Onion (Allium cepa L.) peel extract (OPE) regulates human sperm motility via protein kinase C-mediated activation of the human voltage-gated proton channel

Abstract

Onion (Allium cepa L.) and quercetin protect against oxidative damage and have positive effects on multiple functional parameters of spermatozoa, including viability and motility. However, the associated underlying mechanisms of action have not yet been identified. The aim of this study was to investigate the effect of onion peel extract (OPE) on voltage-gated proton (Hv1) channels, which play a critical role in rapid proton extrusion. This process underlies a wide range of physiological processes, particularly male fertility. The whole-cell patch-clamp technique was used to record the changes in Hv1 currents in HEK293 cells transiently transfected with human Hv1 (HVCN1). The effects of OPE on human sperm motility were also analyzed. OPE significantly activated the outward-rectifying proton currents in a concentration-dependent manner, with an EC50 value of 30 mu g/mL. This effect was largely reversible upon washout. Moreover, OPE induced an increase in the proton current amplitude and decreased the time constant of activation at 0 mV from 4.9 +/- 1.7 to 0.6 +/- 0.1 sec (n = 6). In the presence of OPE, the half-activation voltage (V-1/2) shifted in the negative direction, from 20.1 +/- 5.8 to 5.2 +/- 8.7 mV (n = 6), but the slope was not significantly altered. The OPE-induced current was profoundly inhibited by 10 mu M Zn2+, the most potent Hv1 channel inhibitor, and was also inhibited by treatment with GF109203X, a specific protein kinase C (PKC) inhibitor. Furthermore, sperm motility was significantly increased in the OPE-treated groups. OPE exhibits protective effects on sperm motility, at least partially via regulation of the proton channel. Moreover, similar effects were exerted by quercetin, the major flavonoid in OPE. These results suggest OPE, which is rich in the potent Hv1 channel activator quercetin, as a possible new candidate treatment for human infertility.