Alcohol dehydrogenase III exacerbates liver fibrosis by enhancing stellate cell activation and suppressing natural killer cells in mice

Abstract

The important roles of retinols and their metabolites have recently been emphasized in the interactions between hepatic stellate cells (HSCs) and natural killer (NK) cells. Nevertheless, the expression and role of retinol metabolizing enzyme in both cell types have yet to be clarified. Thus, we investigated the expression of retinol metabolizing enzyme and its role in liver fibrosis. Among several retinol metabolizing enzymes, only alcohol dehydrogenase (ADH) 3 expression was detected in isolated HSCs and NK cells, whereas hepatocytes express all of them. In vitro treatment with 4-methylpyrazole (4-MP), abroad ADH inhibitor, or depletion of the ADH3 gene downregulated collagen and transforming growth factor-β1 (TGF-β1) gene expression, but did not affect α-smooth muscle actin gene expression in cultured HSCs. Additionally, in vitro, treatments with retinol suppressed NK cell activities, whereas inhibition of ADH3 enhanced interferon-γ (IFN-γ) production and cytotoxicity of NK cells against HSCs. In vivo, genetic depletion of the ADH3 gene ameliorated bile duct ligation- and carbon tetrachloride-induced liver fibrosis, in which a higher number of apoptotic HSCs and an enhanced activation of NK cells were detected. Freshly isolated HSCs from ADH3-deficient mice showed reduced expression of collagen and TGF-β1, but enhanced expression of IFN-γ was detected in NK cells from these mice compared with those of control mice. Using reciprocal bone marrow transplantation of wild-type and ADH3-deficient mice, we demonstrated that ADH3 deficiency in both HSCs and NK cells contributed to the suppressed liver fibrosis. Conclusion ADH3 plays important roles in promoting liver fibrosis by enhancing HSC activation and inhibiting NK cell activity, and could be used as a potential therapeutic target for the treatment of liver fibrosis.

Pathobiology of liver fibrosis: a translational success story

“…30 An additional contribution of retinoids may be through the conversion of retinol to retinaldehyde, then retinoic acid, which may be linked to collagen expression and transforming growth factor beta (TGFβ) activation through alcohol dehydrogenase 3. 31 Retinoic acid then sensitises natural killer (NK) cells to kill stellate cells, thereby contributing to an antifibrotic effect. 32 In contrast, retinol released from stellate cells suppresses NK cells, which reduces stellate cell killing and enhances their contribution to fibrosis.…”

Stellate Cells and Hepatic Fibrosis

“…The ADH enzymes within HSCs do not contribute meaningfully to ethanol metabolism within the liver [7]. ADH3, which is the dominant subtype within HSCs, is the major enzyme responsible for the oxidation of retinols released from cytoplasmic droplets, is a positive regulator of HSC activation, and is a negative regulator of NK cells in the setting of liver fibrosis [154]. It is uncertain whether these functions are co-regulated, as ADH3 KO mice develop similar degrees of fibrosis as WT mice [154].…”

“…ADH3, which is the dominant subtype within HSCs, is the major enzyme responsible for the oxidation of retinols released from cytoplasmic droplets, is a positive regulator of HSC activation, and is a negative regulator of NK cells in the setting of liver fibrosis [154]. It is uncertain whether these functions are co-regulated, as ADH3 KO mice develop similar degrees of fibrosis as WT mice [154]. Genome-wide association studies and population-based studies have identified an association between a common polymorphism in PNPLA3 (I148M) and the severity of liver disease including fibrosis and steatosis, initially in NAFLD/NASH but now across a range of liver diseases.…”

Stellate Cells in Alcoholic Hepatitis

“…Several reports suggest that retinol metabolites can activate HSC via activation of latent TGF-β, as well as increase expression of suppressor of cytokine signaling 1 (SOCS 1) and pro-collagen 1α proteins [48]. There is also likely interaction between retinoic acid and suppression of NK cell activity; decreased NK cell activity is associated with decreased apoptosis of activated HSC [47]. While these studies clearly demonstrate a role for the enzymatic machinery of ethanol metabolism in regulation of HSC activation and function, very little is known about the influence of ethanol exposure on these pathways.…”

“…Alcohol dehydrogenase III (ADH III) is the major enzyme responsible for conversion of retinol to retinaldehyde in HSC [47]. Retinaldehyde is further metabolized to retinoic acid by retinaldehyde dehydrogenase enzyme.…”