Evaluation of urine biomarkers of kidney injury in polycystic kidney disease

Abstract

Progressive disruption of renal tubular integrity in the setting of increased cellular proliferation and apoptosis is a feature of autosomal dominant polycystic kidney disease (ADPKD). Here we evaluated the effect of these processes on the expression of Lcn2 (NGAL) and interleukin (IL)-18, markers of tubular injury, in rodent models and in the cyst fluid and urine of patients with ADPKD. Two mouse models where Pkd2 was inactivated, which resulted in early- or adult-onset cysts, were used to evaluate NGAL levels. Further, the Han:SPRD rat model of polycystic disease was used to study IL-18 levels. In four annual serial urine samples collected from 107 patients with ADPKD in the Consortium for Radiologic Imaging for the Study of Polycystic Kidney Disease (CRISP) study, NGAL and IL-18 excretion rates were determined in conjunction with measures of total kidney volume and estimated glomerular filtration rate (eGFR) by the Modification of Diet in Renal Disease equation. Kidneys from affected mice and rats showed prominent expression of NGAL and IL-18/IL-18R, respectively, in epithelial cells lining kidney cysts. In human ADPKD cyst fluid, both NGAL and IL-18 were elevated. In CRISP patients, the mean percentage increase in total kidney volume was 5.4/year and the mean decline in eGFR 2.4 ml/min/year. The trend of increased mean urine NGAL and IL-18 over 3 years was statistically significant; however, there was no association between tertiles of IL-18 or quartiles of NGAL and change in total kidney volume or eGFR over this period. Thus, urinary NGAL and IL-18 excretion is mildly and stably elevated in ADPKD, but does not correlate with changes in total kidney volume or kidney function. This may be due, in part, to the lack of communication between individual cysts and the urinary collecting system in this disorder.

Fig. 1. Expression of LCN2 (NGAL) in orthologous gene mouse models of PKD

(a-c) RNA in situ hybridization for Lcn2 in a Pkd2^WS25/− cystic kidney showing expression (dark reactivity) after hybridization with the Lcn2-specific antisense RNA probe (a, b) in epithelial cyst lining cells and in some pericystic tubules; the sense probe lacks reactivity (c). The boxed area in panel a is shown in higher magnification in panel b (inset is digitally enlarged). (d) The appearance of kidneys from Pkd2^−/−;pCAGGS-PKD2 mice at postnatal day 10. (e, f) Immunocytochemistry showing expression of NGAL in cyst lining epithelia of Pkd2^−/−;pCAGGS-PKD2 mice (NGAL, green; α-tubulin, red; DAPI, blue. Original magnifications: a, c, 10×; b, 40×; e, f, 20×.

Figure 2. IL-18 Expression in Han:SPRD (Cy/+) Rats

(a) Immunocytochemistry showing strong expression of IL-18 (green) in cells lining the cysts in one year old Han:SPRD (Cy/+) rats. (b) IL-18-receptor (IL-18R) expression (red) is also observed in cells adjacent to some cysts. Staining is also seen in the nucleus and interstitium. DAPI, blue. Original magnification, 400×.

Figure 3. Biomarker Distribution in the CRISP Cohort at each visit

Raw Values of NGAL values are in ng/ml and IL-18 values are in pg/ml.