TGF-beta induces G1 phase cell cycle arrest in normal epithelial cells. Most tumor cells are resistant to TGF-beta, which is associated with Ras activation. The role played by a Ras effector, the PI3K/PKB pathway, in the cell cycle was investigated in this thesis.The cdk inhibitor p27KIP1 binds to and inhibits cyclin E-cdk2 in response to TGF-beta. This response is defective in TGF-beta resistant cells. We showed increased PKB activation and cytoplasmic mislocalization of p27 in TGF-beta resistant lines. Transfection of constitutively active PKB conferred TGF-beta resistance. PKB phosphorylates p27 on threonine 157 (T157) and impairs its nuclear import. PI3K/PKB inhibition abolished p27T157 phosphorylation and restored TGF-beta-induced cyclin E-cdk2 inhibition by p27 and G1 arrest. Cytoplasmic p27 mislocalization was associated with PKB activation and with poor patient prognosis in 40% of primary breast cancers. This study has brought to light a novel mechanism whereby PKB impairs p27 function during oncogenesis.In summary, constitutive PKB-dependent phosphorylation of p27 leads to cytoplasmic mislocalization of p27, whereas the assembly function of p27 toward cyclin D1-cdk4 is activated through both PKB-dependent and PKB-independent sequential changes in p27 phosphorylation. PKB over-activation leads to p27 sequestration in cytoplasm or in cyclin D-cdk complexes and may contribute to loss of the cyclin E-cdk2 inhibitory function of p27 and TGF-beta resistance.p27KIP1 also acts as an assembly factor for cyclin D-cdk4 or cdk6 in a mitogen-dependent manner. We showed that following growth factor stimulation, PKB activation precedes cyclin D1-cdk4-p27KIP1 assembly in early G1. Inhibition of the PI3K/PKB pathway resulted in loss of cyclin D1 from p27 complexes and a shift of p27 into cyclin E-cdk2. PKB-dependent p27 phosphorylation increased the ability of p27 to assemble cyclin D1-cdk4. Cyclin D1-bound p27 is discretely phosphorylated at multisites including N-terminal S10 and C-terminal S178/T187 and T198, differing significantly from cdk2-bound p27.