Abstract
High-fat diet-induced obesity (DIO) is associated with fatty liver and elevated IL-6 circulating levels. IL-6 administration in rodents has yielded contradictory results regarding its effects on steatosis progression. In some models of fatty liver disease, high doses of human IL-6 ameliorate the liver steatosis, whereas restoration of IL-6 in DIO IL-6-/- mice up-regulates hepatic lipogenic enzymes and aggravates steatosis. We further examined the effects of chronic low doses of murine IL-6 on hepatic lipid metabolism in WT mice in DIO. IL-6 was delivered twice daily in C57BL/6J DIO mice for 15 days. The status and expression of IL-6-signalling mediators and targets were investigated in relation to the steatosis and lipid content in blood and in liver. IL-6 administration in DIO mice markedly raised circulating levels of lipids, glucose and leptin, elevated fat liver content and aggravated steatosis. Under IL-6 treatment there was hepatic Stat3 activation and increased gene expression of Socs3 and Tnf-alpha whereas the gene expression of endogenous IL-6, IL-6-receptor, Stat3, Cpt1 and the enzymes involved in lipogenesis was suppressed. These data further implicate IL-6 in fatty liver disease modulation in the context of DIO, and indicate that continuous stimulation with IL-6 attenuates the IL-6-receptor response, which is associated with high serum levels of leptin, glucose and lipids, the lowering levels of lipogenic and Cpt1 hepatic enzymes and with increased Tnf-alpha hepatic expression, a scenario evoking that observed in IL-6-/- mice exposed to DIO and in obese Zucker rats.
Highlights
Increased plasma IL-6 levels are normally associated with obesity and fatty liver disease [1,2,3,4], but the involvement of IL-6 in the molecular mechanisms underlying the pathogenesis of lipid and carbohydrate metabolism is not fully understood [5,6,7]
In a recent study we showed that the chronic replacement of IL-6 with physiological doses in IL-6-/- mice seriously aggravates the steatosis induced by a high-fat diet [12]
The fatty changes were more pronounced in HFD-fed mice treated with rIL-6, resulting in a severe macro- and microvesicular steatosis, with predominant accumulation of small lipid droplets in hepatocytes
Summary
Increased plasma IL-6 levels are normally associated with obesity and fatty liver disease [1,2,3,4], but the involvement of IL-6 in the molecular mechanisms underlying the pathogenesis of lipid and carbohydrate metabolism is not fully understood [5,6,7] It is a subject of excited debate in the literature [8,9,10,11,12,13]. The lack of IL-6 predisposes to liver steatosis, reinforcing a priori the idea that IL-6 contributes to alleviating steatosis [12, 22, 24] These beneficial effects were attributed in part to the ability of IL-6 to mediate mitochondrial beta-oxidation of fatty acids, increase the hepatic export of triglycerides and cholesterol, and to its antioxidant, anti-apoptotic effects on hepatocytes [10, 17, 18, 21]
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