Species-specific differential cleavage and polyadenylation of plasminogen activator inhibitor type 1 hnRNA.

Plasminogen activator inhibitor type 1 (PAI-1) is the primary physiologic inhibitor of the naturally occurring plasminogen activators. In higher primates two forms of mature PAI-1 mRNA (3.2 kb and 2.2 kb) arise by alternative cleavage and polyadenylation of PAI-1 hnRNA which is regulated in a tissue-specific fashion in humans. In other mammals only the 3.2 kb mRNA has been detected. The putative downstream polyadenylation site in humans that gives rise to the 3.2 kb PAI-1 mRNA consists of three overlapping copies of the consensus polyadenylation sequence while no consensus polyadenylation sequence is found upstream at a position that could generate the shorter mRNA species. To determine whether differential cleavage and polyadenylation of PAI-1 mRNA is due to species-specific differences in trans-acting factors that process PAI-1 mRNA or to the presence of a nonconsensus polyadenylation site acquired recently during primate evolution we prepared plasmids in which the 3' nontranslated region of the human PAI-1 gene or the mouse PAI-1 cDNA was inserted downstream of the neomycin gene in the plasmid pSV2neo. We show that the 3'-nontranslated region of the human PAI-1 gene but not the mouse PAI-1 cDNA conferred alternative cleavage and polyadenylation to the neomycin gene in transfected human Hep G2 cells as well as mouse NIH3T3 and rat L6 cells.     

Reactive oxygen species upregulate expression of PAI-1 in endothelial cells

Second messengers involved in the signal transduction pathway leading to induction of the plasminogen activator inhibitor (PAI-1) have not yet been well characterized. This study focuses on the mechanisms of regulation of PAI-1 expression by reactive oxygen species (ROS) in human endothelial cells. Inhibition of the tumor necrosis factor alpha (TNFalpha?-induced expression of PAI-1 by antioxidant N-acetyl-L-cysteine (NAC) indicated redox-sensitive mechanisms involved in the signaling pathway. Because TNFalpha induces PAI-1 production in endothelial cells, and NAC attenuated this response, we attempted to investigate the possible involvement of ROS in the activation of PAI-1 by TNFalpha. Upregulation of PAI-1 expression in endothelial cells by the stimulation with TNFalpha (50 ng/ml) or H2O2 (10-200 micro M), observed by measurement of the antigen and mRNA levels, was reversed in the presence of NAC (20mM). The stimulatory effect of ROS was detected also at the level of the PAI-1 promoter in endothelial cells transfected with plasmid p800 LUC containing a PAI-1 promoter fragment (+71 to -800). The PAI-1 promoter activity was increased in the presence of ROS, and was suppressed by up to 75% in the presence of antioxidants. On the basis of this study we can conclude that reactive oxygen species play an important role in a cytokine-induced activation of PAI-1 expression, and may act as a signal transduction messenger.     

Natriuretic peptides reduce plasminogen activator inhibitor-1 expression in human endothelial cells

Plasma concentrations of natriuretic peptides increase in some pathological conditions, but very little is known about the effect of these vasodilator peptides on the regulation of the blood coagulation system. The fundamental role in the regulation of fibrinolysis is played by plasminogen activator inhibitor type 1 (PAI-1). Recent studies demonstrate that natriuretic peptides can modulate PAI-1 expression in bovine aortic smooth muscle cells and rat aortic endothelial cells. In this report, we tested the effect of natriuretic peptides on PAI-1 expression in the human endothelial cell line (EA.hy 926). For this purpose, we treated the cell cultures with ANP, BNP and CNP, and modulation of PAI-1 synthesis was evaluated. We compared the effect of natriuretic peptides on synthesis and release of PAI-1 in unstimulated cells, and after activation with tumour necrosis factor alpha (TNFalpha). Natriuretic peptides abolished TNFalpha - induced upregulation of PAI-1 expression at both the PAI-1 mRNA and the antigen levels. The inhibitory efficiency was higher in the case of CNP when compared to that produced by ANP and BNP, particularly when TNFalpha-stimulated cells were used. We observed an inhibition of stimulatory effect of TNFalpha on PAI-1 expression also at the level of the PAI-1 promoter in cells transfected with a PAI-1 promoter fragment (+71 to -800). The PAI-1 promoter activity was markedly inhibited by C-type natriuretic peptide, already at a very low (0.001 micro M) concentration of the peptide.     

