Macrophage p38α promotes nutritional steatohepatitis through M1 polarization

Background & aims: p38 mitogen-activated protein kinases are important inflammatory factors. p38α alteration has been implicated in both human and mouse inflammatory disease models. Therefore, we aimed to characterize the cell type-specific role of p38α in non-alcoholic steatohepatitis (NASH).

Methods: Human liver tissues were obtained from 27 patients with non-alcoholic fatty liver disease (NAFLD) and 20 control individuals. NASH was established and compared between hepatocyte-specific p38α knockout (p38α^ΔHep), macrophage-specific p38α knockout (p38α^ΔMΦ) and wild-type (p38α^fl/fl) mice fed with high-fat diet (HFD), high-fat/high-cholesterol diet (HFHC), or methionine-and choline-deficient diet (MCD). p38 inhibitors were administered to HFHC-fed wild-type mice for disease treatment.

Results: p38α was significantly upregulated in the liver tissues of patients with NAFLD. Compared to p38α^fl/fl littermates, p38α^ΔHep mice developed significant nutritional steatohepatitis induced by HFD, HFHC or MCD. Meanwhile, p38α^ΔMΦ mice exhibited less severe steatohepatitis and insulin resistance than p38α^fl/fl mice in response to a HFHC or MCD. The effect of macrophage p38α in promoting steatohepatitis was mediated by the induction of pro-inflammatory factors (CXCL2, IL-1β, CXCL10 and IL-6) secreted by M1 macrophages and associated signaling pathways. p38α^ΔMΦ mice exhibited M2 anti-inflammatory polarization as demonstrated by increased CD45⁺F4/80⁺CD11b⁺CD206⁺ M2 macrophages and enhanced arginase activity in liver tissues. Primary hepatocytes from p38α^ΔMΦ mice showed decreased steatosis and inflammatory damage. In a co-culture system, p38α deleted macrophages attenuated steatohepatitic changes in hepatocytes through decreased secretion of pro-inflammatory cytokines (TNF-α, CXCL10 and IL-6), which mediate M1 macrophage polarization in p38α^ΔMΦ mice. Restoration of TNF-α, CXCL10 or IL-6 induced lipid accumulation and inflammatory responses in p38α^fl/fl hepatocytes co-cultured with p38α^ΔMΦ macrophages. Moreover, pharmacological p38 inhibitors suppressed HFHC-induced steatohepatitis.

Conclusions: Macrophage p38α promotes the progression of steatohepatitis by inducing pro-inflammatory cytokine secretion and M1 polarization. p38 inhibition protects against steatohepatitis. LAY SUMMARY: p38 mitogen-activated protein kinases are important inflammatory factors. In the present study, we demonstrated that p38α is upregulated in liver tissues of patients with non-alcoholic fatty liver diseases. Genetic deletion of p38α in macrophages led to ameliorated nutritional steatohepatitis in mice through decreased pro-inflammatory cytokine secretion and increased M2 macrophage polarization.