Angiotensin-converting enzyme is upregulated in the proximal tubules of rats with intense proteinuria

Persistent proteinuria is considered a deleterious prognostic factor in most progressive renal diseases. However, the mechanisms by which proteinuria induces renal damage remain undetermined. Since proximal tubular cells possess all the machinery to generate angiotensin II (Ang II), we approached the hypothesis that proteinuria could elicit the renal activation of the renin-angiotensin system in a model of intense proteinuria and interstitial nephritis induced by protein overload. After uninephrectomy (UNX), Wistar-Kyoto rats received daily injections of 1 g BSA or saline for 8 days. The mean peak of proteinuria was observed at the fourth day (538 +/- 89 versus 3 +/- 1 mg/24 h in UNX controls; n = 12: P < 0.05) and was increased during the whole study period (at the eighth day: 438 +/- 49 mg/24 h; n = 12; P=NS). Morphological examination of the kidneys at the end of the study showed marked tubular lesions (atrophy, vacuolization, dilation, and casts), interstitial infiltration of mononuclear cells, and mesangial expansion. In relation to UNX control rats, renal cortex of BSA-overloaded rats showed an increment in the gene expression of angiotensinogen (2.4-fold) and angiotensin-converting enzyme (ACE) (2.1-fold), as well as a diminution in renin gene expression. No changes were observed in angiotensin type 1 (AT(1)) receptor mRNA expression in both groups of rats. By in situ reverse transcription-polymerase chain reaction and immunohistochemistry, ACE expression (gene and protein) was mainly localized in proximal and distal tubules and in the glomeruli. By immunohistochemistry, angiotensinogen was localized only in proximal tubules, and AT(1) receptor was localized mainly in proximal and distal tubules. In the tubular brush border, an increase in ACE activity was also seen (5.5 +/- 0.5 versus 3.1 +/- 0.7 U/mg protein X 10(-4) in UNX control; n = 7; P < 0.05). Our results show that in the kidney of rats with intense proteinuria, ACE and angiotensinogen were upregulated, while gene expression of renin was inhibited and AT(1) was unmodified. On the whole, these data suggest an increase in Ang II intrarenal generation. Since Ang II can elicit renal cell growth and matrix production through the activation of AT(1) receptor, this peptide may be responsible for the tubulointerstitial lesions occurring in this model. These results suggest a novel mechanism by which proteinuria may participate in the progression of renal diseases.