Cellular responses of BRCA1-defective and triple-negative breast cancer cells and in vitro BRCA1 interactions induced by metallo-intercalator ruthenium(II) complexes containing chloro-substituted phenylazopyridine
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Triple-negative breast cancer (TNBC) is defined by the absence of expression of estrogen receptor, progesterone receptor and human epidermal growth factor receptor 2. Breast cancers with a BRCA1 mutation are also frequently triple-negative. Currently, there is a lack of effective therapies and known specific molecular targets for this aggressive breast cancer subtype. To address this concern, we have explored the cellular responses of BRCA1-defective and triple-negative breast cancer cells, and in vitro BRCA1 interactions induced by the ruthenium(II) complexes containing the bidentate ligand, 5-chloro-2-(phenylazo)pyridine.
Triple-negative MDA-MB-231, BRCA1-defective HCC1937 and BRCA1-competent MCF-7 breast cancer cell lines were treated with ruthenium(II) complexes. The cytoxoxicity of ruthenium-induced breast cancer cells was evaluated by a real time cellular analyzer (RTCA). Cellular uptake of ruthenium complexes was determined by ICP-MS. Cell cycle progression and apoptosis were assessed using propidium iodide and Annexin V flow cytometry. The N-terminal BRCA1 RING protein was used for conformational and functional studies using circular dichroism and in vitro ubiquitination.
HCC1937 cells were significantly more sensitive to the ruthenium complexes than the MDA-MB-231 and MCF-7 cells. Treatment demonstrated a higher degree of cytotoxicity than cisplatin against all three cell lines. Most ruthenium atoms were retained in the nuclear compartment, particularly in HCC1937 cells, after 24 h of incubation, and produced a significant block at the G2/M phase. An increased induction of apoptotic cells as well as an upregulation of p53 mRNA was observed in all tested breast cancer cells. It was of interest that BRCA1 mRNA and replication of BRCA1-defective cells were downregulated. Changes in the conformation and binding constants of ruthenium-BRCA1 adducts were observed, causing inactivation of the RING heterodimer BRCA1/BARD1-mediated E3 ubiquitin ligase activity.
This study has revealed the ability of ruthenium complexes to inhibit cell proliferation, induce cell cycle progression and apoptosis. Ruthenium treatment upregulated the marker genes involved in apoptosis and cell cycle progression while it downregulated BRCA1 mRNA and replication of HCC1937 cells. Our results could provide an alternative approach to finding effective therapeutic ruthenium-based agents with promising anticancer activity, and demonstrated that the BRCA1 RING domain protein was a promising therapeutic target for breast cancers.
KeywordsRuthenium BRCA1 Triple-negative Cell cycle Apoptosis BRCA1 Ubiquitination
BRCA1-associated RING domain 1
Basal-like breast cancer
Breast cancer susceptibility gene 1
Bovine serum albumin
DNA double strand breaks
Dulbecco’s modified Eagle’s medium with 10% fetal bovine serum
Epidermal growth factor receptor
Estrogen responsive DNA element
Fetal bovine serum
Human epidermal growth factor receptor 2
Homologous recombination repair
Heat shock protein
Inductively coupled plasma-mass spectrometer
Mammalian target of rapamycin
Poly (ADP-ribose) polymerase
Pathologic complete response
Quantitative polymerase chain reaction
- RPMI 1640
Roswell Park Memorial Insititute 1640 medium
Real-time cellular analyzer
Reverse transcription polymerase chain reaction
Sodium dodecyl sulphate-polyacrylamide gel electrophoresis
Triple-negative breast cancer.
