Role of Wnt signaling and glycogen synthase kinase-3 beta in adipocyte biology

Adipocyte differentiation from fibroblasts into fat cells (adipogenesis) is regulated by two families of transcription factors: CCAAT/enhancer binding proteins (C/EBPs), members of the basic leucine zipper family, and peroxisome proliferator-activated receptors (PPARs), which belong to the nuclear hormone receptor family. Adipocytes contribute to energy homeostasis with their ability to undergo lipogenesis and lipolysis, insulin-stimulated glucose transport, and secretion of hormones such as leptin and adiponectin. Alterations in adipocyte function play an important role in pathophysiological states, including diabetes and cardiovascular disease. Drosophila wingless (wg) and mouse mammary tumor virus integration-1 (MMTV int-1) are lipid-modified ligands. Wg and MMTV int-1 (together "wnt") bind frizzled receptors, which control polarity in the Drosophila adult cuticle and activate a signaling pathway involving the inhibition of glycogen synthase kinase-3beta (GSK-3beta), originally identified as regulating glycogen synthase activity. Inhibition of GSK-3beta increases the nuclear abundance of beta-catenin, which is then able to interact with the lymphoid enhancer factor/T cell factor (LEF/Tcf) family of transcription factors, increasing their transcriptional activity. Although wnt/beta-catenin signaling is involved in the differentiation of several cell types, a role for writ or beta-catenin signaling had not been established in adipocytes. This research demonstrated that GSK-3beta activity was required during the early stages of adipogenesis and that the expression of beta-catenin decreased during differentiation. It was also observed that ectopic expression of wnt-1 prevented adipogenesis. Furthermore, the ability of wnt signaling to inhibit adipogenesis was at least partially dependent on wnt's ability to suppress the expression and activity of PPARgamma. Conversely, factors that regulated the induction of PPARgamma during adipogenesis negatively influenced beta-catenin expression. Lastly, it was observed that GSK-3beta activity was required to maintain PPARgamma expression in mature adipocytes. Further examination revealed that chronic inhibition of GSK-3beta activity diminished insulin-sensitive glucose uptake. Western blot analysis demonstrated a defect in insulin signaling due to decreased insulin receptora expression. Furthermore, it was observed that decreased perilipin expression correlated with a diminished ability of these cells to undergo hormone-sensitive lipolysis. Taken together, these data demonstrated an important role for writ and GSK-3beta signaling in regulating PPARgamma expression and adipocyte differentiation and physiology.