Specific G2 block in HeLa-S3 cells by neocarzinostatin

Suppression of solid tumor growth by a monoclonal antibody against tumor vasculature in rats: involvement of intravascular thrombosis and fibrinogenesis

We have reported that immunization of rat tumor-derived endothelial cells (TEC) isolated from KMT-17 solid tumors results in the generation of several monoclonal antibodies (MAbs). TES-23, one of these MAbs, recognizes a naturally occurring 80-kDa antigen expressed on endothelial cells of tumor blood vessels. To determine whether such MAbs can suppress solid tumor growth in vivo by impairment of endothelial cells in tumors following direct binding, we tested the biodistribution of (125)I-labeled TES-23 in rats bearing KMT-17 solid tumors. We also examined the effect of treatment using unconjugated TES-23 on tumor growth and histo-pathological changes in tumor tissues. Biodistribution studies showed localization of TES-23 into tumor tissues 60 min after intravenous injection. TES-23 suppressed significantly the growth of KMT-17 solid tumors following administration for 5 days. Histo-pathological examination showed that TES-23 caused degeneration, apoptosis and/or necrosis and denudation of endothelial cells in viable tumor areas following local aggregation and adhesion of lymphocytes, with subsequent intravascular thrombus formation by platelets and fibrin. Our results indicate that TES-23, which recognizes TEC, can target endothelial cells of solid tumor vasculature directly, resulting in growth suppression in vivo by reduction of blood flow due to intravascular thrombosis. Our results also suggest that targeting tumor vasculature is a potentially attractive approach for the treatment of solid tumors.

Inhibition of DNA topoisomerase II by ICRF-193 induces polyploidization by uncoupling chromosome dynamics from other cell cycle events

ICRF-193, a novel noncleavable, complex-stabilizing type topoisomerase (topo) II inhibitor, has been shown to target topo II in mammalian cells (Ishida, R., T. Miki, T. Narita, R. Yui, S. Sato, K. R. Utsumi, K. Tanabe, and T. Andoh. 1991. Cancer Res. 51:4909-4916). With the aim of elucidating the roles of topo II in mammalian cells, we examined the effects of ICRF-193 on the transition through the S phase, when the genome is replicated, and through the M phase, when the replicated genome is condensed and segregated. Replication of the genome did not appear to be affected by the drug because the scheduled synthesis of DNA and activation of cdc2 kinase followed by increase in mitotic index occurred normally, while VP-16, a cleavable, complex-stabilizing type topo II inhibitor, inhibited all these processes. In the M phase, however, late stages of chromosome condensation and segregation were clearly blocked by ICRF-193. Inhibition at the stage of compaction of 300-nm diameter chromatin fibers to 600-nm diameter chromatids was demonstrated using the drug during premature chromosome condensation (PCC) induced in tsBN2 baby hamster kidney cells in early S and G2 phases. In spite of interference with M phase chromosome dynamics, other mitotic events such as activation of cdc2 kinase, spindle apparatus reorganization and disassembly and reassembly of nuclear envelopes occurred, and the cells traversed an unusual M phase termed "absence of chromosome segregation" (ACS)-M phase. Cells then continued through further cell cycle rounds, becoming polyploid and losing viability. This effect of ICRF-193 on the cell cycle was shown to parallel that of inactivation of topo II on the cell cycle of the ts top2 mutant yeast. The results strongly suggest that the essential roles of topo II are confined to the M phase, when the enzyme decatenates intertwined replicated chromosomes. In other phases of the cycle, including the S phase, topo II may thus play a complementary role with topo I in controlling the torsional strain accumulated in various genetic processes.

