Camptothecin induces protein-linked DNA breaks via mammalian DNA topoisomerase I.

Camptothecin, a plant alkaloid with antitumor activity, has been shown to be a potent inhibitor of nucleic acid synthesis and a strong inducer of DNA strand breaks in mammalian cells. Previous studies have shown that camptothecin inhibits purified mammalian DNA topoisomerase I by trapping a reversible enzyme-DNA "cleavable complex" (Hsiang et al., J. Biol. Chem., 260: 14873-14878, 1985). Our present studies, using L1210 cells and SV40-infected monkey cells, have shown that camptothecin-induced strand breaks are protein linked. The linked protein is most likely DNA topoisomerase I as revealed by immunoblot analysis, using antibodies against purified mammalian DNA topoisomerase I. Brief heating of camptothecin-treated cells to 65 degrees C resulted in a rapid reduction of the number of protein-linked DNA breaks. Reversal of the camptothecin-induced topoisomerase I-DNA complex by heat was also observed in an in vitro system by using purified mammalian DNA topoisomerase I. Our results suggest that camptothecin interferes with DNA topoisomerase I both in cultured mammalian cells and in the purified system by trapping a reversible enzyme-DNA cleavable complex.

https://cancerres.aacrjournals.org/content/canres/48/7/1722.full.pdf

Identification of Mammalian DNA Topoisomerase I as an Intracellular Target of the Anticancer Drug Camptothecin

Plants used against cancer - an extension of the work of Jonathan Hartwell.

Topoisomerase I (Top1) is an abundant and essential enzyme. Top1 is the selective target of camptothecins, which are effective anticancer agents. Top1-DNA cleavage complexes can also be trapped by various endogenous and exogenous DNA lesions including mismatches, abasic sites and carcinogenic adducts. Tyrosyl-DNA phosphodiesterase (Tdp1) is one of the repair enzymes for Top1-DNA covalent complexes. Tdp1 forms a multiprotein complex that includes poly(ADP) ribose polymerase (PARP). PARP-deficient cells are hypersensitive to camptothecins and functionally deficient for Tdp1. We will review recent developments in several pathways involved in the repair of Top1 cleavage complexes and the role of Chk1 and Chk2 checkpoint kinases in the cellular responses to Top1 inhibitors. The genes conferring camptothecin hypersensitivity are compiled for humans, budding yeast and fission yeast.

/pdf/repair-of-topoisomerase-i-mediated-dna-damage-4kuo8pzftm.pdf

Repair of topoisomerase I-mediated DNA damage.

BACKGROUND Cephalotoxine esters, including homoharringtonine (HHT), have shown encouraging activity in leukemia in initial studies in China and in later studies in the U.S. METHODS The authors conducted a review of the literature to examine the studies pertinent to HHT in relation to preclinical studies and Phase I-II trials in patients with hematologic malignancies and solid tumors. RESULTS HHT and analogues appear to induce differentiation and apoptosis. Studies from China reported high response rates in patients with leukemia. Trials in the U.S. using short HHT infusions (3-4 mg/m(2) daily for 5 days) resulted in a high incidence of cardiovascular complications that were reduced using continuous infusion schedules of 3-7 mg/m(2) daily for 5-7 days initially, and later lower dose schedules of 2.5 mg/m(2) daily for 7-14 days. Results in solid tumors were negative. However encouraging results were reported in patients with acute myeloid leukemia, myelodysplastic syndrome, acute promyelocytic leukemia, and, most important, chronic myeloid leukemia (CML). In CML patients, HHT has been investigated alone and in combination with interferon-alpha and low-dose cytarabine in late and early chronic phases, with positive results. Additional areas of interest include the potential use of HHT for the treatment of central nervous system leukemia, polycythemia vera, and other nonmalignant conditions such as malaria. New semisynthetic preparations and HHT derivatives that bypass multidrug resistance may improve the efficacy and toxicity profiles, and broaden the range of antitumor efficacy. CONCLUSIONS HHT and its derivatives appear to have promising activity in hematologic malignancies, a finding that needs to be pursued.

Homoharringtonine: History, current research, and future directions

Growth inhibition in the Chinese hamster cell line V79 and in the human lymphoid cell line Raji and induction of sister chromatid exchange(s) (SCE) in V79 cells after treatment with six anticancer drugs [harringtonine (HRT), homoharringtonine (HHRT), camptothecin (CPT), hydroxycamptothecin (HCPT), lycobetaine (LBT), and oxalysine (OXL)] developed in the People's Republic of China were studied. OXL is a new antibiotic; all other drugs are plant extracts. All drugs caused a dose-dependent growth inhibition in both cell types, as evidenced by decreases in plating efficiencies of V79 cells and in viable cell counts of Raji. However, the degree of inhibition differed widely among the drugs. HRT, HHRT, CPT, and HCPT were the most potent growth inhibitors, LBT was next, and OXL was the least effective inhibitor. SCE analyses were made in V79 cells treated with a drug in the presence or absence of the metabolic activation system S9 mixture (S9 mix), except for the HRT assay in which the S9 mix was not used. CPT, HCPT, and LBT induced a dose-dependent increase in SCE frequencies, while HRT, HHRT, and OXL caused no SCE induction at any dose level used. CPT was the most powerful SCE inducer. HCPT induced SCE but at a much reduced rate when compared to that of CPT. LBT was a weak SCE inducer; SCE induction was seen only in cultures treated with 40 micrograms or more LBT/ml. Addition of the S9 mix did not alter SCE frequencies, indicating that the drugs were direct-acting agents. HRT and HHRT were highly toxic, but they induced no increases in SCE frequency, indicating that cytotoxicity of a compound does not necessarily correlate with SCE induction.

F G Wu

Papers

Cytotoxicity and Sister Chromatid Exchanges Induced In Vitro by Six Anticancer Drugs Developed in the People's Republic of China

Ethoxy-erianin phosphate and afatinib synergistically inhibit liver tumor growth and angiogenesis via regulating VEGF and EGFR signaling pathways.