The androgen receptor (AR) is expressed in 60%–70% of breast cancers regardless of estrogen receptor status, and has been proposed as a therapeutic target in breast cancers that retain AR. In this study, the authors aimed to investigate a new treatment strategy using a novel AR inhibitor AZD3514 in breast cancer. AZD3514 alone had a minimal antiproliferative effect on most breast cancer cell lines irrespective of AR expression level, but it downregulated the expressions of DNA damage response (DDR) molecules, including ATM and chk2, which resulted in the accumulation of damaged DNA in some breast cancer cells. Furthermore, AZD3514 enhanced cellular sensitivity to a PARP inhibitor olaparib by blocking the DDR pathway in breast cancer cells. Furthermore, the downregulation of NKX3.1 expression in MDA-MB-468 cells by AZD3514 occurred in parallel with the suppression of ATM–chk2 axis activation, and the suppression of NKX3.1 by AZD3514 was found to result from AZD3514-induced TOPORS upregulation and a resultant increase in NKX3.1 degradation. The study shows posttranslational regulation of NKX3.1 via TOPORS upregulation by AZD3514-induced ATM inactivation–increased olaparib sensitivity in AR-positive and TOPORS-expressing breast cancer cells, and suggests the antitumor effect of AZD3514/olaparib cotreatment is caused by compromised DDR activity in breast cancer cell lines and in a xenograft model. These results provide a rationale for future clinical trials of olaparib/AR inhibitor combination treatment in breast cancer.

This article is featured in Highlights of This Issue, p. 2493

In patients with breast cancer, endocrine therapies targeting estrogen and estrogen receptor (ER) signaling pathways are considered as crucial. However, over a quarter of patients with breast cancer do not express ER and exhibit resistance to endocrine therapy. Although sex steroid hormone receptors, such as, ER and progesterone receptor, are critical for the development and progression of breast cancer, the potential role of androgen receptor (AR) in breast cancer has not been thoroughly elucidated. About 50%–80% of invasive breast cancers (regardless of ER status) express AR and recent studies indicate AR expression is positively associated with a good prognosis. In addition, recent studies showed that AR-expressing triple-negative breast cancer (TNBC) is dependent on AR signaling; thus, targeting AR seems to improve outcomes in TNBC (1–7). Despite the prevalence and clinical significance of AR expression in breast cancer, preclinical evidence supporting the use of AR-targeting agents and potential biomarkers of response to AR inhibitors in breast cancer are lacking (4). Although clinical trials using enzalutamide (NCT01889238, NCT02091960, and NCT02929576), bicalutamide (NCT00468715 and NCT02605486), darolutamide (NCT03383679), enobosarm (NCT02971761), and other AR inhibitors (seviteronel, CR1447, and others) are ongoing, because of a lack of preclinical understanding, AR antagonists are not currently used in standard clinical practice. Nonetheless, more research is required before AR modulation–based strategies are accepted clinically for the treatment of breast cancer.

Some breast cancers are associated with homologous recombination deficiency (HRD), which has been shown to be particularly sensitive to DNA-damaging agents and PARP inhibitors (8–10). Although PARP inhibitors have produced promising results in patients with breast cancer with compromised HR repair (HRR) activities (11–14), only BRCA deficiencies or BRCAness are considered viable PARP inhibitor targets. In fact, olaparib is the first PARP inhibitor with confirmed efficacy for the treatment of breast cancer in patients with a germline BRCA mutation as indicated by the results of a phase III randomized trial, the OlympiAD study (15). The data from the OlympiAD trial resulted in olaparib being approved by the FDA as a single agent for treatment of metastatic breast cancer with BRCA1/2 germline mutation. However, only 5%–10% of the patient population with breast cancer is thought to be BRCAness, and no other HRD marker has been demonstrated to predict sensitivity to PARP inhibitors (16). Therefore, a strategy for extending the usage of PARP inhibitors in breast cancer is required to meet the unmet needs of patients.

Recent studies have demonstrated that AR signaling is a key regulator of DNA damage response (DDR) in prostate cancer, in which AR is the main therapeutic target. AR modulates the transcriptions of a network of DDR genes in prostate cancer (17, 18). Polkinghorn and colleagues showed androgen depletion causes the downregulation of DDR genes and impaired DNA repair, and that AR activation promoted resistance to radiation via rapid repair of IR-induced DNA damage (17). Another group also reported a link between AR and the DNA repair circuit in response to a genotoxic insult (18). These reports suggest AR is involved in DDR activity via the regulation of DDR components. We supposed AR inhibition impedes DDR activity and increases cellular sensitivity to PARP inhibitors, which selectively target cancer cells with defective DDR. Thus, AR inhibition contributes to the cells that may create HRD phenotype, resulting in sensitive to PARP inhibitor. Supporting our hypothesis, Li and colleagues demonstrated that combination of enzalutamide and olaparib downregulated expression of DDR genes resulting in reduced HR efficiency in AR+ prostate cancer cells (19). It provides new insights into the possibility of an expanded BRCAness concept in clinical application of olaparib, but it is unclear how AR inhibition influences on DDR efficiency in breast cancer.

