β-Sitosterol oxidation products attenuate vasorelaxation by increasing reactive oxygen species and cyclooxygenase-2

β-Sitosterol has become a popular cholesterol-lowering functional food product worldwide. β-Sitosterol can be oxidized to β-sitosterol oxidation products (SOPs) during food processing. Little is known about the impact of SOPs and β-sitosterol on the functionality of arteries. This study investigated the effects of SOPs and β-sitosterol on vasorelaxation and the possible cellular mechanisms involved.

By isometric tension measurement, SOPs but not β-sitosterol blunted relaxation induced by acetylcholine or Ca(2+) ionophore A23187 in endothelium-intact aortae. SOPs-impaired vasorelaxation was completely reversed by cyclooxygenase (COX)-2 inhibitor DuP-697, whereas the reversal by COX-1 inhibitor SC-560 was only partial. Western blotting and immunohistochemistry showed that SOPs increased the protein expression of COX-2 but not COX-1 in the endothelium. Using dihydroethidium staining and electron paramagnetic resonance spin trapping techniques, SOPs were found to elevate the level of reactive oxygen species in rat aortic endothelial cells, and the effects were reversed by antioxidants tempol, tiron, or diphenylene iodonium. Consistently, these antioxidants reversed SOPs-induced impairment of endothelium-dependent relaxation. Up-regulation of COX-2 expression by SOPs was also reversed by tempol. Moreover, SOPs attenuated nitric oxide donor sodium nitroprusside-induced relaxation in endothelium-intact, but not endothelium-denuded rings, confirming that SOPs act on the endothelium. Interestingly, a thromboxane-prostanoid (TP) receptor blocker S18886 reversed SOPs-impaired vasorelaxation, suggesting the involvement of TP receptor in mediating the downstream effect. SOPs decreased cGMP production, and the effect could be reversed by inhibiting COX-2 or TP receptor.

This study provides novel experimental evidence showing the harmful effects of SOPs on endothelial function.