Renal cholesterol accumulation: a durable response after acute and subacute renal insults

Abstract

Proximal tubular cholesterol levels rise within 18 hours of diverse forms of acute renal tubular injury (eg, myoglobinuria, ischemia/reperfusion, urinary tract obstruction). These increments serve to protect against further bouts of tubular attack (so-called "acquired cytoresistance"). Whether these cholesterol increments are merely transitory, or persist into the maintenance phase of acute renal failure (ARF), has not been previously defined. Furthermore, whether subacute/insidious tubular injury [eg, cyclosporine A (CSA), tacrolimus toxicity], nontubular injury (eg, acute glomerulonephritis), or physiological stress (eg, mild dehydration) impact renal cholesterol homeostasis have not been addressed. This study sought to resolve these issues. Male CD-1 mice were subjected to glycerol-induced ARF. Renal cortical-free cholesterol (FC) and cholesterol ester (CE) levels were determined 3, 5, 7, or 14 days later, and the values contrasted to prevailing blood-urea nitrogen concentrations. The impact of 40 minutes of unilateral renal ischemia plus reflow (3 to 6 days) on mouse cortical FC/CE content was also assessed. Additionally, FC/CE levels were measured in rat renal cortex either 10 days after CSA or tacrolimus therapy, or 48 hours after induction of nephrotoxic serum nephritis. Finally, the impact of overnight dehydration on mouse renal cortical/medullary FC/CE profiles was determined. Compared to sham-treated animals, glycerol, CSA, tacrolimus, ischemia-reperfusion, and nephrotoxic serum each induced dramatic CE +/- FC elevations, rising as much as 10x control values. In the glycerol model, striking correlations (r </= 0.99) between FC/CE and blood-urea nitrogen levels were observed. The FC/CE increases were specific to damaged kidney (glycerol did not raise hepatic FC/CE; unilateral renal ischemia did not alter contralateral renal FC/CE levels). Overnight dehydration raised renal CE levels, most notably in the medulla.

Conclusions: FC/CE accumulation is a hallmark of the maintenance phase of ischemic and nephrotoxic ARF, and can reflect its severity. That cholesterol accumulation can result from glomerular injury and dehydration suggests that it is a generic renal stress response, with potential relevance extending beyond just the phenomenon of acquired cytoresistance.

Figure 1.

BUN concentrations at 3, 5, 7, or 14 days after induction of glycerol-mediated myohemoglobinuric ARF. The closed bars and shaded bars represent glycerol-treated and control mice, respectively. Decreasing azotemia is noted in the glycerol-treated mice, with near recovery being observed by 14 days after glycerol injection.

Figure 2.

CE levels corresponding with the time-course experiment presented in Figure 1 ▶ . The closed bars and shaded bars represent the glycerol and control mice, respectively. The P values represent unpaired Student’s t-test analysis at each time point. Within each glycerol-treated group, there was significant variation in CE and BUN values: the r values illustrate striking positive correlations between these two parameters at each time point. Individual values for the day 7 and day 14 results are presented in Table 1 ▶ .

Figure 3.

FC levels corresponding with the time-course experiment presented in Figure 1 ▶ . The results with FC analysis primarily mimicked those observed with CE analysis (see Figure 2 ▶ legend for explanations).

Figure 4.

FC and CEs in sham-operated control (cont) mice, and in mice subjected to left (Lt) renal ischemia with the contralateral right (Rt) kidney left in place. Induction of unilateral left renal ischemia induced profound elevations in both FC and CE values, as assessed 3 to 6 days after ischemia (all postischemic values treated as a single group; see text). Note that there was no difference in FC/CE values between kidney samples obtained from the sham-operated (cont) group and the right kidney harvested from mice with left renal ischemia. Thus, the presence of left renal ischemic/reperfusion injury did not impact contralateral CE/FC values.

Figure 5.

FC and CE values in rat kidneys after 10 days of either CSA or tacrolimus (Tacr.) therapy. Both CSA and tacrolimus (Tacr.) induced dramatic CE increments in renal cortex, compared to vehicle-treated controls (cont). In contrast, only small, but statistically significant, increases in FC values were observed (*, P < 0.03; ∧, P < 0.05 versus controls).

Figure 6.

FC and CE at 48 hours after induction of NTS nephritis. NTS (anti-glomerular basement membrane antibody) mediated glomerulonephritis caused a striking increase in CE levels. Although FC levels were slightly higher as well, this did not achieve statistical significance.

Figure 7.

CE levels in mouse renal cortex and medulla under normally hydrated conditions and after overnight dehydration. Under either condition, the medulla demonstrated significantly higher CE values versus cortical values (P by paired Student’s t-test). Overnight dehydration significantly increased CE elevations in both cortex and medulla (P values by unpaired Student’s t-test).