Role of CD36 in cancer progression, stemness, and targeting.

CD36 is highly expressed in diverse tumor types and its expression correlates with advanced stages, poor prognosis, and reduced survival. In cancer cells, CD36: 1) increases fatty acid uptake, reprogramming lipid metabolism; 2) favors cancer cell proliferation, and 3) promotes epithelial-mesenchymal transition. Furthermore, CD36 expression correlates with the expression of cancer stem cell markers and CD36+ cancer cells display increased stemness functional properties, including clonogenicity, chemo- and radioresistance, and metastasis-initiating capability, suggesting CD36 is a marker of the cancer stem cell population. Thus, CD36 has been pointed as a potential therapeutic target in cancer. At present, at least three different types of molecules have been developed for reducing CD36-mediated functions: blocking monoclonal antibodies, small-molecule inhibitors, and compounds that knock-down CD36 expression. Herein, we review the role of CD36 in cancer progression, its participation in stemness control, as well as the efficacy of reported CD36 inhibitors in cancer cell cultures and animal models. Overall, the evidence compiled points that CD36 is a valid target for the development of new anti-cancer therapies.

Targeting effector pathways in RAC1P29S-driven malignant melanoma.

Malignant melanoma is characterized by mutations in a number of driver genes, most notably BRAF and NRAS. Recent genomic analyses revealed that 4-9% of sun-exposed melanomas bear activating mutations in RAC1, which encodes a small GTPase that is known to play key roles in cell proliferation, survival, and migration. The RAC1 protein activates several effector pathways, including Group A p21-activated kinases (PAKs), phosphoinositol-3-kinases (PI3Ks), in particular the beta isoform, and the serum-response factor/myocardin-related transcription factor (SRF/MRTF). Having previously shown that inhibition of Group A PAKs impedes oncogenic signalling from RAC1P29S, we here extend this analysis to examine the roles of PI3Ks and SRF/MRTF in melanocytes and/or in a zebrafish model. We demonstrate that a selective Group A PAK inhibitor (Frax-1036), a pan-PI3K (BKM120), and two PI3Kß inhibitors (TGX221, GSK2636771) impede the growth of melanoma cells driven by mutant RAC1 but not by mutant BRAF, while other PI3K selective inhibitors, including PI3Kα, δ and γ, are less effective. Using these compounds as well as an SRF/MRTF inhibitor (CCG-203,971), we observed similar results in vivo, using embryonic zebrafish development as a readout. These results suggest that targeting Group A PAKs, PI3Kß, and/or SRF/MRTF represent a promising approach to suppress RAC1 signalling in malignant melanoma.

Gpn3 Is Essential for Cell Proliferation of Breast Cancer Cells Independent of Their Malignancy Degree.

Successful therapies for patients with breast cancer often lose their initial effectiveness. Thus, identifying new molecular targets is a constant goal in the fight against breast cancer. Gpn3 is a protein required for RNA polymerase II nuclear targeting in both yeast and human cells. We investigated here the effect of suppressing Gpn3 expression on cell proliferation in a progression series of isogenic cell lines derived from the nontumorigenic MCF-10A breast cells that recapitulate different stages of breast carcinogenesis. Gpn3 protein levels were comparable in all malignant derivatives of the nontumorigenic MCF-10A cells. shRNA-mediated inhibition of Gpn3 expression markedly decreased cell proliferation in all MCF-10A sublines. A fraction of the largest RNA polymerase II subunit Rpb1 was retained in the cytoplasm, but most Rpb1 remained nuclear after suppressing Gpn3 in all cell lines studied. Long-term proliferation experiments in cells with suppressed Gpn3 expression resulted in the eventual loss of all isogenic cell lines but MCF-10CA1d.cl1. In MCF-10CA1d.cl1 cells, Gpn3 knockdown reduced the proliferation of breast cancer stem cells as evaluated by mammosphere assays. After the identification that Gpn3 plays a key role in cell proliferation in mammary epithelial cells independent of the degree of transformation, we also analyzed the importance of Gpn3 in other human breast cancer cell lines from different subtypes. Gpn3 was also required for cell proliferation and nuclear translocation of RNA polymerase II in such cellular models. Altogether, our results show that Gpn3 is essential for breast cancer cell proliferation regardless of the transformation level, indicating that Gpn3 could be considered a molecular target for the development of new antiproliferative therapies. Importantly, our analysis of public data revealed that Gpn3 overexpression was associated with a significant decrease in overall survival in patients with estrogen receptor-positive and Human epidermal growth factor receptor 2 (HER2+) breast cancer, supporting our proposal that targeting Gpn3 could potentially benefit patients with breast cancer.

