Future Anti-Catabolic Therapeutic Targets in Bone Disease

DDIT3 deficiency accelerates bone remodeling during bone healing by enhancing osteoblast and osteoclast differentiation through ULK1-mediated autophagy

The coordination of osteoblasts and osteoclasts is essential for bone remodeling. DNA damage inducible script 3 (DDIT3) is an important regulator of bone and participates in cell differentiation, proliferation, autophagy, and apoptosis. However, its role in bone remodeling remains unexplored. Here, we found that Ddit3 knockout (Ddit3-KO) enhanced both bone formation and resorption. The increased new bone formation and woven bone resorption, i.e., enhanced bone remodeling capacity, was found to accelerate bone defect healing in Ddit3-KO mice. In vitro experiments showed that DDIT3 inhibited both osteoblast differentiation and Raw264.7 cell differentiation by regulating autophagy. Cell coculture assay showed that Ddit3-KO decreased the ratio of receptor activator of nuclear factor-κβ ligand (RANKL) to osteoprotegerin (OPG) in osteoblasts, and Ddit3-KO osteoblasts inhibited osteoclast differentiation. Meanwhile, DDIT3 knockdown (DDIT3-sh) increased receptor activator of nuclear factor-κβ (RANK) expression in Raw264.7 cells, and DDIT3-sh Raw264.7 cells promoted osteoblast differentiation, whereas, DDIT3 overexpression had the opposite effect. Mechanistically, DDIT3 promoted autophagy partly by increasing ULK1 phosphorylation at serine555 (pULK1-S555) and decreasing ULK1 phosphorylation at serine757 (pULK1-S757) in osteoblasts, thereby inhibiting osteoblast differentiation. DDIT3 inhibited autophagy partly by decreasing pULK1-S555 in Raw264.7 cells, thereby suppressing osteoclastic differentiation. Taken together, our data indicate that DDIT3 is one of the elements regulating bone remodeling and bone healing, which may become a potential target in bone defect treatment.

Anti-serpin antibody-mediated regulation of proteases in autoimmune diabetes

Secretion of anti-serpin B13 autoantibodies in young diabetes-prone nonobese diabetic mice is associated with reduced inflammation in pancreatic islets and a slower progression to autoimmune diabetes. Injection of these mice with a monoclonal antibody (mAb) against serpin B13 also leads to fewer inflammatory cells in the islets and more rapid recovery from recent-onset diabetes. The exact mechanism by which anti-serpin activity is protective remains unclear. We found that serpin B13 is expressed in the exocrine component of the mouse pancreas, including the ductal cells. We also found that anti-serpin B13 mAb blocked the inhibitory activity of serpin B13, thereby allowing partial preservation of the function of its target protease. Consistent with the hypothesis that anti-clade B serpin activity blocks the serpin from binding, exposure to exogenous anti-serpin B13 mAb or endogenous anti-serpin B13 autoantibodies resulted in cleavage of the surface molecules CD4 and CD19 in lymphocytes that accumulated in the pancreatic islets and pancreatic lymph nodes but not in the inguinal lymph nodes. This cleavage was inhibited by an E64 protease inhibitor. Consequently, T cells with the truncated form of CD4 secreted reduced levels of interferon-γ. We conclude that anti-serpin antibodies prevent serpin B13 from neutralizing proteases, thereby impairing leukocyte function and reducing the severity of autoimmune inflammation.

Balicatib, a cathepsin K inhibitor, stimulates periosteal bone formation in monkeys

Balicatib, an inhibitor of the osteoclastic enzyme cathepsin K, was tested in ovariectomized monkeys, a model for osteoporosis. As expected, ovariectomy-induced bone mass changes were partially prevented by balicatib treatment. Bone turnover was significantly decreased at most sites, but unlike most bone resorption inhibitors, periosteal bone formation rates were increased.

INTRODUCTION

Selective inhibitors of the osteoclastic enzyme cathepsin K have potential in osteoporosis treatment. This study evaluated the efficacy of balicatib (AAE581), a novel inhibitor of human cathepsin K, on bone mass and dynamic histomorphometric endpoints in ovariectomized monkeys.

