Introductory Review Cellscience Reviews Vol 1 No.4 ISSN 1742-8130

The Complex Interplay Between Fat and Bone

Robert Blank, M.D., Ph.D.

Introduction

Bone is a complex tissue whose properties are influenced by a broad array of factors. Bone serves as a body reservoir of calcium and phosphate, and bone remodeling allows serum concentrations of these ions to be tightly controlled over brief time periods. Superimposed on its role in mineral homeostasis, bone also functions biomechanically to protect vital structures, anchor muscles, and absorb the impacts arising from activity. The last several years have been notable in establishing the importance of mechanical loading, Wnt signaling, and leptin signaling in establishing bone properties.

Further elucidation of the last of these has been provided by the recently published work of Elefteriou and colleagues (Elefteriou et al., 2005), which demonstrates some of the mechanisms by which leptin signaling leads to alterations of bone resorption. In order to contextualize this contribution, I will briefly review some aspects of bone biology. I will then summarize these authors’ findings and conclude by outlining some remaining open questions.

Signaling Pathways in Bone Modeling and Remodeling

In both humans and mice, bone mass changes in a predictable fashion over the lifespan. During childhood and adolescence, total skeletal mass increases, due primarily to linear growth and modeling to increase bone diameter. During young adulthood, bone mass remains relatively stable, followed by progressive loss of skeletal mass in later life (for review, see Seeman, 2001). In humans, peak bone mass is achieved in the 3rd decade, while mice reach skeletal maturity at 4 months (Beamer et al., 1996).

The developmental and regulatory pathways that mediate age-related skeletal changes remain incompletely understood, but several key features are established beyond question. In the normal situation, bone matrix synthesis and bone matrix resorption are tightly coupled processes. This is achieved in part by the fact that osteoclast development requires activation by Rankl, a protein synthesized by osteoblasts (for review, see Teitelbaum, 2000; Ducy et al., 2000). Superimposed on the paracrine signaling between these two cell types are several other environmental and hormonal influences. Mechanical loading is a principal environmental input to bone mass regulation, as first hypothesized by Frost (1987) and subsequently documented by in vivo studies in multiple human and model systems. Accumulating recent evidence points strongly toward osteocytes, the “third cell” present in bone matrix as the locus at which mechanical stimuli are transduced into chemical signals propagated to the osteoblast surface (Bonewald, 2002; Cherian et al. 2003; Cherian et al.,2005; Yang et al., 2005). Hormonal regulation via PTH (for review, see Hodsman et al., 2005; Gensure et al., 2005), IGF-1 (for review, see Rosen, 2004; Clemens & Chernausek, 2004) glucocorticoids (for review see Canalis et al., 2002; Cooper, 2004) and sex steroids (for review, see Bilezikian, 2001; Vanderschueren et al., 2004; Syed & Khosla, 2005) is beyond question. These bone regulatory mechanisms have long been recognized, even though the details have been clarified by recent investigations.

More intriguing because they were previously unsuspected, are the involvement of Wnt and leptin signaling in regulating bone mass. Inactivating mutations of LRP5, a coreceptor for Wnt signaling cause the recessive osteoporosis pseudoglioma syndrome and heterozygous carriers have low bone mass (Gong et al., 1996; Gong et al., 2001). Activating mutations lead to a dominant high bone mass phenotype (Van Wesenbeeck et al, 2003; Boyden et al., 2002; Johnson et al., 1997; Little et al., 2002). Recent work suggests that LRP5 polymorphism may mediate population variability of bone density (Koller et al., 2005).

Leptin, Bone, and the Current Findings

The Karsenty laboratory has been studying leptin’s role in regulating bone mass for close to a decade. These investigators established that both Lep^ob/ob and Lepr^db/db mice exhibit a high vertebral bone mass phenotype arising as a consequence of increased bone formation (Ducy et al., 2000). Further, they demonstrated that leptin’s actions are mediated via neural signaling, as leptin infusion into the CNS could cure the bone phenotype of Lep^ob/ob mice, but that leptin had no direct effect on osteoblasts and that obesity per se was not necessary for the increased bone mass. This was an unexpected observation, for while it was well established that body mass and bone mass are positively correlated, it was equally well known that glucocorticoid excess and sex steroid deficiency lead to loss of bone mass. These mutant animals, while obese on an ad libitum diet, are both hypogonadal and hypercorticosteronemic. Thus, none of the three expected relationships were observed in this series of experiments. Subsequent studies demonstrated that leptin’s effects on feeding behavior and bone mass regulation are mediated by distinct hypothalamic neuronal populations, and furthermore that the leptin antiosteogenic signal is transmitted neurally via a β2 adrenergic pathway (Takeda et al., 2002).

