Tributyltin Engages Multiple Nuclear Receptor Pathways and Suppresses Osteogenesis in Bone Marrow Multipotent Stromal Cells

Organotins are members of the environmental obesogen class of contaminants because they activate peroxisome proliferator-activated receptor gamma (PPAR gamma), the essential regulator of adipogenesis. Exposure to thiazolidinediones (PPAR gamma ligands used to treat type 2 diabetes) is associated with increased fractures. Diminished bone quality likely results from PPAR gamma's role in promoting adipogenesis while suppressing osteogenesis of bone marrow multipotent mesenchymal stromal cells (BM-MSC). We hypothesized that tributyltin (TBT) would be a potent modifier of BM-MSC differentiation and a negative regulator of bone formation. Organotins interact with both PPAR gamma and retinoid X receptors (RXR), suggesting that they activate multiple nuclear receptor pathways. To investigate the role of RXR in the actions of TBT, the effects of PPAR gamma (rosiglitazone) and RXR (bexarotene, LG100268) agonists were compared to the effects of TBT in BMS2 cells and primary mouse BM-MSC cultures. In BMS2 cells, TBT induced the expression of Fabp4, Abca1, and Tgm2 in an RXR-dependent manner. All agonists suppressed osteogenesis in primary mouse BM-MSC cultures, based on decreased alkaline phosphatase activity, mineralization, and expression of osteoblast-related genes. While rosiglitazone and TBT strongly activated adipogenesis, based on lipid accumulation and expression of adipocyte-related genes, the RXR agonists did not. Extending these analyses to other RXR heterodimers showed that TBT and the RXR agonists activated the liver X receptor pathway, whereas rosiglitazone did not. Application of either a PPAR gamma antagonist (T0070907) or an RXR antagonist (HX531) significantly reduced rosiglitazone-induced suppression of bone nodule formation. Only the RXR antagonist significantly reduced LG100268- and TBT-induced bone suppression. The RXR antagonist also inhibited LG100268- and TBT-induced expression of Abca1, an LXR target gene, in primary BM-MSC cultures. These results provide novel evidence that TBT activates multiple nuclear receptor pathways in BM-MSCs, activation of RXR is sufficient to suppress osteogenesis, and TBT suppresses osteogenesis largely through its direct interaction with RXR.