Balance between HNF4a isoforms in the carbohydrate-lipid metabolic switch

University Of California Riverside, Riverside CA

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Abstract

Abstract Hepatocyte Nuclear Factor 4? (HNF4?), a master regulator of liver-specific gene expression, is regulated by two promoters (P1 and P2) which drive expression of two groups of HNF4? isoforms referred to as HNF4?1 and HNF4?7. HNF4? is a known regulator of gluconeogenesis and mutated in maturity onset diabetes of the young one (MODY1). Conventionally, it was thought that HNF4?1, but not HNF4?7, is expressed in the normal adult liver, while HNF4?1 is downregulated and HNF4?7 is upregulated in liver cancer. Now, research in our lab reveals a previously undescribed role for HNF4?7 in the normal adult mouse liver ? one involved in the diurnal variations of lipid and carbohydrate metabolism. More specifically, HNF4?1 appears to be a major driver of gluconeogenesis while HNF4?7 is a driver of ketogenesis: we propose that alterations in the levels of the HNF4? isoforms during the day flip the molecular switch between the two. Our preliminary data also show that HNF4?7 is required for increased levels of circulating ketone bodies in female mice. AMP-Activated Protein Kinase (AMPK), an energy-sensing enzyme, has been shown to phosphorylate HNF4?1 in vitro, but effects in vivo and on HNF4?7 are not known. SIRT1 is a deacetylase that works with AMPK to regulate glucose and lipid metabolism. HNF4?1 is known to be acetylated and our preliminary data suggest that HNF4?7 but not HNF4?1 interacts with SIRT1. Here, we propose to use HNF4?1-expressing (?1HMZ) and HNF4?7-expressing exon swap mice (?7HMZ) to determine the physiological function of the HNF4? isoforms in the switch between gluconeogenesis and ketogenesis, and to characterize the impact of sex on those functions. In Aim 1, we will determine whether intermittent fasting and a ketogenic diet increase the levels of HNF4?7 in the liver, and whether the increase occurs in all hepatocytes, or just a subset. We will determine the consequences of HNF4?7 on gene expression. Kidney and intestines will also be explored. In Aim 2, we will determine whether the AMPK pathway acts in a differential fashion on the HNF4? isoforms to help flip the metabolic switch. Phosphorylation by AMPK and deacetylation by SIRT1 will be explored. Finally, in Aim 3, we will determine whether the estrogen pathway impacts the HNF4? isoforms in female mice and determine the consequences for the metabolic switch. Our compelling preliminary data that the HNF4? isoforms are involved in the switch between gluconeogenesis and ketogenesis shed new light on this basic metabolic process that occurs on a daily basis and under conditions of feeding and fasting. The results from this proposal will illuminate not only the molecular mechanism underlying the switch but also how that mechanism is impacted by sex. The proposed studies have the potential to impact our understanding of numerous metabolic diseases, including diabetes, obesity, fatty liver disease and cancer. Finally, given the fact that ketone bodies serve as a source of fuel for the brain, our results could have a broader impact, including on neurological diseases, such as dementia.