Tyrosine-phosphorylated STAT5 accumulates on podosomes in Hck-transformed fibroblasts and chronic myeloid leukemia cells

Epithelial Mesenchymal Transition (EMT) and Associated Invasive Adhesions in Solid and Haematological Tumours

In solid tumours, cancer cells that undergo epithelial mesenchymal transition (EMT) express characteristic gene expression signatures that promote invasive migration as well as the development of stemness, immunosuppression and drug/radiotherapy resistance, contributing to the formation of currently untreatable metastatic tumours. The cancer traits associated with EMT can be controlled by the signalling nodes at characteristic adhesion sites (focal contacts, invadopodia and microtentacles) where the regulation of cell migration, cell cycle progression and pro-survival signalling converge. In haematological tumours, ample evidence accumulated during the last decade indicates that the development of an EMT-like phenotype is indicative of poor disease prognosis. However, this EMT phenotype has not been directly linked to the assembly of specific forms of adhesions. In the current review we discuss the role of EMT in haematological malignancies and examine its possible link with the progression towards more invasive and aggressive forms of these tumours. We also review the known types of adhesions formed by haematological malignancies and speculate on their possible connection with the EMT phenotype. We postulate that understanding the architecture and regulation of EMT-related adhesions will lead to the discovery of new therapeutic interventions to overcome disease progression and resistance to therapies.

Leukaemia: a model metastatic disease

In contrast to solid cancers, which often require genetic modifications and complex cellular reprogramming for effective metastatic dissemination, leukaemic cells uniquely possess the innate ability for migration and invasion. Dedifferentiated, malignant leukocytes retain the benign leukocytes' capacity for cell motility and survival in the circulation, while acquiring the potential for rapid and uncontrolled cell division. For these reasons, leukaemias, although not traditionally considered as metastatic diseases, are in fact models of highly efficient metastatic spread. Accordingly, they are often aggressive and challenging diseases to treat. In this Perspective, we discuss the key molecular processes that facilitate metastasis in a variety of leukaemic subtypes, the clinical significance of leukaemic invasion into specific tissues and the current pipeline of treatments targeting leukaemia metastasis.

Novel non-canonical role of STAT1 in Natural Killer cell cytotoxicity

STAT1 is an important regulator of NK cell maturation and cytotoxicity. Although the consequences of Stat1-deficiency have been described in detail the underlying molecular functions of STAT1 in NK cells are only partially understood. Here, we describe a novel non-canonical role of STAT1 that was unmasked in NK cells expressing a Stat1-Y701F mutant. This mutation prevents JAK-dependent phosphorylation, subsequent nuclear translocation and cytokine-induced transcriptional activity as verified by RNA-seq analysis. As expected Stat1-Y701F mice displayed impaired NK cell maturation comparable to Stat1^−/− animals. In contrast Stat1-Y701F NK cells exerted a significantly enhanced cytotoxicity in vitro and in vivo compared to Stat1^−/− NK cells in the absence of detectable transcriptional activity. We thus investigated the STAT1 interactome using primary NK cells derived from Stat1^ind mice that inducibly express a FLAG-tagged STAT1. Mass spectrometry revealed that STAT1 directly binds proteins involved in cell junction formation and proteins associated to membrane or membrane-bound vesicles. In line, immunofluorescence studies uncovered the recruitment of STAT1 to the target-cell interphase during NK cell killing. This led us to propose a novel function for STAT1 at the immunological synapse in NK cells regulating tumor surveillance and cytotoxicity.

