- Email: [email protected]
Statins and their potential for osteoporosis
Bone Vol. 29, No. 6 December 2001:495– 497
MINI-REVIEW
Statins and Their Potential for Osteoporosis G. R. MUNDY Department of Endocrinology & Metabolism, University of Texas Health Science Center, San Antonio, TX, USA
reversed by cotreatment with L-mevalonate. These beneficial effects of statins are completely absent in eNOS-deficient mice, indicating that enhanced eNOS activity caused by statins is the mechanism responsible for the beneficial effects. Treatment of human endothelial cells with statins in the presence of L-mevalonate or geranylgeranylpyrophosphate (GGPP), but not farnesylpyrophosphate (FPP) or low-density lipoprotein, was shown to reverse the effects of statins on eNOS expression. 20 In studies examining the mechanisms linking the mevalonate pathway with eNOS expression, Rho activity in endothelial cells was regulated by the use of Clostridium botulinum C3 transferase, or by overexpression of a dominant-negative N19 RhoA mutant, both of which increased eNOS expression.19,20 In contrast, activation of Rho by Escherichia coli cytotoxic necrotizing factor-1 decreased eNOS expression. These elegant studies indicate that Rho negatively regulates eNOS expression and that HMG-CoA reductase inhibitors upregulate eNOS expression by blocking Rho geranylgeranylation, which is necessary for membraneassociated activity. The Rho guanosine triphosphate (GTP)ases are members of the Ras superfamily of low-molecular-weight GTP binding proteins.15 There are at least 14 distinct members of the family, ranging in molecular weight from 20 to 24 kDa, and these can be further subdivided into Rho, Rac, and Cdc42. These GTPases are major substrates for posttranslational modification by isoprenylation, which targets Rho GTPases to the cell membrane. Rho proteins cycle between the active GTP-bound and the inactive GDP-bound state. The key step in the activation of Rho is the attachment of geranylgeraniol, an isoprenoid intermediate of the mevalonate pathway. This serves to translocate inactive Rho from the cytosol to the cell membrane. Statins, which block geranylgeraniol synthesis, inhibit Rho membrane translocation and activity. As noted earlier, this is at least part of the mechanism by which statins affect endothelial cell function and prevent ischemic stroke.10 It may also be related to the effects of statins to increase the transcription of apolipoprotein A-I.22 Other mechanisms have been suggested for the pleiotropic effects of statins on tissues unrelated to their capacity to lower cholesterol, such as the sterol regulatory element-binding proteins (SREBPs),3 which regulate transcription of HMG-CoA reductase and other genes. These membrane-bound transcription factors are released by a proteolytic mechanism regulated by the sterol content of the cell. This topic has been reviewed extensively.3,4 Another mechanism recently described is the inhibition of leukocyte function antigen-1 by binding to a novel allosteric site, and is unrelated to inhibition of HMG-CoA reductase.33
Introduction The observation made several years ago that statins dramatically increase bone formation rates in rodents24 has provoked a much interest. Statins are natural product extracts that inhibit the enzyme, 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase, the rate-limiting step in hepatic cholesterol biosynthesis. As a consequence, they reduce serum cholesterol and the subsequent risk of heart attack.11 These drugs are among the most widely prescribed in Western countries, with more than three million Americans taking a statin every day. Because there is no currently available and acceptable oral agent that stimulates formation of substantial amounts of new bone, this raises the possibility that safe oral agents such as these may be the longsought-after anabolic agent for the treatment of patients with established osteoporosis. Most of the statins exist as prodrugs (lactones), which do not inhibit HMG-CoA reductase but are readily converted by esterases to the active form.25 Although the first statins were natural product extracts, several of the more recent ones are synthetic, and appear to be more powerful and with different pharmacokinetic properties from the original statins. The statins were selected for their capacity to target the liver and most are subject to metabolism by cytochrome P450 enzymes in the liver. Some of these hepatic metabolites are active and some are inactive. Most of the statins are very lipid-soluble and enter cells easily, but some of the newer, synthetic statins, such as pravastatin and robuvastatin, are more water-soluble