Inhibition of thioredoxin signalling by alkyl and aryl 2-imidazolyl disulfide compounds as potential antitumor agents

Abstract

This work describes (1) the identification of alkyl and aryl 2-imidazolyl disulfides that inhibit growth in human cancer cell lines, (2) examines the antitumor activities of these disulfides in xenografted scid mice, (3) characterizes the chemopreventive activity in min mice of the disulfides IV-2, and (4) suggests the potential mechanisms by which these compounds mediate their actions. The alkyl and aryl 2-imidazolyl disulfides are inhibitors of the TR/Trx which regulates cell growth in both normal and cancer cells and is constitutively activated in a number of human primary cancers (Berggren, 1996). We found that all of the disulfides tested were potent inhibitors of growth in a panel of 60 cancer cell lines tested by the NCI. Disulfides IV-2, VI-2, and DLK-36 were found to inhibit growth of HL-60 leukemia and MCF-breast cancer cells implanted in scid mice. However, these disulfides had no significant inhibitory effect on the growth of human HT-29 colon cancer cells implanted in scid mice. Disulfide IV-2 also demonstrated a chemopreventive activity evaluated in min mice fed diet supplemented with 250 ppm IV-2. These disulfides may function through modulation of cellular redox status. It was determined that disulfides IV-2 and DLK-36 decreased GSH levels and that this effect may be due to indirect inhibition of the TR/Trx redox system or a direct reaction of GSH with the disulfides. Importantly, Human Trx regulates transcription factors such as NF-KB and activation of this transcription factor protects cells from apoptosis. Disulfide IV-2 was observed to inhibit the transcriptional activity of NF-KB. This inhibitory activity may sensitize cells to apoptosis. Most of the disulfides tested induced apoptosis at concentrations of twice the IC50 in both MCF-7 breast and HT-20 colon cancer cells. Interestingly, a protein which is secreted by MCF-7 cells into the media can degrade disulfide IV-2. This putative molecule has a molecular weight (m.w) greater than 3000 and is likely redox sensitive because ultra-filtered conditioned media (3000 m.w.) or aged media drastically decreased the degradation or sequestration activity of conditioned media. Plasma from mice treated with disulfides IV-2 or DLK-36 (i.v.) gave the same HPLC profile as seen in in vitro studies. These disulfides could be inhibiting tumor growth by inhibition of the TR/Trx system and/or depletion of GSH. Finally, the p53 tumor suppressor protein appears to be regulated by redox conditions. Recent, evidence suggests p53 may be regulated specifically by the TR redox system. The rat embryo fibroblast cell line, Al-5, expressing wild-type p53 was less sensitive to disulfides than these same cells expressing mutant p53.