Hematopoietic cell kinase (HCK) as a therapeutic target in immune and cancer cells

The hematopoietic cell kinase (HCK) is a member of the SRC family of cytoplasmic tyrosine kinases (SFKs), and is expressed in cells of the myeloid and B-lymphocyte cell lineages. Excessive HCK activation is associated with several types of leukemia and enhances cell proliferation and survival by physical association with oncogenic fusion proteins, and with functional interactions with receptor tyrosine kinases. Elevated HCK activity is also observed in many solid malignancies, including breast and colon cancer, and correlates with decreased patient survival rates. HCK enhances the secretion of growth factors and pro-inflammatory cytokines from myeloid cells, and promotes macrophage polarization towards a wound healing and tumor-promoting alternatively activated phenotype. Within tumor associated macrophages, HCK stimulates the formation of podosomes that facilitate extracellular matrix degradation, which enhance immune and epithelial cell invasion. By virtue of functional cooperation between HCK and bona fide oncogenic tyrosine kinases, excessive HCK activation can also reduce drug efficacy and contribute to chemo-resistance, while genetic ablation of HCK results in minimal physiological consequences in healthy mice. Given its known crystal structure, HCK therefore provides an attractive therapeutic target to both, directly inhibit the growth of cancer cells, and indirectly curb the source of tumor-promoting changes in the tumor microenvironment.

Src family kinases interfere with dimerization of STAT5A through a phosphotyrosine-SH2 domain interaction

Background

Chronic myeloid leukemia (CML) is driven by the expression of the BCR-ABL oncoprotein. STAT5 is a BCR-ABL substrate and persistently activated by tyrosine phosphorylation in CML cells. Activated STAT5 (pSTAT5) drives proliferation and survival of leukemic cells and contributes to initial transformation and maintenance of the disease. In cytokine-induced STAT5 signaling, phosphorylation of STAT5A on Y694 leads to nuclear accumulation of the transcription factor, followed by DNA-binding and gene induction. However, Src-family kinases (SFK) mediate cytoplasmic retention of pSTAT5A leading to attenuated target gene expression and colony formation in CML cells.

Results

In this study we show that autophosphorylation of Y416 in the highly conserved activation loop of SFK generates a potent recruitment site for the SH2 domain of STAT5A. Binding of the SH2 domain to the activation loop is required for STAT5A^Y694 phosphorylation by SFK, but at the same time promotes the persistent cytoplasmic localization of the transcription factor as found in BCR-ABL⁺ leukemia. As a consequence of the complex formation between tyrosine-phosphorylated SFK and the SH2 domain of STAT5A, the dimerization of STAT5A is impaired. We further demonstrate that constitutively active STAT5A^S710F escapes from SFK-mediated cytoplasmic retention by enhancing STAT5A dimer stability.

Conclusion

Our results reveal important structural aspects of cytoplasmic pSTAT5A found in myeloid leukemias and will contribute to the understanding of STAT5A mediated cytoplasmic signaling.

Real-time monitoring of hematopoietic cell interaction with fibronectin fragment: the effect of histone deacetylase inhibitors

Real-time cell analysis (RTCA) system based on measurement of electrical microimpedance has been introduced to monitor adherent cell cultures. We describe its use for real-time analysis of hematopoietic cell adhesion to bone marrow stroma proteins. Cells growing in suspension do not generate any significant change in the microimpedance signal until the surface with embedded microelectrodes is coated with a cell-binding protein. We show that in this case, the microimpedance signal specifically reflects cell binding to the coated surface. The optimized method was used to monitor the effect of two histone deacetylase inhibitors, suberoylanilide hydroxamic acid (SAHA) and tubastatin A, on JURL-MK1 cell adhesion to cell-binding fragment of fibronectin (FNF). Both compounds were used in non-toxic concentrations and induced an increase in the cell adhesivity. The kinetics of this increase was markedly slower for SAHA although tubulin hyperacetylation occurred rapidly for any of the two drugs. The strengthening of cell binding to FNF was paralleled with a decrease of Lyn kinase activity monitored using an anti-phospho-Src family antibody. The inhibition of Src kinase activity with PP2 accordingly enhanced JURL-MK1 cell interaction with FNF. Actin filaments were present at the proximity of the plasma membrane and in numerous membrane protrusions. In some cells, F-actin formed clusters at membrane regions interacting with the coated surface and these clusters colocalized with active Lyn kinase. Our results indicate that the role of Src kinases in the regulation of hematopoetic cell adhesion signaling is similar to that of c-Src in adherent cells.

