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1 Division of Nephrology and Hypertension, Mayo Clinic/Foundation, Rochester, MN, USA
2 Department of Molecular Genetics, Alton Ochsner Medical Foundation, New Orleans, LA, USA
3 Department of Pathology, Mayo Clinic/Foundation, Rochester, MN, USA
* To whom correspondence should be addressed. E-mail: nath.karl{at}mayo.edu.
Heme oxygenase-1 (HO-1), a cytoprotective gene, is commonly induced in renal tubules in the diseased kidney. Since proteinuria is a hallmark for kidney disease, we examined the relationship between proteinuria and tubular induction of HO-1, specifically questioning whether increased trafficking of protein across the renal tubular epithelium, as a consequence of proteinuria, induces tubular expression of HO-1. We examined a model of glomerular proteinuria induced by daily injections of bovine serum albumin (BSA) which is associated with increased tubular uptake of filtered protein, and a model of tubular proteinuria induced by maleate, the latter exhibiting decreased tubular uptake and trafficking of protein. The BSA-model of glomerular proteinuria failed to exhibit induction of HO-1; HO-1 was not induced in proximal tubular epithelial cells exposed to BSA. In contrast, in maleate nephropathy wherein tubular uptake of protein is decreased because of generalized proximal tubular injury induced by maleate, HO-1 was strongly induced in proximal tubules; inhibition of HO activity in maleate nephropathy worsened proteinuria, renal histologic injury, and apoptosis. In renal proximal tubular epithelial cells, maleate induced HO-1 and caused apoptosis, the latter increased when HO activity is inhibited. From these studies, we conclude that expression of HO-1 in the diseased kidney cannot be ascribed to the tubular uptake and metabolism of protein such as albumin, and that the expression of HO-1 in a model of tubular proteinuria reflects a functionally significant stress response to toxin-induced, proximal tubular injury.
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