The physiological effects of the pituitary hormone, adrenocorticotropic hormone (ACTH) on the adrenal are mediated by the melanocortin 2 receptor (MC2R), a G protein coupled receptor (GPCR) that signals via adenylate cyclase to elevate intracellular cyclic AMP (cAMP) levels. The function and expression of the receptor is likely to be a major determinant of the response to ACTH. Following repeated stimulation, the cAMP signal is diminished or desensitized. Prolonged desensitization may involve internalization of the receptor. Internalization may occur by at least two mechanisms - receptor mediated endocytosis via clathrin-coated pits and by caveolae mediated internalization. The mode of internalization for the endogenous MC2R in Y1 cells was determined using radiolabelled ACTH. Treatment of Y1 cells with hypertonic sucrose or with concanavalin A, which inhibit clathrin-mediated endocytosis, blocked internalization. Filipin and nystatin, which inhibit caveolae formation, did not influence internalization. A dominant negative GRK2 inhibited internalization whilst the protein kinase A (PKA) consensus site mutant MC2R (S208A) internalized normally. However, dominant negative V53D β-arrestin-1 did not inhibit ACTH internalization in Y1 cells. In conclusion, it appears that the MC2R in Y1 cells internalizes by a G protein coupled receptor kinase (GRK) dependent clathrin-coated pit mechanism.
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