In this paper, Eleftheriou et al. delineate the mechanisms underlying leptin control of bone mass through an elegant and extensive series of experiments. By characterizing Adrb2 knockout mice lacking the β2 adrenergic receptor (Chruscinski et al., 1999), they establish several novel findings. First, bone resorption as well as bone formation is responsive to β2 adrenergic control. Second, the β2 adrenergic receptor (Chruscinski et al., 1999), they establish several novel findings. First, bone resorption as well as bone formation is responsive to β2 adrenergic regulation, as the β2 pathway stimulates Rankl synthesis, thereby promoting osteoclast maturation. Third, the signal transduction pathway features PKA phosphorylation of the transcription factor Atf4, which then transactivates Rankl. Fourth, this mechanism is necessary for gonadectomy-induced bone loss, as this does not occur in the Adrb2^-/- animals. A further series of experiments demonstrates that a leptin-induced hypothalamic neuropeptide, cocaine amphetamine related transcript (CART, Kristensen et al., 1998), inhibits osteoclastogenesis. This antiresorptive signal is mediated via inhibition of Rankl synthesis by osteoblasts. Disruption of the CART pathway explains why Lep^ob/ob mice exhibit increased bone resorption in conjunction with increased bone formation. In contrast, another obese animal model, the Mc4r^-/- mouse, in which the CART pathway is intact, exhibit a high bone mass phenotype and a reduction in osteoclast surface.

Open Questions and Future Prospects

While Eleftheriou and colleagues’ paper represents a substantial advance in our understanding of the interaction between fat and bone, many unanswered questions remain. First, as the authors themselves point out, the mechanisms by which CART affects Rankl expression remain a black box. In particular, it is not known whether there is cross-talk between osteoclastogenic cytokine signaling and CART signaling. Second, since osteoblasts and adipocytes share a common precursor, it is important to determine whether either fat mass or bone mass feeds back on the developmental trajectory. Third, cross-talk between leptin signaling and mechanical loading, PTH signaling, and IGF-1 remain largely unexplored. Each of these issues addresses the larger question of the extent to which leptin signaling accounts for individual differences in bone mass and fracture susceptibility. Fourth, investigations of leptin and bone have focused on vertebral trabecular bone. However, it is known that vertebral and femoral BMD may be discordant (Beamer et al., 2001). It remains to be seen if leptin's effects on long bones, particularly at cortical bone sites, are similar to those reported.

In spite of the many unanswered questions, this work has advanced our understanding of the mechanisms coupling bone formation and bone resorption. Furthermore, the elucidation of multiple regulatory pathways in bone tissue offers a wealth of targets for possible pharmacologic intervention. This potential has not eluded the many investigators who are actively pursuing related issues. The next few years will no doubt provide both additional answers and still more questions.

REFERENCES

Beamer, W.G., L.R. Donahue, C.J. Rosen, and D.J. Baylink, Genetic variability in adult bone density among inbred strains of mice. Bone, 1996. 18(5): p. 397-403.

Beamer, W.G., Shultz, K.L., Donahue, L.R., Churchill, G.A., Sen, S, Wergedal, J.R., Baylink, D.J., Rosen, C.J.. Quantitative trait loci for femoral and lumbar vertebral bone mineral density in C57BL/6J and C3H/HeJ inbred strains of mice. J Bone Miner Res. 2001 Jul;16(7):1195-206.

Bilezikian, J.P., The role of estrogens in male skeletal development. Reprod Fertil Dev, 2001. 13(4): p. 253-9.

Bonewald, L.F., Osteocytes: A proposed multifunctional bone cell. J Musculoskelet Neuronal Interact, 2002. 2(3): p. 239-41.

Boyden, L.M., J. Mao, J. Belsky, L. Mitzner, A. Farhi, M.A. Mitnick, D. Wu, K. Insogna, and R.P. Lifton, High bone density due to a mutation in LDL-receptor-related protein 5. N Engl J Med, 2002. 346(20): p. 1513-21.

Canalis, E., R.C. Pereira, and A.M. Delany, Effects of glucocorticoids on the skeleton. J Pediatr Endocrinol Metab, 2002. 15 Suppl 5: p. 1341-5.

