| Epidemiologic studies have identified a relationship between dietary intake and cancer; however, the understanding of this relationship at the cellular level, such as the effects of dietary microconstituents on cell signaling events, are poorly understood. The goal of these studies was to investigate the effects of the dietary microconstituents benzyl isothiocyanate (BIT), a dietary detoxification enzyme inducer, and benzo(a)pyrene (BP), a ubiquitous dietary carcinogen, on the activation of the mitogen-activated protein (MAP) kinases: extracellular signal-related kinase (ERK), Jun N-terminal kinase (JNK), and p38 in HT29, human colon adenocarcinoma cells. The involvement of MAP kinase activation in cell death and increases in NAD(P)H:quinone reductase (QR) activity caused by BIT and BP were also investigated. At 25 μM, BIT activated all three kinases, whereas 10 nM BP activated ERK and p38, but not JNK. Using the MAP kinase inhibitors PD98059 and SB203580, the relative contribution of EM JNK, and p38 to QR activity and cell death was also investigated. Treatment of cells with 25 μM BIT or 10 nM BP resulted in increases in QR activity within 24 h of treatment and cell death beginning after about 72 h of treatment. Inhibition of ERK and JNK, but not p38, eliminated BIT-induced QR activity, suggesting that only ERK and JNK contribute to BIT-induced QR activity. In contrast, BP did not activate JM but did activate ERK and p38. Inhibition of ERK eliminated BP-induced QR activity, while inhibition of p38 had no effect on QR activity. These results suggest that only ERK contributes to BP-induced QR activity. Inhibition of the MAP kinases had no effect on preventing cell death by either BIT or BP. Tritiated ([3H]) thymidine incorporation was used to measure DNA synthesis and cell proliferation rate upon BIT and BP treatment. Cells showed a no change compared to controls in thymidine incorporation after 72 h of 25 μM BIT treatment. In contrast, cells showed a 50% increase in [3H] thymidine incorporation after 48 h of 10 nM BP treatment, which was eliminated by ERK inhibition. In summary, treatment of HT29 cells with BIT and BP activates MAP kinases, and kinases are involved in increasing QR activity, but may not mediate the cell death response to BIT and BP. The fact that these microconstituents activate MAP kinases establishes that they can influence cellular signaling pathways. |
