Mechanisms of Melanocortin-2 Receptor (MC2R) Internalization and Recycling in Human Embryonic Kidney (HEK) Cells: Identification of Key Ser/Thr (S/T) Amino Acids

ACTH is the most important stimulus of the adrenal cortex. The precise molecular mechanisms underlying the ACTH response are not yet clarified. The functional ACTH receptor includes melanocortin-2 receptor (MC2R) and MC2R accessory proteins (MRAP). In human embryonic kidney 293/Flp recombinase target cells expressing MC2R, MRAP1 isoforms, and MRAP2, we found that ACTH induced a concentration-dependent and arrestin-, clathrin-, and dynamin-dependent MC2R/MRAP1 internalization, followed by intracellular colocalization with Rab (Ras-like small guanosine triphosphate enzyme)4-, Rab5-, and Rab11-positive recycling endosomes. Preincubation of cells with monensin and brefeldin A revealed that 28% of the internalized receptors were recycled back to the plasma membrane and participated in total accumulation of cAMP. Moreover, certain intracellular Ser and Thr (S/T) residues of MC2R were found to play important roles not only in plasma membrane targeting and function but also in promoting receptor internalization. The S/T residues T131, S140, T204, and S280 were involved in MRAP1-independent cell-surface MC2R expression. Other mutants (S140A, S208A, and S202D) had lower cell-surface expressions in absence of MRAPβ. In addition, T143A and T147D drastically impaired cell-surface expression and function, whereas T131A, T131D, and S280D abrogated MC2R internalization. Thus, the modification of MC2R intracellular S/T residues may positively or negatively regulate its plasma membrane expression and the capacity of ACTH to induce cAMP accumulation. Mutations of T131, T143, T147, and S280 into either A or D had major repercussions on cell-surface expression, cAMP accumulation, and/or internalization parameters, pointing mostly to the second intracellular loop as being crucial for MC2R expression and functional regulation.