Absence of hepatic stellate cell retinoid lipid droplets does not enhance hepatic fibrosis but decreases hepatic carcinogenesis

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Abstract
Objective: Hepatic stellate cells (HSCs) contain a number of bioactive metabolites or their precursors including retinoids in their characteristic lipid droplets. The loss of lipid droplets and retinoids is a hallmark of HSC activation, but it remains unclear whether this loss promotes HSC activation, liver fibrogenesis or carcinogenesis. Design: Spontaneous and experimental fibrogenesis as well as a diethylnitrosamine-induced hepatocarcinogenesis were investigated in lecithin-retinol acyltransferase (LRAT)-deficient mice which lack retinoid-containing lipids droplets in their HSCs. Results: Following HSC activation, LRAT expression was rapidly lost, emphasising its importance in lipid droplet biology in HSCs. Surprisingly, there was no difference in fibrosis induced by bile duct ligation (BDL) or by eight injections of carbon tetrachloride (CCl 4 ) between wild-type and LRAT-deficient mice. To exclude the possibility that the effects on fibrogenesis were missed due to the rapid downregulation of LRAT following HSC activation, acute as well as spontaneous liver fibrosis was investigated. However, there was no increased fibrosis in 3-, 8- and 12-month-old LRAT-deficient mice and in LRAT-deficient mice after a single injection of CCl 4 compared with wild-type mice. To determine whether the absence of retinoids in HSCs affects hepatocarcinogenesis, wild-type and LRAT-deficient mice were injected with diethylnitrosamine. LRAT deficiency decreased diethylnitrosamine-induced injury and tumour load and increased the expression of the retinoic acid responsive genes Cyp26a1, RARb and p21, suggesting that the lower tumour load of LRAT-deficient mice was a result of increased retinoid signalling and subsequent p21-mediated inhibition of proliferation. Conclusions: The absence of retinoid-containing HSC lipid droplets does not promote HSC activation but reduces hepatocarcinogenesis.

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