Merav Darash-Yahana

Role of high expression levels of CXCR4 in tumor growth, vascularization, and metastasis

Hormone refractory metastatic prostate cancer remains an incurable disease. We found that high expression levels of the chemokine receptor CXCR4 correlated with the presence of metastatic disease in prostate cancer patients. Positive staining for CXCL12, the ligand for CXCR4, was mainly present in the tumor‐associated blood vessels and basal cell hyperplasia. Subcutaneous xenografts of PC3 and 22Rv1 prostate tumors that overexpressed CXCR4 in NOD/SCID mice were two‐ to threefold larger in volume and weight vs. controls. Moreover, blood vessel density, functionality, invasiveness of tumors into the surrounding tissues, and metastasis to the lymph node and lung were significantly increased in these tumors. Neutralizing the interactions of CXCL12/CXCR4 in vivo with CXCR4 specific antibodies inhibited the CXCR4‐dependent tumor growth and vascularization. In vitro, CXCL12 induced the proliferation and VEGF secretion but not migration of PC3 and 22Rv1 cells overexpressing CXCR4. Similar effects of CXCR4 overexpression on tumor growth in vivo were also noted in two breast cancer lines, suggesting that the observed effect of CXCR4 is not unique to prostate tumor cells. Thus high levels of the chemokine receptor CXCR4 induce a more aggressive phenotype in prostate cancer cells and identify CXCR4 as a potential therapeutic target in advanced cases of metastatic prostate cancer.

The Chemokine CXCL16 and Its Receptor, CXCR6, as Markers and Promoters of Inflammation-Associated Cancers

Clinical observations and mouse models have suggested that inflammation can be pro-tumorigenic. Since chemokines are critical in leukocyte trafficking, we hypothesized that chemokines play essential roles in inflammation-associated cancers. Screening for 37 chemokines in prostate cancer cell lines and xenografts revealed CXCL16, the ligand for the receptor CXCR6, as the most consistently expressed chemokine. Immunohistochemistry and/or immunofluorescence and confocal imaging of 121 human prostate specimens showed that CXCL16 and CXCR6 were co-expressed, both on prostate cancer cells and adjacent T cells. Expression levels of CXCL16 and CXCR6 on cancer cells correlated with poor prognostic features including high-stage and high-grade, and expression also correlated with post-inflammatory changes in the cancer stroma as revealed by loss of alpha-smooth muscle actin. Moreover, CXCL16 enhanced the growth of CXCR6-expressing cancer and primary CD4 T cells. We studied expression of CXCL16 in an additional 461 specimens covering 12 tumor types, and found that CXCL16 was expressed in multiple human cancers associated with inflammation. Our study is the first to describe the expression of CXCL16/CXCR6 on both cancer cells and adjacent T cells in humans, and to demonstrate correlations between CXCL16 and CXCR6 vs. poor both prognostic features and reactive changes in cancer stoma. Taken together, our data suggest that CXCL16 and CXCR6 may mark cancers arising in an inflammatory milieu and mediate pro-tumorigenic effects of inflammation through direct effects on cancer cell growth and by inducing the migration and proliferation of tumor-associated leukocytes.

Immature Leukemic CD34+CXCR4+ Cells from CML Patients Have Lower Integrin‐Dependent Migration and Adhesion in Response to the Chemokine SDF‐1

Chronic myelogenous leukemia (CML), a malignant myeloproliferative disorder originating from a pluripotent stem cell expressing the bcr‐abl oncogene, is characterized by abnormal release of the expanded, malignant stem cell clone from the bone marrow (BM) into the circulation. Moreover, immature CD34+ CML cells have lower adhesion to stromal cells and fibronectin as well as lower engraftment potential in severe combined immunedeficient (SCID) and nonobese diabetic (NOD)/SCID mice than normal CD34+ cells. We report in this study that leukemic Philadelphia chromosome‐positive (Ph+)CD34+ cells from newly diagnosed CML patients that express the chemokine receptor CXCR4 migrate in response to stromal‐derived factor‐1 (SDF‐1). However, normal Ph‐CD34+CXCR4+ cells derived from the same patient have significantly higher migration levels toward SDF‐1. In contrast to their transwell migration potential, the SDF‐1‐mediated integrin‐dependent polarization and migration of the Ph+CD34+CXCR4+ cells through extracellular matrix‐like gels were significantly lower than for normal cells. Concomitantly, binding of these cells to vascular cell adhesion molecule‐1 or fibronectin, in the presence of SDF‐1, was also substantially lower. These findings suggest a major role for SDF‐1‐mediated, integrin‐dependent BM retention of Ph+CD34+ cells.

