Abstract
Background/Aim: We have reported that p62 (also known as sequestosome 1) is needed for survival/proliferation and tumor formation by aldehyde dehydrogenase 1 (ALDH1) -positive cancer stem cells (CSCs) and that p62high ALDH1A3high expression is associated with a poor prognosis in luminal B breast cancer. However, the association between p62high ALDH1A3high and the benefit from radiotherapy in patients with luminal B breast cancer remains unclear. Materials and Methods: Datasets from the Molecular Taxonomy of Breast Cancer International Consortium (METABRIC) and The Cancer Genome Atlas (TCGA) were downloaded, and data from p62high ALDH1A3high luminal B patients treated without or with radiotherapy were analyzed by Kaplan–Meier and multivariate Cox regression analyses. We also performed an in vitro tumor sphere formation assay after X-ray irradiation using p62-knockdown ALDH1high luminal B BT-474 cells. Results: p62high ALDH1A3high patients had poorer clinical outcomes than other luminal B breast cancer patients treated with radiotherapy. The combination of p62 DsiRNA KD and X-ray irradiation suppressed in vitro tumor sphere formation by ALDH1high BT-474 cells. These results suggest that p62 is involved in the reduced effect of X-ray irradiation on ALDH1-positive luminal B breast CSCs. Conclusion: p62 and ALDH1A3 may serve as prognostic biomarkers for luminal B breast cancer patients treated with radiotherapy. Additionally, the combination of p62 inhibition and radiotherapy could be useful for targeted strategies against ALDH1-positive luminal B breast CSCs.
Breast cancer occurs most often and is the leading cause of cancer-associated mortality in women worldwide (1). Breast cancer is commonly treated with surgery, radiotherapy, endocrine therapy, chemotherapy, and molecular targeted therapy.
X-ray irradiation is considered a major radiotherapy for breast cancer and is effective in reducing recurrence and patient death (2-5). However, relapse after radiotherapy is observed in a certain population of patients with breast cancer, and there are few biomarkers that can predict the effectiveness of radiotherapy. Therefore, it is important to identify effective biomarkers for predicting the effectiveness of radiotherapy for breast cancer at the time of diagnosis.
There are several means of classification of breast cancer including prediction analysis of microarray 50 (PAM50), based on gene expression patterns (6-10). PAM50 classifies breast cancer into six subtypes, including luminal A and luminal B (7, 8, 10). Both the luminal A and luminal B types are estrogen receptor-positive and account for 70-80% of breast cancer cases (11). In addition, many luminal B tumors highly express human epidermal growth factor receptor type 2 and proliferation markers such as Ki-67 (MKI67) (12-14). Luminal B breast cancer is associated with a poorer prognosis and does not show significant responses to radiotherapy, unlike luminal A breast cancer (4, 5, 12-17).
Cancer cell populations contain a small number of cancer stem cells (CSCs), which have stem cell-like properties, such as self-renewal, multipotency and tumorigenicity, and are resistant to chemotherapy and radiotherapy (18-20). Aldehyde dehydrogenase 1 (ALDH1) is known as a CSC marker in several cancer types, including breast cancer (21-26). ALDH1A3, an ALDH1 isoform, contributes significantly to ALDH1 activity in breast cancer cells (27, 28). Patients with high expression of ALDH1 who have received radiotherapy for rectal cancer, head and neck squamous cell carcinoma and squamous cell cervical carcinoma have shown poor outcomes (29-31). Moreover, ALDH1-positive breast CSCs are radioresistant (32-34).
We have previously reported that p62 (sequestosome 1) is essential for survival/proliferation and tumor formation by ALDH1-positive CSCs in luminal B breast cancer and that p62 is involved in cancer progression and contributes to the poor prognosis in p62high ALDH1A3high luminal B breast cancer patients (35). p62 is a multifunctional adapter protein that is involved in various physiological functions, including nuclear factor-B signaling, antioxidant responses and autophagy (36-44). The p62 level predicts radiotherapy resistance in hypopharyngeal carcinoma (45). However, the association between the expression of p62 and the benefit from radiotherapy in patients with ALDH1-positive luminal B breast CSCs remains unclear.
The present study examined the association between the gene expression of p62 and ALDH1A3 and the benefit from radiotherapy in patients with luminal A and B breast cancer. Furthermore, we also examined the combined effect of p62 deficiency and X-ray irradiation on ALDH1-positive CSCs in luminal B breast cancer.
