Isothiocyanates and plant polyphenols as inhibitors of lung and esophageal cancer

Abstract

A group of arylalkyl isothiocyanates were tested for their abilities to inhibit tumorigenicity and DNA methylation induced by both the tobacco-specific nitrosamine, NNK, in A/J mouse lung and the esophageal-specific carcinogen, NMBA, in F344 rat esophagus. In addition, ellagic acid was tested for its ability to inhibit NMBA-induced esophageal tumorigenesis. In the strain A lung tumor model, PEITC effectively inhibited NNK-induced lung tumors at a dose of 5 micromol, but was not inhibitory at lower doses. PPITC, PBITC, PPeITC, and PHITC were all considerably more potent inhibitors of NNK lung tumorigenesis than PEITC, and PHITC was the most potent inhibitor of all. Thus, in the strain A lung tumor model, there was a trend of increased inhibitory efficacy among arylalkyl isothiocyanates with increased alkyl chain length. In the F344 rat esophageal tumor model, PPITC was clearly more potent than PEITC, BITC and PBITC had little inhibitory effect on esophageal tumorigenesis, and in a separate experiment, PHITC actually enhanced esophageal tumorigenesis. Thus, the structure-activity relationships for inhibition of tumorigenesis by arylalkyl isothiocyanates were considerably different in the two animal models. However, the effects of the isothiocyanates on tumorigenesis were well-correlated to their effects on DNA adduct formation in either model. The most likely mechanism of inhibition of tumorigenesis by these isothiocyanates is via inhibition of the cytochrome p450 enzymes responsible for activation of NNK in mouse lung or NMBA in rat esophagus. Ellagic acid was an effective inhibitor of esophageal tumorigenesis, although not as potent as PEITC or PPITC. Like the isothiocyanates, ellagic acid inhibits cytochrome p450-mediated activation of NMBA.