Retinoic Acid Receptor β Loss in Hepatocytes Increases Steatosis and Elevates the Integrated Stress Response in Alcohol-Associated Liver Disease

Abstract: In alcohol-associated liver disease (ALD), hepatic reductions of vitamin A and perturbations in vitamin A metabolism are common. However, the roles that the vitamin A receptors, retinoic acid receptors (RARs), may have in preventing the pathophysiology of ALD remain unclear. Our prior data indicate that a RARβ agonist limits alcohol-related liver disease pathology. Thus, we generated liver-specific AlbCre;RARβ knockout (BKO) mice and compared them to wild-type (WT) mice in an early ALD model. Both strains showed similar ethanol blood level concentrations and ETOH-metabolizing enzymes. However, the livers of pair-fed and ETOH-BKO mice developed higher levels of steatosis and triglycerides than pair-fed-WT and ETOH-WT mice. The increased hepatic steatosis in pair-fed-BKO and ETOH-BKO mice was associated with higher lipid synthesis/trafficking transcripts and lower beta-oxidation transcripts. ETOH-BKO mice also exhibited a higher integrated stress response (ISR) signature, including higher transcript and protein levels of ATF4 and its target, 4EBP1. In human hepatocytes (HepG2) that lack RARβ (RARβ-KO), ETOH treatments resulted in greater reactive oxygen species compared to parental cells. Notably, even without ETOH, ATF4 and 4EBP1 protein levels were higher in RARβ-KO than in parental cells. These 4EBP1 increases were greatly attenuated in cultured ATF4-deficient and RARβ/ATF4-deficient HepG2, suggesting that RARβ is a crucial negative regulator of 4EBP1 through ATF4 in cultured hepatocytes. Here we identify RARβ as a negative regulator of lipid metabolism and cellular stress in ALD