The Fe-S cluster-containing NEET proteins mitoNEET and NAF-1 as chemotherapeutic targets in breast cancer

Significance Cancer is a leading cause of mortality worldwide, with the identification of novel drug targets and chemotherapeutic agents being a high priority in the fight against it. The NEET proteins mitoNEET (mNT) and nutrient-deprivation autophagy factor-1 (NAF-1) were recently shown to be required for cancer cell proliferation. Utilizing a combination of experimental and computational techniques, we identified a derivative of the mitocan cluvenone that binds to NEET proteins at the vicinity of their 2Fe-2S clusters and facilitates their destabilization. The new drug displays a high specificity in the selective killing of human epithelial breast cancer cells, without any apparent effects on normal breast cells. Our results identify the 2Fe-2S clusters of NEET proteins as a novel target in the chemotherapeutic treatment of breast cancer. Identification of novel drug targets and chemotherapeutic agents is a high priority in the fight against cancer. Here, we report that MAD-28, a designed cluvenone (CLV) derivative, binds to and destabilizes two members of a unique class of mitochondrial and endoplasmic reticulum (ER) 2Fe-2S proteins, mitoNEET (mNT) and nutrient-deprivation autophagy factor-1 (NAF-1), recently implicated in cancer cell proliferation. Docking analysis of MAD-28 to mNT/NAF-1 revealed that in contrast to CLV, which formed a hydrogen bond network that stabilized the 2Fe-2S clusters of these proteins, MAD-28 broke the coordinative bond between the His ligand and the cluster’s Fe of mNT/NAF-1. Analysis of MAD-28 performed with control (Michigan Cancer Foundation; MCF-10A) and malignant (M.D. Anderson–metastatic breast; MDA-MB-231 or MCF-7) human epithelial breast cells revealed that MAD-28 had a high specificity in the selective killing of cancer cells, without any apparent effects on normal breast cells. MAD-28 was found to target the mitochondria of cancer cells and displayed a surprising similarity in its effects to the effects of mNT/NAF-1 shRNA suppression in cancer cells, causing a decrease in respiration and mitochondrial membrane potential, as well as an increase in mitochondrial iron content and glycolysis. As expected, if the NEET proteins are targets of MAD-28, cancer cells with suppressed levels of NAF-1 or mNT were less susceptible to the drug. Taken together, our results suggest that NEET proteins are a novel class of drug targets in the chemotherapeutic treatment of breast cancer, and that MAD-28 can now be used as a template for rational drug design for NEET Fe-S cluster-destabilizing anticancer drugs.

Combination of Imatinib with CXCR4 antagonist BKT140 overcomes the protective effect of stroma and targets CML in vitro and in vivo

Functional role of CXCR4 in chronic myelogenous leukemia (CML) progression was evaluated. Elevated CXCR4 significantly increased the in vitro survival and proliferation in response to CXCL12. CXCR4 stimulation resulted in activation of extracellular signal-regulated kinase (Erk)-1/2, Akt, S6K, STAT3, and STAT5 prosurvival signaling pathways. In accordance, we found that in vitro treatment with CXCR4 antagonist BKT140 directly inhibited the cell growth and induced cell death of CML cells. Combination of BKT140 with suboptimal concentrations of imatinib significantly increased the anti-CML effect. BKT140 induced apoptotic cell death, decreasing the levels of HSP70 and HSP90 chaperones and antiapoptotic proteins BCL-2 and BCL-XL, subsequently promoting the release of mitochondrial factors cytochrome c and SMAC/Diablo. Bone marrow (BM) stromal cells (BMSC) markedly increased the proliferation of CML cells and protected them from imatinib-induced apoptosis. Furthermore, BMSCs elevated proto-oncogene BCL6 expression in the CML cells in response to imatinib treatment, suggesting the possible role of BCL6 in stroma-mediated TKI resistance. BKT140 reversed the protective effect of the stroma, effectively promoted apoptosis, and decreased BCL6 levels in CML cells cocultured with BMSCs. BKT140 administration in vivo effectively reduced the growth of subcutaneous K562-produced xenografts. Moreover, the combination of BKT140 with low-dose imatinib markedly inhibited tumor growth, achieving 95% suppression. Taken together, our data indicate the importance of CXCR4/CXCL12 axis in CML growth and CML–BM stroma interaction. CXCR4 inhibition with BKT140 antagonist efficiently cooperated with imatinib in vitro and in vivo. These results provide the rational basis for CXCR4-targeted therapy in combination with TKI to override drug resistance and suppress residual disease. Mol Cancer Ther; 13(5); 1155–69. ©2014 AACR.