and probably depend on specific carrier mechanisms in hepatic cells for entry into these cells. Pleiotropic Effects of the Statins Increasingly, effects of statins are being found that cannot be ascribed to cholesterol-lowering. These so-called pleiotropic effects, as well as their potential mechanisms, have received much recent attention.8,34 They include vasodilative, antithrombotic, antioxidant, antiinflammatory, and antiproliferative effects. One of the best described of the pleiotropic effects of statins comes from an exciting body of work regarding their role in the prevention of cerebral damage following reduced blood supply. Prophylactic treatment of mice with HMG-CoA reductase inhibitors augments cerebral blood flow, reduces cerebral infarct size, and improves neurological function in normocholesterolemic mice.10 These effects of statins are of endothelial nitric oxide synthase (eNOS), associated with upregulation of eNOS, which is not associated with changes in serum cholesterol levels, but is
Mechanisms of Action on Osteoblasts Address for correspondence and reprints: Dr. G. R. Mundy, University of Texas Health Science Center, 7703 Floyd Curl Drive, San Antonio, TX 78229. E-mail: [email protected] © 2001 by Elsevier Science Inc. All rights reserved.
The mechanism by which statins cause effects on bone cell function is a central issue. First, it appears that statins mediate 495
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their effects by increasing expression of the growth factor bone morphogenetic protein-2 (BMP-2) in bone, which in turn leads to osteoblast differentiation and bone formation. The statins were identified as bone-stimulatory agents due to their effects on the BMP-2 promoter.24 All consequences of statin action on bone in vitro can be abrogated by the additon of noggin, the naturally occuring endogenous inhibitor of BMP-2 effects, and the effects of statins on bone formation in vivo are impaired in transgenic mice in which the transgene is the truncated 1B subunit of the BMP receptor linked to the osteocalcin promoter and which are unresponsive to BMP-2.13 These data confirm the central role of BMP-2 in mediating the effects of statins on bone. Thus, statins increase transcription of the BMP-2 gene, and this is the likely distal mechanism responsible for their effects. Perhaps as a consequence of the actions of statins on bone to enhance BMP-2 expression, they can produce prolonged biological effects after brief exposure. Therefore, when used in vitro, a mere 6 h exposure to a statin leads to a prolonged effect on bone formation in bone organ cultures, which is apparent even after 14 days of continuous culture.12 These results are astounding, and suggest that an initial induction of BMP-2 triggers a cascade of factors responsible for the subsequent effects on bone formation. Similar results were found in rats studied in vivo, where statin administration to the skin for 5 days was associated with a 150% increase in bone formation rates 35 days later.14 Again, this suggests that transient exposure to the drug leads to a prolonged effect. It is important to note that the effects of statins on bone do not parallel the effects of statins on the liver. Some statins are very effective in the liver—for example, pravastatin— but have no effect at all on bone. The almost certain reason for this is the low uptake of the more water-soluble statins by bone cells and their poor distribution beyond the liver to the periphery where they can be active in bone. Most statins are extracted by the liver and are subject to first-pass metabolism, and this considerably impairs their bioavailability to nonhepatic sites. Moreover, the effects of some of the newer statins on bone, particularly cerivastatin12,35 and atorvastatin, appear to be much more powerful than the established agents such as lovastatin. Bisphosphonates and the Mevalonate Pathway Independent of our studies on the effects of statins on bone, but occurring at the same time, Rogers and colleagues21,27 showed that, the N2-containing bisphosphonates decrease osteoclast activity by effects on more distal enzymes in the cholesterol biosynthesis pathway. They proposed that this causes osteoclasts to undergo apoptosis, an effect of bisphosphonates that we described several years earlier.17 They and others29 have shown that this effect is due to inhibition of farnesylpyrophosphate synthase activity, a distal enzyme in the mevalonate pathway, although it has been questioned whether osteoclast apoptosis is important.26 These results are intriguing, particularly when considered in light of our studies on the effects of statins on bone formation. The point of interest is that bisphosphonates, drugs that inhibit osteoclastic bone resorption, and statins, compounds that enhance bone formation, both have targets in the same metabolic pathway. We examined the effects of the more potent N2containing bisphosphonates on bone formation in vitro, and found that, at high concentrations, there is an effect on bone formation.12 However, the major action of these drugs is on bone resorption. Conversely, we and others have shown that statins can inhibit osteoclast function,24,27 although our data have shown that the major effect of statins is on bone formation. Bisphosphonates are localized to osteoclasts and taken up by
