Background: In a previous study, nitric oxide synthases (NOS) were found to be strongly expressed in the tubular epithelium of kidneys of a transgenic mouse model of sickle cell disease (alphaHbetaS[betaMDD]). Because NOS activity is often associated with peroxynitrite formation when superoxide radical (.O-2) is present in abundance, we examined the kidneys of sickle cell mice for nitrotyrosine, considered to be a footprint of ONOO-.
Methods: Western blot and immunohistochemistry for nitrotyrosine was carried out. Since peroxynitrite and other reactive oxygen radicals are capable of causing apoptosis, we also performed agarose gel electrophoresis of kidney DNA and TUNEL staining of nuclei, indicators of apoptosis.
Results: Nitration of tyrosine residues of three proteins (kD 66, 57 and 22) was found on Western blot of kidney protein extracts of the sickle cell mice. The degree of tyrosine nitration of the 66 kD protein was not significantly different in the control versus transgenic mice, whereas tyrosine nitration of the 57 and 22 kD proteins was clearly increased in transgenic mice. Strong immunostaining for nitrotyrosine was seen in tubular epithelial cells of the sickle cell mice, in close proximity to positive immunostaining of iNOS. Neither iNOS nor nitrotyrosine was expressed in the control mice. DNA "laddering" was found localized to the same zones of the kidney as nitrotyrosine and iNOS immunostaining. TUNEL assay on mouse kidney tissue sections showed minimal tubular cell apoptosis in normal mouse with hypoxia, mild tubular cell apoptosis in sickle cell mouse in room air, and moderate tubular cell apoptosis in sickle cell mouse with hypoxia.
Conclusions: The observations suggest that ONOO- and perhaps other reactive oxygen species are being produced in the sickle cell kidney. The mechanism may be ischemia/reperfusion due to intermittent vascular occlusion by sickle cells. The resulting hypoxia could result in iNOS activation, superoxide radical and peroxynitrite formation. Two consequences of these reactions appear to be nitration of tyrosine residues of some renal proteins and enhanced apoptosis.