Triple-negative breast cancers (TNBCs) are defined by the lack of expression of an estrogen receptor (ER), a progesterone receptor (PR), and human epidermal growth factor receptor 2 (HER2). They represent approximately 15% of all breast cancers and account for a higher percentage of breast cancer related mortality detected in African and African-American women . TBNCs share genetic and morphological abnormalities with basal-like breast cancer (BLBC), a subgroup of breast cancers defined by gene expression profiling . Breast cancers with a BRCA1 mutation are also frequently triple-negative and basal-like. TNBC has important clinical implications including a typical high grading, and a high rate of proliferation. However, TNBC has a less favorable clinical outcome in terms of the nature and progression, compared with other subtypes of breast cancer. TNBC responds to conventional chemotherapy but relapses more frequently than hormone receptor-positive types, and exhibits poorer outcomes or prognosis . Poly (ADP-ribose) polymerase (PARP), EGFR, and mTOR inhibitors are among the therapeutic agents being studied in patients with TNBC and BRCA1-associated breast cancers . However, the most recent outcomes have not been fruitful. In the adjuvant setting, anthracyclines and taxanes are the drugs of choice for TNBC patients . Neoadjuvant regimens, including platinum drugs in combination with taxane, can achieve high pathologically complete response (pCR) rates in TNBC . Currently, an approach to the use of platinum agents, cisplatin and carboplatin to treat TNBC are being assessed in clinical trials, based on the dysfunction of BRCA1 and its pathways associated with a specific DNA-repair defect . TNBC patients with decreased BRCA1 expression can be sensitized to cisplatin . In other word, cisplatin treatment has improved outcomes for some TNBC patients .
Although it is widely used in cancer chemotherapy, application of cisplatin is somewhat limited because of its severe toxicity and also because of the development of drug resistances . Moreover, cisplatin-induced secondary mutations in the tumors of BRCA1 mutation carriers have been shown to confer resistance to such a platinum-based drug . These limitations have consequently motivated extensive investigations into alternative metal-based cancer therapies. Ruthenium compounds seem to be the most promising alternatives to platinum complexes for new-generation therapies . Ruthenium compounds offer potential benefits to the antitumor platinum(II) complexes such as reduced toxicity, no cross-resistance and they have a different spectrum of activity. The low toxicity of ruthenium drugs is attributable to similar ligand exchange kinetics to those of the platinum(II) complexes, and different oxidation states under physiological conditions. In addition, ruthenium is capable of mimicking iron in binding to carrier proteins such as transferrin, that has been postulated to be a specific metal (mainly iron) delivery mechanism to cancer cells that require higher iron requirement. As a result, ruthenium compounds could be well suited for cancer treatment [12, 13, 14, 15, 16].
The cell lines, including human breast adenocarcinoma cells [MCF-7 (BRCA1 wild-type, ER positive); MDA-MB-231 (BRCA1 wild-type, TNBC); and HCC1937 [BRCA1 mutant (5382insC), TNBC], were purchased from the American Type Culture Collections (ATCC, Rockville, MD, USA). HCC1937 cells were grown in Roswell Park Memorial Insititute 1640 medium (RPMI 1640) (Life Technologies, Paisley, UK) without phenol red. MCF-7 and MDA-MB-231 cells were grown in Dulbecco’s modified Eagle’s medium (DMEM) (Life Technologies, Paisley, UK) without phenol red. All media were supplemented with 10% fetal bovine serum (FBS) and 1% penicillin-streptomycin. All cell lines were cultured at a constant temperature of 37°C in a 5% carbon dioxide (CO2) humidified atmosphere.
