Induction of the intestinal membrane transporters ABCA1 and ABCG8 by 27-hydroxycholesterol through a redox mechanism

Objective: : We tested the effect of 27-hydroxycholesterol (27OHC) on the expression and synthesis of two membrane transporters involved in sterols extrusion from the intestinal epithelium into the gut lumen: ATP-binding cassette A1 (ABCA1) and G8 (ABCG8). Special attention was given to ABCG8, a key player in the intestinal cell discharge of plant sterols. Methods: Differentiated CaCo-2 intestinal cells were supplemented with 27OHC, and added to the cell incubation medium at a final concentration of 1 or 5 mu M. These 27OHC externally added amounts were proven to reach intracellular oxysterol levels within the range of those normally recovered in the human peripheral blood. Results: : An up-regulation of theABCA1 ABCA1 and ABCG8 mRNAs was observed in the CaCo-2 cells supplemented with 27OHC. Moreover, both 1 mu M and 5 mu M 27OHC induced a net, and steady, statistically significant, increase of both ABCA1 and ABCG8 protein levels. Of interest, the cellular pre-treatment with diphenylene iodonium, a selective inhibitor of NADPH oxidase, i.e. a major intracellular source of reactive oxygen species, fully inhibited the 27OHC enhancement of both ABCA1 and ABCG8 protein synthesis. Conclusion: : This in vitro study shows for the first time that the addition of 27OHC to intestinal epithelial cells up-regulates ABCG8, the transporter discharging plant sterols into the gut lumen, besides confirming to induce ABCA1 as well. Importantly, the 27OHC-dependent up-regulation of the two transporters appears to involve a redox mechanism rather than the canonical liver-X-receptors-dependent pathway.