AKT1 mediates bypass of the G1/S checkpoint after genotoxic stress in normal human cells

Abstract

Certain forms of hexavalent chromium [Cr(VI)] are human carcinogens. Our recent work has shown that a broad range protein tyrosine phosphatase (PTP) inhibitor, sodium orthovanadate (SOV), abrogated both Cr(VI)-induced growth arrest and clonogenic lethality. Notably, SOV enhanced Cr(VI) mutation frequency, ostensibly through forced survival of genetically damaged cells. In the present study, co-treatment with this PTP inhibitor bypassed the Cr(VI)-induced G1/S checkpoint arrest in diploid human lung fibroblasts (HLF). Moreover, the PTP inhibitor abrogated the Cr(VI)-induced decrease in the expression of key effectors of the G1/S checkpoint [Cyclin D1, phospho Ser 807/811 Rb (pRB), p27]. Cr(VI)-induced G1 arrest was associated with the cytoplasmic appearance of pRb and the nuclear localization of p27, both of which were reversed by the PTP inhibitor. The PTP inhibitor’s reversal of G1/S checkpoint effector localization after Cr exposure was found to be Akt1-dependent, as this was abrogated by transfection with either akt1 siRNA or an Akt1-kinase dead plasmid. Furthermore, Akt1 activation alone was sufficient to induce G1/S checkpoint bypass and to prevent Cr(VI)-induced changes in pRb and p27 localization. In conclusion, this work establishes Akt1 activation to be both sufficient to bypass the Cr(VI)-induced G1/S checkpoint, as well as necessary for the observed PTP inhibitor effects on key mediators of the G1/S transition. The potential for Akt to bypass G1/S checkpoint arrest in the face of genotoxic damage could increase genomic instability, which is a hallmark of neoplastic progression.