Dietary Phytochemicals as a Potential Source for Targeting Cancer Stem Cells

The efficacy of natural bioactive compounds against prostate cancer: Molecular targets and synergistic activities

Globally, prostate cancer (PCa) is regarded as a challenging health issue, and the number of PCa patients continues to rise despite the availability of effective treatments in recent decades. The current therapy with chemotherapeutic drugs has been largely ineffective due to multidrug resistance and the conventional treatment has restricted drug accessibility to malignant tissues, necessitating a higher dosage resulting in increased cytotoxicity. Plant-derived bioactive compounds have recently attracted a great deal of attention in the field of PCa treatment due to their potent effects on several molecular targets and synergistic effects with anti-PCa drugs. This review emphasizes the molecular mechanism of phytochemicals on PCa cells, the synergistic effects of compound-drug interactions, and stem cell targeting for PCa treatment. Some potential compounds, such as curcumin, phenethyl-isothiocyanate, fisetin, baicalein, berberine, lutein, and many others, exert an anti-PCa effect via inhibiting proliferation, metastasis, cell cycle progression, and normal apoptosis pathways. In addition, multiple studies have demonstrated that the isolated natural compounds: d-limonene, paeonol, lanreotide, artesunate, and bicalutamide have potential synergistic effects. Further, a significant number of natural compounds effectively target PCa stem cells. However, further high-quality studies are needed to firmly establish the clinical efficacy of these phytochemicals against PCa.

Identification of a novel combination treatment strategy in clear cell renal cell carcinoma stem cells with shikonin and ipilimumab

Background

Management of clear cell renal cell carcinoma (ccRCC) has changed rapidly in recent years with the advent of immune checkpoint inhibitors (ICIs). However, only a limited number of patients can sustainably respond to immune checkpoint inhibitors and many patients develop resistance to therapy, creating an additional need for therapeutic strategies to improve the efficacy of systemic therapies.

Methods

Binding probability and target genes prediction using online databases, invasion, migration, and apoptosis assays as well as the inhibition of cancer stem cells (CSCs) markers in ccRCC cell lines were used to select the most promising phytochemicals (PTCs). Mixed lymphocyte tumor cell culture (MLTC) system and flow cytometry were performed to confirm the potential combination strategy. The potential immunotherapeutic targets and novel CSC markers were identified via the NanoString analysis. The mRNA and protein expression, immune signatures as well as survival characteristics of the marker in ccRCC were analyzed via bioinformation analysis.

Results

Shikonin was selected as the most promising beneficial combination partner among 11 PTCs for ipilimumab for the treatment of ccRCC patients due to its strong inhibitory effect on CSCs, the significant reduction of FoxP3+ Treg cells in peripheral blood mononuclear cells (PBMCs) of patients and activation of the endogenous effector CD3+CD8+ and CD3+CD4+ T cells in response to the recognition of tumor specific antigens. Based on NanoString analysis VCAM1, CXCL1 and IL8 were explored as potential immunotherapeutic targets and novel CSC markers in ccRCC. The expression of VCAM1 was higher in the tumor tissue both at mRNA and protein levels in ccRCC compared with normal tissue, and was significantly positively correlated with immune signatures and survival characteristics in ccRCC patients.

Conclusion

We propose that a combination of shikonin and ipilimumab could be a promising treatment strategy and VCAM1 a novel immunotherapeutic target for the treatment of ccRCC.

Targeting oxeiptosis-mediated tumor suppression: a novel approach to treat colorectal cancers by sanguinarine

Oxeiptosis is a recently identified reactive oxygen species (ROS)-sensitive, caspase independent, non-inflammatory regulated cell death pathway. The activation of Kelch-like ECH-associated protein 1-Phosphoglycerate mutase 5-Apoptosis inducing factor mitochondria associated 1 (KEAP1-PGAM5-AIFM1) pathway is the key signaling event in the execution of oxeiptosis. In the present study, we demonstrate that sanguinarine (SNG), a quaternary benzophenanthridine alkaloid, induces oxeiptosis in human colorectal cancer (CRC) cells via ROS, specifically hydrogen peroxide (H₂O₂)-dependent activation of KEAP1-PGAM5-AIFM1 signaling axis. Whilst, knockdown of KEAP1, PGAM5, and AIFM1 largely abolishes SNG-induced oxeiptosis, hence reinforcing the importance of the role of this pathway in the SNG-mediated cytotoxicity. Moreover, extracellular addition of H₂O₂ sensitizes SNG-induced oxeiptosis in CRC cells, while removal of intracellular ROS by ROS scavengers, not only alleviated the overproduction of ROS caused by SNG, but also reversed the biochemical events associated with oxeiptosis. Finally, in vivo study demonstrates that SNG effectively reduces the tumor growth in HT-29 xenograft mouse model through features associated with oxeiptosis. This study highlights oxeiptosis as a novel tumor suppressive mechanism and further investigation of the role of oxeiptosis in cancer treatment is warranted.

