Identification and Molecular Characterization of a m5 Muscarinic Receptor in A2058 Human Melanoma Cells

Abstract

We report the identification and biochemical characterization of an endogenous m5 muscarinic acetylcholine receptor (mAChR) in the A2058 human melanoma cell line. This is the first demonstration of a m5AChR outside the central nervous system. The unusual effector coupling of this endogenous m5AChR is presented. The coding region amplified by polymerase chain reaction was identical to the known m5AChR sequence. Binding studies indicated a K_d of 99 ± 6 pM and a B_max of 45 ± 4 fmol/mg membrane protein. This m5AChR coupled to stimulation of arachidonic acid release and to a 50% inhibition of forskolin-stimulated cAMP accumulation. The inhibition of cAMP production was insensitive to pertussis toxin treatment, but was dependent upon extracellular calcium. In contrast to the odd mAChR pattern, no cAMP was produced in response to carbachol (CC) stimulation. Moreover, no release of inositol phosphates could be measured after CC treatment despite the presence of at least 2 phospholipase C isoforms in A2058 cells. CC-stimulated arachidonic acid release (EC₅₀ = 17.8 ± 0.1 μM) was dependent upon external Ca²⁺, with marked reduction after coincubation with EGTA, Co²⁺, or high doses of verapamil (IC₅₀ = 166 μM) or diltiazem (IC₅₀ = 243 μM). Brief exposure to phorbol 12-myristate 13-acetate augmented CC-stimulated arachidonic acid release, whereas prolonged phorbol 12-myristate 13-acetate treatment resulted in down-regulation of release. Activation of the m5AChR resulted in Ca²⁺ influx that was attenuated by muscarinic antagonism and removal of extracellular Ca²⁺. A2058 cells exposed to CC had no alteration of cell shape or growth potential in monolayer culture, however, a statistically significant reduction in density-independent growth was observed over the range of CC concentrations from 0.1 to 100 μM. This endogenous m5AChR has a novel signal transduction coupling profile and receptor activation reduces clonogenic potential.