Background: Renal diseases are conventionally classified into acute and chronic disorders. We questioned whether acute, reversible, renal insults may be induced to incite a chronic scarring process, employing as an acute insult the glycerol model of heme protein-induced renal injury.
Methods: Rats were subjected to weekly injections of hypertonic glycerol for up to six months. Renal function was serially determined, and the effect of such insults on renal histology and renal expression of collagen and fibrogenic cytokines was assessed.
Results: After the first injection of glycerol, which, expectedly, induced a prompt fall in the glomerular filtration rate (GFR), subsequent injections encountered a remarkable renal resistance in that the fall in GFR was markedly blunted. This resistance to acute decline in renal function in rats subjected to repetitive injections of glycerol was accompanied by less necrosis and apoptosis of renal tubular epithelial cells after such injections. The attenuation in the fall in GFR in response to repetitive exposure to glycerol-induced heme protein injury was maintained for up to six months. A progressive decline in GFR appeared after three months and was accompanied by histologic tubulointerstitial injury, the latter assessed at six months. These kidneys demonstrated up-regulation of collagen I, III, and IV in conjunction with increased expression of the oxidant-inducible, chemotactic cytokine, monocyte chemoattractant protein-1 (MCP-1), and the oxidant-inducible, fibrogenic cytokine, transforming growth factor-beta1 (TGF-beta1). The exposure of the kidney to a single injection of hypertonic glycerol increased the expression of both cytokines some three to five days following this exposure, while the exposure of NRK 49F cells in culture to an iron-dependent model of oxidative stress also increased expression of TGF-beta1 and collagen mRNAs.
Conclusions: We conclude that this nephrotoxic insult, repetitively administered, encounters a resistance in the kidney such that the expected fall in GFR does not occur. However, with time, such resistance is accompanied by a decrease in GFR, the latter associated with chronic tubulointerstitial disease. Thus, a long-term cost is exacted, either along with, or as a consequence of, such resistance. We suggest that chronic up-regulation of such oxidant-inducible genes such as TGF-beta1 and MCP-1 contributes to tubulointerstitial disease, and iron-mediated oxidative stress may directly induce TGF-beta1.