In this study, we evaluated the effect of AZD3514, a selective androgen receptor downregulator, as a monotherapy and in combination with olaparib in breast cancer cells, in the hope that the information gained would help extend usage of PARP and AR inhibitors in breast cancer treatment. In addition, we explored the mechanisms responsible for the effects of AZD3514/olaparib treatment, especially with respect to the link between AR inhibition and DDR activity in breast cancer cells. Finally, we attempted to identify a marker of sensitivity to AZD3514/olaparib treatment. This is the first study to demonstrate the underlying mechanisms of AR inhibition, especially on DDR capacity. In addition, we suggest that AR and PARP inhibitor combination therapy could be effective by inducing the HRD phenotype in patients with AR+ breast cancer.

Reagents

AZD3514 and olaparib were kindly provided by AstraZeneca.

Cell lines

Six human breast cancer cell lines (MDA-MB-157, -231, BT-549, HCC70, HCC1143, and Hs578T) authenticated by short tandem repeat analysis, were purchased from the ATCC. Another 6 breast cancer cells (MDA-MB-453, -468, BT-474, MCF7, T47D, and SKBr3) verified by DNA fingerprinting analysis, were purchased from the Korean Cell Line Bank (Seoul, Korea). All cell lines were banked, cultured as described previously (20), and passaged for less than 6 months before use.

Cell growth inhibition assay

The cell viabilities at the early time point (120 hours) were determined using an MTT assay as described previously (12). Cells were exposed to AZD3514 or olaparib alone or in combination at various concentrations for 5 days. Combination index (CI) was calculated using CalcuSyn software (Biosoft). Drug synergism was defined as a CI value at ED75 of <1 as determined by the Chou–Talalay method (21). The long-term efficacies of AZD3514, olaparib, or AZD3514/olaparib were assessed using a colony formation assay (CFA). Cells were treated with specific concentrations of each drug, and then cultured until colonies formed (14 days). Colonies were counted using a GELCOUN Tumor Colony Counter (Oxford Optronix Ltd.).

Western blot analysis

Total and phosphorylated (p) protein expression levels after 5 days of treatment and protein expression levels were determined by Western blotting as described previously (22). The following primary antibodies were purchased from Cell Signaling Technology; antibodies against HER2, caspase 3, ATR, PR, AKT, p-AKT, ERK, p-ERK, p-chk1, p-chk2, p-ATM, and p-ATR. Antibodies against AR, ATM, chk1, chk2, RAD51, TOPORS, ERα, cyclin B1, and cdc2 were obtained from Santa Cruz Biotechnology. Anti-ERCC1 antibody was obtained from Thermo Fisher Scientific, anti-NKX3.1 antibody from Abcam, anti-p-histone H2A.X antibody (clone JBW301) from Millipore, and anti-PARP antibody from BD Biosciences. Anti-α-tubulin and anti-actin antibodies (Sigma Aldrich) were used as controls.

Cell-cycle analysis

The DNA contents of cells (1 × 104 cells) treated with AZD3514 and/or olaparib were determined using a FACSCalibur flow cytometer (BD Biosciences) after propidium iodide staining.

siRNA transfection

siRNA specific for TOPORS and nonspecific control siRNA were purchased from Genolutions. Lipofectamine 2000 (Invitrogen) was used for the transfection according to the manufacturer's instructions. The sequence of the AR-specific siRNA was 5′-AAGAAGGCCAGUUGUAUGGAC-3′, the sequence of the TOPORS-specific siRNA was 5′- CAAGGAGCCUGUCUAGUAAUU-3′, and the sequence of the nonspecific control siRNA was 5′-AATTCTCCGAACGTGTCACG-3′.

RT-PCR and real-time PCR

Total RNA was isolated using TRI reagent (Molecular Research Center) as described previously (12). cDNA was synthesized by RT-PCR using ImProm-II reverse transcriptase (Promega). qRT-PCR was performed using an iCycler iQ Detection System (Bio-Rad Laboratories, Inc.) and SYBR Green. The mRNA expression levels of ATM were normalized relative to actin cDNA and fold changes in expression were calculated versus controls. The sequences of the ATM-specific primer were 5′-AGCACAGAAGTGCCTCCAAT-3′ (forward) and 5′-GCCAATACTGGACTGGTGCT-3′ (reverse).

Comet assays

Degree of DNA damage was assessed using an alkaline comet assay using the Trevigen Comet Assay Kit (Trevigen). Tail moments were measured using the Comet Assay IV Program (Andor Technology).

Immunofluorescence assay

RAD51 and γH2AX foci formation were evaluated following AZD3514 and/or olaparib treatment as described previously (12). Foci were visualized using a Nikon A1 confocal laser scanning microscope (Nikon). At least 100 cells were counted and the percentages of cells with more than five RAD51 or γH2AX foci were calculated.

IHC and TUNEL assay

IHC was performed using anti-rabbit antibodies against Ki-67 (GeneTex) and NKX3.1 (Abcam) at a dilution of 1:100. The terminal deoxynucleotidyl transferase–mediated dUTP nick end labeling (TUNEL) assay was conducted using the ApopTag In situ Apoptosis Detection Kit (Chemicon International) as described previously (12).

