DNA minor groove binding agents interfere with topoisomerase II mediated lesions induced by epipodophyllotoxin derivative VM-26 and acridine derivative m-AMSA in nuclei from L1210 cells

rRNA transcription is integral to phase separation and maintenance of nucleolar structure

Transcription of ribosomal RNA (rRNA) by RNA Polymerase (Pol) I in the nucleolus is necessary for ribosome biogenesis, which is intimately tied to cell growth and proliferation. Perturbation of ribosome biogenesis results in tissue specific disorders termed ribosomopathies in association with alterations in nucleolar structure. However, how rRNA transcription and ribosome biogenesis regulate nucleolar structure during normal development and in the pathogenesis of disease remains poorly understood. Here we show that homozygous null mutations in Pol I subunits required for rRNA transcription and ribosome biogenesis lead to preimplantation lethality. Moreover, we discovered that Polr1a-/-, Polr1b-/-, Polr1c-/- and Polr1d-/- mutants exhibit defects in the structure of their nucleoli, as evidenced by a decrease in number of nucleolar precursor bodies and a concomitant increase in nucleolar volume, which results in a single condensed nucleolus. Pharmacological inhibition of Pol I in preimplantation and midgestation embryos, as well as in hiPSCs, similarly results in a single condensed nucleolus or fragmented nucleoli. We find that when Pol I function and rRNA transcription is inhibited, the viscosity of the granular compartment of the nucleolus increases, which disrupts its phase separation properties, leading to a single condensed nucleolus. However, if a cell progresses through mitosis, the absence of rRNA transcription prevents reassembly of the nucleolus and manifests as fragmented nucleoli. Taken together, our data suggests that Pol I function and rRNA transcription are required for maintaining nucleolar structure and integrity during development and in the pathogenesis of disease.

Macrocyclic Zinc(II) Complexes for Selective Recognition of Nucleobases in Single- and Double-Stranded Polynucleotides

The model study of zinc enzyme by Zn2+–cyclen complexes (cyclen = 1, 4, 7, 10-tetraazacyclododecane) disclosed the intrinsic properties of zinc(II) as having strong anion affinities and yet the resulting Zn2+–anion bonds have a labile nature. The basic understanding has evolved into novel selective nucleobase recognition by the Zn2+–cyclen complexes. The Zn2+–aromatic pendant cyclen complexes selectively and effectively bind to thymine T (or uracil U) in single- and double-stranded DNA (or RNA). The Zn2+ complexes work like molecular zippers to break A–T pairs in double-stranded DNA, as proven by various physicochemical and DNA footprinting measurements. Moreover, these Zn2+–complexes affect relevant biochemical and ultimately biological properties such as inhibition of a transcriptional factor and antimicrobial activities.

Tumor Repression of VCaP Xenografts by a Pyrrole-Imidazole Polyamide

Pyrrole-imidazole (Py-Im) polyamides are high affinity DNA-binding small molecules that can inhibit protein-DNA interactions. In VCaP cells, a human prostate cancer cell line overexpressing both AR and the TMPRSS2-ERG gene fusion, an androgen response element (ARE)-targeted Py-Im polyamide significantly downregulates AR driven gene expression. Polyamide exposure to VCaP cells reduced proliferation without causing DNA damage. Py-Im polyamide treatment also reduced tumor growth in a VCaP mouse xenograft model. In addition to the effects on AR regulated transcription, RNA-seq analysis revealed inhibition of topoisomerase-DNA binding as a potential mechanism that contributes to the antitumor effects of polyamides in cell culture and in xenografts. These studies support the therapeutic potential of Py-Im polyamides to target multiple aspects of transcriptional regulation in prostate cancers without genotoxic stress.

Organoplatinum(II) complexes with nucleobase motifs as inhibitors of human topoisomerase II catalytic activity

Platinum(II) complexes bearing acetylide ligands containing nucleobase motifs are prepared and their impact on human topoisomerase II (TopoII) is evaluated. Both platinum(II) complexes [Pt(II)(C^N^N)(C≡CCH₂R)] (1a-c) and [Pt(II)(tBu₃terpy)(C≡CCH₂R)](+) (2a-c) (C^N^N=6-phenyl-2,2'-bipyridyl, tBu₃terpy=4,4',4''-tri-tert-butyl-2,2':6',2''-terpyridyl, and R=(a) adenine, (b) thymine, and (c) 2-amino-6-chloropurine) are stable in aqueous solutions for 48 hours at room temperature. The binding constants (K) for the platinum(II) complexes towards calf thymus DNA are in the order of 10⁵ dm³ mol⁻¹ as estimated by using UV/Vis absorption spectroscopy. Of the complexes examined, only complexes 1a-c are found to behave as intercalators. Both complexes 1a-c and 2a-c inhibit TopoII-induced relaxation of supercoiled DNA, while 2c is the most potent TopoII inhibitors among the tested compounds. Inhibition of DNA relaxation is detected at nanomolar concentrations of 2c. All of the platinum(II) complexes are cytotoxic to human cancer cells with IC₅₀ values of 0.5-13.7 μM, while they are less toxic against normal cells CCD-19 Lu.

A series of alpha-heterocyclic carboxaldehyde thiosemicarbazones inhibit topoisomerase IIalpha catalytic activity

A series of novel thiosemicarbazone derivatives bearing condensed heterocyclic carboxaldehyde moieties were designed and synthesized. Among them, TSC24 exhibited broad antiproliferative activity in a panel of human tumor cells and suppressed tumor growth in mice. The mechanism research revealed that TSC24 was not only an iron chelator but also a topoisomerase IIalpha catalytic inhibitor. Its inhibition on topoisomerase IIalpha was due to direct interaction with the ATPase domain of topoisomerase IIalpha which led to the block of ATP hydrolysis. Molecular docking predicted that TSC24 might bind at the ATP binding site, which was confirmed by the competitive inhibition assay. These results about the mechanisms involved in the anticancer activities of thiosemicarbazones will aid in the rational design of novel topoisomerase II-targeted drugs and will provide insights into the discovery and development of novel cancer therapeutics based on the dual activity to chelate iron and to inhibit the catalytic activity of topoisomerase IIalpha.

