Changes in free and esterified cholesterol - Hallmarks of acute renal tubular injury and acquired cytoresistance

Acute tubular cell injury is accompanied by plasma membrane phospholipid br eakdown. Although cholesterol is a dominant membrane lipid which interdigit ates with, and impacts, phospholipid homeostasis, its fate during the induc tion and recovery phases of acute renal failure (ARF) has remained ill defi ned. The present study was performed to ascertain whether altered cholester ol expression is a hallmark of evolving tubular damage. Using gas chromatog raphic analysis, free cholesterol (FC) and esterified cholesterol (CE) were quantified in: 1) isolated mouse proximal tubule segments (PTS) after 30 m inutes of hypoxic or oxidant (ferrous ammonium sulfate) injury; 2) cultured proximal tubule (HK-2) cells after 4 or 18 hours of either ATP depletion/C a2+ ionophore- or ferrous ammonium sulfate-mediated injury; and 3) in renal cortex 18 hours after induction of glycerol-induced myoglobinuric ARF, a t ime corresponding to the so-called "acquired cytoresistance" state tie, res istance to further renal damage). Hypoxic and oxidant injury each induced s imilar to 33% decrements in CE (but not FC) levels in PTS, corresponding wi th lethal cell injury (similar to 50 to 60% LDH release). When comparable C E declines were induced in normal PTS by exogenous cholesterol esterase tre atment, proportionate lethal cell injury resulted. During models of slowly evolving HK-2 cell injury, progressive CE increments occurred: these were f irst noted at 4 hours, and reached similar to 600% by 18 hours. In vivo myo globinuric ARF produced comparable renal cortical CE (and to a lesser exten t FC) increments. Renal CE accumulation strikingly correlated with the seve rity of ARF (eg, blood urea nitrogen versus CE; r, 0.84). Mevastatin blocke d cholesterol accumulation in injured HK-2 cells, indicating de novo synthe sis was responsible. Acute tubule injury first lowers, then raises, tubule cholesterol content. Based on previous observations that cholesterol has cy toprotectant properties, the present findings have potential relevance for both the induction and maintenance phases of ARF.