Siah2 Protein Mediates Early Events in Commitment to an Adipogenic Pathway

Adipose tissue expansion occurs by increasing the size of existing adipocytes or by increasing the number of adipocytes via adipogenesis. Adipose tissue dysfunction in obesity is associated with adipocyte hypertrophy and impaired adipogenesis. We recently demonstrated that deletion of the ubiquitin ligase Siah2 is associated with enlarged adipocytes in lean or obese mice. In this study, we find that adipogenesis is impaired in 3T3-L1 preadipocytes stably transfected with Siah2 shRNA and that overexpression of Siah2 in non-precursor fibroblasts promotes adipogenesis. In the 3T3-L1 model, loss of Siah2 is associated with sustained beta-catenin expression post-induction, but depletion of beta-catenin only partially restores PPAR gamma expression and adipocyte formation. Using wild-type and Siah2(-/-) adipose tissue and adipose stromal vascular cells, we observe that Siah2 influences the expression of several factors that control adipogenesis, including Wnt pathway genes, beta-catenin, Zfp432, and Bmp-4. Consistent with increased beta-catenin levels in shSiah2 preadipocytes, Wnt10b is elevated in Siah2(-/-) adipose tissue and remains elevated in Siah2(-/-) primary stromal cells after addition of the induction mixture. However, addition of BMP-4 to Siah2(-/-) stromal cells reduces Wnt10b expression, reduces Zfp521 protein levels, and increases expression of Zfp423, a transcriptional regulator of peroxisome proliferator-activated receptor gamma expression that controls commitment to adipogenesis and is repressed by Zfp521. These results indicate that Siah2 acts upstream of BMP-4 to regulate factors that control the commitment of adipocyte progenitors to an adipogenic pathway. Our findings reveal an essential role for Siah2 in the early events that signal undifferentiated progenitor cells to become mature adipocytes.