Chi F, Tsukamoto H, Xu J
Department of Pathology, Southern California Research Center for ALPD and Cirrhosis, Keck School of Medicine of the University of Southern California, Los Angeles, CA, USA
Macrophage proinflammatory (M1) activation is the key event in pathogenesis of alcoholic steatohepatitis (ASH), while alternative (M2) activation is associated with tumor promotion and progression. Notch is highly conserved pathway integrates environmental cues to specify cell fate. Notch signaling has been shown to regulate macrophage polarization in vitro, and aberrant Notch activity in hepatocytes leads to liver cancer. To study the mechanism through with Notch regulates macrophage activation in alcoholic liver disease, we generated macrophage Notch1-deficient (N1KO) and Notch2-deficient (N2KO) mice by crossing Notch1flox/flox and Notch2flox/flox mice with LysM-Cre mice, respectively. ASH was induced by intragastric feeding high-fat diet plus alcohol, and liver cancer by i.p. injection of diethylnitrosamine (DEN). We find that Notch1, but not Notch2 pathway is activated in macrophages isolated from the ASH mice. In cell culture, N1KO attenuates LPS-induced hepatic macrophage M1 activation, but it has minimal effect on IL-4 induced M2 activation. N2KO does not prevent LPS- nor IL-4-induced macrophage activations. ChIP-seq reveals Notch1 intracellular domain (NICD1) binding to promoters of M1 genes and mitochondrial enzyme pyruvate dehydrogenase phosphatase 1 (Pdp1). The increased PDP1 expression leads to activation of pyruvate dehydrogenase (PDH) in M1 macrophages, resulting in increased glucose oxidation and mitochondrial ROS. The change of metabolism is also associated with Notch1 dependent upregulation of mitochondrion encoded respiratory chain components. Inhibition of Notch1 or Pdp1 abrogates glucose oxidation, mitochondrial ROS, and M1 gene expression. In vivo, myeloid N1KO, not N2KO, attenuates hepatic macrophage M1 activation and inflammation in ASH mice. However, N2KO decreases while N1KO aggravates DEN-induced, alcohol-promoted liver cancer. Expression of Notch1 and Notch2 receptors is elevated in liver tumors compared to non-tumor bearing liver tissues. Paradoxically, expression of Notch ligands is low in the tumors relatively to the liver tissues. In conclusion, our studies disclose isoform-specific effects of myeloid Notch on hepatic macrophage differentiation, liver inflammation, and tumorigenesis. Notch1 dictates M1 activation by coupling transcriptional upregulation of M1 genes with metabolic upregulation of mitochondrial oxidative phosphorylation and ROS to augment the M1 gene induction. Myeloid Notch1 and Notch2 have contrasted effects on liver tumorigenesis through a mechanism(s) yet to be identified.