METHODS

Eighty adult female Macaca fascicularis underwent bilateral ovariectomies and were dosed twice daily by oral gavage with balicatib at 0, 3, 10, and 50 mg/kg for 18 months (groups O, L, M, H, respectively). Approximately 1 month after treatment initiation, the 50 mg/kg dose was decreased to 30 mg/kg. Twenty animals underwent sham-ovariectomies (group S). Bone mass was measured at 3-6 month intervals. At 18 months, vertebra and femur were collected for histomorphometry.

RESULTS

In both spine and femur, group O animals lost BMD and all other groups gained BMD between 0 and 18 months. In balicatib-treated animals, BMD change in the spine was intermediate between group S and O, with groups L and M significantly different from group O. In femur, all three doses of balicatib significantly increased BMD gain relative to group O and group mean values were also higher than group S. Most histomorphometric indices of bone turnover in vertebra and femoral neck were significantly lower than group O with balicatib treatment, except that periosteal bone formation rates (Ps.BFR) were significantly higher. Ps.BFR in mid-femur was also significantly increased by treatment.

CONCLUSIONS

Balicatib partially prevented ovariectomy-induced changes in bone mass, inhibited bone turnover at most sites, and had an unexpected stimulatory effect on periosteal bone formation.

Balicatib, a cathepsin K inhibitor, stimulates periosteal bone formation in monkeys

Balicatib, an inhibitor of the osteoclastic enzyme cathepsin K, was tested in ovariectomized monkeys, a model for osteoporosis. As expected, ovariectomy-induced bone mass changes were partially prevented by balicatib treatment. Bone turnover was significantly decreased at most sites, but unlike most bone resorption inhibitors, periosteal bone formation rates were increased.

INTRODUCTION

Selective inhibitors of the osteoclastic enzyme cathepsin K have potential in osteoporosis treatment. This study evaluated the efficacy of balicatib (AAE581), a novel inhibitor of human cathepsin K, on bone mass and dynamic histomorphometric endpoints in ovariectomized monkeys.

METHODS

Eighty adult female Macaca fascicularis underwent bilateral ovariectomies and were dosed twice daily by oral gavage with balicatib at 0, 3, 10, and 50 mg/kg for 18 months (groups O, L, M, H, respectively). Approximately 1 month after treatment initiation, the 50 mg/kg dose was decreased to 30 mg/kg. Twenty animals underwent sham-ovariectomies (group S). Bone mass was measured at 3-6 month intervals. At 18 months, vertebra and femur were collected for histomorphometry.

RESULTS

In both spine and femur, group O animals lost bone mineral density (BMD), and all other groups gained BMD between 0 and 18 months. In balicatib-treated animals, BMD change in the spine was intermediate between group S and O, with groups L and M significantly different from group O. In femur, all three doses of balicatib significantly increased BMD gain relative to group O, and group mean values were also higher than group S. Most histomorphometric indices of bone turnover in vertebra and femoral neck were significantly lower than group O with balicatib treatment, except that periosteal bone formation rates (Ps.BFR) were significantly higher. Ps.BFR in mid-femur was also significantly increased by treatment.

CONCLUSIONS

Balicatib partially prevented ovariectomy-induced changes in bone mass, inhibited bone turnover at most sites, and had an unexpected stimulatory effect on periosteal bone formation.

New treatment targets in osteoporosis

Postmenopausal osteoporosis is characterized by bone remodeling alterations with an imbalance between excessive bone resorption and inadequate bone formation. At present, osteoporosis treatment rests on bone resorption inhibitors and, more specifically, on bisphosphonates. However, the introduction of anabolic agents such as parathyroid hormone that stimulate bone formation has expanded the range of treatment options. New treatment targets have been identified via improved knowledge on bone pathophysiology, bone remodeling, bone cells and intracellular signaling pathways. RANKL inhibition by anti-RANKL antibodies is undergoing considerable development as a treatment for osteoporosis. Also under development are anti-catabolic drugs that target the molecular mechanisms involved in bone resorption, including cathepsin K inhibitors and integrin alpha(v)beta(3) antagonists. The identification of new pathways involved in bone formation is directing clinical research efforts toward the development of anabolic agents. The signaling pathways involved in bone formation, most notably the Wnt-pathway, hold considerable promise as treatment targets in conditions characterized by insufficient bone formation. Current focuses of interest include antibodies against naturally occurring Wnt-pathway antagonists (e.g., sclerostin and Dkk1) and modulators of parathyroid hormone production (calcilytic agents). Thus, active research is ongoing to improve the treatment of osteoporosis, a disease whose high prevalence and considerable functional and socioeconomic impact will raise formidable challenges in the near future.

