Introduction
Breast cancer is categorized into luminal A, luminal B, ERBB2/HER2, basal-like and triple-negative (TN) subtypes. Approximately 15–20% of breast cancers are classified as triple negative breast cancers (TNBC). TNBC is clinically characterized as a more aggressive cancer and has a poor prognosis with a five-year overall survival (OS) of 78.5%. It is highly prevalent in African-American [
1] and premenopausal women [
2] and BRCA1/2 mutation carriers [
2]. TNBC patients often develop tumor recurrence, which typically occurs within the first 3 years after diagnosis [
2]. Due to low or absent expression level of estrogen receptors (ER), progesterone receptors (PR) and the HER2 protein, endocrine therapies and anti-HER2 antibody treatment such as trastuzumab are ineffective in treating TNBC [
3]. As a result, cytotoxic chemotherapy is the backbone of systemic therapy in TNBC. Chemotherapy regimen for TNBC is usually a combination treatment of anthracyclines, alkylators, and taxanes and can be given in a neoadjuvant and/or adjuvant setting. Neoadjuvant chemotherapy is the standard approach to reduce tumor burden and evaluate chemo-efficacy prior to surgical resection in high-risk TNBC, while adjuvant is done after surgery [
3]. TNBCs have high inter- and intra-tumor heterogeneity [
4] resulting in very different treatment responses to standard chemotherapy. The survival rates for chemo-resistant metastatic TNBC patients have not been improved significantly over the past 30 years, and thus there is a pressing unmet need to develop new innovative strategies to control TNBCs in the clinic.
According to gene expression profiling, TNBC can be classified into two distinct basal-like (BL1 and BL2), a mesenchymal-like and a luminal androgen receptor (LAR) subtype [
5]. AR is detected in 12–55% of TNBC tumors [
6,
7]. LAR tumors are similar to breast tumors termed molecular apocrine [
8], and they constitute approximately 16% of TNBCs. LAR tumors exhibit a 9-fold greater AR protein expression compared to all other subtypes and possess enriched expression of AR downstream targets and coactivators [
9]. Interestingly, AR expression has also been detected in non-LAR TNBCs, albeit to low levels [
10,
11]. The role of AR is complex in breast cancer: AR possesses anti-tumor activity in estrogen receptor positive (ER
+) breast cancer. However, AR stimulates tumor cell progression in ER-negative (ER
−) breast cancers and is becoming an emerging molecular target in TNBC [
12,
13].
The TNBC subtype is an umbrella term, consisting of all breast cancers that are not classified as luminal, ER+ or HER2 amplified subtypes [
14]. Evaluating further subtypes within TNBC based on molecular characteristics may provide therapeutic insights for unique subtypes of TNBC patients. Using the molecular-based sub-classifications, FDA-approved tumor actionable drugs may be repurposed to treat chemo-resistant subtype-specific TNBC tumors. Bicalutamide and enzalutamide are FDA-approved AR antagonists for prostate cancer treatment [
15]. AR
+ TNBCs respond to these AR antagonists’ treatment in vitro and in vivo [
11,
16]. AR targeting therapies with bicalutamide or enzalutamide demonstrated early promise because single-agent AR inhibition exhibited modest efficacy in completed AR
+ TNBC clinical trials. There are an additional 18 ongoing clinical trials targeting AR inhibition for AR
+ metastatic or locally advanced AR
+ TNBC in the United States [
14]. Thus, there is an urgent need to improve the efficacy of AR antagonism.
In this study, we found that ceritinib, an FDA-approved drug for lung cancers [
17], efficiently inhibited growth of LAR TNBC cells. In addition, we identified Activated CDC42 Kinase 1 (ACK1) as a major ceritinib target in LAR TNBC cells and demonstrated that ceritinib inhibited RTK-ACK1/FAK-AR signaling pathway in LAR TNBC cells. Finally, we developed a novel therapeutic strategy for AR
+ TNBC tumors and a combination treatment for AR
− or AR
low TNBCs.