Src family kinases mediate cytoplasmic retention of activated STAT5 in BCR-ABL-positive cells

Persistent activation of the Abl tyrosine kinase in the BCR-ABL fusion protein is the major cause of chronic myeloid leukemia (CML). Among many other substrates BCR-ABL phosphorylates STAT5 and Src family kinases (SFK). Activated pSTAT5 is essential for initial transformation and maintenance of the disease. Cytokine-induced phosphorylation on tyrosine 694 typically leads to nuclear accumulation of pSTAT5 and target gene expression. We verified that in BCR-ABL-positive progenitor cells from a CML patient and in K562 cells pSTAT5 is cytoplasmic. However, upon ectopic expression of BCR-ABL p210 in non-myeloid cells, co-transfected STAT5A is phosphorylated on Y694 and localized in the nucleus arguing for an additional factor mediating cytoplasmic retention in CML cells. Expression of the SFK v-Src, Hck or Lyn together with STAT5A results in phosphorylation on Y694 and cytoplasmic retention. Upon coexpression of BCR-ABL and individual SFK the cytoplasmic retention of activated STAT5A mediated by v-Src and Hck but not Lyn is dominant over nuclear translocation induced by BCR-ABL. Cytoplasmic retention depends on the kinase activity of SFK and is mediated through the interaction of the SH2 domain of STAT5A with the SFK. Interestingly, nuclear accumulation of STAT5A as a result of activation by FLT3-ITD, an oncogene found in acute myeloid leukemia, cannot be prevented by coexpression of SFK. Importantly, inhibition of SFK in K562 cells restored nuclear accumulation of pSTAT5A, enhanced STAT5 target gene expression and increased colony formation. Thus, SFK mediate cytoplasmic retention of pSTAT5A in BCR-ABL-positive cells. Cytoplasmic pSTAT5A in CML cells might balance the controversial functions of STAT5 in cellular senescence and differentiation versus G1/S progression and survival.

Polarised apical-like intracellular sorting and trafficking regulates invadopodia formation and degradation of the extracellular matrix in cancer cells

Invadopodia are proteolytically active protrusions formed by invasive tumoral cells when grown on an extracellular matrix (ECM) substratum. A current challenge is to understand how proteolytic activity is so precisely localised at discrete sites of the plasma membrane to produce focalised ECM degradation at invadopodia. Indeed, a number of components including metalloproteases need to be directed to invadopodia to ensure proper segregation of proteolytic activities. We recently found invadopodia to feature the properties of cholesterol-rich membrane domains (a.k.a. lipid drafts) and that ECM degradation depends on the tight control of cholesterol homeostasis. Since apically directed polarised sorting and transport in epithelial cells relies on segregation of proteins into lipid rafts at the Golgi complex, we hypothesised that invadopodia-dependent ECM degradation might also rely on lipid raft-dependent polarised transport routes. To investigate this issue we undertook a three-pronged approach. First, we found that microtubule depolymerisation, which is known to disrupt polarised transport in polarised cells, strongly inhibited invadopodia formation, while not affecting overall protein transport. In the second approach we found that glycosylphosphatidylinositol-anchored green fluorescent protein (an apical model protein), but not vesicular stomatitis virus G-protein or influenza virus hemagglutinin (both model basolateral model cargoes), was transported to sites of ECM degradation. Finally, RNAi-mediated knock-down of proteins known to specifically regulate polarised apical or basolateral transport in epithelial cells, such as caveolin 1 and annexin XIIIB or clathrin, respectively, demonstrated that the selective inhibition of the apical, but not the basolateral, transport route impairs invadopodia formation and ECM degradation. Taken together, our findings suggest that invadopodia are apical-like membrane domains, where signal transduction and local membrane remodelling events might be temporally and spatially confined via selective raft-dependent apical transport routes.

