Zinc Chemoprevention of Arsenic Co-Carcinogenesis

There is strong evidence that in addition to the known direct carcinogenic actions of high arsenic concentrations, low, non-carcinogenic levels of arsenic synergize with other DNA-damaging agents such as benzo[a]pyrene and ultraviolet radiation (UVR) to enhance carcinogenesis. This observation has led to the identification of arsenic as a co-carcinogen. One mode of action to account for arsenic co-carcinogenesis is inhibition of DNA repair proteins containing zinc finger DNA binding motifs. Research from our lab demonstrates that: i) zinc finger proteins with e3 cysteine residues are direct molecular targets for arsenite binding, including xeroderma pigmentosum group A (XPA) and poly(ADP-ribose)polymerase (PARP)-1, which are the critical proteins for nucleotide excision repair (NER) and base excision repair (BER) respectively; ii) low levels of arsenite enhance UVR-induced photoproducts and oxidative DNA damage; and iii) supplemental zinc prevents arsenite effects on PARP-1 inhibition, enhancement of UVR-induced DNA damage and mutations. We hypothesize that zinc supplement will prevent arsenite enhancement of UVR-induced skin tumors by restoring the DNA repair capabilities in both BER and NER pathways. The proposed study will address a novel mechanism by which zinc may act to prevent arsenic-augmented carcinogenesis through interaction with distinct molecular targets in DNA repair pathways. In Aim 1, we will use multiple analytical approaches to definitively examine the competitive interaction of arsenite and zinc with synthetic peptides representing the PARP-1 and XPA zinc finger domains and with purified protein. These findings will be coupled with functional assays to establish the consequences of any identified differences in arsenite affinity for target proteins and zinc interactions. Aim 2 will analyze PARP-1 and XPA isolated from treated cells to measure arsenic and zinc binding to protein and the corresponding functional consequences to reveal the specific mechanism by which zinc reverses the arsenic effects. In Aim 3, we will validate PARP-1 and XPA as arsenic targets in vivo using biochemical methods, and test the hypothesis that zinc will prevent arsenic enhancement of UVR-induced DNA damage and skin tumors. The outcomes from the proposed studies are expected to demonstrate the capability of zinc to restore PARP-1 and XPA activity leading to improved genomic integrity and a decrease in the arsenic contribution to co-carcinogenesis. Although zinc has been considered chemoprotective largely through its reported general antioxidant activities, this proposal is innovative by focusing on a novel mechanism by which zinc prevents arsenite binding with specific zinc finger DNA repair protein targets, thus reversing the arsenite inhibition of DNA repair. This study will provide the solid foundations for a clear mechanistic understanding of how supplemental zinc reduces arsenic co-carcinogenesis, and provide the proof of principle for the potential of zinc supplement to prevent arsenic co-carcinogenesis. If validated, supplemental zinc could represent a low cost and easily implemented strategy for chemoprevention in arsenic exposed populations.