β-Catenin Directly Sequesters Adipocytic and Insulin Sensitizing Activities but Not Osteoblastic Activity of PPARγ2 in Marrow Mesenchymal Stem Cells

Lineage allocation of the marrow mesenchymal stem cells (MSCs) to osteoblasts and adipocytes is dependent on both Wnt signaling and PPAR gamma 2 activity. Activation of PPAR gamma 2, an essential regulator of energy metabolism and insulin sensitivity, stimulates adipocyte and suppresses osteoblast differentiation and bone formation, and correlates with decreased bone mass and increased fracture rate. In contrast, activation of Wnt signaling promotes osteoblast differentiation, augments bone accrual and reduces total body fat. This study examined the cross-talk between PPAR gamma 2 and beta-catenin, a key mediator of canonical Wnt signaling, on MSC lineage determination. Rosiglitazone-activated PPAR gamma 2 induced rapid proteolytic degradation of beta-catenin, which was prevented by either inhibiting glycogen synthase kinase 3 beta (GSK3 beta) activity, or blocking pro-adipocytic activity of PPAR gamma 2 using selective antagonist GW9662 or mutation within PPAR gamma 2 protein. Stabilization of beta-catenin suppressed PPAR gamma 2 pro-adipocytic but not anti-osteoblastic activity. Moreover, beta-catenin stabilization decreased PPAR gamma 2-mediated insulin signaling as measured by insulin receptor and FoxO1 gene expression, and protein levels of phosphorylated Akt (pAkt). Cellular knockdown of beta-catenin with siRNA increased expression of adipocyte but did not affect osteoblast gene markers. Interestingly, the expression of Wnt10b was suppressed by anti-osteoblastic, but not by pro-adipocytic activity of PPAR gamma 2. Moreover, beta-catenin stabilization in the presence of activated PPAR gamma 2 did not restore Wnt10b expression indicating a dominant role of PPAR gamma 2 in negative regulation of pro-osteoblastic activity of Wnt signaling. In conclusion, beta-catenin and PPAR gamma 2 are in cross-talk which results in sequestration of pro-adipocytic and insulin sensitizing activity. The anti-osteoblastic activity of PPAR gamma 2 is independent of this interaction.