PTH Signaling through LRP6

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 DESCRIPTION (provided by applicant): Osteoporosis, the leading cause of fractures in the United States, is commonly associated with the population who have developed hyperlipidemia and associated atherosclerosis. It is believed that the same bioactive oxidized lipids (oxlipids) and low-density lipoprotein (oxLDL) that promote atherosclerosis also adversely affect bone. However, how these bioactive lipids act on bone cells remain largely unknown. Recent human genetic studies demonstrated that LRP6 mutations are associated with premature osteoporosis, hyperlipidemia, and early onset coronary artery disease. In our preliminary study, we have found that LRP6 on plasma membrane of MSCs directly binds oxlipids and oxLDL followed by LRP6 endocytosis. MSCs treated with oxlipids had impaired differentiation toward osteoblasts and reduced responses to PTH, BMPs and Wnts, indicating that LRP6 may mediate the action of bioactive lipids on bone. We hypothesize that increased oxidized lipids in bone microenvironment binds LRP6 on osteoblastic lineage of cells and induces LRP6 endocytosis, leading to the unavailability of LRP6 to transduce the signals of PTH, Wnt, and BMP for their osteogenic function. The hypothesis will be tested by the following Specific Aims. Aim 1 will examine the endocytosis of LRP6 in osteoblast lineage cells in response to bioactive oxidized lipids. We will examine whether cell surface LRP6 expression in bone marrow MSCs is reduced in response to increased bioactive lipids in wild type mice and ldlr-/- mice fed HFD vs CHD. We will examine LRP6 endocytosis stimulated by oxLDL and individual oxlipids in cultured bone marrow MSCs and calvarial preosteoblasts by monitoring LRP6 subcellular localization and cell surface LRP6 expression. Aim 2 will examine the specificity of oxlipid-LRP6 binding and the molecular pathway mediating oxidized lipid-induced LRP6 endocytosis in osteoblast lineage cells. We will define if LRP6 directly binds oxlipids using the established protein lipid overlay assay and 125I- oxLDL-cell binding assay. The key pathways mediating oxlipids-induced LRP6 endocytosis in MSCs will also be examined. Aim 3 will determine if inhibition of LRP6 endocytosis increases the osteogenic function of MSCs and restores the PTH action on bone during hyperlipidemia. We will evaluate whether blocking the oxlipid-LRP6 binding or inhibiting LRP6 endocytosis restores the impaired osteogenic function of bone marrow MSCs and the anabolic action of PTH on bone in hyperlipidemia mice. The study will provide new understanding of the mechanisms by which osteoporosis and hyperlipidemia/ atherosclerosis co-develop and yield new strategies to simultaneously treat both disorders.