Functional analysis and transcriptional regulation of porcine six transmembrane epithelial antigen of prostate 4 (STEAP4) gene and its novel variant in hepatocytes

Six-transmembrane epithelial antigen of prostate 4 (STEAP4) plays a critical role in modulating inflammatory response and protecting metabolic function. However, the role of STEAP4 in hepatocytes is not well understood, and the mechanism of STEAP4 action remains elusive. Here, we report the molecular characterization of porcine STEAP4 and its novel splice variant (STEAP4v), then the metabolic and anti-inflammatory roles of porcine STEAP4 and STEAP4v were investigated in HepG2 liver cells. The results revealed that overexpression of STEAP4, but not STEAP4v, suppresses triglyceride (TG) content and ameliorates the up-regulation of the transcription of the genes necessary for de novo lipogenesis and gluconeogenesis elicited by FFAs treatment. In RAW264.7 macrophage cells, transient transfection of STEAP4v, to a greater extent than STEAP4, repressed the transcription of TNFα and IL-6. In HepG2 cells with LPS treatment, the endogenous mRNA and protein levels of STEAP4 and STEAP4v were up-regulated, which was accompanied by a concurrent increase in C/EBPβ mRNA and protein levels. Thirdly, the functional regulation of STEAP4 was explored, which revealed that the porcine STEAP4 promoter activity was significantly up-regulated by C/EBPβ. The progressive deletions and mutations demonstrated that the C/EBPβ binding motif situated at −73/−59 bp is an essential component required for promoter activity of STEAP4 gene. Chromatin immunoprecipitation (ChIP) assays determined that C/EBPβ can directly interact with the steap4 promoter DNA. In conclusion, our data demonstrated that C/EBPβ directly regulates the roles of STEAP4 and its novel variant in attenuating lipogenesis, gluconeogenesis or/and inflammation elicited by FFAs or LPS in hepatocytes.