High concentrations of retinoic acid (RA) are associated with increased fractures and decreased cortical bone, but the role of RA in skeletal maintenance is incompletely characterized. Mice with a knockout of the gene for the transcriptional represser TGIF had enhanced responsiveness to RA and complex changes to the skeleton. The goal of this project is to determine the role of RA signaling and TGIF in the low bone mass phenotype of this mouse model. The low bone mass in Tgif null mice is characterized by increased osteoclasts and decreased osteoblasts. TGIF is a transcriptional corepressor of both TGFbeta and retinoic acid pathways; so enhanced responsiveness to either factor could cause the skeletal phenotype. TGFB signaling was first tested, since TGFS decreases osteoblast function and increases osteoclast activity, resulting in lowered bone mass. However, systemic treatment with a TGFS inhibitor did not correct the low bone mass in Tgif null mice. Tgif null osteoblasts also have an increased ratio of RANK ligand to osteoprotegerin, which could cause increased osteoclast formation and contribute to the skeletal actions of RA.
Aim 1 : Test role of RA in low bone mass in Tgif null mice. Mice will be treated with a RA inhibitor to rescue low bone mass, Tgif null osteoblasts and osteoclasts will be tested in vitro for increased responsiveness to retinoic acid.
Aim 2 : Test effects of TGIF on RA regulation of osteoclast formation. RANK ligand and osteoprotegerin regulation by TGIF and RA will be determined in vitro. Tgif null osteoclasts will be tested for cell- autonomous changes in RA-sensitivity in co-cultures of osteoblasts with osteoclast precursors. The contribution of RANK ligand to low bone mass in Tgif null mice will be determined by treating the mice with the inhibitor Opg-Fc. Lack of TGIF causes low bone mineral density, reduced osteoblast differentiation and activity, as well as increased osteoclast activity in mice. Thus, TGIF may have a role in controlling normal bone remodeling in response to retinoic acids. Understanding the mechanisms of action of RA and TGIF in bone will provide insight into the pathogenesis of diseases associated with low bone mass, such as osteoporosis, and clarify the role of retinoic acid signaling, both to endogenous and to pharmacological ligands, in skeletal homeostasis. ? ? ?