Differential usage of VLA-4 and CXCR4 by CD3+CD56+ NKT cells and CD56+CD16+ NK cells regulates their interaction with endothelial cells

The mechanism that regulates the preferential accumulation of NKT cells in the BM is unknown. The BM endothelium constitutively expresses selectins, the integrin ligands VCAM‐1 and ICAM‐1, andthe chemokine CXCL12. Both NK and NKT subsets of cells exhibited similar tethering and rolling interactions on both P‐selectin and E‐selectin and expressed similar levels of the integrins, VLA‐4 and LFA‐1. Although NKT cells express higher levels of CXCR4 than NK cells, CXCL12 (the ligand for CXCR4) rapidly stimulates similar levels of adhesion of NK and NKT cells to VCAM‐1 and ICAM‐1. In both subsets, the arrest on VCAM‐1 was dependent on high affinity VLA‐4 and the homing of these cells to the BM of NOD/SCID was VLA‐4‐dependent. However, as opposed to the situation for NK cells, CXCL12 preferentially triggers, under shear flow, the rolling on VCAM‐1 and transendothelial migration of NKT cells. Moreover, over‐expression of high levels of CXCR4 on the YT NK cell line enables them to migrate in response to CXCL12. This study therefore suggests an important role for CXCR4 levels of expression and for VLA‐4 in regulating the accumulation of NKT cells in the BM.

Breast cancer tumorigenicity is dependent on high expression levels of NAF-1 and the lability of its Fe-S clusters.

Significance Elevated expression of the iron–sulfur (Fe-S) protein nutrient-deprivation autophagy factor-1 (NAF-1) is associated with the progression of multiple cancer types. Here we demonstrate that the lability of the Fe-S cluster of NAF-1 plays a key role in promoting breast cancer cell proliferation, tumor growth, and resistance of cancer cells to oxidative stress. Our study establishes an important role for the unique 3Cys-1His Fe-S cluster coordination structure of NAF-1 in promoting the development of breast cancer tumors and suggests the potential use of drugs that suppress NAF-1 accumulation or stabilize its cluster in the treatment of cancers that display high expression levels of NAF-1. Iron–sulfur (Fe-S) proteins are thought to play an important role in cancer cells mediating redox reactions, DNA replication, and telomere maintenance. Nutrient-deprivation autophagy factor-1 (NAF-1) is a 2Fe-2S protein associated with the progression of multiple cancer types. It is unique among Fe-S proteins because of its 3Cys-1His cluster coordination structure that allows it to be relatively stable, as well as to transfer its clusters to apo-acceptor proteins. Here, we report that overexpression of NAF-1 in xenograft breast cancer tumors results in a dramatic augmentation in tumor size and aggressiveness and that NAF-1 overexpression enhances the tolerance of cancer cells to oxidative stress. Remarkably, overexpression of a NAF-1 mutant with a single point mutation that stabilizes the NAF-1 cluster, NAF-1(H114C), in xenograft breast cancer tumors results in a dramatic decrease in tumor size that is accompanied by enhanced mitochondrial iron and reactive oxygen accumulation and reduced cellular tolerance to oxidative stress. Furthermore, treating breast cancer cells with pioglitazone that stabilizes the 3Cys-1His cluster of NAF-1 results in a similar effect on mitochondrial iron and reactive oxygen species accumulation. Taken together, our findings point to a key role for the unique 3Cys-1His cluster of NAF-1 in promoting rapid tumor growth through cellular resistance to oxidative stress. Cluster transfer reactions mediated by the overexpressed NAF-1 protein are therefore critical for inducing oxidative stress tolerance in cancer cells, leading to rapid tumor growth, and drugs that stabilize the NAF-1 cluster could be used as part of a treatment strategy for cancers that display high NAF-1 expression.

Activation of apoptosis in NAF-1-deficient human epithelial breast cancer cells.

