Opposite and independent actions of cyclic AMP and transforming growth factor β in the regulation of type 1 plasminogen activator inhibitor expression

Abstract

We have investigated the mechanisms by which type 1 plasminogen activator inhibitor (PAI-1) is regulated by transforming growth factor β (TGF-β) and by epidermal growth factor (EGF) in CCL64 mink lung epithelial cells. BSC-1 monkey kidney epithelial cells. mouse embryo fibroblast (AKR-2B 84A) cells and normal rat kidney fibroblasts (NRK). TGF-β increases PAI-1 expression in all four cell lines, and EGF acts synergistically with TGF-β to increase PAI-1 expression in CCL64 cells but not in the other three cell lines. Here we show that PAI-1 expression can be regulated independently through two different signal transduction pathways. One pathway involves protein kinase C and is stimulated by the tumour promoter phorbol myristate acetate (PMA). Whereas preincubation with PMA completely eliminated PMA-induced PAI-1 synthesis and secretion in both CCL64 and BSC-1 cells, this treatment had no effect on TGF-β- and EGF-induced PAI-1 levels. Therefore we conclude that protein kinase C does not mediate the effects of either EGF or TGF-β on PAI-1 expression. The expression of PAI-1 was decreased by agents increasing intracellular cyclic AMP: (cAMP) cholera toxin, forskolin and dibutyryl cAMP lowered both the basal level and the TGF-β- and PMA-induced levels of PAI-1 expression. These effects of cAMP-elevating agents and of TGF-β on PAI-1 protein synthesis were also reflected in changes in TGF-β- induced PAI-1 gene transcription, as measured by nuclear run-on. These results show that PAI-1 gene expression is sensitive to high levels of intracellular cAMP and that this effect occurs at the transcriptional level. Although increased intracellular cAMP concentrations decrease the absolute level of PAI-1 expression, the ability of TGF-β- and EGF to induce PAI-1 gene expression is unchanged. These results are discussed in relation to the observation that sensitivity to cAMP is a common feature of TGF-β-regulated genes.