A negative reciprocal regulatory axis between cyclin D1 and HNF4α modulates cell cycle progression and metabolism in the liver

Hepatocyte nuclear factor 4 alpha (HNF4 alpha) is a master regulator of liver function and a tumor suppressor in hepatocellular carcinoma (HCC). In this study, we explore the reciprocal negative regulation of HNF4 alpha and cyclin D1, a key cell cycle protein in the liver. Transcriptomic analysis of cultured hepatocyte and HCC cells found that cyclin D1 knockdown induced the expression of a large network of HNF4 alpha-regulated genes. Chromatin immunoprecipitation-sequencing (ChIP-seq) demonstrated that cyclin D1 inhibits the binding of HNF4 alpha to thousands of targets in the liver, thereby diminishing the expression of associated genes that regulate diverse metabolic activities. Conversely, acute HNF4 alpha deletion in the liver induces cyclin D1 and hepatocyte cell cycle pro-gression; concurrent cyclin D1 ablation blocked this proliferation, suggesting that HNF4 alpha maintains proliferative quiescence in the liver, at least, in part, via repression of cyclin D1. Acute cyclin D1 deletion in the regenerating liver markedly inhibited hepatocyte proliferation after partial hepatectomy, confirming its pivotal role in cell cycle progression in this in vivo model, and enhanced the expression of HNF4 alpha target proteins. Hepatocyte cyclin D1 gene ablation caused markedly increased postprandial liver glycogen levels (in a HNF4 alpha-dependent fashion), indicating that the cyclin D1-HNF4 alpha axis regulates glucose metabolism in response to feed-ing. In AML12 hepatocytes, cyclin D1 depletion led to increased glucose uptake, which was negated if HNF4 alpha was depleted simul-taneously, and markedly elevated glycogen synthesis. To summarize, mutual repression by cyclin D1 and HNF4 alpha coordinately controls the cell cycle machinery and metabolism in the liver.