Gadd45-α and Gadd45-γ utilize p38 and JNK signaling pathways to induce cell cycle G2/M arrest in Hep-G2 hepatoma cells

The Gadd45 family of proteins, which includes alpha, beta, and gamma isoforms, has recently been shown to play a role in the G2/M cell cycle checkpoint in response to DNA damage; however, the mechanisms by which Gadd45 proteins inhibit cell cycle control are not fully understood. Using immunohistochemical analysis, we found that protein expression of Gadd45 gamma, but not Gadd45 alpha, was down-regulated in hepatocellular carcinoma. We thus investigated possible mechanisms by which Gadd45 alpha and Gadd45 gamma might differentially induce G2/M arrest in the human hepatoma Hep-G2 cell line. Flow cytometric analysis revealed significant G2/M arrest in cells transfected with either Gadd45 alpha or Gadd45 gamma. Importantly, we found that expression of either Gadd45 alpha or Gadd45 gamma activated the P38 and JNK kinase pathways to induce G2/M arrest. Taken together, these findings suggest that the induction of G2/M arrest by Gadd45 alpha or Gadd45 gamma involves activation of two distinct signaling pathways in Hep-G2 hepatoma cell lines.