Notch ligand, JAG1, is evolutionarily conserved target of canonical WNT signaling pathway in progenitor cells

WNT, Notch, FGF, and Hedgehog signaling pathways network together during embryogenesis, tissue regeneration, and carcinogenesis. Association of Notch ligands with Notch receptors on neighboring cells leads to cleavage of Notch receptors by metalloprotease and γ-secretase to induce nuclear translocation of Notch intracellular domain (NICD). Nuclear complex, consisting of CSL (RBPSUH), NICD, Mastermind (MAML), p300 and histone acetyltransferase (HAT), then induces transcriptional activation of Notch target genes, such as HES1, HES5, HES7, HEY1, HEY2 and HEYL. Here, we searched for TCF/LEF-binding site within the promoter region of Notch ligand genes, including DLL1, DLL3, DLL4, JAG1 and JAG2. Because TCF/LEF-binding sites were identified within human JAG1 promoter based on bioinformatics and human intelligence, comparative genomics analyses on JAG1 orthologs were further performed. Chimpanzee JAG1 gene, consisting of 26 exons, was identified within NW_120319.1 genome sequence. XM_525264.1 and XM_514517.1 were not the correct coding sequences for chimpanzee JAG1. Chimpanzee JAG1 gene was found to encode a 1218-amino-acid protein showing 99.5% and 96.2% total-amino-acid identity with human JAG1 and mouse Jag1, respectively. Phylogenetic analysis revealed that JAG1 orthologs were more conserved than those of other Notch ligands. JAG1 gene was identified as evolutionarily conserved target of WNT/β-catenin signaling pathway based on the conservation of double TCF/LEF-binding sites within 5'-promoter region of mammalian JAG1 orthologs. Human JAG1 mRNA was expressed in embryonic stem (ES) cells, neural tissues, lung carcinoid, gastric cancer, pancreatic cancer, colon cancer, and also in squamous cell carcinoma (SCC) of skin, oral cavity, esophagus, head and neck. JAG1 expression on progenitor cells due to canonical WNT signaling activation induces self-renewal of stem cells due to Notch signaling activation. JAG1, functioning as WNT-dependent Notch signaling activator, is the key molecule maintaining the homeostasis of stem and progenitor cells.