Ectopic expression of the cAMP‐responsive element binding protein inhibits phorbol ester‐mediated induction of tissue‐type plasminogen activator gene expression

The human tissue‐type plasminogen activator (t‐PA) gene is regulated in a cell‐type dependent manner. The t‐PA gene is transcriptionally induced by the phorbol ester PMA in HeLa cells, but suppressed by PMA in HT‐1080 cells. A cAMP responsive element (tPACRE) and a Sp‐1 site located within the proximal t‐PA gene promoter are functionally important in both cell systems. HeLa and HT‐1080 cells contain a different repertoire of factors that associate with the tPACRE. In HT‐1080 cells, CREB and c‐Jun are the two major t‐PACRE binding proteins identified, while activating transcription factor 2 (ATF‐2) is a predominant t‐PACRE binding protein in HeLa cells. To determine whether alteration in the distribution of tPACRE binding proteins would influence the differential regulation of the t‐PA gene in these cells, the tPACRE binding profiles in these two cell systems were manipulated by over expressing ATF‐2 in HT‐1080 cells and CREB in HeLa cells. Supershift experiments confirmed that the overexpression of these factors resulted in binding to the tPACRE site. However, the presence of ATF‐2 in HT‐1080 cells did not affect either constitutive or PMA‐mediated suppression of the endogenous t‐PA gene. In contrast, enforced tPACRE‐binding activity of CREB in HeLa cells significantly reduced the magnitude of PMA‐mediated induction of t‐PA mRNA in HeLa cells. These results indicate that the introduction of CREB into HeLa cells disrupts the regulation of the t‐PA gene.