Four Missense Mutations in the Ghrelin Receptor Result in Distinct Pharmacological Abnormalities

Abstract

The growth hormone secretagogue receptor (GHSR) plays an important role in regulating food intake and energy homeostasis. In this study, we compared the pharmacological properties of four reported variants of the human GHSR (I134T, V160M, A204E, and F279L) with those of the wild-type receptor. Corresponding recombinant receptors were transiently expressed in either human embryonic kidney 293 or COS-7 cells. Basal as well as ligand-induced signaling was assessed by luciferase reporter gene assays and measurement of inositol phosphate production. In addition, receptor expression levels were monitored by whole-cell enzyme-linked immunosorbent assay. Ligand-independent signaling of the wild-type GHSR is significantly reduced with introduction of either the V160M or F279L substitutions, whereas basal activity of the A204E mutant is not detectable. Ghrelin potency is markedly increased at the V160M mutant, whereas the I134T variant is unresponsive to this endogenous agonist. In contrast, the I134T mutant responds to a known GHSR inverse agonist, [D-Arg(1), D-Phe(5), D-Trp(7,9), Leu(11)]-substance P (SP-analog), albeit with reduced efficacy. Activity of the SP-analog at the V160M and F279L mutants is comparable to the wild type (WT) value. The overall expression level of each of the four GHSR variants is reduced relative to WT; however, the ratio between the intracellular and plasma membrane receptor density remains comparable. Treatment with the SP-analog significantly increases cell surface expression of each receptor with the exception of the A204E variant. Taken together, our studies reveal that naturally occurring GHSR mutations affect a wide range of pharmacologic properties. The physiological impact of these alterations within selected populations (e.g., obese, lean individuals) as well as the pharmacogenomic consequences of corresponding mutations remain to be further investigated.