**7. Effects of phytochemicals through DNA repair modulation and their interaction with alkylating agents used as chemotherapeutic**

Some DNA-damaging agents (specially alkylating agents) are used in cancer therapy due to their ability to induce DNA damage and subsequently apoptosis of tumor cells (Maynard et al., 2009). The efficacy of DNA damage-based cancer therapy is modulated by DNA repair pathways. Therefore these pathways may, attenuate the therapeutic effects of alkylating agents. These drugs are usually classified in two classes: monofunctional (e.g., N-methyl-N\_-nitro-N-nitrosoguanidine [MNNG], temozolomide [TMZ] and dacarbazine) and bifunctional alkylating agents (e.g., carmustine [BCNU], cyclophosphamide, lomustine [CCNU] and fotemustine).

BCNU, induces several kinds of DNA damage such as cross-linking, strand breaks and modified bases (Kondo et al., 2010). Between alkylation damage, N7 alkyl-guanine is the most abundant (around 90% of the total alkylation events) and O6 alkyl-guanine is the less frequent. O6 alkyl-guanine, if not repaired before cell division, can form base pairs with T, generating GCAT transitions mutations by action of MMR pathway. O6 alkyl-guanine is the lesion mainly responsible for the cytotoxic and mutagenic effects of these alkylating drugs. However O6 alkyl-guanine can be repaired by MGMT. In human colorectal adenoma, reduced MGMT activity has been found. Therefore, more mutations occur when cells are treated with alkylating agent (Lees *et al.*, 2004; Lees *et al.*, 2002). However, high levels of MGMT where found in some other tumors (Baer et al., 1993). In the context of therapy with alkylating agents, inhibition of MGMT activity in tumor cells is desirable. The coadjuvant drug O6-benzylguanine (O6-BG) inactivates MGMT, acting as a pseudosubstrate. Effect of O6-BG has been investigated when in combination with an alkylating drug to increase its efficacy (Liu *et al.*, 2002).

N7-alkylG is the most frequent alkylation damage however is not considered to be as mutagenic as O6-alkylG because it is efficiently repaired by BER pathway. N-methylpurine-DNAglycoslase (MPG), the only glycosylase that recognizes alkylation lesions in animal cells, removes N7-alkylG by hydrolysis of the N-glycosylic bond creating an AP site that is repaired by the other enzymes of BER pathway (Drablos et al., 2004). An overexpression of MPG increases the production of AP sites. If the levels of the other enzymes of BER pathway remain unaltered, repair of AP sites is low and accumulation of these lesions becomes cytotoxic and mutagenic. Therefore, an overexpression of MPG or a decrease in other enzymes involved in the subsequent steps in BER pathway, increases cytotoxicity of alkylating drugs. This cytoxicity becomes more relevant when cells are resistant to cytotoxicity from O6-alkylation.

MGMT and MMR have contrasting effects on DNA O6-alkylG. While MGMT is an efficient mechanism of repair, MMR in contrast, does not remove the alkylated base but introduce more lesions like strand break in an attempt to repair the mismatch. Accumulation of strand breaks may activate apoptotic pathways, leading to cell death. In some cells, resistance to alkylating agents can be mediated by MGMT and MMR. Active MGMT and loss of MMR pathway protect cells against the cell death induced by methylating chemotherapeutic drugs (Allan and Travis, 2005). Depletion of MGMT activity (for example, by O6-BG or by epigenetic silencing of the MGMT gene) and intact MMR system results in reversion of resistance with high sensitivity to the cytotoxic effects of alkylating drugs (Casorelli et al., 2008; Esteller et al., 2000; Hegi et al., 2005). Tumors with low MGMT level are more sensitive to alkylating chemotherapy (Bandres et al., 2005). Therefore,DNA repair mechanisms may be understood as a promising target to develop new cancer treatments (Helleday *et al.*, 2008; Jiricny, 2006). Different strategies have been developed to enhance the efficacy of chemotherapy using alkylating agents. In tumor cells, inhibition of MGMT activity and/or BER pathway decreases resistance to alkylating drugs (Drablos et al., 2004; Jaiswal et al., 2011; Middleton and Margison, 2003). Downregulation of DNA repair pathways (except MMR) may increase efficacy of alkylating agents, decreasing the amount of drug needed for chemotherapy and consequently reduction of the side effects (Kondo et al., 2010).

Dietary agents that modulate MGMT expression and/or BER pathway may play an important role in chemotherapy when in combination with alkylating agents. However this subject, in contrast to chemoprevention, has received little attention. Some studies have been that dietary agents may increase MGMT activity. Niture *et al.*, (2006), investigated the potential ability of some Indian medicinal plants extracts to modulate MGMT activity and expression in human peripheral blood lymphocytes and cancer cell lines. The results showed that both the ethanolic and aqueous extracts from neem (*Azadirachta indica*), holy basil (*Ocimum sanctum*), winter cherry (*Withania somnifera*), and oregano (*Origanum majorana*) increased MGMT expression and its activity. Extracts from gooseberry (*Emblica officinalis*), common basil (*Ocimum basilicum*), and spearmint (*Mentha viridis*) also increased MGMT levels, however to a smaller extent. Later, the same author reported that some phytochemicals such as curcumin, silymarin and resveratrol increase protein expression as well as activity of MGMT in lymphocytes and cancer cell lines (Niture *et al.*, 2007). In tumor cells a number of genes are abnormally methylated. Some dietary agents, such as genistein and epigallocatechin-3-gallate showed the ability to reactivate some methylation-silenced genes in cancer cells like MGMT due to a direct inhibition of DNA methyltransferase (Fang et al., 2005; Fang et al., 2003). Recently, Billson et al., (2009), demonstrated that a high vegetable intake in humans decreases MGMT activity in normal colorectal mucosa. To understand the real role of dietary agents on chemotherapy when in combination with alkylating drugs more studies are need.