Discussion
Advanced screening and imaging technology, effective chemotherapy, targeted therapy and immunotherapy have extended lives of breast cancer patients significantly. About 98% of patients with early-stage breast cancer can survive for 10–15 years or longer (
https://www.breastcancer.org/symptoms/types/triple-negative). The five-year survival rate of breast cancer is about 90% in Western countries. TNBC is the most aggressive subtype of breast cancer and has the worst outcomes. The five-year overall survival of TNBCs is 78.5%. Even worse, the five-year survival rate for locally advanced and metastatic TNBC is only 11%. In the last decade, the success of PD-L1/PD-1 inhibitors, PARP inhibitors, and anti-Trop-2 antibody drug conjugates yielded several targeted treatment options for TNBC patients. Many TNBC tumors do not express biomarkers that would benefit from these recently FDA-approved targeted therapies (
https://www.breastcancer.org/symptoms/types/triple-negative). Therefore, there are unmet medical needs to develop new targeted drugs or strategies for TNBC. Furthermore, acquired resistance to targeted therapies remains a large obstacle in developing effective therapeutics. Synergistic drug combinations are becoming promising new avenues to overcome predicted drug resistance.
There have been impressive advancements in targeted therapy development for TNBC. BRCA1 is a risk gene linked to TNBC. TNBC tumors carrying BRCA mutations, 10-15% of TNBC tumors, are sensitive to poly-ADP-ribose polymerase (PARP) inhibitor therapy [
41]. Olaparib is an example of PARP1 inhibitor that has been shown to provide a significant benefit over standard chemotherapy for metastatic, germline BRCA mutated HER2-negative breast cancers [
41]. Recently, Olaparib received FDA approval to treat advanced-stage TNBC with a BRCA1 or BRCA2 mutation [
41]. In addition, PD-L1 is expressed in approximately 40% of TNBC tumors and TNBC-associated tumor stromal and infiltrating immune cells in the tumor microenvironment [
42]. Atezolizumab is an anti-PD-L1 monoclonal antibody. Adding atezolizumab to nab-paclitaxel chemotherapy significantly improved PD-L1 positive TNBC cohort median progression-free survival and a longer overall survival [
43]. Atezolizumab in combination with the albumin-bound paclitaxel or nab-paclitaxel was recently approved to treat advanced or metastatic PD-L1-positive TNBC [
43]. The success of PD-L1/PD-1 inhibitors, PARP inhibitors, and anti-Trop-2 antibody drug conjugates, demonstrates that targeted drugs have important therapeutic implications in TNBC but warrant further clinical consideration for the treatment of specific subsets of TNBC patients [
44]. Despite these impressive clinical successes, 60% of TNBC patients’ tumors do not express PD-L1 and about 20% of TNBC patients carry germline BRCA1/2 mutations [
45]. As a result, many TNBC patients would not benefit from these recently FDA-approved targeted therapies. It is known that 40% of TNBC patients develop metastatic disease and mortality from the cancer [
2]. Therefore, there are unmet medical needs to develop targeted drugs for TNBCs.
Luminal androgen receptor (LAR) represents a subtype of TNBC characterized by the presence of AR signaling. Asian patients with TNBC tumors have a higher prevalence of the LAR subtype (23% in Chinese compared to 12% in White cohorts) [
46]. Androgens activate the AR and commonly thought as male hormones [
47], but they are detectable in the circulation of women [
48]. The level of circulated testosterone in women is about 5% of those observed in men [
49,
50]. AR
+ TNBC patients hardly benefit from current standard chemotherapy regimens. Therefore, discovery of novel targeted and combination therapeutic approaches for this subset of tumors are of great clinical relevance. For example, PIK3CA mutations are highly prevalent in TNBC LAR subtype. The combination of AR antagonist bicalutamide with PI3K inhibitor GDC-0941 or the dual PI3K/mTOR inhibitor GDC-0980 reduced LAR xenograft tumor growth [
51].
AR stimulates tumor development and progression in ER
− breast cancers [
12,
13]. AR signal transduction pathways include the canonical AR signaling and androgen-independent AR signaling pathways. In the canonical AR signaling pathway, androgens such as DHT cross the cell membrane and drive target gene transcription [
52]. In addition, RTKs can initiate androgen-independent ACK1-AR signaling pathways. This androgen-independent AR signaling does not respond to classic AR antagonists including bicalutamide and enzalutamide. RTKs activate an intracellular tyrosine kinase ACK1 and promote oncogene transcription [
28,
30]. RTK-activated androgen-independent AR signaling may partially complement or compete with canonical AR signaling activity, which results in only modest efficacy of AR antagonism in AR positive TNBC treatments. Current AR-targeting studies focus on the blockade of the canonical AR signaling pathway, while the role of androgen independent AR signaling pathway in TNBCs is neglected. We proposed here that the canonical AR signaling and androgen independent AR signaling pathways should be blocked concordantly to improve the response of AR antagonist in AR
+ TNBC. We further showed that the combination strategy inhibits drug resistance through suppression of the CSC phenotype.
