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The alcohol-induced depletion of hepatic retinoid stores correlates with the progression of liver injury; however, the mechanisms underlying alcohol's effects have not been fully elucidated. Our goal was to gain a mechanistic understanding of alcohol-induced hepatic retinoid depletion. Wild-type and mutant mice were continuously fed alcohol through Lieber-DeCarli liquid diets, with matched control animals pair fed an isocaloric alcohol-free diet to ensure equal nutrient and calorie intake between groups. A systematic analysis of tissue retinol and retinyl ester levels was performed with HPLC, complemented by gene and protein expression analyses. Our results delineated 2 phases of alcohol-induced depletion of hepatic retinoid. Initially, ~15% of hepatic retinoid content was mobilized from the liver, causing extrahepatic tissue retinoid levels to increase. Subsequently, there was a precipitous drop in hepatic retinoid content (>60%), without further retinoid accumulation in the periphery. Follow-up studies in mutant mice revealed roles for RBP, CRBP1, and CD36 in retinoid mobilization and extrahepatic retinoid uptake, as well as a role for CYP2E1 in the catabolism of hepatic retinoid. In summary, alcohol has abiphasic effect on hepatic retinoid stores, characterized by an initial phase of rapid mobilization to extrahepatic tissues followed by extensive catabolism within the liver. [ABSTRACT FROM AUTHOR]

Liver fatty acid binding protein (L-Fabp) modulates lipid trafficking in enterocytes, hepatocytes, and hepatic stellate cells (HSCs). We examined hepatocyte vs. HSC L-Fabp deletion in hepatic metabolic adaptation and fibrotic injury. Floxed L-Fabp mice were bred to different transgenic Cre mice or injected with adeno-associated virus type 8 (AAV8) Cre and fed diets to promote steatosis and fibrosis or were subjected to either bile duct ligation or CCl4 injury. Albumin-Cre-mediated L-Fabp deletion revealed recombination in hepatocytes and HSCs; these findings were confirmed with 2 other floxed alleles. Glial fibrillary acid protein-Cre and platelet-derived growth factor receptor β-Cre-mediated L-Fabp deletion demonstrated recombination only in HSCs. Mice with albumin promoter-driven Cre recombinase (Alb-Cre)-mediated or AAV8-mediated L-Fabp deletion were protected against food withdrawal-induced steatosis. Mice with Alb-Cre-mediated L-Fabp deletion were protected against high saturated fat-induced steatosis and fibrosis, phenocopying germline L-Fabp-/- mice. Mice with HSC-specific L-Fabp deletion exhibited retinyl ester depletion yet demonstrated no alterations in fibrosis. On the other hand, fibrogenic resolution after CCl4 administration was impaired in mice with Alb-Cre-mediated L-Fabp deletion. These findings suggest cell type-specific roles for L-Fabp in mitigating hepatic steatosis and in modulating fibrogenic injury and reversal. [ABSTRACT FROM AUTHOR]

Retinoic acid signaling is required for maintaining a range of cellular processes, including cell differentiation, proliferation, and apoptosis. We investigated the actions of all-trans-retinoic acid (atRA) signaling in pancreatic β-cells of adult mice. atRA signaling was ablated in β-cells by overexpressing a dominant-negative retinoic acid receptor (RAR)-α mutant (RARdn) using an inducible Cre-Lox system under the control of the pancreas duodenal homeobox gene promoter. Our studies establish that hypomorphism for RAR in β-cells leads to an age-dependent decrease in plasma insulin in the fed state and in response to a glucose challenge. Glucose-stimulated insulin secretion was also impaired in islets isolated from mice expressing RARdn. Among genes that are atRA responsive, Glut2 and Gck mRNA levels were decreased in isolated islets from RARdn-expressing mice. Histologic analyses of RARdn-expressing pancreata revealed a decrease in β-cell mass and insulin per β-cell 1 mo after induction of the RARdn. Our results indicate that atRA signaling mediated by RARs is required in the adult pancreas for maintaining both β-cell function and mass, and provide insights into molecular mechanisms underlying these actions. [ABSTRACT FROM AUTHOR]

The mammalian embryo relies on maternal circulating retinoids (vitamin A derivatives) for development. β-Carotene is the major human dietary provitamin A. β-Carotene-15,15'-oxygenase (CMOI) has been proposed as the main enzyme generating retinoid from β-carotene in vivo. CMOI is expressed in embryonic tissues, suggesting that β-carotene provides retinoids locally during development. We performed loss of CMOI function studies in mice lacking retinol-binding protein (RBP), an established model of embryonic vitamin A deficiency (VAD). We show that, unexpectedly, lack of CMOI in the developing tissues further exacerbates the severity of VAD and thus the embryonic malformations of RBP-/- mice. Since β-carotene was not present in any of the mouse diets, we unveiled a novel action of CMOI independent from its β-carotene cleavage activity. We also show for the first time that CMOI exerts an additional function on retinoid metabolism by influencing retinyl ester formation via modulation of lecithin:retinol acyltransferase (LRAT) activity, at least in developing tissues. Finally, we demonstrate unequivocally that β-carotene can serve as an alternative vitamin A source for the in situ synthesis of retinoids in developing tissues by the action of CMOI. [ABSTRACT FROM AUTHOR]

The physiology of two metabolites of vitamin A is understood in substantial detail: retinaldehyde functions as the universal chromophore in the vertebrate and invertebrate eye; retinoic acid regulates a set of vertebrate transcription factors, the retinoic acid receptor superfamily. The third member of this retinoid triumvirate is retinol. While functioning as the precursor of retinaldehyde and retinoic acid, a growing body of evidence suggests a far more fundamental role for retinol in signal transduction. Here we show that retinol is essential for the metabolic fitness of mitochondria. When cells were deprived of retinol, respiration and ATP synthesis defaulted to basal levels. They recovered to significantly higher energy output as soon as retinol was restored to physiological concentration, without the need for metabolic conversion to other retinoids. Retinol emerged as an essential cofactor of protein kinase Cδ (PKCδ), without which this enzyme failed to be activated in mitochondria. Furthermore, retinol needed to physically bind PKCδ, because mutation of the retinol binding site rendered PKCδ unresponsive to Rol, while retaining responsiveness to phorbol ester. The PKCδ/retinol complex signaled the pyruvate dehydrogenase complex for enhanced flux of pyruvate into the Krebs cycle. The baseline response was reduced hi vitamin A-deficient lecithin:retinol acyl transferase-knockout mice, but this was corrected within 3 h by intraperitoneal injection of vitamin A; this suggests that vitamin A is physiologically important. These results illuminate a hitherto unsuspected role of vitamin A in mitochondrial bioenergetics of mammals, acting as a nutritional sensor. As such, retinol is of fundamental importance for energy homeostasis. The data provide a mechanistic explanation to the nearly 100-yr-old question of why vitamin A deficiency causes so many pathologies that are independent of retinoic acid action. [ABSTRACT FROM AUTHOR]