Fatty acid synthase phosphorylation: a novel therapeutic target in HER2-overexpressing breast cancer cells

Introduction

The human epidermal growth factor receptor 2 (HER2) is a validated therapeutic target in breast cancer. Heterodimerization of HER2 with other HER family members results in enhanced tyrosine phosphorylation and activation of signal transduction pathways. HER2 overexpression increases the translation of fatty acid synthase (FASN), and FASN overexpression markedly increases HER2 signaling, which results in enhanced cell growth. However, the molecular mechanism and regulation of HER2 and FASN interaction are not well defined. Lapatinib is a small-molecule tyrosine kinase inhibitor that blocks phosphorylation of the epidermal growth factor receptor and HER2 in breast cancer cells, resulting in apoptosis. We hypothesized that FASN is directly phosphorylated by HER2, resulting in enhanced signaling and tumor progression in breast cancer cells.

Methods

Using mass spectrometry, we identified FASN as one of the proteins that is dephosphorylated by lapatinib in SKBR3 breast cancer cells. Immunofluorescence, immunoprecipitation, Western blotting, a kinase assay, a FASN enzymatic activity assay, an invasion assay, a cell viability assay and zymography were used to determine the role of FASN phosphorylation in invasion of SKBR3 and BT474 cells. The FASN inhibitor C75 and small interfering RNA were used to downregulate FASN expression and/or activity.

Results

Our data demonstrated that FASN is phosphorylated when it is in complex with HER2. FASN phosphorylation was induced by heregulin in HER2-overexpressing SKBR3 and BT474 breast cancer cells. Heregulin-induced FASN phosphorylation resulted in increased FASN enzymatic activity, which was inhibited by lapatinib. The FASN inhibitor C75 suppressed FASN activity by directly inhibiting HER2 and FASN phosphorylation. Blocking FASN phosphorylation and activity by lapatinib or C75 suppressed the activity of matrix metallopeptidase 9 and inhibited invasion of SKBR3 and BT474 cells.

Conclusions

FASN phosphorylation by HER2 plays an important role in breast cancer progression and may be a novel therapeutic target in HER2-overexpressing breast cancer cells.

Intracrine VEGF signalling mediates colorectal cancer cell migration and invasion

Background:

Vascular endothelial growth factor (VEGF) and its receptors (VEGFRs) are key regulators of angiogenesis, affecting endothelial cell survival and function. However, the effect of VEGF-VEGFR signalling on tumour cell function is not well understood. Our previous studies in colorectal cancer (CRC) cells have demonstrated an intracrine VEGF/VEGFR1 signalling mechanism that mediates CRC cell survival and chemo-sensitivity. Since extracellular VEGF signalling regulates migration of endothelial cells and various tumour cells, we attempted to determine whether intracrine VEGF signalling affects CRC cell motility.

Methods:

Migration and invasion of CRC cells, with and without VEGF or VEGFR1 depletion, were assayed using transwell migration chambers. Changes in cell morphology, epithelial-mesenchymal transition (EMT) markers, and markers of cell motility were assessed by immunostaining and western blot.

Results:

Depletion of intracellular VEGF and VEGFR1 in multiple CRC cell lines led to strong inhibition of migration and invasion of CRC cells. Except for Twist, there were no significant differences in markers of EMT between control and VEGF/VEGFR1-depleted CRC cells. However, VEGF/VEGFR1-depleted CRC cells demonstrated a significant reduction in levels of phosphorylated focal adhesion kinase and its upstream regulators pcMET and pEGFR.

Conclusions:

Inhibition of intracrine VEGF signalling strongly inhibits CRC cell migration and invasion by regulating proteins involved in cell motility.

Rictor phosphorylation on the Thr-1135 site does not require mammalian target of rapamycin complex 2

In animal cells, growth factors coordinate cell proliferation and survival by regulating the phosphoinositide 3-kinase/Akt signaling pathway. Deregulation of this signaling pathway is common in a variety of human cancers. The PI3K-dependent signaling kinase complex defined as mammalian target of rapamycin complex 2 (mTORC2) functions as a regulatory Ser-473 kinase of Akt. We find that activation of mTORC2 by growth factor signaling is linked to the specific phosphorylation of its component rictor on Thr-1135. The phosphorylation of this site is induced by the growth factor stimulation and expression of the oncogenic forms of ras or PI3K. Rictor phosphorylation is sensitive to the inhibition of PI3K, mTOR, or expression of integrin-linked kinase. The substitution of wild-type rictor with its specific phospho-mutants in rictor null mouse embryonic fibroblasts did not alter the growth factor-dependent phosphorylation of Akt, indicating that the rictor Thr-1135 phosphorylation is not critical in the regulation of the mTORC2 kinase activity. We found that this rictor phosphorylation takes place in the mTORC2-deficient cells, suggesting that this modification might play a role in the regulation of not only mTORC2 but also the mTORC2-independent function of rictor.

