Introduction
Taxanes are important drugs for treatment of breast cancer [
1‐
3]. These drugs bind to tubulin and suppress spindle microtubule dynamics, which leads to cell cycle arrest in G2/M phase followed by apoptosis [
4‐
6].
Several mechanisms of taxane resistance have been described, including overexpression of the drug efflux pump MDR-1/P-gp, HER-2 overexpression, tubulin mutation, and variable expression of tubulin isotypes and stathmin [
4,
7‐
12]. Microtubule-associated protein-tau (MAPT), which is implicated in the pathogenesis of Alzheimer's disease, is associated with another mechanism of taxane resistance. MAPT binds to both the outer and inner surfaces of microtubules, leading to tubulin assembly and microtubule stabilization. Since taxanes also bind to the inner surface of microtubules, MAPT obstructs the function of the drug [
5,
6,
13,
14]. Rouzier
et al. found that low MAPT expression was associated with higher rates of a pathologic complete response to preoperative paclitaxel and 5-fluorouracil, doxorubicin, cyclophosphamide (paclitaxel/FAC) chemotherapy [
5]. This group also showed that MAPT overexpression was correlated with resistance to paclitaxel and that knockdown of MAPT with small interfering RNA (siRNA) reversed the resistance to taxanes
in vitro [
5].
MAPT has six isoforms that are spliced from a single gene. These isoforms differ by having three or four conserved repeat motifs in the microtubule-binding domain and none, one or two insertions in the N-terminal projection domain. Isoforms with four C-terminal repeats have a higher affinity for microtubules than isoforms with three such repeats [
13‐
17]. However, the function of each isoform is unknown.
Previous experimental studies have shown that MAPT expression is increased by estrogen
in vitro and
in vivo [
18,
19], and clinical studies have shown a positive correlation of MAPT levels with estrogen receptors (ER) expression [
20,
21]. Jonna
et al. found that estrogen stimulation upregulated MAPT mRNA in MCF-7 cells in microarray analysis [
22], and the MAPT gene is considered to contain an imperfect ER response element upstream of its promoter. The ER plays a key role in the development and progression of breast cancer, but it is unknown if ER stimulation induces MAPT expression in breast cancer cells.
Hormonal drugs play an important role in breast cancer therapy. The selective ER inhibitor, fulvestrant, inhibits estrogen signaling through the ER in two ways: by competing with estradiol binding to the ER, and by increasing the turnover of ER to decrease the ER protein level in breast cancer cells. In contrast, tamoxifen, a selective ER modulator, is an ER antagonist but often displays estrogen-like agonist activity [
22‐
24]. Therefore, fulvestrant and tamoxifen may have different effects on MAPT expression via the ER.
Previous
in vitro studies show that tamoxifen has an antagonistic effect on anti-cancer drugs [
25,
26]. Several clinical studies that used tamoxifen for hormone therapy have found that it has an antagonistic effect on chemotherapy drugs when it is used concurrently with them, and that the results of the combined use of tamoxifen with chemotherapy drugs is inferior, compared with using the drugs sequentially [
27‐
30]. The effect of combination treatment using other modern hormone therapies, such as aromatase inhibitors or fulvestrant, has not been examined thoroughly.
In this study, we examined the relationship between the MAPT expression and the sensitivity to taxanes, the effect of ER expression or modulation on MAPT expression, and the combined impact of hormones and taxanes on anti-cancer activity and taxane resistance in breast cancer cell lines.
Materials and methods
Cell culture and agents
Twelve human breast cancer cell lines were used in the study: MCF-7, MDA-MB-231, SK-BR-3 and ZR75-1 were obtained from the American Type Culture Collection (Rockville, MD, USA); YMB1-E was kindly provided by the Tohoku University Institute of Development, Aging and Cancer Cell Resource Center for Biomedical Research; and MDA-MB-134-VI, HCC38, HCC1143, HCC1569, HCC1806, HCC1937 and HCC3153 were kindly provided by Adi F. Gazdar (Hamon Center for Therapeutic Oncology Research and Department of Pathology, University of Texas Southwestern Medical Center at Dallas, Dallas, TX, USA). Cells were maintained at 37°C in 5% CO2 in RPMI-1640 medium (Sigma-Aldrich, St. Louis, MO, USA) containing 10% heat-inactivated fetal bovine serum and 1% penicillin-streptomycin.
Paclitaxel, docetaxel, fulvestrant, 17-β estradiol and tamoxifen were purchased from Sigma-Aldrich. Vinorelbine and doxorubicin were obtained from Kyowa Hakkoh (Tokyo, Japan). Cells were cultured in a phenol-free medium containing 10% dextran-coated, charcoal-treated FCS (Thermo Scientific, Waltham, MA, USA) and then treated with the above agents alone or in combination.
