EPHB6 augments both development and drug sensitivity of triple-negative breast cancer tumours.

Triple-negative breast cancer (TNBC) tumours that lack expression of oestrogen, and progesterone receptors, and do not overexpress the HER2 receptor represent the most aggressive breast cancer subtype, which is characterised by the resistance to therapy in frequently relapsing tumours and a high rate of patient mortality. This is likely due to the resistance of slowly proliferating tumour-initiating cells (TICs), and understanding molecular mechanisms that control TICs behaviour is crucial for the development of effective therapeutic approaches. Here, we present our novel findings, indicating that an intrinsically catalytically inactive member of the Eph group of receptor tyrosine kinases, EPHB6, partially suppresses the epithelial-mesenchymal transition in TNBC cells, while also promoting expansion of TICs. Our work reveals that EPHB6 interacts with the GRB2 adapter protein and that its effect on enhancing cell proliferation is mediated by the activation of the RAS-ERK pathway, which allows it to elevate the expression of the TIC-related transcription factor, OCT4. Consistent with this, suppression of either ERK or OCT4 activities blocks EPHB6-induced pro-proliferative responses. In line with its ability to trigger propagation of TICs, EPHB6 accelerates tumour growth, potentiates tumour initiation and increases TIC populations in xenograft models of TNBC. Remarkably, EPHB6 also suppresses tumour drug resistance to DNA-damaging therapy, probably by forcing TICs into a more proliferative, drug-sensitive state. In agreement, patients with higher EPHB6 expression in their tumours have a better chance for recurrence-free survival. These observations describe an entirely new mechanism that governs TNBC and suggest that it may be beneficial to enhance EPHB6 action concurrent with applying a conventional DNA-damaging treatment, as it would decrease drug resistance and improve tumour elimination.

The monoamine oxidase-A inhibitor clorgyline promotes a mesenchymal-to-epithelial transition in the MDA-MB-231 breast cancer cell line.

Monoamine oxidase-A (MAO-A) dysfunction has been historically associated with depression. Recently, depression as well as altered MAO-A expression have both been associated with a poor prognosis in cancers, although the mechanism involved remains ambiguous. For example, MAO-A mRNA is repressed across cancers, yet MAO-A protein and levels of serotonin, a substrate of MAO-A implicated in depression, are paradoxically increased in malignancies, including breast cancer. The effect of clorgyline (CLG), a selective inhibitor of MAO-A, on malignant behaviour, expression of transitional markers, and biochemical correlates was examined in two human breast carcinoma cell lines, i.e. the epithelial, oestrogen receptor (ER)-positive MCF-7 cell line and the post-EMT (mesenchymal), ER-negative MDA-MB-231 cell line. CLG exerted little effect on malignant behaviour in MCF-7 cells, but inhibited proliferation and anchorage-independent growth, and increased invasiveness and active migration of MDA-MB-231 cells. CLG induced the expression of the mesenchymal marker vimentin in MCF-7 cells, but not in MDA-MB-231 cells. In contrast, CLG induced the epithelial protein marker E-cadherin in both cell lines, with a more robust effect in MDA-MB-231 cells (where a nuclear E-cadherin signal was also detected). This effect appears to be independent of any canonical Snai1-mediated regulation of E-cadherin mRNA expression. CLG interfered with the ß-catenin/[phospho]GSK-3ß complex as well as the E-cadherin/ß-catenin complex in both cell lines cells, but, again, the effect was more robust in MDA-MB-231 cells. Parallel studies revealed a general lack of effect of CLG on the ER-negative, epithelial Au565 breast cancer cell line. Thus, any effect of CLG on metastatic behaviours appears to rely on the cell's EMT status rather than on the cell's ER status. These data suggest that inactivation of MAO-A triggers a mesenchymal-to-epithelial transition in MDA-MB-231 cells via a non-canonical mechanism. This potentially implicates an MAO-A-sensitive step in advanced breast cancer and should be borne in mind when considering pharmacological treatment options for co-morbid depression in breast cancer patients.

EphB receptors trigger Akt activation and suppress Fas receptor-induced apoptosis in malignant T lymphocytes.