Materials and Methods
Molecular Taxonomy of Breast Cancer International Consortium (METABRIC) dataset. The METABRIC dataset (46, 47) was downloaded from cBioPortal (48, 49) on January 12, 2021. The clinicopathological data of these patients have been summarized previously (50-52). The METABRIC dataset has disease-specific survival (DSS) data with mRNA expression levels (n=1,423) and data regarding radiotherapy (luminal A: without radiotherapy, n=192, with radiotherapy, n=269; luminal B: without radiotherapy, n=115, with radiotherapy, n=224).
The Cancer Genome Atlas (TCGA) PanCancer Atlas dataset. TCGA PanCancer Atlas (53-56) data were downloaded from cBioPortal on April 5, 2023. The clinicopathological data of these patients have been summarized previously (51). The TCGA PanCancer Atlas dataset contains DSS data with mRNA expression levels (n=880) and data regarding radiotherapy (luminal A: without radiotherapy, n=205, with radiotherapy, n=250; luminal B: without radiotherapy, n=77, with radiotherapy, n=97).
Prognostic analyses. The Kaplan–Meier method, log-rank (Cochran–Mantel–Haenszel) test and multivariate Cox regression analyses of DSS were performed as previously described (35, 50-52, 57-64). Briefly, to divide patients into groups of high and low p62 and ALDH1A3 gene expression, receiver operating characteristic curves were drawn using DSS data, and the Youden index was used as the optimal cutoff threshold (Table I). In multivariate Cox regression analyses, age at diagnosis was used as a confounding factor. Two-sided p-values below 0.05 were considered to indicate a significant difference. The statistical analyses were carried out using BellCurve for Excel ver. 4.05 (Social Survey Research Information Co., Ltd., SSRI, Tokyo, Japan).
Cell culture. BT-474 human luminal B breast cancer cells were obtained from the American Type Culture Collection (Manassas, VA, USA) and cultured as previously described (35) with 10% fetal bovine serum (FBS; Nichirei Biosciences Inc., Tokyo, Japan).
DsiRNA knockdown. p62 knockdown (KD) in BT-474 cells was performed using Dicer-Substrate siRNA (DsiRNA), which led to long-term suppression of gene expression (65). The sequences were as follows. p62 DsiRNA: sense strand: 5′-GUGAACUCCAGUC CCUACAGAUGCC-3′ and antisense strand: 5′-GGCAUCUGUA GGGACUGGAGUUCACCU-3′; and negative control DsiRNA: sense strand: 5′-CGUUAAUCGCGUAUAAUACGCGUAT-3′ and antisense strand: 5′-AUACGCGUAUUAUACGCGAUUAACGAC-3′ (Integrated DNA Technologies, Inc., IDT, Coralville, IA, USA). Transfection was performed using Lipofectamine RNAiMAX (Thermo Fisher Scientific, Inc., Waltham, MA, USA) and OPTI-MEM (Gibco, Thermo Fisher Scientific Inc.). The cells were transfected two times, 48 and 24 h before the ALDEFLUOR assay. KD efficiency was monitored using western blotting as previously described (35, 50, 57, 59). The following primary antibodies were used: mouse monoclonal anti-p62 (610833, 1:10,000; BD Biosciences, San Jose, CA, USA); mouse monoclonal anti-β-actin (60008-1-Ig, 1:10,000; ProteinTech Group, Inc., Rosemont, IL, USA). Goat anti-mouse IgG (7076S, 1:5,000; Cell Signaling Technology, Inc., Danvers, MA, USA) was used as secondary antibody.
X-ray irradiation and in vitro tumor sphere formation. ALDH1high cells were isolated from BT-474 cells after p62 DsiRNA KD using the ALDEFLUOR assay according to our previous reports (35, 50, 57-59, 61-63, 66). The in vitro tumor sphere formation assay was then conducted with ALDH1high cells upon p62 KD as previously described (35). The isolated ALDH1high BT-474 cells were plated in ultralow-attachment 96-well plates (5×103 cells/well) (Greiner Bio-One GmbH, Frickenhausen, Germany). After 20 h, the plated cells were irradiated with 4 or 8 Gy of X-ray (0.596 Gy/min) using Faxitron CellRad (Faxitron, Tucson, AZ, USA). The irradiated cells were cultured for 9 days after the day of seeding. Whole-well and tiled images were acquired and analyzed using a BZ-X800 microscope (Keyence, Osaka, Japan) and BZ-X800 analyzer (Keyence). The area of tumor-spheres larger than 1,256 μm2 was analyzed.