In vivo study

In vivo experimentation was conducted in the animal facility of Seoul National University (Seoul, South Korea) in accord with institutional guidelines after obtaining prior approval from the Institutional Animal Care and Use Committee (IACUC) committee (SNU-140422-13). Female Balb/c athymic nude mice 6 weeks old were tested to measure the in vivo activities of AZD3514 and/or olaparib. MDA-MB-468 cells (1 × 107) were subcutaneously transplanted into each mouse, and drugs were administered when tumor volumes reached 200 mm3. AZD3514 (50 mg/kg) and/or olaparib (30 mg/kg) were administered via oral gavage once daily for 28 consecutive days. Tumor volumes was calculated using [(width)2 × (height)]/2, and mice were sacrificed with CO2 at the end of the observation period. Tumors were excised and preserved for further analysis as described previously (20).

Statistical analysis

All experiments were repeated at least three times. Results are presented as mean ± SEs. The analysis was conducted using SigmaPlot version 9.0, and the two-sided Student t test was used for group comparisons. Statistical significance was accepted for P < 0.01.

AZD3514 suppressed DDR activation in breast cancer cells

Because AR is a potential therapeutic target in breast cancer, we observed AR protein expression levels in 12 breast cancer cell lines (Supplementary Fig. S1). All cell lines including hormone receptor–positive, HER2-positive and TNBC cell lines expressed AR. Although AZD3514 at concentrations of <10 μmol/L reduced cell survival by 50% in some breast cancer cell lines, it had mild antiproliferative effects on most of the breast cancer cell lines, irrespective of AR expression level (Supplementary Fig. S2). Antiproliferative effect through AR inhibition by AZD3514 was not prominent in breast cancer cells as monotherapy. However, AZD3514 downregulated the expression of DDR molecules (p-ATM, p-chk1, and p-chk2) in a number of breast cancer cell lines. ATM mRNA expression levels were significantly suppressed in MDA-MB-468 and MCF7 cells by AZD3514, but no change was observed in MDA-MB-453 or HCC1143 cells (Fig. 1A). Concordantly, activation of the ATM–chk2 axis was significantly suppressed in MDA-MB-468 and MCF7 cells but not in MDA-MB-453 and HCC1143 by AZD3514 (Fig. 1B). These results support the roles of AR in DDR activation in breast cancer cells.

Figure 1.

AR inhibition downregulates DNA repair molecules. A, The expressions of ATM mRNA in breast cancer cells were analyzed by qRT-PCR after DHT or AZD3514 treatment. ATM expression levels were normalized versus actin and renormalized by value at untreated controls, and are presented in the bar graph with SEs (n = 3). B, DDR protein expressions were assessed by Western blotting after treating cells with increasing concentrations of AZD3514 for 5 days.

Figure 1.

AR inhibition downregulates DNA repair molecules. A, The expressions of ATM mRNA in breast cancer cells were analyzed by qRT-PCR after DHT or AZD3514 treatment. ATM expression levels were normalized versus actin and renormalized by value at untreated controls, and are presented in the bar graph with SEs (n = 3). B, DDR protein expressions were assessed by Western blotting after treating cells with increasing concentrations of AZD3514 for 5 days.

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AZD3514 enhanced the antitumor effect of olaparib in four of the nine breast cancer cell lines

Our data revealed that AR inhibition suppressed the expression and activation of ATM–chk2 axis, and AR signaling has been reported to promote DDR by upregulating DDR gene expression (17). Accordingly, we hypothesized that AR inhibition by AZD3514 might enhance the antitumor effect of PARP inhibition by compromising DNA repair activity. To determine whether AR inhibition increases the antitumor effect of PARP inhibitor in breast cancer cells, the growth-inhibitory effects of AZD3514 with/without olaparib was evaluated using an MTT assay. Breast cancer cell lines exhibited various levels of response to AZD3514/olaparib treatment irrespective of subtype or AR expression level (Table 1). On the basis of the results obtained, MDA-MB-453 and MDA-MB-468 cells were chosen for further experiments; the antiproliferative effect of AZD3514/olaparib was confirmed using a CFA. AZD3514/olaparib treatment suppressed the proliferation of MDA-MB-468 but not that of MDA-MB-453 cells (Fig. 2A). MCF7 cells showed AZD3514 and olaparib acted synergistically, while HCC1143 cells showed they acted in an antagonistic manner (Supplementary Fig. S3A). The effects of AZD3514/olaparib treatment on cell-cycle progression were determined by a cell-cycle analysis. The results obtained showed that AZD3514/olaparib synergistically induced G2–M cell-cycle arrest and apoptosis in MDA-MB-468 and MCF7 cells (Fig. 2B; Supplementary Fig. S3B). However, MDA-MB-453 and HCC1143 cells with CI values of >1 were unaffected by AZD3514/olaparib. Consistent with cell-cycle data, combination treatment induced the expressions of G2–M cell-cycle molecules, including cyclin B1 and cdc2, in only MDA-MB-468 cells. Increases in PARP and caspase-3 cleavage were also clearly detected in MDA-MB-468 cells treated with AZD3514/olaparib (Fig. 2C). These results show that combination treatment increased the antitumor effect by inducing G2–M cell-cycle arrest and the apoptosis of MDA-MB-468 cells.