Real-time cell growth profiling
The real time growth kinetics of MCF-7, MDA-MB-231 and HCC1937 cells towards the metal-complex treatments were examined using the Real-Time Cellular Analyzer (RTCA) (xCELLigence System, Roche Applied Science, Mannheim, Germany). RTCA utilizes an E-plate that contains interdigitated micro-electrodes integrated on its bottom. The cell number, viability, morphology and degree of adherence of cells in contact with the electrodes will affect the local ionic environment leading to an increase in the electrode impedance. This is represented as the Cell Index (CI) and reflects a calculation (via an internal system algorithm) of the frequency-dependent electrode impedance with or without attached healthy cells present on the surface of the wells. For each experiment, 100 μL of medium was added into 96-well E-plates and the background readings were recorded. A cell suspension (100 μL) at a cell density of 5 × 104 cells/well was added to each well of the E-plate. The attachment, spreading and proliferation of the cells was monitored every 15 min over the following 7 h for the MCF-7 cells and HCC1937 cells and for 18 h with the MDA-MB-231 cells (this allows for cell attachment, spreading and cells entering their logarithmic growth phase). When the cells entered the logarithimic phase, the plate was removed from the RTCA machines. The cells were washed once with PBS to remove any cell debris, and either fresh medium containing various concentrations of the metal complexes or fresh medium (control) was added to each well. The plate was reinserted into the RTCA machine and the proliferation of the cells were further assessed every 15 min for the next 24 h. After a 24 h incubation time, the medium containing the metal complexes was removed, the wells were again washed once with PBS and fresh medium was added to all wells. The plate was then reinserted into the RTCA machine for a further 24 h to assess the degree of cellular recovery in the absence of the metal complexes. Experiments were performed in triplicate.
Cellular uptake and distribution
IC 50 values (μM) for 1, 2, and cisplatin on MCF-7, MDA-MB-231 and HCC1937 cells after 24 h of treatment (data reflect the mean and ± SD of results from three separate experiments, each performed in triplicate)
42.2 ± 8 *,**
128.2 ± 7 *,**
23.4 ± 7 *,**
10.7 ± 0.6 *,**
14.1 ± 0.5 *,**
9.9 ± 0.2 *,**
8.2 ± 0.1 *,**
13.2 ± 0.3 *,**
1.8 ± 0.1 *,**
Cell cycle analysis
About 106 cells were seeded into 6-well culture plates. Cells were incubated in the absence and the presence of the IC50 concentration of 1 and 2 for 24 h. Following incubation the cells were trypsinized, washed twice with 0.5 mL of PBS and centrifuged at 300 g for 5 min and then 106cells were collected and fixed in cold 70% ethanol at -20°C overnight. The fixed cells were washed twice with PBS. The cell pellets were resuspended in 1 mL of PBS (100 μg/mL of RNase A, 50 μg/mL of PI, and 0.1% of Triton-X 100), and then further incubated at 37°C in the dark for 30 min. The fluorescence of 20000 cells was measured using a FACSCanto flow cytometer. The cell cycle distribution was analyzed with MultiCycle software. The proportions of cells in the sub-G1, G0/G1, S, and G2/M phases were represented as DNA histograms.
Annexin V apoptosis detection assay
About 106 cells were seeded into 6-well culture plates. Cells were incubated in the absence and the presence of the IC50 concentrations of 1 and 2 for 24 h. Following incubation the cells were trypsinized, washed twice with 0.5 mL of PBS and centrifuged at 300 g for 5 min. The pellet was resuspended in 100 mL of 1× Annexin-binding buffer. Alexa Fluor 488 Annexin V, 5 μL, and 1 μL of PI (100 μg/mL) were added to each cell suspension which were then further incubated at room temperature for 15 min. Then, 400 μL of 1× Annexin-binding buffer was added and mixed gently. Annexin V binding was analyzed on a FACSCanto flow cytometer equipped with a fluorescence emission at 530 and 575 nm using a fluorescence excitation at 488 nm.
Cellular BRCA1 damage using QPCR
About 106 cells were incubated with various concentrations of 1 or 2 at 37°C for 48 h in 5% CO2. Genomic DNA of the ruthenium-treated or untreated (control) cells was isolated, and the 3426-bp fragment of the BRCA1 exon 11 of the cells was then amplified by PCR, electrophoresed on 1% agarose gel, stained with ethidium bromide and then visualized under UV light . The quantitative PCR (QPCR) method was used to assess the polymerase inhibiting effect of DNA ruthenation. The amplification products were quantified using a Bio-Rad Molecular Imager, and the amount of DNA amplification (%) was plotted as a function of the concentration .
