Fas-associated death domain protein interacts with methyl-CpG binding domain protein 4: a potential link between genome surveillance and apoptosis

Fas-associated death domain protein (FADD) is an adaptor protein bridging death receptors with initiator caspases. Thus, its function and localization are assumed to be cytoplasmic, although the localization of endogenous FADD has not been reported. Surprisingly, the data presented here demonstrate that FADD is mainly nuclear in several adherent cell lines. Its accumulation in the nucleus and export to the cytoplasm required the phosphorylation site Ser-194, which was also required for its interaction with the nucleocytoplasmic shuttling protein exportin-5. Within the nucleus, FADD interacted with the methyl-CpG binding domain protein 4 (MBD4), which excises thymine from GT mismatches in methylated regions of chromatin. The MBD4-interacting mismatch repair factor MLH1 was also found in a complex with FADD. The FADD-MBD4 interaction involved the death effector domain of FADD and a region of MBD4 adjacent to the glycosylase domain. The FADD-binding region of MBD4 was downstream of a frameshift mutation that occurs in a significant fraction of human colorectal carcinomas. Consistent with the idea that MBD4 can signal to an apoptotic effector, MBD4 regulated DNA damage-, Fas ligand-, and cell detachment-induced apoptosis. The nuclear localization of FADD and its interaction with a genome surveillance/DNA repair protein that can regulate apoptosis suggests a novel function of FADD distinct from direct participation in death receptor signaling complexes.

Mechanisms of disease: methyl-binding domain proteins as potential therapeutic targets in cancer

The methyl-CpG-binding domain (MBD) proteins 'read' and interpret the methylation moieties on DNA, and thus are critical mediators of many epigenetic processes. Currently, the MBD family comprises five members; MBD1, MBD2, MBD3, MBD4 and MeCP2. Although not a 'classical' MBD protein, Kaiso also mediates transcriptional repression by using zinc finger domains to bind its targets. Since DNA hypermethylation is a well-recognized mechanism underlying gene silencing events in both tumorigenesis and drug resistance, it is likely that the MBD proteins may be important modulators of tumorigenesis. We review the recent work addressing this possibility, and discuss several of the MBD proteins as potentially excellent novel therapeutic targets.

New approaches for modelling cancer mechanisms in the mouse

Mouse models of human cancer are vital to our understanding of the neoplastic process, and to advances in both basic and clinical research. Indeed, models of many of the major human tumours are now available and are subject to constant revision to more faithfully recapitulate human disease. Despite these advances, it is important to recognize that limitations do exist to the current range of models. The principal approach to modelling has relied upon the use of constitutive gene knockouts, which can often result in embryonic lethality, can potentially be affected by developmental compensation, and which do not mimic the sporadic development of a tumour expanding from a single cell in an otherwise normal environment. Furthermore, simple knockouts are usually designed to lead to loss of protein function, whereas a subset of cancer-causing mutations clearly results in gain of function. These drawbacks are well recognized and this review describes some of the approaches used to address these issues. Key amongst these is the development of conditional alleles that precisely mimic the mutations found in vivo, and which can be spatially and tissue-specifically controlled using 'smart' systems such as the tetracycline system and Cre-Lox technology. Examples of genes being manipulated in this way include Ki-Ras, Myc, and p53. These new developments in modelling mean that any mutant allele can potentially be turned on or off, or over- or under-expressed, in any tissue at any stage of the life-cycle of the mouse. This will no doubt lead to ever more accurate and powerful mouse models to dissect the genetic pathways that lead to cancer.

Potentiation of melphalan activity in the KHT sarcoma by the radiosensitizer RSU 1069

The radiation sensitizer misonidazole (MISO) has been shown to potentiate the cytotoxic action of a variety of anti-cancer agents. Even larger enhancement ratios than those observed with MISO have been found with certain other nitroimidazoles. One agent reported to be particularly effective in combination with the chemotherapeutic agent melphalan is the sensitizer RSU 1069. The present studies therefore were designed to evaluate the effect of combining these two agents in the treatment of intramuscularly growing KHT sarcomas. Tumor response was assessed using an in vivo to in vitro clonogenic cell survival assay. When given at times ranging from 60 min before to 30 min after melphalan exposure, RSU 1069 was found to increase the tumoricidal activity of the chemotherapeutic agent. Complete dose response curves combining RSU 1069 and a range of melphalan doses then were determined. For comparison the effects of combining MISO or benznidazole (BENZO) with melphalan also were evaluated. All sensitizers were administered i.p. either 30 min before (BENZO) or simultaneously with (MISO, RSU 1069) the chemotherapeutic agent. Survival of clonogenic tumor cells assessed 22 to 24 hr after treatment was used to assay tumor response. When combined with melphalan, doses of RSU 1069 (0.38 mmol/kg), BENZO (0.3 mmol/kg) and MISO (5.0 mmol/kg) were found to yield dose modifying factors of 1.6, 1.5, and 1.4, respectively. These results indicate that potentiation of melphalan activity occurs at RSU 1069 doses which are approximately 10-fold lower than those of MISO, making this sensitizer as effective a potentiator of melphalan as so far tested in the KHT sarcoma.