Table 1.

Combination effect of AZD3514 and olaparib in breast cancer cell lines

Cell lineSubtypeER ExpressionPR ExpressionHER2 ExpressionAR ExpressionAZD3514 IC50 (μmol/L, mean ± SD)Olaparib IC50 (μmol/L, mean ± SD)1:1 Combination IC50 (μmol/L, mean ± SD)Combination index (CI value at ED75)
MCF7 Luminal − Normal Weakly positive 7.72 ± 0.1 >10 3.64 ± 0.007 0.24 
T47D Luminal Normal ++ 8.25 ± 0.101 >10 >10 0.63 
BT-474 HER2+ Amplified ++ >10 >10 9.822 ± 0.134 3.14 
MDA-MB-453 HER2+ − − Amplified +++ 8.08 ± 0.54 4.52 ± 0.009 3.1 ± 0.01 1.46 
HCC1143 TNBC − − Normal Weakly positive >10 >10 >10 2.25 
MDA-MB-468 TNBC − − Normal >10 5.8 ± 0.02 2 ± 0.05 0.49 
MDA-MB-231 TNBC − − Normal 7.3 ± 0.024 >10 3.14 ± 0.017 0.39 
BT-549 TNBC − − Normal ++ >10 >10 8.4 ± 0.155 5.75 
Hs578T TNBC − − Normal >10 >10 >10 2.78 
Cell lineSubtypeER ExpressionPR ExpressionHER2 ExpressionAR ExpressionAZD3514 IC50 (μmol/L, mean ± SD)Olaparib IC50 (μmol/L, mean ± SD)1:1 Combination IC50 (μmol/L, mean ± SD)Combination index (CI value at ED75)
MCF7 Luminal − Normal Weakly positive 7.72 ± 0.1 >10 3.64 ± 0.007 0.24 
T47D Luminal Normal ++ 8.25 ± 0.101 >10 >10 0.63 
BT-474 HER2+ Amplified ++ >10 >10 9.822 ± 0.134 3.14 
MDA-MB-453 HER2+ − − Amplified +++ 8.08 ± 0.54 4.52 ± 0.009 3.1 ± 0.01 1.46 
HCC1143 TNBC − − Normal Weakly positive >10 >10 >10 2.25 
MDA-MB-468 TNBC − − Normal >10 5.8 ± 0.02 2 ± 0.05 0.49 
MDA-MB-231 TNBC − − Normal 7.3 ± 0.024 >10 3.14 ± 0.017 0.39 
BT-549 TNBC − − Normal ++ >10 >10 8.4 ± 0.155 5.75 
Hs578T TNBC − − Normal >10 >10 >10 2.78 
Figure 2.

AZD3514 plus olaparib induces G2–M cell-cycle arrest and cell death in MDA-MB-468 cells. A, The growth-inhibitory effects of AZD3514/olaparib were evaluated using a CFA. The cells were incubated with the indicated concentrations of AZD3514 and/or olaparib for 14 days, and percentages of surviving cells were calculated. Results are presented with SEs (n = 3; **, P < 0.001). B, Cells were exposed to AZD3514 and/or olaparib at the indicated doses for 5 days, and percentages of cells in the G1, G2–M phase, and percentages of apoptotic cells were calculated. Results are presented with SEs (n = 3; *, P < 0.005). C, The expression levels of cell-cycle–related proteins, PARP, and caspase-3 cleavages were determined by Western blotting.

Figure 2.

AZD3514 plus olaparib induces G2–M cell-cycle arrest and cell death in MDA-MB-468 cells. A, The growth-inhibitory effects of AZD3514/olaparib were evaluated using a CFA. The cells were incubated with the indicated concentrations of AZD3514 and/or olaparib for 14 days, and percentages of surviving cells were calculated. Results are presented with SEs (n = 3; **, P < 0.001). B, Cells were exposed to AZD3514 and/or olaparib at the indicated doses for 5 days, and percentages of cells in the G1, G2–M phase, and percentages of apoptotic cells were calculated. Results are presented with SEs (n = 3; *, P < 0.005). C, The expression levels of cell-cycle–related proteins, PARP, and caspase-3 cleavages were determined by Western blotting.

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AZD3514/olaparib impaired DNA damage repair capacity.