Intracrine VEGF Signaling Mediates the Activity of Prosurvival Pathways in Human Colorectal Cancer Cells

The effects of vascular endothelial growth factor-A (VEGF-A/VEGF) and its receptors on endothelial cells function have been studied extensively, but their effects on tumor cells are less well defined. Studies of human colorectal cancer cells where the VEGF gene has been deleted suggest an intracellular role of VEGF as a cell survival factor. In this study, we investigated the role of intracrine VEGF signaling in colorectal cancer cell survival. In human colorectal cancer cells, RNAi-mediated depletion of VEGF decreased cell survival and enhanced sensitivity to chemotherapy. Unbiased reverse phase protein array studies and subsequent validation experiments indicated that impaired cell survival was a consequence of disrupted AKT and ERK1/2 (MAPK3/1) signaling, as evidenced by reduced phosphorylation. Inhibition of paracrine or autocrine VEGF signaling had no effect on phospho-AKT or phospho-ERK1/2 levels, indicating that VEGF mediates cell survival via an intracellular mechanism. Notably, RNAi-mediated depletion of VEGF receptor VEGFR1/FLT1 replicated the effects of VEGF depletion on phospho-AKT and phospho-ERK1/2 levels. Together, these studies show how VEGF functions as an intracrine survival factor in colorectal cancer cells, demonstrating its distinct role in colorectal cancer cell survival. Cancer Res; 76(10); 3014-24. ©2016 AACR.

Abstract 3255: Inhibition of intracrine VEGF signaling prevents colorectal cancer cell migration and invasion

Introduction: Vascular endothelial growth factor (VEGF) is a key regulator of angiogenesis and vascular function. The mechanisms of VEGF signaling in angiogenesis have been extensively studied. However, its effects on tumor cell function remain to be elucidated. Our studies on depletion of VEGF by somatic knockout of the VEGF gene or by siRNA in human colorectal cancer (CRC) cell lines have demonstrated that loss of VEGF expression led to significantly decreased cell growth, increased apoptosis, and enhanced chemo-sensitivity of CRC cells through an intracrine signaling mechanism. These intracrine effects were mediated through VEGF by regulation of the activity of multiple receptor tyrosine kinases (e.g. EGFR, cMET) and downstream AKT signaling. Similar effects were observed by depletion of VEGF receptor 1 (VEGFR1), supporting the role of a novel intracellular VEGF-VEGFR1 complex in CRC cell survival. Since VEGF signaling has been shown to effect endothelial and tumor cell migration, we attempted to determine if inhibition of intracrine VEGF signaling affected CRC cell motility.

Methods: Migration and invasion of CRC cells, with and without VEGF depletion, were assayed using Boyden chambers and a serum gradient. Changes in cell morphology and epithelial to mesenchymal transition (EMT) markers and markers of cell motility were assessed by microscopy, immunostaining, and western blot analyses.

Results: Depletion of VEGF by siRNAs in multiple CRC cell lines led to a very strong inhibition of migration and invasion of CRC cells. Such inhibition was not observed when CRC cells were treated with bevacizumab to inhibit paracrine or autocrine signaling. Further examination revealed no significant changes in cell morphology. Other than Twist, there were no significant differences between specific markers of EMT indicating that differences in migration and invasion were not due to classic alterations in EMT. However, significant changes in activation of mediators of cell motility were observed. VEGF depleted CRC cells demonstrated significantly lower levels of phosphorylated focal adhesion kinase (p-FAK) compared to cells with normal VEGF expression. Analyses of upstream regulators indicated strong reduction in either p-cMET or p-SRC depending on cell type.

Conclusions: Inhibition of intracrine VEGF signaling strongly inhibits CRC cell migration and invasion by regulating proteins involved in cell motility.