Macrophage mesenchymal migration requires podosome stabilization by filamin A

Filamin A (FLNa) is a cross-linker of actin filaments and serves as a scaffold protein mostly involved in the regulation of actin polymerization. It is distributed ubiquitously, and null mutations have strong consequences on embryonic development in humans, with organ defects which suggest deficiencies in cell migration. We have reported previously that macrophages, the archetypal migratory cells, use the protease- and podosome-dependent mesenchymal migration mode in dense three-dimensional environments, whereas they use the protease- and podosome-independent amoeboid mode in more porous matrices. Because FLNa has been shown to localize to podosomes, we hypothesized that the defects seen in patients carrying FLNa mutations could be related to the capacity of certain cell types to form podosomes. Using strategies based on FLNa knock-out, knockdown, and rescue, we show that FLNa (i) is involved in podosome stability and their organization as rosettes and three-dimensional podosomes, (ii) regulates the proteolysis of the matrix mediated by podosomes in macrophages, (iii) is required for podosome rosette formation triggered by Hck, and (iv) is necessary for mesenchymal migration but dispensable for amoeboid migration. These new functions assigned to FLNa, particularly its role in mesenchymal migration, could be directly related to the defects in cell migration described during the embryonic development in FLNa-defective patients.

The podosome marker protein Tks5 regulates macrophage invasive behavior

Tks5 is a Src substrate and adaptor protein previously recognized for its regulation of cancer cell invasion through modulation of specialized adhesion structures called podosomes/invadopodia. Here we show for the first time that Tks5 localizes to the podosomes of primary macrophages, and that Tks5 protein levels increase concurrently with podosome deposition during the differentiation of monocytes into macrophages. Similar results are reported for model THP-1 cells, which differentiate into macrophages and form proteolytically active podosomes in response to a PKC signaling agonist (PMA) and with sensitivity to a PKC inhibitor (bisindolylmaleimide). Genetic manipulation of Tks5 expression (silencing and overexpression) in stable THP-1 cell lines does not independently alter this macrophage differentiation process. Nor do these cells lose the ability to focalize F-actin and its accessory proteins into podosome-like structures following PMA treatment. However, Tks5 directly controls podosome-associated gelatin degradation and invasion through collective changes in adhesion, chemotaxis, and the expression/proteolytic activity of MMP9. The Src family kinase-dependent phosphorylation of Tks5 is also implicated in the regulation of THP-1 macrophage invasive behavior. These results therefore define a previously unappreciated function of Tks5 signaling specific to the functional attributes of the macrophage podosome in adhesion, motility, and extracellular matrix-remodeling.

Dynamics and non-canonical aspects of JAK/STAT signalling

The Janus kinase/signal transducer and activator of transcription (JAK/STAT) pathway directly links ligand-binding to a membrane-bound receptor with the activation of a transcription factor. This signalling module enables the cell to rapidly initiate a transcriptional response to external stimulation. The main components of this evolutionary conserved module are cytokines that specifically bind to cytokine receptors leading to the activation of receptor-associated Janus tyrosine kinases (JAKs). The receptor-bound JAKs activate STAT transcription factors through phosphorylation of a single tyrosine residue. Activated STAT dimers translocate into the nucleus to induce target gene expression. In this article we will review current opinions on the molecular mechanism and on intracellular dynamics of JAK/STAT signalling with a special focus on the cytokine receptor glycoprotein 130 (gp130) and STAT3. In particular we will concentrate on non-canonical aspects of Jak/STAT signalling including preassembled receptor complexes, preformed STAT dimers, STAT trafficking and non-canonical functions of STATs.

Doubles game: Src-Stat3 versus p53-PTEN in cellular migration and invasion

We have recently shown that Src induces the formation of podosomes and cell invasion by suppressing endogenous p53, while enhanced p53 strongly represses the Src-induced invasive phenotype. However, the mechanism by which Src and p53 play antagonistic roles in cell invasion is unknown. Here we show that the Stat3 oncogene is a required downstream effector of Src in inducing podosome structures and related invasive phenotypes. Stat3 promotes Src phenotypes through the suppression of p53 and the p53-inducible protein caldesmon, a known podosome antagonist. In contrast, enhanced p53 attenuates Stat3 function and Src-induced podosome formation by upregulating the tumor suppressor PTEN. PTEN, through the inactivation of Src/Stat3 function, also stabilizes the podosome-antagonizing p53/caldesmon axis, thereby further enhancing the anti-invasive potential of the cell. Furthermore, the protein phosphatase activity of PTEN plays a major role in the negative regulation of the Src/Stat3 pathway and represses podosome formation. Our data suggest that cellular invasiveness is dependent on the balance between two opposing forces: the proinvasive oncogenes Src-Stat3 and the anti-invasive tumor suppressors p53-PTEN.

