Melanin-Concentrating Hormone Receptor-1 is Enriched in Lipid Rafts and the Effects of Lipid Raft Integrity on Receptor Signaling

Abstract

The melanin-concentrating hormone receptor-1 (MCHR-1) is a member of the G protein-coupled receptor (GPCR) superfamily, and functions in the regulation of food consumption and energy metabolism. MCHR-1 is expressed in neural, pancreatic, and fat tissue. Fat cells begin their journey as pre-adipocytes, culminating in the formation of mature adipocytes as differentiation occurs. Of interest to note, fat cells accumulate caveolae markers -caveolin-1 and cholesterol -during this process. Certain GPCRs and their associated downstream signaling molecules co-localize with caveolae membranes. This thesis seeks to demonstrate that MCHR-1 is enriched in lipid rafts and that a possible consequence of this may be altered signal transduction in obese individuals, such as downregulated ERK 1/2-mediated leptin transcription. Increased amounts of adipose tissue, and thus caveolae, may play a central role in the regulation of MCHR-1 signaling. The first aim of this project addressed the question of MCHR-1 localization to lipid rafts. To test this, caveolae membranes were isolated via sucrose density gradient ultracentrifugation. Subsequent fractionation and western blotting confirmed that MCHR-1 is enriched in lipid raft fractions containing caveolin-1. The second objective assessed ligand dependence on MCHR-1 localization. MCH exposure (1.0-µM) had no obvious effect on receptor localization to lipid raft fractions containing caveolin-1 over an expanded time course. The third aim examined the effect of lipid rafts on MCHR-1 signaling. Pharmacological disruption of caveolae by cholesterol depletion with methyl­ ?-cyclodextrin (M?CD) dampened MCH-mediated ERK 1/2 activation, suggesting that lipid rafts may significantly impact the regulation of MCHR-1 signaling in cells.