Citation Format: Rajat Bhattacharya, Ling Xia, Fan Fan, Rui Wang, Delphine Boulbes, Xiang-Cang Ye, Lee Ellis. Inhibition of intracrine VEGF signaling prevents colorectal cancer cell migration and invasion. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 3255.

Abstract P3-09-03: microRNAs and Target Gene Networks in Breast Cancer Cell Lines

Background: MicroRNAs (miRNAs) are a class of small endogenous RNA molecules that post-transcriptionally inhibit gene expression. Several miRNAs have been involved in the regulation of several cellular processes during oncogenesis.

Objective: We simultaneously analyzed 18 different breast cancer cell lines (BCC) for miRNA and mRNA expression profiles in order to identify regulatory miRNA target gene networks and novel miRNA-directed therapeutic targets.

Materials and Methods: We preprocessed the mRNA and miRNA expression data using R software (justRMA) and performed quantile normalization using background-subtracted log ratio distributions across arrays. We identified 39 hsa-miRNAs with the largest variation across samples and examined the Spearman correlations of miRNAs with the Affymetrix probesets. We calculated false discovery rates for probe sets with stringent correlation coefficients. We performed hierarchical clustering using the 39 miRNAs and correlated dendograms branches with basal/luminal classes. Potential miRNA target genes were identified. We performed pathway analyses by using all 39 miRNA target genes. Western blot analyses were performed to confirm regulation of specific proteins. Results: We demonstrated 39 deregulated miRNAs and 10612 mRNA targets genes in BCC. Twenty-three miRNAs were upregulated in basal and downregulated in the luminal groups targeting 6790 genes, and 16 miRNAs downregulated in basal and upregulated in luminal cells, targeting 3822 genes. The first direct comparison screening for the 39 miRNAs and all mRNAs identified 29 direct pairs where the correlation magnitude was above 0.9. MiR-221/222, -31, -143/145, -34a, -200 were the most and consistently deregulated miRNAs in all luminal and basal like BCC. We identified genes, such as CD59, PDGF-β and discoidin domain receptor tyrosine kinase 1-DDR1 and different cancer network pathways, such as cell cycle and cell surface markers, potentially regulated by a subset of these miRNAs. Interestingly, we found that the expression of single genes might be regulated by several miRNAs.

Conclusions: We found a specific miRNA signature and respective genome targets that well defined different clusters of BCC. Breast cancer cell lines are associated with changes in expression of multiple miRNAs that, in turn, disrupt a network of genes that either activate or inhibit important cellular functions. A better understanding of the network of genes and cellular pathways regulated by these miRNAs will enable to understand breast cancer pathogenesis and miRNA based breast cancer therapy.

Citation Information: Cancer Res 2010;70(24 Suppl):Abstract nr P3-09-03.

Abstract 2337: ADAM17 mediation of cancer stem cell-ness and chemo-resistance in colorectal cancer

INTRODUCTION: Despite the multitude of drug options, most patients with metastatic colorectal cancer (mCRC) die within 3 years of diagnosis. Response to systemic therapy is not durable (< 1year) and drug resistance is inevitable. There is accumulating evidence for the existence of cancer stem cells (CSCs) in CRC, which are now believed to mediate chemo-resistance. The molecular mechanisms that regulate the CSC phenotype in CRC have not been fully elucidated. The metalloproteinase ADAM17 [also known as TNFα-converting enzyme (TACE)], an enzyme involved in Notch pathway activation, has been shown to be overexpressed in CRC and mediate cell proliferation and chemo-resistance in CRC cells. However, the role of ADAM17 in mediating the CSC phenotype in CRC has not been characterized. The present study sought to determine the role of CRC-associated ADAM17 in mediating chemo-resistance of CRC cells by regulating the CSC phenotype. METHODS: Most preclinical studies used established cancer cell lines that have been cultured in vitro for decades. Our laboratory has shown that these cell lines are not suitable for consistent isolation of CSCs (Fan et al, 2014, BJC in press). To circumvent this problem, we developed a new model using CRC cells either freshly isolated from patient-derived xenografts (PDX) or early passage of newly established Human CRC Primary cell lines (HCP) derived from the PDXs. With this newly established HCP cell line model, we used siRNA knockdown or a synthetic peptide inhibitor TAPI-2, to determine the effect of blocking ADAM17 on the CSC phenotype and chemo-resistance in CRC cells. RESULTS: ADAM17 inhibition, either by siRNA or by TAPI-2, reduced the protein levels of cleaved Notch1 (NICD) and its downstream target HES-1 in CRC cells. Levels of proteins that are involved in other CSC-associated pathways were not altered by ADAM17 inhibition, suggesting that the Notch pathway is the major stem-cell pathway activated by ADAM17. Furthermore, ADAM17 inhibition caused a decrease in the CSC-phenotype in CRC cells as determined by the sphere formation assay and the Aldefluor assay (a standard assay for identifying CSCs). In addition, the lethal dose 50 (LD50) of 5-fluorouracil (5-FU) was decreased by 3 fold when pre-incubating CRC cells with the ADAM17 inhibitor TAPI-2. We also showed that the protein levels of ADAM17 were higher in our chemo-resistant cells (cells that survive continuous exposure to chemotherapy) compared to control. CONCLUSION: Our studies demonstrated the role of ADAM17 in promoting the CSC phenotype and chemo-resistance in CRC cells. Utilization of drugs that inhibit ADAM17 activity may increase the therapeutic benefit to patients with mCRC, and potentially other solid malignancies.

