Abstract
Epithelial cancer cells can undergo an epithelial-mesenchymal transition (EMT), a complex genetic program that enables cells to break free from the primary tumor, breach the basement membrane, invade through the stroma and metastasize to distant organs. Myoferlin (MYOF), a protein involved in plasma membrane function and repair, is overexpressed in several invasive cancer cell lines. Depletion of myoferlin in the human breast cancer cell line MDA-MB-231 (MDA-231MYOFKD) reduced migration and invasion and caused the cells to revert to an epithelial phenotype. To test if this mesenchymal-epithelial transition was durable, MDA-231MYOFKD cells were treated with TGF-β1, a potent stimulus of EMT. After 48 hr with TGF-β1, MDA-231MYOFKD cells underwent an EMT. TGF-β1 treatment also decreased directional cell motility toward more random migration, similar to the highly invasive control cells. To probe the potential mechanism of MYOF function, we examined TGF-β1 receptor signaling. MDA-MB-231 growth and survival has been previously shown to be regulated by autocrine TGF-β1. We hypothesized that MYOF depletion may result in the dysregulation of TGF-β1 signaling, thwarting EMT. To investigate this hypothesis, we examined production of endogenous TGF-β1 and observed a decrease in TGF-β1 protein secretion and mRNA transcription. To determine if TGF-β1 was required to maintain the mesenchymal phenotype, TGF-β receptor signaling was inhibited with a small molecule inhibitor, resulting in decreased expression of several mesenchymal markers. These results identify a novel pathway in the regulation of autocrine TGF-β signaling and a mechanism by which MYOF regulates cellular phenotype and invasive capacity of human breast cancer cells.
Highlights
Metastatic breast cancer is the second leading cause of cancer mortality in women; second only to lung adenocarcinoma, the most lethal of all malignancies [1]
To investigate the plasticity of the cellular phenotype with reduced MYOF, MDA-231LVC and MDA-231MYOFKD were treated with 2 ng/mL transforming growth factor-β1 (TGF-β1), a potent stimulus of epithelialmesenchymal transition (EMT)
While the morphology of MDA231LVC was unchanged from the elongated mesenchymal morphology with TGF-β1 treatment, MDA-231MYOFKD cells exhibited a dramatic change from a cobblestonelike epithelial morphology to an elongated morphology, reduced cell-cell adhesion, and more pronounced, organized vimentin intermediate filaments, similar to the MDA-231LVC cellular phenotype (Figure 1A)
Summary
Metastatic breast cancer is the second leading cause of cancer mortality in women; second only to lung adenocarcinoma, the most lethal of all malignancies [1]. EMT is characterized by changes in cell morphology and protein expression such as down regulation of E-cadherin, the major epithelial cell-cell adhesion protein, and upregulation of vimentin, the principal mesenchymal intermediate filament protein [3]. Silencing of MYOF in the invasive breast cancer cell line MDA-MB-231 produced a dramatic morphology change from a mesenchymal to an epithelial phenotype. This change was accompanied by changes in protein expression, including upregulation of E-cadherin and downregulation of vimentin and fibronectin, indicating the cells had undergone a mesenchymal to epithelial transition (MET) [13]. Cells with MYOF knock down formed smaller, smooth edged tumors that did not invade into surrounding tissue in a mouse xenograft model, with similar results in a study on Lewis lung carcinoma [16, 17]
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