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The ability of phytosterol compounds to reduce plasma serum cholesterol levels in humans is well investigated. However, phytosterols are structurally similar to cholesterol with a double bond at the C5-6 position and are therefore susceptible to oxidation. Much research has been carried out on the biological effects of cholesterol oxidation products (COPs) in vitro. In contrast, there is less known about phytosterol oxidation products (POPs). From previous studies, it is apparent that oxidized derivatives of the phytosterols, β-sitosterol and stigmasterol, are cytotoxic in vitro but are less potent than their COP counterparts. In the present study, the cytotoxic and apoptotic potential of oxidized derivatives of dihydrobrassicasterol (DHB) including 5α,6α-epoxyergostan-3β-ol (α-epoxide), 5β,6β-epoxyergostan-3β-ol (β-epoxide), ergost-5-en-7-on-3β-ol (7-keto), ergost-5-ene-3β,7β-diol (7-β-OH), and ergostane-3β,5α,6β-triol (triol) were evaluated in the U937 and HepG2 cell lines. In general, 7-keto, 7-β-OH, and triol derivatives had a significant cytotoxic impact on U937 and HepG2 cells. The oxides appear to be more toxic toward U937 cells. In line with previous findings, the POPs investigated in this study were less potent than the equivalent COPs. The results add to the body of data on the toxicity of individual POPs.

Dietary exposure to phytosterols has increased in recent years due to the incorporation of these compounds into cholesterol-lowering products. Previous studies have investigated the cytotoxic effects of the oxidized derivatives of β-sitosterol and determined that phytosterol oxidation products (POP) have a similar but less potent toxicity compared to their cholesterol equivalents. In the present study, the cytotoxicity of the oxidized derivatives of stigmasterol were investigated in the U937 cell line. The stigmasta-5,22-diene-3β,7β-diol (7β-OH), 5,6-epoxystigmasta-22,23-diol (epoxydiol), 5,6,22,23-diepoxystigmastane (diepoxide), and (22R,23R)-stigmast-5-ene-3β,22,23-triol (22R,23R-triol) derivatives were identified as the most cytotoxic, and the mode of cell death was identified as apoptosis in cells incubated with 7β-OH, epoxydiol, and diepoxide stigmasterol. The antioxidants α-tocopherol, γ-tocopherol, and β-carotene did not protect against apoptosis induced by 7β-OH and diepoxide stigmasterol; however, α-tocopherol was found to protect against epoxydiol-induced apoptosis. The cellular antioxidant, glutathione, was depleted and the apoptotic protein, Bcl-2, was down-regulated by the stigmasterol oxides identified as apoptotic.