AP-1 proteins mediate hyperglycemia-induced activation of the human TGF-β1 promoter in mesangial cells

Abstract

Hyperglycemia-induced overproduction of the prosclerotic cytokine transforming growth factor-β1 (TGF-β1) has been implicated in the pathogenesis of diabetic nephropathy. Because high glucose and phorbol esters (PMA) increase TGF-β1 mRNA levels in mesangial cells, this study was designed to characterize these effects on the human TGF-β1 promoter activity. With the use of luciferase reporter gene constructs containing TGF-β1 5'-flanking sequence (from -453 to + 11 bp) transfected into mesangial cells, it was found that 30 mM glucose induced a nearly twofold increase in TGF-β1 promoter activity after 24 h of incubation in human and porcine mesangial cells. Stimulation by PMA was more effective (2.3-fold). Mutagenesis in either one of the two or both activating protein-1 (AP-1) binding sites abolished the high glucose and the PMA effect. Furthermore, addition of the AP-1 inhibitor curcumin obliterated the glucose response. Corresponding experiments revealed that the transcription factor stimulating protein 1 was not involved in mediating the glucose effect. The high glucose-induced TGF-β1 promoter activation was also prevented by inhibitors of protein kinase C and p38 mitogen-activated proteinkinase. Electrophoretic mobility shift assays with oligonucleotides containing one of the two AP-1 binding sites showed that glucose treatment markedly enhanced the binding activity of nuclear proteins of mesangial cells, particularly to box B. Supershift assays demonstrated that JunD and c-Fos were present in the protein-DNA complexes under control and hyperglycemic conditions. The functional and structural results show that glucose regulates human TGF-β1 gene expression through two adjacent AP-1 binding sites and gives rise to the involvement of protein kinase C and p38 mitogen-activated proteinkinase in hyperglycemia-induced TGF-β1 gene expression.