The cJun NH 2 -terminal kinase (JNK) signaling pathway in the liver promotes systemic changes in metabolism by regulating peroxisome proliferator-activated receptor α (PPARα)-dependent expression of the hepatokine fibroblast growth factor 21 (FGF21). Hepatocyte-specific gene ablation studies demonstrated that the Mapk9 gene (encoding JNK2) plays a key mechanistic role. Mutually exclusive inclusion of exons 7a and 7b yields expression of the isoforms JNK2α and JNK2β. Here we demonstrate that Fgf21 gene expression and metabolic regulation are primarily regulated by the JNK2α isoform. To identify relevant substrates of JNK2α, we performed a quantitative phosphoproteomic study of livers isolated from control mice, mice with JNK deficiency in hepatocytes, and mice that express only JNK2α or JNK2β in hepatocytes. We identified the JNK substrate retinoid X receptor α (RXRα) as a protein that exhibited JNK2α-promoted phosphorylation in vivo. RXRα functions as a heterodimeric partner of PPARα and may therefore mediate the effects of JNK2α signaling on Fgf21 expression. To test this hypothesis, we established mice with hepatocyte-specific expression of wild-type or mutated RXRα proteins. We found that the RXRα phosphorylation site Ser 260 was required for suppression of Fgf21 gene expression. Collectively, these data establish a JNK-mediated signaling pathway that regulates hepatic Fgf21 expression.

The HER2 + subtype of human breast cancer is associated with the malignant transformation of luminal ductal cells of the mammary epithelium. The sequence analysis of tumor DNA identifies loss of function mutations and deletions of the MAP2 K4 and MAP2 K7 genes that encode direct activators of the JUN NH 2 -terminal kinase (JNK). We report that in vitro studies of human mammary epithelial cells with CRISPR-induced mutations in the MAPK and MAP2K components of the JNK pathway caused no change in growth in 2D culture, but these mutations promoted epithelial cell proliferation in 3D culture. Analysis of gene expression signatures in 3D culture demonstrated similar changes caused by HER2 activation and JNK pathway loss. The mechanism of signal transduction cross-talk may be mediated, in part, by JNK-suppressed expression of integrin α6β4 that binds HER2 and amplifies HER2 signaling. These data suggest that HER2 activation and JNK pathway loss may synergize to promote breast cancer. To test this hypothesis, we performed in vivo studies using a mouse model of HER2 + breast cancer with Cre/loxP -mediated ablation of genes encoding JNK ( Mapk8 and Mapk9 ) and the MAP2K ( Map2 k4 and Map2 k7 ) that activate JNK in mammary epithelial cells. Kaplan–Meier analysis of tumor development demonstrated that JNK pathway deficiency promotes HER2 + -driven breast cancer. Collectively, these data identify JNK pathway genes as potential suppressors for HER2 + breast cancer.