Bar graph data are presented as the mean±standard error of the mean (SEM) of three independent experiments, and data were analyzed by two-way ANOVA followed by Tukey’s test for post hoc analysis. Two-sided p-values below 0.05 were considered to indicate significant difference. In the beeswarm plots, all sphere sizes larger than 1,256 μm2 in all wells from three independent experiments were plotted. Beeswarm plots show the size distribution of individual spheres in detail. Bar graphs were drawn in GraphPad Prism ver. 9.5.1 (GraphPad Software, Boston, MA, USA) and beeswarm plots were drawn in R ver. 4.2.1 (The R Foundation for Statistical Computing, Vienna, Austria). The representative images of the tumor spheres were edited to brightness +50 with Microsoft photo ver.2023.11090.12017.0 (Microsoft Corporation, Redmond, WA, USA). The statistical analyses were carried out using BellCurve for Excel ver. 4.05 (SSRI).
Results
Luminal B breast cancer patients with high p62 expression had poorer clinical outcomes after radiotherapy than those with low expression. To assess the relationship between p62 expression and the benefit from radiotherapy in the luminal A and B subtypes, we analyzed the METABRIC and TCGA datasets by the Kaplan–Meier method and log-rank tests. Both without and with radiotherapy, p62high patients had poorer clinical outcomes than p62low patients in the luminal A group in the METABRIC dataset (Figure 1A). There was no significant difference in the survival rate of p62high or p62low patients treated either without or with radiotherapy in the luminal A group in the TCGA dataset (Figure 1B). However, there was no significant difference in the survival rate of p62high and p62low patients treated without radiotherapy, whereas p62high patients had poorer clinical outcomes than p62low patients treated with radiotherapy in the luminal B group in both datasets (Figure 1A and B).
Next, multivariate Cox regression analyses were performed with age at diagnosis as a confounding factor. p62high patients tended to have worse clinical outcomes than p62low patients treated with radiotherapy in the luminal B group in both datasets (Table II).
These results suggest that p62high luminal B breast cancer patients may not benefit from radiotherapy, whereas there is no association between p62high expression and the benefit of radiotherapy in luminal A breast cancer patients.
p62high and ALDH1A3high patients treated with radiotherapy had poorer clinical outcomes than other patients with luminal B breast cancer. We previously reported that p62high ALDH1A3high patients had poorer clinical outcomes than other patients with luminal B breast cancer (35). Therefore, we examined the prognosis of p62high ALDH1A3high patients treated without or with radiotherapy. p62high ALDH1A3high patients treated without and with radiotherapy had poorer clinical outcomes than other luminal A breast cancer patients in the METABRIC dataset (Figure 2A). However, there was no significant difference in the survival rate of p62high ALDH1A3high patients and other patients treated without or with radiotherapy in the luminal A group in the TCGA dataset (Figure 2B). However, there was no significant difference in the survival rate of p62high ALDH1A3high patients and other patients treated without radiotherapy, whereas p62high ALDH1A3high patients had poorer clinical outcomes than other patients treated with radiotherapy in the luminal B group in both the METABRIC and TCGA datasets (Figure 2A and B). The multivariate Cox regression analyses with age at diagnosis as a confounding factor using the same datasets showed worse clinical outcomes in p62high ALDH1A3high patients than other patients treated with radiotherapy in the luminal B group in the METABRIC dataset, and the same trend was observed in the TCGA dataset (Table II).
These results suggest that p62high contributes to the reduced efficacy of radiotherapy in patients with ALDH1A3high luminal B breast cancer, whereas there is no relationship between p62high and the benefit of radiotherapy in ALDH1A3high luminal A breast cancer.
The combination of p62 DsiRNA KD and X-ray irradiation suppressed in vitro tumor sphere formation by ALDH1high BT-474 cells. Based on the above results, we examined whether p62 is involved in the reduced efficacy of X-ray irradiation against ALDH1-positive luminal B breast CSCs. Therefore, we performed an in vitro tumor sphere formation assay using p62-deficient ALDH1high luminal B BT-474 cells exposed to X-ray irradiation. p62 is highly expressed in BT-474 cells (35). Western blotting analysis confirmed that the transfection of p62 DsiRNA into BT-474 cells efficiently reduced the p62 protein expression (Figure 3A). p62 DsiRNA suppressed the area of tumor sphere formation by ALDH1high BT-474 cells, consistent with our previous study (35) (main effect for p62 DsiRNA KD: p<0.001, Figure 3A and B). Tumor sphere formation by ALDH1high BT-474 cells was also suppressed by X-ray irradiation in a Gy-dependent manner (main effect for X-ray irradiation: p<0.001, Figure 3A and B). Furthermore, the combination of p62 DsiRNA KD and X-ray irradiation additively suppressed tumor sphere formation by ALDH1high BT-474 cells (interaction between p62 DsiRNA KD and X-ray irradiation was not statistically significant, Figure 3A and B). We also showed the size distribution of individual spheres as beeswarm plots (Figure 3C).
