Involvement of JNK signaling pathway in lipopolysaccharide-induced complement C3 transcriptional activation from amphioxus Branchiostoma belcheri.

Complement C3 is a pivotal component of three cascades of complement activation. C3 in circulation is mainly provided by the hepatic cecum. The expression and secretion of C3 by hepatocytes is increased during acute inflammation. The detailed information on the regulationary mechanism underlying C3 transcriptional activation is limited. Here, we characterized the 5'-flanking region of the amphioxus C3 gene. To functionally analyze the upstream regulatory region of the C3 gene, a series of luciferase reporter gene constructs containing deleted or mutant regulatory elements were prepared. Using luciferase assay, we revealed that a potential C-JUN-1 binding sites within the proximal promoter region were necessary for full activation of the C3 promoter, whereas NF-κB, AP-1, C-JUN-2 and NFAT transcription factor binding sites played roles in governing the promoter activity at a homeostatic level. Our data also indicated that sp600125, a c-Jun N-terminal kinase (JNK) inhibitor, decreased lipopolysaccharide (LPS)-stimulated C3 promoter activity, mRNA expression and protein secretion using western blotting and quantitative real-time PCR analysis. These findings demonstrated that JNK signaling pathway is involved in the regulation of C3 gene transcription by targeting C-JUN transcription factor binding sites in the 5'-flanking promoter region, leading to LPS-induced C3 activation and therefore providing a potential target for regulating C3 expression.