Abstract

To study the role of mitogen-activated protein kinase in the regulation of M₂ receptors, we studied the effect of platelet-derived growth factor (PDGF) on M₂ receptor gene expression. PDGF (4 ng/ml) caused a time-dependent decrease in M₂ receptor number and in m2 receptor mRNA levels in HEL 299 cells. The PDGF-induced loss in m2 mRNA required de novo protein synthesis and occurred through a decrease in the rate of transcription of the m2 receptor gene. The down-regulation of M₂receptors was not accompanied by an uncoupling of the remaining receptors, indicating a large receptor reserve in these cells. Preincubations with the phosphatidylinositol 3-kinase inhibitor wortmannin, the protein kinase C inhibitor GF 109203X and the cAMP-dependent protein kinase inhibitor H-8 did not attenuate PDGF-induced down-regulation, indicating a lack of involvement of these enzymes in the down-regulation process. Activation of the extracellular signal-regulated protein kinase (ERK) 1 and 2 proteins was measured by an “in gel” phosphorylation assay. Carbachol did not activate ERK1 or 2, whereas PDGF and 4β-phorbol 13,14-dibutyrate resulted in a large increase in ERK1 and 2 activity along with a decrease in m2 mRNA. Preincubation with PD 098059, an inhibitor of mitogen-activated protein kinase kinase, inhibited PDGF- and 4β-phorbol 13,14-dibutyrate-mediated activation of ERK 1 and 2 in a concentration-dependent manner. The inhibitory action of PD 098059 was reflected at the mRNA level attenuating both PDGF- and 4β-phorbol 13,14-dibutyrate-mediated decreases in m2 mRNA. These results suggest a role of ERK1 and 2 in the regulation of muscarinic m2 receptor gene expression.