OBJECTIVETerminal stage renal failure is the final common fate of chronic nephropathies independent of the type of initial insult. Abnormally filtered proteins have an intrinsic renal toxicity linked to their over-reabsorption by proximal tubular cells and activation of tubular-dependent pathways of interstitial inflammation and fibrosis. The functional importance of tubulointerstitial events in progressive renal disease is supported by evidence that the severity of tubular interstitial damage strongly correlates with the risk of renal failure. The present study aimed to investigate the expressive tendency of some pro-fibrosis genetic factors mRNA, including thrombospondin-1 (TSP-1), transforming growth factor-beta1 (TGF-beta1), connective tissue growth factor (CTGF) and the major component of extracellular matrix-fibronectin (FN) in renal tissues at different time points during early stage of renal lesions caused by proteinuria in bovine serum albumin (BSA) injection-induced proteinuria overload nephrotic young rats and the significance of these factors on tubulointerstitial fibrosis development.

METHODSFemale young Wistar rats aged 3-4 weeks with proteinuria overload nephrosis induced by BSA (1.0 g/d) injected intraperitoneally were used as experimental models. The 80 young rats were divided into control group (n = 40) and BSA injected group (n = 40). At different time points (weeks 1, 2, 3 and 4), the urinary protein was measured by Coomassie brilliant blue colorimetric assay; the renal tissues morphologic changes were evaluated after HE staining; the P(65)/Rel-A, TSP-1, TGF-beta1 and CTGF mRNA expression in renal tissues was determined by in situ hybridization method and the FN mRNA expression was detected by Northern blot. The experimental data were evaluated by statistics software SPSS10.0.

RESULTS(1) Three to four weeks after BSA injection, heavy proteinuria was observed in the rats of BSA group (week 3: 104.3 +/- 21.8 mg/24 h; week 4: 131.1 +/- 18.3 mg/24 h). The proteinuria deteriorated progressively afterwards. Histopathological examination revealed that inflammatory cells infiltrated into tubulointerstitial areas extensively, protein casts were seen in tubules and edema occurred in tubulointerstitial areas. (2) In situ hybridization showed that NF-kappaB (P(65)/Rel-A) mRNA expression was up-regulated progressively in nuclei of tubular epithelial cells, the semi-quantitative scores (at 1, 2, 3 and 4 weeks) were 2.33 +/- 0.20, 2.76 +/- 0.12, 2.96 +/- 0.19, and 3.76 +/- 0.18, respectively (F = 37.34, P < 0.01). (3) At 1 week after BSA injection, the TSP-1 mRNA expression appeared in glomeruli and increased, but was light in tubulointerstitial areas, its expressive peak was observed at week 2, and declined to mild after weeks 3 and 4. The semi-quantitative scores at different time points suggested that TSP-1 mRNA was expressed mainly in early stage of lesion in this model, then, this tendency turned to a flat roof smoothly. (4) TGF-beta1 and CTGF mRNA was up-regulated simultaneously in tubular epithelial cells (F = 8.80, P < 0.01 F = 19.41, P < 0.01). (5) Northern blotting showed that FN mRNA was considerably up-regulated at second week in the kidneys of rats in BSA group, 2.7-fold higher at week 4 than that at week 1 in BSA group rats, and was 3.6-fold higher than that of control group.

CONCLUSIONThe present study suggested that NF-kappaB (P(65)/Rel-A) mRNA expression and its activity was enhanced significantly by proteinuria-loading and synchronized with high expression of TSP-1, TGF-beta1, and CTGF mRNA in the kidney, at the same time, FN mRNA was up-regulated in renal tissues and an aggravating tendency in tubulointerstitial lesions was observed in nephrotic young rats with heavy proteinuria.