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Fibroblast proliferation necessitates the presence of mitogen-activated protein kinases P42 map kinase (mapK) and P44 mapK, which also ensures the change in state of the S phase to G(0) phase during cell differentiation. Inactivation of MAP kinase hinders cell growth and differentiation. It is observed that the expression of any one isoform of MAP kinase RNA antisense results in the suppression of P42 mapK and P44 mapK. The mechanism of suppression is affected through the RNA duplexes formation, as a response to rapid degradation. Protooncogene product Raf-1, a kinase cascade transmitting growth factor, plays role in the activation of MAPK kinase kinase, a MAP kinase activator characterized by dual-specificity.

Indirect immunofluorescence microscopy was employed to prove that p42mapk and p44mapk which are mitogen-activated protein kinases are responsible for the rapid translocation to the nucleus as a reaction to strong mitogens. Their activator p45mapkk continues to be cytoplasmic. These results support the hypothesis that these kinases have a critical role to play in the transmission of surface receptor-mediated signals to the nucleus.

Rivard, Nathalie, Marie-Josee Boucher, Claude Asselin, and Gilles L'Allemain. MAP kinase cascade is required for p27 downregulation and S phase entry in fibroblasts and epithelial cells. Am. J. Physiol. 277 (Cell Physiol. 46): C652-C664, 1999.--The present report delineates the critical pathway in the [G.sub.1] phase involved in downregulation of [p27.sup.Kip1], a cyclin-dependent kinase inhibitor, which plays a pivotal role in controlling entry into the S phase of the cell cycle. In resting CCL39 fibroblasts and IEC-6 intestinal epithelial cells, protein levels of [p27.sup.Kip1] were elevated but dramatically decreased on serum stimulation, along with hyperphosphorylation of pRb and increased CDK2 activity. In both cell types, expression of ras resulted in an increase of basal and serum-stimulated E2F-dependent transcriptional activity and a reduction in [p27.sup.Kip1] protein levels as well. The role of the mitogen-activated protein (MAP) kinase cascade in [p27.sup.Kip1] reduction and S phase reentry was reinforced by the blockades of serum-induced E2F-dependent transcriptional activity and [p27.sup.Kip1] downregulation with the MKK-1/2 inhibitor PD-98059. In both cell lines, downregulation of [p27.sup.Kip1] was associated with a repression of its synthesis, an event mediated by the p42/p44 MAP kinase pathway. Using an antisense approach, we demonstrated that [p27.sup.Kip1] may control cell cycle exit in both cell types. These data indicate that activation of the MAP kinase cascade is required for S phase entry and [p27.sup.Kip1] downregulation in fibroblasts and epithelial cells. growth control; cell cycle; [G.sub.1]-to-S phase transition; growth signaling