Bone Vol. 29, No. 6 December 2001:495– 497
them, which may explain why their effects on the mevalonate pathway are more apparent in osteoclasts.28 An alternative explanation is that there are many side pathways branching from the main mevalonate pathway, and different components are responsible for the effects observed on osteoblasts and osteoclasts by inhibiting different enzymes in the pathway. Perhaps the most important point is that these results point to the importance of the mevalonate pathway in bone formation. The data suggest that there may be other molecular targets within this pathway that may be useful target for drug discovery. This important concept awaits further testing and investigation. Effects of Statins on Bone in Humans Many questions have been raised, however, as a consequence of animal studies that have been done. The key immediate issue is whether these findings have relevance to humans. A number of investigators have looked retrospectively at the large available databases to determine whether there is evidence supporting effects of statins on the human skeleton. Bauer and Cummings examined their large data bases at UCSF to determine whether there was any previously unrecognized association between statin use and skeletal status. In the SOF and FIT data bases they found that there was a possible relationship between statin use, bone mineral density, and subsequent fractures, and presented their findings during the oral program of the ASBMR annual scientific meeting in December 1999.2 Since that time, there have been a number of other published reports of observational studies suggesting a beneficial effect on fracture rates and bone mineral density.6,7,9,23,32 On the other hand, several abstracts were presented at ASBMR 2000 (one from the same data base as a positive published report) suggesting no effect5,18,30 and, in addition, the data of Van Staa et al. have recently been published.31 The current significance of these findings is difficult to determine. Most of these data bases consist of a majority of patients who have used lovastatin, the earliest of the statins, and probably one of the least effective on bone when given orally. The results are also diluted by the use of pravastatin, which has very little effect on bone. As a consequence, it is not clear whether these results provide meaningful information on the issue of whether statins benefit bone mass in humans. Bauer et al. performed a metaanalysis on eight observational studies, and concluded that the data were consistent with a protective effect of statins on hip and nonspine fractures.1 The recently published study by Van Staa et al.31 was accompanied by a thoughtful editorial reviewing the published clinical data,16 which concluded “the available evidence seems consistent with a protective effect of statins on risk of hip fracture, but this finding also could have been caused by an uncontrolled confounding.” The only way in which this issue can be addressed definitively is by randomized, controlled trials in patients using appropriate doses and agents. References 1. Bauer, D. C., Black, D. M., and van der Klift, M. Statin use and fracture: A meta-analysis of 8 observational studies. Bone 28(Suppl.):OR79; 2001. 2. Bauer, D. C., Mundy, G. R., Jamal, S. A., Black, D. M., Cauley, J. A., Harris, F., Duong, T., Cummings, S. R. Statin use, bone mass and fracture: An analysis of two prospective studies [abstract]. J Bone Miner Res 14(Suppl.)1188; 1999. 3. Brown, M. S. and Goldstein, J. L. The SREBP pathway: Regulation of cholesterol metabolism by cell 89:331–340; 1997. 4. Brown, M. S. and Goldstein, J. L. Sterol regulatory element binding proteins (SREBPs): Controllers of lipid synthesis and cellular uptake. Nutr Rev 56(Suppl.)S1–S3; 1998. 5. Cauley, J. A., Jackson, R., Pettinger, M., LaCroix, A., Bauer, D., Chen, Z., Daugherty, S., Hsia, J., Lewis, C. E., McGowan, J., McNeeley, S. G., and
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7. 8. 9.
