Melanocortin receptor-mediated mobilization of intracellular free calcium in HEK293 cells

Abstract

Mouse melanocortin receptors, MC1-R, MC3-R, MC4-R, and MC5-R, when expressed in HEK293 cells and stimulated with either α-melanocyte-stimulating hormone (α-MSH) or desacetyl-α-MSH, mediate increases in intracellular free calcium concentration ([Ca2+]i) with EC50 values between 0.3 and 4.3 nM. The increase in [Ca2+]i is cholera toxin sensitive and pertussis toxin insensitive. The mechanism involves calcium mobilization from intracellular stores without a transient rise in inositol trisphosphate. Mouse agouti protein (55 nM) is a competitive antagonist of α-MSH (6-fold) and desacetyl-α-MSH (8-fold), coupling the mMC1-R to increased [Ca2+]i. Agouti protein (55 nM) significantly increased the EC50 for α-MSH (3-fold), and 550 nM agouti protein significantly increased the EC50 for desacetyl-α-MSH (4-fold), coupling the mMC4-R to a rise in [Ca2+]i. However, agouti protein antagonism of the MC4-R may not be competitive since there was a trend for the maximum response to also increase. There was no significant antagonism of the MC3-R and MC5-R by agouti protein (55 nM). Understanding the physiological relevance of the transduction of a calcium signal by melanocortin peptides may be important for future development of therapeutic targeting of the melanocortin receptors.