Project Summary Osteotoxic side effects of glucocorticoids, such as osteoporosis and osteonecrosis, limit clinical use. Children are a particularly vulnerable population as 30-50% of children on chronic glucocorticoids develop an osteotoxic side effect and there are no-FDA approved treatments for children. Although there have been extensive studies about glucocorticoid effects on the bone in the mature skeleton focusing largely on osteoclasts and osteoblasts, knowledge gaps still exist which in the growing skeleton. Bone, particularly growing bone, is highly vascularized. A unifying histological feature of osteoporosis, osteonecrosis, and impaired skeletal growth, is reduced bone vasculature. Glucocorticoids impair skeletal angiogenesis, which correlates with skeletal fragility in animal models. The purpose of this study is to elucidate the key affected cell and intracellular signaling mechanism involved in glucocorticoid suppression of angiogenesis and its relation to osteotoxic side effects. We have established a young glucocorticoid-osteotoxic mouse model. We have found that glucocorticoids drastically reduce type H vessels, a specific subtype of blood vessels associated with osteogenesis. Building on our prior work demonstrating that Trap+ preosteoclasts secrete platelet-derived growth factor type BB (PDGF-BB), which recruits endothelial precursor cells (EPCs) to form type H blood vessels, we found that glucocorticoids suppress Pdgfb transcription by interfering with binding of nuclear factor kappa beta (NF-κB) to the Pdgfb promoter. Decreased PDGF-BB was associated with a decreased number of type H vessels, number of mature osteoblasts, and decreased bone volume. We hypothesize that glucocorticoid suppression of preosteoclast PDGF-BB via inactivation of NF-κB is the cause of glucocorticoid impairment of skeletal angiogenesis. In this proposal, we will dissect the key cellular mechanism involved in glucocorticoid suppression of angiogenesis. Specifically, we will 1) Demonstrate preosteoclasts are the major cell type in glucocorticoid-suppression of skeletal angiogenesis. 2) Determine the mechanism of glucocorticoid-suppression of NF-κB-mediated preosteoclast Pdgfb transcription. 3) Examine efficacy of increasing preosteoclast PDGF-BB for preventing glucocorticoid-suppression of skeletal angiogenesis. Determination of bone-specific factors that regulate angiogenesis, which is critical to the growing skeleton, will allow targeted drug therapy to treat and/or prevent osteotoxic side effects of glucocorticoids in children. Furthermore, identification of the mechanism of glucocorticoid-suppression of angiogenesis will advance our fundamental knowledge and expand future studies specifically on osteonecrosis, aid in drug-development for therapeutic interventions, and provide insight into mechanisms of other off-target tissue side effects observed with chronic glucocorticoid usage.