Emerging roles of Ruk/CIN85 in vesicle-mediated transport, adhesion, migration and malignancy

Ruk/CIN85 is an adaptor protein. Similar to many other proteins of this type, Ruk/CIN85 is known to take part in multiple cellular processes including signal transduction, vesicle-mediated transport, cytoskeleton remodelling, programmed cell death and viral infection. Recent studies have also revealed the potential importance of Ruk/CIN85 in cancer cell invasiveness. In this review we summarize the various roles of this protein as well as the potential contribution of Ruk/CIN85 to malignancy and the invasiveness of cancer cells. In the last section of the paper we also speculate on the utility of Ruk/CIN85 as a target for novel anti-cancer therapies.

Three-dimensional migration of macrophages requires Hck for podosome organization and extracellular matrix proteolysis

Tissue infiltration of phagocytes exacerbates several human pathologies including chronic inflammations or cancers. However, the mechanisms involved in macrophage migration through interstitial tissues are poorly understood. We investigated the role of Hck, a Src-family kinase involved in the organization of matrix adhesion and degradation structures called podosomes. In Hck(-/-) mice submitted to peritonitis, we found that macrophages accumulated in interstitial tissues and barely reached the peritoneal cavity. In vitro, 3-dimensional (3D) migration and matrix degradation abilities, 2 protease-dependent properties of bone marrow-derived macrophages (BMDMs), were affected in Hck(-/-) BMDMs. These macrophages formed few and undersized podosome rosettes and, consequently, had reduced matrix proteolysis operating underneath despite normal expression and activity of matrix metalloproteases. Finally, in fibroblasts unable to infiltrate matrix, ectopic expression of Hck provided the gain-of-3D migration function, which correlated positively with formation of podosome rosettes. In conclusion, spatial organization of podosomes as large rosettes, proteolytic degradation of extracellular matrix, and 3D migration appeared to be functionally linked and regulated by Hck in macrophages. Hck, as the first protein combining a phagocyte-limited expression with a role in 3D migration, could be a target for new anti-inflammatory and antitumor molecules.

Proteins recruited by SH3 domains of Ruk/CIN85 adaptor identified by LC-MS/MS

Background

Ruk/CIN85 is a mammalian adaptor molecule with three SH3 domains. Using its SH3 domains Ruk/CIN85 can cluster multiple proteins and protein complexes, and, consequently, facilitates organisation of elaborate protein interaction networks with diverse regulatory roles. Previous research linked Ruk/CIN85 with the regulation of vesicle-mediated transport and cancer cell invasiveness. Despite the recent findings, precise molecular functions of Ruk/CIN85 in these processes remain largely elusive and further research is hampered by a lack of complete lists of its partner proteins.

Results

In the present study we employed a LC-MS/MS-based experimental pipeline to identify a considerable number (over 100) of proteins recruited by the SH3 domains of Ruk/CIN85 in vitro. Most of these identifications are novel Ruk/CIN85 interaction candidates. The identified proteins have diverse molecular architectures and can interact with other proteins, as well as with lipids and nucleic acids. Some of the identified proteins possess enzymatic activities. Functional profiling analyses and literature mining demonstrate that many of the proteins recruited by the SH3 domains of Ruk/CIN85 identified in this work were involved in the regulation of membranes and cytoskeletal structures necessary for vesicle-mediated transport and cancer cell invasiveness. Several groups of the proteins were also associated with few other cellular processes not previously related to Ruk/CIN85, most prominently with cell division.