Enhanced topoisomerase II targeting by annamycin and related 4-demethoxy anthracycline analogues

Targeting topoisomerase II (topo II) is regarded as an important component of the pleiotropic mechanism of action of anthracycline drugs. Here, we show that 4-demethoxy analogues of doxorubicin, including annamycin, exhibit a greater ability to trap topo II cleavage complexes than doxorubicin and some other 4-methoxy analogues. In leukemic CEM cells with wild-type topo II, annamycin induced substantial levels of topo II–mediated DNA-protein cross-links (15-37% of total DNA for 0.5-50 μmol/L drug), whereas doxorubicin-induced DNA-protein cross-links were marginal (0-4%). In CEM/VM-1 cells that harbor mutated, drug-resistant topo II, both 4-methoxy and 4-demethoxy drugs produced marginal DNA-protein cross-links. Annamycin, but not doxorubicin, formed topo II–mediated DNA-protein cross-links also in isolated CEM nuclei. In disparity with the unequal DNA-protein cross-link induction, both drugs induced comparable levels of DNA strand breaks in CEM cells. Compared with CEM, drug cytotoxicity against CEM/VM-1 cells was reduced 10.5- to 13.8-fold for 4-demethoxy analogues but only 3.8- to 5.5-fold for 4-methoxy drugs. Hence, growth inhibition by 4-demethoxy analogues seems more dependent on the presence of wild-type topo II. The enhanced topo II targeting by 4-demethoxy analogues was accompanied by a profound induction of apoptotic DNA fragmentation in leukemic CEM cells. Normal WI-38 fibroblasts, however, were markedly more resistant to annamycin-induced DNA-protein cross-links, apoptosis, and growth inhibition. The enhanced topo II targeting by 4-demethoxy doxorubicin analogues underscores the mechanistic diversity of anthracycline drugs. This diversity needs to be recognized as a factor in responses to drugs such as annamycin and doxorubicin.

A novel topoisomerase II poison GL331 preferentially induces DNA cleavage at (C/G)T sites and can cause telomere DNA damage

PURPOSE

Topoisomerase II (Topo II) preferentially cuts DNA at alternating purine-pyrimidine repeats. Different Topo II poisons may affect Topo II to produce distinct drug-specific DNA cleavage patterns. GL331 is a new podophyllotoxin derivative exhibiting potent Topo II-poisoning activity. Therefore, the sequence selectivity of GL331-induced DNA cleavage was determined.

METHODS

Human gastric adenocarcinoma SC-M1 cells were treated with GL331, and the resultant DNA fragments were isolated by SDS-K+ precipitation. These DNA fragments were further cloned and sequenced to exhibit GL331-induced DNA cleavage sites. In addition, the telomere damage was detected by Southern blot analyses using a (TTAGGG)4 probe. GL331's effect on telomerase was examined using the TRAP assay.

RESULTS

The selective sequences of GL331-induced DNA cleavage were analyzed. The first nucleotide 3'-terminal to the cleavage sites was preferentially C or G and followed by the second nucleotide T. More than 50% of GL331-induced DNA cleavage fragments exhibited AT-rich sequences in the first 20 nucleotides. In addition, the telomeric damage was observed both from GL331-treated SC-M1 cells and in vitro incubation of genomic DNA with GL331 and purified human Topo II. Although GL331 treatment reduced cellular telomerase activity, in vitro reaction data suggested that GL331 was not a telomerase inhibitor.

CONCLUSION

GL331 preferentially induced Topo II-mediated DNA cleavage at (C/G)T sites. Because the telomeric repeat sequence contains GL331's GT preference site, the telomere was identified as one of the targets of GL331-induced DNA damage.

Catalytic inhibition of DNA topoisomerase II by N-benzyladriamycin (AD 288)

N-Benzyladriamycin (AD 288) is a highly lipophilic, semi-synthetic congener of doxorubicin (DOX). Unlike DOX, which stimulates double-stranded DNA scission by stabilizing topoisomerase II/DNA cleavable complexes, AD 288 is a catalytic inhibitor of topoisomerase II, capable of preventing topoisomerase II activity on DNA. The concentration of AD 288 required to inhibit the topoisomerase II-catalyzed decatenation of linked networks of kinetoplast DNA was comparable to that for DOX. However, AD 288 did not stabilize cleavable complex formation or stimulate topoisomerase II-mediated DNA cleavage. In addition, AD 288 inhibited the formation of cleavable complexes by etoposide in a concentration-dependent manner. Human CCRF-CEM cells and murine J774. 2 cells exhibiting resistance against DOX, teniposide, or 3'-hydroxy-3'-deaminodoxorubicin through reduced topoisomerase II activity remained sensitive to AD 288. These studies suggest that AD 288 inhibits topoisomerase II activity by preventing the initial non-covalent binding of topoisomerase II to DNA. Since AD 288 is a potent DNA intercalator, catalytic inhibition is achieved by prohibiting access of the enzyme to DNA binding sites. These results also demonstrate that specific substitutions on the aminosugar of DOX can alter the mechanism of topoisomerase II inhibition.

Selective recognition of consecutive G sequence in double-stranded DNA by a zinc(II)-macrocyclic tetraamine complex appended with an anthraquinone

A zinc (II) complex with a macrocyclic tetraamine appended with an anthraquinone ((9,10-anthraquinon-2-yl)methyl-1,4,7,10-tetraazacyclododecane, ZnL, anthraquinonyl-cyclen) selectively recognizes consecutive G sequence in double-stranded DNA. The affinity of the Zn2+-anthraquinonyl-cyclen to consecutive dG groups in DNA was disclosed by comparison of K(app) values (=[DNA-bound ZnL]/[uncomplexed ZnL][uncomplexed nucleobase in DNA]) determined by the UV spectrophotometric titrations at pH 8, I=0.1 (NaNO3), and 25 degrees C for poly(dG) x poly(dC) (K(app) = 1.5 x 10(5) M(-1)), poly(dG-dC)2 (2.8 x 10(4) M(-1)), poly(dA-dT)2 (4.3 x 10(4) M(-1)), and calf thymus DNA (2.8 x 10(4) M(-1)). The corresponding K(app) values with the Zn2+-free ligand were 5.3 x 10(3) M(-1), 7.4 x 10(3) M(-1), 7.4 x 10(3) M(-1), and 5.9 x 10(3) M(-1), respectively. The selective recognition of consecutive G sequence was concluded from the DNase I footprinting of SV40 early promotor DNA fraction (197 bp) containing a TATA box and six GC boxes. The present finding is in remarkable contrast to the previous selective T-recognition by Zn2+-cyclen complexes appended with acridine, quinoline(s), and naphthalene(s) [J. Am. Chem. Soc. 121 (1999) 5426]. While the Zn2+-acridinyl-cyclen inhibited TATA binding protein from interacting with a TATA box consensus DNA [J. Inorg. Biochem. 79 (2000) 253], the present Zn2+-anthraquinonyl-cyclen inhibited the Sp1 transcriptional factor protein from interacting with a GC box-consensus DNA.

