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STXBP6, reciprocally regulated with autophagy, reduces triple negative breast cancer aggressiveness

ABSTRACT: Abstract Background Although autophagy plays a dual role in suppressing or promoting certain cancers, the nature of its involvement in breast cancers remains unclear. Here, we investigated the function of STXBP6, a protein regulating the autophagy?associated SNARE complex, in triple negative breast cancer (TNBC). Results We report that STXBP6 is profoundly downregulated in TNBC specimens in association with reduced overall patient survival. Notably, we found that STXBP6 promoter was specifically hyper?methylated in TNBC specimens. Ectopic expression of STXBP6 inhibited TNBC cell proliferation in cellular and mouse models. Mass spectrometric analysis revealed physical interactions of STXBP6 with a number of autophagy?related proteins including SNX27, a molecule involved in endocytosis of plasma membrane receptors and protein trafficking. Overexpression of STXBP6 elicited autophagy through inhibition of mTORC1 signaling. Reciprocally, induction of autophagy rescued STXBP6 expression by inhibiting EZH2 and altering STXBP6 methylation. The mutual regulation between STXBP6 and autophagy was replicated in luminal breast cancer cells only when estrogen receptor (ER) activation was abrogated. Ectopic expression of STXBP6 significantly reduced TNBC cells’ migratory ability in vitro and tumor metastasis in vivo. Conclusions Our results unveil a role of STXBP6 in TNBC that highlights a new paradigm in autophagy regulation. Our results significantly enhance the understanding of the mechanisms of TNBC aggressiveness, which might help in designing novel therapies targeting TNBC.

SUBMITTER: Lenka G

PROVIDER: S-EPMC7418817 | biostudies-literature | 2020 Jul

REPOSITORIES: biostudies-literature

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Project description:IntroductionCancer stem cells (CSCs) drive the initiation, maintenance, and therapy response of breast tumors. CD49f is expressed in breast CSCs and functions in the maintenance of stemness. Thus, blockade of CD49f is a potential therapeutic approach for targeting breast CSCs. In the present study, we aimed to repurpose drugs as CD49f antagonists.Materials and methodsWe performed consensus molecular docking using a subdomain of CD49f that is critical for heterodimerization and a collection of pharmochemicals clinically tested. Molecular dynamics simulations were employed to further characterize drug-target binding. Using MDA-MB-231 cells, we evaluated the effects of potential CD49f antagonists on 1) cell adhesion to laminin; 2) mammosphere formation; and 3) cell viability. We analyzed the effects of the drug with better CSC-selectivity on the activation of CD49f-downstream signaling by Western blot (WB) and co-immunoprecipitation. Expressions of the stem cell markers CD44 and SOX2 were analyzed by flow cytometry and WB, respectively. Transactivation of SOX2 promoter was evaluated by luciferase reporter assays. Changes in the number of CSCs were assessed by limiting-dilution xenotransplantation.ResultsPranlukast, a drug used to treat asthma, bound to CD49f in silico and inhibited the adhesion of CD49f+ MDA-MB-231 cells to laminin, indicating that it antagonizes CD49f-containing integrins. Molecular dynamics analysis showed that pranlukast binding induces conformational changes in CD49f that affect its interaction with β1-integrin subunit and constrained the conformational dynamics of the heterodimer. Pranlukast decreased the clonogenicity of breast cancer cells on mammosphere formation assay but had no impact on the viability of bulk tumor cells. Brief exposure of MDA-MB-231 cells to pranlukast altered CD49f-dependent signaling, reducing focal adhesion kinase (FAK) and phosphatidylinositol 3-kinase (PI3K) activation. Further, pranlukast-treated cells showed decreased CD44 and SOX2 expression, SOX2 promoter transactivation, and in vivo tumorigenicity, supporting that this drug reduces the frequency of CSC.ConclusionOur results support the function of pranlukast as a CD49f antagonist that reduces the CSC population in triple-negative breast cancer cells. The pharmacokinetics and toxicology of this drug have already been established, rendering a potential adjuvant therapy for breast cancer patients.

Project description:The tumor microenvironment (TME) plays key roles in promoting disease progression in the aggressive triple-negative subtype of breast cancer (TNBC; Basal/Basal-like). Here, we took an integrative approach and determined the impact of tumor-stroma-inflammation networks on pro-metastatic phenotypes in TNBC. With the TCGA dataset we found that the pro-inflammatory cytokines tumor necrosis factor ? (TNF?) and interleukin 1? (IL-1?), as well as their target pro-metastatic chemokines CXCL8 (IL-8), CCL2 (MCP-1), and CCL5 (RANTES) were expressed at significantly higher levels in basal patients than luminal-A patients. Then, we found that TNF?- or IL-1?-stimulated co-cultures of TNBC cells (MDA-MB-231, MDA-MB-468, BT-549) with mesenchymal stem cells (MSCs) expressed significantly higher levels of CXCL8 compared to non-stimulated co-cultures or each cell type alone, with or without cytokine stimulation. CXCL8 was also up-regulated in TNBC co-cultures with breast cancer-associated fibroblasts (CAFs) derived from patients. CCL2 and CCL5 also reached the highest expression levels in TNF?/IL-1?-stimulated TNBC:MSC/CAF co-cultures. The elevations in CXCL8 and CCL2 expression partly depended on direct physical contacts between the tumor cells and the MSCs/CAFs, whereas CCL5 up-regulation was entirely dependent on cell-to-cell contacts. Supernatants of TNF?-stimulated TNBC:MSC "Contact" co-cultures induced robust endothelial cell migration and sprouting. TNBC cells co-cultured with MSCs and TNF? gained migration-related morphology and potent migratory properties; they also became more invasive when co-cultured with MSCs/CAFs in the presence of TNF?. Using siRNA to CXCL8, we found that CXCL8 was significantly involved in mediating the pro-metastatic activities gained by TNF?-stimulated TNBC:MSC "Contact" co-cultures: angiogenesis, migration-related morphology of the tumor cells, as well as cancer cell migration and invasion. Importantly, TNF? stimulation of TNBC:MSC "Contact" co-cultures in vitro has increased the aggressiveness of the tumor cells in vivo, leading to higher incidence of mice with lung metastases than non-stimulated TNBC:MSC co-cultures. Similar tumor-stromal-inflammation networks established in-culture with luminal-A cells demonstrated less effective or differently-active pro-metastatic functions than those of TNBC cells. Overall, our studies identify novel tumor-stroma-inflammation networks that may promote TNBC aggressiveness by increasing the pro-malignancy potential of the TME and of the tumor cells themselves, and reveal key roles for CXCL8 in mediating these metastasis-promoting activities.

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STXBP6, reciprocally regulated with autophagy, reduces triple negative breast cancer aggressiveness

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