Ceritinib is an ALK inhibitor approved by the FDA for the treatment of ALK
+ metastatic non–small-cell lung cancer (NSCLC) in 2014 [
53]. In our report, we provide evidence for a mechanism through which ceritinib exerts its effects. We identified Activated CDC42 Kinase 1 (ACK1) as a major ceritinib target in LAR TNBC cells and demonstrated that ceritinib inhibited the RTK-ACK1-AR axis in LAR TNBC cells. Consistent with this, ACK expression positively correlated with AR, Her3 (ERBB3), and WNT signaling proteins such as Axin 1, β-catenin and cyclin D1.
Due to the observed inhibitory effects of ceritinib in the AR signaling cascade, we proposed that an AR inhibitor and ceritinib would be synergistic and more effective in a combination strategy. In this study, we discovered that the combination of enzalutmide and ceritinib is an effective regimen in AR+ TNBC patients and the combination of PTX and ceritinib is an effective therapeutic strategy in AR− and ARlow TNBC. The efficacy of these combination strategies were evaluated in multiple model systems. Furthermore, the diverse breast cancer cell lines and tumor specimens in our study addressed any potential cell line specific effects of the proposed combination drug strategies. We demonstrated these novel drug combinations were effective in a highly translational model system using intact patient-derived xenografted tumors, indicating our observations are translatable to the clinical setting. Furthermore, although paclitaxel is a commonly used therapeutic in breast cancer systemic chemotherapy regimens, resistance to paclitaxel results in tumor recurrence and drug resistance, limiting efficacy of paclitaxel-based strategies. In our study, we revealed ceritinib to inhibit FAK and YB-1 mediated mechanisms that contribute to paclitaxel resistance.
This study has crucial implications in the field of drug and target discovery in the context of breast cancer. The results from our study have immediate implications for clinical translation for this difficult-to-treat breast cancer subtype, which would benefit significantly from effective targeted therapeutic regimens. Our innovative approach to test repurposed FDA-approved drugs in new clinical contexts can be applied to all cancer tumor types and is not limited to breast cancer.
Methods
Experimental animals
All the mice were housed in a pathogen-free animal room under standard conditions with free access to water and food (standard chow diet and water ad libitum). All of the procedures were approved by the Institutional Animal Care and Use Committee of the LSU Health Science Center at New Orleans.
Antibodies, cell lines and chemicals
MDA-MB-231, MCF7, T47D, SKBR3, BT474, BT-20, MDA-MB-468, Hs578t and MDA-MB-453 and MDA-MB-157 cells purchased from the American Type Culture Collection (Manassas, VA). MFM 223, SUM159PE and SUM185PE cells were kindly provided by Dr. Jennifer A. Pietenpol (Vanderbilt-Ingram Cancer Center). ACK1, AR, EGFR, Her2, Her3, Mcl-1, Bcl-2, Puma, YB-1, FAK, AKT, Anti-phospho-AKT (Ser473), anti-phospho-S6K1 (S79), Anti-phospho-4-EBP1, Anti-phospho-LRP6, Anti-phospho-mTOR, Anti-phospho-YB-1, Anti-phospho-FAK (Y397), anti- ribosomal protein S6, anti-ribosomal protein S6, BCL-xL and anti-cleaved Casp3 antibodies were obtained from Cell Signaling Technology (Beverly, MA). Anti-phospho-ribosomal protein S6 (S235/236), mouse anti-ERK antibody and Anti-phospho-ERK were purchased from Santa Cruz Biotechnology (Santa Cruz, CA). Anti-Vinculin was purchased from Sigma. Secondary anti-mouse IgG with horseradish peroxidase was from Calbiochem. Secondary anti-rabbit IgG with horseradish peroxidase was from GE Healthcare. Ceritinib and Paclitaxel were purchased from APExBIO. Bicalutamide (97%) purchased from ACROS Organics. The 133 FDA-approved drugs were kindly provided from NCI/DTP Approved Oncology Drugs Plated Set (AODVIII).
