Meprin-alpha in chronic diabetic nephropathy: interaction with the renin-angiotensin axis

Meprin (MEP) A is a metalloendopeptidase that is present in the renal proximal tubule brush-border membrane (BBM) and that colocalizes with angiotensin-converting enzyme (ACE). The MEP beta-chain gene locus on chromosome 18 has been linked to a heightened risk of diabetic nephropathy (DN) in patients with type 2 diabetes. This study evaluated 1) whether MEP-alpha and MEP-beta gene and protein expression are altered in db/db mice before the onset of DN and 2) the role of MEP-alpha in the pathogenesis of DN and the impact of the renin-angiotensin system on this interaction in two experimental models of diabetes. MEP-alpha and MEP-beta gene and protein expression were evaluated in db/db mice, 13-14 wk of age, compared with lean C57BLKS/J littermate animals. A treatment study was then performed in which db/db mice and controls were assigned to one of three groups: control (C) water, no therapy; ACE inhibitor therapy, enalapril (EN)-treated water, 50 mg/l; ANG II receptor type 1 blocker (ARB) therapy, losartan (LOS)-treated water, 500 mg/l. Treatment was started at 8 wk of age and continued for 52 wk. Male Sprague-Dawley rats with diabetes for 52 wk following a single dose of streptozocin (STZ; 60 mg/kg) were also studied. At 13.5 wk of age, MEP-alpha and MEP-beta kidney mRNA abundance and protein expression were significantly lower in db/db mice compared with lean controls, with greater changes in MEP-beta (P < 0.05). In the treatment study, EN ameliorated and LOS exacerbated DN in db/db mice. BBM MEP A enzymatic activity and MEP-alpha protein content were lower in db/db mice vs. control nonobese mice at 52 wk (P < 0.02). EN-treated db/db mice showed increased MEP A activity, MEP-alpha content in BBM, decreased urinary MEP-alpha excretion, and enhanced BBM staining for MEP-alpha protein vs. C and LOS-treated db/db mice. In nonobese mice, EN and LOS treatment had no effect on MEP-alpha expression. In rats with STZ-induced diabetes for 52 wk, urinary MEP-alpha excretion was increased and MEP A activity and MEP-alpha protein content per milligram of BBM protein were decreased compared with age-matched control animals (P < 0.05). These results indicate that db/db mice manifest decreased MEP-alpha and MEP-beta gene and protein expression, before the development of overt kidney disease. Moreover, in db/db mice with DN and rats with STZ-diabetes, there was an inverse relationship between renal MEP-alpha content and the severity of the renal injury. Treatment with an ACE inhibitor was more effective than ARB in ameliorating DN in db/db mice, a change that correlated with alterations in urinary excretion and BBM content of MEP-alpha. MEP-alpha may play a role in the pathogenesis of DN and the benefits of ACE inhibitor therapy on the progression of diabetic kidney disease may be related, in part, to its impact on renal MEP-alpha expression.