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Passaro, M. Statin use and bone mineral density(BMD) in older women: The Women’s Health Initiative Observational Study (WH I-OS). J Bone Miner Res 15:(Suppl.)1068; 2000. Chan, K. A., Andrade, S. E., Boles, M., Buist, D. S., Chase, G. A., Donahue, J. G., Goodman, M. J., Gurwitz, J. H., LaCroix, A. Z., and Platt, R. Inhibitors of hydroxymethylglutaryl-coenzyme A reductase and risk of fracture among older women. Lancet 355(9222):2185; 2000. Cummings, S. R., and Bauer, D. C. Do statins prevent both cardiovascular disease and fracture? JAMA 283(24):3255–3257; 2000. Davignon, J. Methods and endpoint issues in clinical development of lipidacting agents with pleiotropic effects. Am J Cardiol 85(7):919 –920; 2000. Edwards, C. J., Hart, D. J., and Spector, T. D. Oral statins and increased bone-mineral density in postmenopausal women. Lancet 355(9222):2218 – 2219; 2000. Endres, M., Laufs, U., Huang, Z., Nakamura, T., Huang, P., Moskowitz, M. A., and Liao, J. K. Stroke protection by 3-hydroxy-3-methylglutaryl (HMG)-CoA reductase inhibitors mediated by endothelial nitric oxide synthase. Proc Natl Acad Sci USA 95(15):8880 – 8885; 1998. Farnier, M. and Davignon, J. Current and future treatment of hyperlipidemia. The role of statins. Am J Cardiol 82(4B):3J–10J; 1998. Garrett, I. R., Escobedo, A., and Mundy, G. R. N2-containing bisphosphonates risedronate and ibandronate stimulate bone formation in organ culture. J Bone Miner Res 14:(Suppl. 1):#SU373; 1999. Garrett, I. R., Esparza, J., Chen, D., Zhao, M., Gutierrez, G., Escobedo, A., Horn, D., and Mundy, G. R. Statins mediate their effects on osteoblasts by inhibition of HMG-CoA reductase and ultimately BMP2. J Bone Miner Res 15(Suppl. 1):S225; 2001. Gutierrez, G., Garrett, I. R., Rossini, G., Castano, M., Chapa, G., Escobedo, A., Esparza, J., Horn, D., Qiao, M., Taylor S., Lalka, D., and Mundy, G. R. Dermal application of lovastatin to rats causes greater increases in bone formation and plasma concentrations than when administered by oral gavage [abstract]. Bone Miner Res 15(Suppl. 1):SU397; 2001. Hall, A. Rho GTPases and the actin cytoskeleton. Science 279(5350):509 –514; 1998. Hennessy, S. and Strom, B. L. Statins and fracture risk. JAMA 285(14):1888 – 1889; 2001. Hughes, D. E., Wright, K. R., Uy, H. L., Sasaki, A., Yoneda, T., Roodman, G. D., Mundy, G. R., and Boyce, B. F. Bisphosphonates promote apoptosis in murine osteoclasts in vitro and in vivo. J Bone Miner Res 10(10):1478 –1487; 1995. LaCroix, A. Z., Cauley, J. A., Jackson, R., McGowan, J., Pettinger, M., Hsia, J., Chen, Z., Lewis, C., Bauer, D. C., Daugherty, S., McNeeley, S. G., and Passaro, M. Does statin use reduce risk of fracture in postmenopausal women? Results from the Women’s Health Initiative Observational Study. J Bone Miner Res 15(Suppl):1066; 2000. Laufs, U., Gertz, K., Huang, P., Nickenig, G., Bohm, M., Dimagl, U., and Endres, M. Atorvastatin upregulates type III nitric oxide synthase in thrombocytes, decreases platelet activation, and protects from cerebral ischemia in normocholesterolemic mice. Stroke 31(10):2442–2449; 2000. Laufs, U. and Liao, J. K Post-transcriptional regulation of endothelial nitric oxide synthase mRNA stability by Rho GTPase. J Biol Chem 273(37):24266 – 24271; 1998 Luckman, S. P., Hughes, D. E., Coxon, F. P., Graham, R., Russell, G., and Rogers, M. J. Nitrogen-containing bisphosphonates inhibit the mevalonate