Phospho-RTK Array analysis
To determine which receptor tyrosine kinases (RTKs) are targeted by ceritinib, the Human RTK Phosphorylation Antibody Array C1 Kit (RayBiotech, Cat. No. AAH-PRTK-1-2) was used. MDA-MB-453 cells were grown in DMEM medium containing 10% fetal bovine serum (FBS) and penicillin/streptomycin. When cells reached 70 ~ 80% confluence, media were changed to DMEM medium supplemented with DMSO, or 10 μM ceritinib. After 4 h incubation, cell lysates were prepared using lysis buffer containing protease and phosphatase inhibitors. After blocking for 1 hour at room temperature, the array membranes incubated with 700 μg of protein lysate overnight at 4 °C. Next, the arrays washed and incubated with biotin-conjugated phospho-tyrosine detection antibody overnight at 4 °C. Finally, the arrays incubated with a horseradish peroxidase-conjugated streptavidin (1:1000) at room temperature for 2 h. The phosphorylated RTKs detected and captured by ChemiDoc™ XRS+ Imaging Systems (Bio-Rad). The densitometric values of phospho-RTKs determined by an image lab software (Bio-Rad). The relative intensities of the duplicated spots were normalized to positive control spots. Values represent the mean of duplicate spots for each protein after normalization.
Drug treatments for TNBC cells
Tumor cells were seeded onto 6-well plates at a density of 300,000 cells per well. After culturing in complete DMEM medium for 16 hours, media was replaced with fresh DMEM containing 10% FBS and 10 μM ceritinib for 4 hours. In control conditions, media was replaced with fresh DMEM containing 10% FBS. Cells at indicated time points were lysed in radioimmunoprecipitation (RIPA) lysis buffer with protease inhibitor cocktail (Roche) and phosphatase inhibitors. Protein concentration was determined using a bicinchoninic acid (BCA) protein assay (Thermo, Waltham, MA). Samples were boiled in SDS sample buffer for 10 minutes and stored at − 80 °C until analysis.
Cell viability assay
TNBC cells were seeded (3000–5000 cells per well) in 96-well plates. Growth medium was replaced with either fresh medium (DMSO as a control) or medium containing the drugs after overnight growth. Cell viability was determined in quadruplicate using the MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) assay, a colorimetric assay according to the conversion of tetrazolium salts to blue formazan products by active cells, in 96-well plates at the indicated time points. The replicates normalized to the control wells. To further confirm the results of MTT assay, the experiments were repeated using Cell Counting Kit 8 (WST-8 / CCK8) (Abcam, ab228554). The data analysis performed using Prism software (GraphPad Software). Data represent the mean ± SEM. Student’s t test was used to analyze the data, and a p-value of < 0.05 was considered statistically significant. * P < 0.05; ** P < 0.01, *** P < 0.0001.
Ki 67 staining
Tissue sections were deparaffinized with xylene and re-hydrated through descending grades of alcohol up to water. Then, non-enzymatic antigen retrieval in Citrate buffer, pH 6.0 for 30 minutes at 95 °C, and endogenous peroxidase quenching with H2O2 in Methanol for 30 minutes were done. Sections were washed three times in PBS at interval of 10 minutes before blocking with 5% normal goat serum in 0.1% PBS/BSA. Tissues were incubated with Anti-Ki67 antibody (1:100 dilution; Cat# ab833, Abcam) overnight. After PBS washing, sections were incubated with a biotinylated anti-rabbit secondary IgG for 60 minutes, incubated with avidin-biotin-peroxidase (ABC) complexes, and developed with diaminobenzidine (Sigma).
Analysis of MDA-MB 231 xenograft tumor development and metastasis
MDA-MB 231 (1 × 106 cells) with 50% Matrigel™ were injected into Number 4 mammary gland of 4-5 weeks old NSG (Jackson Laboratory). Tumors were measured by a caliper. When tumors reached a size of ∼50 mm3 the mice were randomly distributed into four groups (seven mice in each group): (1) untreated control, (2) ceritinib (25 mg/kg bodyweight) alone, (3) paclitaxel (PTX) alone (10 mg/kg of bodyweight), and (4) ceritinib (25 mg/kg of bodyweight) + PTX (10 mg/kg of bodyweight). Ceritinib was dissolved in sesame oil and administrated by oral gavage once daily for 2 weeks. PTX (10 mg/Kg) was injected intra-peritoneally twice a week. Tumor volume measured twice a week after the initial injection, and the volumes were calculated using the formula (π x length x width1 x width2 /6). Mice were euthanized when they became moribund, or when they lost 20% weight. All organs examined for the presence of tumors and metastases at autopsy. Metastatic tumors were defined as nodules identified at secondary sites with a diameter ≥ 0.2 mm. To examine whether lungs infiltrated by metastases, lungs were fixed in formalin and embedded in paraffin. Sections stained with hematoxylin & eosin (H&E) staining. Data grouped and plotted using GraphPad Prism 8.
