Comprehensive study of altered proteomic landscape in proximal renal tubular epithelial cells in response to calcium oxalate monohydrate crystals

Kidney stone is a common disease with prevalence and incidence increasing across the world that seriously affects human health. Generally, it can cause urinary retention, renal pelvis and ureteral hydrops, ureteral dilatation, renal function damage, and infection. The prevalence of kidney stones exhibited a significant increasing trend in the past decades in the mainland of China [1, 2]. The latest research shows that the prevalence of Chinese adult kidney stones is 5.8%, and the life-time prevalence in South China is as high as 26.6% [3]. Kidney stone disease mainly affects people aged 30–50 years old, and half the patients suffered kidney stone recurrence [4]. Kidney stone seriously affects the health and quality of life of patients, and has become a public health problem across the world.

Calcium Oxalate Monohydrate (COM) accounts for over 80% of the incidence of stones [5]. COM caused renal tubular epithelial cells expressing a variety of macromolecules [6], thus affecting adhesion, aggregation and growth of calcium oxalate crystals plays an important role in the formation of kidney stones [6, 7]. Studies reported that the COM crystal-cell interaction stimulates the expression of NADPH oxidase in renal tubular epithelial cells, triggers the massive production of reactive oxygen species, activates the nuclear factor-κB signal transduction pathway, and releases a large number of inflammatory factors and activates the inflammatory cascade, links to local intrarenal inflammation and kidney injury [8‐11].

Recently, the mechanism of kidney stone formation has been increasingly concerned. Serval studies demonstrated that kidney stones are initiated by COM crystals deposition due to supersaturation of urinary calcium and oxalate ions [12]. These deposits may act as nidus for stone growth, adhere to apical surface of renal tubular epithelial cells via several crystal-binding molecules or potential crystal receptors [13]. Calcium oxalate stone matrix has been reported to include a large number of protein molecules, which were identified and evidenced to promote the aggregation, nucleation and growth of calcium oxalate crystals in kidney, which ultimately leads to the formation of stones [6, 7, 14‐16]. CD44, OPN, MCP-1 and HA are the most widely studied proteins which calcium oxalate crystal attachment depends upon [8, 17]. However, various theories of pathogenesis of human kidney stones suggest that calcium oxalate stone formation is a multistep process which is too complex for simple understanding, a large number of protein molecules involved are still unknown and uninvestigated.

Crystal-cell interaction model is widely used for kidney stone research for better understanding of the pathogenic mechanisms of kidney stone formation [18]. MDCK renal tubular cells (a cell line derived from dog kidney exhibiting distal renal tubule phenotype) and HK-2 cells (an immortalized human kidney proximal tubule epithelial cell line) are the most frequently used cells in crystal-cell interaction model [18]. Although several previous studies have found some new candidate proteins, further studies are necessary, since the COM-crystals prepared by calcium chloride dihydrate and sodium oxalate are different from the clinical COM-stone samples and use of cells from different species of the nephron may result in different findings. We used HK-2 cells for it was derived from adult human kidney proximal tubule which is the major site of renal oxalate handling [19], represents a major potential advantage over currently available animal or human embryonic derived cell lines [20].

In this study, we aim to use TMT (Tandem Mass Tag)–based quantitative proteomics analysis to investigate the effects of calcium oxalate crystal on the differential protein expression profiles of human renal tubular epithelial cell HK-2, screen differentially expressed protein molecules and initially explore their functional roles in calcium oxalate stone formation and its resulting renal damage.

Kidney stone is a common urological disease with high incidence, which remains a common health problem worldwide. At present, the mechanism of kidney stone formation is not fully understood. One of the favored theories suggests that the calcium oxalate-induced injury to renal tubular epithelial cells promotes the adherence and acumination of calcium oxalate crystals results in stone formation [27]. Given proteins are the major component of kidney stone organic matrix and considered to play a regulatory function in cell-crystal interactions and lithogenesis inside the kidney [28‐31], we selected immortalized human proximal tubular epithelial cells HK-2 exposure to COM crystals to generate the cell-crystal interaction model and analyzed the altered proteomic landscape in HK-2 cells in response to COM adhesion.

