On the Role of c-Jun in the Induction of PAI-1 Gene Expression by Phorbol Ester, Serum, and IL-1α in HepG2 Cells

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Abstract
—We have characterized the regulation of plasminogen activator inhibitor-1 (PAI-1) gene expression by phorbol 12-myristate 13-acetate (PMA), serum, and interleukin-1α (IL-1α) in the human hepatoma cell line HepG2. PMA, serum, and IL-1α induced a rapid and transient 28-fold (PMA), 9-fold (serum), and 23-fold (IL-1α) increase in PAI-1 mRNA, peaking after ≈4 hours. These inductions of PAI-1 mRNA accumulation were reduced by pretreatment of the HepG2 cells with the protein tyrosine kinase inhibitor genistein. Conversely, stimulation of tyrosine phosphorylation by sodium orthovanadate, an inhibitor of protein tyrosine phosphatases, caused an increase in PAI-1 mRNA levels. The effects of PMA, serum, and IL-1α on PAI-1 mRNA expression have been compared with their ability to modulate the expression of a chloramphenicol acetyltransferase (CAT) reporter plasmid, which was under control of the −489 to +75 region of the PAI-1 promoter, and stably transfected into HepG2 cells. This region of the PAI-1 promoter was previously found to contain a tetradecanoyl phorbol acetate–response element (TRE; between −58 and −50) necessary for PMA responsiveness and with a high affinity for c-Jun homodimers. Whereas incubation of these transfected HepG2 cells with PMA and serum showed an induction profile of CAT mRNA similar to that of PAI-1 mRNA, hardly any induction of CAT mRNA was found with IL-1α. In line with these findings, IL-1α poorly induced c-Jun homodimer binding to the PAI-1 TRE in gel mobility-shift assays. Pretreatment of HepG2 cells with the protein kinase C inhibitor Ro 31-8220 or the mitogen-activated protein kinase kinase (MAPKK) 1,2 activity blocker PD98059 selectively suppressed the induction of PAI-1 (and CAT) expression by PMA, but not that by IL-1α. In contrast, the protein tyrosine kinase inhibitor herbimycin A blocked PAI-1 mRNA induction by IL-1 α only. We propose 2 separate PAI-1 inductory pathways for PMA and IL-1α in HepG2, both involving protein tyrosine kinase activation; the serum-induced signaling pathway may (partially) overlap with the PMA-activated protein kinase C/mitogen-activated protein kinase kinase pathway, leading to c-Jun homodimer binding to the PAI-1 TRE.