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

Sima Rahman,Yalin Lu,Piotr J Czernik,Beata Lecka-Czernik,Xing-Ming Shi

doi:10.1371/journal.pone.0051746

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.

Highlights

Regulation of marrow mesenchymal stem cells (MSCs) fate toward adipocyte or osteoblast lineage involves multiple mechanisms including modulation of lineage-specific transcription factors [1]
A detailed analysis of bcatenin gene expression as a function of time following Rosi treatment showed that a decrease in the level of b-catenin transcript occurred relatively late, when cells acquired phenotype of fully differentiated adipocytes marked by significant accumulation of fat droplets and expression of lipid metabolism gene markers (Table 1) [14]
Since PPARc activation with anti-diabetic Rosi increases insulin signaling in adipocytes, we examined the effect of b-catenin stabilization on the expression of gene markers of this pathway

Summary

Introduction

Regulation of marrow MSC fate toward adipocyte or osteoblast lineage involves multiple mechanisms including modulation of lineage-specific transcription factors [1]. Such modulation may comprise of direct interactions between transcription factors and their co-modulators, which is often coordinated by changes in the activity of signaling pathways. The example of such interaction includes regulation of Wnt signaling and PPARc2 activity. PPARc1 is ubiquitously expressed, whereas PPARc2 expression is restricted to adipocytes, including marrow adipocytes [2,4] Both isoforms have overlapping transcriptional activities, PPARc2 seems to be more specific for lipids and carbohydrates metabolism. The studies of the PPARc role in marrow MSCs differentiation suggest PPARc2 function in commitment to adipocyte lineage, while PPARc1 in control of osteoblast differentiation and production of mineralized matrix [4,8,9]

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Results

Conclusion