Expression of MRP1 and GSTP1-1 modulate the acute cellular response to treatment with the chemopreventive isothiocyanate, sulforaphane

A major component of the anticarcinogenic activity of the dietary chemopreventive agent sulforaphane (SFN) is attributed to its ability to induce expression of phase II detoxification genes containing the antioxidant response element (ARE) within their promoters. Because SFN is a reactive electrophile--readily forming conjugates with glutathione (GSH)--we asked whether expression of glutathione S-transferase (GST) P1-1 and the GSH conjugate efflux pump, multidrug resistance or resistance-associated protein (MRP) 1, would significantly modify the cellular response to SFN exposure. This was investigated using GST- and MRP1-poor parental MCF7 cells and transgenic derivatives expressing GSTP1-1 and/or MRP1. Compared with parental cells, expression of GSTP1-1 alone enhanced the rate of intracellular accumulation of SFN and its glutathione conjugate, SFN-SG--an effect that was associated with increased ARE-containing reporter gene induction. Expression of MRP1 greatly reduced SFN/SFN-SG accumulation and resulted in significant attenuation of SFN-mediated induction of ARE-containing reporter and endogenous gene expression. Coexpression of GSTP1-1 with MRP1 further reduced the level of induction. Depletion of GSH prior to SFN treatment or the substitution of tert-butylhydroquinone for SFN abolished the effects of MRP1/GSTP1-1 on ARE-containing gene induction-indicating that these effects are GSH dependent. Lastly, analysis of NF-E2-related factor 2 (Nrf2)--a transcription factor operating via binding to the ARE--showed that the increased levels of Nrf2 following SFN treatment were considerably less sustained in MRP1-expressing, especially those coexpressing GSTP1-1, than in MRP1-poor cells. These results suggest that the regulating effects of MRP1 and GSTP1-1 expression on SFN-dependent induction of phase II genes are ultimately mediated by altering nuclear Nrf2 levels.