Small interfering RNA
Expression levels of MAPT and ER alpha were knocked down by transfection of the cells with two anti-MAPT siRNAs (5'-CGG GAC TGG AAG CGA TGA CAA-3' and 5'-CCG CCA GGA GTT CGA AGT GAT-3'; Qiagen, Valencia, CA, USA) and an anti-ER alpha siRNA (5'-GAG ACT TGA ATT AAT AAG TGA-3'; Qiagen), respectively. Scrambled siRNA (AllStars Negative Control siRNA, Qiagen) was used as the control. Transfection of siRNA was performed using HiPerfect Transfection Reagent (Qiagen) according to the manufacturer's protocol.
mRNA and protein expression analysis
Total RNA was extracted from cell pellets and cDNA was synthesized using a High Capacity cDNA Reverse Transcription Kit (Applied Biosystems, Foster City, CA, USA) in accordance with the manufacturer's protocol. Quantitative real-time PCR was performed using the Step One™ Real-Time PCR System (Applied Biosystems) with 18S rRNA as an internal control (Applied Biosystems). The sequences of primers and the probe were as follows: MAPT, forward primer: 5'-TAG GCA ACA TCC ATC ATA AAC CA-3'; reverse primer: 5'-TCG ACT GGA CTC TGT CCT TGA A-3'; and the FAM-TAMR probe: 5'-TGG CCA GGT GGA AG-3' (Invitrogen, Carlsbad, CA, USA). Data were analyzed using the relative standard curve method.
Samples from cultured cells were prepared for Western blot analysis, as previously described [
31]. The samples were separated on a NuPAGE Bis-Tris Gel 4 to 12% (Invitrogen) and electroblotted onto a polyvinylidene fluoride membrane. Primary antibodies for Western blotting were as follows: MAPT (T1029, United States Biological, Swampscott, MA, USA) [
5,
6]; ERα (Santa Cruz Biotechnology, Santa Cruz, CA, USA); and actin (Sigma-Aldrich). Blots were exposed to a horseradish peroxidase-conjugated secondary antibody (Santa Cruz Biotechnology) with development using enhanced chemiluminescence detection (ECL Kit, Amersham Pharmacia Biotech, Chandler, AZ, USA).
Effects of agents on cells
A cell viability assay was performed as previously described [
31], in which IC
50 values were determined for the anti-proliferative activity of each drug. Experiments were performed independently four times and the data shown are the average of the four assays. The combination effect of two agents was evaluated using the Combination Index (C.I), which was calculated using Calcusyn software (Biosoft, Cambridge, UK). The definition of C.I is as follows: C.I = (D)1/(Dx)1 + (D)2/(Dx)2 + (D)1(D)2/(Dx)1(Dx)2, where (Dx)1 is the dose of Drug 1 alone required to produce an X% effect; (D)1 is the dose of Drug 1 required to produce the same X% effect in combination with Drug 2; (Dx)2 is the dose of Drug 2 alone required to produce an X% effect; and (D)2 is the dose of Drug 2 required to produce the same X% effect in combination with Drug 1. C.I < 1, 1 and > 1 indicates a synergistic effect, an additive effect, and an antagonistic effect, respectively. Cell cycle effects were examined by flow cytometry as previously described [
32,
33].
Immunofluorescence
Cells were fixed in 4% paraformaldehyde, washed with cold PBS, and incubated in PBS containing 0.1% Triton X-100. After permeabilization, the cells were incubated in blocking buffer (PBS containing 0.1% Tween-20 and 3% BSA) containing antibodies against α-tubulin (Sigma-Aldrich). After washing with 0.1% Tween PBS, the cells were incubated in blocking buffer containing an anti-mouse AlexaFluor 488-conjugated secondary antibody (green) (Invitrogen). After washing again with 0.1% Tween PBS, the cells were incubated in PBS containing DAPI (blue) (Invitrogen). Immunofluorescence microscopy was performed using Biozero (Keyence, Osaka, Japan).
