The role of mTOR in SAF-1-mediated VEGF expression and breast cancer progression
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Breast cancer is a heterogeneous disease with a huge impact on the community evident from its high indices of morbidity and mortality worldwide. In recent years, research has been extensively directed toward unravelling the molecular basis of the pathogenesis in different molecular subtypes of breast cancer. Better understanding of the different breast cancer cells and the subsequent improvement of diagnostic and therapeutic modalities of the disease resulted in a promising decline in its morbidity and mortality. In addition to surgery, radiotherapy, and conventional chemotherapy, treatment of breast cancer, nowadays, involves more targeted therapeutic options. Treatment options of breast cancer are determined mainly by the tumor stage and molecular subtypes of the tumor cells. Invasive potentials of a tumor involve also its ability to recruit blood supply through a process known as angiogenesis. Angiogenesis is an intricate process mediated by the interaction of many players including the tumor cells and stroma as well as the endothelial cells in adjacent blood vessels. Vascular endothelial growth factor (VEGF) is one of the main proangiogenic molecules secreted by breast cancer cells to promote angiogenesis through binding to the vascular endothelial growth factor receptors (VEGFR) on target cells. Although angiogenesis is a hallmark of invasive solid tumors, it is also required in some normal physiological conditions. Targeting angiogenesis indiscriminately imposes a huge risk of enormous adverse effects. Therefore, the goal of research today is to explore potential mechanisms to counteract angiogenesis selectively in the tumor cells. Secretion of VEGF by breast cancer cells is activated by several stimuli including hypoxia, metabolic stress and inflammatory conditions. Hypoxia is one of the main drivers for both physiological and pathological angiogenesis. Hypoxia induces VEGF expression in hypoxic cells through the action of hypoxia inducible factor-1 (HIF-1). However, evidence indicates that breast cancer cells are capable of secreting VEGF at early stages before hypoxia ensues in the tumor mass. Evidence shows also that breast cancer cells secrete VEGF even under normoxic conditions, which suggests that VEGF expression in breast cancer, particularly at early stages, is mediated by a mechanism(s) other than hypoxia. The goal of our research is to investigate some of these potential mechanism(s). Unravelling these mechanism(s) could pave the road for potentially novel therapeutic modalities in the treatment of breast cancer. Previous work in our lab has identified a role of serum amyloid activating factor-1 (SAF-1), a transcription factor, in overexpression of VEGF in some breast cancer cells. Here, we report that the SAF-1-mediated VEGF expression in breast cancer cells is repressed by Kruppel like factor-4 (KLF-4) transcription factor. Our findings suggest also that KLF-4 is potentially involved in the repression of the VEGF expression in a SAF-1-independent manner. We found that the level of KLF-4 is lower in breast cancer cells compared to normal breast cells. Therefore, we further investigated possible mechanism(s) for upregulation of KLF-4 or downregulation of SAF-1 in breast cancer cells to curtail VEGF expression and counteract angiogenesis and thence progression of breast cancer. There is evidence in literature that inhibition of a serine-threonine kinase called mammalian target of rapamycin (mTOR) exerts antiproliferative effect in vascular smooth muscle cells (VSMC). Evidence showed that the effect of mTOR inhibition on VSMC is mediated by upregulation of KLF-4 through a yet unknown mechanism. Thus, we have explored a potential role of mTOR inhibition in upregulation of KLF-4 in breast cancer cells and inhibition of VEGF expression. VEGF secreted by breast cancer cells not only stimulates proliferation and migration of the endothelial cells, but also promotes proliferation and migration of the secreting breast cancer cells themselves. Mounting evidence substantiates the beneficial role of mTOR inhibition in breast cancer. Our results show a novel mechanism of upregulating KLF-4 and inhibition of VEGF expression in breast cancer cells through inhibition of mTOR. Nonetheless, we noted that the effects of mTOR inhibition on breast cancer cells, including upregulation of KLF-4 and inhibition of proliferation and migration, are variable among different breast cell lines as well as among different mTOR inhibitors. One of the possible explanation of the variable response to mTOR inhibitors is the rebound upregulation of proteins in the mTOR pathway which imposes a risk of emergence of resistance or refractoriness to treatment with mTOR inhibition. The role of active mTOR in the invasiveness of breast cancer is well known, yet relatively little is known about the role and impact of total mTOR protein. We found that total mTOR protein level is higher in breast cancer cells compared to their noncancerous counterparts. mTOR protein is particularly high in the ER+ breast cancer cells which constitute the majority of breast cancer cells subtypes. High mTOR protein level in breast cancer cells could be attributed to decreased mTOR protein degradation, increased mTOR gene expression, or both. Our results indicate a defective degradation of mTOR in breast cancer cells compared to normal cells, which could, at least in part, explain why mTOR protein is high in breast cancer cells. Our results show also that transcription of mTOR gene is elevated in the ER+ breast cancer cells. Our study revealed that promoter region of mTOR in the ER+ breast cancer cells has a truncated dinucleotide tandem repeat region. Tandem repeats are a potential site for regulation of transcription of genes. Shortening of this region could be a possible mechanism of increased transcription of mTOR gene in the ER+ breast cancer cells. Moreover, we have revealed a novel mechanism of increasing mTOR degradation as well as inhibition of MTOR transcription in the ER+ breast cancer cells by treatment with metformin the antidiabetic mTOR inhibitor. In correlation, metformin treatment induced a profound effect on upregulation of KLF-4 and inhibition of proliferation and migration of the breast cancer cells, particularly, the ER+ subtype. These findings could be utilized in the optimization of chemotherapeutic regimens of breast cancer.
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