Treatment of hematopoietic malignancies often requires allogeneic bone marrow transplantation, and the subsequent graft-versus-leukemia response is crucial for the elimination of malignant cells. Cytotoxic T lymphocytes and NK cells responsible for the immunoelimination express Fas ligand and strongly rely on the induction of Fas receptor-mediated apoptosis for their action. Although cancer cells are removed successfully by graft-versus-leukemia reactions in myeloid malignancies, their efficiency is low in T cell leukemias. This may be partially because of the ability of malignant T cells to escape apoptosis. Our work shows that Eph family receptor EphB3 is consistently expressed by malignant T lymphocytes, most frequently in combination with EphB6, and that stimulation with their common ligands, ephrin-B1 and ephrin-B2, strongly suppresses Fas-induced apoptosis in these cells. This effect is associated with Akt activation and with the inhibition of the Fas receptor-initiated caspase proteolytic cascade. Akt proved to be crucial for the prosurvival response, because inhibition of Akt, but not of other molecules central to T cell biology, including Src kinases, MEK1 and MEK2, blocked the antiapoptotic effect. Overall, this demonstrates a new role for EphB receptors in the protection of malignant T cells from Fas-induced apoptosis through Akt engagement and prevention of caspase activation. Because Fas-triggered apoptosis is actively involved in the graft-versus-leukemia response and cytotoxic T cells express ephrin-Bs, our observations suggest that EphB receptors are likely to support immunoevasivenes of T cell malignancies and may represent promising targets for therapies, aiming to enhance immunoelimination of cancerous T cells.

Dancing with the dead: Eph receptors and their kinase-null partners.

Eph receptor tyrosine kinases and their ligands, ephrins, are membrane proteins coordinating a wide range of biological functions both in developing embryos and in adult multicellular organisms. Numerous studies have implicated Eph receptors in the induction of opposing responses, including cell adhesion or repulsion, support or inhibition of cell proliferation and cell migration, and progression or suppression of multiple malignancies. Similar to other receptor tyrosine kinases, Eph receptors rely on their ability to catalyze tyrosine phosphorylation for signal transduction. Interestingly, however, Eph receptors also actively utilize three kinase-deficient receptor tyrosine kinases, EphB6, EphA10, and Ryk, in their signaling network. The accumulating evidence suggests that the unusual flexibility of the Eph family, allowing it to initiate antagonistic responses, might be partially explained by the influence of the kinase-dead participants and that the exact outcome of an Eph-mediated action is likely to be defined by the balance between the signaling of catalytically potent and catalytically null receptors. We discuss in this minireview the emerging functions of the kinase-dead EphB6, EphA10, and Ryk receptors both in normal biological responses and in malignancy, and analyze currently available information related to the molecular mechanisms of their action in the context of the Eph family.

The EphB6 receptor cooperates with c-Cbl to regulate the behavior of breast cancer cells.

Cancer invasiveness plays a major role in the mortality of patients with solid tumors, and deregulated cell adhesion and migration are suspected to drive invasive behavior. Since Eph receptor tyrosine kinases control both cell attachment and migration, they may act to define the level of cancer invasiveness. EphB6 is an unusual Eph receptor, lacking catalytic capacity due to alterations in its kinase domain. Interestingly, increased metastatic activity is associated with reduced EphB6 receptor expression in several tumor types, including breast cancer. This emphasizes the potential of EphB6 to act as a suppressor of cancer aggressiveness; however, the mechanism of its action is not well understood. We show that restoration of EphB6 expression in invasive breast cancer cells supports actin-dependent spreading and attachment and blocks invasiveness. EphB6 stimulation induces its tyrosine phosphorylation, which is crucial for its function and is mediated by the EphB4 receptor. This is accompanied by EphB6-c-Cbl interaction and phosphorylation of c-Cbl partner, the Abl kinase. Cbl silencing suppresses Abl phosphorylation, cell adhesion, and morphologic changes and blocks the ability of EphB6 to inhibit invasiveness, confirming its importance for EphB6 activity. Despite its crucial role in EphB6 responses, EphB4 also acts in an EphB6-independent manner to enhance invasive activity, suggesting that cancer invasiveness may be defined by the balance in the EphB6-EphB4 system. Overall, our observations suggest a new role for EphB6 in suppressing cancer invasiveness through c-Cbl-dependent signaling, morphologic changes, and cell attachment and indicate that EphB6 may represent a useful prognostic marker and a promising target for therapeutic approaches.