Pharmacogenetics of OATP (SLC21/SLCO), OAT and OCT (SLC22) and PEPT (SLC15) Pharmacogenetics of OATP (SLC21/SLCO), OAT and OCT (SLC22) and PEPT (SLC15) transporters in the intestine, liver and kidney AbstractThe role of carrier-mediated transport in determining the pharmacokinetics of drugs has become increasingly evident with the discovery of genetic variants that affect expression and/or function of a given drug transporter. Drug transporters are expressed at numerous epithelial barriers, such as intestinal epithelial cells, hepatocytes, renal tubular cells and at the blood-brain barrier. Several recent studies have associated alterations in substrate uptake with the presence of SNPs. Here, we summarize the current knowledge on the functional and phenotypic consequences of genetic variation in intestinally, hepatically and renally expressed members of the organic anion-transporting polypeptide family (OATPs; SLC21/SLCO family), the organic anion and organic cation transporters (OATs/OCTs; SLC22 family) and the peptide transporter-1 (PEPT1; SLC15 family).

Folates are essential for nucleic acid synthesis and are particularly required in rapidly proliferating tissues, such as intestinal epithelium and hemopoietic cells. Availability of dietary folates is determined by their absorption across the intestinal epithelium, mediated by the proton-coupled folate transporter (PCFT) at the apical enterocyte membranes. Whereas transport properties of PCFT are well characterized, regulation of PCFT gene expression remains less elucidated. We have studied the mechanisms that regulate PCFT promoter activity and expression in intestine-derived cells. PCFT mRNA levels are increased in Caco-2 cells treated with 1,25-dihydroxyvitamin D 3 (vitamin D 3 ) in a dose-dependent fashion, and the duodenal rat Pcft mRNA expression is induced by vitamin D 3 ex vivo. The PCFT promoter region is transactivated by the vitamin D receptor (VDR) and its heterodimeric partner retinoid X receptor-␣

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Genotoxic tolerance to low-level exposure of monofunctional alkylating agents is compound specific, with the mechanism pertaining to alkyl-induced genotoxic threshold response as yet unknown. N-methylpurine DNA glycosylase (MPG), an initiator glycosylase of the base excision repair (BER) pathway, typically repairs alkyl-induced DNA adducts, many of which are associated with genomic instability and tumorigenic risk. Here we demonstrate the involvement of MPG in modulating the genotoxic threshold response induced by the Sn2 alkylating agent ethyl methanesulfonate (EMS) and not the Sn1 alkylating agent N-ethyl-N-nitrosourea (ENU) in human lymphoblastoid cells and suggest the lack of N7-ethylguanine adduct repair as a key factor attributable to an observed increase in EMS-induced chromosome damage. Moreover, an increase in MPG messenger RNA expression levels in response to EMS and not ENU doses administered below the low-observed effect level substantiates the proposed specific involvement of MPG in relation to EMS-induced genotoxicity. We further report an unexpected dose-dependent decrease in the mutation frequency of the MPG-deficient cell line M09B when challenged with ENU, a response deemed consequential to a pronounced dose-dependent increase in the number of apoptotic cells relative to wild type. Collectively, these findings implicate the differential involvement of MPG-directed BER as a primary mechanism of action for the chromosome damage threshold response and cytotoxicity induced by alkane sulfonates and N-nitrosourea compounds, respectively.

It has been long assumed that DNA damage is induced in a linear manner with respect to the dose of a direct acting genotoxin. Thus, it is implied that direct acting genotoxic agents induce DNA damage at even the lowest of concentrations and that no "safe" dose range exists. The linear (non-threshold) paradigm has led to the one-hit model being developed. This "one hit" scenario can be interpreted such that a single DNA damaging event in a cell has the capability to induce a single point mutation in that cell which could (if positioned in a key growth controlling gene) lead to increased proliferation, leading ultimately to the formation of a tumour. There are many groups (including our own) who, for a decade or more, have argued, that low dose exposures to direct acting genotoxins may be tolerated by cells through homeostatic mechanisms such as DNA repair. This argument stems from the existence of evolutionary adaptive mechanisms that allow organisms to adapt to low levels of exogenous sources of genotoxins. We have been particularly interested in the genotoxic effects of known mutagens at low dose exposures in human cells and have identified for the first time, in vitro genotoxic thresholds for several mutagenic alkylating agents (Doak et al., 2007). Our working hypothesis is that DNA repair is primarily responsible for these thresholded effects at low doses by removing low levels of DNA damage but becoming saturated at higher doses. We are currently assessing the roles of base excision repair (BER) and methylguanine-DNA methyltransferase (MGMT) for roles in the identified thresholds (Doak et al., 2008). This research area is currently important as it assesses whether "safe" exposure levels to mutagenic chemicals can exist and allows risk assessment using appropriate safety factors to define such exposure levels. Given human variation, the mechanistic basis for genotoxic thresholds (e.g. DNA repair) has to be well defined in order that susceptible individuals are considered. In terms of industrial exposures to known mutagens, knowing the dose relationships and protective mechanisms involved, offers the possibility of screening workers for susceptibility to mutation through examining DNA repair gene polymorphisms. Hence, thresholds may exist for certain mutagens, but there will undoubtedly be human subpopulations who are more at risk from low dose exposures than others and who should not be exposed, if possible. By studying polymorphisms in DNA repair genes, susceptible individuals may be identified, and additional safety factors appropriately targeted to these populations.