1. Department of Pharmacology, School of Basic Medical Science, Nanjing Medical University, Nanjing, Jiangsu 211166, China;
2. Jiangsu Key Laboratory of Drug Screening, China Pharmaceutical University, Nanjing, Jiangsu 210009, China;
3. Laboratory of Molecular Design and Drug Discovery, School of Science, China Pharmaceutical University, Nanjing, Jiangsu 211198, China;
4. Department of Immunology, Nanjing Medical University, Nanjing, Jiangsu 211166, China;
5. Key Laboratory of Rare Metabolic Diseases, Nanjing Medical University, Nanjing, Jiangsu 211166, China;
6. Department of Rehabilitation Medicine, Jiangsu Province People's Hospital and Nanjing Medical University First Affiliated Hospital, Nanjing, Jiangsu 210029, China.
*Yaru Li and Mingming Chen contributed equally to this work.
Background: Inflammation and steatosis are the main pathological features of alcoholic liver disease (ALD), in which, inflammation is one of the critical drivers for the initiation and development of alcoholic steatosis. NIK, an inflammatory pathway component activated by inflammatory cytokines, was suspected to link inflammation to hepatic steatosis during ALD. However, the underlying pathogenesis is not well-elucidated.
Methods: Alcoholic steatosis was induced in mice by chronic-plus-binge ethanol feeding. Both the loss- and gain-of-function experiments by the hepatocyte-specific deletion, pharmacological inhibition and adenoviral transfection of NIK were utilized to elucidate the role of NIK in alcoholic steatosis. Rate of fatty acid oxidation was assessed in vivo and in vitro. PPARα agonists or antagonists of MEK1/2 and ERK1/2 were used to identify the NIK-induced regulation of PPARα, MEK1/2, and ERK1/2. The potential interactions between NIK, MEK1/2, ERK1/2 and PPARα and the phosphorylation of PPARα were clarified by immunoprecipitation, immunoblotting and far-western blotting analysis.
Results: Hepatocyte-specific deletion of NIK protected mice from alcoholic steatosis by sustaining hepatic fatty acid oxidation. Moreover, overexpression of NIK contributed to hepatic lipid accumulation with disrupted fatty acid oxidation. The pathological effect of NIK in ALD may be attributed to the suppression of PPARα, the main controller of fatty acid oxidation in the liver, because PPARα agonists reversed NIK-mediated hepatic steatosis and malfunction of fatty acid oxidation. Mechanistically, NIK recruited MEK1/2 and ERK1/2 to form a complex that catalyzed the inhibitory phosphorylation of PPARα. Importantly, pharmacological intervention against NIK significantly attenuated alcoholic steatosis in ethanol-fed mice.
Conclusions: NIK targeting PPARα via MEK1/2 and ERK1/2 disrupts hepatic fatty acid oxidation and exhibits high value in ALD therapy.
Keywords: carnitine palmitoyl transferase 1α, mitogen-activated protein kinase/extracellular signal-regulated kinase kinase, extracellular signal-regulated kinase, proinflammatory cytokine