Corticotropin releasing hormone (CRH) and its related peptides, urocortins (UCN), are now emerging as critical regulators of energy balance and homeostasis. These peptides exert their effects through activation of two types of CRH receptors (CRH-R1 and CRH-R2). The role of these peptides as regulators of mammalian thermogenesis and their effects on brown adipose tissue (BAT) have been primarily attributed to modulation of brain centres controlling sympathetic outflow. However, direct effects of CRH and UCNs on brown adipocytes have not been investigated. In this study we have demonstrated that mRNA of both types of CRH-Rs is expressed in mouse BAT and in a brown adipocyte cell line (T37i). Immunofluorescence confocal microscopy demonstrated that CRH-Rs are not uniformly distributed over the cell membrane, but are also present in cytoplasmic compartments and around the nucleus, potentially indicating newly synthesised unprocessed receptor, internalised mature CRH-R or cytoplasmic forms of specific CRH-R variants. Western blot analysis identified multiple immunoreactive proteins indicative of receptor heterogeneity and potentially differential post-translational processing. Exposure of T37i adipocytes to CRH led to significant increase in cAMP levels and glycerol release in a dose-dependent manner, via a PKA-dependent mechanism. Maximum CRH effects were observed at 1 nM, whereas CRH concentrations >10 nM has smaller or insignificant effect on cAMP and glycerol release. Further studies showed that CRH induces phosphorylation of hormone sensitive lipase (HSL) at Ser-563 and its subsequent translocation toward lipid droplets. This was associated with redistribution of perillipin on the lipid droplets. In conclusion, this is the first report to show a direct effect of CRH on brown adipocyte lipolysis. This signalling pathway appears to be inactive at high CRH concentrations, raising the possibility that excess CRH availability might modulate distinct biological effects of BAT.
03 - 07 May 2008
European Society of Endocrinology