Liver fatty-acid-binding protein in heart and kidney allograft recipients in relation to kidney function

Mammalian intracellular fatty-acid-binding proteins (FABPs), a large multigene family, encode 14-kD proteins that are members of a superfamily of lipid-binding proteins. FABPs are tissue specific. Liver-type FABP (L-FABP) can be filtered through the glomerulus owing to its small molecular size, similar to cystatin C, but it is reabsorbed by proximal tubule epithelial cells like other small proteins. In the human kidney, L-FABP is expressed predominantly in proximal tubules. It had been suggested that the presence of L-FABP in urine reflects hypoxic conditions resulting from decreased peritubular capillary flow, serving as a marker of acute kidney injury. The aim of this study was to assess urinary L-FABP in 111 heart and 76 kidney transplant recipients in relation to kidney function. Complete blood count, urea, fasting glucose, creatinine, and the N-terminal fragment of brain natriuretic protein were studied by standard laboratory methods; L-FABP and cystatin C, by ELISA using commercially available kits. Kidney transplant recipients displayed significantly higher L-FABP than heart recipients. Upon univariate analysis, urinary L-FABP correlated, with serum creatinine, cystatin C and estimated glomerular filtration ratio (eGFR) in kidney allograft recipients. However, in heart transplant recipients it was not related to kidney function, as reflected by creatinine or eGFR; was strongly related to cystatin C (r=0.34; P<.001) and urinary creatinine (r=-0.29; P<.01), and NGAL (r=0.29; P<.01). Upon multiple regression analysis, the best predictor of urinary L-FABP in kidney allograft recipients, was eGFR whereas in heart recipients, no parameter independently predicted L-FABP. Successful heart transplantation is associated with kidney injury as reflected by a reduced eGFR; however, in this population, L-FABP did not serve as a marker of kidney function. In contrast, in kidney allograft recipients, L-FABP may be a potential early marker for impaired kidney function/injury.