The Constitutive Active/Androstane Receptor Facilitates Unique Phenobarbital-Induced Expression Changes of Genes Involved in Key Pathways in Precancerous Liver and Liver Tumors

Our overall goal is to elucidate progressive changes, in expression and methylation status, of genes which play key roles in phenobarbital (PB)-induced liver tumorigenesis, with an emphasis on their potential to affect signaling through critical pathways involved in the regulation of cell growth and differentiation. PB-elicited unique expression changes of genes, including some of those identified previously as exhibiting regions of altered DNA methylation, were discerned in precancerous liver tissue and/or individual liver tumors from susceptible constitutive active/androstane receptor (CAR) wild-type (WT) compared with resistant CAR knockout (KO) mice. Many of these function in crucial cancer-related processes, for example, angiogenesis, apoptosis, cell cycle, DNA methylation, Hedgehog signaling, invasion/metastasis, Notch signaling, and Wnt signaling. Furthermore, a subset of the uniquely altered genes contained CAR response elements (CAREs). This included Gadd45b, a coactivator of CAR and inhibitor of apoptosis, and two DNA methyltransferases (Dnmt1, Dnmt3a). The presence of CAREs in Dnmts suggests a potential direct link between PB and altered DNA methylation. The current data are juxtaposed with the effects of PB on DNA methylation and gene expression which occurred uniquely in liver tumor-prone B6C3F1 mice, as compared with the resistant C57BL/6, following 2 or 4 weeks of treatment. Collectively, these data reveal a comprehensive view of PB-elicited molecular alterations (i.e., changes in gene expression and DNA methylation) that can facilitate hepatocarcinogenesis. Notably, candidate genes for initial fingerprints of early and late stages of PB-induced tumorigenesis are proposed.