Genotoxicity of several clinically used topoisomerase II inhibitors

DNA topoisomerase II is an essential nuclear enzyme that modulates DNA topology during multiple cellular processes such as DNA replication and chromosome segregation. Several important clinical antitumor drugs and antibiotics act through inhibition of topoisomerase II. There are a number of different steps in the action of topoisomerase II, all of which are potential targets for inhibition through drugs and also for cellular and genetic toxicity as well as for mutagenesis. We have investigated and compared the genotoxicity and mutagenicity of the mechanistically different topoisomerase II inhibitors m-amsacrine, mitoxantrone, etoposide, genistein, ICRF 193, and berenil using the in vitro micronucleus test, single cell gelelectrophoresis (comet assay) and the mutation assay (tk-locus) in L5178Y mouse lymphoma cells. All six compounds induced micronuclei and all except berenil were mutagenic. M-amsacrine, mitoxantrone, etopside and genistein induced DNA migration in the comet assay, whereas ICRF 193 was only weakly positive and berenil was negative in this test. Our results are in good agreement with the compounds' proposed mechanisms of interaction with topoisomerase II.

Controlling gene expression by zinc(II)-macrocyclic tetraamine complexes

The zinc(II) complexes of 12-membered macrocyclic tetraamines (1,4,7,10-tetraazacyclododecane, cyclen) appended with one or two aryl-methyl group(s) (quinolyl-methyl, naphthyl-methyl, and acridinyl-methyl) selectively bind to thymines in a TATA box of the SV40 early promoter region and thus inhibit the binding of a transcriptional factor, TATA binding protein. These Zn2+-cyclen derivatives also act as inhibitors of DNA-targeted enzymes, type I and type II topoisomerases. They also exhibited strong antimicrobial activities for the gram-positive bacterial strain. These biochemical and biological properties were compared with those of conventionally established AT-recognizing drugs, distamycin A and DAPI. The Zn2+-cyclen complexes are a new type of small molecular, genetic transcriptional regulation factor.

Synthetic DNA minor groove-binding drugs

In this review, both cationic and neutral synthetic ligands that bind in the minor groove of DNA are discussed. Certain bis-distamycins and related lexitropsins show activities against human immunodeficiency virus (HIV)-1 and HIV-2 at low nanomolar concentrations. DAPI binds strongly to AT-containing polymers and is located in the minor groove of DNA. DAPI intercalates in DNA sequences that do not contain at least three consecutive AT bp. Berenil can also exhibit intercalative, as well as minor groove binding, properties depending on sequence. Furan-containing analogues of berenil play an important role in their activities against Pneumocystis carinii and Cryptosporidium parvuam infections in vivo. Pt(II)-berenil conjugates show a good activity profile against HL60 and U-937 human leukemic cells. Pt-pentamidine shows higher antiproliferative activity against small cell lung, non-small cell lung, and melanoma cancer cell lines compared with many other tumor cell lines. trans-Butenamidine shows good anti-P. carinii activity in rats. Pentamidine is used against P. carinii pneumonia in individuals infected with HIV who are at high risk from this infection. A comparison of the cytotoxic potencies of adozelesin, bizelesin, carzelesin, cisplatin, and doxorubicin indicates that adozelesin is a potent analog of CC-1065. Naturally occurring pyrrolo[2,1-c][l,4]benzodiazepines such as anthramycin have a 2- to 3-bp sequence specificity, but a synthetic PBD dimer spans 6 bp, actively recognizing a central 5'-GATC sequence. The crosslinking efficiency of PBD dimers is much greater than that of other major groove crosslinkers, such as cisplatin, melphalan, etc. Neothramycin is used clinically for the treatment of superficial carcinoma of the bladder.

Catalytic inhibitors of DNA topoisomerase II

Catalytic inhibitors of mammalian DNA topoisomerase II have been found recently in natural and synthetic compounds. These compounds target the enzyme within the cell and inhibit various genetic processes involving the enzyme, such as DNA replication and chromosome dynamics, and thus proved to be good probes for the functional analyses of the enzyme in a variety of eukaryotes from yeast to mammals. Catalytic inhibitors were shown to be antagonists against topoisomerase II poisons. Thus bis(2,6-dioxopiperazines) have a potential to overcome cardiac toxicity caused by potent antitumor anthracycline antibiotics such as doxorubicin and daunorubicin. ICRF-187, a (+)-enantiomer of racemic ICRF-159, has been used in clinics in European countries as cardioprotector. Furthermore, bis(2,6-dioxopiperazines) enhance the efficacy of topoisomerase II poisons by reducing their side effects in preclinical and clinical settings. Bis(2,6-dioxopiperazines) per se among others have antitumor activity, and one of their derivatives, MST-16 or Sobuzoxane, bis(N1-isobutyloxycarbonyloxymethyl-2, 6-dioxopiperazine), has been developed in Japan as an anticancer drug used for malignant lymphomas and adult T-cell leukemia in clinics.

Bis(2,6-dioxopiperazines), catalytic inhibitors of DNA topoisomerase II, as molecular probes, cardioprotectors and antitumor drugs

Bis(2,6-dioxopiperazines) and other catalytic inhibitors of mammalian DNA topoisomerase II have recently been found in natural and synthetic compounds. These compounds target the enzyme within the cell and inhibit various genetic processes involving the enzyme such as DNA replication and chromosome dynamics and thus proved to be good probes for the functional analyses of the enzyme in a variety of eucaryotes from yeast to mammals. Catalytic inhibitors were shown to be antagonists against topoisomerase II poisons under some conditions, but to be synergistic under others. Bis(2,6-dioxopiperazines) have a potential to overcome cardiac toxicity caused by potent antitumor anthracycline antibiotics such as doxorubicin and daunorubicin. ICRF-187, +enantiomer of racemic ICRF-159, has been used in EU countries as cardioprotector in cancer clinics. Furthermore, bis(2,6-dioxopiperazines) enhance the efficacy of antitumor topoisomerase II poisons, e.g. anthracycline antibiotics such as daunorubicin and doxorubicin, by reducing their side effects and by allowing dose escalation of the antitumor drugs in preclinical and clinical settings. Besides bis(2,6-dioxopiperazines) per se having antitumor activity, and one of their derivatives, MST-16 or sobuzoxane, bis(N1-isobutyloxycarbonyloxymethyl-2,6-dioxopiperazine), has been developed in Japan and used in clinics as anticancer drug for malignant lymphomas and adult T-cell leukemia (ATL). Further developments of bis(2,6-dioxopiperazines) as antimetastatic agents are expected.

