Emerging Roles for the Transforming Growth Factor-β Superfamily in Regulating Adiposity and Energy Expenditure

Members of the TGF-β superfamily regulate many aspects of development, including adipogenesis. Studies in cells and animal models have characterized the effects of superfamily signaling on adipocyte development, adiposity, and energy expenditure. Although bone morphogenetic protein (BMP) 4 is generally considered a protein that promotes the differentiation of white adipocytes, BMP7 has emerged as a selective regulator of brown adipogenesis. Conversely, TGF-β and activin A inhibit adipocyte development, a process augmented in TGF-β-treated cells by Smads 6 and 7, negative regulators of canonical TGF-β signaling. Other superfamily members have mixed effects on adipogenesis depending on cell culture conditions, the timing of expression, and the cell type, and many of these effects occur by altering the expression or activities of proteins that control the adipogenic cascade, including members of the CCAAT/enhancer binding protein family and peroxisome proliferator-activated receptor-γ. BMP7, growth differentiation factor (GDF) 8, and GDF3 are versatile in their mechanisms of action, and altering their normal expression characteristics has significant effects on adiposity in vivo. In addition to their roles in adipogenesis, activins and BMP7 regulate energy expenditure by affecting the expression of genes that contribute to mitochondrial biogenesis and function. GDF8 signals through its own receptors during adipogenesis while antagonizing BMP7, an example of a ligand from one major branch of the superfamily regulating the other. With such intricate relationships that ultimately affect adiposity, TGF-β superfamily signaling holds considerable promise as a target for treating human obesity and its comorbidities.