Thyroid Hormone Receptor Sumoylation Is Required for Preadipocyte Differentiation and Proliferation

Background: Thyroid hormone receptor (TR) sumoylation is essential for thyroid hormone regulation of gene expression. Results: TR sumoylation mutants impair differentiation though down-regulation of C/EBPs, constitutive interaction with NCoR, interference with PPAR signaling, and disruption of the Wnt canonical signaling pathway important for preadipocyte proliferation. Conclusion: TR sumoylation site mutations impair preadipocyte proliferation and differentiation. Significance: TR sumoylation is required for adipogenesis. Thyroid hormone and thyroid hormone receptor (TR) play an essential role in metabolic regulation. However, the role of TR in adipogenesis has not been established. We reported previously that TR sumoylation is essential for TR-mediated gene regulation and that mutation of either of the two sites in TR or any of the three sites in TR reduces TR sumoylation. Here, we transfected TR sumoylation site mutants into human primary preadiocytes and the mouse 3T3L1 preadipocyte cell line to determine the role of TR sumoylation in adipogenesis. Reduced sumoylation of TR or TR resulted in fewer and smaller lipid droplets and reduced proliferation of preadipocytes. TR sumoylation mutations, compared with wild-type TR, results in reduced C/EBP expression and reduced PPAR(2) mRNA and protein levels. TR sumoylation mutants recruited NCoR and disrupted PPAR-mediated perilipin1 (Plin1) gene expression, associated with impaired lipid droplet formation. Expression of NCoRID, a mutant NCoR lacking the TR interaction domain, partially rescued the delayed adipogenesis and restored Plin1 gene expression and adipogenesis. TR sumoylation site mutants impaired Wnt/-catenin signaling pathways and the proliferation of primary human preadipocytes. Expression of the TR K146Q sumoylation site mutant down-regulated the essential genes required for canonical Wnt signal-mediated proliferation, including Wnt ligands, Fzds, -catenin, LEF1, and CCND1. Additionally, the TR K146Q mutant enhanced the canonical Wnt signaling inhibitor Dickkopf-related protein 1 (DKK1). Our data demonstrate that TR sumoylation is required for activation of the Wnt canonical signaling pathway during preadipocyte proliferation and enhances the PPAR signaling that promotes differentiation.