The fundamental ribosomal RNA transcription initiation factor-IB (TIF-IB, SL1, factor D) binds to the rRNA core promoter primarily by minor groove contacts

Acanthamoeba castellanii transcription initiation factor-IB (TIF-IB) is the TATA-binding protein-containing transcription factor that binds the rRNA promoter to form the committed complex. Minor groove-specific drugs inhibit TIF-IB binding, with higher concentrations needed to disrupt preformed complexes because of drug exclusion by bound TIF-IB. TIF-IB/DNA interactions were mapped by hydroxyl radical and uranyl nitrate footprinting. TIF-IB contacts four minor grooves in its binding site. TIF-IB and DNA wrap around each other in a right-handed superhelix of high pitch, so the upstream and downstream contacts are on opposite faces of the helix. Dimethyl sulfate protection assays revealed limited contact with a few guanines in the major groove. This detailed analysis suggests significant DNA conformation dependence of the interaction.

DNA binding properties of minor groove binders and their influence on the topoisomerase II cleavage reaction

We present titrations of the human delta beta-globin gene region with DNA minor groove binders netropsin, bisnetropsin, distamycin, chromomycin and four bis-quaternary ammonium compounds in the presence of calf thymus topoisomerase II and DNase I. With increasing ligand concentration, stimulation and inhibition of enzyme activity were detected and quantitatively evaluated. Additionally we show a second type of stimulation, the appearance of strong new topoisomerase II cleavage sites at high ligand concentrations. The specific binding sites of the minor groove binders of the DNA sequence and their microscopic binding constants were determined from DNase I footprints. A binding mechanism for minor groove binders is proposed in order to explain these results especially when ligand concentration is increased.

DNA topoisomerase II rescue by catalytic inhibitors: a new strategy to improve the antitumor selectivity of etoposide

The nuclear enzyme DNA topoisomerase II (topo II) is the target of important antitumor agents such as etoposide. Recent work has classified topo II targeting drugs into either topo II poisons that act by stabilizing enzyme-DNA cleavable complexes leading to DNA breaks, or topo II catalytic inhibitors that act at stages in the catalytic cycle of the enzyme where both DNA strands are intact and, therefore, do not cause DNA breaks. Accordingly, catalytic inhibitors are known to abrogate DNA damage and cytotoxicity caused by topo II poisons. In this commentary, we have focused on the possibilities of enabling high-dose therapy with the topo II poison etoposide by protection of normal tissue with catalytic inhibitors, analogous to folinic acid rescue in high-dose methotrexate treatment. Thus, we have demonstrated recently that (+)-1,2-bis(3,5-dioxopiperazinyl-1-yl)propane (ICRF-187) enabled a 3- to 4-fold dose escalation of etoposide in mice. Two high-dose etoposide models are described, namely use of the weak base chloroquine in tumors with acidic extracellular pH and targeting of CNS tumors with protection of normal tissue by the bisdioxopiperazine ICRF-187. In conclusion, high supralethal doses of topo II poisons in combination with catalytic inhibitor protection form a new strategy to improve the antitumor selectivity of etoposide and other topo II poisons. Such an approach may be used to overcome problems with drug resistance and drug penetration.

Is reduced accumulation of Hoechst 33342 in multidrug resistant cells related to P-glycoprotein activity?

Although bisbenzimidazole-DNA interactions have been studied in solution, little information has been available in living cells. The reduced accumulation of the nuclear dye Hoechst 33342 (H342) in cells with multidrug resistant (MDR) phenotype suggested its possible use in a functional test for detection of these cells. We performed experiments to elucidate the mechanisms involved in the H342-exclusion from resistant cells. As contradictory results have been reported in literature, we compared the entire fluorescence spectra of H342 in solution and in intact living cells under different experimental conditions. The study was performed by fluorescence image cytometry. This technique allow accurate quantification of the amount of H342 bound to DNA in living cells. The dye uptake was followed in sensitive and resistant cells, a lymphoblastoid cell line, CCRF-CEM, and its resistant variant selected with vinblastine CEM/VLB100 under conditions that could modulate H342-cell binding. Competition experiments with sodium azide, verapamil, and vinblastine indicated that resistant cells did not differ in the number of possible binding sites for H342. The obtained results ruled out the possibility of discriminating cells on the basis of a spectral shift. Two modes of binding, differing in their affinity for the dye, seem to co-exist in intact cells. Although it clearly appeared that the P-glycoprotein expressed in MDR cells was mainly responsible for the H342-exclusion, other mechanisms might also be involved.

Antimicrobial activity of 9-oxo and 9-thio acridines: correlation with interacalation into DNA and effects on macromolecular biosynthesis

The antimicrobial activity of several new 9-acridinones and 9-thioalkylacridines towards Escherichia coli, Staphylococcus aureus, Mycobacterium smegmatis and Candida albicans was investigated. Minimal inhibitory, bactericidal and fungicidal concentrations were determined using a microplate assay which enabled inhibitory, bactericidal and fungicidal indices to be calculated. These indices facilitated structure/activity relationship studies. DNA-intercalating capability and DNA supercoiling inhibitory effects as well as inhibitory effects on macromolecular synthesis were determined. Results showed that intercalation into DNA, which is the mechanism of action usually postulated for acridines, cannot be correlated with the properties examined. However, inhibition of RNA synthesis may be involved in the antimicrobial activity of the drugs.

Selective inhibition of topoisomerases from Pneumocystis carinii compared with that of topoisomerases from mammalian cells

Type I and II topoisomerase activities were partially purified from Pneumocystis carinii. The catalytic (strand-passing) activities of both enzymes were selectively inhibited by members of a series of dicationic-substituted bis-benzimidazoles compared with those of topoisomerases of mammalian (calf thymus) origin. The most active inhibitors of the parasite enzymes were also highly effective in an in vivo animal model of P. carinii pneumonia. Selected dicationic-substituted bis-benzimidazoles also strongly inhibited the induction of the topoisomerase I- and II-mediated cleavable complex, suggesting that the biologically active DNA minor groove-binding molecules inhibit the enzyme-DNA binding step of the topoisomerase reaction sequence. The apparent selectivities for the parasite enzymes and the low levels of toxicity to mammalian cells for the biologically active bis-benzimidazoles suggest that these compounds hold promise as effective therapeutic agents in the treatment of a life-threatening AIDS-related disease, P. carinii pneumonia.