Cherian, P.P., B. Cheng, S. Gu, E. Sprague, L.F. Bonewald, and J.X. Jiang, Effects of mechanical strain on the function of Gap junctions in osteocytes are mediated through the prostaglandin EP2 receptor. J Biol Chem, 2003. 278(44): p. 43146-56.

Cherian, P.P., A.J. Siller-Jackson, S. Gu, X. Wang, L.F. Bonewald, E. Sprague, and J.X. Jiang, Mechanical Strain Opens Connexin 43 Hemichannels in Osteocytes: A Novel Mechanism for the Release of Prostaglandin. Mol Biol Cell, 2005.

Chruscinski, A.J., D.K. Rohrer, E. Schauble, K.H. Desai, D. Bernstein, and B.K. Kobilka, Targeted disruption of the beta2 adrenergic receptor gene. J Biol Chem, 1999. 274(24): p. 16694-700.

Clemens, T.L., and S.D. Chernausek, Genetic strategies for elucidating insulin-like growth factor action in bone. Growth Horm IGF Res, 2004. 14(3): p. 195-9.

Cooper, M.S., Sensitivity of bone to glucocorticoids. Clin Sci (Lond), 2004. 107(2): p. 111-23.

Ducy, P., T. Schinke, and G. Karsenty, The osteoblast: a sophisticated fibroblast under central surveillance. Science, 2000. 289(5484): p. 1501-4.

Ducy, P., M. Amling, S. Takeda, M. Priemel, A.F. Schilling, F.T. Beil, J. Shen, C. Vinson, J.M. Rueger, and G. Karsenty, Leptin inhibits bone formation through a hypothalamic relay: a central control of bone mass. Cell, 2000. 100(2): p. 197-207.

Elefteriou, F., J.D. Ahn, S. Takeda, M. Starbuck, X. Yang, X. Liu, H. Kondo, W.G. Richards, T.W. Bannon, M. Noda, K. Clement, C. Vaisse, and G. Karsenty, Leptin regulation of bone resorption by the sympathetic nervous system and CART. Nature, 2005. 434(7032): p. 514-20.

Frost, H.M., Bone "mass" and the "mechanostat": a proposal. Anat Rec, 1987. 219(1): p. 1-9.

Gensure, R.C., T.J. Gardella, and H. Juppner, Parathyroid hormone and parathyroid hormone-related peptide, and their receptors. Biochem Biophys Res Commun, 2005. 328(3): p. 666-78.

Gong, Y., M. Vikkula, L. Boon, J. Liu, P. Beighton, R. Ramesar, L. Peltonen, H. Somer, T. Hirose, B. Dallapiccola, A. De Paepe, W. Swoboda, B. Zabel, A. Superti-Furga, B. Steinmann, H.G. Brunner, A. Jans, R.G. Boles, W. Adkins, M.J. van den Boogaard, B.R. Olsen, and M.L. Warman, Osteoporosis-pseudoglioma syndrome, a disorder affecting skeletal strength and vision, is assigned to chromosome region 11q12-13. Am J Hum Genet, 1996. 59(1): p. 146-51.

Gong, Y., R.B. Slee, N. Fukai, G. Rawadi, S. Roman-Roman, A.M. Reginato, H. Wang, T. Cundy, F.H. Glorieux, D. Lev, M. Zacharin, K. Oexle, J. Marcelino, W. Suwairi, S. Heeger, G. Sabatakos, S. Apte, W.N. Adkins, J. Allgrove, M. Arslan-Kirchner, J.A. Batch, P. Beighton, G.C. Black, R.G. Boles, L.M. Boon, C. Borrone, H.G. Brunner, G.F. Carle, B. Dallapiccola, A. De Paepe, B. Floege, M.L. Halfhide, B. Hall, R.C. Hennekam, T. Hirose, A. Jans, H. Juppner, C.A. Kim, K. Keppler-Noreuil, A. Kohlschuetter, D. LaCombe, M. Lambert, E. Lemyre, T. Letteboer, L. Peltonen, R.S. Ramesar, M. Romanengo, H. Somer, E. Steichen-Gersdorf, B. Steinmann, B. Sullivan, A. Superti-Furga, W. Swoboda, M.J. van den Boogaard, W. Van Hul, M. Vikkula, M. Votruba, B. Zabel, T. Garcia, R. Baron, B.R. Olsen, and M.L. Warman, LDL receptor-related protein 5 (LRP5) affects bone accrual and eye development. Cell, 2001. 107(4): p. 513-23.

