Role of PI3K/Akt and MEK/ERK signaling pathways in sulforaphane - and erucin- induced phase II enzymes and MRP2 transcription, G2/M arrest and cell death in Caco-2 cells

Isothiocyanate sulforaphane is an extensively studied cancer chemopreventive agent in human diet. In this study, the effects of sulforaphane (SFN) and its sulfide analog, erucin (ERN), have been examined on the induction of the phase II enzymes, quinine oxidoreductase (NQO1) and UDP-glucuronosyl transferase (UGT1A1), multidrug transporter (MRP2), cell cycle arrest and cell death in human colon adenocarcinoma Caco-2 cells. Additionally, the roles of PI3K/Akt and MEK/ERK signaling pathways have been assessed in these sulforaphane- and erucin-induced events. Although erucin and sulforaphane have similar IC50 values (21 and 23 μM after 72 h treatment), erucin was more effective in the induction of G2/M accumulation, depletion of mitochondrial potential, induction of cell death and mRNA induction of phase II enzymes and MRP2. Erucin (20 μM) induced the mRNAs of NQO1, UGT1A1 and MRP2 by 11.1-, 11.6- and 6.7-fold, whereas sulforaphane (20 μM) induced 3.3-, 5.3- and 2.2-fold, respectively. Both erucin and sulforaphane induced activation (phosphorylation) of ERK1/2 and Akt kinases but had no effect on JNK and p38 activation. Erucin-induced phase II enzyme transcriptions were decreased by PI3K and MEK1 inhibitors (LY294002 and PD98059), but the decreases in sulforaphane-induced transcription were less marked. Erucin induced a large increase in G2/M cell number than sulforaphane. The ability of kinase inhibitors to overcome G2/M block was low with the exception of PD98059 in sulforaphane-treated cells. Both, sulforaphane and erucin at high concentrations induced accumulation of sub-G1 cells, cell death and dissipation of mitochondrial membrane potential. Taken together, these results demonstrate that PI3K/Akt and MEK/ERK signals are important intracellular mediators in erucin- and sulforaphane-mediated phase II enzyme transcription and cell cycle arrest in Caco-2 cells