Interleukin-19 as a translational indicator of renal injury
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Accurate detection and prediction of renal injury are central not only to improving renal disease management but also for the development of new strategies to assess drug safety in pre-clinical and clinical testing. In this study, we utilised the well-characterised and differentiated human renal proximal tubule cell line, RPTEC/TERT1 in an attempt to identify markers of renal injury, independent of the mechanism of toxicity. We chose zoledronate as a representative nephrotoxic agent to examine global transcriptomic alterations using a daily repeat bolus protocol over 14 days, reflective of sub-acute or chronic injury. We identified alterations in targets of the cholesterol and mevalonate biosynthetic pathways reflective of zoledronate specific effects. We also identified interleukin-19 (IL-19) among other inflammatory signals such as SERPINA3 and DEFB4 utilising microarray analysis. Release of IL-19 protein was highly induced by an additional four nephrotoxic agents, at magnitudes greater than the characterised marker of renal injury, lipocalin-2. We also demonstrate a large increase in levels of IL-19 in urine of patients with chronic kidney disease, which significantly correlated with estimated glomerular filtration rate levels. We suggest IL-19 as a potential new translational marker of renal injury.
KeywordsNephrotoxicity Chronic kidney disease Proximal tubule Biomarker
The search for new and better predictive markers of nephrotoxicity, particularly those which can predict an adverse effect before any long lasting damage occurs, is a fundamental strategy behind many studies involved not only in disease monitoring but also those seeking to build better systems for pre-clinical and clinical nephrotoxicity compound screening. Animal studies are used on a routine basis to assess organ specific injury as part of drug development and disease investigative programs. Despite the potential for species specific differences, these models have played a significant role in the identification and development of relevant biomarkers such as lipocalin-2 (LCN2) a protein marker of renal injury (Kieran et al. 2003; Mishra et al. 2003). It is important to understand that these models have limitations in their ability to reflect and predict tissue specific injury in humans (Olson et al. 2000), which has led investigators to explore other model systems to model tissue injury and explore biomarker development. Particular attention has been given to developing better in vitro culture of highly differentiated human cells.
Within the kidney, the proximal tubule plays a major role in the reabsorption and secretion of key substances such as glucose, amino acids and certain metabolites. It is also particularly sensitive to injury and has a prominent role in the pathology of many conditions where the kidney is a target of injury (Thevenod 2003; Vallon 2011; Witzgall 1999). Primary cell culture models represent in vivo function to a high degree but are limited by their loss of differentiation over time and their limited proliferative capacity (Brown et al. 2008; Courjault-Gautier et al. 1995; Detrisac et al. 1984; Sharpe and Dockrell 2012). This has led to the development of many immortalised human proximal tubular epithelial cells such as the RPTEC/TERT1 cell line (Wieser et al. 2008), which represents one of the most similar to primary culture and highly differentiated cell line available (Aschauer et al. 2013; Wieser et al. 2008). These cells also have the added benefit of forming stable differentiated monolayers capable of being maintained over many weeks allowing previously difficult long-term exposure protocols to be performed with relative ease (Limonciel et al. 2011; Wilmes et al. 2013). The use of cells with a highly differentiated phenotype to study human biomarker development represents a complementary approach to those currently applied to clinical sample sets. Distinct advantages include high throughput and the ability to use global molecular screening or “Omics” strategies in well-controlled experimental designs that not only identify important targets for further validation but also give important mechanistic insight, which is often difficult to ascertain from in vivo animal and human studies.
The aim of this study was to investigate sub-acute or chronic toxic exposure in RPTEC/TERT1 cells up to 14 days in order to identify markers that could represent or predict toxic exposure in vivo. Using the bisphosphonate zoledronate as a model stimulus of renal injury, transcriptomic analysis identified interleukin-19 (IL-19) as the most highly differentially regulated gene. Moreover, IL-19 could be detected in the supernatant medium and displayed time and concentration dependent profiles. IL-19 could also be measured in urine samples from patients with chronic kidney disease (CKD) and correlated with renal function and other urinary markers of renal injury including NAG and LCN2. This work is an important example of how identification of a target molecule in highly differentiated in vitro culture can be used to explore new and better markers of human renal injury.
New strategies are being continuously developed to identify more specific and predictive markers of renal injury. Targets are ideally released into the extracellular environment allowing detection in readily accessible bio-fluids such as urine. The characteristics of such markers are typical of those peptide signals found as part of inflammatory responses. Bearing this in mind, using our highly differentiated in vitro model, we examined the nephrotoxic effects of zoledronate across a dosing regimen of IC10 and NOEL concentrations at 14 days to reflect or recapitulate sub-acute or chronic injury responses. Using transcriptomic analysis, we identified a number of pathways and genes with biomarker potential and focused on interleukin-19, an inflammatory peptide as the highest regulated gene on transcriptomic analysis.
Interleukin-19 is a member of the IL-10 family of cytokines which have indispensable functions in many inflammatory processes (Ouyang et al. 2011). It is also a member of the IL-20 subfamily and binds to the IL-20 receptor to regulate processes such as anti-microbial, wound healing and tissue remodelling (Ouyang et al. 2011). Cellular sources have been characterised as of myeloid and epithelial origin fitting with the significant levels found in the RPTEC/TERT1 renal epithelial cell system. The precise function of IL-19 in our system is difficult to ascertain given the in vitro nature of the model, but we have demonstrated that recombinant IL-19 protein induces the release of IL-6 and TNF-α from these cells (data not shown). The signalling mechanisms responsible for the induction of IL-19 were suggested by IPA analysis to include IL-17. Interestingly, IL-17 when used in combination with TNF-α has been observed to induce many of the genes altered in our model system, including IL-19, DEFB4 and LCN2 (Johnston et al. 2013). While insights into how IL-19 is regulated are important, in the context of biomarker discovery, it is equally important to characterise whether such signals can be used as predictive markers of tissue specific injury. Evidence to support our hypothesis that IL-19 expression in renal cells can reflect toxicity, independent of the type of injury or dysfunction, has recently been reported (Hsu et al. 2013). This study demonstrated increased expression of IL-19 using in vivo mouse models of ischaemia reperfusion and HgCl2-induced acute kidney injury. This study, however, did not look at changes in IL-19 in any clinical samples.
When we examined urine from patients with chronic kidney disease, we demonstrated a large increase in IL-19 protein levels as compared to control volunteers, with levels of change similar to other urinary markers of renal damage such as protein, NAG and LCN2. We also observed a more significant negative correlation of IL-19 with estimated GFR than when compared to LCN2, the implication of which is interesting to speculate upon. From our urinary analysis, it is unclear what the cellular source for IL-19 is but as NAG levels are increased in these samples indicating proximal tubular injury, it is possible these cells may be acting as a source. Some of our unpublished observations have demonstrated lack of IL-19 expression from liver- and CNS-derived cells in response to toxic stimuli, hinting at some degree of renal specificity. This, however, would have to be explored in greater detail. Levels of IL-19 expression from RPTEC/TERT1 cells seemed to precede general toxicity (data not shown) but whether this is the case in human urinary samples is unclear. We can say, however, that based on our results that it is very likely IL-19 is a novel clinical marker of ongoing proximal tubular injury. Lastly, it should also be highlighted that translation of targets identified in vitro to an in vivo human patient cohort gives great confidence to future studies which aim to use such cellular models to predict renal injury in humans.
This project was funded by the European Union’s 7th Framework Programme (FP7/2007–2013) under grant agreement No. 202222, Predict-IV. The funding agency had no input into study design and in the decision to publish.
Conflict of interest
The authors declare no conflicts of interest.