The Mevalonate Pathway during Acute Tubular Injury

Abstract

Renal injury evokes tubular cholesterol accumulation, mediated in part by increased HMG CoA reductase (HMGCR) levels. The present study was undertaken to define potential molecular determinants of these changes and to ascertain the relative importance of increased cholesterol production versus mevalonate pathway-driven protein prenylation, on the emergence of the so-called postrenal injury "cytoresistant state." Cultured proximal tubule (HK-2) cells were subjected to Fe or ATP depletion injury, followed 1 to 24 hours later by assessments of: 1) sterol transcription factor expression (SREBP)-1 and -2); 2) HMGCR mRNA levels; and 3) Ras/Rho prenylation. HMGCR mRNA and Ras/Rho prenylation were also assessed after in vivo ischemic and Fe-mediated renal damage. Using specific inhibitors, the relative importance of protein prenylation versus terminal cholesterol synthesis on HK-2 cell susceptibility to injury was also assessed. Acute injury induced HK-2 cell SREBP disruption and reductions in HMGCR mRNA. Renal cortical HMGCR mRNA also fell in response to either in vivo ischemic or Fe-mediated oxidant damage. At 24 hours after in vitro/in vivo injury, a time of cholesterol buildup, no increase in Ras/Rho prenylation was observed. Prenylation inhibitors did not sensitize HK-2 cells to injury. Conversely, squalene synthase (terminal cholesterol synthesis) blockade sensitized HK-2 cells to both Fe and ATP depletion attack. We concluded that: 1) acute tubular cell injury can destroy SREBPs and lower HMGCR mRNA. This suggests that posttranscriptional/translational events are responsible for HMGCR enzyme and cholesterol accumulation after renal damage. 2) Injury-induced cholesterol accumulation appears dissociated from increased protein prenylation. 3) Cholesterol accumulation, per se, seems to be the dominant mechanism by which the mevalonate pathway contributes to the postrenal injury cytoresistant state.

Experimental Glomerulopathy Alters Renal Cortical Cholesterol, SR-B1, ABCA1, and HMG CoA Reductase Expression

“…By analogy with the SR-B1 and ABCA1 changes, marked suppression of renal cholesterol synthesis, via a down-regulation of HMGCR enzyme, would also be expected. 37,39 Hence, the final goal of this study was to seek confirmation for this last assumption. Paradoxically, results opposite to those that were predicted were obtained: HMGCR protein mass, as assessed by Western blot, was elevated in the setting of both NTS and PHN.…”

“…After completing 2-day incubations, the cells were washed with Hanks' balanced salt solution, they were recovered with a cell scraper, and proteins were extracted for Western blotting, as previously described. 39 [Note: addition of normal mouse serum to HK-2 cells causes an approximate 25% in cellular cholesterol levels via increased import (unpublished observations; RZ). Hence, feedback inhibition of the SR-B1 importer might be expected].…”

“…39 The PHN model was selected for analysis because it, unlike the NTS model, was associated with progressive proteinuria and cholesterol accumulation. cDNA bands were visualized and quantified by densitometry.…”

“…cDNA bands were visualized and quantified by densitometry. 39 HMGCR cDNA bands were expressed as a ratio to the simultaneously obtained GADPH cDNA bands. RNA samples obtained from a total of four controls/five PHN kidneys and nine controls/eight PHN kidneys were probed at the 10-and 30-day time points, respectively.…”

HMG–CoA Reductase Activation and Urinary Pellet Cholesterol Elevations in Acute Kidney Injury

“…A seemingly constant consequence of ischemic or toxic acute kidney injury (AKI) is an upregulation of renal cortical HMG-CoA reductase (HMGCR) activity (1)(2)(3)(4)(5)(6)(7)(8)(9)(10). Within 12 to 24 hours post-AKI induction, this culminates in an approximately 20% to 40% increase in renal cortical cholesterol content.…”