Role of CAGA boxes in the plasminogen activator inhibitor-1 promoter in mediating oxidized low-density lipoprotein-induced transcriptional activation in mesangial cells

PAI-1 in tissue fibrosis

Fibrosis is defined as a fibroproliferative or abnormal fibroblast activation-related disease. Deregulation of wound healing leads to hyperactivation of fibroblasts and excessive accumulation of extracellular matrix (ECM) proteins in the wound area, the pathological manifestation of fibrosis. The accumulation of excessive levels of collagen in the ECM depends on two factors: an increased rate of collagen synthesis and or decreased rate of collagen degradation by cellular proteolytic activities. The urokinase/tissue type plasminogen activator (uPA/tPA) and plasmin play significant roles in the cellular proteolytic degradation of ECM proteins and the maintenance of tissue homeostasis. The activities of uPA/tPA/plasmin and plasmin-dependent MMPs rely mostly on the activity of a potent inhibitor of uPA/tPA, plasminogen activator inhibitor-1 (PAI-1). Under normal physiologic conditions, PAI-1 controls the activities of uPA/tPA/plasmin/MMP proteolytic activities and thus maintains the tissue homeostasis. During wound healing, elevated levels of PAI-1 inhibit uPA/tPA/plasmin and plasmin-dependent MMP activities, and, thus, help expedite wound healing. In contrast to this scenario, under pathologic conditions, excessive PAI-1 contributes to excessive accumulation of collagen and other ECM protein in the wound area, and thus preserves scarring. While the level of PAI-1 is significantly elevated in fibrotic tissues, lack of PAI-1 protects different organs from fibrosis in response to injury-related profibrotic signals. Thus, PAI-1 is implicated in the pathology of fibrosis in different organs including the heart, lung, kidney, liver, and skin. Paradoxically, PAI-1 deficiency promotes spontaneous cardiac-selective fibrosis. In this review, we discuss the significance of PAI-1 in the pathogenesis of fibrosis in multiple organs.

Bone morphogenetic protein-7 inhibits proximal tubular epithelial cell Smad3 signaling via increased SnoN expression

Bone morphogenetic protein-7 (BMP-7) improves outcome in animal models of fibrotic renal disease by opposing transforming growth factor beta1 (TGF-beta)-dependent fibrosis. However, the underlying mechanisms remain obscure. Here, we studied the effect of BMP-7 on response to TGF-beta in the proximal tubular cell line HK-2 (PTC). BMP-7 specifically limited Smad3 but not Smad2 signaling. BMP-7 did not inhibit Smad3 phosphorylation or nuclear accumulation, nor did BMP-7 alter phosphorylated Smad3 dephosphorylation or degradation. However, BMP-7 treatment reduced Smad3 DNA binding to a consensus Smad binding element probe, and chromatin immunoprecipitation showed reduced Smad3 binding to the plasminogen activator inhibitor-1 promoter in PTCs treated with BMP-7 and TGF-beta compared with TGF-beta alone. Degradation of the transcriptional repressor SnoN has recently been shown to be necessary for Smad3 (but not Smad2) signaling. SnoN expression was transiently lost in PTCs after TGF-beta stimulation, but BMP-7 prevented this. Furthermore, BMP-7 had no effect on Smad3 signaling after siRNA-mediated SnoN knockdown, whereas prevention of SnoN degradation with the proteasome inhibitor MG132 reproduced the inhibitory action of BMP-7 on Smad3 signaling. We conclude that BMP-7 prevents TGF-beta-mediated loss of the transcriptional repressor SnoN and hence specifically limits Smad3 DNA binding, altering the balance of transcriptional responses to TGF-beta in PTCs. These results provide an important mechanistic insight into a key regulator of TGF-beta signaling.

Lovastatin inhibits oxidized low-density lipoprotein-induced plasminogen activator inhibitor and transforming growth factor-beta1 expression via a decrease in Ras/extracellular signal-regulated kinase activity in mesangial cells

Oxidized low-density lipoprotein (Ox-LDL) might be involved in the progression of renal disease. Ox-LDL stimulation of plasminogen activator inhibitor-1 (PAI-1) expression via transforming growth factor-beta (TGF-beta)/Smad signaling in mesangial cells required activation of extracellular signal-regulated kinase (ERK). Mevalonate depletion by 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase inhibitors, or statins, decreases the levels of farnesyl pyrophosphate (FPP) for isoprenylation of Ras. We postulate that statins may ameliorate the Ox-LDL-induced mesangial matrix accumulation by inhibiting Ras/ERK activation with subsequent downregulation of TGF-beta target genes. Quiescent mesangial cells were incubated for 18 h with and without the presence of lovastatin before 50 microg/mL of Ox-LDL treatment for 1 h. Lovastatin inhibited markedly the stimulatory effects of Ox-LDL on ERK1/2 activation, nuclear Smad3 expression, TGF-beta1 and PAI-1 mRNA and protein expression, and PAI-1 luciferase activity. These inhibitory effects of lovastatin were reversed almost completely by mevalonate or FPP. Similar to lovastatin, FTI-277, which is an inhibitor of Ras farnesylation, decreased the Ox-LDL-induced activation of ERK/Smad3 and induction of TGF-beta1/PAI-1. These results indicate that lovastatin prevents the Ox-LDL-induced Ras/ERK activation that results in inhibition of Smad3 activation in mesangial cells with subsequent downregulation of TGF-beta target genes. Thus, statins seem to have antifibrotic effects through their anti-TGF-beta response that are relevant in the treatment of chronic renal disease with dyslipidemia.