Combination of guanine arabinoside and Bcl-2 inhibitor YC137 overcomes the cytarabine resistance in HL-60 leukemia cell line.

Cytarabine (ara-C) is the key agent for treating acute myeloid leukemia. After being transported into leukemic cells, ara-C is phosphorylated, by several enzymes including deoxycytidine kinase (dCK), to ara-C triphosphate (ara-CTP), an active metabolite, and then incorporated into DNA, thereby inhibiting DNA synthesis. Therefore, the cytotoxicity of ara-C depends on the production of ara-CTP and the induction of apoptosis.

Combined low-dose cytarabine, melphalan and mitoxantrone for older patients with acute myeloid leukemia or high-risk myelodysplastic syndrome.

Background: Low-dose cytarabine (ara-C) has been used to treat older patients with acute myeloid leukemia (AML) or high-risk myelodysplastic syndrome (MDS), but has resulted in complete remission for <20% of cases. A pilot study of the efficacy of a combination chemotherapy using low-dose ara-C, melphalan (Mel), and mitoxantrone (Mit) was conducted.

Patients And Methods: The treatment comprised ara-C (10 mg/m2) twice daily, melphalan (2 mg/body) every other day, and mitoxantrone (3 mg/m2) every 3 days.

Defective Mre11-dependent activation of Chk2 by ataxia telangiectasia mutated in colorectal carcinoma cells in response to replication-dependent DNA double strand breaks.

The Mre11.Rad50.Nbs1 (MRN) complex binds DNA double strand breaks to repair DNA and activate checkpoints.

Inhibition of glutathione synthesis overcomes Bcl-2-mediated topoisomerase inhibitor resistance and induces nonapoptotic cell death via mitochondrial-independent pathway.

Bcl-2 protein plays a critical role in inhibiting anticancer drug-induced apoptosis. We found that Bcl-2 overexpression is associated with a nearly 3-fold increase in cellular glutathione levels and with increased resistance to cell death after treatment with etoposide or SN-38, a derivative of camptothecin, in leukemia 697 cells with wild-type p53. Treatment of Bcl-2-overexpressing 697 cells (697-Bcl-2) with buthionine sulfoximine (BSO), an inhibitor of glutathione synthesis, reduced cellular glutathione levels and completely abolished Bcl-2-mediated drug resistance.

Arsenic trioxide circumvents multidrug resistance based on different mechanisms in human leukemia cell lines.

To determine the antitumor effect of arsenic trioxide (As2O3) on multidrug-resistant cells, we applied 3 human leukemia cell lines: daunorubicin (DNR)-resistant cell line K562/D1-9, which overexpresses p-glycoprotein (Pgp); DNR and 1-beta-D-arabinofuranosylcytosine (Ara-C) double-resistant cell line HL60/AD, which overexpresses multidrug resistance-associated protein (MRP1); and Bcl-2-transfected pre-B lineage leukemia cell line 697/Bcl-2. Interestingly, K562/D1-9 showed collateral sensitivity. Only HL60/AD showed small cross resistance, but 697/Bcl-2 had no resistance to As2O3.

Tetrandrine induces early G1 arrest in human colon carcinoma cells by down-regulating the activity and inducing the degradation of G1-S-specific cyclin-dependent kinases and by inducing p53 and p21Cip1.

Tetrandrine is an antitumor alkaloid isolated from the root of Stephania tetrandra. We find that micromolar concentrations of tetrandrine irreversibly inhibit the proliferation of human colon carcinoma cells in MTT and clonogenic assays by arresting cells in G(1). Tetrandrine induces G(1) arrest before the restriction point in nocodazole- and serum-starved synchronized HT29 cells, without affecting the G(1)-S transition in aphidicolin-synchronized cells.

Repair of and checkpoint response to topoisomerase I-mediated DNA damage.

Topoisomerase I (Top1) catalyzes two transesterification reactions: single-strand DNA cleavage and religation that are normally coupled for the relaxation of DNA supercoiling in transcribing and replicating chromatin. A variety of endogenous DNA modifications, potent anticancer drugs and carcinogens uncouple these two reactions, resulting in the accumulation of Top1 cleavage complexes. Top1 cleavage complexes damage DNA and kill cells by generating replication-mediated DNA double-strand breaks (DSBs) and by stalling transcription complexes.

Phosphorylation of histone H2AX and activation of Mre11, Rad50, and Nbs1 in response to replication-dependent DNA double-strand breaks induced by mammalian DNA topoisomerase I cleavage complexes.

DNA double-strand breaks originating from diverse causes in eukaryotic cells are accompanied by the formation of phosphorylated H2AX (gammaH2AX) foci. Here we show that gammaH2AX formation is also a cellular response to topoisomerase I cleavage complexes known to induce DNA double-strand breaks during replication. In HCT116 human carcinoma cells exposed to the topoisomerase I inhibitor camptothecin, the resulting gammaH2AX formation can be prevented with the phosphatidylinositol 3-OH kinase-related kinase inhibitor wortmannin; however, in contrast to ionizing radiation, only camptothecin-induced gammaH2AX formation can be prevented with the DNA replication inhibitor aphidicolin and enhanced with the checkpoint abrogator 7-hydroxystaurosporine.

UCN-01 inhibits p53 up-regulation and abrogates gamma-radiation-induced G(2)-M checkpoint independently of p53 by targeting both of the checkpoint kinases, Chk2 and Chk1.

UCN-01 (7-hydroxystaurosporine) is a cell-cycle checkpoint abrogator that sensitizes cells to ionizing radiation (IR) and chemotherapeutic agents. It has been shown previously that UCN-01 abrogates DNA-damage-induced G(2) checkpoint most selectively in p53-defective cells, by primarily targeting Chk1. Here we show that UCN-01 prevented IR-induced p53 up-regulation and p53 phosphorylation on serine 20, a site previously identified for Chk2 (or/and Chk1) kinase.