Proteinuria induces tubular cell turnover: A potential mechanism for tubular atrophy

Background. Proteinuria and tubular atrophy have both been closely linked w ith progressive renal failure. We hypothesized that apoptosis may be induce d by tubular cell exposure to heavy proteinuria, potentially leading to tub ular atrophy. Apoptosis was studied in a rat model of "pure" proteinuria, w hich does not induce renal impairment, namely protein-overload proteinuria. Methods. Adult female Lewis rats underwent intraperitoneal injection of 2 g of bovine serum albumin (BSA, N = 16) or sham saline injections (controls, N = 8) daily for seven days. Apoptosis was assessed at day 7 in tissue sec tions using in situ end labeling (ISEL) and electron microscopy. ISEL-posit ive nuclei (apoptotic particles) were counted in blinded fashion using imag e analysis with NIH Image. Cell proliferation was assessed by detection of mRNA for histone by in situ hybridization, followed by counting of positive cells using NIH Image. Results. Animals injected with saline showed very low levels of apoptosis o n image analysis. BSA-injected rats had heavy proteinuria and showed both c ortical and medullary apoptosis on ISEL. This was predominantly seen in the tubules and, to a lesser extent, in the interstitial compartment. Overall, the animals injected with BSA showed a significant 30-fold increase in the number of cortical apoptotic particles. Electron microscopy of tubular cel ls in a BSA-injected animal showed a progression of ultrastructural changes consistent with tubular cell apoptosis. The BSA-injected animals also disp layed a significant increase in proximal tubular cell proliferation. This i ncreased proliferation was less marked than the degree of apoptosis. Conclusion. Protein-overload proteinuria in rats induces tubular cell apopt osis. This effect is only partially balanced by proliferation and potential ly provides a direct mechanism whereby heavy proteinuria can induce tubular atrophy and progressive renal failure.