Ph.D. Brandeis 1985; B.A. Johns Hopkins 1979
Nitrosamine Carcinogens. The long term goal of this research is to understand fundamental and applied aspects of the chemistry of carcinogenic nitrosamines. Nitrosamines are a large class of compounds of varied structure to which there is human exposure through endogenous formation and environmental sources. Most of the nitrosamines investigated are carcinogenic or mutagenic in the Ames test when activated by liver microsomes. Nitrosamines are found in groundwater, foods, personal care products, and tobacco products and are encountered in a number of industrial environs, in particular in rubber manufacturing and curing facilities, metal and leather working concerns. The mechanism by which the simplest nitrosamines manifest their deleterious actions is reasonably well understood. They tend to target the oxygen atoms of DNA by means of the adducts they form they cause replicative polymerases to mis-insert opposite these lesions leading to mutation. Some fundamental aspects are not well understood - for example why the diazonium ions formed from nitrosamines tend to target the oxygen atoms and why there are mutation/adduct deposition hotspots. This proposal contains experimental approaches by which these issues can finally be resolved. The proposal also seeks to understand the more complex chemistry of a 'non-simple' nitrosamine - N-Nitrosomorpholine (NMOR) - to which there is human exposure. Recent work from this laboratory has demonstrated that a metabolite of NMOR decomposes to give nucleoside adducts of novel structure, ones that contain a pendant aldehyde. These slowly decompose to hydroxyethyl lesions. Such pendant alhehyde adducts are likely widely encounted from sources such as other nitrosamines and the products of DNA oxidation and lipid peroxidation as well as other environmental toxicants. It is intended to understand the damage spectrum derived from the metabolite, how it evolves with time and also to construct the novel adducts in oligonucleotides and to study their structure and cross-linking activities by NMR and trapping studies. The combination of novel technical and synthetic approaches are integrated toward a complete and detailed understanding of fundamental aspects of DNA damage and structures that have broad application.
Cancer Chemoprevention. Many compounds of diverse structure are known to have biochemical properties that imbue them with cancer chemopreventive potential. These compounds tend to elevate the cell’s ability to blunt assault by carcinogenic agents of exogenous and endogenous origin. A large body of work over the last 20 years has indicated that a major transcriptional pathway for activation of the phase 2 xenobiotic metabolizing enzymes that manifest this protection is mediated by transcription factor Nrf2. The availability of Nrf2 to activate transcription is controlled by a chaperone protein - Keap1 - a cysteine-rich protein that has come under increasing scrutiny of late. Keap1 is believed to be a target for compounds with cancer chemopreventive properties. It is generally believed that the covalent modification of Keap1 by these compounds is part of the process involved in facilitating the ability of Nrf2 to activate the transcription of genes that augment cellular defense. It is generally not known for any given chemopreventive, whether it is the chemopreventive agent, or some derivative thereof that is involved in covalent modification of Keap1.
Dithiolethiones are a class of cancer chemopreventives, two of which, oltipraz and anetholedithione, have been investigated in chemoprevention clinical trials. The chemical mechanism by which these compounds activate the Nrf2 pathway is unknown. Work by the PI’s group has recently shown that the major reductive metabolite of oltipraz activates the transcription of phase 2 enzymes with a potency comparable to oltipraz itself. It has recently further been demonstrated that H2O2, which is generated from the major metabolite in a GSH-stimulated redox cycle, is an important messenger for chemopreventive phase 2 enzyme induction by the metabolite and oltipraz itself. A downstream metabolite(s) has also been shown to induce phase 2 enzymes with comparable potency, but is not a redox active species. So, it may induce in a chemically distinctly different way, in so far as it acts through the Keap1/Nrf2 pathway.
The involvement of H2O2 as a messenger for dithiolethiones generally is not certain, though preliminary experiments suggest it is in the case of anetholedithione. The target Cys on Keap1 of H2O2 and celluar oxidants in general is not known. It is also unclear whether oltipraz may interact separately with Keap1, independent of its metabolites. The contribution of each of the metabolites of oltipraz to the global gene expression induced by oltipraz is also uncertain. The group is currently working toward clarifying the mechanistic aspects by which the cancer chemopreventive effects of these compounds are manifest.