Regulation of the expression of type 1 plasminogen activator inhibitor in Hep G2 cells by epidermal growth factor

To identify factors potentially influencing expression of type 1 plasminogen activator inhibitor (PAI-1), we characterized the human tissue-specific distribution of PAI-1 mRNA and the influence of epidermal growth factor (EGF) on expression of steady state levels of PAI-1 mRNA and secretion of PAI-1 by Hep G2 cells. Two species of PAI-1 mRNA (3.2 and 2.2 kilobases) were detected, and the ratio of the two varied among tissues (3 to 5:1) in contrast to the 1:1 ratio detected in Hep G2 cells. Expression of PAI-1 mRNA was inversely related to the distribution of tissue-type plasminogen activator mRNA (2.3 kilobases). Nu-Serum, a growth media supplement, increased steady state levels of PAI-1 mRNA 5-fold within 3 h. Factors responsible were found to be trypsin-sensitive and dialysis-resistant. Antisera to EGF attenuated Nu-Serum-induced increases of PAI-1 mRNA by 57%, suggesting that EGF or EGF homologous peptides contributed to the response. EGF elicited increases of PAI-1 mRNA levels in a dose-dependent manner. Induction was rapid (7-fold at 3 h with 5 ng/ml) and complete within 10 h. The response was not attenuated by cycloheximide (25 micrograms/ml). Factor X and glyceraldehyde-3-phosphate dehydrogenase mRNA did not increase. Increased levels of PAI-1 antigen were detected in conditioned media of Hep G2 cells by 4 h and were maximal at 8 h (6-fold). We conclude that the expression of PAI-1 mRNA is tissue-specific and regulated by epidermal growth factor in Hep G2 cells.     

Transforming growth factor beta inhibits plasminogen activator (PA) activity and stimulates production of urokinase-type PA, PA inhibitor-1 mRNA, and protein in rat osteoblast-like cells.

Transforming growth factor beta (TGF beta) treatment of rat osteoblast-rich calvarial cells or of the clonal osteogenic sarcoma cells, UMR 106-01, resulted in dose-dependent inhibition of plasminogen activator (PA) activity, and increased production of 3.2 kb mRNA and protein for PA inhibitor -1 (PAI-1). Although tissue-type PA (tPA) protein was not measured, TGF beta did not influence production of mRNA for tPA. Production of 2.3 kb mRNA for urokinase-type PA (uPA) was also increased by TGF beta in a dose-dependent manner. The effects of TGF beta on synthesis of mRNA for PAI-1 and uPA were maintained when protein synthesis was inhibited, and were abolished by inhibition of RNA synthesis. Although uPA had not been detected previously as a product of rat osteoblasts, treatment of lysates of osteoblast-like cells with plasmin yielded a band of PA activity on reverse fibrin autography, corresponding to a low Mr form of uPA. Untreated conditioned media from normal osteoblasts or UMR 106-01 cells contained no significant TGF beta activity, but activity could be detected in acidified medium. Treatment of conditioned media with plasmin resulted in activation of approximately 50% of the TGF beta detectable in acidified media. The results identify several effects of TGF beta on the PA-PA inhibitor system in osteoblasts. Net regulation of tPA activity through the stimulatory actions of several calciotropic hormones and the promotion of PAI-1 formation by TGF beta could determine the amount of osteoblast-derived TGF beta activated locally in bone. Stimulation of osteoblast production of mRNA for uPA could reflect effects on the synthesis of sc-uPA, a precursor for the active form of the enzyme.     

Cyclic adenosine monophosphate induces plasminogen activator inhibitor-1 expression in human mast cells

Plaminogen activator inhibitor-1 (PAI-1), the key physiological inhibitor of the plasmin fibrinolytic system, plays important roles in the pathogenesis of asthma. Mast cells (MCs) are crucial effector cells and a major source of PAI-1 for asthma. Cyclic adenosine monophosphate (cAMP) is the important regulator of MCs; however, its effects on PAI-1 expression in MCs remain unknown. We reported cAMP/protein kinase A pathway positively regulates PAI-1 expression through cAMP-response element binding protein binding to hypoxia response element-1 at −158 to −153 bp of human PAI-1 promoter in human MCs. Moreover, cAMP synergistically augments PAI-1 expression with ionomycin- or IgE receptor cross-linking-mediated stimulation.