Targeting defective NO/cGMP signaling as novel therapy for diabetic osteoporosis

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 DESCRIPTION (provided by applicant): Bone loss in diabetes has received surprisingly little attention, particularly considering that osteoporosis affects ~20% of patients with type 1 diabetes (T1DM) and poor bone quality is also a problem in type 2 diabetes. Bone loss is due to reduced osteoblastic bone formation, not increased osteoclastic resorption, thus, anti- resorptive therapies are inadequate. The molecular mechanisms of defective osteoblast function(s) are not well defined, but may include excessive production of reactive oxygen species (ROS). Nitric oxide (NO) plays an important role in osteoblasts, and clinical trials suggest that NO donors improve bone mineral density in post- menopausal women; however, conventional NO donors enhance oxidative stress. We previously showed that the NO/cGMP/protein kinase G (PKG) signaling pathway regulates osteoblast proliferation and survival. We found increased NADPH oxidase (NOX4) expression, excess ROS generation, decreased NO/cGMP production, and reduced PKG expression in bones and osteoblasts from mice with streptozotocin-induced T1DM, compared to control mice. Treatment with the cGMP-elevating agent cinaciguat, which is active even under high oxidative stress, largely restored defective proliferation and differentiation in diabetic (pre)osteo- blasts, and improved bone formation and trabecular bone volume in mice with T1DM. We hypothesize that defective NO/cGMP/PKG signaling-secondary in part to oxidative damage of pathway enzymes-contributes to bone loss in diabetes; restoring NO/cGMP signaling and/or reducing oxidative stress could be effective treatment(s) for diabetes-associated osteoporosis. The Specific Aims are to: (i) determine mechanisms and consequences of impaired NO/cGMP/PKG signaling in diabetic (pre)osteoblasts; (ii) analyze effects of cGMP- elevating agents on diabetes-induced bone loss in vivo; and (iii) examine the role of PKG and NOX4 in diabetes-induced bone loss. We will study the effects of diabetes on NO synthase and guanylate cyclase oxidation and post-translational modifications, and on PKG transcriptional regulation in osteoblasts. We will compare several approaches to prevent bone loss in mice with T1DM by treating the mice with: (i) cinaciguat; (ii) cobinamide, a vitamin B12 analog and potent anti-oxidant; (iii) the novel NO donor nitrosyl-cobinamide, which generates cobinamide on releasing NO; and (iv) combinations of cobinamide and cinaciguat. We will analyze the drugs' effects on bone formation and architecture, with micro-CT, histomorphometry, biomechanical testing, and gene expression profiling, and compare their effects on proliferation, differentiation, and survival of diabetic (pre)osteoblasts ex vivo. We will determine if mice expressing constitutively-active PKG1/2 and NOX4 knock-out mice are protected from diabetic bone loss, and examine if insulin mediates bone-protective effects via PKG, using (pre)osteoblast-specific PKG1/2 knock-out mice. These studies could be paradigm shifting, because they may define a novel mechanism for diabetic bone loss and provide a rational basis for testing cGMP-elevating drugs in diabetic osteoporosis, where they could provide a novel, anabolic treatment strategy.