Stimulation of site-specific topoisomerase II-mediated DNA cleavage by an N-methylpyrrolecarboxamide-anilinoacridine conjugate: relation to DNA binding

The DNA binding properties and effects on topoisomerase II of MePyGA, an anilinoacridine derivative bearing an N-methylpyrrolecarboxamide unit at position 1', have been compared with those of its precursor glycylanilinoacridine and the structurally related antileukaemic drug amsacrine. Electric linear dichroism spectroscopy reveals that MePyGA intercalates its acridine chromophore between DNA base pairs with a preference for GC-rich sequences, whereas both its structural analogue lacking the N-methylpyrrole unit and amsacrine intercalate into DNA without any strong sequence preference. The effects of the test drug on the catalytic activities of topoisomerase II were studied in vitro using purified calf thymus enzyme and 32P-labeled DNA. MePyGA stabilizes the topoisomerase II-DNA covalent complex and stimulates the cutting of DNA at a subset of preexisting topoisomerase II cleavage sites. The removal of the N-methylpyrrole unit abolishes both the GC-preferential binding to DNA and the topoisomerase II-mediated DNA cleavage. MePyGA and amsacrine stimulate the cleavage of DNA by topoisomerase II at different places: cleavage stimulated by amsacrine is consistent with the expected adenine requirement at position +1 whereas the predominant sites of DNA cleavage stimulated by MePyGA contain a cytosine at position +/- 1. This is the first instance where an anilinoacridine derivative differing only by the nature of the substituent at position 1' has been found to affect the catalytic activity of topoisomerase II differently. The spectroscopic and biochemical data lead to the conclusion that two functional domains can be identified in MePyGA: its anilino group can be regarded as a skeletal core to which are connected (i) the tricyclic acridine moiety which represents the DNA-binding domain and (ii) the N-methylpyrrole moiety which constitutes the topoisomerase II-targeted domain. The structure of the substituent at position 1' of the anilinoacridine chromophore evidently determines the location of the sites of DNA cleavage by topoisomerase II. These findings provide guidance for the synthesis and development of new topoisomerase II-targeted antitumor anilinoacridine derivatives.

CC-1065 bonding to intracellular and purified SV40 DNA: site specificity and functional effects

CC-1065 is a minor-groove bonding agent capable of forming covalent adducts with the N-3 position of adenines within A-T-rich regions of duplex DNA. By examining the formation and location of CC-1065 adducts within the simian virus 40 (SV40) DNA molecule, the present study marks the first time that the precise sites of CC-1065 lesions have been identified at the level of eukaryotic genomic DNA. In naked DNA preparations, r values (moles of drug/mole of nucleotide base pair) > or = 0.0015 effected, after thermal treatment, a measurable decrease in intact supercoiled form I, as well as increases in forms II and III, indicating that both single-strand and apparent double-strand damage had occurred. A similar pattern of damage was observed in SV40-infected cells, albeit at higher CC-1065 levels. The amount of CC-1065 required to produce a 50% loss in form I was > 2-fold higher in infected cells (r = 0.029) than with purified DNA samples (r = 0.013). The appearance of double-strand damage at low drug levels suggested a high specificity of CC-1065 bonding to localized regions of the genome. The precise location of these CC-1065 adduction sites was examined by three methods: sequence analysis of the entire genome (GenBank), DNA polymerase termination assay of specific fragments of SV40, and restriction enzyme digestion analysis of the entire SV40 molecule. When sequence analysis of the entire genome was performed by examining both strands for the presence of the consensus CC-1065 binding sequence 5'-A/T-A/T-A/T-A/T-A*-3'[Reynolds et al. (1985) Biochemistry 24, 6228-6247], 294 single-strand adduction sites were predicted, compared to 20 sites where CC-1065 should bond to both strands within a 30-base-pair window and at which, when heated, a double-strand break should occur. DNA polymerase termination assay of actual adduction sites was performed on restriction fragments of SV40 DNA pretreated with CC-1065 in infected cells or in purified supercoiled DNA preparations and selected on the basis of the sequence analysis (i.e., regions 2510-2730, 3701-3920, 4400-4659, 4020-4320, and 5163-65). In general, double-strand lesions were detected in similar regions of the genome by the DNA termination assay and by sequence analysis. When restriction enzyme digestion and the DNA polymerase termination assay were compared throughout the genome, nearly identical patterns of adduct formation were observed. Interestingly, similar alkylation patterns were observed with either naked or infected cell DNA.(ABSTRACT TRUNCATED AT 400 WORDS)

Mode of DNA binding of bis-benzimidazoles and related structures studied by electric linear dichroism

The binding mode of a series of bis-benzimidazole analogues of Hoechst 33258 to a variety of DNAs and polynucleotides has been investigated by electric linear dichroism. Two groups of compounds were examined: (i) benzoxazole and pyridoimidazole derivatives and (ii) pyridoimidazole analogs substituted with an N-alkoxyalkyl group either directed towards the minor groove or directed away from the minor groove. The ELD data indicate that the mode of binding of these drugs varies significantly with the sequence of the target DNA sequence. The DNA binding properties of these drugs are related to their topoisomerase inhibitory properties.

Antagonistic effect of aclarubicin on camptothecin induced cytotoxicity: role of topoisomerase I

The cellular target of camptothecin and several of its derivatives has been identified as topoisomerase I. Central to the cytotoxic action of camptothecin is the drug's ability to stimulate formation of topoisomerase I mediated DNA cleavages. Here we demonstrate that the intercalating antitumor agent aclarubicin inhibits camptothecin induced DNA single strand breaks in cells as measured by alkaline elution. When purified topoisomerase I was reacted with DNA, aclarubicin inhibited the formation of enzyme mediated DNA breaks induced by camptothecin. High aclarubicin concentrations (10 and 100 microM) caused a slight stimulation of topoisomerase I mediated DNA cleavage at a few distinct DNA sites. The cytotoxicity associated with camptothecin treatment measured in clonogenic assays was antagonized by preincubation with aclarubicin. This inhibitory effect of aclarubicin upon camptothecin action holds implications for the scheduling of aclarubicin in combination therapy with anticancer agents directed against topoisomerase I. Aclarubicin also inhibits the effect of topoisomerase II directed agents [such as etoposide (VP16), amsacrine (mAMSA), etc.] suggesting that aclarubicin acts against the two topoisomerases.

DNA recognition by lexitropsins, minor groove binding agents

Consideration is given to alternative approaches to the development of DNA sequence selective binding agents because of their potential applications in diagnosis and treatment of cancer as well as in molecular biology. The concept of lexitropsins, or information-reading molecules, is introduced within the antigene strategy as an alternative to, and complementary with, the antisense approach for cellular intervention and gene control. The chemical, physical and pharmacological factors involved in the design of effective lexitropsins are discussed and illustrated with experimental results. Among the factors contributing to the molecular recognition processes are: the presence and disposition of hydrogen bond accepting and donating groups, ligand shape, chirality, stereochemistry, flexibility and charge. For longer ligands, such as are required to target unique sequences in biological systems (14-16 base pairs), the critical feature is the phasing or spatial correspondence between repeat units in the ligand and the receptor. The recently discovered 2:1 lexitropsin-DNA binding motif provides a further refinement in molecular recognition in permitting discrimination between GC and CG base pairs. The application of these factors in the design and synthesis of novel agents which exhibit anticancer, antiviral and antiretroviral properties, and inhibition of critical cellular enzymes including topoisomerases is discussed. The emerging evidence of a relationship between sequence selectivity of the new agents and the biological responses they invoked is also described.

