Nuclear Receptor Small Heterodimer Partner Inhibits Proinflammatory Macrophage Polarization in Liver Disease
Authors: Ahamed Forkan , Magee Nancy , Zhang Yuxia 
Published: April 2020

Objective

Macrophages are the first responding cells to hepatocyte injuries and play a central role in innate immune response during the pathogenesis of liver diseases. Macrophage polarization to pro‐inflammatory M1 or anti‐inflammatory M2 sub‐populations is a critical event of inflammatory response which balances tissue injury and repair. Various types of signaling pathways have been identified that direct macrophage polarization; however, the regulatory mechanisms that coordinate the pathways are still not well understood. In this study we investigated the role of small heterodimer partner (Shp), an orphan nuclear receptor, in regulating the signaling pathways involved in macrophage polarization during the development of liver diseases.

Methods

Primary macrophages isolated from mouse liver were differentiated into pro‐inflammatory M1 or anti‐inflammatory M2 macrophages and Shp expression was determined during the differentiation. Mouse macrophage cell line RAW 264.7 overexpressed with Shp was treated with lipopolysaccharide (LPS) to evaluate inflammatory response. For the in vivo studies, we generated macrophage‐specific Shp deficient mice (Shp‐MKO represents Shp flox/flox; LysMCre+/−) and littermate controls (Shp‐WT represents Shp flox/flox; LysMCre−/−). Two different animal models were used to assess inflammatory responses in the liver, including high fat, cholesterol, and fructose (HFCF) diet‐induced nonalcoholic steatohepatitis (NASH) model and LPS‐induced acute inflammatory model.

Results

Shp expression was down‐regulated during pro‐inflammatory M1 macrophage differentiation and up‐regulated in anti‐inflammatory M2 macrophage differentiation. Overexpression of Shp in Raw 264.7 cells resulted in inhibition of LPS‐induced activation of NF‐κB pathway compared to control cells. In addition, Shp overexpression downregulated MAP kinase pathway activation by inhibiting the phosphorylation of MKK4, JNK and c‐jun. Consistently, the in vivo study showed that both NF‐κB and MAP kinase signaling pathways were highly activated in the Shp‐MKO with the increased serum levels of TNF‐α and CCL2 after LPS treatment. Strikingly, macrophage polarization toward pro‐inflammatory M1 stage was enhanced in the Shp‐MKO liver in comparison to the Shp‐WT, which is evidenced by increased iNOS, IL‐6 and TNF‐α production upon LPS injection. On the molecular level we found that SHP interacted with proteins involved in NF‐κB and MAP kinase pathways and thereby coordinated macrophage polarization. Finally, without affecting hepatocyte steatosis Shp‐MKO mice were more susceptible to HFCF‐induced NASH progression with monocyte infiltration and liver fibrosis.

Conclusion

Our study uncovered a unique role of SHP in regulating macrophage polarization that couples both NF‐κB and MAP kinase pathways, which impacts the pathogenesis of acute and chronic inflammatory liver diseases.

Support or Funding Information

This project was supported by grants NCI K22CA184146, R01DK119131 and NIH P20 GM103549.