ALPHA(2) ADRENERGIC-RECEPTOR SUBTYPES EXPRESSED IN CHINESE-HAMSTER OVARY CELLS ACTIVATE DIFFERENTIALLY MITOGEN-ACTIVATED PROTEIN-KINASE BY A P21(RAS) INDEPENDENT PATHWAY

Epinephrine stimulation of rat alpha(2D), alpha(2B), and alpha(2C) adrenergic receptor subtypes, expressed stably in Chinese hamster ovary (CHO) cells, caused a rapid, transient activation of mitogen-activated protein kinase (MAPK), with subtype-specific different efficiencies. The order of activation was CHO-2B similar to CHO-2D >> CHO-2C. Pertussis toxin blocked the stimulation of MAPK enzymatic activity and the parallel MAPK phosphorylation, demonstrating that these responses are mediated by pertussis toxin-sensitive G(i) proteins. Contrary to what has been reported for the alpha(2A) subtype expressed in rat-1 fibroblasts, epinephrine did not cause any detectable activation of p21(ras) in the CHO transfectants. Furthermore, combined application of epinephrine and phorbol myristate acetate had a potent cooperative but not additive effect in clones CHO-2D and CHO-2B but not in CHO-2C, suggesting that protein kinase C is probably differently involved in the signaling by the three alpha(2) receptor subtypes. These results show that in CHO cells, the different alpha(2) adrenergic receptor subtypes utilize differential pathways to activate MAPK in a p21(ras)-independent way.