ABSTRACT Maintaining iron (Fe) ion and reactive oxygen species homeostasis is essential for cellular function, mitochondrial integrity and the regulation of cell death pathways, and is recognized as a key process underlying the molecular basis of aging and various diseases, such as diabetes, neurodegenerative diseases and cancer. Nutrient-deprivation autophagy factor 1 (NAF-1; also known as CISD2) belongs to a newly discovered class of Fe-sulfur proteins that are localized to the outer mitochondrial membrane and the endoplasmic reticulum. It has been implicated in regulating homeostasis of Fe ions, as well as the activation of autophagy through interaction with BCL-2. Here we show that small hairpin (sh)RNA-mediated suppression of NAF-1 results in the activation of apoptosis in epithelial breast cancer cells and xenograft tumors. Suppression of NAF-1 resulted in increased uptake of Fe ions into cells, a metabolic shift that rendered cells more susceptible to a glycolysis inhibitor, and the activation of cellular stress pathways that are associated with HIF1α. Our studies suggest that NAF-1 is a major player in the metabolic regulation of breast cancer cells through its effects on cellular Fe ion distribution, mitochondrial metabolism and the induction of apoptosis. Summary: NAF-1 is a major player in the metabolic regulation of breast cancer cells through its effects on cellular Fe ion distribution, mitochondrial metabolism and the induction of apoptosis.

Interactions between mitoNEET and NAF-1 in cells

The NEET proteins mitoNEET (mNT) and nutrient-deprivation autophagy factor-1 (NAF-1) are required for cancer cell proliferation and resistance to oxidative stress. NAF-1 and mNT are also implicated in a number of other human pathologies including diabetes, neurodegeneration and cardiovascular disease, as well as in development, differentiation and aging. Previous studies suggested that mNT and NAF-1 could function in the same pathway in mammalian cells, preventing the over-accumulation of iron and reactive oxygen species (ROS) in mitochondria. Nevertheless, it is unknown whether these two proteins directly interact in cells, and how they mediate their function. Here we demonstrate, using yeast two-hybrid, in vivo bimolecular fluorescence complementation (BiFC), direct coupling analysis (DCA), RNA-sequencing, ROS and iron imaging, and single and double shRNA lines with suppressed mNT, NAF-1 and mNT/NAF-1 expression, that mNT and NAF-1 directly interact in mammalian cells and could function in the same cellular pathway. We further show using an in vitro cluster transfer assay that mNT can transfer its clusters to NAF-1. Our study highlights the possibility that mNT and NAF-1 function as part of an iron-sulfur (2Fe-2S) cluster relay to maintain the levels of iron and Fe-S clusters under control in the mitochondria of mammalian cells, thereby preventing the activation of apoptosis and/or autophagy and supporting cellular proliferation.

Structure of the human monomeric NEET protein MiNT and its role in regulating iron and reactive oxygen species in cancer cells

Significance NEET proteins belong to a unique family of iron-sulfur (Fe-S) proteins that regulate iron and reactive oxygen homeostasis and are involved in the progression of cancer, diabetes, neurodegeneration, and aging. Of the three human NEET proteins, the structure and function of the mitochondrial matrix-localized mitochondrial inner NEET protein (MiNT) are unknown. Here, we show that MiNT is a pseudosymmetrical monomeric protein that contains two distinct Fe-S cluster-binding motifs. MiNT transfers its clusters to the human mitochondrial ferredoxins FDX1/adrenodoxin and FDX2 and is required for regulating iron and reactive oxygen species levels in the mitochondria. Our study demonstrates that although MiNT differs in structure and localization from its homodimeric NEET counterparts, it nevertheless cooperates with them in the same important pathway. The NEET family is a relatively new class of three related [2Fe-2S] proteins (CISD1–3), important in human health and disease. While there has been growing interest in the homodimeric gene products of CISD1 (mitoNEET) and CISD2 (NAF-1), the importance of the inner mitochondrial CISD3 protein has only recently been recognized in cancer. The CISD3 gene encodes for a monomeric protein that contains two [2Fe-2S] CDGSH motifs, which we term mitochondrial inner NEET protein (MiNT). It folds with a pseudosymmetrical fold that provides a hydrophobic motif on one side and a relatively hydrophilic surface on the diametrically opposed surface. Interestingly, as shown by molecular dynamics simulation, the protein displays distinct asymmetrical backbone motions, unlike its homodimeric counterparts that face the cytosolic side of the outer mitochondrial membrane/endoplasmic reticulum (ER). However, like its counterparts, our biological studies indicate that knockdown of MiNT leads to increased accumulation of mitochondrial labile iron, as well as increased mitochondrial reactive oxygen production. Taken together, our study suggests that the MiNT protein functions in the same pathway as its homodimeric counterparts (mitoNEET and NAF-1), and could be a key player in this pathway within the mitochondria. As such, it represents a target for anticancer or antidiabetic drug development.