Modulation of DNA supercoiling by interaction with netropsin and other minor groove binders

The assay of DNA unwinding by ethidium, followed by sedimentation velocity techniques, was applied to complexes of supercoiled plasmid DNA with different non-intercalating drugs which strongly and sequence-specifically bind to DNA. Compared with the behaviour of naked DNA, most of the complexes exhibit an increase in the critical EB/nucleotide binding ratio associated with the principal minimum in the sedimentation profile. Using netropsin (Nt) as the paradigm of the minor groove binders investigated, the drug-induced alterations in various structural parameters of both the relaxed and supercoiled form of DNA are described. Whereas winding number, helical repeat (both being defined with reference to a surface normal), and linking number of the superhelical DNA remain constant in our experiments, its twist number, surface twist, number of superhelical turns as well as the absolute values of linking number difference, superhelix density, and writhing number increase on binding of Nt. Correspondingly, compared with the naked relaxed DNA a higher linking number (or twist number, or winding number), a higher average duplex winding angle and a lower helical repeat have to be assigned to the relaxed Nt-DNA complex. The various minor groove binders investigated were found to differ considerably in their efficiency to alter the structure of supercoiled DNA.

Transcriptional regulation of differentiation, selective toxicity and ATGCAAAT binding of bisbenzimidazole derivatives in human melanoma cells

To study the relationship between the structure of minor groove ligands and their affinity for specific DNA sequences that regulate gene transcription, three analogues of the A-T-specific DNA minor groove ligands Hoechst 33258 and Hoechst 33342 were synthesized with 5, 8 or 12 carbons in an aliphatic chain attached to the phenolic oxygen of the molecule. There was a striking bimodal relationship between toxicity to HeLa cells and the lipophilicity of the five analogues, toxicity being low for the compounds with a free hydroxyl (Hoechst 33258) or a 12-carbon substituent, yet high for the 5-carbon analogue. Selective killing of human melanoma cells compared with normal fibroblasts was observed for the Hoechst analogue with a 12-carbon chain attached. Hoechst 33258 itself was selectively toxic for the MM96E melanoma cell line compared with other cell lines, induced a highly dendritic morphology, increased tyrosinase activity and tyrosinase mRNA but decreased the level of gp75 (TRP-1) mRNA; message for a third pigment gene, Pmel-17, was unchanged. Tyrosinase activity was decreased in the resistant A2058 melanoma cell line and transcription was affected to a lesser extent than in MM96E. Expression of gp75 protein and two intermediate filament proteins was inhibited by Hoechst 33258 in MM96E cells. There was no major difference in the amount of 125I-Hoechst 33258 taken up by sensitive and resistant cells. Of the five derivatives studied, the parent drug Hoechst 33258 and the 2-carbon analogue (Hoechst 33342) were found to have the most inhibitory effect on affinity of octamer binding proteins for the ATGCAAAT consensus sequence found in the promoter region of certain genes associated with proliferation and differentiation. In contrast to Distamycin A (also an A-T-specific minor groove ligand), Hoechst 33258 displaced proteins already bound to the octamer motif. The G-C ligand chromomycin A3 exhibited a different spectrum of cell toxicity and tyrosinase stimulation compared with Hoechst 33258. Chromomycin A3 but not Hoechst 33258, strongly inhibited the zinc-dependent transcriptional activity of the sheep metallothionein-Ia promoter in reporter gene assays of transfected cells. Since the six metal-responsive elements of the promoter are GC-rich, this provides independent evidence for the sequence-specificity of transcriptional inactivation by one of these drugs in melanoma cells. Overall, the results suggest that Hoechst 33258 acts by inhibiting the transcription of specific genes, cell lines evidently differing in the accessibility to drugs of certain A-T-rich sequences.

DNA minor groove-binding ligands: a different class of mammalian DNA topoisomerase I inhibitors

A number of DNA minor groove-binding ligands (MGBLs) are known to exhibit antitumor and antimicrobial activities. We show that DNA topoisomerase (Topo) I may be a pharmacological target of MGBLs. In the presence of calf thymus Topo I, MGBLs induced limited but highly specific single-strand DNA breaks. The 3' ends of the broken DNA strands are covalently linked to Topo I polypeptides. Protein-linked DNA breaks are readily reversed by a brief heating to 65 degrees C or the addition of 0.5 M NaCl. These results suggest that MGBLs, like camptothecin, abort Topo I reactions by trapping reversible cleavable complexes. The sites of cleavage induced by MGBLs are distinctly different from those induced by camptothecin. Two of the major cleavage sites have been sequenced and shown to be highly A + T-rich, suggesting the possible involvement of a Topo I-drug-DNA ternary complex at the sites of cleavage. Different MGBLs also exhibit varying efficiency in inducing Topo I-cleavable complexes, and the order of efficiency is as follows: Hoechst 33342 and 33258 >> distamycin A > berenil > netropsin. The lack of correlation between DNA binding and cleavage efficiency suggest that, in addition to binding to the minor grooves of DNA, MGBLs must also interact with Topo I in trapping Topo I-cleavable complexes.

Binding to DNA of selected lexitropsins and effects on prokaryotic topoisomerase activity

The binding behaviour toward DNA of some minor groove binders related to distamycin was studied by means of circular dichroism. In addition their influence on the activity of topoisomerases isolated from Streptomyces noursei has been investigated. The monocationic imidazole containing ligands (lexitropsins) show a decreased affinity to AT pairs but an increased affinity to GC pairs which contrasts the AT-preferred binding of Dst-2 and Dst-3. For the monocationic triimidazole containing lexitropsin the affinity for GC over AT pairs was most pronounced. It was also found that the imidazole containing lexitropsins are inhibitors of topoisomerases. These minor groove binders interfere more strongly with the DNA gyrase activity than with the prokaryotic topoisomerase I. Our results indicate that Dst-3 most effectively inhibits gyrase and topoisomerase I activity. However, the inhibitory effect is neither related to the base pair specificity nor to the binding strength of different ligands. The mechanism of interference of minor groove binders with topoisomerase activity is more complex. It is considered that different factors, such as the nature of the ligand together with their DNA binding parameters and the target sequences of the enzymes play a role in the inhibitory effects of minor groove binders.

