Metabolic responses of light and taste receptors - unexpected actions of GPCRs in adipocytes.

The G-protein-coupled receptor (GPCR) superfamily includes sensory receptors that can detect and respond to taste and light. Recent investigations have identified key metabolic roles for such receptors in tissues considered 'non-sensory' such as adipose tissue. The major functions of white and brown adipose tissues include energy storage/release and thermogenesis, respectively. These processes are tightly controlled by GPCR pathways that serve to maintain energy homeostasis. Opsins 3 and 4 are GPCRs activated by blue light and in adipocytes control lipolysis as well as affect brown adipocyte activity. Furthermore, Opsin 3 signals to regulate the conversion of white to thermogenic beige/BRITE (Brown-in-white) adipocytes. Taste receptors that respond to fatty acids, sweet and bitter are expressed in adipocytes as well as in taste buds. Ffar2 and the long chain fatty acid receptor GPR120 are highly expressed in white adipocytes and the human tongue. In adipose tissue Ffar2 mediates the metabolic effects of butyrate and propionate produced by the gut microbiome. GPR120 is highly expressed in brown adipose tissue and regulates fatty acid oxidation and mitochondrial function. The type I taste receptor Tas1r3 senses sweet and umami, is expressed in adipocytes and on obesogenic diets Tas1r3 global gene knockout protects from metabolic dysfunction. Type II taste receptors that sense bitter are expressed by adipocytes and bitter agonists have been found to modulate adipocyte differentiation and lipid storage levels. This review explores recent unexpected findings of light and taste receptors in adipocytes and examines effects of their signaling in the control of adipose tissue biology.