Stem Cell Models for Breast and Colon Cancer: Experimental Approach for Drug Discovery

The progression of the early stages of female breast and colon cancer to metastatic disease represents a major cause of mortality in women. Multi-drug chemotherapy and/or pathway selective targeted therapy are notable for their off-target effects and are associated with spontaneous and/or acquired chemotherapy resistance and the emergence of premalignant chemo-resistant cancer-initiating stem cells. The stem cell populations are responsible for the evolution of therapy-resistant metastatic disease. These limitations emphasize an unmet need to develop reliable drug-resistant cancer stem cell models as novel experimental approaches for therapeutic alternatives in drug discovery platforms. Drug-resistant stem cell models for breast and colon cancer subtypes exhibit progressive growth in the presence of cytotoxic chemo-endocrine therapeutics. The resistant cells exhibit upregulated expressions of stem cell-selective cellular and molecular markers. Dietary phytochemicals, nutritional herbs and their constituent bioactive compounds have documented growth inhibitory efficacy for cancer stem cells. The mechanistic leads for the stem cell-targeted efficacy of naturally occurring agents validates the present experimental approaches for new drug discovery as therapeutic alternatives for therapy-resistant breast and colon cancer. The present review provides a systematic discussion of published evidence on (i) conventional/targeted therapy for breast and colon cancer, (ii) cellular and molecular characterization of stem cell models and (iii) validation of the stem cell models as an experimental approach for novel drug discovery of therapeutic alternatives for therapy-resistant cancers.

Stem Cell Models for Cancer Therapy

Metastatic progression of female breast and colon cancer represents a major cause of mortality in women. Spontaneous/acquired resistance to conventional and targeted chemo-endocrine therapy is associated with the emergence of drug-resistant tumor-initiating cancer stem cell populations. The cancer-initiating premalignant stem cells exhibit activation of select cancer cell signaling pathways and undergo epithelial-mesenchymal transition, leading to the evolution of a metastatic phenotype. The development of reliable cancer stem cell models provides valuable experimental approaches to identify novel testable therapeutic alternatives for therapy-resistant cancer. Drug-resistant stem cell models for molecular subtypes of clinical breast cancer and for genetically predisposed colon cancer are developed by selecting epithelial cells that survive in the presence of cytostatic concentrations of relevant therapeutic agents. These putative stem cells are characterized by the expression status of select cellular and molecular stem cell markers. The stem cell models are utilized as experimental approaches to examine the stem-cell-targeted growth inhibitory efficacy of naturally occurring dietary phytochemicals. The present review provides a systematic discussion on (i) conceptual and experimental aspects relevant to the chemo-endocrine therapy of breast and colon cancer, (ii) molecular/cellular aspects of cancer stem cells and (iii) potential stem-cell-targeting lead compounds as testable alternatives against the progression of therapy-resistant breast and colon cancer.

The Divergent Effects of Ovarian Steroid Hormones in the MCF-7 Model for Luminal A Breast Cancer: Mechanistic Leads for Therapy

The growth modulating effects of the ovarian steroid hormones 17β-estradiol (E₂) and progesterone (PRG) on endocrine-responsive target tissues are well established. In hormone-receptor-positive breast cancer, E₂ functions as a potent growth promoter, while the function of PRG is less defined. In the hormone-receptor-positive Luminal A and Luminal B molecular subtypes of clinical breast cancer, conventional endocrine therapy predominantly targets estrogen receptor function and estrogen biosynthesis and/or growth factor receptors. These therapeutic options are associated with systemic toxicity, acquired tumor resistance, and the emergence of drug-resistant cancer stem cells, facilitating the progression of therapy-resistant disease. The limitations of targeted endocrine therapy emphasize the identification of nontoxic testable alternatives. In the human breast, carcinoma-derived hormone-receptor-positive MCF-7 model treatment with E₂ within the physiological concentration range of 1 nM to 20 nM induces progressive growth, upregulated cell cycle progression, and downregulated cellular apoptosis. In contrast, treatment with PRG at the equimolar concentration range exhibits dose-dependent growth inhibition, downregulated cell-cycle progression, and upregulated cellular apoptosis. Nontoxic nutritional herbs at their respective maximum cytostatic concentrations (IC₉₀) effectively increase the E₂ metabolite ratio in favor of the anti-proliferative metabolite. The long-term exposure to the selective estrogen-receptor modulator tamoxifen selects a drug-resistant phenotype, exhibiting increased expressions of stem cell markers. The present review discusses the published evidence relevant to hormone metabolism, growth modulation by hormone metabolites, drug-resistant stem cells, and growth-inhibitory efficacy of nutritional herbs. Collectively, this evidence provides proof of the concept for future research directions that are focused on novel therapeutic options for endocrine therapy-resistant breast cancer that may operate via E₂- and/or PRG-mediated growth regulation.

