The impact of spermine competition on the efficacy of DNA-binding Fe(II), Co(II), and Cu(II) complexes of dimeric chromomycin A(3)

Synergistic binding of actinomycin D and echinomycin to DNA mismatch sites and their combined anti-tumour effects

Combination cancer chemotherapy is one of the most useful treatment methods to achieve a synergistic effect and reduce the toxicity of dosing with a single drug. Here, we use a combination of two well-established anticancer DNA intercalators, actinomycin D (ActD) and echinomycin (Echi), to screen their binding capabilities with DNA duplexes containing different mismatches embedded within Watson-Crick base-pairs. We have found that combining ActD and Echi preferentially stabilised thymine-related T:T mismatches. The enhanced stability of the DNA duplex-drug complexes is mainly due to the cooperative binding of the two drugs to the mismatch duplex, with many stacking interactions between the two different drug molecules. Since the repair of thymine-related mismatches is less efficient in mismatch repair (MMR)-deficient cancer cells, we have also demonstrated that the combination of ActD and Echi exhibits enhanced synergistic effects against MMR-deficient HCT116 cells and synergy is maintained in a MMR-related MLH1 gene knockdown in SW620 cells. We further accessed the clinical potential of the two-drug combination approach with a xenograft mouse model of a colorectal MMR-deficient cancer, which has resulted in a significant synergistic anti-tumour effect. The current study provides a novel approach for the development of combination chemotherapy for the treatment of cancers related to DNA-mismatches.

Association of antitumor antibiotic Mithramycin with Mn2+ and the potential cellular targets of Mithramycin after association with Mn2+

Mithramycin (MTR), an aureolic acid group of antitumor antibiotic is used for the treatment of several types of tumors. We have reported here the association of MTR with an essential micronutrient, manganese (Mn(2+)). Spectroscopic methods have been used to characterize and understand the kinetics and mechanism of complex formation between them. MTR forms a single type of complex with Mn(2+) in the mole ratio of 2:1 [MTR: Mn(2+)] via a two step kinetic process. Circular dichroism (CD) spectroscopic study indicates that the complex [(MTR)2 Mn(2+)] has a right-handed twist conformation similar in structure with the complexes reported for Mg(2+) and Zn(2+). This conformation allows binding via minor groove of DNA with (G, C) base preference during the interaction with double-stranded B-DNA. Using absorbance, fluorescence, and CD spectroscopy we have shown that [(MTR)2 Mn(2+)] complex binds to double-stranded DNA with an apparent dissociation constant of 32 μM and binding site size of 0.2 (drug/nucleotide). It binds to chicken liver chromatin with apparent dissociation constant value 298 μM. Presence of histone proteins in chromatin inhibits the accessibility of the complex for chromosomal DNA. We have also shown that MTR binds to Mn(2+) containing metalloenzyme manganese superoxide dismutase from Escherichia coli.

The structural basis of actinomycin D-binding induces nucleotide flipping out, a sharp bend and a left-handed twist in CGG triplet repeats

The potent anticancer drug actinomycin D (ActD) functions by intercalating into DNA at GpC sites, thereby interrupting essential biological processes including replication and transcription. Certain neurological diseases are correlated with the expansion of (CGG)_n trinucleotide sequences, which contain many contiguous GpC sites separated by a single G:G mispair. To characterize the binding of ActD to CGG triplet repeat sequences, the structural basis for the strong binding of ActD to neighbouring GpC sites flanking a G:G mismatch has been determined based on the crystal structure of ActD bound to ATGCGGCAT, which contains a CGG triplet sequence. The binding of ActD molecules to GCGGC causes many unexpected conformational changes including nucleotide flipping out, a sharp bend and a left-handed twist in the DNA helix via a two site-binding model. Heat denaturation, circular dichroism and surface plasmon resonance analyses showed that adjacent GpC sequences flanking a G:G mismatch are preferred ActD-binding sites. In addition, ActD was shown to bind the hairpin conformation of (CGG)₁₆ in a pairwise combination and with greater stability than that of other DNA intercalators. Our results provide evidence of a possible biological consequence of ActD binding to CGG triplet repeat sequences.

The binding of the Co(II) complex of dimeric chromomycin A3 to GC sites with flanking G:G mismatches

Some neurological diseases are correlated with expansion of (CXG)n trinucleotide repeats, which contain many contiguous GpC flanked by mismatched X/X base pair. This study focused on the binding of the Co(II) complex of dimeric chromomycin A3(Chro), Co(II)(Chro)2, to DNA with CXG trinucleotide repeats. The present study showed that GC sites with flanking G:G mismatches provide an excellent binding site for Co(II)(Chro)2 as shown by surface plasmon resonance and fluorescence analysis, compared to GC sites with flanking A:A, T:T, or C:C mismatches. In addition, we measured the ability of Co(II)(Chro)2 to act on the hairpin DNA of (CGG)16. We observed that Co(II)(Chro)2 could stabilize and trap the cruciform conformation of (CGG)16. Furthermore, two Co(II)(Chro)2 molecules may bind at the two GpC sites separated by at least one GC site in the hairpin structure of (CGG)16. In a synthetic self-priming DNA model, 5'-(CGG)16(CCG)6-3', Co(II)(Chro)2 can interfere with the expansion process of CGG triplet repeats, as shown by a gel electrophoretic expansion assay. Here, we first report the acting of Co(II)(Chro)2, the groove-binding drug, to trinucleotide repeats. Our results provide the possible biological consequence of Co(II)(Chro)2 bound to CGG triplet repeat sequences.

