self-stimulate their own expression, IL-1 and TNF-{alpha} stimulate each as well as IL-6 production, and the production of IL-6 and TNF-{alpha} are suppressed by E. Thus, small changes in any of these OB-derived cytokines could amplify the whole system. An additional parameter to consider is the report that E induces the production of a decoy receptor for IL-1, termed IL-1R (type II), and decreases the expression of the IL-1-signaling receptor gene (IL-1RI) (32). In E-deficient states, the expression of the signaling gene would be increased and that of the decoy receptor would be reduced, which should enhance the OCL responsiveness to IL-1.

Osteoprotegerin and Its Ligand
Recently, a novel secreted glycoprotein, OPG, a soluble non-membrane-bound member of the TNF-R superfamily, has been identified by screening a fetal rat intestine cDNA library. After a series of studies, OPG now is believed to play a major role in bone biology (42). The OPG lacks a transmembrane domain, and thus is secreted as a soluble factor. Both the rat protein and a human homolog have been identified, and both are potent inhibitors of osteoclastogenesis (see Fig. 2Go) (43). Transgenic mice that overexpress OPG display a phenotype of nonlethal osteopetrosis associated with decreased osteoclastogenesis (43, 44). Further, OPG knockout mice have markedly increased OCL activity and severe osteoporosis characterized by decreased trabecular bone mass and cortical thinning (45).

Recent studies have also identified the natural ligand for OPG termed "OPG-L" (46, 47). OPG-L is a TNF-related cytokine that is produced by a variety of cells, including bone marrow stromal cells and OBs. Interestingly, OPG-L is identical to TRANCE/RANKL, which was recently identified as a factor enhancing T cell growth and dendritic cell function (48). As outlined in Fig. 2Go, many investigators speculate that OPG-L is likely the long-sought ligand mediating an essential signal from both marrow stromal cells and OBs to the OCL progenitor cells to initiate their differentiation into mature OCLs. OPG-L, in the presence of M-CSF, can replace the requirement for stromal cells in the spleen coculture model for in vitro osteoclastogenesis (46, 47). The osteoclastogenesis effects of OPG-L are blocked in vitro and in vivo by OPG. Interestingly, E treatment of human OB cells increased OPG mRNA steady state levels and OPG protein secretion, and these effects were inhibited by the E antagonist, ICI 182,780 (19).

Thus, while ablation or neutralization of certain bone-resorbing cytokines (IL-1, IL-6, TNF-{alpha}) in animal models prevents sex hormone deficiency-induced bone loss (38, 49), OPG is unique because, in contrast to other cytokines affecting osteoclastogenesis (42), its production is strongly regulated by E, its overexpression by gene transfection results in osteopetrosis, and its targeted deletion results in osteoporosis (45). Taken together, the evidence is compelling that the OPG/OPGL system acts downstream to IL-1, TNF, IL-6, and PGE2 and thus may be the final effector mediating the effects of cytokines on the regulation of osteoclastogenesis, and thus, an important mediator of the effect of E deficiency on bone cell functions. More studies to confirm this interesting possibility are needed.

The TGF-ß and IGF Families
There are two other growth factor families that are believed to also play major roles in bone cell coupling and E regulation. Both TGF-ß and IGFs are found in large concentrations in the bone matrix. Space limitations only allow brief review of their role, but more extensive reviews are available (4, 5, 8, 50). In vivo studies have demonstrated that TGF-ß is involved in osteogenesis and chondrogenesis and that local injections of TGF-ß into rodents caused increased bone mass (4, 8, 50). Interestingly, when injected subperiosteally into the femur of young rats, TGF-ß induced the differentiation of periosteal mesenchymal cells into OB cells (50). Similarly, TGF-ß treatment of mice increased calvarial thickness and mineralized bone formation. OBs and OCLs have been demonstrated to synthesize TGF-ß in vitro, and this production has been shown to be induced by E (5). The TGF-ßs, in turn, have been shown to regulate gene expression and other cellular activities of the OBs and OCLs (5, 50, 51). The BMPs compose an extensive portion of the TGF-ß superfamily and are an integral component in growth and development. Select members of this superfamily are thought to act sequentially in regulating endochondrial bone development, and BMP-6 has been shown to be regulated by E whereas E has no effect on the other BMPs (50). IGF-I and II are also found in high concentrations in the skeleton, are produced in OB cells, and are regulated by E (4, 5, 8). Interestingly, E also regulates one of the binding proteins for the IGFs, IGFBP-4, which in turn regulates the biological activity of the IGFs (4, 5). The exact roles of TGF-ß and the IGFs in OB-to-OCL communication, cell differentiation, and bone loss caused by E deprivation in the skeleton remain to be defined.

