MAP kinase cascade is required for p27 downregulation and S phase entry infibroblasts and epithelial cells

The present report delineates the critical pathway in the G(1) phase involv ed in downregulation of p27(Kip1), a cyclin-dependent kinase inhibitor, whi ch plays a pivotal role in controlling entry into the S phase of the cell c ycle. In resting CCL39 fibroblasts and IEC-6 intestinal epithelial cells, p rotein levels of p27(Kip1) were elevated but dramatically decreased on seru m stimulation, along with hyperphosphorylation of pRb and increased CDK2 ac tivity. In both cell types, expression of ras resulted in an increase of ba sal and serum-stimulated E2F-dependent transcriptional activity and a reduc tion in p27(Kip1) protein levels as well. The role of the mitogen-activated protein (MAP) kinase cascade in p27(Kip1) reduction and S phase reentry wa s reinforced by the blockades of serum-induced E2F-dependent transcriptiona l activity and p27(Kip1) downregulation with the MKK-1/2 inhibitor PD-98059 . In both cell lines, downregulation of p27(Kip1) was associated with a rep ression of its synthesis, an event mediated by the p42/p44 MAP kinase pathw ay. Using an antisense approach, we demonstrated that p27(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(Kip1) downreg ulation in fibroblasts and epithelial cells.