Chemoprevention by Black Raspberry of Oral Cancer Induced by Tobacco Carcinogens: Translational Studies

Tobacco use remains a high risk factor for oral squamous cell carcinoma (OSCC) resulting from exposure to potent tobacco carcinogens including polycyclic aromatic hydrocarbons (PAHs) such as dibenzo[a,l]pyrene (DB[a,l]P), benzo[a]pyrene (B[a]P) and tobacco-specific nitrosamines (TSNA) such as N’-nitrosonornicotine (NNN). To assess effects of these carcinogens on oral mucosa, we developed a novel OSCC mouse model using DB[a,l]P and its fjord region diol epoxide (DB[a,l]PDE). Importantly, we showed that dietary intervention with freeze-dried black raspberries (BRB) powder inhibited carcinogen-induced DNA damage, mutagenesis and carcinogenesis in the mouse oral cavity. We have also established that BRB reduce formation and/or enhance repair of bulky adducts in vitro, but, the mechanisms by which BRB reduce DNA damage remain to be fully elucidated. Considering the varied structures of DNA adducts, we will focus on BRB effects on the nucleotide excision repair (NER) and base-excision repair (BER) enzymes. Based on the abundant phenolic compounds in BRB e.g. anthocyanins, it is logical to propose that BRB help preserve the cellular redox poise, enhance redox scavenging and thus function to prevent oxidative DNA damage; our data support this proposition. In addition to their cytoprotective functions, a specific cellular reducing equivalent-NADPH-functions in reductive biosynthetic reactions including conversion of ribonucleotides to deoxyribonucleotides (dNTPs) that are essential components for DNA repair. Based on these data, we hypothesize that BRB reduce DNA damage (cf. Scheme 1, Significance) in a multimodal fashion: 1) BRB enhance NER and BER function via preservation of key substrates and cofactors i.e. dNTPs and Mg2+, while maintaining an optimal, non-oxidized environment conducive to DNA repair; 2) BRB preserve the cellular redox status via efficient scavenging of reactive species that can inhibit DNA repair enzymes thereby reducing oxidative DNA damage. The proposed mechanistic studies entail two complementary, yet independent, Specific Aims: Aim 1A will investigate the effects of BRB on repair (NER, BER) of covalent tobacco carcinogen-DNA adducts and 8-OXO-dG in primary human oral keratinocyte cells that have been transfected with these adducts using our established assay. Aim 1B will determine the capacity of BRB to prevent oxidative damage in wild-type (OGG1+/+, a component of BER enzymes) and knockout (OGG1-/-) MEF cells. Concurrent studies, applicable to both subaims, will assess BRB effect on preservation of dNTP pools and key cellular redox poise parameters in a continuum of validated cells ranging from primary human oral keratinocytes, oral leukoplakia and OSCC cells. Aim 2 will determine for the first time the effects of local BRB delivery on formation of covalent DNA adducts and 8-OXO-dG in buccal cells of healthy smokers. The results could serve as the framework for future chemopreventive trials for addicted smokers who are unable to quit as well as non- or former-smokers who are exposed to environmental carcinogens.