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pathway and prevent post-translational prenylation of GTP-binding proteins, including rats. J Bone Miner Res 13(4):581–589; 1998. Martin, G., Duez, H., Blanquart, C., Berezowski, V., Poulain, P., Fruchart, J. C., Najib-Fruchart, J., Glineur, C., and Staels, B. Statin-induced inhibition of the Rho-signaling pathway activates PPARalpha and induces HDL apoA-I. J Clin Invest 107(11):1423–1432; 2001. Meier, C. R., Schlienger, R. G., Kraenzlin, M. E., Schlegel, B., and Jick, H. HMG-CoA reductase inhibitors and the risk of fractures. JAMA 283(24):3205; 2000. Mundy, G., Garrett, R., Harris, S., Chan, J., Chen, D., Rossini, G., Boyce, B., Zhao, M., and Gutierrez, G. Stimulation of bone formation in vitro and in rodents by statins. Science 286(5446):1946 –1949; 1999. Rao, S., Porter, D. C., Chen, X., Herliczek, T., Lowe, M., and Keyomarsi, K. Lovastatin-mediated G1 arrest is through inhibition of the proteasome, independent of hydroxymethyl glutaryl-CoA reductase. Proc Natl Acad Sci USA 96(14):7797–7802; 1999. Reszka, A. A., Halasy-Nagy, J. M., Masarachia, P. J., and Rodan, G. A. Bisphosphonates act directly on the osteoclast to induce caspase cleavage of mst1 kinase during apoptosis. A link between inhibition of the mevalonate pathway and regulation of an apoptosis-promoting kinase. J Biol Chem 274(49):34967–34973; 1999. Rogers, M. J., Watts, D. J., and Russell, R. G. Overview of bisphosphonates. Cancer 80(Suppl. 8):1652–1660; 1997. Sato, M., Grasser, W., Endo, N., Akins, R., Simmons, H., Thompson, D. D., Golub, E., and Rodan, G. A. Bisphosphonate action. Alendronate localization in rat bone and effects on osteoclast ultrastructure. J Clin Invest 88(6):2095– 2105; 1991. Van Beek, E, Pieterman, E., Cohen, L., Lowik, C., and Papapoulos, S. Farnesyl pyrophosphate synthase is the molecular target of nitrogen-containing bisphosphonates. Biochem Biophys Res Commun 264(1):108 –111; 1999. Van Staa, T. P., Wegman, S. L. J., de Vries, F., Leufkens, H. G. M., and Cooper, C. Use of statins and risk of fractures. J Bone Miner Res 2000; 15(Suppl.):1067. Van Staa, T. P., Wegman, S., de Vries, F., Leufkens, B., and Cooper, C. Use of statins and risk of fractures. JAMA 285(14):1850 –1855; 2001. Wang, P. S., Solomon, D. H., Mogun, H., and Avorn, J. HMG-CoA reductase inhibitors and the risk of hip fractures in elderly patients. JAMA 283(24):3211– 3216; 2000. Weitz-Schmidt G., Welzenbach, K., Brinkmann, V., Kamata, T., Kallen, J., Bruns, C., Cottens, S., Takada, Y., and Hommel, U. Statins selectively inhibit leukocyte function antigen-1 by binding to a novel regulatory integrin site. Nature Med 7(6):687– 692; 2001. Wheeler, D. C. Are there potential non-lipid-lowering uses of statins? Drugs 56(4):517–522; 1998. Wilkie, D., Bowman, B., Lyga, A., Bagi, C. M., Miller, S. C., Ranges, G. E., and Carley, W. Cerivastatin increases cortical bone formation in OVX rats. Am Soc Bone Miner Res 15(Suppl. 1):#M390; 2000.
