Osteoporosis is the most common human bone disorder. It is estimated that more than 10 million women and men in the United States have osteoporosis and 20 million more are at risk for osteoporotic fractures. Animal and human studies have shown that, when administered intermittently at low doses, both PTH and PTH-related protein (PTHrP) effectively stimulate bone formation. The bone anabolic activity of PTH and PTHrP is associated with their capacity to stimulate the type 1 PTH/PTHrP receptor (PTHI Rc), expressed in both osteoblasts and bone marrow stromal cells. PTH1 Rc is coupled to several signaling pathways, leading to activation of protein kinase A (PKA), PKC and mitogen-activated protein kinases ERK1 and ERK2. While the Gs/cAMP/PKA signaling pathway seems to be necessary, and possibly sufficient, to stimulate an anabolic response in the skeleton, recent studies have established the combined actions of PKC and beta-arrestin2 in the desensitization of PTH 1 Rc function. The central hypothesis of this research proposal is that the cellular responses to PTH and PTHrP depend not only on PTH1 Rc-mediated signal transduction, but also on additional interactions with arrestins, endocytosis and cellular trafficking of ligand/receptor beta-arrestin2 complexes. Specifically, we hypothesize that continuous and selective activation of the Gs/camp/PKA pathway will induce expression of genes, proteins and cellular responses associated with the anabolic activity on bone. The development of non-desensitizing PTHrP analogs displaying selective and sustained CAMP signaling permits the direct testing of this hypothesis. We therefore propose to: 1) identify specific interactions between PTH1 Rc and beta-arrestin2; 2) determine the role of beta-arrestin2 in mediating PTH 1Rc signal transduction and cellular trafficking in osteoblasts and stromal cells; 3) determine the effect of selective and continuous stimulation of cAMP signaling on gene/protein expression by osteoblasts and stromal cells; 4) determine the effect of selective and continuous stimulation of cAMP signaling on osteoblasts and stromal cells proliferation and apoptosis. These studies will advance the understanding of the molecular mechanisms underlying the cellular actions of PTH and PTHrP and may lead to the development of novel PTH/PTHrP-based anabolic compounds.
