Prorenin from renal tubules is a major driver of diabetic kidney disease

The renin-angiotensin-system (RAS) plays a critical role in diabetic nephropathy, and inhibitors of the RAS are central components in its therapy. Renin circulates in the blood in its active form but also in the form of its enzymatically inactive precursor prorenin. In humans, the plasma prorenin concentration exceeds that of active renin several times. While the plasma concentration of active renin is unchanged or even reduced in diabetic patients, the prorenin concentration increases significantly and it has been shown that high prorenin levels are associated with diabetic microvascular damage. Why prorenin increases in diabetes while active renin is reduced is unclear. Previous studies suggest that there may be formation of prorenin in the tubular system of diabetic kidneys. To investigate the functional consequences of possible tubular renin formation in diabetes, we generated mice with inducible tubule-specific deletion of the renin gene (tubule-renin KO). Under control conditions, tubule-renin KO had no apparent phenotype and plasma and tissue levels of renin and prorenin were similar to those of mice with intact tubular renin (control mice). In control mice type-1 diabetes (streptozotocin, STZ) for 8 to 12 weeks stimulated tubular renin mRNA and protein expression, especially in distal nephron segments including collecting ducts. This increase in renin synthesis in renal tubules was markedly attenuated or even absent in tubule-renin KO mice. Similar to diabetic patients, plasma prorenin was markedly elevated in diabetic control mice. This stimulation of plasma prorenin was absent in mice with tubule-specific deletion of the renin gene. Moreover, the high prorenin levels in renal tissue, which were observed in diabetic control mice, were markedly reduced in tubule-renin KO. Noteworthy, plasma renin activity was not reduced in tubule-renin KO compared with controls, suggesting that renal tubules of diabetic mice mainly release prorenin. Kidneys of control mice with intact tubular renin showed classical signs of diabetic renal damage, such as albuminuria, mesangial expansion, fibrosis, inflammation and capillary rarefaction. All of these parameters were significantly ameliorated in tubule-renin KO, indicating that tubular renin, most likely in its prorenin form, significantly aggravates renal damage in diabetes. These data provide clear evidence for the first time that the tubular renin system is an additional source for circulating prorenin in diabetes, hereby providing an explanation for the paradox regulation of active renin and prorenin in diabetes. Moreover, the data show that tubular renin markedly contributes to the progression of diabetic nephropathy.