Abstract

Previous studies in hepatocyte-derived cell lines and the whole liver established that the aryl hydrocarbon receptor (AhR) can disrupt G₁-phase cell cycle progression following exposure to persistent AhR agonists, such as TCDD (dioxin, 2,3,7,8-tetrachlorodibenzo-p-dioxin). Growth arrest was attributed to inhibition of G₁-phase cyclin-dependent kinase 2 (CDK2) activity. The present study examined the effect of TCDD exposure on liver regeneration following 70% partial hepatectomy in mice lacking the Cip/Kip inhibitors p21^Cip1 or p27^Kip1 responsible for regulating CDK2 activity. Assessment of the regenerative process in wild-type, p21^Cip1 knockout, and p27^Kip1 knockout mice confirmed that TCDD-induced inhibition of liver regeneration is entirely dependent on p21^Cip1 expression. Compared with wild-type mice, the absence of p21^Cip1 expression completely abrogated the TCDD inhibition, and accelerated hepatocyte progression through G₁ phase during the regenerative process. Analysis of the transcriptional response determined that increased p21^Cip1 expression during liver regeneration involved an AhR-dependent mechanism. Chromatin immunoprecipitation studies revealed that p21^Cip1 induction required AhR binding to the newly characterized nonconsensus xenobiotic response element, in conjunction with the tumor suppressor protein Kruppel-like factor 6 functioning as an AhR binding partner. The evidence also suggests that AhR functionality following partial hepatectomy is dependent on a p21^Cip1-regulated signaling process, intimately linking AhR biology to the G₁-phase cell cycle program.