Date Received: May 29, 2001 Date Revised: July 2, 2001 Date Accepted: July 3, 2001
MINI-REVIEW
Statins and Their Potential for Osteoporosis G. R. MUNDY Department of Endocrinology & Metabolism, University of Texas Health Science Center, San Antonio, TX, USA
reversed by cotreatment with L-mevalonate. These beneficial effects of statins are completely absent in eNOS-deficient mice, indicating that enhanced eNOS activity caused by statins is the mechanism responsible for the beneficial effects. Treatment of human endothelial cells with statins in the presence of L-mevalonate or geranylgeranylpyrophosphate (GGPP), but not farnesylpyrophosphate (FPP) or low-density lipoprotein, was shown to reverse the effects of statins on eNOS expression. 20 In studies examining the mechanisms linking the mevalonate pathway with eNOS expression, Rho activity in endothelial cells was regulated by the use of Clostridium botulinum C3 transferase, or by overexpression of a dominant-negative N19 RhoA mutant, both of which increased eNOS expression.19,20 In contrast, activation of Rho by Escherichia coli cytotoxic necrotizing factor-1 decreased eNOS expression. These elegant studies indicate that Rho negatively regulates eNOS expression and that HMG-CoA reductase inhibitors upregulate eNOS expression by blocking Rho geranylgeranylation, which is necessary for membraneassociated activity. The Rho guanosine triphosphate (GTP)ases are members of the Ras superfamily of low-molecular-weight GTP binding proteins.15 There are at least 14 distinct members of the family, ranging in molecular weight from 20 to 24 kDa, and these can be further subdivided into Rho, Rac, and Cdc42. These GTPases are major substrates for posttranslational modification by isoprenylation, which targets Rho GTPases to the cell membrane. Rho proteins cycle between the active GTP-bound and the inactive GDP-bound state. The key step in the activation of Rho is the attachment of geranylgeraniol, an isoprenoid intermediate of the mevalonate pathway. This serves to translocate inactive Rho from the cytosol to the cell membrane. Statins, which block geranylgeraniol synthesis, inhibit Rho membrane translocation and activity. As noted earlier, this is at least part of the mechanism by which statins affect endothelial cell function and prevent ischemic stroke.10 It may also be related to the effects of statins to increase the transcription of apolipoprotein A-I.22 Other mechanisms have been suggested for the pleiotropic effects of statins on tissues unrelated to their capacity to lower cholesterol, such as the sterol regulatory element-binding proteins (SREBPs),3 which regulate transcription of HMG-CoA reductase and other genes. These membrane-bound transcription factors are released by a proteolytic mechanism regulated by the sterol content of the cell. This topic has been reviewed extensively.3,4 Another mechanism recently described is the inhibition of leukocyte function antigen-1 by binding to a novel allosteric site, and is unrelated to inhibition of HMG-CoA reductase.33
Introduction The observation made several years ago that statins dramatically increase bone formation rates in rodents24 has provoked a much interest. Statins are natural product extracts that inhibit the enzyme, 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase, the rate-limiting step in hepatic cholesterol biosynthesis. As a consequence, they reduce serum cholesterol and the subsequent risk of heart attack.11 These drugs are among the most widely prescribed in Western countries, with more than three million Americans taking a statin every day. Because there is no currently available and acceptable oral agent that stimulates formation of substantial amounts of new bone, this raises the possibility that safe oral agents such as these may be the longsought-after anabolic agent for the treatment of patients with established osteoporosis. Most of the statins exist as prodrugs (lactones), which do not inhibit HMG-CoA reductase but are readily converted by esterases to the active form.25 Although the first statins were natural product extracts, several of the more recent ones are synthetic, and appear to be more powerful and with different pharmacokinetic properties from the original statins. The statins were selected for their capacity to target the liver and most are subject to metabolism by cytochrome P450 enzymes in the liver. Some of these hepatic metabolites are active and some are inactive. Most of the statins are very lipid-soluble and enter cells easily, but some of the newer, synthetic statins, such as pravastatin and robuvastatin, are more water-soluble and probably depend on specific carrier mechanisms in hepatic cells for entry into these cells. Pleiotropic Effects of the Statins Increasingly, effects of statins are being found that cannot be ascribed to cholesterol-lowering. These so-called pleiotropic effects, as well as their potential mechanisms, have received much recent attention.8,34 They include vasodilative, antithrombotic, antioxidant, antiinflammatory, and antiproliferative effects. One of the best described of the pleiotropic effects of statins comes from an exciting body of work regarding their role in the prevention of cerebral damage following reduced blood supply. Prophylactic treatment of mice with HMG-CoA reductase inhibitors augments cerebral blood flow, reduces cerebral infarct size, and improves neurological function in normocholesterolemic mice.10 These effects of statins are of endothelial nitric oxide synthase (eNOS), associated with upregulation of eNOS, which is not associated with changes in serum cholesterol levels, but is
Mechanisms of Action on Osteoblasts Address for correspondence and reprints: Dr. G. R. Mundy, University of Texas Health Science Center, 7703 Floyd Curl Drive, San Antonio, TX 78229. E-mail: [email protected] © 2001 by Elsevier Science Inc. All rights reserved.