Hodsman, A.B., D.C. Bauer, D. Dempster, L. Dian, D.A. Hanley, S.T. Harris, D. Kendler, M.R. McClung, P.D. Miller, W.P. Olszynski, E. Orwoll, and C.K. Yuen, Parathyroid Hormone and Teriparatide for the Treatment of Osteoporosis: A Review of the Evidence and Suggested Guidelines for Its Use. Endocr Rev, 2005.

Johnson, M.L., G. Gong, W. Kimberling, S.M. Recker, D.B. Kimmel, and R.B. Recker, Linkage of a gene causing high bone mass to human chromosome 11 (11q12- 13). Am J Hum Genet, 1997. 60(6): p. 1326-32.

Koller, D.L., S. Ichikawa, M.L. Johnson, D. Lai, X. Xuei, H.J. Edenberg, P.M. Conneally, S.L. Hui, C.C. Johnston, M. Peacock, T. Foroud, and M.J. Econs, Contribution of the LRP5 gene to normal variation in peak BMD in women. J Bone Miner Res, 2005. 20(1): p. 75-80.

Kristensen, P., M.E. Judge, L. Thim, U. Ribel, K.N. Christjansen, B.S. Wulff, J.T. Clausen, P.B. Jensen, O.D. Madsen, N. Vrang, P.J. Larsen, and S. Hastrup, Hypothalamic CART is a new anorectic peptide regulated by leptin. Nature, 1998. 393(6680): p. 72-6.

Little, R.D., J.P. Carulli, R.G. Del Mastro, J. Dupuis, M. Osborne, C. Folz, S.P. Manning, P.M. Swain, S.C. Zhao, B. Eustace, M.M. Lappe, L. Spitzer, S. Zweier, K. Braunschweiger, Y. Benchekroun, X. Hu, R. Adair, L. Chee, M.G. FitzGerald, C. Tulig, A. Caruso, N. Tzellas, A. Bawa, B. Franklin, S. McGuire, X. Nogues, G. Gong, K.M. Allen, A. Anisowicz, A.J. Morales, P.T. Lomedico, S.M. Recker, P. Van Eerdewegh, R.R. Recker, and M.L. Johnson, A mutation in the LDL receptor-related protein 5 gene results in the autosomal dominant high-bone-mass trait. Am J Hum Genet, 2002. 70(1): p. 11-9.

Rosen, C.J., Insulin-like growth factor I and bone mineral density: experience from animal models and human observational studies. Best Pract Res Clin Endocrinol Metab, 2004. 18(3): p. 423-35.

Seeman, E., Clinical review 137: Sexual dimorphism in skeletal size, density, and strength. J Clin Endocrinol Metab, 2001. 86(10): p. 4576-84.

Syed, F., and S. Khosla, Mechanisms of sex steroid effects on bone. Biochem Biophys Res Commun, 2005. 328(3): p. 688-96.

Takeda, S., F. Elefteriou, R. Levasseur, X. Liu, L. Zhao, K.L. Parker, D. Armstrong, P. Ducy, and G. Karsenty, Leptin regulates bone formation via the sympathetic nervous system. Cell, 2002. 111(3): p. 305-17.

Teitelbaum, S.L., Bone resorption by osteoclasts. Science, 2000. 289(5484): p. 1504-8.

Vanderschueren, D., L. Vandenput, S. Boonen, M.K. Lindberg, R. Bouillon, and C. Ohlsson, Androgens and bone. Endocr Rev, 2004. 25(3): p. 389-425.

Van Wesenbeeck, L., E. Cleiren, J. Gram, R.K. Beals, O. Benichou, D. Scopelliti, L. Key, T. Renton, C. Bartels, Y. Gong, M.L. Warman, M.C. De Vernejoul, J. Bollerslev, and W. Van Hul, Six novel missense mutations in the LDL receptor-related protein 5 (LRP5) gene in different conditions with an increased bone density. Am J Hum Genet, 2003. 72(3): p. 763-71.

Yang, W., Y. Lu, I. Kalajzic, D. Guo, M.A. Harris, J. Gluhak-Heinrich, S. Kotha, L.F. Bonewald, J.Q. Feng, D.W. Rowe, C.H. Turner, A.G. Robling, and S.E. Harris, Dentin matrix protein 1 gene cis-regulation: Use In osteocytes to characterize local responses to mechanical loading in vitro and In vivo. J Biol Chem, 2005.