ACTION OF OTHER ESTROGENS ON THE SKELETON

SERMs
Studies with synthetic estrogens, including SERMs, such as raloxifene and tamoxifen, are not only providing new insights into the actions of E in bone, but also are identifying new therapeutic estrogens to prevent osteoporosis without the negative side effects of the native estrogens (8, 52). These compounds have tissue-specific actions and can serve as either E antagonists or agonists. Certain SERMs mimic the action of E in bone cells and blood lipids, while antagonizing E action in reproductive cells. Other SERMs, such as raloxifene, are E agonists in bone but have a neutral effect on endometrial tissue (8, 52). While SERMs bind the receptor at the same site and often with the same affinity as E, they induce different conformational changes in the receptor molecule which, in turn, alters which transcriptional activation domains on the ER molecule will be active (53). In addition, the SERM-ER complex can interact with different coactivators/corepressors to generate different patterns of gene expression than the native E-receptor complex. For instance, E regulates the expression of the IL-6 gene not by direct interaction of the ER complex with the ERE on DNA, but by binding and inhibiting the activity of transcription factors, such as nuclear factor-kß and C/EBPß, which are required for IL-6 gene transcription (8, 22). In addition, raloxifene has been shown to activate the gene for transforming growth factor-ß3 in human osteosarcoma MG-63 cells transfected with a mutated ER-{alpha} that lacked the DNA-binding domain in the ER-{alpha} receptor. Finally, both ER-{alpha} and ER-ß interact with AP-1 sites in DNA via interactions with the transcription factors, fos and jun. However, when native estradiol is present, ER-{alpha} activates transcription from the AP-1 site, while ER-ß inhibits transcription (26, 28, 53, 54, 55, 56). In contrast, in the presence of SERMs such as raloxifene, ER-ß stimulates transcription from the AP-1 site, whereas ER-{alpha} inhibits transcription. Thus, the presence of either ER-{alpha} or ER-ß, or different ratios of the two, may have a significant impact on the response of the OBs or OCLs to the particular estrogenic steroid. The exact mechanisms of action of these SERMs appear to be complex, but may well provide new insights into E action in the skeleton and other tissues.

E Metabolites
The possible role of E metabolites in bone biology should be mentioned, since these can become the major serum estrogens in postmenopausal women. Estradiol (E2) is metabolized mainly to estrone, the primary estrogenic metabolite in the circulation of postmenopausal women. The estrone is then hydroxylated, primarily at the C-2 or C-16{alpha} sites. The C-16 hydroxylation leads to the production of 16{alpha}-hydroxyestrone (16{alpha}-OHE), an E agonist that binds the ER, enhances uterine weight in OVX rats, alters gonadotropin secretion in OVX rats, stimulates breast cell proliferation, as does E (57). Further, 16{alpha}-OHE, but not 2-OHE, decreased bone turnover and tibial growth in OVX rats, similar to E, but had much less of an effect than did E on increasing uterine weight and development. Interestingly, there is some evidence that women who generate less 16{alpha}-OHE than 2-OHE are at greater risk for developing osteoporosis and show greater bone loss than women who have high levels of 16{alpha}-OHE. Further, the ratio of 16{alpha}-OHE to 2-OHE correlates positively with spinal bone mineral density (58). These results indicate that 16{alpha}-OHE has E agonist activities on the skeleton and may play a role in the incidence (i.e. protection against) of postmenopausal osteoporosis.

