Novel Insights Into Normal Aryl Hydrocarbon Receptor Biology Through the Regulation of Stanniocalcin 2

Abstract

Proper hepatocyte function is vital for survival; hence unrepaired destruction of the parenchymal tissue leading to liver decompensation is devastating. Therefore, understanding the homeostatic process regulating liver regeneration is clinically important, and evidence that the Aryl hydrocarbon Receptor (AhR) can promote cell survival following intrinsic apoptotic stimuli is integral to the regenerative process. Recent evidence suggests that the AhR promotes cell survival through the PI3K-Akt/PKB axis in the absence of an exogenous ligand. However this study was performed using a cancer cell line that does not accurately represent normal liver biology. Therefore, I hypothesize that the AhR mechanistically contributes to liver homeostasis through the regulation of genes hitherto not associated with AhR functions in the absence of an exogenous ligand. To test this hypothesis the current studies utilize primary hepatocytes to identify survival mechanisms consistent with normal AhR biology. Taking advantage of the Cre-lox system to manipulate AhR status, I report here that primary hepatocyte apoptosis is preferentially suppressed in cells expressing the AhR. Studies revealed that Akt/PKB activation previously linked to AhR dependent cell survival was not involved. Likewise, expression profiling of 84 key genes involved in apoptosis failed to account for the differential apoptotic susceptibility associated with AhR status. However, a comprehensive microarray analysis designed to identify immediate and direct changes in the transcriptome concomitant with AhR loss identified Stanniocalcin 2 (Stc2) as a novel receptor target gene previously reported to have a cytoprotective role in endoplasmic reticulum stress. The Stc2 promoter contains multiple xenobiotic response elements (XRE) clustered in a 250 bp region that was shown to recruit the AhR by chromatin immunoprecipitation. Interestingly, Stc2 gene expression is refractory to classic exogenous AhR agonists, but responds to cellular stress in an AhR-dependent mechanism consistent with a process promoting cell survival. In conclusion, AhR mediated transcriptional control of Stc2 represents a novel target gene associated with normal AhR biology that contributes mechanistically to liver homeostasis.