Real-time quantitative RT-PCR
The breast cancer cells were plated and cultured in complete medium and allowed to grow for 48 h followed by the addition of the IC50 concentrations of 1 and 2. The cells were further incubated at 37°C. The cells were harvested and the total RNA was extracted from cultured cells using the RNeasy® Mini Kit (Qiagen, Germany). cDNA was obtained by reverse transcription of total RNA using QuantiTech® Reverse Transcription (Qiagen, Germany). The primer sequences were as follows:
BRCA1: 5/-GCCAGTTGGTTGATTTCCACC-3/ (forward) and 5/-GTCAAATGTGCTCCCCAAAAGC-3/ (reverse)
p53: 5/-GGTCTCCCCAAGGCGCACTGG-3/ (forward) and 5/-AGGCTGGGGGCACAGCAGGCC-3/ (reverse)
p21: 5/-GACACCACTGGAGGGTGACT-3/ (forward) and 5/-CAGGTCCACATGGTCTTCCT-3/ (reverse)
β-Actin: 5/-GGACTTCGAGCAAGAGATGG-3/ (forward) and 5/-AGCACTGTGTTGGCGTACAG-3/ (reverse).
Real-time PCR reactions were then carried out in a total volume of 25 μL including 100 ng of the cDNA template, 12.5 μL of QuantiFast SYBR green PCR master mix, and the final concentration of primers of 0.5 μM. The PCR conditions were as follows: 5 min at 95°C, and 35 cycles of 10 sec at 95°C, 30 sec at 60°C. Fluorescence was measured during the annealing step on an ABI-Prism 7300 analytical thermal cycler (Applied Biosystems). Data were analyzed according to the 2-∆∆CT method , and normalized by β-Actin mRNA expression in each sample. Experiments were performed in triplicate.
Plasmid constructions, expression and purification
The N-terminal BRCA1 RING domain protein containing the 304 amino acid residues was prepared by PCR-mediated cloning as previously described . The purified protein was identified on 12% Coomassie blue-stained SDS-PAGE and subsequently confirmed by sequencing the tryptic digested peptides.
The N-terminal BRCA1 (1–304) proteins (10 μM) were prepared in deionized water, according to the Bradford assay using BSA as standard. ZnCl2, ruthenium polypyridyl complexes (1 and 2) were prepared as 1 mM stock solutions in deionized water. BRCA1 protein with and without pre-incubation of 3 mol-equivalent ratio of Zn2+ were treated with 1 and 2 at various concentrations. Metal-dependent folding of the protein was monitored by acquiring a CD spectrum over a range of 200–260 nm using a Jasco J720 spectropolarimeter (Japan Spectroscopic Co., Ltd., Hachioji City, Japan). Measurements of ruthenium complex binding were carried out at 20°C using a 0.1 cm quartz cuvette. The spectrum was averaged from five separate spectra with a step size of 0.1 nm, a 2 s response time and a 1 nm bandwidth. Data were baseline-corrected by the subtraction of each metal complex concentration. The secondary structures of proteins were predicted using the CONTIN program . The effect of ruthenium complex binding on the protein conformation was determined in the absence and presence of a 3 mol-equivalent ratio of Zn2+ to protein. The binding constant was determined as described previously .
In vitro ubiquitination assay and western bloting
The ubiquitin ligase reactions (20 μL) contained 20 μM Ub, 300 nM E1, 5 μM UbcH5c, 3 μg BRCA1 or Ru-BRCA1 adduct, and 3 μg BARD1 in a buffer [50 mM Tris (pH 7.5), 0.5 mM DTT, 5 mM ATP, 2.5 mM MgCl2, and 5 μM ZnCl2]. Two separate reactions were incubated at 37°C for 3 h, and then terminated by adding an equal volume of SDS-loading dye before electrophoresis on 10% SDS-PAGE. The separated protein was then transfered to the PVDF membrane and immunodetected with anti-His6 HRP (Horseradish Peroxidase) conjugated (chemiluminescent method, QIAGEN) at a dilution of 1:2000 performed according to the manufacturer’s protocol. The blot was detected by chemiluminescence (SuperSignal TM, Pierce) on X-ray film. The relative E3 ligase activity of the BRCA1 adducts was quantified by normalizing the density of an apparent band of the ubiquitinated-protein conjugates to that of the parental BRCA1 as the control, using a Bio-Rad GS-700 Imaging Densitometer.