Conclusion

Obtained data support the notion that Ruk/CIN85 regulates vesicle-mediated transport and cancer cell invasiveness through the assembly of multimeric protein complexes governing coordinated remodelling of membranes and underlying cytoskeletal structures, and imply its important roles in formation of coated vesicles and biogenesis of invadopodia. In addition, this study points to potential involvement of Ruk/CIN85 in other cellular processes, chiefly in cell division.

The oncogenic activity of the Src family kinase Hck requires the cooperative action of the plasma membrane- and lysosome-associated isoforms

Hck is a phagocyte specific proto-oncogene of the Src family expressed as two isoforms, p59Hck and p61Hck. It plays a critical role in Bcr/Abl-chronic myeloid leukaemia and is able to transform fibroblasts in vitro. However, the tumourigenic activity of Hck and the respective oncogenic functions of Hck isoforms have not been examined. Tet-Off fibroblasts expressing constitutively active mutants of p59Hck and p61Hck together or individually were used. In contrast to cells expressing p59Hck(ca) or p61Hck(ca) alone, cells expressing both isoforms were transformed in vitro and induced tumour formation in 90% of nude mice within 2 weeks. This is the first demonstration of (i) the tumourigenic activity of Hck in mice, (ii) the cooperative action of the two Hck isoforms in vitro and in vivo. To our knowledge, this is the first example of a transforming activity 'split' in two requisite isoforms.

Podosomes and Invadopodia: Related structures with Common Protein Components that May Promote Breast Cancer Cellular Invasion

A rate-limiting step in breast cancer progression is acquisition of the invasive phenotype, which can precede metastasis. Expression of cell-surface proteases at the leading edge of a migrating cell provides cells with a mechanism to cross tissue barriers. A newly appreciated mechanism that may be relevant for breast cancer cell invasion is the formation of invadopodia, well-defined structures that project from the ventral membrane and promote degradation of the extracellular matrix, allowing the cell to cross a tissue barrier. Recently, there has been some controversy and discussion as to whether invadopodia, which are associated with carcinoma cells, are related to a similar structure called podosomes, which are associated with normal cells. Invadopodia and podosomes share many common characteristics, including a similar size, shape, subcellular localization and an ability to promote invasion. These two structures also share many common protein components, which we outline herein. It has been speculated that podosomes may be precursors to invadopodia and by extension both structures may be relevant to cancer cell invasion. Here, we compare and contrast the protein components of invadopodia and podosomes and discuss a potential role for these proteins and the evidence that supports a role for invadopodia and podosomes in breast cancer invasion.

Invadopodia: at the cutting edge of tumour invasion

Invasion of tissues by malignant tumours is facilitated by tumour cell migration and degradation of extracellular matrix (ECM) barriers. Several invasive neoplasms, including head and neck squamous cell carcinoma, breast carcinoma, melanoma and glioma, contain tumour cells that can form actin-rich protrusions with ECM proteolytic activity called invadopodia. These dynamic organelle-like structures adhere to, and digest, collagens, laminins and fibronectin. Invadopodia are dependent on multiple transmembrane, cytoplasmic and secreted proteins engaged in cell adhesion, signal transduction, actin assembly, membrane regulation and ECM proteolysis. Strategies aimed at disrupting invadopodia could form the basis of novel anti-invasive therapies for treating patients. Here we review the molecular basis of invadopodia formation with particular emphasis on the intracellular signaling networks that are essential for invadopodia activity and examine the potential role of these structures in glioma invasion.

Stat5 as a diagnostic marker for leukemia

The Jak-Stat-Socs pathway is an important component of cytokine receptor signaling. Not surprisingly, perturbation of this pathway is implicated in diseases of hematopoietic and immune origin, including leukemia, lymphoma and immune deficiencies. This review examines the role of a key component of this pathway, Stat5. This has been shown to be activated in a variety of leukemias and myeloproliferative disorders, including downstream of a range of key oncogenes where it has been shown to play an important role in mediating their effects. Therefore, Stat5 represents a useful pan-leukemia/myeloproliferative disorder diagnostic marker and key therapeutic end point, as well as representing an attractive therapeutic target for these disorders.