Inorganic arsenic exposure induces E2F-dependent G0/G1 arrest via an increase in retinoblastoma family protein p130 in B-cell lymphoma A20 cells.

Inorganic arsenic exerts toxic effect on multiple systems including the immune system. We previously showed in a study on mouse thymocytes and B-cell lymphoma A20 cells that arsenite induces cell cycle arrest at G0/G1 by suppressing expression of E2F-target genes. In this study, we furthermore investigated the involvement of retinoblastoma (RB) family proteins in E2F-dependent cell cycle arrest by arsenite. Arsenite exposure of A20 cells was showed to increase the protein level of p130, a RB family member, without changing the mRNA level. Suppression of arsenite-induced p130 by siRNA reduced the G0/G1 phase, indicating that p130 accumulation is responsible for arsenite-induced G0/G1 arrest. The accumulated p130 was shown to increase the p130 complex with E2F4, a transcription-suppressing E2F. Comparison by Western blotting of arsenite-induced p130 and p130 accumulated by a proteasome inhibitor suggested that arsenite-induced p130 is hypophosphorylated and hypoubiquitinated and refractory to proteasome-dependent degradation. We also showed that arsenite increases mRNA and protein of p16(INK4a), an inhibitor of CDK4/6 that phosphorylates p130. Down-regulation of arsenite-induced p16(INK4a) by siRNA suppressed the p130 accumulation. We propose a novel mechanism in which arsenite inhibits phosphorylation/ubiquitin-dependent proteasome degradation of p130 by inducing p16(INK4a) and the accumulated p130 causes cell cycle arrest with E2F4.