Cellular and molecular effects of α-tocopheryloxybutyrate: Lessons for the design of vitamin E analog for cancer prevention

α-Tocopherol and its synthetic derivative, α-tocopheryl succinate (α-TS), are known to inhibit proliferation of cancer cells. α-TS is considered a more desirable anticancer agent because of the ability to induce apoptosis. It has been established previously that the whole intact α-TS molecule is necessary for its pro-apoptotic activity. For this reason, α-TS is not suitable for oral use because the ester bond linking succinate to tocopherol is subject to hydrolysis by intestinal esterases. One approach to overcome this problem is to replace the ester bond with an ether bond, since the latter is resistant to esterase-mediated hydrolysis. α-Tocopheryloxybutyrate (α-TOB) is the ether analog of α-TS. In this study, we compared the potency of α-TS and α-TOB using a panel of bioassays: cell growth, TUNEL labelling for apoptosis, PARP cleavage, caspase-3 and caspase-9 activation, as well as Akt and JNK phosphorylation. The experiments were carried out in two human prostate cancer cell lines: LNCaP and PC-3. Our results showed that α-TOB was capable of inhibiting cell growth and inducing apoptosis, although α-TOB was less active than α-TS on an equimolar basis. In general, it took twice as much α-TOB as α-TS to achieve the same response. Nonetheless, these two compounds shared the same mechanism of targeting the Akt and JNK signaling pathways, and activating the intrinsic cell death mediators of caspase-9 and caspase-3. Cellular analysis of α-TS and α-TOB showed that α-TOB was taken up as efficiently as α-TS (if not more so), suggesting that the lower activity of α-TOB is an inherent property of the molecule and not due to impaired uptake. Additional evidence is provided to show that α-TS may act at the membrane level to interfere with Akt phosphorylation, although the exact nature of this disruption remains unclear. The future design of new anticancer tocopherol analogs should incorporate the ether linkage of the side chain for esterase resistance as well as other structural modifications for enhanced blocking of membrane signaling.