Glucose-Regulated Protein GRP78 Controls Key Hunger Receptor MC4R Activity

The melanocortin-4 receptor (MC4R) is a critical G protein-coupled receptor (GPCR) predominantly expressed in the hypothalamus, playing a central role in regulating energy homeostasis, food intake, and body weight. Dysregulation of MC4R signaling is strongly linked to severe obesity in humans, highlighting the need to understand its intricate regulatory mechanisms. Despite its importance, the full spectrum of proteins that directly interact with and modulate MC4R function, particularly in response to metabolic cues, remains largely unknown, representing a significant knowledge gap this study addresses.

The study definitively demonstrated that GRP78 directly binds to the MC4R, influencing its cell surface expression and signaling efficacy. Co-immunoprecipitation experiments revealed a robust interaction between endogenous GRP78 and MC4R in hypothalamic cells, which was significantly enhanced under conditions of ER stress. > Overexpression of GRP78 in HEK293 cells led to a significant reduction in MC4R-mediated cAMP production by approximately 43% (p<0.01) compared to control cells, indicating attenuated receptor activity. Conversely, genetic knockdown of GRP78 using siRNA resulted in a 2.5-fold increase (p<0.005) in MC4R cell surface expression and a 68% enhancement (p<0.001) in downstream cAMP signaling in response to the MC4R agonist α-MSH. These findings suggest that GRP78 acts as a negative regulator of MC4R function, potentially by retaining the receptor in the ER or promoting its degradation, thereby limiting its availability at the cell surface.