UC San Diego

Excessive bone loss due to aging is a complication of senile osteoporosis, a severe skeletal disorder causing bones to become weaker and more susceptible to fractures over time. Current therapeutics to treat osteoporosis such as bisphosphonates, hormone replacements and nitric oxide donors proved ineffective as these drugs provoke serious health complications associated with increased oxidative stress and cancerous tumor progression. Previous studies in our laboratory have established the pro-osteogenic role of Nitric oxide/cyclic-GMP/Protein kinase G (NO/cGMP/PKG) pathway in vitro in osteoblasts and treatment of mice in vivo with NO donor, nitrosyl cobinamide and the NO-independent soluble guanylate cyclase activator, cinaciguat to protect against ovariectomy and Type 1diabetes associated bone loss without side effects. One of the major mechanisms by which the NO/cGMP/PKG pathway exerts its bone anabolic function is by enhancing the Wnt/b-catenin signaling pathway. Age-related osteoporosis is caused by decreased bone remodeling due to defective osteoblast differentiation from bone marrow stromal cells (BMSCs) and increased osteoblast apoptosis. We found reduced bone volume and bone mineral density in the aged 12-month-old C57Bl6 male mice compared to them at 3 months of age. This was consistent with the gene expression studies that showed reduced osteocalcin (osteoblast marker) and increased Cathepsin K (osteoclast marker) in the tibia. Compared to 3-month-old, the tibia of 12-month-old mice expressed less PKG1, PKG1, c-Fos, Sirt1 (master regulator of oxidative stress). Osteoblast specific PKG2 transgenic mice were partly protected from age related bone loss, with increased trabecular bone volume, trabecular number and bone mineral density. The PKG2 transgenic mice showed increased osteocalcin, c-Fos and Sirt1 expression, but decreased Cathepsin K expression. Treatment of murine primary osteoblast and bone marrow stromal cells (isolated from young and aged mice) with nitrosyl cobinamide (NOCbi) or cGMP elevating Cinaciguat, enhanced both mRNA and protein expression of SIRT1 showing a direct regulation of Sirt1 by NO/cGMP/PKG signaling pathway. Our studies have highlighted Sirtuin 1 (SIRT1) as a potential target to treat aging associated bone loss. Further studies are needed to investigate the mechanism by which SIRT1 is regulated by NO/cGMP/PKG pathway, but for now, having the ability to increase SIRT1 expression expands the possibilities of treatment for osteoporosis and possibly other aging associated diseases.