Enzalutamide and ceritinib treatment for SUM159 xenograft mice
SUM159PE (5 × 106 cells) with 50% Matrigel™ were injected into Number 4 mammary glands of 4-5 week old SCID mice (Jackson Laboratory). Tumors were measured by a caliper. When tumors reached ∼100 mm3 the mice were randomly distributed into four groups that included seven mice in each group: untreated control, ceritinib (50 mg/kg bodyweight) alone, enzalutamide alone (25 mg/kg of bodyweight), and ceritinib (50 mg/kg bodyweight) + enzalutamide (25 mg/kg of bodyweight). Drugs were administrated by oral garage once daily for 2 weeks. Tumor volume was measured twice a week after the initial injection. All the mouse experiments were performed in accordance with procedures and guidelines approved by Institutional Animal Care and Use Committee of the LSU Health Science Center at New Orleans.
Patient-derived xenografts
The AR-positive triple negative patient-derived tumor, designated as TU-4EA-LNb, was derived from a metastatic lymph node after mastectomy surgery with lymph node dissection from an African-American patient. TU-4EA-LNb was established in 4–6 week old SCID/Beige (CB17.Cg-PrkdcscidLyst bg/Crl) mice provided from Jackson Laboratory, a mouse type chosen to optimize tumor take. Tumor volume was measured using a digital caliper. Intact tumor pieces (3 × 3 mm2) were coated with Matrigel™ (BD Biosciences) and implanted unilaterally in the mammary fat pads of SCID/Beige mice. When tumors reached ∼100 mm3 the mice were randomly distributed into four groups: (1) untreated control, (2) ceritinib (50 mg/kg bodyweight) alone, (3) enzalutamide alone (25 mg/kg of bodyweight), and (4) ceritinib (50 mg/kg bodyweight) + enzalutamide (25 mg/kg of bodyweight). Drugs were administrated by oral gavage once daily for 2 weeks. Tumor volume was measured twice a week after the initial injection. For ex vivo analysis, TU-4EA-LNb tumor pieces were plated in cell culture dishes and covered in DMEM containing 10% FBS and drug treatments or vehicle controls. After the designated treatment time explants were collected, mechanically disrupted using scissors, and enzymatically digested using Trizol. Tumor pieces of additional PDX models established by the Burow lab were utilized to evaluate baseline expression of AR and ACK1: TU-BcX-2 K1 (T8, T11), TU-BcX-4 M4 (Tb2, Tb3), TU-BcX-4QX (Tb1, Tb2), TU-BcX-4IC (T1, T3) and TU-BcX-4EA-LNb (passage 4). The number following ‘T’ in the nomenclature of the PDX models after ‘TU-BcX-‘denotes the number of times the tumor had been serially transplanted in mice before the tumor was removed for analysis. All the mouse experiments performed in accordance with procedures and guidelines approved by Institutional Animal Care and Use Committee of the LSU Health Science Center at New Orleans. PDX tissues were procured through the Louisiana Cancer Research Consortium Biospecimen Core and processed following NIH regulations and institutional guidelines of Tulane University with IRB exemption status. All animal procedures were conducted in compliance with State and Federal laws, standards of the U.S. Department of Health and Human Services, and guidelines established by the Tulane University Animal Care and Use Committee. The facilities and laboratory animals programs of Tulane University are accredited by the Association for the Assessment and Accreditation of Laboratory Animal Care.
Mouse tumor tissue protein analysis
Fresh isolated tumor samples were snap frozen in liquid nitrogen. The extracts were prepared by grinding tissue into a fine powder in liquid nitrogen and subsequently dissolved in modified RIPA buffer supplemented with protease and phosphatase inhibitors. The lysates cleared by centrifuge at 13,000 rpm for 15 min and the supernatant was collected as extracted protein. Protein concentrations were determined using BCA protein assay. Equal amount of protein lysates separated by SDS-PAGE and then transferred to a PVDF membrane and detected with various antibodies.
Statistical analysis
Data are shown as means ± SEM if not otherwise indicated. Two-tailed unpaired Student’s t-test was applied for statistical analysis to compare the two groups of interest and P < 0.05 was considered statistically significant unless otherwise stated. Graphical information performed using GraphPad Prism software (GraphPad Software Inc., San Diego, CA).
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