The role of monoclonal antibody A7 as a drug modifier in cancer therapy

An anticancer antibiotic, neocarzinostatin (NCS), was covalently conjugated to the murine monoclonal antibody A7 (mAb A7), which recognizes the glycoprotein on the cell surface of human colon cancer. The biological and pharmacological properties of the conjugate (A7-NCS) were examined and compared with those of unconjugated NCS. A7-NCS exhibited a strong binding and cytotoxicity to the cell and an antigen-specific tumor accumulation. Significant tumoricidal effects in vivo were observed in the antigen-positive tumor-bearing mice treated with A7-NCS, whereas NCS mixed with mAb A7 and NCS alone were relatively ineffective. In the antigen-negative tumor, the tumoricidal effect of A7-NCS was lower than in the antigen-positive tumor. The NCS concentration in blood and tumor were significantly elevated by conjugation to mAb A7. The NCS localization in tumor was higher in the antigen-positive tumor than in the antigen-negative tumor. Death due to acute toxicity was observed at a dose of 20 units (U) NCS in mice treated with unconjugated NCS, whereas toxicity was seen with a much higher dose of NCS (100 U) if the drug was conjugated to the mAb. These findings show that mAb A7 confers more favorable pharmacological properties on an anticancer drug, making it potentially more useful for cancer chemotherapy.

Mode of action of C-1027, a new macromolecular antitumor antibiotic with highly potent cytotoxicity, on human hepatoma BEL-7402 cells

C-1027, a new macromolecular peptide antitumor antibiotic produced by a Streptomyces strain, was extremely cytotoxic to cultured cancer cells and markedly inhibited the growth of transplantable tumors in mice. As determined by tritium-labeled precursor-incorporation assay, C-1027 strongly inhibited DNA and RNA synthesis in hepatoma BEL-7402 cells without affecting protein synthesis. After incubation with the hepatoma cells for 4 h, IC50 values for [3H]-thymidine and [3H]-uridine incorporation were 0.00012 and 0.00032 microM, respectively. After 30 min incubation, C-1027 showed much stronger inhibition of [3H]-thymidine incorporation than did Adriamycin, mitomycin C or methotrexate, even at a concentration 10,000 times lower. The effect of C-1027 on pBR322 DNA suggested that the drug could cause single- or double-strand scission of DNA. As determined by flow cytometry, C-1027 delayed the progression of hepatoma cells through the S-phase and blocked the cells at G2+M. Cytological study showed that C-1027 caused a drastic reduction of the mitotic index within 1 h and that an overshot of the mitotic index occurred at 48 h. Our results indicate that C-1027 is an interesting compound with highly potent activity on cellular DNA.

Induction of DNA lesions, chromosomal aberrations, and G2 delay by bromo- and chlorodeoxyuridine

The presence of lesions in DNA of CHO cells, substituted with BrdUrd or CldUrd, has been investigated in a direct way by alkaline sucrose gradient and nucleoid sedimentation analysis and indirectly by screening for induced CAs. The influence of inhibitors of DNA repair (Caff and 3AMB) or DNA synthesis (HU) on the frequencies of such aberrations has been estimated. No randomly located DNA breaks could be detected under neutral conditions, but BrdUrd-substituted DNA was found to contain numerous alkali labile sites. At high concentrations, CldUrd causes G2 delay, similar to the action of known DNA-damaging agents. The extent of delay depends on the pattern of incorporation of the analog, i.e., incorporation for 2 cell cycles causes the longest delay, growth for 12 hr in CldUrd followed by 12 hr in dThd- containing medium gives less delay, and delay is not significant when the cells are incubated in the analog for only 12 hr prior to fixation. Numerous chromatid-type aberrations are present in cells incubated at the highest CldUrd concentration and their induction follows the pattern of induction of G2 delay, indicating the involvement of a common lesion. 3AMB, and HU increase the number of CAs when added 2 hr before fixation. The significance of these results is discussed.