Transcriptome-based identification of lovastatin as a breast cancer stem cell-targeting drug.

BACKGROUND: Breast cancer is a neoplastic disease with high morbidity and mortality in women worldwide. Breast cancer stem cells (CSCs) have a significant function in tumor growth, recurrence, and therapeutic resistance. Thus, CSCs have been pointed as targets of new therapies for breast cancer. Herein, we aimed to repurpose certain drugs as breast CSC-targeting agents. METHODS: We compared a consensus breast CSC signature with the transcriptomic changes that were induced by over 1300 bioactive compounds using Connectivity Map. The effects of the selected drugs on SOX2 promoter transactivation, SOX2 expression, viability, clonogenicity, and ALDH activity in breast cancer cells were analyzed by luciferase assay, western blot, MTT assay, mammosphere formation assay, and ALDEFLUOR® test, respectively. Gene Set Enrichment Analysis (GSEA) was performed using the gene expression data from mammary tumors of mice that were treated with lovastatin. RESULTS: Five drugs (fasudil, pivmecillinam, ursolic acid, 16,16-dimethylprostaglandin E2, and lovastatin) induced signatures that correlated negatively with the query CSC signature. In vitro, lovastatin inhibited SOX2 promoter transactivation, and reduced the efficiency of mammosphere formation and the percentage of ALDH+ cells. Mevalonate mitigated the effects of lovastatin, suggesting that the targeting of CSCs by lovastatin was mediated by the inhibition of its reported target, 3-hydroxy-3-methyl-glutaryl-coenzyme A reductase (HMGCR). By GSEA, lovastatin downregulated genes that are involved in stemness and invasiveness in mammary tumors, corroborating our in vitro findings. CONCLUSION: Lovastatin is a breast CSC-targeting drug. The inhibition of HMGCR might develop new adjuvant therapeutic strategies for breast tumors.

CCR5 Governs DNA Damage Repair and Breast Cancer Stem Cell Expansion.

The functional significance of the chemokine receptor CCR5 in human breast cancer epithelial cells is poorly understood. Here, we report that CCR5 expression in human breast cancer correlates with poor outcome. CCR5+ breast cancer epithelial cells formed mammospheres and initiated tumors with >60-fold greater efficiency in mice. Reintroduction of CCR5 expression into CCR5-negative breast cancer cells promoted tumor metastases and induced DNA repair gene expression and activity. CCR5 antagonists Maraviroc and Vicriviroc dramatically enhanced cell killing mediated by DNA-damaging chemotherapeutic agents. Single-cell analysis revealed CCR5 governs PI3K/Akt, ribosomal biogenesis, and cell survival signaling. As CCR5 augments DNA repair and is reexpressed selectively on cancerous, but not normal breast epithelial cells, CCR5 inhibitors may enhance the tumor-specific activities of DNA damage response-based treatments, allowing a dose reduction of standard chemotherapy and radiation.Significance: This study offers a preclinical rationale to reposition CCR5 inhibitors to improve the treatment of breast cancer, based on their ability to enhance the tumor-specific activities of DNA-damaging chemotherapies administered in that disease. Cancer Res; 78(7); 1657-71. ©2018 AACR.

Virtual screening-driven repositioning of etoposide as CD44 antagonist in breast cancer cells.

CD44 is a receptor for hyaluronan (HA) that promotes epithelial-to-mesenchymal transition (EMT), induces cancer stem cell (CSC) expansion, and favors metastasis. Thus, CD44 is a target for the development of antineoplastic agents. In order to repurpose drugs as CD44 antagonists, we performed consensus-docking studies using the HA-binding domain of CD44 and 11,421 molecules. Drugs that performed best in docking were examined in molecular dynamics simulations, identifying etoposide as a potential CD44 antagonist. Ligand competition and cell adhesion assays in MDA-MB-231 cells demonstrated that etoposide decreased cell binding to HA as effectively as a blocking antibody. Etoposide-treated MDA-MB-231 cells developed an epithelial morphology; increased their expression of E-cadherin; and reduced their levels of EMT-associated genes and cell migration. By gene expression analysis, etoposide reverted an EMT signature similarly to CD44 knockdown, whereas other topoisomerase II (TOP2) inhibitors did not. Moreover, etoposide decreased the proportion of CD44+/CD24- cells, lowered chemoresistance, and blocked mammosphere formation. Our data indicate that etoposide blocks CD44 activation, impairing key cellular functions that drive malignancy, thus rendering it a candidate for further translational studies and a potential lead compound in the development of new CD44 antagonists.