URINARY FOLLISTATIN-LIKE 3 IS A MARKER REFLECTING THE SEVERITY OF TUBULAR DAMAGE IN PATIENTS WITH ACUTE KIDNEY INJURY

Acute kidney injury (AKI) is a high-risk condition that can progress to chronic kidney disease (CKD) and is commonly triggered by renal ischemia, sepsis, and nephrotoxic substances. AKI affects renal prognosis and life prognosis, even if its severity is mild. Although AKI is defined by changes in serum creatinine and urine output, the poor sensitivity and delayed kinetics of creatinine can postpone diagnosis and treatment. Hence, early, non-invasive biomarkers are needed to guide risk stratification and management. Follistatin-like 3 (FSTL3) is an activin-binding protein that, like follistatin, antagonizes the function of activin A, a member of the TGF-β superfamily. In animal models of AKI, blockade of activin A with exogenous follistatin attenuates renal injury and improves renal function, suggesting that endogenous activin A negatively regulates renal tubular repair after AKI. While follistatin functions as a local regulator of activin A in multiple tissues, the role of FSTL3 in the kidney remains unclear.

We investigated the localization of FSTL3 in normal human kidneys and examined whether urinary FSTL3 could be a useful biomarker for AKI. In a prospective observational study, we enrolled patients with AKI (n = 160) and healthy adults (n = 19). The diagnosis and severity of AKI were determined using the AKI criteria outlined in the KDIGO guidelines. Serum and urinary FSTL3 levels at the time of AKI diagnosis were measured by ELISA. The correlations between urinary FSTL3 and clinical parameters were analyzed. Immunohistochemistry for FSTL3 was performed on histologically normal sections from nephrectomy specimens (renal cell carcinoma), with segment markers including Na-Cl cotransporter (NCC), uromodulin, aquaporin-1 (AQP1), megalin, and aquaporin-2 (AQP2).

FSTL3 was localized in renal tubules of normal kidneys. FSTL3-producing cells were positive for Na-Cl cotransporter and uromodulin, but negative for aquaporin 1, megalin, and aquaporin 2, indicating predominant expression in distal convoluted tubules and the thick ascending limb. Urinary FSTL3, undetectable in healthy adults, was significantly increased in patients with AKI (0.00 ± 0.00 vs. 20.67 ± 30.98 ng/mL, p <0.001). Urinary FSTL3 levels were increased according to the severity of AKI in the KDIGO classification (stage 1: 11.37 ± 12.42 ng/mL; stage 2: 10.04 ± 9.29 ng/mL; stage 3: 20.70 ± 17.26 ng/mL). Furthermore, urinary FSTL3 levels were significantly elevated in patients who required renal replacement therapy (RRT) compared with those who did not (21.37 ± 16.82 vs. 11.79 ± 14.26 ng/mL, p < 0.001), and in patients who progressed to irreversible end-stage renal disease (ESRD) compared with those who did not (28.35 ± 14.08 vs. 17.70 ± 17.00 ng/mL, p < 0.01). Urinary FSTL3 levels were significantly correlated with urinary protein level, urinary NAG, urinary α1-microglobulin, urinary β2-microglobulin, urinary NGAL, KIM-1, and serum FSTL3, but not with L-FABP. 

FSTL3 is localized in the distal tubules, and the significant increase in FSTL3 in the urine of AKI patients reflects disease severity, RRT requirement, and progression to ESRD. These findings support urinary FSTL3 as a novel, non-invasive biomarker for monitoring the severity of acute tubular injury and for risk stratification in AKI.