Data processing and statistical analysis
Values are shown as the standard error of the mean unless indicated otherwise. Data were analyzed and, where appropriate, the significance of the differences between the mean values was determined using one-way ANOVA. A probability of 0.01 was deemed statistically significant. The following notation was used throughout: * p < 0.01, relative to control.
Anti-proliferative effects of ruthenium(II) polypyridyl complexes with the bidentate ligand 5-chloro-2-(phenylazo)pyridine
Differential cellular accumulation
[Ru(Clazpy)2phen]Cl2.8H2O (2) produced a significant block at the G2/M phase with prominent induction of apoptosis in triple-negative MDA-MB-231 cells
Ruthenium (II) complexes slightly blocked BRCA1 amplification
mRNA expression induced by the ruthenium(II) complexes
Changes in secondary structure of the N-terminal BRCA1 RING protein induced by metallo-intercalator ruthenium(II) complexes
Presently, two main controversial aspects are whether ruthenium compounds target DNA or proteins. However, a major limitation in this framework is their unknown mode of action, and the lack of a specific biological target. Extensive investigations of the ruthenium-based compounds has mainly focused on the characterization of ruthenium-DNA adducts and has paid less attention to other potential cellular targets. For ruthenium-based drug candidates, their precise mechanisms of action as anticancer agents remain comparatively unexplored. There is evidence to indicate that ruthenium compounds might directly interfere with specific proteins involved in the signal transduction pathways, cell adhesion and migration processes [21, 22]. However, the molecular mechanism and the signaling pathways still remain unidentified. Recently, a novel approach for cancer therapy involved alterations to the double strand breaks (DSBs) repair processes by which the cancerous cells with dysfunctional DNA repair pathways to accumulate high levels of DNA damage that eventually resulted in major genomic instability and cell death [31, 32, 33, 34, 35]. The breast cancer suppressor protein 1 (BRCA1)-mediated the DNA repair pathway, one of the important DNA damage response pathways, that plays a vital role in the maintenance of the genome integrity. Several lines of evidence have demonstrated that cancerous cells with inactivated BRCA1 had a defect in the repair of DSBs [36, 37], and this conferred hypersensitivity to the cancerous cells towards platinum-based chemotherapy drugs and γ-irridation [24, 25, 38, 39]. To our knowledge, the effect of the ruthenium(II) complexes on a DNA repair protein BRCA1 has not been studied. Therefore, it was of great interest to investigate the effect of the ruthenium(II) complexes with the bidentate ligand 5-chloro-2-(phenylazo)pyridine, 1 and 2, on BRCA1 binding and its secondary structure.
The N-terminal BRCA1 protein-mediated E3 ligase is inactivated by the ruthenium-based drugs
Recently, EGFR, mTOR, and Poly (ADP-ribose) polymerase (PARP) inhibitors were among the therapeutic agents being studied in patients with TNBC and BRCA1-associated breast cancers . However, the outcomes of those inhibitors have not been successful. Therefore, the development of new targeted therapies for TNBC is clearly needed to help this patient population. It has been reported that different platinum-based regimens showed higher chemosensitivity against TNBC compared with other breast cancer subtypes, both in the neoadjuvant and metastatic settings [8, 42]. In addition, treatment with platinum-based drugs alone or in combination with other anticancer compound has improved outcomes for TNBC patients. However, the platinum drugs are limited by their severe toxicity and tendency to induce drug resistance . Furthermore, cisplatin-induced secondary mutations in the tumors of BRCA1 mutation carriers have been shown to confer resistance to such a platinum-based drug [11, 44]. For these reason, ruthenium-based drugs are being developed and have offered potential advantages over the antitumor platinum(II) complexes including reduced toxicity, a novel mechanism of action, the prospect of no cross-resistance and a different spectrum of activity . Several lines of evidence have indicated that ruthenium polypyridyl complexes inhibit the proliferation of cells by inhibiting cell proliferation, cell cycle progression and inducing apoptosis [16, 19, 45, 46].