The mechanism by which statins cause effects on bone cell function is a central issue. First, it appears that statins mediate 495
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G. R. Mundy et al. Statins and their potential for osteoporosis
their effects by increasing expression of the growth factor bone morphogenetic protein-2 (BMP-2) in bone, which in turn leads to osteoblast differentiation and bone formation. The statins were identified as bone-stimulatory agents due to their effects on the BMP-2 promoter.24 All consequences of statin action on bone in vitro can be abrogated by the additon of noggin, the naturally occuring endogenous inhibitor of BMP-2 effects, and the effects of statins on bone formation in vivo are impaired in transgenic mice in which the transgene is the truncated 1B subunit of the BMP receptor linked to the osteocalcin promoter and which are unresponsive to BMP-2.13 These data confirm the central role of BMP-2 in mediating the effects of statins on bone. Thus, statins increase transcription of the BMP-2 gene, and this is the likely distal mechanism responsible for their effects. Perhaps as a consequence of the actions of statins on bone to enhance BMP-2 expression, they can produce prolonged biological effects after brief exposure. Therefore, when used in vitro, a mere 6 h exposure to a statin leads to a prolonged effect on bone formation in bone organ cultures, which is apparent even after 14 days of continuous culture.12 These results are astounding, and suggest that an initial induction of BMP-2 triggers a cascade of factors responsible for the subsequent effects on bone formation. Similar results were found in rats studied in vivo, where statin administration to the skin for 5 days was associated with a 150% increase in bone formation rates 35 days later.14 Again, this suggests that transient exposure to the drug leads to a prolonged effect. It is important to note that the effects of statins on bone do not parallel the effects of statins on the liver. Some statins are very effective in the liver—for example, pravastatin— but have no effect at all on bone. The almost certain reason for this is the low uptake of the more water-soluble statins by bone cells and their poor distribution beyond the liver to the periphery where they can be active in bone. Most statins are extracted by the liver and are subject to first-pass metabolism, and this considerably impairs their bioavailability to nonhepatic sites. Moreover, the effects of some of the newer statins on bone, particularly cerivastatin12,35 and atorvastatin, appear to be much more powerful than the established agents such as lovastatin. Bisphosphonates and the Mevalonate Pathway Independent of our studies on the effects of statins on bone, but occurring at the same time, Rogers and colleagues21,27 showed that, the N2-containing bisphosphonates decrease osteoclast activity by effects on more distal enzymes in the cholesterol biosynthesis pathway. They proposed that this causes osteoclasts to undergo apoptosis, an effect of bisphosphonates that we described several years earlier.17 They and others29 have shown that this effect is due to inhibition of farnesylpyrophosphate synthase activity, a distal enzyme in the mevalonate pathway, although it has been questioned whether osteoclast apoptosis is important.26 These results are intriguing, particularly when considered in light of our studies on the effects of statins on bone formation. The point of interest is that bisphosphonates, drugs that inhibit osteoclastic bone resorption, and statins, compounds that enhance bone formation, both have targets in the same metabolic pathway. We examined the effects of the more potent N2containing bisphosphonates on bone formation in vitro, and found that, at high concentrations, there is an effect on bone formation.12 However, the major action of these drugs is on bone resorption. Conversely, we and others have shown that statins can inhibit osteoclast function,24,27 although our data have shown that the major effect of statins is on bone formation. Bisphosphonates are localized to osteoclasts and taken up by
