SUN-LB119 Role of HNF4a Isoforms in the Carbohydrate/Lipid Switch in the Liver and Responsiveness to AMPK

Abstract

Hepatocyte Nuclear Factor 4α (HNF4α), the master regulator of liver-specific gene expression, is regulated by two promoters (P1 and P2) which drive expression of two groups of HNF4α isoforms referred to here as HNF4α1 and HNF4α7. HNF4α is a known regulator of gluconeogenesis and is mutated in maturity onset diabetes of the young one (MODY1). Conventionally, it was thought that HNF4α1, but not HNF4α7, is expressed in the normal adult liver, with HNF4α1 downregulated and HNF4α7 upregulated in liver cancer. Now, we identify a previously undescribed role for HNF4α7 in the normal adult mouse liver - one involved in the diurnal variations of lipid and carbohydrate metabolism. More specifically, HNF4α1 appears to be a major driver of gluconeogenesis while HNF4α7 is a driver of ketogenesis: we hypothesize that alterations in the levels of the HNF4α isoforms during the day function as a molecular switch between the two. Moreover, our preliminary data show that HNF4α7 is required for increased levels of circulating ketone bodies in female mice, suggesting interactions with the estrogen pathway. AMP-Activated Protein Kinase (AMPK), an energy-sensing kinase that also plays a major role in carbohydrate and lipid metabolism, has been shown to phosphorylate HNF4α1 in vitro, but effects in vivo and on HNF4α7 are not known. In order to investigate the impact of AMPK on HNF4α isoforms, we employed HNF4α isoform-specific mice α7HMZ (express only HNF4α7) and α1HMZ mice (express only HNF4α1), as well as heterozygous mice which express both. Intraperitoneal injection of the mice with AMPK activator AICAR leads to a rapid decrease in glucose. Interestingly, half the α7HMZ males and all the females began seizing 30 min post injection, while very few α1HMZ males/females and none of the heterozygous mice seized. Moreover, there were differences in the survival of the different genotypes: a third of α1HMZ mice die within 24hrs, while two thirds of α7HMZ mice die within a week, with all heterozygous mice surviving. We suspect the seizures could be due to an electrolyte imbalance exacerbated by AICAR or extremely low glucose caused by AICAR. The α7HMZ females have significantly lower potassium levels compared to α1HMZ and wildtype mice. Additionally, AMPK is known to regulate Na+/glucose transporters, and HNF4α1 is expressed in the proximal tubules in the kidney (responsible for Na+ uptake). To elucidate the cause of the seizures, AICAR injections were repeated with α1HMZ males followed by a glucose or saline gavage. Interestingly, half of the glucose-gavaged mice died within 24hrs, while all of the saline-gavaged mice survived. Our work underscores the critical role that the HNF4α isoforms play in the metabolic switch, and suggests that the kidney as well as the liver could be involved.