Reduced Esterification Rather Than Increased Hydrolysis Is Causative for Loss of Hepatic Retinoids Upon CCl₄-Induced...

Advanced liver disease leads to liver fibrosis that is characterised by the activation of non-parenchymal stellate cells, accumulation of extracellular matrix proteins, and the loss of hepatic vitamin A stores. To date, the molecular mechanisms and enzymes mediating the loss of hepatic vitamin A stores are incompletely understood.

Using a fibrosis mouse model induced by the hepatotoxin carbon tetrachloride (CCl₄), we investigated which cellular processes in the liver mediate the loss of hepatic retinyl ester stores. We found that repeated CCl₄ injections into mice over six weeks led to a biphasic change in plasma retinol levels that were increased after three and decreased after six weeks as compared to control mice. As expected, livers of mice receiving CCl₄ injections showed increased expression of pro-fibrogenic genes that were accompanied by decreased hepatic retinoid content, which was mainly due to loss of retinoids in non-parenchymal cells (NPCs). In the liver and NPCs, decreased retinyl ester levels correlated with reduced gene expression of lecithin:retinol acyltransferase and reduced hepatic ex vivo retinol acyltransferase activity. Conversely, gene expression of lipases known to exhibit retinyl ester hydrolase activity (REHA) remained unchanged or was decreased, consistent with decreased neutral REHA in homogenates of respective livers and lysates of isolated NPCs, respectively. Albeit hepatic expression levels of marker proteins for autophagosomal and lysosomal membranes were increased, gene expression of the major acidic retinyl ester hydrolase, lysosomal acid lipase, as well as ex vivo acidic REHA were reduced in NPC lysates.

Together, these results indicate that the loss of hepatic retinyl ester stores upon liver injury and stellate cell activation is rather a consequence of reduced retinol esterification than neutral or acidic hydrolysis.