Prorenin accumulation and activation in human endothelial cells - Importance of mannose 6-phosphate receptors

ACE inhibitors improve endothelial dysfunction, possibly by blocking endoth elial angiotensin production. Prorenin, through its binding and activation by endothelial mannose 6-phosphate (M6P) receptors, may contribute to this production. Here, we investigated this possibility as well as prorenin acti vation kinetics, the nature of the prorenin-activating enzyme, and M6P rece ptor-independent prorenin binding. Human umbilical vein endothelial cells ( HUVECs) were incubated with wild-type prorenin, K/A-2 prorenin tin which Ly s42 is mutated to Ala, thereby preventing cleavage by known proteases), M6P -free prorenin, and nonglycosylated prorenin, with or without M6P, protease inhibitors, or angiotensinogen. HUVECs bound only M6P-containing prorenin (K-d 0.9 +/-0.1 nmol/L, maximum number of binding sites [B-max] 1010 +/- 50 receptors/cell). At 37 degreesC, because of M6P receptor recycling, the am ount of prorenin internalized via M6P receptors was > 25 times B-max, Insid e the cells, wild-type and K/A-2 prorenin were proteolytically activated to renin. Renin was subsequently degraded. Protease inhibitors interfered wit h the latter but not with prorenin activation, thereby indicating that the activating enzyme is different from any of the known prorenin-activating en zymes. Incubation with angiotensinogen did not lead to endothelial angioten sin generation, inasmuch as HUVECs were unable to internalize angiotensinog en. Most likely, therefore, in the absence of angiotensinogen synthesis or endocytosis, M6P receptor-mediated prorenin internalization by endothelial cells represents prorenin clearance.