MECHANISMS OF IRON-INDUCED PROXIMAL TUBULE INJURY IN RAT REMNANT KIDNEY

xThe proposition that proximal tubule (PT) iron accumulation may cause PT injury by lysosomal destabilization or reactive oxygen species generation in human and animal chronic renal disease was examined in partially nephrectomized [remnant kidney (RK)] and sham-operated (SO) Wistar rats. Electron microscopic histochemistry with horseradish peroxidase indicated iron uptake into and release from lysosomes. PT cytoplasmic iron was seen in RK but not in SO by energy-dispersive X-ray spectrometry. Total (9.66 +/- 1.89 vs. 3.30 +/- 0.31 nmol/mg protein; P < 0.01), low-molecular-weight (1.39 +/- 0.09 vs. 0.91 +/- 0.07; P < 0.001), and catalytic iron (1.88 +/- 0.27 vs. 1.28 +/- 0.04; P = 0.05) were higher in RK cytoplasm than in SO. Lysosomal enzyme activity was greater in RK than in SO [e.g., N-acetyl-beta-D-glucosaminidase (NAG): 0.75 +/- 0.05 vs. 0.57 +/- 0.06 mu mol p-nitrophenol . h(-1) . mg protein(-1); P < 0.05] and was increased further by chronic iron loading (e.g., RK and NAG: 0.84 +/- 0.04 vs. 0.60 +/- 0.07; P < 0.05). There was no enzymatic evidence of lysosomal fragility, and chronic iron loading of RK decreased fragility as assessed by NAG release (1.36 +/- 0.14 vs. 2.17 +/- 0.14; P < 0.05). The lipid peroxide, malondialdehyde, was increased by iron loading in RK cytoplasm (1.72 +/- 0.27 vs. 0.81 +/- 0.14 nmol/mg protein; P < 0.05) but not in the lysosomal fraction (4.60 +/- 0.69 vs. 5.43 +/- 1.47; not significant). In rat RK, filtered transferrin iron is reabsorbed by endocytosis across PT luminal membrane into lysosomes, from which reactive iron is released to cause cytoplasmic peroxidative damage, despite preservation of lysosomal function and stability.