Fig. 1.

OPN3 and OPN4 mediate effects of light exposure on adipocyte metabolism. Exposure of brown adipocytes to white light, through the action of Opsin 3 (OPN3) increases expression of Cpt1, along with increased mitochondrial respiration and glucose uptake. OPN3 activation in white adipocytes by blue light (480 nm) induces expression of uncoupling protein 1 (Ucp1) and PGC-1α, indicative of conversion to a beige/BRITE phenotype. OPN3 is required for normal lipolysis in response to cold exposure through induction of cAMP and phosphorylation of hormone sensitive lipase (HSL). Similarly, OPN4 in white adipocytes is required for normal lipolysis through phosphorylation of HSL. The fatty acids released by lipolysis provide an energy substrate for BAT thermogenesis as well as directly activating UCP1