G-ALPHA-12 AND G-ALPHA-13 REGULATE EXTRACELLULAR SIGNAL-REGULATED KINASE AND C-JUN KINASE PATHWAYS BY DIFFERENT MECHANISMS IN COS-7 CELLS

Many growth factors and agonists for G protein-coupled receptors activ ate mitogen-activated protein (MAP) kinase pathways, including the ext racellular signal-regulated kinase (ERK) pathway and the c-Jun kinase (JNK) pathway. Transient transfection of dominant negative and constit utively active pathway components in COS-7 cells shows that two G prot ein subunits, G alpha 12 and G alpha 13, inhibit the ERK pathway and s timulate the JNK pathway. Constitutively active (GTPase-deficient) G a lpha 12 and G alpha 13 both inhibit ERK pathway activation by epiderma l growth factor. A G alpha 13/alpha z chimera, which responds to stimu lation by G(i)-coupled receptors, mediates inhibition of ERK via such a receptor, the dopamine-2 receptor. In addition, expression of a domi nant negative mutant of the GTPase, Cdc42, blocks activation of the JN K pathway by G alpha 12 and G alpha 13 but does not alter inhibition o f ERK activation by the same G alpha proteins; conversely, mutationall y activated Cdc42 stimulates the JNK pathway but has no effect on the ERK pathway, Our results show that different mechanisms mediate two ef fects of G alpha 12 and G alpha 13: the ERK pathway inhibition is medi ated at the level of MAP kinase kinase in a Ras- and Raf-independent f ashion, whereas the JNK pathway stimulation is mediated by Cdc42.