AR suppression by AZD3514 significantly downregulated DDR gene expression, which could explain the sensitivity of breast cancer cells to PARP inhibitors (Fig. 1). In MDA-MB-468 cells, AZD3514/olaparib treatment decreased ATM–chk2 axis activity, the expressions of DDR proteins (e.g., RAD51 and ERCC1; Fig. 3A), led to a dramatic reduction in RAD51 foci formation, and increased the number of γH2AX foci (Fig. 3B). Furthermore, AZD3514/olaparib treatment led to an accumulation of DNA damage in these cells (Fig. 3C). The same experiments were also performed in MCF7 and HCC1143 cells. In MCF7 cells, AZD3514/olaparib suppressed DDR proteins and increased DNA damage levels, whereas this was not observed in HCC1143 cells (Supplementary Fig. S3C and S3D). These results indicate that the mechanism responsible for the enhancement of cellular sensitivity to olaparib by AZD3514 involves abrogation of the HRR pathway in some breast cancer cells.

Figure 3.

AZD3514 enhances olaparib-induced DNA damage accumulation by compromising DNA damage repair response. A, Cells were treated with AZD3514 and/or olaparib for 5 days, and Western blotting was conducted to assess the expression levels of DDR proteins. B, An immunofluorescence assay was conducted to evaluate RAD51 and γH2AX foci formation after AZD3514 and/or olaparib treatment. At least 100 nuclei were assessed per experiment. The percentages of cells containing more than 5 RAD51 and γH2AX foci in three experiments are presented as a bar graph with SEs, *, P < 0.005; **, P < 0.001. C, Degree of DNA damage accumulation was assessed using an alkaline comet assay. Cells were treated with AZD3514 and/or olaparib for 5 days, and degrees of DNA damage accumulation were analyzed using the alkaline comet assay. Mean tail moments were calculated for three independent experiments and are represented as a bar graph. Column entries are the means of three independent experiments; bars represent ± SEs (*, P < 0.005).

Figure 3.

AZD3514 enhances olaparib-induced DNA damage accumulation by compromising DNA damage repair response. A, Cells were treated with AZD3514 and/or olaparib for 5 days, and Western blotting was conducted to assess the expression levels of DDR proteins. B, An immunofluorescence assay was conducted to evaluate RAD51 and γH2AX foci formation after AZD3514 and/or olaparib treatment. At least 100 nuclei were assessed per experiment. The percentages of cells containing more than 5 RAD51 and γH2AX foci in three experiments are presented as a bar graph with SEs, *, P < 0.005; **, P < 0.001. C, Degree of DNA damage accumulation was assessed using an alkaline comet assay. Cells were treated with AZD3514 and/or olaparib for 5 days, and degrees of DNA damage accumulation were analyzed using the alkaline comet assay. Mean tail moments were calculated for three independent experiments and are represented as a bar graph. Column entries are the means of three independent experiments; bars represent ± SEs (*, P < 0.005).

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AZD3514 inhibited ATM activation by downregulating NKX3.1 expression negatively associated with TOPORS expression.

To determine how AR inhibition regulates ATM activation in MDA-MB-468 cells, we focused on molecules directly targeted by AR that are known to regulate DDR activation. Bowen and colleagues demonstrated NKX3.1 enhances DDR by increasing ATM activation in prostate cancer cells (23–26). Therefore, we hypothesized that NKX3.1 expression may affect the antiproliferative effects of AZD3514/olaparib in MDA-MB-468 cells by inactivating DDR induced by ATM. Interestingly, although both treatment-naïve MDA-MB-453 and MDA-MB-468 cells expressed NKX3.1, AZD3514 downregulated NKX3.1 expression in only MDA-MB-468 cells (Fig. 4A). Furthermore, this downregulation of NKX3.1 expression cooccurred with suppressed activation of the ATM–chk2 axis. Because NKX3.1 expression was downregulated in MDA-MB-468 cells only after AR inhibition, we focused on the posttranslational modification of NKX3.1, which can be regulated by TOPORS, an E3 ubiquitin ligase (27). To determine whether different levels of NKX3.1 suppression after AZD3514 treatment were the result of TOPORS induction by AZD3514 and a subsequent increase in NKX3.1 degradation, we measured TOPORS levels by Western blotting. As was expected, TOPORS levels differed in MDA-MB-453 and MDA-MB-468 cells, and AZD3514 increased TOPORS expression only in MDA-MB-468 cells, which expressed TOPORS in the basal state (Fig. 4B). To determine whether the downregulation of NKX3.1 by TOPORS occurs mechanically via ubiquitination, MDA-MB-468 cells were cotreated with MG132 (a proteasomal inhibitor) and AZD3514. An immunoprecipitation assay was then conducted to confirm whether NKX3.1 ubiquitination influenced ATM activity. The results showed that MG132 cotreatment inhibited AZD3514-induced NKX3.1 degradation and led to maintenance of ATM activation (Fig. 4C). In addition, TOPORS knockdown stabilized NKX3.1 expression levels and maintained ATM–chk2 axis activation in MDA-MB-468 cells treated with AZD3514 (Fig. 4D). These modulations of DDR activities by AZD3514-induced TOPORS upregulation and resultant increases in NKX3.1 were also observed when AR expression was depleted by AR-specific siRNA treatment (Fig. 4E). Thus, DDR modulation through TOPORS–NKX3.1 regulation is a direct effect of AR-directed therapy. To determine whether TOPORS silencing adversely affects the activity of AZD3514/olaparib by modulating ATM activation, the viabilities of TOPORS siRNA knocked down MDA-MB-468 cells treated with AZD3514 with/without olaparib were measured using the CFA. TOPORS depletion increased resistance to AZD3514/olaparib in MDA-MB-468 cells (Fig. 4F). Furthermore, similar results were observed for MCF7 cells (Supplementary Fig. S4). Our data indicate that posttranslational regulation of NKX3.1 via the modulation of TOPORS expression by AZD3514 induced ATM inactivation, and that this increased sensitivity to olaparib in AR-positive, TOPORS-expressing breast cancer cells.

