The Multifaceted Role of the Lysosomal Protease Cathepsins in Kidney Disease

Renal fibrosis is the inevitable consequence of an excessive accumulation of extracellular matrix that occurs in virtually every type of chronic kidney disease. The pathogenesis of renal fibrosis is a progressive process that ultimately leads to end-stage renal failure, a devastating disorder that requires dialysis or kidney transplantation. In a simplistic view, renal fibrosis represents a failed wound-healing process of the kidney tissue after chronic, sustained injury. Several cellular pathways, including mesangial and fibroblast activation as well as tubular epithelial-mesenchymal transition, have been identified as the major avenues for the generation of the matrix-producing cells in diseased conditions. Among the many fibrogenic factors that regulate renal fibrotic process, transforming growth factor-beta (TGF-beta) is one that plays a central role. Although defective matrix degradation may contribute to tissue scarring, the exact action and mechanisms of the matrix-degrading enzymes in the injured kidney have become increasingly complicated. Recent discoveries on endogenous antifibrotic factors have evolved novel strategies aimed at antagonizing the fibrogenic action of TGF-beta/Smad signaling. Many therapeutic interventions appear effective in animal models; however, translation of these promising results into humans in the clinical setting remains a daunting task. This mini-review attempts to highlight the recent progress in our understanding of the cellular and molecular pathways leading to renal fibrosis, and discusses the challenges and opportunities in developing therapeutic strategies.

The current staging system for chronic kidney disease is based primarily on estimated glomerular filtration rate (eGFR) with lower eGFR associated with higher risk of adverse outcomes. Although proteinuria is also associated with adverse outcomes, it is not used to refine risk estimates of adverse events in this current system. To determine the association between reduced GFR, proteinuria, and adverse clinical outcomes. Community-based cohort study with participants identified from a province-wide laboratory registry that includes eGFR and proteinuria measurements from Alberta, Canada, between 2002 and 2007. There were 920 985 adults who had at least 1 outpatient serum creatinine measurement and who did not require renal replacement treatment at baseline. Proteinuria was assessed by urine dipstick or albumin-creatinine ratio (ACR). All-cause mortality, myocardial infarction, and progression to kidney failure. The majority of individuals (89.1%) had an eGFR of 60 mL/min/1.73 m(2) or greater. Over median follow-up of 35 months (range, 0-59 months), 27 959 participants (3.0%) died. The fully adjusted rate of all-cause mortality was higher in study participants with lower eGFRs or heavier proteinuria. Adjusted mortality rates were more than 2-fold higher among individuals with heavy proteinuria measured by urine dipstick and eGFR of 60 mL/min/1.73 m(2) or greater, as compared with those with eGFR of 45 to 59.9 mL/min/1.73 m(2) and normal protein excretion (rate, 7.2 [95% CI, 6.6-7.8] vs 2.9 [95% CI, 2.7-3.0] per 1000 person-years, respectively; rate ratio, 2.5 [95% CI, 2.3-2.7]). Similar results were observed when proteinuria was measured by ACR (15.9 [95% CI, 14.0-18.1] and 7.0 [95% CI, 6.4-7.6] per 1000 person-years for heavy and absent proteinuria, respectively; rate ratio, 2.3 [95% CI, 2.0-2.6]) and for the outcomes of hospitalization with acute myocardial infarction, end-stage renal disease, and doubling of serum creatinine level. The risks of mortality, myocardial infarction, and progression to kidney failure associated with a given level of eGFR are independently increased in patients with higher levels of proteinuria.

Acute kidney injury (AKI) is a widespread problem of epidemic status. Compelling evidence indicates that the incidence of AKI is rapidly increasing, particularly among hospitalized patients with acute illness and those undergoing major surgery. This increase might be partially attributable to greater recognition of AKI, improved ascertainment in administrative data and greater sensitivity of consensus diagnostic and classification schemes. Other causes could be an ageing population, increasing incidences of cardiovascular disease, diabetes mellitus and chronic kidney disease (CKD), and an expanding characterization of modifiable risk factors, such as sepsis, administration of contrast media and exposure to nephrotoxins. The sequelae of AKI are severe and characterized by increased risk of short-term and long-term mortality, incident CKD and accelerated progression to end-stage renal disease. AKI-associated mortality is decreasing, but remains unacceptably high. Moreover, the absolute number of patients dying as a result of AKI is increasing as the incidence of the disorder increases, and few proven effective preventative or therapeutic interventions exist. Survivors of AKI, particularly those who remain on renal replacement therapy, often have reduced quality of life and consume substantially greater health-care resources than the general population as a result of longer hospitalizations, unplanned intensive care unit admissions and rehospitalizations.