Degradation of HeLa cell chromatin by neocarzinostatin and its chromophore

Chromatin is the in vivo target site for neocarzinostatin, a DNA strand scission antitumor drug. The effect of neocarzinostatin and its active chromophore component on HeLa cell chromatin is described here. Chromatin consisting of a mixture of mono-, di-, tri- and larger nucleosome fragments is prepared by micrococcal nuclease digestion of HeLa cell nuclei. Drug-induced conversion of chromatin to smaller sized fragments is measured by electrophoresis of the DNA on non-denaturing 4% polyacrylamide gels. Chromatin breakdown measured under these conditions is double-stranded in nature. In the presence of 2 mM dithiothreitol, neocarzinostatin causes degradation of large chromatin fragments and a loss of distinct nucleosome peaks. Detection of chromatin breakdown by neocarzinostatin is dependent upon the concentration of chromatin in the assay. When chromatin is increased from 14 to 70 micrograms/ml, changes in the larger fragments caused by 100 micrograms/ml neocarzinostatin become less obvious are are almost undetectable at 140 micrograms/ml chromatin. No change is observed when chromatin is treated with either neocarzinostatin or its chromophore in the absence of dithiothreitol. For detectable levels of chromatin degradation, 10 micrograms/ml neocarzinostatin is required compared to only 2.5 microgram/ml chromosome (expressed in microgram equivalent neocarzinostatin). Such degradation also occurs more rapidly with chromophore than with neocarzinostatin. Digestion of chromatin with neocarzinostatin continues for at least 30 min at 37 degrees C, while similar degradation caused by chromophore is complete in 1 min. Neocarzinostatin levels which actively degrade isolated chromatin can also effect release of soluble chromatin from intact nuclei. The released chromatin can serve as a substrate for micrococcal nuclease digestion. Such chromatin studies should prove useful in characterizing the mechanism of action of DNA reactive drugs such as neocarzinostatin.

A pharmacokinetic simulation model for chemotherapy of brain tumor with an antitumor protein antibiotic, neocarzinostatin. Theoretical considerations behind a two-compartment model for continuous infusion via an internal carotid artery

A pharmacokinetic two-compartment model for the treatment of brain tumors in man was simulated with the aid of a computer. The parameters necessary for the simulations such as inactivation rate constant, elimination rate constant, distribution volume, blood volume, cerebral blood flow, and cytotoxic drug concentration were either determined in this study or obtained from the literature. A proteinaceous antitumor antibiotic, neocarzinostatin (NCS), was utilized as a prototype drug because it has features making it advantageous in the treatment of brain tumor. In particular, NCS has an extremely short half-life in serum (t 1/2 less than or equal to 3 s), while it is relatively stable in the cerebrospinal fluid (CSF) (t 1/2 approximately 50 s). Therefore, the drug level in the cerebral compartment can be made adequately high with an appropriate infusion velocity into the cerebral compartment; however, it was possible to keep the plasma level of the drug much lower than the toxic level. Thus, few side-effects should result. In an in vitro study, NCS was found to exhibit its cytotoxicity to glioblastoma cells at a concentration as low as 0.005 microgram/ml. In contrast, the cytotoxicity was not apparent for the normal glia cells at 0.1 microgram/ml. The model being considered in this investigation is a two-compartment model, which consists of the cerebral compartment and the rest of the circulatory system of the body. In this case the drug is infused via an internal carotid artery. The results of pharmacokinetic simulation and dose regimens for NCs are presented, based on the effective concentration of the drug to glioblastoma cells in culture and the available pharmacological parameters.

The molecular basis of drug-induced G2 arrest in mammalian cells

The purpose of this review was to focus mainly on the molecular events related to the progression of cells through the G2 period to examine the cause for G2-arrest in mammalian cells after exposure to various anticancer drugs. With few exceptions, most of the eukaryotic cells exhibit a G2 period in their life cycles. The G2 period, which separates S phase from mitosis, represents the time necessary for the synthesis of the various components related to the condensation of chromosomes, assembly of the mitotic spindle, and cytokinesis. Continued synthesis of RNA and protein is necessary for the successful completion of G2 and the initiation of mitosis. Inhibition of RNA and protein synthesis, replacement of phenylalanine by its analog paraversible G2 arrest in cultured cells. Exposure of cells to certain antineoplastic drugs also blocks cells preferentially in G2. This irreversible drug-induced G2 arrest is associated with extensive chromosome damage. The G2-arrested cells were found to be deficient in certain proteins that may be specific for the G2-mitotic transition. These mitotic or chromosome condensation factors synthesized during the G2 period, reach their maximum levels at mitosis. A preliminary characterization of the chromosome condensation factor revealed that it is a heat labile, Ca2+-sensitive, nondialyzable protein with a sedimentation value of 4-5S.

