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

Abstract

Lineage allocation of the marrow mesenchymal stem cells (MSCs) to osteoblasts and adipocytes is dependent on both Wnt signaling and PPARγ2 activity. Activation of PPARγ2, 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 PPARγ2 and β-catenin, a key mediator of canonical Wnt signaling, on MSC lineage determination. Rosiglitazone-activated PPARγ2 induced rapid proteolytic degradation of β-catenin, which was prevented by either inhibiting glycogen synthase kinase 3 beta (GSK3β) activity, or blocking pro-adipocytic activity of PPARγ2 using selective antagonist GW9662 or mutation within PPARγ2 protein. Stabilization of β-catenin suppressed PPARγ2 pro-adipocytic but not anti-osteoblastic activity. Moreover, β-catenin stabilization decreased PPARγ2-mediated insulin signaling as measured by insulin receptor and FoxO1 gene expression, and protein levels of phosphorylated Akt (pAkt). Cellular knockdown of β-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 PPARγ2. Moreover, β-catenin stabilization in the presence of activated PPARγ2 did not restore Wnt10b expression indicating a dominant role of PPARγ2 in negative regulation of pro-osteoblastic activity of Wnt signaling. In conclusion, β-catenin and PPARγ2 are in cross-talk which results in sequestration of pro-adipocytic and insulin sensitizing activity. The anti-osteoblastic activity of PPARγ2 is independent of this interaction.

Sphingosine-1-phosphate inhibits differentiation of C3H10T1/2 cells into adipocyte

“…In addition, Petrie et al [32,33] reported that S1P or FTY720 improved the osseous tissue growth in vivo. Cawthorn et al [4] and Rahman et al [5] showed that the Wnt/b-catenin signaling pathway regulated not only the inhibition of adipogenesis, but also the stimulation of osteoblastogenesis in MSC. Although the involvement of Wnt in S1P-modulated MSC differentiation remains unclear, our understanding of the mechanism of S1P for MSC differentiation is important for bone regeneration.…”

“…To confirm the reduction of cAMP by S1P triggered the inhibition of adipogenic differentiation of C3H10T1/2 cells, we further examined the effect of S1P on cAMPindependent adipogenic differentiation by rosiglitazone, a directional PPARc agonist [5]. Since C/EBPb is the upstream gene of PPARc, rosiglitazone did not increase C/EBPb mRNA and protein levels, and S1P did not influence their levels, compared to adipogenic differentiation cocktail (Fig.…”

“…Thus, driving the commitment to MSC differentiation is important for regeneration and repair of tissues. It is generally believed that the induction of MSC differentiation into adipocyte leads to the inhibition of osteogenic differentiation and vice versa [4,5]. Therefore, inhibition of adipogenesis may have beneficial effects on osteogenesis.…”

Skeletal integration of energy homeostasis: Translational implications

“…Rosiglitazone forces MSCs toward the adipogenic lineage and blocks osteogenesis, in part by increasing β-catenin degradation, leading to enhanced marrow adiposity and bone loss [57–59]. This has led to the tenet that there must be exclusive commitment toward one cell type over another.…”

Reciprocal Regulation of PPARγ and RUNX2 Activities in Marrow Mesenchymal Stem Cells: Fine Balance between p38 MAPK and Protein Phosphatase 5

“…The lineage commitment can be triggered by environmental cues consisting of BMP and Wnt signaling for osteoblast and fatty acids and hormonal stimulation for adipocytes resulting in transcriptional upregulation of two key proteins: the runt-related transcription factor 2/core-binding factor-α1 (RUNX2/CBFA1; also known as AML3 and PEBP2αA) and the peroxisome proliferator-activated receptor gamma2 (PPARγ2 or NR1C3) [2]. Reciprocal regulation of expression of these two transcription factors includes the activity of Wnt signaling which upregulates RUNX2 and suppresses PPARγ2, and suppressive effect of PPARγ2 on Wnt signaling activity and RUNX2 expression [3, 4, 2]. An additional mechanism which operates at the level of activity of both proteins includes TAZ (transcriptional coactivator with PDZ-binding motif) which co-activates RUNX2-dependent gene transcription while repressing PPARγ-dependent gene transcription [5].…”