In this study, we first systematically screened the DEPs profiles in COM-HK-2 model by TMT-labeled quantitative proteomics analysis. Of the 1141 identified DEPs, 699 (61.3%) proteins were up-regulated, 442 (38.7%) proteins were down-regulated. By contrast, a similar previous study performed by Chen et al. in 2010 [32] demonstrated that only 12 DEPs were identified in HK-2 cells after COM crystals stimulation. This different observation may due to the limits of methodology used by Chen et al. in their study. Here, gel-free TMT-based quantitative proteomic approach uses isobaric labels and allows for genome-wide quantitation of differences in protein expression levels [33], proved to be a more reliable and reproducible technology to analyze complex samples.

Several of proteins have been characterized to play critical roles in kidney stone formation, however, the mechanisms involved are far from clear. Tamm-Horsfall protein (THP) was identified as a kidney-specific protein, serve as a key regulator in promoting the aggregation of calcium-salt crystals and stone formation [34, 35], which is a potent immunomodulatory molecule and a disease biomarker in the urinary system [35]. Osteopontin (OPN) is another important component of calcium oxalate stone matrix [29], plays an important role in preventing the formation of calcium oxalate monohydrate (COM) kidney stones, which controls switching of calcium oxalate monohydrate morphologies in urine [36] to promote the formation of stones by promoting crystal adhesion and mediating oxidative stress and apoptosis [17, 37].

In this study, we identified that the LZTS1 (leucine zipper hypothetical tumor suppressor gene 1) protein, a full-length 596 amino acid protein with a molecular weight of 67 kD, was significantly upregulated in COM adhered HK-2 cells. Though the role and mechanism involved of LZTS1 protein was well studied in various types of malignant tumors such as stomach, lung, bladder, ovary, and kidney [38], the functional role of LZTS1 in kidney stone disease is unreported. The HES1 (hairy enhancer of split 1) protein, the core effector of NOTCH signaling pathway and considered to be a good indicator of the activation of the NOTCH signaling pathway [39, 40], exhibited significant reduce expression in COM adhered HK-2 cells. Present study is the first report to demonstrate the relation between the HES1 protein and kidney stone formation and its associated kidney injury, however, further study of the mechanism involved is in need.

Our bioinformatics analysis indicated that the GO terms of cellular process, cell part, organelles, binding and catalytic activity are significant changed in COM adhered HK-2 cells, suggested that these processes or cell structures and its related proteins are widely involved in kidney stone formation. The enriched signaling pathway identified by KEEG analysis were regulation of the actin cytoskeleton, tight junction and focal adhesion. Of these, the regulation of actin cytoskeleton showed the most significance, which plays vital roles in response to extracellular signals, spatially and temporally regulates adhesions, protrusion, contraction, and retraction [41]. Meanwhile, we analyzed the protein interaction networks of the selected DEPs and found that the CFL1, ACTN and MYH9 were the 3 high-degree hub nodes and may be involved in the pathological processes of HK-2 cells in response to the COM crystals adhesion. These findings suggested that the cell structure- and cell actin cytoskeleton dynamics had been significantly changed by COM crystals.

We screened and identified key protein molecules that may be involved in the formation of calcium oxalate kidney stones, and revealed the possible signaling pathways and related disease processes, providing important potential targets and interactions for further elucidation of the pathogenesis of kidney stones. However, despite these comprehensive bioinformatics analyses, the current study has several limitations. HK-2 cells are not normal proximal tubular cells, which is a big limitation of the present study. Furthermore, the in vitro COM concentration could not perfectly mimic the status of crystals in kidney, the dose-dependent and time-dependent effects of COM stimulation were not involved. In our future investigations, we hope to perform additional proteomics analyses on normal proximal tubular cells/animal models of different time points and experimental validation to enrich this study.