Different modes of anthracycline interaction with topoisomerase II. Separate structures critical for DNA-cleavage, and for overcoming topoisomerase II-related drug resistance

In contrast to the classic anthracyclines (doxorubicin and daunorubicin), aclarubicin (ACLA) does not stimulate topoisomerase II (topo II) mediated DNA-cleavage. This distinction may be important with respect to topo II-related drug resistance, and the aim of this study was to clarify drug-structures responsible for this difference. Various ACLA analogs were tested for: (a) interaction with purified topo II, (b) induction of DNA cleavage in cells, (c) cellular uptake and (d) cytotoxicity. A remarkable distinction was seen between analogs containing the chromophore aklavinone (AKV) (e.g. ACLA) which have a carboxymethyl group (COOCH3) at C-10 and drugs with a beta-rhodomycinone (RMN) chromophore with hydroxyl groups at C-10 and at C-11. Thus, RMN-containing analogs, including the aglycone RMN itself, effectively stimulated topo II-mediated DNA cleavage. In contrast, AKV-containing drugs inhibited DNA cleavage and antagonized cytotoxicity mediated by RMN-containing drugs. In OC-NYH/VM cells, exhibiting multidrug resistance due to an altered topo II phenotype (at-MDR), cross-resistance was only seen to the RMN-containing drugs whereas no cross-resistance was seen to the non-DNA cleaving AKV-containing compounds. Thus, our data show that one domain in the anthracycline is of particular importance for the interaction with topo II, namely the positions C-10 and C-11 in the chromophore, and further that at-MDR was circumvented by a COOCH3 substitution at position C-10. These findings may provide guidance for the synthesis and development of new analogs with activity in at-MDR cells.

Mixed micelles as proliposomes for the solubilization of teniposide

The aqueous solubility of teniposide in detergent and phospholipid mixed micelles was investigated as functions of the detergents and lipids composing the mixed micelles, the molar ratio of detergent to phospholipid, and the total lipid concentration of the system. The polarity, the charge of the phospholipid, and its saturation affected the solubilization potential of the micelles. Physical chemical factors such as the pH, ionic strength, and temperature of the dispersion medium also altered the solubilization capacity of the system. The results are explained by the changes occurring in the critical micelle concentration and packing arrangements of the aggregates. The desired solubility of teniposide can be achieved by adjusting the studied parameters to the optimum values. Teniposide-containing mixed micelles were spontaneously converted to drug-containing vesicles upon aqueous dilution; therefore, the precipitation of the drug was totally eliminated. In conclusion, mixed micelles as proliposomes can be a suitable drug carrier system for insoluble compounds such as teniposide.

Effects of analogs of the DNA minor groove binder Hoechst 33258 on topoisomerase II and I mediated activities

By contrast with other DNA minor groove binders, Hoechst 33258 inhibited topoisomerase-mediated activity in intact cells. To determine whether specific structural alterations could modify the topoisomerase reactivity of this drug, a series of analogs of Hoechst 33258 (compound 1) was examined. When the relative DNA binding affinities (Ka) of these agents were determined, compound 1 had the highest Ka while agents with substitutions in either of the benzimidazole moieties showed reduced affinity. Whether these changes in DNA binding correlated with topoisomerase inhibitory potency was next examined. In isolated nuclei, 25 microM of agents 1, 5 and 7 reduced VM-26 induced cross-links by 64, 65 and 83%, compared with 15 to 25% reductions by agents 2, 3, 4 and 6, respectively. The structural modification common to the less active compounds was the substitution of an oxygen for nitrogen at either position 1 or 2. On the basis of these results, agents 1, 2, 3 and 7, representing a range of inhibitory potency, were chosen for further analyses. Cross-link induction by m-AMSA and camptothecin in isolated nuclei, as well as by VM-26 in intact cells, was inhibited to a greater extent by agents 1 and 7 than 2 or 3. Additionally, all four drugs inhibited relaxation of pBR 322 DNA induced by both topoisomerases, although topoisomerase I was 2 to 5-fold more sensitive than topoisomerase II. A linear correlation was observed between the logarithms of the Ka value of compounds 1, 2 and 3 and their IC25 values for both topoisomerases, suggesting a strong dependence on DNA binding affinity for enzyme inhibition. Nevertheless, agent 7, despite having less affinity for calf thymus DNA than 1, was the most potent topoisomerase inhibitor tested in intact cells and in isolated enzyme systems. Thus, retention of nitrogen at positions 1 and 2 as well as the addition of nitrogen at position 16 was associated with increased topoisomerase inhibitory potency.

Structure and dynamics of ligand-template interactions of topoisomerase inhibitory analogs of Hoechst 33258: high field 1H-NMR and restrained molecular mechanics studies

The binding characteristics of Hoechst 33258 (1), a synthetic bis-benzimidazole, and its structural analog 2, with one of the benzimidazoles replaced by a pyridoimidazole, to the self-complementary decadeoxyribonucleotide sequences d(CGCAATTGCG)2 (A) and d-(CATGGCCATG)2 (B) respectively, were examined using high field 1H-NMR techniques. Selective complexation induced chemical shift changes, the presence of exchange signals and intermolecular NOE contacts between the ligands and the minor groove protons of the oligonucleotides suggest the preferred binding sites as the centrally located AATT segment for complex A1, and the CCAT segment for complex B2. The B-type conformations of the two DNA duplexes are preserved upon complexation, as confirmed by the 2D-NOESY based sequential connectivities involving DNA base and sugar protons. Close intermolecular NOE based contacts between the ligands and their respective DNA sequences were further refined to model the ligand-DNA complexes starting from the computer generated B-type structures for the oligonucleotides. Force field calculations of ligand-DNA interaction energies indicate a more favorable contribution from the van der Waals energy component in the case of complex A1 consistent with its stronger net binding compared with the complex B2. Overall, the incorporation of a pyridinic nitrogen in Hoechst 33258 structure alters its selectivity for base pair recognition from A.T to G.C, resulting largely from the formation of a hydrogen bond between the new basic center and the 2-NH2 group of a guanosine moiety. The rates for the exchange of ligands between the two equivalent binding sites (AATT for 1, and CCAT for 2) of the self-complementary DNA sequences, are estimated from analyses of coalescence of NMR signals to be 189s-1 at 301 K for A1 and 79s-1 at 297 K for B2; which correspond to delta G++ of 13.8 and 18.6 kcal.mol-1 respectively.

Biological activity and molecular interaction of a netropsin-acridine hybrid ligand with chromatin and topoisomerase II

A hybrid molecule, which combines an anilinoacridine chromophore related to the antitumour drug amsacrine (m-AMSA) and a bispyrrole moiety analogous to the antiviral agent netropsin, has been examined for its ability to bind chromatin and to modulate the activity of topoisomerase II. The results show that the presence of histones does not alter the bimodal DNA binding process. Intercalation of the acridine and groove binding of the netropsin part of the drug are both observed with chromatin preparations. Moreover, the hybrid has a clear topoisomerase II-DNA cleavable complex-inducing activity close to that of m-AMSA. The role of the two parts of the hybrid ligand is discussed in relation to ternary complex formation. Two cell lines (L1210 leukemia and MCF7 mammary carcinoma) were compared in their sensitivity to the tested ligand. The drug, which appears to be an efficient growth inhibitor of leukemic cells in vitro, reveals moderate activity against P388 leukemia in vivo. The biological activity of the hybrid may derive from a mechanism that involves DNA binding and topoisomerase II inhibition. This study demonstrates that agents which intercalate and bind to the minor groove of DNA simultaneously represent a new class of drugs interfering with topoisomerase II and provide opportunities for the development of new antitumour agents.

