BCR-ABL down-regulates the DNA repair protein DNA-PKcs.

This study demonstrates in both stable and inducible BCR-ABL-expressing hematopoietic cells a down-regulation of the major mammalian DNA repair protein DNA-PKcs by BCR-ABL. Similar results were found in BCR-ABL CD34(+) cells from patients with chronic myelogenous leukemia (CML). DNA-PKcs down-regulation is a proteasome-dependent degradation that requires tyrosine kinase activity and is associated with a marked DNA repair deficiency along with increased sensitivity to ionizing radiation. The conjunction of a major DNA repair deficiency and a resistance to apoptosis, both induced by BCR-ABL, provides a new mechanism to explain how secondary genetic alterations can accumulate in CML, eventually leading to blast crisis. The down-regulation of DNA-PKcs was reversible in CD34(+) CML cells suggesting that this approach might offer a novel and powerful therapeutic strategy in this disease, especially to delay the blast crisis. (Blood. 2001;97:2084-2090)

Transfer of Ku86 RNA antisense decreases the radioresistance of human fibroblasts.

Ku86 has been shown to be involved in DNA double-strand break (DSB) repair and radiosensitivity in rodents, but its role in human cells is still under investigation. The purpose of this study was to evaluate the radiosensitivity and DSB repair after transfection of a Ku86-antisense in a human fibroblast cell line. Simian virus 40-transformed MRC5V1 human fibroblasts were transfected with a vector (pcDNA3) containing a Ku86-antisense cDNA. The main endpoints were Ku86 protein level, Ku DNA end-binding and DNA protein kinase activity, clonogenic survival, and DSB repair kinetics. After transfection of the Ku86-antisense, decreased Ku86 protein expression, Ku DNA end-binding activity, and DNA protein kinase activity were observed in the uncloned cellular population. The fibroblasts transfected with the Ku86-antisense showed also a radiosensitive phenotype, with a surviving fraction at 2 Gy of 0.29 compared with 0.75 for the control and 20% of unrepaired DSB observed at 24 hours after irradiation compared with 0% for the control. Several clones were also isolated with a decreased level of Ku86 protein, a surviving fraction at 2 Gy between 0.05 and 0.40, and 10-20% of unrepaired DSB at 24 hours. This study is the first to show the implication of Ku86 in DSB repair and in the radiosensitivity of human cells. This investigation strongly suggests that Ku86 could constitute an appealing target for combining gene therapy and radiation therapy.

Radiation-induced expression of functional Fas ligand in EBV-positive human nasopharyngeal carcinoma cells.

Ionizing radiation remains a major therapeutic tool against human cancers, especially epithelial tumors, which account for the majority of human malignancies. Although Fas and Fas-L are essential determinants of apoptosis, few data support their role in the cytotoxic effect of ionizing radiation. Epstein-Barr-virus (EBV)-positive nasopharyngeal carcinoma (NPC) were chosen to address this question owing to their known sensitivity to ionizing radiation and their constitutive expression of the Fas-receptor. We here report that, in xenografted NPC cells, Fas-L expression, which was very low in basal conditions, was dramatically increased by tumor irradiation. Both the Fas receptor and the Fas ligand were found to be functional in this model, and a high proportion of irradiated NPC cells underwent apoptosis following tumor irradiation. Induction of Fas-L expression and apoptosis were observed for doses as low as 2 Gy. These data show an increase in Fas-L expression upon irradiation exposure, and strongly suggest that, in some epithelial malignancies, Fas-mediated apoptosis can play a major role in the anti-tumor effect of ionizing radiation, in the range of doses used for therapeutic applications.

Large-scale preparation, purification, and crystallization of UDP-N-acetylmuramoyl-L-alanine: D-glutamate ligase from Escherichia coli.

The UDP-N-acetylmuramoyl-L-alanine:D-glutamate ligase from Escherichia coli, an enzyme involved in the biosynthesis of the bacterial peptidoglycan monomer unit, was overproduced and purified to homogeneity on a large scale, yielding 4 mg of protein per liter of bacterial culture. Crystals of the complex with the substrate UDP-MurNAc-L-Ala were grown by the hanging drop method using ammonium sulfate as the precipitant. They are tetragonal with cell dimensions a = b = 65.5 A and c = 134.59 A, space group P4(1) or P4(3), and contain one monomer of 46,842 Da in the asymmetric unit. In order to use the multiple-wavelength anomalous diffraction method for phasing, a selenomethionine derivative of the protein has also been overproduced, purified, and crystallized.

The complete cDNA sequence encoding dog gastric lipase.

Oligodeoxyribonucleotide ligation to single-stranded cDNA (SLIC) and polymerase chain reaction (PCR) techniques were used to clone an entire dog gastric lipase (DGL) cDNA. The size of the cDNA is confirmed by Northern blot analysis. The DGL is synthesized as a 379-amino acid mature polypeptide with a molecular mass of 43176 Da which is preceded by a 19-amino acid signal sequence located at the NH2-terminus. Comparison of the signal sequences reveals a high degree of similitude between the DGL, the human gastric lipase (HGL), the rabbit gastric lipase (RGL) and the rat lingual lipase (RLL).

Study of the reaction mechanism of the D-glutamic acid-adding enzyme from Escherichia coli.

The D-glutamic acid-adding enzyme of Escherichia coli, or MurD, was purified from an overproducing strain and a few aspects of its reaction mechanism were studied. The existence of a reactive cysteinyl residue was shown by the following experiments: (1) two thiol-modifying reagents, (5,5'-dithiobis)2-nitrobenzoic acid and 2-nitro-5-thiocyanobenzoic acid, inactivated the enzyme; (2) incubation with tetranitromethane led to inactivation and to the appearance of cysteic acid (not to 3-nitrotyrosine); (3) in each case, ATP or UDP-MurNAc-L-Ala (but not D-glutamic acid) protected the enzyme from inactivation. The existence of a reactive lysyl residue was shown by the action of 2,4,6-trinitrobenzenesulfonic acid, a reagent specific for lysyl residues present in phosphate-binding sites. The formation of an acyl phosphate intermediate was consistent with three types of results: (1) the molecular isotope exchange reaction, which took place only in the presence of phosphate, but which was not strictly dependent on the presence of ADP; (2) a release of phosphate, measured by the molybdate assay, observed when the enzyme was incubated with ATP and UDP-MurNAc-L-Ala (without D-glutamic acid); (3) the appearance of a new radioactive compound (besides ATP and Pi) after incubation for a few minutes with UDP-MurNAc-L-Ala and [gamma-32P]ATP. Finally, the fact that phosphinate 1 was a good inhibitor of the enzyme (IC50 = 0.7 microM) strongly suggested that a tetrahedral transition state follows the acyl phosphate in the reaction pathway.