Differential regulation of Jun N-terminal kinase and p38MAP kinase by Gα₁₂

Abstract

Based on the findings that the overexpression of the wild-type Gα₁₂ (Gα₁₂WT) result in the oncogenic transformation of NIH3T3 cells in a serum-dependent manner, a model system has been established in which the mitogenic and subsequent cell transformation pathways activated by Gα₁₂ can be turned on or off by the addition or removal of serum. Using this model system, our previous studies have shown that the stimulation of Gα₁₂WT or the expression of an activated mutant of Gα₁₂ (Gα₁₂QL) leads to increased cell proliferation and subsequent oncogenic transformation of NIH3T3 cells, as well as persistent activation of Jun N-terminal kinases (JNKs). In the present studies, we show that the stimulation of Gα₁₂WT or the expression of Gα₁₂QL results in a potent inhibition of p38MAPK, and that the mechanism by which Gα₁₂ inhibits p38MAPK activity involves the dual specificity kinases upstream of p38MAPK. The results indicate that Gα₁₂ attenuates the activation of MKK3 and MKK4, which are known to stimulate only p38MAPK or p38MAPK and JNK, respectively. The results also suggest that Gα₁₂ activates JNKs specifically through the stimulation of the JNK-specific upstream kinase MKK7. These findings demonstrate for the first time that Gα₁₂ differentially regulates JNK and p38MAPK by specifically activating MKK7, while inhibiting MKK3 and MKK4 in NIH3T3 cells. Since the stimulation of p38MAPK is often associated with apoptotic responses, our findings suggest that Gα₁₂ stimulates cell proliferation and neoplastic transformation of NIH3T3 cells by attenuating p38MAPK-associated apoptotic responses, while activating the mitogenic responses through the stimulation of ERK- and JNK-mediated signaling pathways.