A1 adenosine receptor-mediated PKC and p42/p44 MAPK signaling in mouse coronary artery smooth muscle cells

Abstract

The A1 adenosine receptor (A1AR) is coupled to Gi/Go proteins, but the downstream signaling pathways in smooth muscle cells are unclear. This study was performed in coronary artery smooth muscle cells (CASMCs) isolated from the mouse heart [A1AR wild type (A1WT) and A1AR knockout (A1KO)] to delineate A1AR signaling through the PKC pathway. In A1WT cells, treatment with (2 S)- N6-(2-endo-norbornyl)adenosine (ENBA; 10−5M) increased A1AR expression by 150%, which was inhibited significantly by the A1AR antagonist 1,3-dipropyl-8-cyclopentylxanthine (10−6M), but not in A1KO CASMCs. PKC isoforms were identified by Western blot analysis in the cytosolic and membrane fractions of cell homogenates of CASMCs. In A1WT and A1KO cells, significant levels of basal PKC-α were detected in the cytosolic fraction. Treatment with the A1AR agonist ENBA (10−5M) translocated PKC-α from the cytosolic to membrane fraction significantly in A1WT but not A1KO cells. Phospholipase C isoforms (βI, βIII, and γ1) were analyzed using specific antibodies where ENBA treatment led to the increased expression of PLC-βIII in A1WT CASMCs while having no effect in A1KO CASMCs. In A1WT cells, ENBA increased PKC-α expression and p42/p44 MAPK (ERK1/2) phospohorylation by 135% and 145%, respectively. These effects of ENBA were blocked by Gö-6976 (PKC-α inhibitor) and PD-98059 (p42/p44 MAPK inhibitor). We conclude that A1AR stimulation by ENBA activates the PKC-α signaling pathway, leading to p42/p44 MAPK phosphorylation in CASMCs.