The (pro)renin receptor ((P)RR), also known as ATP6AP2, is an accessory protein of the vacuolar H+-ATPase (V-ATPase), which is required for V-ATPase integrity and function. V-ATPases play an important role in acidifying intracellular compartments that are important for multiple cellular events such as proteolytic processing of proinsulin. To investigate whether the (P)RR affects prorenin-to-renin conversion, we studied the consequences of (P)RR knockdown in (pro)renin-synthesizing human mast cells (HMC-1). HMC-1 cells were transfected with siRNA against the (P)RR using Nucleofector® kit L with Amaxa® Nucleofector device, and the medium and cellular renin/prorenin content were measured by immunoradiometric assay (Cisbio, France) after 48, 72 and 96 hours (four experiments in duplicate). (P)RR siRNA (but not mock transfection or negative control siRNA) decreased the (P)RR protein levels by up to 95% from 18-96 hours after transfection. Renin medium levels amounted to 9.61.6, 13.13.3 and 14.54.1 pg/mg protein at 48, 72 and 96 hours, whereas the concomitant prorenin levels were 19.64.1, 31.510.4 and 36.811.1 pg/mg protein. Cells exclusively contained renin, and the renin content at 48 hours (28131 pg/mg protein) was not different from that at 72 and 96 hours. Whereas negative control siRNA did not affect these levels, (P)RR siRNA increased renin release at 72 and 96 hours 5-7-fold, and doubled the prorenin release at these time points. As a consequence, the % of total renin (=renin + prorenin) released by HMC-1 as renin doubled. No siRNA effects were noted on the cellular renin content. In conclusion, (P)RR knockdown affects both prorenin-renin conversion and (pro)renin release, resulting in a net rise of the renin/prorenin ratio in the medium.