The value of urinary RBP4 in the diagnosis of FSGS and other renal diseases

Abstract

Background: Unequivocal diagnosis of FSGS can only be made with a renal biopsy, which is an invasive, risk-associated medical procedure. The discovery of noninvasive molecular biomarkers for the diagnosis of FSGS remains an important scientific goal. This study examines the urinary proteome of FSGS patients and reference groups, in order to identify urinary protein expression alterations indicative of FSGS.Methods: Urine samples were collected from subjects representing FSGS, IgA nephropathy (IgAN), clear cell renal cell carcinoma (ccRCC), chromophobe renal cell carcinoma (chRCC), and healthy control group, respectively. The samples were subjected to SWATH-MS proteomics analysis. ELISA was utilized to validate the expression level of Retinol-binding protein 4 (uRBP4) in FSGS and reference samples (IgAN, ccRCC, chRCC, prostate cancer and healthy subjects). Results:The MS study identified 194 (FSGS), 179 (IgAN), 271 (ccRCC), 255 (chRCC), and 275 (healthy controls) urinary proteins. The comparative proteomic analysis revealed that urinary Retinol-binding protein 4 (uRBP4) clearly discriminates FSGS from the rest of the groups. Increased levels of uRBP4 in FSGS urine specimens were also evidenced by ELISA. Significantly elevated levels of uRBP4 were also observed for IgAN, ccRCC and chRCC versus healthy individuals. Conclusions:Determining FSGS diagnosis based on uRBP4 expression alone is not possible. Specific uRBP4 concentration cut-off can be applied to accurately distinguish individuals with renal disorder (in general) from healthy subjects. Possibly, urinary RBP4 could serve as a screening biomarker identifying people at risk of renal disorders, who should undergo more detailed diagnostics.

The Clinical Significance of Urinary Retinol-Binding Protein 4: A Review

“…In normal, voided urine, RBP4 presents at a relatively high abundance and constitutes nearly 8.5% of the urinary proteome [ 18 ]. RBP4 molecules can be detected in total urine, urine supernatant (debris-deprived urine), as well as in urinary extracellular vesicles (EVs, formerly called exosomes) [ 19 , 20 , 21 ]. There are three RBP4 isoforms encountered in human urine, namely full-length RBP4 and its two C-terminal truncated derivatives, RBP4-L (lacking Leu-183) and RBP4-LL (deprived of Leu-183 and Leu-182).…”

“…The protein was significantly more abundant in FSGS than in IgAN patients. Interestingly, uRBP4 concentration ranges in IgAN overlapped with those of disease controls, i.e., renal cell carcinomas (clear cell renal cell carcinoma, ccRCC; chromophobe renal cell carcinoma, chRCC) ( Supplementary Table S2 ) [ 21 ]. Kalantari et al employed two different quantitative proteomic strategies to characterize the urinary proteome of early-stage IgAN (class II–IV according to the Haas and Lee classification) and advanced IgAN (class V).…”

“…Recently, our group analyzed uRBP4 expression patterns in RCC subjects (ccRCC and chRCC) and compared them with uRBP4 levels in healthy controls and men with prostate cancer (disease control group). Our data demonstrated that uRBP4 levels are significantly increased in RCC and that RCC diagnosis cannot be stated based on uRBP4 alone due to overlapping expression patterns between RCC and reference groups [ 21 ]. Because some RCC patients presented “normal” levels of uRBP4, we hypothesize that tubular injury may not be an obvious feature of RCC tumors.…”