ACTIONS OF OTHER SEX STEROIDS

Only a brief mention of the actions of the other two classes of sex steroids, androgens and progesterone, can be made. The increase in bone mass at puberty in boys is associated with increases in markers of bone formation and is closely linked to pubertal stage, suggesting that testicular androgen production contributes to the pubertal increase in bone mass (8). As in the case of E, there is evidence that androgens inhibit bone resorption and have receptors in the bone-resorbing OCLs and inhibit their activity in vitro. Thus, orchiectomy, like ovariectomy, is associated with increased bone resorption and rapid bone loss (8). Since human OB and OCL cells have been demonstrated to have ARs and because dehydrotestosterone (DHT) has been shown to have mitogenic effects on normal and transformed OB-like cells in most OB cell lines (depending on the differentiation state of the OB), it is probable that androgens act on OB cells to regulate their activity and differentiation (8). Recent studies have shown that DHT can activate an androgen response element-chloramphenicol acetyltransferase reporter construct and regulate OB cell proliferation and bone matrix protein production (59). In addition, DHT and testosterone inhibited IL-6 production and the IL-6 receptor gene promoter activity in murine bone marrow-derived stromal cells (60), as well as IL-6 production by human OB cells (61). In addition to testosterone and DHT, there is some evidence that adrenal androgens may also have significant effects on bone metabolism (60, 62). The effects of progesterone on bone metabolism are much less clear than those of E or androgens. However, PRs have been identified in primary human osteoblastic cells, human osteosarcoma cells, and immortalized fetal OB cells (for a review, see Ref. 8). Clinical studies using progesterone to prevent bone loss, however, have found variable results, with some studies showing an effect of progesterone in prevention of spinal or cortical bone loss, whereas other studies have found no significant benefit of progesterone treatment on postmenopausal bone loss (8). Further studies are required to determine the exact role of progesterone and androgens in bone biology.

CONCLUSIONS

Figure 3Go summarizes the potential pathways of the E regulation of bone resorption, as described in this minireview. Although the ultimate action of E in the skeleton appears to occur at the level of bone resorption due to the regulation of OCL activity as well as differentiation of the OCL precursors, one has to consider the critical role of the OB cells. Most of the growth factors/cytokines that mediate E action on bone resorption as well as OCL differentiation and activity are produced by OB cells and are under E regulation. Therefore, any defects in the production of these OB-derived factors or in the regulation of this production by E, would be reflected in the overall bone resorptive activity found in oophorectomy, postmenopause, and bone disease. This review presents only a sampling of the rapidly developing field of sex steroids and cytokine action on bone. Readers are encouraged to read further in the more extensive reviews and new papers listed in the reference section. Since osteoporosis affects millions of women and men and costs the United States approximately 14 billion dollars a year in medical care, investigations into new drugs, both steroidal and other, to prevent and even reverse bone loss, will be a challenging and exciting endeavor for scientists in the years to come. To succeed in this endeavor, the molecular actions and interactions of the sex steroids, SERMS, and growth factors/cytokines on the skeleton and bone diseases need to be fully elucidated. This field will present exciting challenges for basic scientists and clinical investigators in the years to come.



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Figure 3. Potential Pathways in the E Regulation of Bone Resorption

The IL-1, IL-6, and TNF-{alpha} inhibit apoptosis and extend the life span of OCLs. In contrast, TGF-ß increases osteoclast apoptosis (5 7 ).

 

FOOTNOTES

Address requests for reprints to: Dr. Thomas Spelsberg, Department of Biochemistry and Molecular Biology, Mayo Clinic, 1601 A Guggenheim Building, Rochester, Minnesota 55905-0001.

This study was supported by NIH Grants AG-04875 and AR-43627.

Received for publication February 25, 1999. Revision received March 18, 1999. Accepted for publication March 22, 1999.

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