Figure 4.

TOPORS expression increases the combination effects of AZD3514 and olaparib by inducing ATM inactivation via the posttranslational suppression of NKX3.1. A, Cells were incubated with different doses of AZD3514 for 5 days, and the levels of NKX3.1, ATM, and chk2 proteins were analyzed by Western blotting. B, TOPORS expression was assessed by Western blotting following AZD3514 treatment for 5 days. C, Posttranslation modification of NKX3.1 protein by ubiquitination in MDA-MB-468 cells was analyzed by treating 10 μmol/L of MG132 with/without AZD3514 for 5 days. D, MDA-MB-468 cells were transfected with nonspecific control or TOPORS-specific siRNA, and exposed to AZD3514 for 5 days. Protein levels changed in a TOPORS protein level–dependent manner after AZD3514 treatment. E, Cells were transfected with AR-specific or nonspecific siRNA and the levels of TOPORS, NKX3.1, and DDR proteins were then examined by Western blotting. F, TOPORS silencing using TOPORS-specific siRNA decreased cellular sensitivity to AZD3514/olaparib inhibition. CFA was conducted using cells transfected with siRNA targeting TOPORS or nonspecific control siRNA for 3 days and then treated with AZD3514 and/or olaparib for 14 days. Cell viability percentages were calculated and are presented in a bar graph with SEs (n = 3; **, P < 0.001).

Figure 4.

TOPORS expression increases the combination effects of AZD3514 and olaparib by inducing ATM inactivation via the posttranslational suppression of NKX3.1. A, Cells were incubated with different doses of AZD3514 for 5 days, and the levels of NKX3.1, ATM, and chk2 proteins were analyzed by Western blotting. B, TOPORS expression was assessed by Western blotting following AZD3514 treatment for 5 days. C, Posttranslation modification of NKX3.1 protein by ubiquitination in MDA-MB-468 cells was analyzed by treating 10 μmol/L of MG132 with/without AZD3514 for 5 days. D, MDA-MB-468 cells were transfected with nonspecific control or TOPORS-specific siRNA, and exposed to AZD3514 for 5 days. Protein levels changed in a TOPORS protein level–dependent manner after AZD3514 treatment. E, Cells were transfected with AR-specific or nonspecific siRNA and the levels of TOPORS, NKX3.1, and DDR proteins were then examined by Western blotting. F, TOPORS silencing using TOPORS-specific siRNA decreased cellular sensitivity to AZD3514/olaparib inhibition. CFA was conducted using cells transfected with siRNA targeting TOPORS or nonspecific control siRNA for 3 days and then treated with AZD3514 and/or olaparib for 14 days. Cell viability percentages were calculated and are presented in a bar graph with SEs (n = 3; **, P < 0.001).

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Coadministration of AZD3514 and olaparib significantly impeded tumor growth in an in vivo mouse xenograft model.

Treatment with AZD3514 plus olaparib significantly reduced tumor growth during and after treatment (Fig. 5A). As shown in Fig. 5B, cotreatment with AZD3514 and olaparib was well tolerated. Consistent with our in vitro findings, tumor tissues from mice treated with AZD3514/olaparib showed lower Ki-67 expression that those of mice treated with vehicle, AZD3514, or olaparib, and a TUNEL assay showed increased apoptosis (Fig. 5C). The protein levels of NKX3.1 in tumor tissues from mice treated with AZD3514/olaparib were dramatically lower than in other treatment groups, whereas TOPORS expression levels were significantly increased in tissues from AZD3514 alone or in combination with olaparib (Fig. 5C). Furthermore, levels of proteins related to proliferation (e.g., AKT and ERK) were reduced by AZD3514/olaparib treatment and PARP and caspase-3 cleavage were elevated (Fig. 5D). In addition, AZD3514/olaparib treatment suppressed the expression of DNA damage repair proteins associated with decreased levels of NKX3.1 induced by TOPORS induction in tumor tissues, which resulted in increased γH2AX levels. These findings demonstrate that the antitumor effect of AZD3514/olaparib involves compromising DDR in breast cancer cell lines and in our xenograft model.

Figure 5.