In this study, we used triple-negative MDA-MB-231, BRCA1-mutated HCC1937 and sporadic BRCA1-competent MCF-7 cell lines as models of breast cancer cell growth and progression. Dynamic evaluation by the RTCA system showed that 1 and 2 quickly inhibited the proliferation of MCF-7, MDA-MB-231 and HCC1937 cancer cells within a few hours after treatment with the ruthenium complexes. This indicated a direct cytotoxic response towards these complexes. A continuous drop in the CI was observed at high concentrations of the complexes. Comparing the IC50 values of 1 and 2, 2 appeared to have a higher cytotoxicity against all three breast cancer cell lines than 1. Each ruthenium complex was differently absorbed by these cells and had distinct modulations on the arrest of cell cycle progression. 2 induced a significant block at the G2/M cell cycle arrest with a pronounced increase in apoptotic cells in the triple-negative MDA-MB-231 and BRCA1-defective cells. Our data may be attributed to the larger size, lipophilic characteristics of the polypyridyl ligand, and thereby enhancing their passage through the cell membrane. It has been reported that the intracellular uptake is to a major extent determined by the carrier ligand [15, 18]. The hydrophobicity of the ruthenium-based drugs could also minimize the impact of the decreased accumulation of resistance mechanisms . It has been shown that NAMI-A  inhibited the invasion and metastasis of cancer cells by arresting them at the G2/M stage and that it is a likely consequence of the accumulation of an inactive phosphorylated form of Cdk1, caused by the lack of Cdc25 phosphatase activity . In addition, RAPTA-C inhibited cell proliferation by triggering the G2/M phase cell cycle arrest and the subsequent apoptosis . It is well established that cell cycle progression is a tightly ordered and regulated process that involves multiple cellular checkpoints. These checkpoints respond to a variety of growth and transduction signals of the cells. In response to DNA damage, the checkpoints delay or stop in the cell cycle, at critical points before or during DNA replication (G1/S and intra S) and before cell division (G2/M).
A significant increase in apoptotic cells in the triple-negative MDA-MB-231 cells could result from an alternative breast cancer progression pathway defined through over-expression of the epidermal growth factor (EGF)-induced nuclear factor κB (NF-κB) that can be activated for ER- negative breast cancer cells . NF-κB controls the cell-cycle progression by modulating the action of cell-cycle regulatory proteins. In addition, this triple-negative cell could trigger multiple pathways towards apoptosis, including those involving endonuclease G, caspases, and c-Jun N-terminal kinase . MDA-MB-231 cells that lacked ER appeared to be less sensitive to tested metal-based drugs compared with its counterpart ER-harboring the MCF-7 cells. This data could be interpreted to mean that the ER plays a vital role in cell proliferation and cell viability of breast cancers. In the ER-positive breast cancer cells, ER exists in an inactive status as a complex with an inhibitory heat shock protein 90 (hsp90). Upon estrogen binding, hsp90 is released, and ER converts to an active conformation that interacts with its responsive DNA element, ERE. This ER-ERE interaction leads to the expression of hormone-responsive genes . It was also a surprise, to find that HCC1937 cells that harbored a BRCA1 mutation (5382insC) and lacked an estrogen receptor were 10–20 fold more sensitive than the MDA-MB-231 cells. This implied that the increase in the ruthenium sensitivity in BRCA1 defective breast cancer cells might be related to a dysfunctional BRCA1 that is unable to repair DNA damage produced by ruthenium treatment, and ultimately led to breast cancer cell death . Recently, the loss of BRCA1 led to an increase in the expression of the epidermal growth factor receptor (EGFR) in mammary epithelial cells, and inhibition of EGFR prevented ER-negative cancers in BRCA1-mutant mice .
