Plasmin in renal interstitial fibrosis: innocent or guilty?

Epithelial-mesenchymal plasticity in kidney fibrosis

Epithelial-mesenchymal transition (EMT) is an important biological process contributing to kidney fibrosis and chronic kidney disease. This process is characterized by decreased epithelial phenotypes/markers and increased mesenchymal phenotypes/markers. Tubular epithelial cells (TECs) are commonly susceptible to EMT by various stimuli, for example, transforming growth factor-β (TGF-β), cellular communication network factor 2, angiotensin-II, fibroblast growth factor-2, oncostatin M, matrix metalloproteinase-2, tissue plasminogen activator (t-PA), plasmin, interleukin-1β, and reactive oxygen species. Similarly, glomerular podocytes can undergo EMT via these stimuli and by high glucose condition in diabetic kidney disease. EMT of TECs and podocytes leads to tubulointerstitial fibrosis and glomerulosclerosis, respectively. Signaling pathways involved in EMT-mediated kidney fibrosis are diverse and complex. TGF-β1/Smad and Wnt/β-catenin pathways are the major venues triggering EMT in TECs and podocytes. These two pathways thus serve as the major therapeutic targets against EMT-mediated kidney fibrosis. To date, a number of EMT inhibitors have been identified and characterized. As expected, the majority of these EMT inhibitors affect TGF-β1/Smad and Wnt/β-catenin pathways. In addition to kidney fibrosis, these EMT-targeted antifibrotic inhibitors are expected to be effective for treatment against fibrosis in other organs/tissues.

Inhibition of lysosomal protease cathepsin D reduces renal fibrosis in murine chronic kidney disease

During chronic kidney disease (CKD) there is a dysregulation of extracellular matrix (ECM) homeostasis leading to renal fibrosis. Lysosomal proteases such as cathepsins (Cts) regulate this process in other organs, however, their role in CKD is still unknown. Here we describe a novel role for cathepsins in CKD. CtsD and B were located in distal and proximal tubular cells respectively in human disease. Administration of CtsD (Pepstatin A) but not B inhibitor (Ca074-Me), in two mouse CKD models, UUO and chronic ischemia reperfusion injury, led to a reduction in fibrosis. No changes in collagen transcription or myofibroblasts numbers were observed. Pepstatin A administration resulted in increased extracellular urokinase and collagen degradation. In vitro and in vivo administration of chloroquine, an endo/lysosomal inhibitor, mimicked Pepstatin A effect on renal fibrosis. Therefore, we propose a mechanism by which CtsD inhibition leads to increased collagenolytic activity due to an impairment in lysosomal recycling. This results in increased extracellular activity of enzymes such as urokinase, triggering a proteolytic cascade, which culminates in more ECM degradation. Taken together these results suggest that inhibition of lysosomal proteases, such as CtsD, could be a new therapeutic approach to reduce renal fibrosis and slow progression of CKD.

Fibrosis in Atrial Fibrillation - Role of Reactive Species and MPO

Atrial fibrosis with enhanced turnover and deposition of matrix proteins leads to inhomogeneous atrial electrical conduction and gives rise to electrical reentry circuits resulting in atrial fibrillation. The multifactorial pathogenesis of atrial fibrosis involves resident cardiac cells as well as infiltrating leukocytes, both generating and sequestering matrix metalloproteinases (MMPs), a key enzyme family involved in fibrosis. A growing body of evidence points toward an important role of reactive oxygen species (ROS) in the release and activation of pro-MMPs and the stimulation of pro-fibrotic cascades. Myeloperoxidase (MPO), a bactericidal enzyme released from activated polymorphonuclear neutrophils (PMN) is not only associated with a variety of cardiovascular diseases, but has also been shown to be mechanistically linked to atrial fibrosis and fibrillation. MPO catalyzes the generation of reactive species like hypochlorous acid, which affect intracellular signaling cascades in various cells and advance activation of pro-MMPs and deposition of atrial collagen resulting in atrial arrhythmias. Thus, inflammatory mechanisms effectively promote atrial structural remodeling and importantly contribute to the initiation and perpetuation of atrial fibrillation.

Tissue-type plasminogen activator deficiency attenuates peritoneal fibrosis in mice

Peritoneal fibrosis (PF) is an important complication of peritoneal dialysis therapy. The present study was performed to examine the mechanisms of PF in view of the plasminogen activator (PA)/plasmin/matrix metalloproteinase (MMP) cascade. PF was induced in tissue-type PA (tPA) deficient mice and wild-type mice by intraperitoneal injection of chlorhexidine gluconate. Mice were killed on day 21, and tissue samples were taken. Histopathological studies were performed. Plasmin activity, gelatinases activity, and the levels of tPA, transforming growth factor-β1 (TGF-β1), and MMP-2 mRNA were determined. Protein levels of MMP-3, tissue inhibitor of metalloproteinases (TIMP)-1, -2, and -3, phospho-Smad3, membrane-type 1 (MT1)-MMP, and MT3-MMP were also studied. On day 21, tPA +/+ mice showed severe PF, whereas tPA −/− mice showed milder change. Submesothelial basement membranes were dissolved in tPA +/+ mice while they were relatively preserved in tPA −/− mice. The levels of macrophage infiltration, staining for α-smooth muscle actin (α-SMA) and collagen type III, and vascular density were all significantly lower in tPA −/− mice than in tPA +/+ mice. The levels of plasmin activity, pro- and active MMP-2, mRNA expression of tPA and TGF-β1, and phospho-Smad3 protein were also lower in tPA −/− mice. No difference was observed between the two groups concerning the protein levels of MMP-3, TIMP-1, TIMP-2, TIMP-3, MT1-MMP, or MT3-MMP. These results indicate that the presence of tPA enhances inflammation, angiogenesis, and fibrogenesis in the peritoneum of the PF model mice. Activation of the PA/plasmin/MMP cascade may play a pivotal role in the pathogenesis of PF.

Effects of pioglitazone and candesartan on renal fibrosis and the intrarenal plasmin cascade in spontaneously hypercholesterolemic rats

The profibrotic effect of plasminogen activator inhibitor-1 (PAI-1) in renal fibrosis is widely recognized, but its mechanism remains controversial especially in chronic progressive kidney disease. In the present study, pioglitazone (Pio) and candesartan (CD), which are reported to inhibit PAI-1, were administered to spontaneously hypercholesterolemic (SHC) rats, a model of chronic progressive kidney disease. Therapeutic effects and effects on the intrarenal plasmin cascade were examined. Eight-wk-old SHC rats were used as controls. Oral administration of vehicle alone, Pio, or CD was performed starting at 8 wk of age and was continued for 24 wk. The degree of renal fibrosis was evaluated by sirius red staining of kidney sections and by total collagen assay of renal homogenates. The renal PAI-1 protein level was assessed by Western blotting, and plasmin activity was analyzed by chromogenic assay and casein gel zymography. Urinary protein and blood urea nitrogen were significantly increased in the vehicle-treated group, but the increase was attenuated in the Pio- and CD-treated groups. This correlated well with the degree of fibrosis as assessed by sirius red staining and total collagen assay. The PAI-1 protein level was also increased significantly in the vehicle-treated group, and the increase was attenuated in the Pio- and CD-treated groups. Despite the presumed plasmin-inhibitory function of PAI-1, plasmin activity changed in parallel with PAI-1. These results suggest that Pio and CD inhibit PAI-1 and exert renoprotective effects against chronic progressive renal disease, but its action is independent of the regulatory function on plasmin activity.