These results suggest that p62 is involved in the reduced efficacy of X-ray irradiation against ALDH1-positive luminal B breast CSCs.
Discussion
In this study, we showed that p62 deficiency and X-ray irradiation additively suppressed tumor sphere formation by ALDH1-positive luminal B breast CSCs. X-rays damage cancer cells by causing direct DNA double strand breaks and inducing production of reactive oxygen species (ROS) (67-70). Reports on the relationship between p62 expression and the effects of radiation are as follows: Arai et al. reported that p62 is involved in radiotherapy resistance via p62-dependent antioxidant signaling in hypopharyngeal carcinoma (45). Moreover, Li et al. reported that activation of the p62-Keap1-NRF2 pathway protects A549 cells against radiation-induced ferroptosis (71). However, there have also been reports that p62 contributes to radiosensitivity through suppressing chromatin ubiquitination and delaying DNA damage repair (72) or promoting autophagy-dependent senescence (73). Thus, the reduced effect of X-ray irradiation on ALDH1-positive luminal B breast CSCs may be due to protection from ROS damage by p62-Keap1-NRF2 antioxidant signals.
p62 is a multifunctional scaffold protein that binds to PKCλ/ι (36-44). Interestingly, we have reported that PKCλ/ι is involved in the regulation of the stemness properties of ALDH1A3-positive CSCs in breast and pancreatic cancer (58, 59, 63, 66). Thus, PKCλ/ι may contribute to the reduced effectiveness of radiotherapy against ALDH1A3-positive luminal B CSCs via interaction with p62.
In the present study, we revealed that p62high ALDH1A3high luminal B breast cancer is less susceptible to radiotherapy by analyzing two gene expression datasets. It has been reported that ALDH1high patients receiving radiotherapy for rectal cancer, head and neck squamous cell carcinoma and squamous cell cervical carcinoma have poorer outcomes (29-31). Thus, further research into the prognosis of p62high ALDH1A3high patients who receive radiotherapy for those cancers is needed. In addition to radiotherapy, luminal B breast cancer is usually treated by endocrine therapy, chemotherapy, and molecular targeted therapy. Therefore, revealing the effect of these drug therapies on p62high ALDH1A3high luminal B breast cancer is important.
Conclusion
In the present study, we revealed that p62high ALDH1A3high luminal B breast cancer is less susceptible to radiotherapy. We also showed that p62 deficiency and X-ray irradiation additively suppressed tumor sphere formation by ALDH1-positive luminal B breast CSCs. Therefore, we can conclude that p62 and ALDH1A3 are potential prognostic biomarkers for use in luminal B breast cancer patients treated with radiotherapy and that the combination of p62 inhibition and radiotherapy could be useful for targeted therapy against ALDH1-positive luminal B breast CSCs.
Acknowledgements
The Authors are extremely grateful to Dr. Nozaki Y. of Tokyo University of Science for the helpful discussion.
Footnotes
Authors’ Contributions
A.O., Y.M., Y.T., Y.N. C.O., and K.S. performed bioinformatic analyses; A.O., A.M., T.K. and Y.H. performed experiments; K.K, and M.T. performed X-ray irradiation; A.O., A.M., Y.M., Y.T. and K.S. performed data analyses; M.T., S.T., K.S. and S.O. supplied experimental materials and resources; A.O. and K.A. conceived the study; A.O. and K.A. drafted the manuscript; all the Authors contributed to the discussion and review of the final manuscript; and all the Authors approved the final manuscript.
Conflicts of Interest
The Authors declare that they have no conflicts of interest related to the contents of this article.
Funding
This work was supported by a Grant-in-Aid for Scientific Research (C) from the JSPS (20K07207) (K.A.), the JST Moonshot R&D (JPMJPS2022) (S.O.), a Tokyo University of Science Grant for President’s Research Promotion (K.A.), a Grant-in-Aid for Research Activity Start-up (21K20732) (S.T.), a Grant-in-Aid for Early-Career Scientists (23K14352) (S.T.), a Grant-in-Aid for Special Research in Subsidies for ordinary expenses of private schools from The Promotion and Mutual Aid Corporation for Private Schools of Japan (K.S.), a Grant from the Institute for Environmental & Gender-specific Medicine, Juntendo University (K.S.), JST SPRING (JPMJSP2151) (A.O., Y.N.), and Nagai Memorial Research Scholarship from the Pharmaceutical Society of Japan (A.O.).
- Received October 27, 2023.
- Revision received November 21, 2023.
- Accepted November 22, 2023.
- Copyright © 2024 International Institute of Anticancer Research (Dr. George J. Delinasios), All rights reserved.
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