Evaluation of 6-mercaptopurine in a cell culture model of adaptable triple-negative breast cancer with metastatic potential

TET Enzymes and 5hmC Levels in Carcinogenesis and Progression of Breast Cancer: Potential Therapeutic Targets

Breast Cancer (BC) was the most common female cancer in incidence and mortality worldwide in 2020. Similarly, BC was the top female cancer in the USA in 2022. Risk factors include earlier age at menarche, oral contraceptive use, hormone replacement therapy, high body mass index, and mutations in BRCA1/2 genes, among others. BC is classified into Luminal A, Luminal B, HER2-like, and Basal-like subtypes. These BC subtypes present differences in gene expression signatures, which can impact clinical behavior, treatment response, aggressiveness, metastasis, and survival of patients. Therefore, it is necessary to understand the epigenetic molecular mechanism of transcriptional regulation in BC, such as DNA demethylation. Ten-Eleven Translocation (TET) enzymes catalyze the oxidation of 5-methylcytosine (5mC) to 5-hydroxymethylcytosine (5hmC) on DNA, which in turn inhibits or promotes the gene expression. Interestingly, the expression of TET enzymes as well as the levels of the 5hmC epigenetic mark are altered in several types of human cancers, including BC. Several studies have demonstrated that TET enzymes and 5hmC play a key role in the regulation of gene expression in BC, directly (dependent or independent of DNA de-methylation) or indirectly (via interaction with other proteins such as transcription factors). In this review, we describe our recent understanding of the regulatory and physiological function of the TET enzymes, as well as their potential role as biomarkers in BC biology.

Sensitization of Resistant Cells with a BET Bromodomain Inhibitor in a Cell Culture Model of Deep Intrinsic Resistance in Breast Cancer

We treated highly metabolically adaptable (SUM149-MA) triple-negative inflammatory breast cancer cells and their control parental SUM149-Luc cell line with JQ1 for long periods to determine its efficacy at inhibiting therapy-resistant cells. After 20 days of treatment with 1-2 µM of JQ1, which killed majority of cells in the parental cell line, a large number of SUM149-MA cells survived, consistent with their pan-resistant nature. Interestingly, though, the JQ1 treatment sensitized resistant cancer cells in both the SUM149-MA and SUM149-Luc cell lines to subsequent treatment with doxorubicin and paclitaxel. To measure JQ1-mediated sensitization of resistant cancer cells, we first eradicated approximately 99% of relatively chemotherapy-sensitive cancer cells in culture dishes by long treatments with doxorubicin or paclitaxel, and then analyzed the remaining resistant cells for survival and growth into colonies. In addition, combination, rather than sequential, treatment with JQ1 and doxorubicin was also effective in overcoming resistance. Notably, Western blotting showed that JQ1-treated cancer cells had significantly lower levels of PD-L1 protein than did untreated cells, indicating that JQ1 treatment may reduce tumor-mediated immune suppression and improve the response to immunotherapy targeting PD-L1. Finally, JQ1 treatment with a low 62.5 nM dose sensitized another resistant cell line, FC-IBC02-MA, to treatment with doxorubicin and paclitaxel.

Gold nanoparticles loaded chitosan encapsulate 6-mercaptopurine as a novel nanocomposite for chemo-photothermal therapy on breast cancer

Background

As a promising strategy to overcome the therapeutic disadvantages of 6-mercaptopurine (6MP), we proposed the encapsulation of 6MP in chitosan nanoparticles (CNPs) to form the 6MP-CNPs complexes. The encapsulation was followed by the loading of complexes on gold nanoparticles (AuNPs) to generate a novel 6MP-CNPs-AuNPs nanocomposite to facilitate the chemo-photothermal therapeutic effect.

Methods

CNPs were produced based on the ionic gelation method of tripolyphosphate (TPP). Moreover, 6MP-CNPs composite were prepared by the modified ionic gelation method and then loaded on AuNPs which were synthesized according to the standard wet chemical method using trisodium citrate as a reducing and capping agent. The synthesized nanocomposites were characterized by UV–VIS spectroscopy, dynamic light scattering, Fourier transform infrared spectroscopy, and transmission electron microscopy. The potential cytotoxicity of the prepared nanocomposites on MCF7 cell line was carried out using Sulphorhodamine-B (SRB) assay.

Results

Optimization of CNPs, 6MP-CNPs, and 6MP-CNPs-AuNPs revealed 130 ± 10, 200 ± 20, and 25 ± 5 nm particle size diameters with narrow size distributions and exhibited high stability with zeta potential 36.9 ± 4.11, 37, and 44.4 mV, respectively. The encapsulation efficiency of 6MP was found to be 57%. The cytotoxicity of 6MP-CNPs and 6MP-CNPs-AuNPs on breast cell line MCF7 was significantly increased and reached IC50 of 9.3 and 8.7 µM, respectively. The co-therapeutic effect of the nanocomposites resulted in an improvement of the therapeutic efficacy compared to the individual effect of chemo- and photothermal therapy. Irradiation of 6MP-CNPs and 6MP-CNPs-AuNPs with a diode laser (DPSS laser, 532 nm) was found to have more inhibition on cell viability with a decrease in IC50 to 5 and 4.4 µM, respectively.

Conclusion

The Chemo-Photothermal co-therapy treatment with novel prepared nanocomposite exhibits maximum therapeutic efficacy and limits the dosage-related side effects of 6MP.

