High levels of EGFR prevent sulforaphane-induced reactive oxygen species-mediated apoptosis in non-small-cell lung cancer cells

Abstract

BackgroundSulforaphane (SFN) has been shown to induce the production of reactive oxygen species (ROS) and inhibit epidermal growth factor receptor (EGFR)–mediated signaling in non-small-cell lung cancer (NSCLC). NSCLC cells harboring constitutively active EGFR mutations are more sensitive to SFN treatment than cells with wild-type EGFR, but whether NSCLC cells with high levels of EGFR expression are more resistant or sensitive to SFN treatment is not known. Study designWe employed a pair of cell lines, CL1-0 and CL1-5, which have the same genetic background but different levels of EGFR expression, to examine the effects of high EGFR level in the sensitivity to SFN. MethodsThe effect of SFN on cell viability and tumorigenicity was examined by trypan blue dye-exclusion assay, clonogenic assays, flow cytometry, and immunoblotting in vitro as well as tumorigenicity study in vivo. ROS levels in cells were assessed by flow cytometry using the ROS-reactive fluorescent indicator CM-H2DCFDA. Knockdown of EGFR in CL1-5 cells was infected with an EGFR-targeting small hairpin (interfering) RNA (shRNA)–containing lentivirus. ResultsWe present evidence that cells with high-level EGFR expression (CL1-5) are more resistant to SFN treatment than those with low-level expression (CL1-0). SFN treatment produced a similar increase in ROS and caused arrest of a cell population at S-phase accompanied by the induction of γH2AX, a DNA damage-response marker, in both cell sublines. However, SFN induced apoptosis only in the high-EGFR-expressing CL1-0 subline. Pretreatment with the antioxidant N-acetyl-L-cysteine prevented SFN-induced apoptosis in CL1-0 cells and production of γH2AX in both CL1-0 and CL1-5 cells. shRNA-mediated knockdown of EGFR in CL1-5 cells rendered the cells susceptible to SFN-induced apoptosis. ConclusionThe cellular effects produced by SFN in NSCLC cells are largely mediated by SFN-induced production of ROS. Cells with higher levels of EGFR were more resistant to SFN treatment and showed resistance to SFN-induced apoptosis, suggesting that high EGFR levels protect cells from SFN-induced apoptosis. Despite this, we found that SFN retained the ability to inhibit the growth of NSCLC tumors with high-level EGFR expression in vivo.