Rapamycin and the transcription factor C/EBPbeta as a switch in osteoclast differentiation: implications for lytic bone diseases

Lytic bone diseases and in particular osteoporosis are common age-related diseases characterized by enhanced bone fragility due to loss of bone density. Increasingly, osteoporosis poses a major global health-care problem due to the growth of the elderly population. Recently, it was found that the gene regulatory transcription factor CCAAT/enhancer binding protein beta (C/EBPbeta) is involved in bone metabolism. C/EBPbeta occurs as different protein isoforms of variable amino terminal length, and regulation of the C/EBPbeta isoform ratio balance was found to represent an important factor in osteoclast differentiation and bone homeostasis. Interestingly, adjustment of the C/EBPbeta isoform ratio by the process of translational control is downstream of the mammalian target of rapamycin kinase (mTOR), a sensor of the nutritional status and a target of immunosuppressive and anticancer drugs. The findings imply that modulating the process of translational control of C/EBPbeta isoform expression could represent a novel therapeutic approach in osteolytic bone diseases, including cancer and infection-induced bone loss.

Transcription factor C/EBPbeta isoform ratio regulates osteoclastogenesis through MafB

Disequilibrium between bone-forming osteoblasts and bone-resorbing osteoclasts is central to many bone diseases. Here, we show that dysregulated expression of translationally controlled isoforms of CCAAT/enhancer-binding protein β (C/EBPβ) differentially affect bone mass. Alternative translation initiation that is controlled by the mammalian target of rapamycin (mTOR) pathway generates long transactivating (LAP^*, LAP) and a short repressive (LIP) isoforms from a single C/EBPβ transcript. Rapamycin, an inhibitor of mTOR signalling increases the ratio of LAP over LIP and inhibits osteoclastogenesis in wild type (WT) but not in C/EBPβ null (c/ebpβ^−/−) or in LIP knock-in (L/L) osteoclast precursors. C/EBPβ mutant mouse strains exhibit increased bone resorption and attenuated expression of MafB, a negative regulator of osteoclastogenesis. Ectopic expression of LAP and LIP in monocytes differentially affect the MafB promoter activity, MafB gene expression and dramatically affect osteoclastogenesis. These data show that mTOR regulates osteoclast formation by modulating the C/EBPβ isoform ratio, which in turn affects osteoclastogenesis by regulating MafB expression.

Cathepsin K inhibitors for osteoporosis and potential off-target effects

Cathepsin K is a highly potent collagenase and the predominant papain-like cysteine protease expressed in osteoclasts. Cathepsin K deficiencies in humans and mice have underlined the central role of this protease in bone resorption and, thus, have rendered the enzyme as an attractive target for anti-resorptive osteoporosis therapy. In the past decade, a lot of efforts have been made in developing highly potent, selective and orally applicable cathepsin K inhibitors. Some of these inhibitors have passed preclinical studies and are presently in clinical trials at different stages of advancement. The development of the inhibitors and preliminary results of the clinical trials revealed problems and lessons concerning the in situ specificity of the compounds and their tissue targeting. In this review, we briefly summarize the history of cathepsin K research and discuss the current development of cathepsin K inhibitors as novel anti-resorptives for the treatment of osteoporosis. We also discuss potential off-target effects of cathepsin K inhibition and alternative applications of cathepsin K inhibitors in arthritis, atherosclerosis, blood pressure regulation, obesity and cancer.

Natural product inhibitors of protein-protein interactions mediated by Src-family SH2 domains

In this Letter, we report the natural products salvianolic acid A, salvianolic acid B, and caftaric acid as inhibitors of the protein-protein interactions mediated by the SH2 domains of the Src-family kinases Src and Lck, two established disease targets. Moreover, we propose a binding mode for the inhibitors based on molecular modeling, which will facilitate chemical optimization efforts of these important lead structures for drug discovery.