Contribution of lysosomal cysteine proteases in cardiac and renal diseases

Abstract

Cardiac and renal diseases (CRDs) are characterized by extensive remodeling of the extracellular matrix (ECM) architecture of the cardiorenal system. Among the many extracellular proteolytic enzymes present in cardiorenal cells and involved in ECM remodeling, members of the matrix metalloproteinase family and serine protease family have received the most attention. However, recent findings from laboratory and clinical studies have indicated that cysteine protease cathepsins also participate in pathogenesis of the heart and kidney. Deficiency and pharmacological inhibition of cathepsins have allowed their in vivo evaluation in the setting of pathological conditions. Furthermore, recent studies evaluating the feasibility of cathepsins as a diagnostic tool have suggested that the serum levels of cathepsins L, S and K and their endogenous inhibitor cystatin C have predictive value as biomarkers in patients with coronary artery disease and heart and renal failure. The goal of this review is to highlight recent discoveries regarding the contributions of cathepsins in CRDs, particularly hypertensive heart failure and proteinuric kidney disease.

Key Words: Cysteine proteases; Cathepsins; Cystatin C; Extracellular matrix proteins; Cardiovascular disease; Inflammation

CYSTEINE PROTEASE CATHEPSINS IN A NUTSHELL

Cysteine cathepsins are papain family members of the cysteine protease superfamily. In humans, 11 members have been identified: cathepsins B, C, F, H, K, L, O, S, V, W and X, all of which share a conserved active three-dimensional pocket formed with cysteine, histidine and asparagine residues. Cathepsins are synthesized as proenzymes. Procathepsin is formed after removal of the prepeptide during the passage to the endoplasmic reticulum. Cysteine cathepsins were originally identified as proteases acting in the lysosome. More recent investigations have identified nontraditional roles for cathepsins in the extracellular space, as well as in the intracellular spaces[1-4]. These cathepsins have been demonstrated to play an important role in extracellular matrix (ECM) remodeling and have been implicated in the development and progression of cardiac and renal diseases (CRDs)[5-9]. Furthermore, evaluations of the feasibility of cathepsins as a diagnostic tool have revealed that the serum levels of several cathepsins seem to be promising biomarkers in the diagnosis of coronary artery disease, cardiac fibrosis and renal dysfunction[10-13].

The roles of cathepsins in atherosclerosis-based vascular disease and ischemic heart disease process have been covered by recent comprehensive reviews[14,15]. This review examines several issues in regard to cysteine cathepsin molecular function and participation in cardiorenal pathological processes, especially with respect to their potential application as diagnostic and/or prognostic markers and drug targets.

CATHEPSINS AND CYSTATIN C IN CRDS

Cardiac disease

More than a decade ago, cathepsin B, L and H were the first cathepsins identified in the myocardium of a rat model of acute myocardial infarction[16]. Crie and other groups reported a change in the cathepsin B protein and activity in cardiac injuries[17,18]. Failing human myocardium with dilated cardiomyopathy has been shown to overexpress cathepsin B mRNA and protein[19]. In studies by Cheng et al[6,7,20], it was reported that while normal cardiac tissues contained little or no cathepsin K or cathepsin S, these proteins were abundantly expressed in cardiac myocytes, macrophages, intracoronary smooth muscle cells (SMCs) and endothelial cells (ECs) of humans and animals with failing myocardium and hypertension. In addition to cardiac tissues, activated interstitial myofibroblasts have been shown to express the elastases cathepsin S and K in myxomatous heart valves[21]. Cell culture experiments have shown that proinflammatory mediators, including interleukin (IL)-1β or/and tumor-necrosis factor (TFN)-α, stimulate cathepsin S and cathepsin K gene expression and increased elastolytic and collagenolytic activity in neonatal cardiomyocytes, myxomatous valve-derived myofibroblasts and macrophages. This suggests that the inflammatory processes that prevail during cardiac remodeling locally increase the presence of the active form of these cathepsins. The ability of cardiomyocytes and macrophages to use cathepsins to degrade elastin and collagen support a role for these proteases in the cardiac wall and in valve alterations in humans and animals. Novel insights into cathepsin function have been gained by the generation and in-depth analysis of knockout and transgenic mice. Cathepsin L deficiency has been shown to result in cardiac chamber dilation and impaired cardiac function[22]. The human cathepsin L transgenic heart has been shown to exhibit a decrease in overload-induced hypertrophic response, apoptosis and fibrosis through blocking of the AKT/GSK3β signaling pathway in two models of aortic banding and angiotensin II infusion[23]. Recently, in vivo and in vitro studies demonstrated that overexpression of cystatin C or exposing fibroblasts to cystatin protein resulted in an inhibition of cathepsin B and accumulation of collagens and fibronectin[24]. Although there have been numerous genetic studies on the functions of cathepsins, most of the molecular mechanisms underlying the roles of cathepsins in cardiac diseases remain unclear.

Kidney disease

Kidney cancer: The role of cysteine cathepsins in kidney cancer pathogenesis has been covered by a recent comprehensive review[25]. Here, we will focus on the role of cathepsins in several non-tumor-associated proteinuric kidney diseases, as described below.