Drug-Resistant Stem Cells: Novel Approach for Colon Cancer Therapy

BACKGROUND

Next to breast cancer, advanced stage metastatic colon cancer represents a major cause for mortality in women. Germline or somatic mutations in tumor suppressor genes or in DNA mismatch repair genes represent risk factors for genetic predisposition of colon cancer that are also detectable in sporadic colon cancer. Conventional chemotherapy for colon cancer includes combination of 5-fluoro-uracil with oxaliplatin and irinotecan or targeted therapy with non-steroid anti-inflammatory drugs and selective cyclooxygenase-2 inhibitors. Major limitations of these therapeutic interventions are associated with systemic toxicity, acquired tumor resistance and the emergence of drug resistant stem cells that favor initiation, progression and metastasis of therapy-resistant disease. These limitations emphasize an unmet need to identify tumor stem cell selective testable alternatives. Drug-resistant stem cell models facilitate the identification of new testable alternatives from natural phytochemicals and herbal formulations. The goal of this review is to provide an overview relevant to the current status of conventional/targeted therapy, the role of cancer stem cells and the status of testable alternatives for therapy-resistant colon cancer. Experimental models: Hyper-proliferative and tumorigenic cell lines from genetically predisposed colonic tissues of female mice represent experimental models. Chemotherapeutic agents select drug-resistant phenotypes that exhibit upregulated expressions of cellular and molecular stem cell markers. Mechanistically distinct natural phytochemicals effectively inhibit stem cell growth and downregulate the expressions of stem cell markers.

CONCLUSIONS

The present review discusses the status of colon cancer therapy and inherent limitations, cancer stem cell biology, potential lead compounds and their advantages over chemotherapy. The present experimental approaches will facilitate the identification of pharmacological and naturally-occurring agents as lead compounds for stem cell targeted therapy of colon cancer.

Reversal of cisplatin resistance by neferine/isoliensinine and their combinatorial regimens with cisplatin-induced apoptosis in cisplatin-resistant colon cancer stem cells (CSCs)

Cisplatin chemotherapy to the colorectal cancer cells (CRCs) is accompanied by dose-limiting adverse effects along with the acquisition of drug resistance implicating low therapeutic outcomes. The present study is aimed to evaluate the chemosensitizing efficacy of neferine/isoliensinine or combinatorial regimen of neferine/isoliensinine with cisplatin against CSCs (cisplatin resistant colon stem cells). CSCs were developed using pulse exposure of cisplatin to parental HCT-15 cells. Neferine/isoliensinine or combinatorial regimens of Neferine/isoliensinine and cisplatin exhibited a stronger cytotoxic activity against CSCs compared to control. IC₅₀ doses were found to be 6.5 μM for neferine, 12.5 μM for isoliensinine, and 120 μM for cisplatin respectively. Furthermore, the combinatorial regimen of a low dose of cisplatin (40 μM) with 4 μM neferine/8 μM isoliensinine induced cell death in a synergistic manner as described by isobologram. Neferine/isoliensinine could confer extensive intracellular reactive oxygen species generation in CSCs. Neferine/isoliensinine or combinatorial regimens dissipated mitochondrial membrane potential and enhanced intracellular [Ca²⁺ ]i, which were measured by spectroflurimetry. Furthermore, these combinatorial regimens induced a significant increase in the sub G0 phase of cell cycle arrest and PI uptake and alleviated the expression of ERCC1 in CSCs. Combinatorial regimens or neferine/isoliensinine treatments downregulated the cell survival protein expression (PI3K/pAkt/mTOR) and activated mitochondria-mediated apoptosis by upregulating Bax, cytochrome c, caspase-3, and PARP cleavage expression while downregulating the BCl-2 expression in CSCs. Our study confirms the chemosensitizing efficacy of neferine/isoliensinine or combinatorial regimens of neferine/isoliensinine with a low dose of cisplatin against CSCs.