An uptake of fluorescein isothiocyanate labeled neocarzinostatin into the cancer and normal cells

An uptake of fluorescein isothiocyanate labeled neocarzinostatin into normal and cancerous epithelial cells from bladder was investigated. Results showed that neocarzinostatin traversed the cell membrane into cytosol and nuclei, and it appeared to have a preferential cytotoxicity for the cancer cell.

DNA damage and repair in relation to cell killing in neocarzinostatin-treated HeLa cells

To elucidate the mechanism of the cell killing activity of neocarzinostatin on mammalian cells, the drug-induced damage of DNA and its repair were examined. Very low doses of neocarzinostatin, at which high survival of cells was observed, clearly produced single-strand breaks of DNA and decomposition of the 'DNA complex', but these damages appeared to be repaired almost completely. At higher doses of neocarzinostatin, single-strand breaks were repaired to a considerable extent while double-strand breaks seemed not to be repaired. The number of non-repairable single-strand breaks was about twice that of double-strand breaks. This implies that single-strand breaks are repaired except for those constituting double-strand breaks. Although at low levels of neocarzinostatin repair of double-strand breaks may occur, the correlation existing between the colony-forming ability of cells treated with neocarzinostatin and non-repairable DNA breakage suggests that production of a small number of critical non-repairable double-strand breaks per cell may be responsible for the cell killing activity of the drug.

Neocarzinostatin induction of DNA repair synthesis in HeLa cells and isolated nuclei

The antitumor antibiotic neocarzinostatin that causes DNA strand breaks in vivo and in vitro is shown to induce DNA repair synthesis in HeLa S3 cells. In the repair assay, the parental DNA was prelabeled with 32P and a density label (bromodeoxyuridine) was introduced into the new synthesized DNA. Quantitation of the repair synthesis as measured by the incorporation of [3H]thymidine into the light parental DNA at varying doses of the drug indicate that there is a significant repair response at low levels of the drug (0.2--0.5 microgram/ml) which cause DNA strand breakage and inhibition of DNA synthesis. In isolated HeLa nuclei neocarzinostatin stimulates the incorporation of dTMP many-fold. This enhancement of dTMP incorporation, which requires the presence of a sulfhydryl agent, is a consequence of the drug-induced DNA strand breakage and is in the parental DNA. These results suggest that an intact cell membrane is not required for DNA strand breakage and its subsequent repair.

The relationship between DNA strand-scission and DNA synthesis inhibition in HeLa cells treated with neocarzinostatin

Neocarzinostatin inhibits DNA synthesis in HeLa S3 cells and induces the rapid limited breakage of cellular DNA. The fragmentation of cellular DNA appears to precede the inhibition of DNA synthesis. Cells treated with drug at 37 degrees C for 10 min and then washed free of drug show similar levels of inhibition of DNA synthesis or cell growth, or of strand-scission of DNA as when cells were not washed. If cells are preincubated with neocarzinostatin at 0 degrees C before washing, the subsequent incubation of 37 degrees C results in no inhibition of DNA synthesis or cell growth, or cutting of DNA. Isolated nuclei or cell lysates derived from neocarzinostatin-treated HeLa S3 cells are inhibited in DNA synthesis but this can be overcome in cell lysates by adding activated DNA. A cytoplasmic fraction from drug-treated cells can stimulate DNA synthesis by nuclei isolated from untreated cells, whereas nuclei from drug-treated cells are not stimulated by the cytoplasmic fraction from untreated cells. By contrast, neocarzinostatin does not inhibit DNA synthesis when incubated with isolated nuclei, but it can be shown that under these conditions the DNA is already degraded and is not further fragmented by the drug. These data suggest that the drug's ability to induce breakage of cellular DNA in HeLa S3 cells is an essential aspect of its inhibition of DNA replication and may be responsible for the cytotoxic and growth-inhibiting actions of neocarzinostatin.