Posttranscriptional regulation of expression of plasminogen activator inhibitor type-1 by cAMP in HepG2 liver cells

Abstract

Altered expression of plasminogen activator inhibitor type-1 (PAI-1), a physiologic fibrinolysis inhibitor, is implicated in atherosclerosis. Cyclic adenosine monophosphate (cAMP) alters PAI-1 expression in several cells. Nevertheless, posttranscriptional regulation of PAI-1 has not been elucidated. To determine whether cAMP affects PAI-1 expression at posttranscriptional level, we determined promoter activity, mRNA levels, 3'-untranslated region (UTR) activity and protein levels of PAI-1 using HepG2 cells. cAMP decreased PAI-1 promoter activity at 24 h and mRNA expression at 4 h while it increased mRNA expression and accumulation of PAI-1 protein into media at 24 h. Human PAI-1 mRNA exists in two subspecies (3.2 and 2.2 kb), and cAMP increased baseline luciferase activity of 3'-UTR of the 3.2 kb PAI-1 mRNA [3'-UTR (+1358-3176)] and 1 kb fragment of 3'-terminus of 3'-UTR of 3.2 kb mRNA [3'-UTR (+2177-3176)]. cAMP increased PAI-1 protein expression despite decrease in promoter activity, presumably by regulating PAI-1 expression at the posttranscriptional level and thereby affecting mRNA stability. The 53-nt fragment in 3'-UTR (+2591 to +2643 nt) was involved in posttranscriptional regulation by cAMP. Thus, cAMP can stabilize 3.2 kb PAI-1 mRNA mediated by specific effects on 3'-UTR, and these effects are associated with increased expression of PAI-1 protein.