Inorganic arsenic exposure induces E 2 F ‐dependent G 0/ G 1 arrest via an increase in retinoblastoma family protein p130 in B ‐cell lymphoma A 20 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 A 20 cells that arsenite induces cell cycle arrest at G 0/ G 1 by suppressing expression of E 2 F ‐target genes. In this study, we furthermore investigated the involvement of retinoblastoma ( RB ) family proteins in E 2 F ‐dependent cell cycle arrest by arsenite. Arsenite exposure of A 20 cells was showed to increase the protein level of p130, a RB family member, without changing the m RNA level. Suppression of arsenite‐induced p130 by si RNA reduced the G 0/ G 1 phase, indicating that p130 accumulation is responsible for arsenite‐induced G 0/ G 1 arrest. The accumulated p130 was shown to increase the p130 complex with E 2 F 4, a transcription‐suppressing E 2 F . Comparison by W estern 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 m RNA and protein of p16 INK4a , an inhibitor of CDK 4/6 that phosphorylates p130. Down‐regulation of arsenite‐induced p16 INK4a by si RNA 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 E 2 F 4.