In cancer cells, damage to the BRCA1 gene by the ruthenium-based drugs could lead to a loss of its functions and ultimately result in the death of the cancer cell. To address this concern, we further investigated whether these metallo-intercalator ruthenium complexes had any effect on cellular DNA damage that was affected by the BRCA1 gene. It was a surprise when both ruthenium complexes at their IC50 values caused a dramatic decrease in BRCA1 replication in HCC1937 cells while they produced only slight damage at equimolar concentrations or even at their IC50 concentration in MCF-7 and MDA-MB-231 cells. This may be in part due to the biology and cellular responses of each cancer subtype to ruthenium treatment [2, 53]. This has provided the first evidence with regard to the inhibition of DNA replication of the BRCA1 gene in BRCA1-defective HCC1937 cells that was induced by this class of the ruthenium complexes, and this closely correlated to our previous result performed with the platinum-based drugs . However, a lower dose of ruthenium complexes is required than for either cisplatin or carboplatin to achieve the 50% inhibition of DNA replication in such a cell line. A higher degree of inhibition of BRCA1 polymerization was induced by 2 in HCC1937 than in MCF-7 and MDA-MB-231 and this is most probably linked to the increased accumulation of 2 in the nuclear compartment of cancerous cells. Consequently, it can more easily damage the DNA and signal transduction pathways involved in cell cycle progression and apoptosis.
The quantitative RT-PCR data demonstrated that treatment with 1, upregulated the expression of p53 and BRCA1 mRNA in all tested cell lines while 2 slightly downregulated the expression of BRCA1 mRNA in BRCA1-mutated HCC1937 cells. An increase in p53 mRNA reflected cellular responses to DNA-damaging agents, presumably to allow the cells to perform critical repair functions prior to passing through the cell cycle . In this situation, it is likely that these cell lines use p53-dependent apoptosis pathways . In the present study, it revealed that 2 did suppress or damage the BRCA1 gene in the HCC1937 cell lines, and the inhibition of transcription by 2 was a critical determinant of cell-cycle arrest in the G2/M phase because cells could not synthesize the mRNA necessary to pass into mitosis, and this eventually led to apoptosis. Low BRCA1 mRNA expression was correlated with an increased response rate and median overall survival in cisplatin-based chemotherapy or chemoradiotherapy . This might lead to an insufficiency of BRCA1 function in cancer cells. In contrast, the up-regulation of BRCA1 mRNA induced by 1 in both MCF-7 and MDA-MB-231 cells could partly contribute to cellular resistance to 1. In addition, the over-expression of BRCA1 mRNA in MCF-7 cells has also been reported to have increased resistance to cisplatin . The levels of BRCA1 mRNA expression predicted outcomes following cisplatin-chemotherapy . Cancer patients treated with cisplatin, or those with low or intermediate levels of BRCA1 mRNA also attained a significantly better response, disease-free survival and overall survival than those with high levels [59, 60].