AZD3514/olaparib treatment exerts antitumor effects in an MDA-MB-468 xenograft model. A, An MDA-MB-468 xenograft mouse model was established and mice were treated with 50 mg/kg AZD3514 (n = 8), 30 mg/kg olaparib (n = 8), or 50 mg/kg AZD3514 plus 30 mg/kg olaparib (n = 8), or vehicle only (n = 8) daily for 28 days. Tumor volumes were assessed every other day and are presented in the graph with SEs. B, Mouse body weights were measured to assess treatment toxicities. C, IHC staining for Ki-67, hematoxylin and eosin staining (H&E), and TUNEL assays were performed. The expression levels of NKX3.1 and TOPORS were examined in tumor tissues. Representative images are presented (400× original magnification). Scale bars represent 50 μm. D, The expression levels of proteins associated with proliferation, apoptosis, and DNA damage response were evaluated by Western blotting.

Figure 5.

AZD3514/olaparib treatment exerts antitumor effects in an MDA-MB-468 xenograft model. A, An MDA-MB-468 xenograft mouse model was established and mice were treated with 50 mg/kg AZD3514 (n = 8), 30 mg/kg olaparib (n = 8), or 50 mg/kg AZD3514 plus 30 mg/kg olaparib (n = 8), or vehicle only (n = 8) daily for 28 days. Tumor volumes were assessed every other day and are presented in the graph with SEs. B, Mouse body weights were measured to assess treatment toxicities. C, IHC staining for Ki-67, hematoxylin and eosin staining (H&E), and TUNEL assays were performed. The expression levels of NKX3.1 and TOPORS were examined in tumor tissues. Representative images are presented (400× original magnification). Scale bars represent 50 μm. D, The expression levels of proteins associated with proliferation, apoptosis, and DNA damage response were evaluated by Western blotting.

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In this study, we aimed to investigate a new treatment strategy using a novel AR inhibitor AZD3514 in breast cancer. Our in vitro and in vivo results from this study aid understanding of the impact of AR signaling on DDR in breast cancer. This article is the first to show that a link exists between AR and DDR activity in breast cancer. Furthermore, it suggests combinational AR/PARP inhibitor strategies be considered in breast cancer. We believe this preclinical research study provides fundamental data for future clinical trials of AR inhibition.

In phase II single-arm trial (NCT01889238) of an AR inhibitor enzalutamide showed clinical benefit in 47% of patients with TNBC with AR expression and an androgen-related gene signature (28, 29). On the basis of the promising results of phase II trial and preclinical data, phase III trial of enzalutamide in combination with paclitaxel or as monotherapy in TNBC (NCT02929576) is ongoing, and these studies suggested that enzalutamide might be a novel therapeutic strategy for TNBC. However, the identification of androgen-related gene expression profiles is a complex and costly process (3, 30), and AR-related subtyping of breast cancer is not clearly classified from other subtypes of breast cancer, thus targeting AR to treat breast cancer presents many hurdle in standard clinical practice. According to our results, targeting AR is not effective enough to suppress cell viability even in cells expressing AR, and thus, new therapeutic strategies are required to extend the use of AR inhibitors in AR-expressing patients with breast cancer. To fulfill this unmet need, some attempts have been made using AR inhibitors in combination with other targeted therapeutic agents, such as, aromatase inhibitor and NYP-BEZ235 (a PI3K and mTOR dual inhibitor; refs. 31–33). However, these approaches are also limited in context- and subtype-specific cases.

AZD3514 inhibits AR signaling through androgen-dependent and -independent mechanisms (34–36). It was developed by AstraZeneca as a second-generation anti-androgen, and some reports have described the action mechanisms of AZD3514 in castration-resistant prostate cancer (CRPC) cells. Enzalutamide (Medivation) inhibits AR nuclear translocation, DNA binding, and coactivator recruitment by binding with high affinity to the ligand-binding domain of AR, whereas AZD3514 inhibits AR nuclear translocation and transcriptional activity, and downregulates AR levels (35). In a phase I clinical trial on patients with CRPC, AZD3514 reduced PSA activity in 17%–25% of patients, although its development was later discontinued due to tolerability issues (35, 37). However, the modulation of DDR activities by AZD3514-induced AR inhibition assessed in this study was also observed after AR depletion using siRNA (Fig. 4E). Thus, the DDR-modulating effect of AZD3514 is not additional effect, but rather a cellular mechanism of AR. Therefore, dual targeting of AR and PARP in breast cancer based on downregulation of DDR offers a promising treatment strategy for patients with breast cancer.

NKX3.1 is a haploinsufficient androgen-regulated tumor suppressor gene that is downregulated in prostate carcinoma. The Cancer Genome Atlas reported 64 of 333 patients with prostate cancer (19%) showed a genetic alteration in the NKX3.1 gene, and it has been well established that loss of NKX3.1 is associated with prostate carcinoma progression (38–40). Some previous studies demonstrated that NKX3.1 may also play a broader role in cellular response to DNA damage (24–26, 40). Furthermore, depletion of NKX3.1 in prostate cancer cells was found to be associated with a significant decrease in p-ATM levels (25), and Bowen and colleagues found NKX3.1 binds to ATM and accelerates ATM activation (23). In this study, protein levels of NKX3.1 were suppressed by AZD3514 treatment in sensitive MDA-MB-468 and MCF7 cells, and these suppressions corresponded to significant decreases in the levels of p-ATM and p-chk2. These observations indicate AR inhibition modulates DDR activity in an ATM–chk2 activation dependent manner by downregulating NKX3.1 protein, a direct target of AR signaling.

