Isothiocyanates: mechanism of cancer chemopreventive action.

Dietary and synthetic isothiocyanates have cancer chemopreventive activity. Dietary isothiocyanates are formed from glucosinolate precursors of ingested green vegetables. Isothiocyanates are absorbed across intestinal cell membranes by passive diffusion and bind reversibly to plasma protein thiols by thiocarbamoylation. Free isothiocyanate enters cells and is converted to the glutathione conjugate by glutathione S-transferases (GSTs). The glutathione conjugate is exported from cells by multidrug resistance proteins (MRPs), and metabolized in the mercapturic acid pathway to the corresponding mercapturic acid. The isothiocyanate is reformed by fragmentation of mercapturic acid pathway metabolites; it is inactivated by slow hydrolysis to the corresponding amine that is inactive in chemoprevention. Depletion of cellular glutathione and protein thiocarbamoylation activates signal transduction for cancer chemoprevention. Isothiocyanates inhibited and inactivated cytochrome P450 isoforms. They induced increased expression of GST, NADPH: quinone oxidoreductase, aldo-keto reductase and gamma-glutamylcysteine synthetase. These responses were coordinated at the transcription level by nuclear factor-erythroid 2 p45-related factor-2 acting through the antioxidant/electrophile enhancer response element and stimulated by the mitogen-activated protein kinase/extracellular signal-regulated kinase kinase kinase-1 and c-Jun N-terminal kinase-1 (JNK1) pathway. Isothiocyanates also induced apoptosis of pre-cancerous cells and tumor cells activated by caspase-8 and potentiated by JNK1. The chemopreventive activity of isothiocyanates is influenced by the isothiocyanate bioavailability-as is toxicity, GST polymorphism, protein thiocarbamoylation and probably also by MRP expression. These features of isothiocyanate metabolism and chemoprevention deserve further investigation.