Novel mechanisms of regulation of the expression and transcriptional activity of hepatocyte nuclear factor 4 alpha

Hepatocyte nuclear factor 4 alpha (HNF4 alpha) is a master regulator of development and function of digestive tissues. The HNF4A gene uses two separate promoters P1 and P2, with P1 products predominant in adult liver, whereas P2 products prevalent in fetal liver, pancreas, and liver/colon cancer. To date, the mechanisms for the regulation of HNF4A and the dynamic switch of P1-HNF4 alpha and P2-HNF4 alpha during ontogenesis and carcinogenesis are still obscure. Our study validated the previously reported self-stimulation of P1-HNF4 alpha but invalidated the reported synergism between HNF4 alpha and HNF1 alpha. HNF4A-AS1, a long noncoding RNA, is localized between the P2 and P1 promoters of HNF4A. We identified critical roles of P1-HNF4 alpha in regulating the expression of HNF4A-AS1 and its mouse ortholog Hnf4a-os. Paired box 6 (PAX6), a master regulator of pancreas development overexpressed in colon cancer, cooperated with HNF1 alpha to induce P2-HNF4 alpha but antagonized HNF4 alpha in HNF4A-AS1 expression. Thus, PAX6 may be important in determining ontogenic and carcinogenic changes of P2-HNF4 alpha and HNF4A-AS1 in the pancreas and intestine. We also interrogated transactivation activities on multiple gene targets by multiple known and novel HNF4 alpha mutants identified in patients with maturity onset diabetes of the young 1 (MODY1) and liver cancer. Particularly, HNF4 alpha-D78A and HNF4 alpha-G79S, two mutants found in liver cancer with mutations in DNA-binding domain, displayed highly gene-specific transactivation activities. Interestingly, HNF4 alpha-Q277X, a MODY1 truncation mutant, antagonized the transactivation activities of HNF1 alpha and farnesoid X receptor, key regulators of insulin secretion. Taken together, our study provides novel mechanistic insights regarding the transcriptional regulation and transactivation activity of HNF4 alpha in digestive tissues.