Soluble Urokinase Receptor and Acute Kidney Injury

Abstract

Background: Acute kidney injury is common, with a major effect on morbidity and health care utilization. Soluble urokinase plasminogen activator receptor (suPAR) is a signaling glycoprotein thought to be involved in the pathogenesis of kidney disease. We investigated whether a high level of suPAR predisposed patients to acute kidney injury in multiple clinical contexts, and we used experimental models to identify mechanisms by which suPAR acts and to assess it as a therapeutic target.

Methods: We measured plasma levels of suPAR preprocedurally in patients who underwent coronary angiography and patients who underwent cardiac surgery and at the time of admission to the intensive care unit in critically ill patients. We assessed the risk of acute kidney injury at 7 days as the primary outcome and acute kidney injury or death at 90 days as a secondary outcome, according to quartile of suPAR level. In experimental studies, we used a monoclonal antibody to urokinase plasminogen activator receptor (uPAR) as a therapeutic strategy to attenuate acute kidney injury in transgenic mice receiving contrast material. We also assessed cellular bioenergetics and generation of reactive oxygen species in human kidney proximal tubular (HK-2) cells that were exposed to recombinant suPAR.

Results: The suPAR level was assessed in 3827 patients who were undergoing coronary angiography, 250 who were undergoing cardiac surgery, and 692 who were critically ill. Acute kidney injury developed in 318 patients (8%) who had undergone coronary angiography. The highest suPAR quartile (vs. the lowest) had an adjusted odds ratio of 2.66 (95% confidence interval [CI], 1.77 to 3.99) for acute kidney injury and 2.29 (95% CI, 1.71 to 3.06) for acute kidney injury or death at 90 days. Findings were similar in the surgical and critically ill cohorts. The suPAR-overexpressing mice that were given contrast material had greater functional and histologic evidence of acute kidney injury than wild-type mice. The suPAR-treated HK-2 cells showed heightened energetic demand and mitochondrial superoxide generation. Pretreatment with a uPAR monoclonal antibody attenuated kidney injury in suPAR-overexpressing mice and normalized bioenergetic changes in HK-2 cells.

Conclusions: High suPAR levels were associated with acute kidney injury in various clinical and experimental contexts. (Funded by the National Institutes of Health and others.).

Figure 1 (facing page).. Risk of Acute Kidney Injury after Coronary Angiography.

Panel A shows the odds ratios and 95% confidence intervals (CIs; I bars) for acute kidney injury according to quartiles of soluble urokinase plasminogen activator receptor (suPAR) level before the procedure. Model 1 was unadjusted; model 2 was adjusted for age, sex, race, smoking history, diabetes mellitus, congestive heart failure, hypertension, and cohort (EmCAB [Emory Cardiovascular Biobank] or CASABLANCA [Catheter Sampled Blood Archive in Cardiovascular Diseases]); and model 3 incorporated the aforementioned variables in addition to acute myocardial infarction, revascularization, volume of contrast material, and baseline kidney function (estimated glomerular filtration rate). Quartile 1 was the reference group (1.00) in all models, with a suPAR level of less than 2475 pg per milliliter. The suPAR levels in quartiles 2, 3, and 4 were 2475 to 3198 pg per milliliter, 3199 to 4183 pg per milliliter, and 4184 pg per milliliter or more, respectively. Panel B shows the odds ratios for acute kidney injury per 1 unit natural log of suPAR according to subgroup in the unadjusted analysis (model 1). Stage 3 chronic kidney disease was defined as an estimated glomerular filtration rate of less than 60 ml per minute per 1.73 m² of body-surface area.

Figure 2 (facing page).. Acute Kidney Injury in Wild-Type and Transgenic Mice before and after Treatment with Anti-uPAR Monoclonal Antibody.

Panels A through F show representative kidney histologic findings, on high-power view, with the use of periodic acid–Schiff stain in samples obtained from wild-type mice and suPAR-transgenic mice at baseline (Panels A and D) and 48 hours after the administration of iohexol (Panels B, C, E, and F) stratified according to treatment (IgG isotype, in Panels B and E; or urokinase plasminogen activator receptor [uPAR] monoclonal antibody, in Panels C and F). Wild-type mice and suPAR-transgenic mice had largely normal kidney morphologic features at baseline. At 48 hours after iohexol administration, tubular vacuolization could be seen in all wild-type and suPAR-transgenic mice (arrows). The suPAR-transgenic mice that received IgG (Panel E) had more severe renal injuries than mice in any other studied groups. The suPAR-transgenic mice that received the uPAR monoclonal antibody (Panel F) had significantly less severe tubular vacuolization than their counterparts that received the IgG isotype (Panel E). Panel G shows serum creatinine levels measured before and after the administration of contrast material. To convert the values for creatinine to micromoles per liter, multiply by 88.4. As compared with baseline, the serum creatinine level at 24 hours after iohexol injection was increased in all examined groups. The suPAR-transgenic mice that received the IgG isotype had much higher creatinine levels than mice in any other groups. There was no significant between-group difference at baseline. Panel H shows a semiquantitative scoring system that accounts for glomerular and tubular changes associated with acute kidney injury; kidney-injury scores range from 0 to 12, with higher scores indicating more severe kidney injury. (One mouse in the suPAR-transgenic group that received IgG died unexpectedly, so data are shown for 19 mice.) The analyses in Panels G and H were conducted with two-way analysis of variance. In Panels G and H, bars represent means, and I bars ±1 SD; circles or squares indicate values in individual mice.