Previous studies from our laboratory indicated that expression of the MLH1 DNA mismatch repair (MMR) gene was necessary to restore cytotoxicity and an efficient G 2 arrest in HCT116 human colon cancer cells, as well as Mlh1 ؊/؊ murine embryonic fibroblasts, after treatment with 5-fluoro-2-deoxyuridine (FdUrd). Here, we show that an identical phenomenon occurred when expression of MSH2, the other major MMR gene, was restored in HEC59 human endometrial carcinoma cells or was present in adenovirus E1A-immortalized Msh2 In addition to its roles in correcting DNA replication errors and editing recombination intermediates, DNA mismatch repair (MMR) 1 can process numerous DNA lesions (1-4). In fact, an intact MMR system is required for the lethality of specific DNA-damaging agents such as N-methyl-NЈ-nitro-N-nitrosoguanidine (MNNG), 6-thioguanine (6-TG), and cisplatin (5-7). MMR also mediates the lethality of fluoropyrimidines (FPs) such as 5-fluorouracil (FU) and 5-fluoro-2Ј-deoxyuridine (FdUrd) (8, 9). Inactivation of MMR allows resistance to the cytotoxic effects of these agents, a phenomenon referred to as "damage tolerance" (10 -13). Importantly, this enables cancer cells to uncouple persistent DNA damage from cell death, resulting in increased drug resistance (14 -16).The two major gene products that comprise MMR are MSH2 (which heterodimerizes with MSH3 or MSH6 to recognize mispairs and loops in DNA) and MLH1 (which heterodimerizes with PMS2 or MLH3 to act as a molecular matchmaker between the MSH2 complex and other DNA repair/replication and perhaps cell cycle factors) (17, 18). Defects in these two genes account for most cases of hereditary non-polyposis colorectal cancer, a familial condition with a predisposition to cancers of the colon, endometrium, stomach, ovary, and biliary tracts (19), as well as sporadic tumors of the colon (20), endometrium (21), stomach (22), head and neck (23), and prostate (24).Others and we (8, 9) have demonstrated that cells deficient in MLH1 are resistant to the cytotoxic effects of FU and FdUrd. Because FPs are the agents of choice in the treatment of colorectal cancer, understanding potential resistance mechanisms is important. FPs exert cytotoxic effects through incorporation into RNA and/or DNA, as well as inhibition of thymidylate synthase (TS). The inhibition of TS, which is the central enzyme of de novo pyrimidine synthesis, leads to decreases in intracellular dTTP pools; this depletion results in immediate cytostatic effects (via inhibition of DNA synthesis) and alters dNTP pool sizes (thus increasing the error rate of DNA polymerase) (25). A hallmark of MMR deficiency is instability in the length of repetitive sequences in DNA, referred to as microsatellite instability (MSI). This reflects the inability of MMRdeficient cells to correct insertions and deletions in their DNA that result from polymerase slippage at these sequences (26). It is also an easily measured clinical marker. Due to the resistance of MMR-deficient (i.e. MSI ϩ ) cancer cells to FU and FdUrd, one woul...

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Nature 364, 82-84). In this study, we show that interaction of n-alkanols and general anesthetics with PKC␣ results in dramatically different effects on membraneassociated compared with lipid-independent enzyme activity. Furthermore, the effects on membrane-associated PKC␣ differ markedly depending on whether activity is induced by diacylglycerol or phorbol ester and also on n-alkanol chain length. PKC␣ contains two distinct phorbol ester binding regions of low and high affinity for the activator, respectively (Slater, S. J., Ho, C., Kelly, M. B., Larkin, J. D., Taddeo, F. J., Yeager, M. D., and Stubbs, C. D. (1996) J. Biol. Chem. 271, 4627-4631). Short chain n-alkanols competed for low affinity phorbol ester binding to the enzyme, resulting in reduced enzyme activity, whereas high affinity phorbol ester binding was unaffected. Long chain n-alkanols not only competed for low affinity phorbol ester binding but also enhanced high affinity phorbol ester binding. Furthermore, long chain n-alkanols enhanced phorbol ester induced PKC␣ activity. This effect of long chain n-alkanols was similar to that of diacylglycerol, although the nalkanols alone were weak activators of the enzyme. The cellular effects of n-alkanols and general anesthetics on PKC-mediated processes will therefore depend in a complex manner on the locality of the enzyme (e.g. cytoskeletal or membrane-associated) and activator type, apart from any isoform-specific differences. Furthermore, effects mediated by interaction with the region on the enzyme possessing low affinity for phorbol esters represent a novel mechanism for the regulation of PKC activity.