To further characterize whether these ruthenium complexes affected the BRCA1 protein, we performed ruthenium-BRCA1 interactions in vitro, and utilized the N-terminal BRCA1 RING domain protein (1–304 amino acid residues without mutating BRCA1 that play a vital role in E3 ubiquitin ligase activity) as a representative of all breast cancer cell lines used in this study. Our study on the Ru-BRCA1 interactions indicated that 1 and 2 both interacted with the holo-form of the BRCA1 protein and affected the overall conformation of the Zn2+-bound BRCA1. In addition, the effect of 2 was more effective than 1. This might be attributable to the chemical structure of the phenanthroline ligand of 2 that is more planar and hydrophobic than the bipyridyl ligand of 1. However, this observation is not the explanation for the anticancer platinum drug cisplatin . In that case the affinity of Zn2+ or cisplatin to the holo-form of the BRCA1 RING protein has been demonstrated to occur in a different way. The Pt binding to BRCA1 had a binding constant of 3.00 ×106 M-1, equivalent to that of the Zn2+ binding (2.79 ×106 M-1). The calculated free energy of cisplatin and Zn2+ bindings were -8.68 and -8.64 kcal mol-1, respectively. The difference of the binding constant and free energy can be explained by the compact structure of the protein molecule. In addition, changes in the structural consequences of some protein conformations in the presence of ruthenium complexes, such as the binding of the ruthenium(III) complex KP1019 to cytochrome c induced conformational changes in the protein. This conformational change was subsequently expected to play a vital role in the biological activity of cytochrome c, in particular, in its ability to induce cell apoptosis .
For BRCA1-mediated ubiquitination, a reduction of the BRCA1 E3 ligase activity by both ruthenium complexes could reflect an altered interaction between the RING heterodimer domains of BRCA1 and BARD1. A slight difference in the reactivity toward the BRCA1 RING domain protein may be due to the geometry of each ruthenium complex . This observation also agreed very well with the above mentioned ruthenium binding to the N-terminal BRCA1 RING domain protein. It is also notable that inactivation of BRCA1 E3 ligase activity induced by 1 and 2 was similar to that induced by the platinum-based drugs . BRCA1 plays a vital role in the maintenance of genomic integrity through multiple functions including DNA damage repair, transcriptional regulation, a cell cycle checkpoint and protein ubiquitination. Cancerous cells with inactivated BRCA1 had defects in the repair of DNA double strand breaks (DSBs). These cells have increased sensitivity to DNA-damaging agents that eventually result in major genomic instability and cell death. Several investigations have revealed the relevance of the BRCA1-mediated ubiquitination to DNA repair functions. Mutations in the BRCA1 RING domain resulted in a loss of the E3 ubiquitin ligase activity, and conferred hypersensitivity of the cancerous cells to chemotherapy that causes DNA damage [24, 25, 61]. Therefore, targeting the BRCA1 RING domain protein through the disruption of the BRCA1 E3 ligase activity by this class of ruthenium complexes might be an effective approach to eradicate breast cancers. However, future trials need to be considered before utilizing BRCA1 as a promising therapeutic target for breast cancer treatment.
This study revealed the ability of ruthenium complexes to inhibit cell proliferation, induce cell cycle progression and apoptosis. Ruthenium treatment upregulated the marker genes involved in apoptosis and cell cycle progression while it downregulated BRCA1 mRNA and replication of BRCA1-defective breast cancer cells. Changes in conformation and the binding constant of ruthenium-BRCA1 adducts were observed, causing inactivation of the RING heterodimer BRCA1/BARD1-mediated E3 ubiquitin ligase activity. Our results could provide an alternative approach to finding effective therapeutic ruthenium-based agents with promising anticancer activity, and have identified the BRCA1 RING domain protein as a promising therapeutic target for breast cancers.
This work was financed by grants from the National Research Council of Thailand (PHA5011990045S, PHA5111990041S, PHA550011S and PHA560065S), the Graduate School, Prince of Songkla University, Center of Excellence for Innovation in Chemistry (PERCH-CIC), Commission on Higher Education, Ministry of Education, and the Thailand Research Fund under the Royal Golden Jubilee Ph.D. Program (to P.T., 5.G.PS/49/F.2). The authors would like to thank Prof. Chartchai Krittanai, Institute of Molecular Biosciences, Mahidol University for CD. We are grateful to Dr. Brian Hodgson for assistance with the English, and the Pharmaceutical Laboratory Service Center, Faculty of Pharmaceutical Sciences, Prince of Songkla University for research facilities.
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