Netropsin and bis-netropsin analogs as inhibitors of the catalytic activity of mammalian DNA topoisomerase II and topoisomerase cleavable complexes

This study examined the ability of netropsin and related minor groove binders to interfere with the actions of DNA topoisomerases II and I. We evaluated a series of netropsin dimers linked with flexible aliphatic chains of different lengths. These agents are potentially able to occupy longer stretches of DNA than the parental drug as a result of bidentate binding. Both netropsin and its dimers were found: (i) to inhibit the catalytic activity of isolated topoisomerase II and (ii) to interfere with the stabilization of the cleavable complexes of topoisomerase II and I in nuclei. Dimers with linkers consisting of 0-4 and 6-9 methylene groups (n) were far more inhibitory than netropsin against isolated enzyme and in the nuclear system. The compound with n = 5 was less active than netropsin in both assays while the dimer with n = 10 inhibited only the isolated enzyme. The comparison of dimers with fixed linker length (n = 2) but varying number of N-methylpyrrole residues (from 1 to 3) revealed that the inhibitory properties were enhanced with increasing number of N-methylpyrrole units. For dimers with varying linker length, drug ability to inhibit catalytic activity of isolated topoisomerase II was positively correlated with calf thymus DNA association constants. In contrast, no such correlation existed in nuclei. However, the inhibitory effects in the nuclear system were correlated with the association constants for poly(dAdT). The results indicate that bidentate binding can significantly enhance anti-topoisomerase activity of netropsin related dimeric minor groove binders. However, other factors such as the length of the linker, the number of pyrrole moieties and the nature of the target (isolated enzyme/DNA versus chromatin in nuclei) also contribute to these activities.

Significance and measurement of DNA double strand breaks in mammalian cells

Techniques for measuring DNA double strand breaks in mammalian cells are being used increasingly by researchers studying both physiological processes, such as recombination, replication, and apoptosis, as well as pathological processes, such as clastogenesis induced by ionizing radiation, chemotherapeutic drugs, and chemical toxicants. In this review we evaluate commonly used assays for measuring DNA double strand breaks, focusing on neutral filter elution and pulsed field gel electrophoresis, and explore the advantages and limitations of applying these techniques to problems of current interest in carcinogenesis and genetic toxicology.

Viscometry of alkaline cell lysates--the hitherto simplest short-term test for chromatin-interactive agents? Investigations in rat thymic and splenic cells

Alkaline (AL) lysates from thymic cells (T-cells) and splenic cells (S-cells) of the rat were measured by low-shearing glass capillary viscometry. AL-viscometry was compared to the nucleoid sedimentation technique and the alkaline unwinding method. The results obtained in cells treated by (a) DNA strand breaking agents (X-rays, UV-light, doxorubicin, bleomycin, hydrogenperoxide, methylmethanesulfonate (MMS)), (b) intercalating and/or cross-linking substances (ethidium bromide, actinomycin D, mitomycin C, bisbenzimide), (c) the DNA repair inhibitor 3-aminobenzamide (3-AB) and (d) hyperthermia suggest that AL-viscometry may be considered as a very simple, rapid and inexpensive preliminary short-term test for detecting chromatin-interactive agents. Whereas agent- and cell-specific characteristics can be also revealed by AL-viscometry, quantification of specific lesions and conclusions as to the mechanisms of action require additional assays.

Cell cycle synchronization: reversible induction of G2 synchrony in cultured rodent and human diploid fibroblasts

In accord with a set of prespecified principles of cell synchrony induction, a three-step procedure was developed to arrest cells reversibly in the G2 phase of the cell cycle. Cultures of Chinese hamster ovary (CHO) cells were presynchronized in early S phase by sequential treatment with isoleucine deficiency and hydroxyurea blockades; then they were switched to medium supplemented with either of two agents that inhibit DNA topoisomerase II activity by different mechanisms, Hoechst 33342 at 7.5 micrograms/ml for 12 hr or VM-26 at 0.5 micrograms/ml for 8 hr. Up to 95% of the cells accumulated in G2 phase under those conditions. After switch of Hoechst 33342-treated cells to drug-free medium, the cells divided as a highly synchronized cohort of cells within 3 hr. Up to 85% of the cells in a culture of human diploid dermal fibroblasts (HSF-55 cells) could be accumulated in G2 phase by placing cells presynchronized in early-S phase in medium containing Hoechst 33342 at 0.1 micrograms/ml for 10 hr. Reversal of G2 arrest in the HSF-55 cultures resulted in cells dividing synchronously over 3.5 hr. By varying the concentration of Hoechst 33342 and the duration of the treatment period, it was possible to alter the position within G2 phase at which cells accumulated. This synchronization protocol should greatly facilitate study of G2/M biochemical events in mammalian cells, in particular, those associated with cdc2 gene regulation of the onset of mitosis.

Effects of minor groove binding drugs on camptothecin-induced DNA lesions in L1210 nuclei

Topoisomerase I inhibition detected in mammalian cells can be correlated with reduced tumor growth. Camptothecin specifically inhibits topoisomerase I by stabilization of a covalently linked DNA-enzyme complex and associated DNA single-strand breaks. Whether perturbations in nuclear DNA structure can alter camptothecin-induced DNA damage was examined using the non-intercalative DNA minor groove binders distamycin, Hoechst 33258 and DAPI (4',6-diamidino-2-phenylindole). L1210 nuclei were treated with camptothecin alone or in the presence of single minor groove binders. DNA-protein crosslinks and single-strand breaks were determined using potassium-sodium dodecyl sulfate precipitation and alkaline elution respectively. Distamycin produced a dose-dependent decrease in DNA-protein crosslinks and strand breaks. This effect was reduced if nuclei were treated with camptothecin prior to distamycin addition. Distamycin was unable to reverse lesions once induced or to prevent repair of damage upon camptothecin removal. Hoechst 33258 and DAPI also decreased camptothecin-induced DNA damage. The order of inhibitory potency was: distamycin greater than Hoechst greater than DAPI. This order corresponded to the molecular weights as well as to the size of the nucleotide binding sites of the drugs. Identifying agents which alter such DNA lesions should provide better understanding of the chemotherapeutic activity of camptothecin as well as help elucidate new leads for drug combinations of improved therapeutic benefit.