Regulation of apoptosis by α-subunits of G12 and G13 proteins via apoptosis signal-regulating kinase-1

Many growth factors and G protein-coupled receptors activate mitogen-activated protein (MAP) kinase pathways. The MAP kinase pathways are involved in the regulation of the ubiquitous process of apoptosis or programmed cell death. Two related MAP kinase kinase kinases, apoptosis-signal regulating kinase 1 (ASK1) and MAP kinase kinase kinase 1 (MEKK1), stimulate c-Jun kinase (JNK) activity and induce apoptosis, Transient transfection of dominant negative and constitutively active components of the JNK pathway in COS-7 cells showed that two G protein subunits, G alpha 12 and G alpha 13, stimulated the JNK pathway in a ASK1- and MEKK1-dependent manner. Moreover, the mutationally activated G alpha 12 and G alpha 13 stimulated the kinase activity of ASK1. Both G alpha 12 and G alpha 13 employ small GTPases, Cdc42 and Rac1, to transduce signal to MEKK1 and, subsequently, to JNK, However, activation of JNK by Cdc42 and Rac1 did not require ASK1. Additionally, ASK1 and MEKK1 are involved in the apoptosis induced by G alpha 12 and G alpha 13, We conclude that G alpha 12 and G alpha 13 can induce apoptosis using two separate MAP kinase pathways; one is initiated by ASK1, and the other is initiated by MEKK1, Furthermore, Bcl-2 can block apoptosis induced by G alpha 12 and G alpha 13. This death-sparing function was associated with increased Bcl-2 phosphorylation, suggesting that phosphorylation of Bcl-2 may be a critical mechanism protecting cells from G alpha 12- and G alpha 13-induced apoptosis.