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them, which may explain why their effects on the mevalonate pathway are more apparent in osteoclasts.28 An alternative explanation is that there are many side pathways branching from the main mevalonate pathway, and different components are responsible for the effects observed on osteoblasts and osteoclasts by inhibiting different enzymes in the pathway. Perhaps the most important point is that these results point to the importance of the mevalonate pathway in bone formation. The data suggest that there may be other molecular targets within this pathway that may be useful target for drug discovery. This important concept awaits further testing and investigation. Effects of Statins on Bone in Humans Many questions have been raised, however, as a consequence of animal studies that have been done. The key immediate issue is whether these findings have relevance to humans. A number of investigators have looked retrospectively at the large available databases to determine whether there is evidence supporting effects of statins on the human skeleton. Bauer and Cummings examined their large data bases at UCSF to determine whether there was any previously unrecognized association between statin use and skeletal status. In the SOF and FIT data bases they found that there was a possible relationship between statin use, bone mineral density, and subsequent fractures, and presented their findings during the oral program of the ASBMR annual scientific meeting in December 1999.2 Since that time, there have been a number of other published reports of observational studies suggesting a beneficial effect on fracture rates and bone mineral density.6,7,9,23,32 On the other hand, several abstracts were presented at ASBMR 2000 (one from the same data base as a positive published report) suggesting no effect5,18,30 and, in addition, the data of Van Staa et al. have recently been published.31 The current significance of these findings is difficult to determine. Most of these data bases consist of a majority of patients who have used lovastatin, the earliest of the statins, and probably one of the least effective on bone when given orally. The results are also diluted by the use of pravastatin, which has very little effect on bone. As a consequence, it is not clear whether these results provide meaningful information on the issue of whether statins benefit bone mass in humans. Bauer et al. performed a metaanalysis on eight observational studies, and concluded that the data were consistent with a protective effect of statins on hip and nonspine fractures.1 The recently published study by Van Staa et al.31 was accompanied by a thoughtful editorial reviewing the published clinical data,16 which concluded “the available evidence seems consistent with a protective effect of statins on risk of hip fracture, but this finding also could have been caused by an uncontrolled confounding.” The only way in which this issue can be addressed definitively is by randomized, controlled trials in patients using appropriate doses and agents. References 1. Bauer, D. C., Black, D. M., and van der Klift, M. Statin use and fracture: A meta-analysis of 8 observational studies. Bone 28(Suppl.):OR79; 2001. 2. Bauer, D. C., Mundy, G. R., Jamal, S. A., Black, D. M., Cauley, J. A., Harris, F., Duong, T., Cummings, S. R. Statin use, bone mass and fracture: An analysis of two prospective studies [abstract]. J Bone Miner Res 14(Suppl.)1188; 1999. 3. Brown, M. S. and Goldstein, J. L. The SREBP pathway: Regulation of cholesterol metabolism by cell 89:331–340; 1997. 4. Brown, M. S. and Goldstein, J. L. Sterol regulatory element binding proteins (SREBPs): Controllers of lipid synthesis and cellular uptake. Nutr Rev 56(Suppl.)S1–S3; 1998. 5. Cauley, J. A., Jackson, R., Pettinger, M., LaCroix, A., Bauer, D., Chen, Z., Daugherty, S., Hsia, J., Lewis, C. E., McGowan, J., McNeeley, S. G., and
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Date Received: May 29, 2001 Date Revised: July 2, 2001 Date Accepted: July 3, 2001