Highlights d A method to target or deplete FAPs locally in skeletal muscle is described d Depletion of FAPs results in a regenerative deficit in skeletal muscle d Depletion of FAPs results in skeletal muscle atrophy d MuSC number declines in muscles in which FAPs have been depleted

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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The metabolism and efficacy of 5-fluorouracil (FUra) and other fluorinated pyrimidine (FP) derivatives have been intensively investigated for over fifty years. FUra and its antimetabolites can be incorporated at RNA-and DNA-levels, with RNA level incorporation provoking toxic responses in human normal tissue, and DNA-level antimetabolite formation and incorporation believed primarily responsible for tumour-selective responses. Attempts to direct FUra into DNA-level antimetabolites, based on mechanism-of-action studies, have led to gradual improvements in tumour therapy. These include the use of leukovorin to stabilize the inhibitory thymidylate synthase-5-fluoro-2′-deoxyuridine 5′ monophoshate (FdUMP)-5,10-methylene tetrahydrofolate (5,10-CH2FH4) trimeric complex. FUra incorporated into DNA also contributes to antitumour activity in preclinical and clinical studies. This review examines our current state of knowledge regarding the mechanistic aspects of FUra:Gua lesion detection by DNA mismatch repair (MMR) machinery that ultimately results in lethality. MMR-dependent direct cell death signalling or futile cycle responses will be discussed. As 10-30% of sporadic colon and endometrial tumours display MMR defects as a result of human MutL homologue-1 (hMLH1) promoter hypermethylation, we discuss the use and manipulation of the hypomethylating agent, 5-fluorodeoxycytidine (FdCyd), and our ability to manipulate its metabolism using the cytidine or deoxycytidylate (dCMP) deaminase inhibitors, tetrahydrouridine or deoxytetrahydrouridine, respectively, as a method for re-expression of hMLH1 and re-sensitization of tumours to FP therapy. (2009) Keywords: 5-fluorouracil; DNA mismatch repair; thymidylate synthase; hypermethylation; hMLH1; MMR/c-Abl/p73a/ GADD45a signalling Abbreviations: 5,10-CH2FH4, 5,10-methylene tetrahydrofolate; 5′-dRP, 5′-deoxyribose phosphate; AM, adaptors/mediators; AP, apyrimidinic/apurinic site; APE, apurinic/apyrimidinic endonuclease; ATM, ataxia telangiectasia mutated; ATR, ataxia telangiectasia-and-rad3-related; BER, base excision repair; CD, cytosine deaminase; dCMP, deoxycytidylate; dCMPD, deoxycytidylate deaminase; dH4Urd, deoxytetrahydrouridine; DS, damage sensors; DSBs, DNA double-strand breaks; dThyd, thymidine; ES, embryonic stem; FdCyd, 5-fluoro-2′-deoxycyticine; FdUDP, 5-fluoro-2′-deoxyuridine 5′-diphosphate; FdUMP, 5-fluoro-2′-deoxyuridine 5′-monophosphate; FdUTP, 5-fluoro-2′-deoxyuridine-5′-triphosphate; FdUrd, 5-fluoro-2′-deoxyuridine; FEN1, flap-endonuclease 1; FPs, fluorinated pyrimidines; FUDP, 5-fluorouridine 5′-diphosphate; FUra (or 5-FU), 5-fluorouracil; FUrd, fluorouridine; FUTP, 5-fluorouridine-5′-triphosphate; GADD45, growth arrest and DNA damage-inducible-45 gene/ protein; H4Urd, 3,4,5,6-tetrahydrouridine; hMLH1, human MutL homologue-1; hMSH2, human MutS homologue-2; hMSH3, human MutS homologue-3; hMSH6, human MutS homologue-6; hPMS2, postmeiotic segregation increased 2; IDL, insertion/deletion loop-type; MBD4, methyl-CpG binding domain protein 4; British Journal of Phar...