Unlike prostate cancer, less than 1% of patients with breast cancer have a genetic alteration in NKX3.1. In fact, all nine breast cancer cell lines using in this study expressed NKX3.1 protein, and no genetic alteration was found in NKX3.1 (Supplementary Fig. S5). Thus, the genetic and protein expressional statuses of NKX3.1 do not appear to determine the combinatorial effects of AR plus PARP inhibitor via the modulation of ATM–chk2 dependent DDR activity. Interestingly, we found NKX3.1 protein levels were downregulated by AZD3514 in MDA-MB-468 and MCF7 cells, which exhibited synergistic sensitivity to AZD3514/olaparib, whereas NKX3.1 protein levels were not downregulated by AZD3514 in MDA-MB-453 and HCC1143. To identify the different mechanisms responsible for downregulating NKX3.1 protein expression after AR inhibition, we assessed the possibility of the posttranslational modification–based regulation of NKX3.1. TOPORS, a robust E3 ubiquitin ligase is known to ubiquitinate NKX3.1 in prostate cancer (25, 27, 41). Guan and colleagues demonstrated overexpression of TOPORS leads to NKX3.1 ubiquitination, and that knockdown of TOPORS leads to a higher steady-state level of NKX3.1 and extends its half-life (27). We found NKX3.1 protein levels were maintained by inhibiting proteasomal degradation even in the presence of AZD3514, which suggests NKX3.1 protein was mediated by posttranslational modification induced by AR inhibition. Furthermore, NKX3.1 protein and TOPORS protein levels were found to be inversely related. These observations suggest that TOPORS functioning as a negative regulator of NKX3.1 may be involved in regulation of DDR activity by controlling NKX3.1 protein expression (Fig. 6). In fact, MDA-MB-453 and HCC1143 cells exhibited an antagonistic effect between AZD3514 and olaparib have no protein expression of TOPORS, which can sustain DDR activation via retaining NXK3.1 protein expression. Subsequently, TOPORS deficiency contributes to decreased sensitivity to PARP inhibitor, olaparib. These findings indicate TOPORS is a key molecule in terms of the synergism shown by combinatorial AR and PARP inhibitor treatment, and suggest that TOPORS might serve as a predictive marker to select patients likely to benefit from an AR/PARP inhibitor treatment strategy.

Figure 6.

Proposed model for the action mechanism of AZD3514 on DDR modulation.

Figure 6.

Proposed model for the action mechanism of AZD3514 on DDR modulation.

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In this study, we evaluated the antitumor activities of AZD3514 in breast cancer cell lines and in a xenograft model. AR inhibition by AZD3514 alone did not effectively suppress cell proliferation, but increased levels of DNA damage accumulation due to the inactivation of DDR molecules, especially of ATM and chk2. AZD3514 treatment led to HRD in cells and enhanced sensitivity to PARP inhibitor. Interestingly, AR-expressing breast cancer cells exhibited different responses to AZD3514/olaparib that depended on the downregulation of NKX3.1 protein expression by AR inhibition. In TOPORS-expressing breast cancer cells, NKX3.1 protein levels were suppressed by TOPORS expression–induced AR inhibition. Thus, AR inhibition induced TOPORS expression, and led to the proteosomal degradation of NKX3.1. Subsequently, reduced levels of NKX3.1 led to the loss of DDR activity, and finally increased sensitivity to PARP inhibitor.

This study shows how AR inhibition effects DDR activity in breast cancer, in addition its findings suggests TOPORS baseline expression might be useful for predicting response to the combined inhibitions of AR and PARP in breast cancer. Hopefully, this new strategy will result in the use of combined AR and PARP inhibitors for the treatment of breast cancer, and encourage others to undertake clinical trials to explore this potential strategy.

S.-A. Im reports receiving a commercial research support from Research Fund from AstraZeneca and is also a consultant/advisory board member for AstraZeneca, Roche, Hanmi, Novartis, Pfizer, and Spectrum. No potential conflicts of interest were disclosed by the other authors.

Conception and design: A. Min, K.-H. Lee, M.J. O'Connor, S.-A. Im

Development of methodology: A. Min

Acquisition of data (provided animals, acquired and managed patients, provided facilities, etc.): A. Min

Analysis and interpretation of data (e.g., statistical analysis, biostatistics, computational analysis): A. Min, K.-H. Lee, Y. Yang, M.J. O'Connor, S.-A. Im

Writing, review, and/or revision of the manuscript: A. Min, K.-H. Lee, D.K. Kim, K.J. Suh, Y. Yang, S.-A. Im

Administrative, technical, or material support (i.e., reporting or organizing data, constructing databases): A. Min, H. Jang, S. Kim, K.-H. Lee, D.K. Kim

Study supervision: S.-A. Im

This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and future Planning (2015R1A2A2A01004655) and also supported by Doosan Yonkang Foundation (30-2013-0140; to S.A. Im). This research was partly supported by the SNUH Research Fund (03-2016-0030; to S.A. Im).

The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

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