Discussion
In this study, we obtained several new findings on MAPT expression and the effects of taxanes. First, the sensitivity to taxanes is influenced by MAPT protein isoforms of less than 70 kDa. MAPT has six isoforms that give bands representing molecules ranging in size from 50 to 70 kDa, and MAPT protein isoforms have a significant impact on taxane sensitivity since they have different affinities for microtubules and different antagonistic effects on taxane [
13‐
16]. Rouzier
et al. provided the first report of the correlation of MAPT expression with the remission rate in subjects who received perioperative chemotherapy with a regimen including paclitaxel, and identified MAPT as a predictor of sensitivity to taxanes [
5]. However, subsequent studies did not support the utility of MAPT as a predictor of the effect of taxanes [
20,
21,
35,
36]. Recently, Pusztai
et al. performed a large-scale phase III clinical trial to compare doxorubicin and cyclophosphamide (AC) and AC followed by four courses of paclitaxel as adjuvant chemotherapy after surgery for breast cancer [
36]. The prognosis of patients with MAPT expression was better than that of patients with no MAPT expression, although the utility of MAPT as a predictor of the taxane effect was not shown [
36]. In clinical studies, RT-PCR and immunostaining are used for analysis of MAPT expression. The results of our
in vitro study indicated that expression of MAPT protein isoforms less than 70 KDa had the most influence on sensitivity to taxanes. A discrepancy between MAPT mRNA expression and protein expression has been found previously [
37], and thus analysis of MAPT mRNA expression may not be appropriate for examining the utility of MAPT as a predictor of taxane sensitivity. Furthermore, the status of the expression of different MAPT protein isoforms is important in determining sensitivity to taxanes, but immunohistochemistry cannot be used to evaluate each isoform. MAPT isoform expression in breast cancer tissues must be examined in detail to determine the exact correlation between MAPT expression and response to taxanes.
The second finding in the study involved clarification of the effect of the ER on the MAPT protein level in breast cancer cells. Clinical studies have suggested that MAPT expression has a positive correlation with ER expression and is influenced by ER signaling [
20,
21]. In our study in ER-positive and MAPT-positive breast cancer cell lines, expression of MAPT protein isoforms of less than 70 kDa, which have a large impact on sensitivity to taxanes, was affected by ER signaling. Furthermore, treatment of MAPT- and ER-positive cells with tamoxifen or fulvestrant had different effects on MAPT expression via the ER, which suggests that these drugs can alter cellular sensitivity to taxanes. In clinical treatment for breast cancer, the advantages and disadvantages of concomitant use of chemotherapeutic drugs and endocrine therapy have long been discussed. Several clinical studies of tamoxifen as hormone therapy have found that an antagonistic effect on concurrent chemotherapeutic agents, and that the results of giving tamoxifen concurrently with these agents are inferior to that of sequential administration [
27‐
30]. These results suggest that concomitant chemotherapy and endocrine therapy should be avoided clinically. However, the effect of combination treatment using other modern hormone therapies, such as aromatase inhibitors or fulvestrant, has not been examined thoroughly.
Our third finding supports and complements the current idea on concomitant use of chemotherapy and endocrine therapy, and indicated a new possibility for concomitant use. Tamoxifen is an ER antagonist that also has estrogen-like agonist activity. It has been used as hormone drug for long-term therapy, but its effect are complicated and incompletely understood. Our results suggested that the effect of tamoxifen on ER signaling differs depending on the dose. MAPT protein expression was increased at low concentrations of tamoxifen of 500 nM - 1 μM, but decreased at higher concentrations. Several factors associated with resistance to chemotherapy via regulation by ER signaling have been identified [
4,
22,
38‐
40]. Tamoxifen is thought to exert an antagonistic effect in concomitant use with chemotherapeutic drugs by increasing the expression of these factors via an agonistic effect on the ER. Active metabolites of tamoxifen also have different functions compared with the parent drug [
41‐
43], and more detailed studies are needed to determine how tamoxifen and its metabolites influence chemotherapy
in vivo and
in vitro.
Fulvestrant decreased ER and MAPT expression at all concentrations. An MTS assay, flow cytometry, and immunofluorescence all showed that the combination of fulvestrant and taxanes had a synergistic effect, consistent with the finding of Sui
et al. that fulvestrant combined with paclitaxel was effective in breast cancer cells
in vitro [
24]. Fulvestrant assists taxane function by downregulating the ER and ER-regulated factors associated with taxane resistance, and the combination of fulvestrant with taxanes increases the sensitivity of MAPT- and ER-positive breast cancer cells to taxanes.
ER-positive breast cancers clinically show a lower sensitivity to chemotherapy than do ER-negative breast cancers. This may be caused by the ER itself or by ER modulation of factors that result in resistance to chemotherapy. Our study indicates that the combination of modern hormone therapy with modern chemotherapy may become an effective therapy to ER-positive breast cancers.
Acknowledgements
This study was supported by a Grant-in-Aid for Scientific Research (c) from the Ministry of Education, Culture, Sports, Science and Technology of Japan. We thank Dr. Adi F. Gazdar for providing several breast cancer cell lines.
Competing interests
The authors declare that they have no competing interests.
Authors' contributions
NT, FH, TF, TS and HD designed this study. HI wrote the manuscript with NT. HI and TN performed the experiments. HY and JS analyzed the data. ST and SM gave expert advice throughout the study.