Graphical Abstract

Sensitization of Resistant Breast Cancer Cells with a Jumonji Family Histone Demethylase Inhibitor

Simple Summary

Using a cell culture model of resistant breast cancer cells with the phenotype that is often responsible for the early relapse of triple-negative breast cancer, namely, the persistence of these cells in reversible quiescence under a variety of challenges, we found that reprogramming the epigenome by treatment with JIB-04, a small-molecule inhibitor of Jumonji-family histone demethylases, sensitized resistant cells. We used this model of deep intrinsic resistance featuring many molecular mechanisms of achieving this phenotype to perform lengthy evaluations of less cytotoxic doses of JIB-04. We found that resistant cells derived from triple-negative inflammatory breast cancer cell lines were either much more sensitive to JIB-04 than the parental cell line or altered by the treatment such that they became sensitive to the chemotherapeutic drugs paclitaxel and doxorubicin. Notably, JIB-04 exposure increased PD-L1 expression in cancer cells, which means that JIB-04 may have clinical applications in improving the responses of triple-negative breast cancer to anti-PD-L1 therapy.

Abstract

In the present study, we evaluated JIB-04, a small-molecule epigenetic inhibitor initially discovered to inhibit cancer growth, to determine its ability to affect deep intrinsic resistance in a breast cancer model. The model was based on a function-based approach to the selection of cancer cells in a cell culture that can survive a variety of challenges in prolonged, but reversible, quiescence. These resistant cancer cells possessed a variety of mechanisms, including modifications of the epigenome and transcriptome, for generating a high degree of cellular heterogeneity. We found that long pretreatment with JIB-04 sensitized resistant triple-negative inflammatory breast cancer cells and their parental cell line SUM149 to the chemotherapeutic drugs doxorubicin and paclitaxel. Resistant cancer cells derived from another inflammatory breast cancer cell line, FC-IBC02, were considerably more sensitive to JIB-04 than the parental cell line. Investigating a mechanism of sensitization, we found that JIB-04 exposure increased the expression of PD-L1 in resistant cells, suggesting that JIB-04 may also sensitize resistant breast cancer cells to anti-PD-L1 immune therapy. Finally, these results support the usefulness of a cell culture-based experimental strategy for evaluating anticancer agents, such as JIB-04, that may halt cancer evolution and prevent the development of cancer resistance to currently used therapies.

Specific MRP4 Inhibitor Ceefourin-1 Enhances Apoptosis Induced by 6-Mercaptopurine in Jurkat Leukemic Cells, but Not in Normal Lymphoblast Cell Line CRL-1991

Background and objectives: The multidrug resistance protein 4 (MRP4) is a member of the ABC transporter, which has been extensively related to many types of cancer including leukemia. MRP4 overexpression and activity over the efflux of some chemotherapeutic drugs are the main causes of chemoresistance. 6-mercaptopurine (6-MP) is a chemotherapeutic drug widely used in the consolidation and maintenance phases of leukemia treatment. However, 6-MP is a substrate of MRP4, which decreases its chemotherapeutic efficacy. Current research is focused on the development of MRP4 inhibitors to combat chemoresistance by allowing the accumulation of the drug substrates inside the cells. To date, the only specific MRP4 inhibitor that has been developed is ceefourin-1, which has been reported to inhibit MRP4 in many cancer cells and which makes it an excellent candidate to enhance the activity of 6-MP in a combined treatment in vitro of leukemic cells. Materials and methods: in the present work, we determined the enhancing activity of ceefourin-1 on the antiproliferative and apoptotic effect of 6-MP in leukemic Jurkat cells by trypan blue assay and flow cytometry. Besides, we determined the 6-MP and ceefourin-1 binding sites into MRP4 by molecular docking and molecular dynamics. Results: ceefourin-1 enhanced the apoptotic activity of 6-MP in Jurkat cells, while in CRL-1991 cells both antiproliferative and apoptotic effect were significantly lower. Ceefourin-1 additively cooperates with 6-MP to induce apoptosis in leukemic cells, but normal lymphoblast CRl-1991 showed resistance to both drugs. Conclusion: ceefourin-1 and 6-MP cooperates to trigger apoptosis in leukemic Jurkat cells, but the full mechanism needs to be elucidated in further works. In addition, our perspective is to test the cooperation between ceefourin-1 and 6-MP in samples from patients and healthy donnors.

Inhibition of resistant triple-negative breast cancer cells with low-dose 6-mercaptopurine and 5-azacitidine

Highly adaptable breast cancer cells that can opportunistically switch between proliferation and quiescence are often responsible for disease relapse. We have developed a function-based selection strategy for such resistant cells, exemplified by SUM149-MA and FC-IBC02-MA triple-negative breast cancer cells. We have also reported that a lengthy treatment with low-dose 6-mercaptopurine, a clinically useful anti-inflammatory drug, inhibits such resistant cells. To more rigorously test the clinical suitability of 6-mercaptopurine, here we investigated effects of further lowering its dose and the possibility of overcoming resistance to single-drug treatment by combining the drug with another ribonucleoside analog 5-azacitidine. We found that that a lengthy treatment with 1 μM 5-azacitidine, without a significant effect on cell proliferation, sensitized cancer cells to the inhibitory effects of low-dose 6-mercaptopurine. Importantly, treatment for several weeks with low doses of 6-mercaptopurine and/or 5-azacitidine did not render cancer cells resistant to chemotherapeutic drugs doxorubicin or paclitaxel. In fact, the cells became more sensitive to chemotherapeutic drugs upon treatment with 6-mercaptopurine and/or 5-azacitidine. Our analyses of protein markers of epithelial-to-mesenchymal transition indicated that treatments with 6-mercaptopurine and/or 5-azacitidine do not significantly reverse this process in our model. Our results showed that safe drugs such as low-dose 6-mercaptopurine singly or combined with 5-azacitidine, which are suitable for use prior to disease relapse, have a potential of inhibiting highly resistant triple-negative breast cancer cells.