Citation Format: Rui Wang, Fan Fan, Delphine Boulbes, Rajat Bhattacharya, Xiang-Cang Ye, Ling Xia, Lee Ellis. ADAM17 mediation of cancer stem cell-ness and chemo-resistance in colorectal cancer. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 2337. doi:10.1158/1538-7445.AM2015-2337

Abstract 3474: Depletion of SPECC1L inhibits colorectal cancer cell proliferation

Introduction: Despite the numerous drugs available for patients with metastatic colorectal cancer (mCRC), median overall survival for this group of patients remains at ~20-24 months, with no significant advances in the last 7 years. In the US ~50,000 patients die each year from mCRC refractory to systemic therapy. Inhibiting angiogenesis as a therapy has led to a great deal of enthusiasm. However, the overall benefit of classical-antiangiogenic therapy remains modest and has not lived up to their expectations in treating CRC. Our recent studies suggest that VEGF intracrine signaling, rather than autocrine/paracrine signaling, regulates cell survival in CRC cells. Further studies to understand the significance and mechanisms of this novel function have led us to identify factors that intracellularly interact with VEGF. Our studies further indicate that one such interacting protein, SPECC1L, may have a significant role in CRC cell proliferation and may be a potential target in mCRC therapy.

Methods: Lysates from CRC cells expressing Myc-tagged VEGF protein were immunoprecipitated and analyzed by mass spectrometry to identify VEGF-interacting protiens. SPECC1L was identified as a co-precipitated protein with high level of confidence. SPECC1L was depleted using siRNA and effects of such depletion on CRC cell growth and morphology were measured by cell growth assays (MTT), microscopy, FACS and western blot analyses. Localization of the protein and its interaction with microtubules and actin were visualized by immunostaining of FLAG-tagged recombinant SPECC1L protein.

Results: SPECC1L was identified as a protein that co-immunoprecipitated with Myc-tagged VEGF in CRC cells using mass spectroscopy. Previous literature suggests a role for SPECC1L in cell division. As a fraction of VEGF overexpressing CRC cells have a large multinucleated phenotype, likely arising due to defects in cell division, it was hypothesized that a VEGF mediated regulation of SPECC1L may lead to such phenotype. Depletion of SPECC1L by siRNAs in multiple CRC cell lines led to strong defects in cell division. The effects of SPECC1L depletion were manifested as accumulation of doublet-cells failing to complete cytokinesis following mitosis and resulted in reduced cell proliferation. Failure to complete cell division also led to the formation of multinucleated cells and enhanced cell death.

Conclusions: Inhibition of SPECC1L strongly inhibits CRC cell proliferation and enhances cell death. Thus targeting SPECC1L has the potential for developing therapeutics that reduce viability of CRC cells and improve survival of colorectal cancer patients.

*** These studies were supported by the Gillson-Longenbaugh Foundation.

Citation Format: Rajat Bhattacharya, Ling Xia, Fan Fan, Rui Wang, Delphine Boulbes, Xiang-Cang Ye, David Hawke, Lee Ellis. Depletion of SPECC1L inhibits colorectal cancer cell proliferation [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 3474. doi:10.1158/1538-7445.AM2017-3474