Regulation of Cyclooxygenase- and Cytochrome P450-derived Eicosanoids by Heme Oxygenase in the Rat Kidney

Heme oxygenase enzymes (HO-1 and HO-2) catalyze the conversion of heme to biliverdin, free iron, and carbon monoxide (CO). Heme and products derived from its metabolism potentially influence renal function and blood pressure by affecting the expression and/or activity of hemeproteins, including cytochrome P450 (CYP4A) monooxygenases and cyclooxygenases (COX-1 and COX-2). We studied HO isoform expression and examined the effect of HO-1 induction by SnCl(2) on CYP4A and COX expression and activity in the rat kidney. HO-1 protein levels in kidney tissues from untreated rats were barely detectable, whereas HO-2 protein was expressed in all kidney structures examined and its levels were higher in the outer medulla followed by the inner medulla/papilla and cortex. HO-2 expression along the nephron followed its regional distribution, ie, the highest levels were detected in the medullary thick ascending limb (mTAL) and inner medullary collecting ducts followed by proximal tubules. SnCl(2) Treatment did not significantly affect HO-2 expression or distribution; however, it markedly increased HO-1 protein in the inner and outer medulla, specifically, in the inner medullary collecting ducts and mTAL. CYP4A expression and 20-hydroxyeicosatetraenoic acid (20-HETE) synthesis were the highest in the outer medulla followed by the cortex and inner medulla/papilla. SnCl(2) treatment reduced cortical and inner medullary CYP4A protein levels by 60% and 50% and inhibited 20-HETE synthesis by 90% and 60%, respectively. Despite a significant induction of HO-1 protein in the outer medulla, CYP4A expression and 20-HETE synthesis were hardly affected. SnCl(2) treatment did not affect COX-1 expression but markedly reduced cortical and medullary COX-2 protein levels. We conclude that HO isoform expression is segmented within the kidney and along the nephron and that treatment with an HO-1 inducer suppressed the levels of CYP4A and COX-2 proteins in a tissue-specific manner with concomitant effects on their activity. Such interactions may play an important role in the regulation of renal function.