Our interest is in the molecular basis of cancer, and we have approached this problem by studying intracellular pathways which participate in the transduction of proliferative signals. We have previously shown that growth promoting pathways activated by G protein-coupled receptors were shown to involve activation of the ras proto-oncogene and 72raf. The latter initiates activity from a serine-threonine kinase cascade that converges in the activation of extracellular signal-regulated kinases (ERKs) or MAP kinases, and ultimately regulates the expression of genes essential for proliferation. We have demonstrated that beta-gamma subunits of G proteins, not G-alpha, act in a ras-dependent manner to stimulate MAP kinases, converging at this level with the signaling route utilized by tyrosine kinase-growth factor receptors. Thus, activation of either type of receptor would be expected to elicit a similar response at the level of nuclear transcription factors. We have observed that activation of G protein-coupled receptors in NIH 3T3 cells induces a distinct pattern of expression of immediate early genes of the jun and fos family. These responses did not correlate with the activation of MAP kinases. We found that triggering G protein coupled receptors, but not PDGF potently stimulate the activity of a novel family of enzymes closely related to MAP kinases, known as jun kinases (JNKs). We found that G protein-coupled receptors can signal through pathways leading to the activation of JNK, thus diverging at this level with those pathways utilized by receptors of the tyrosine kinase class. These observations led us to hypothesize that JNK and MAP kinase might be differentially regulated, and prompted us to investigate the biochemical route controlling JNK. Using a model system to explore the mechanism of activation of JNK, we found that Ras could weakly activate JNK, utilizing a pathway distinct from that regulating MAPK. We showed that expression of mutationally activated forms of the small GTP- binding proteins Rac1 and Cdc42 initiate an independent kinase cascade leading to JNK activation, and have also shown that Rac and Cdc42 are an integral part of the signaling route linking G protein coupled receptors as well as receptors for inflammatory cytokines and epidermal growth factor to JNK. Furthermore, we have shown that JNK is potently activated by a number of naturally occurring human oncogenes, and recently published reports support a role for Rac1 in metastasis. Thus, we believe that our findings unveiling the existence of a novel signaling pathway communicating the cell surface to the nucleus have helped to identify a number of potential candidates as targets for therapeutic intervention in cancer.
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