Tissue-type plasminogen activator (t-PA) gene expression is regulated by the tumor-promoting phorbol ester, phorbol-12-myristate 13-acetate (PMA), by cyclic AMP analogues, and the cAMP agonist, forskolin. Based on nuclear "run-on" transcription assays, t-PA expression is modulated by PMA on the level of transcription. 8-Bromo-cyclic AMP and forskolin do not induce t-PA gene transcription alone but act synergistically with PMA. These effects are confirmed by transient expression assays in HeLa cells employing deletion mutants of the t-PA gene promoter fused to the chloramphenicol acetyltransferase (CAT) reporter gene. Constitutive expression and most of the PMA-mediated induction requires sequences downstream of position -145. DNase I protection ("footprint") analysis of this region reveals two protein-binding sites: one between position -102 and -115, differing from the consensus sequence of the cAMP-responsive element (CRE) by the substitution of an adenine for a guanine in the middle of the core motif (TGACATCA), and another, located in the first exon (between position +60 and +74), displaying homology to the consensus sequence of the activator protein 2- (AP-2) binding site (CCCCACCCCC). Base substitutions in the core of either the CRE-like element or the AP-2 site suppress constitutive CAT expression by over 80%, whereas the relative PMA- and PMA plus cAMP-mediated responses are retained. CAT expression is below the detection limit when both elements are mutagenized together. Hence, the CRE-like element and the exon-located AP-2-binding site have a cooperative impact on basal transcription, but each element can independently convey the effect of activators of the protein kinase C- and A-dependent pathways of signal transduction. The results of band-shift analysis and competition titration experiments demonstrate that the CRE-like element acts as a low affinity binding site for the same proteins which recognize the authentic CRE.
Amanitins Base Sequence