Urinary fatty acid-binding protein 1: an early predictive biomarker of kidney injury

Abstract

In the development of novel therapeutic strategies for kidney disease, new renal biomarkers for early detection and accurate evaluation of renal injury are urgently required for both acute kidney injury (AKI) and chronic kidney disease (CKD). Fatty acid-binding protein 1 (FABP1) is expressed in renal proximal tubule cells and shed into urine in response to hypoxia caused by decreased peritubular capillary blood flow. To clarify the role of urinary FABP1 in kidney disease, we established human FABP1 transgenic mice and evaluated the responses of FABP1 to several AKI and CKD models. Moreover, there are accumulating clinical data that urinary FABP1 can detect human AKI earlier than serum creatinine and can distinguish the risk population for AKI. Investigation with "humanized" FABP1 transgenic mice and measurement of clinical samples allowed us to develop urinary FABP1 as a new renal biomarker. Further clinical studies are necessary to confirm the potential of urinary FABP1 for clinical application.

Figures

Fig. 1.

Immunohistochemical analysis of human fatty acid-binding protein 1 (FABP1) transgenic (Tg) mice. Immunohistochemical analysis was performed with the antibody that reacts with mouse and human FABP1. Because of the silencing sequence in the upstream promoter region, mouse FABP1 expression in C57BL/6 wild-type (WT) kidney was absent under normal conditions (A). Renal ischemia-reperfusion (I/R) injury (C) or exposure to cisplatin (CP; E) did not induce FABP1 expression in WT kidney. In human FABP1 Tg mouse kidney, constitutional expression of FABP1 protein was found under normal conditions (B) and FABP1 expression was amplified and expanded in the S3 segment of the cortex 24 h after renal I/R (D). In 20 mg/kg CP-injected Tg kidney (F), FABP1 expression of damaged tubules was diminished or disappeared, whereas viable tubules remained positive 72 h after CP injection. AKI, acute kidney injury.

Fig. 2.

Responses of renal biomarkers in animal ischemic AKI. AKI was induced by several different ischemia times (0, 5, 15, 30 min), and the responses of blood urea nitrogen (BUN; A), acute tubular necrosis (ATN) score (B), urinary FABP1 (C), and urinary N-acetyl-d-glucosaminidase (NAG; D) were evaluated. Urinary FABP1 levels even at 3 h could reflect the injury in ischemic AKI, whereas BUN and urinary NAG could not predict the severity of AKI. Original data from Negishi et al. (37).

Fig. 3.

Responses of renal biomarkers in animal CP-induced AKI. Different doses of CP (0, 5, 10, 20 mg/kg) were administered, and the responses of BUN (A), ATN score (B), urinary FABP1 (C), and urinary NAG (D) were evaluated. BUN and urinary FABP1 could reflect the severity of AKI; however, urinary FABP1 responded earlier than BUN. Original data from Negishi et al. (37).

Fig. 4.

Urinary FABP1 levels in radiocontrast medium (RCM)-induced AKI. Urinary FABP1 measurement of 66 patients who had a serum creatinine level >1.2 mg/dl (>106 μmol/l) and

Fig. 5.

Correlation of urinary markers and tubulointerstitial injury. Correlations between urinary markers [protein (top), NAG (middle), and FABP1 (bottom)] and fibrotic area and F4/80 positively stained area in adenine-induced tubulointerstitial injury model are shown. •, Adenine diet group; ▴, treatment group; ○, normal diet group. Original data from Tanaka et al. (49).

Fig. 6.

Urinary FABP1 levels of responders and nonresponders in cyclooxygenase 2 (COX2) inhibitor-induced tubulointerstitial injury model. A: COX2 inhibitor-induced low peritubular capillary blood flow determined the animals as responders or nonresponders. B: responders (urinary FABP1 >50 μg/g creatinine) showed larger interstitial fibrosis area than nonresponders (<50 μg/g creatinine). Original data from Tanaka et al. (50). Quantitative analysis was performed by the computer-aided evaluation program AIS (Fujifilm, Tokyo, Japan). Asterisk denotes P < 0.05.

Fig. 7.

Correlations between urinary FABP1 level and peritubular capillary blood flow and ischemia time of the transplanted kidney. Urinary FABP1 was compared with 1/blood flow (A) and ischemia time (B). Where renal ischemia is more severe, 1/blood flow is larger. The ischemic time in living-related renal transplantation was defined as the period between the clamp of the donor's renal artery and the appearance of virgin urine from the recipient's ureter. Original data from Yamamoto et al. (60).

Fig. 8.

Double immunostaining of human (h)FABP1 and 4-hydroxyhexenal (HHE) in animal AKI models. Double immunostaining of hFABP1 (brown) and HHE (purple) in I/R-AKI (A) and CP-AKI (B) is shown. HHE accumulation was found in severely damaged tubular cells, and relatively preserved tubular cells showed positive staining of FABP1. Original data partly from Negishi et al. (36).

Fig. 9.

Urinary FABP1, plasma FABP1, and urine-to-plasma ratio of FABP1 in CP-AKI mice. Urinary and plasma FABP1 levels were examined in CP-AKI (20 mg/kg). Urine and plasma levels of hFABP1 increased after CP injection. However, urinary level was much higher than plasma level and urine-to-plasma ratio increased >25-fold compared with 0 h. Original data from Negishi et al. (36).

Fig. 10.

A strategy for biomarker development. CKD, chronic kidney disease; LPS, lipopolysaccharide; CLP, cecal ligation and puncture; CPB, cardiopulmonary bypass; UUO, unilateral ureter obligation; Alb, albumin; FGS, focal glomerulosclerosis; DM diabetes mellitus; IgAN, IgA nephropathy.