Accelerated decomposition of 4-hydroxycyclophosphamide by human serum albumin

Cyclophosphamide, a widely used anticancer agent, requires initial metabolic activation to 4-hydroxycyclophosphamide (4-OHCP) to elicit its activity. The rate of decomposition of cis-4-OHCP was much faster in plasma than in buffer at pH 7.4. This plasma activity was not affected by treatment with acid (pH 1.3) or heat (60 degrees C for 30 min). The activity was retained in the macromolecular fraction (greater than 10,000) but not in the filtrate. Serum albumin was identified as the catalyst for the elimination step that generates phosphoramide mustard from aldophosphamide; albumin had no effect on the rate of ring opening of cis-4-OHCP to aldophosphamide. This catalytic activity was dependent on serum albumin concentration and independent of pH over the range of 6.5 to 7.5, in contrast to the buffer-catalyzed reaction. The catalytic rate constants kcat (pH 7.4, 37 degrees C) for phosphate buffer, human serum albumin, and bovine serum albumin were 1.13, 285, and 83 M-1 min-1, respectively. Pretreatment of cis-4-OHCP with serum albumin resulted in a time-dependent decrease in cytotoxic activity against L1210 tumor cells in vitro. These data suggest that the albumin-catalyzed reaction of cis-4-OHCP in plasma represents an important pathway for the transformation of cyclophosphamide metabolites and further emphasize the importance of considering phosphoramide mustard generated extracellularly versus intracellularly and the respective contributions of extracellular and intracellular phosphoramide mustard to cyclophosphamide cytotoxicity in vivo.

In vivo interaction between radiation and 1-(2-chloroethyl)-3-cyclohexyl-1-nitrosourea in the absence or presence of misonidazole in mice

The effect of combining the radiosensitizer misonidazole (MISO) with 1-(2-chloroethyl)-3-cyclohexyl-1-nitrosourea (CCNU) prior to radiation on the response of various KHT sarcoma cell subpopulations was evaluated. Centrifugal elutriation was used to obtain homogeneous populations of cells with respect to their cell cycle position from suspensions prepared directly from treated solid KHT tumors. Survival in these subpopulations was determined using an in vivo to in vitro cloning assay. In situ treatment consisted of either (a) CCNU (3.0 mg/kg) plus MISO (1.0 mg/g), (b) radiation (15 Gy) alone, or (c) CCNU plus MISO preceding the radiation by 24 hr. In the latter protocol, one treatment effectively eliminated those cells being preferentially spared by the other. The CCNU-MISO followed by radiation combination not only reduced the treatment resistance due to hypoxia and cell cycle position but also left all cell subpopulations in the tumor equally sensitive to the treatment. To determine the nature of the interaction between these agents in detail, the extent of cell killing following such a treatment regimen was evaluated using isoeffect plot (isobologram) analysis. KHT sarcoma-bearing C3H mice were treated with various doses of CCNU either alone or in simultaneous combination with a 1.0-mg/g dose of MISO 24 hr prior to receiving a range of radiation doses. Combining CCNU and radiation led to significantly enhanced tumor cell killing. When evaluated by isoeffect plot analysis, the data points resulting from this combination fell well below the envelope of additivity, indicating that a supraadditive interaction between these two agents had occurred in the tumor cells. An even greater interaction was observed in the KHT sarcomas when MISO was combined with CCNU prior to irradiation. The findings of effective elimination of treatment-resistant subpopulations along with a supraadditive tumor cell kill offer, at least in part, an explanation for the previously described therapeutic advantage observed for the CCNU-MISO-radiation protocol.

Enhancement of the antitumor efficacy of lomustine by the radiosensitizer RSU 1069

Previous investigations have shown that combining the radiation sensitizer misonidazole with conventional alkylating chemotherapeutic agents can lead to a therapeutic advantage. More recently, another sensitizer, RSU 1069, has been reported to give an enhancement of antitumor agent efficacy similar to that observed with misonidazole, but at an approximately tenfold lower sensitizer dose. One chemotherapeutic agent whose activity has been modified by sensitizers to a greater extent in tumors than in critical normal tissues is the nitrosourea lomustine (CCNU). The present studies evaluated the therapeutic benefit of combining RSU 1069 and CCNU in KHT sarcoma-bearing C3H/HeJ mice. The drugs were administered ip, and tumor response was assessed by measuring the survival of clonogenic KHT cells 22-24 hours after treatment. Normal tissue toxicity was determined using peripheral wbc counts 3 days after treatment and a 30-day lethality assay. Combining CCNU with a 0.38-mmol/kg dose of RSU 1069 increased tumor cell killing by a factor of approximately 1.9. Wbc toxicity and 30-day animal lethality increased with CCNU dose, but the addition of RSU 1069 enhanced either endpoint only slightly (factor of 1.0-1.2). The addition of RSU 1069 to CCNU treatment, therefore, led to a significant therapeutic benefit.