The crucial role of divalent metal ions in the DNA-acting efficacy and inhibition of the transcription of dimeric chromomycin A3

Chromomycin A3 (Chro) is capable of forming a stable dimeric complex via chelation with Ni(II), Fe(II) and Co(II). According to the circular dichroism study, the dimer conformations are significantly different among the Fe(II)-, Co(II)-, and Ni(II)-containing dimeric Chro complexes; however, the dimer conformations were preserved at high temperatures. Furthermore, we conducted a systematic study to determine the effects of these divalent metal ions on the DNA-acting efficacy of dimeric Chro, including its DNA-binding affinity, DNA stabilization capacity, DNA cleavage activity, and the inhibition of transcription both in vitro and within cells. Kinetic analyses using surface plasmon resonance (SPR) showed that Ni^II(Chro)₂ exhibited the highest K_a with a value of 1.26×10⁷ M⁻¹, which is approximately 1.6- and 3.7-fold higher than the K_a values obtained for Co^II(Chro)₂ and Fe^II(Chro)₂, respectively. The T_m and ΔG values for the DNA duplex increased after the addition of drug complexes in the following order: Ni^II(Chro)₂>Co^II(Chro)₂>Fe^II(Chro)₂. In the DNA integrity assays, the DNA cleavage rate of Co^II(Chro)₂ (1.2×10⁻³ s⁻¹) is higher than those of Fe^II(Chro)₂ and Ni^II(Chro)₂, which were calculated to be 1×10⁻⁴ and 3.1×10⁻⁴ s⁻¹, respectively. Consistent with the SPR and UV melting results, Ni^II(Chro)₂ possesses the highest inhibitory effect on in vitro transcription and c-myc transcription within cells compared to Co^II(Chro)₂ and Fe^II(Chro)₂. By comparing the cytotoxicity among Co^II(Chro)₂, Fe^II(Chro)₂, and Ni^II(Chro)₂ to several cancer cell lines, our studies concluded that Ni^II(Chro)₂ displayed more potential antitumor activities than Co^II(Chro)₂ and Fe^II(Chro)₂ did due to its higher DNA-acting efficacy. Changes to the divalent metal ions in the dimeric Chro complexes have been correlated with improved anticancer profiles. The availability of new metal derivatives of Chro may introduce new possibilities for exploiting the unique properties of this class of compounds for therapeutic applications.

Conformational changes in DNA upon ligand binding monitored by circular dichroism

Circular dichroism (CD) spectroscopy is an optical technique that measures the difference in the absorption of left and right circularly polarized light. This technique has been widely employed in the studies of nucleic acids structures and the use of it to monitor conformational polymorphism of DNA has grown tremendously in the past few decades. DNA may undergo conformational changes to B-form, A-form, Z-form, quadruplexes, triplexes and other structures as a result of the binding process to different compounds. Here we review the recent CD spectroscopic studies of the induction of DNA conformational changes by different ligands, which includes metal derivative complex of aureolic family drugs, actinomycin D, neomycin, cisplatin, and polyamine. It is clear that CD spectroscopy is extremely sensitive and relatively inexpensive, as compared with other techniques. These studies show that CD spectroscopy is a powerful technique to monitor DNA conformational changes resulting from drug binding and also shows its potential to be a drug-screening platform in the future.

Association of aureolic acid antibiotic, chromomycin A3 with Cu2+ and its negative effect upon DNA binding property of the antibiotic

Here we have examined the association of an aureolic acid antibiotic, chromomycin A3 (CHR), with Cu(2+). CHR forms a high affinity 2:1 (CHR:Cu(2+)) complex with dissociation constant of 0.08 × 10(-10) M(2) at 25°C, pH 8.0. The affinity of CHR for Cu(2+) is higher than those for Mg(2+) and Zn(2+) reported earlier from our laboratory. CHR binds preferentially to Cu(2+) in presence of equimolar amount of Zn(2+). Complex formation between CHR and Cu(2+) is an entropy driven endothermic process. Difference between calorimetric and van't Hoff enthalpies indicate the presence of multiple equilibria, supported from biphasic nature of the kinetics of association. Circular dichroism spectroscopy show that [(CHR)(2):Cu(2+)] complex assumes a structure different from either of the Mg(2+) and Zn(2+) complex reported earlier. Both [(CHR)(2):Mg(2+)] and [(CHR)(2):Zn(2+)] complexes are known to bind DNA. In contrast, [(CHR)(2):Cu(2+)] complex does not interact with double helical DNA, verified by means of Isothermal Titration Calorimetry of its association with calf thymus DNA and the double stranded decamer (5'-CCGGCGCCGG-3'). In order to interact with double helical DNA, the (antibiotic)(2) : metal (Mg(2+) and Zn(2+)) complexes require a isohelical conformation. Nuclear Magnetic Resonance spectroscopy shows that the Cu(2+) complex adopts a distorted octahedral structure, which cannot assume the required conformation to bind to the DNA. This report demonstrates the negative effect of a bivalent metal upon the DNA binding property of CHR, which otherwise binds to DNA in presence of metals like Mg(2+) and Zn(2+). The results also indicate that CHR has a potential for chelation therapy in Cu(2+) accumulation diseases. However cytotoxicity of the antibiotic might restrict the use.