Proteinuric kidney disease: More than a decade ago, it was reported that the levels of cathepsins B, L and H are altered in tubules of the rat kidney in response to injuries[26-29]. We reported that proximal tubular cells and podocytes of proteinuric failing kidneys contain much more cathepsin S protein than normal kidneys. In contrast, the tubular cells from the kidneys of patients with early stage diabetic and hypertensive hypertrophy show decreased activity of cathepsins B and L[28,30], suggesting that the roles of cathepsins in kidney disease might vary among the different cathepsins and different stages. On the other hand, cathepsin L is relatively well known to play a role in proteinuric kidney disease. In this regard, an article by Sever et al[2] provides new information of great interest. The authors demonstrate that cathepsin L deficiency impairs cleavage of dynamin at an evolutionarily conserved site, reducing the reorganization of the podocyte actin cytoskeleton and ameliorating proteinuria. Recent findings from in vitro experiments have shown that cathepsin L is involved in the podocyte migration coordinated by α3 integrin and degradation of the Rho GTPase regulator synaptopodin in podocytes[3,31]. In addition to podocytes and tubular cells, glomerular epithelial cells also express cathepsin L upon exposure to IL-4 and IL-13.

Pharmacological alternative to cathepsins and cystatin C in CRDs

Numerous cardiovascular drugs have been designed to target the expression and activity of cathepsins. Several recent groups, including our own, have demonstrated that the expression and activity of cathepsins S, K and L were increased in the failing myocardium and kidney of hypertensive animal models; these changes were targeted by the lipid-lowering and angiotensin II receptor-blocking drugs olmesartan and statin via the reduction of inflammatory actions and the reduction of the nicotinamide adenine dinucleotide phosphate-oxidase (NADPH) oxidase-dependent superoxide production[7,20,32]. Simvastatin inhibited cardiac hypertrophy and fibrosis in apolipoprotein-deficient mice fed a high-fat diet by increasing peroxisome proliferator-activated receptor α and γ expressions and reducing the expressions of matrix metalloproteinase (MMP)-9 and cathepsin S[33]. In addition, recent studies have shown that gallic acid also prevents lysosomal dysfunction by inhibiting increases in cathepsins B and D activity in myocardial injuries in animal models[34,35].

Circulating cathepsins or cystatin C as biomarkers for CRDs

Recent studies have evaluated the use of serum cathepsin levels as a diagnostic tool for heart and renal diseases. The most extensively described cysteine protease in renal insufficiency is the endogenous inhibitor cystatin C. Cystatin C is used to evaluate renal function and to detect the prevalence of dysfunction, particularly metabolic diseases[36]. Several human studies have indicated that serum cystatin C and cathepsin L are sensitive new predictors of potential kidney injury[13,37], particularly contrast-induced nephropathy. Interestingly, urinary cathepsin B activity is also increased in patients with membranous glomerulonephritis[38]. On the other hand, the feasibility of using cystatin C as a tool for predicting cardiac remodeling and clinical outcomes in patients with heart failure was recently explored. High levels of cystatin C were associated with an increased risk of heart failure, such an association may be limited to hypertensive individuals[39]. Manzano-Fernandez et al[12] reported that cystatin C can predict long-term outcomes in patients with acute heart failure. Although the data are still preliminary, these findings indicate that several cathepsins and/or cystatin C may serve as useful biomarkers in the diagnosis of CRDs.

CONCLUSION

Among cathepsins, cysteine protease cathepsins have been described to play a role in several heart and kidney diseases, including hypertensive heart failure and proteinuric kidney disease. Figure 1 shows an overview of potential regulations and functions for cathepsins in the pathogenesis of cardiorenal system. The data from laboratory and clinical studies showed that cysteine cathepsins B, K, L and S are mainly expressed in cardiorenal system cells, including cardiac myocytes, interstitial myofibroblasts, intracoronary SMCs and ECs, podocytes and tubular cells, and also to a lesser degree in CRDs-associated inflammatory macrophages. In vitro experiments revealed that, although these cells express negligible levels of cathepsins S, K and L, their incubation with inflammatory cytokines, such as IL-1β, TNF-α, and angiotensin II, causes these cells to significantly express and secrete these cathepsins and to exhibit their collagenolytic and elastolytic activities. Several recent reports, related primarily to the remodeling of intracellular and extracellular proteins, have established an important role for cysteine in CRDs in genetically altered mice. Clinical findings have indicated a possible role for cathepsins as biomarkers in the diagnosis of CRDs and other diseases. Furthermore, pharmacological inhibition of cathepsin activity and expression as well as MMPs and SPs by several cardiovascular drugs has also shown protective effects in hypertensive heart failure and renal failure. We therefore propose that multiple proteases, including MMPs and SPs, work in concert during the initiation, progression and complications of CRDs.

Figure 1 Roles of cathepsins and cystatin C in the pathogenesis of heart and kidney diseases. A schematic model of cathepsin regulation and function in cardiorenal cells is shown. Tissue and circulating cathepsins (Cats) were used as biomarkers for the diagnosis of cardiorenal disease. Ang I: Angiotensin II; AT1R: Ang II type 1 receptor; IL-1β: Interleukin-1β; TNF-α:Tumor necrosis factor-α; TNFR: TNF-α receptor; ILR: IL-1β receptor; ECM: Extracellular matrix; NADPH: Nicotinamide adenine dinucleotide phosphate.