Newest findings on the oldest oncogene; how activated src does it

Advancing Src kinase inhibition: From structural design to therapeutic innovation - A comprehensive review

Src kinase, a non-receptor tyrosine kinase implicated in cellular signaling networks, plays a pivotal role in tumor progression and therapeutic resistance. Despite intensive research efforts spanning decades, no Src-selective kinase inhibitors have yet entered clinical use, highlighting the challenges in developing targeted therapeutics. Here we review recent advances in small-molecule Src inhibitor development, focusing on structural design strategies, binding mechanisms, and therapeutic applications. We analyze emerging approaches including fragment-based drug design, allosteric targeting, and substrate-competitive inhibition that have yielded promising new scaffold classes. Special attention is given to innovations in achieving isozyme selectivity, particularly through exploitation of non-ATP binding pockets and covalent inhibition strategies. Integration of artificial intelligence, living organoid platforms, and targeted protein degradation technologies is accelerating inhibitor optimization. We discuss key challenges in Src inhibitor development, including the need for enhanced selectivity, reduced off-target effects, and improved resistance profiles. Our analysis reveals promising directions for future therapeutic development, emphasizing the importance of rational design principles guided by structural insights and emerging technologies. These findings provide a framework for developing next-generation Src inhibitors with improved clinical potential.

Endothelial c-Src Mediates Neovascular Tuft Formation in Oxygen-Induced Retinopathy

Vascular retinopathy, characterized by abnormal blood vessel growth in the retina, frequently results in vision impairment or loss. Neovascular tufts, a distinctive pathologic feature of this condition, are highly leaky blood vessel structures, exacerbating secondary complications. Despite their clinical significance, the mechanisms underlying tuft development are not fully elucidated, posing challenges for effective management and treatment of vascular retinopathy. This study investigates the role of cellular (c)-Src in neovascular tuft formation. Although c-Src is a pivotal regulator in developmental angiogenesis within the retinal vasculature, its specific role in governing pathologic retinal angiogenesis remains to be fully understood. Herein, the oxygen-induced retinopathy model was used for neovascular tuft formation in both Cre-mediated vascular-specific c-Src knockout mice and wild-type littermates. High-resolution imaging and analysis of isolated retinas were conducted. c-Src depletion demonstrated a significant reduction in neovascular tufts within the oxygen-induced retinopathy model. This decrease in tuft formation was observed independently of any alterations in cell death, cell proliferation, or cell adhesion, and the absence of c-Src did not impact tuft pericyte coverage and junctional morphology. These findings underline the critical role of c-Src in the pathogenesis of neovascular tufts in vascular retinopathy. Understanding the molecular mechanisms involving c-Src may offer valuable insights for the development of targeted therapies aimed at mitigating vision-threatening complications associated with retinopathy.

Identification and characterization of STAT family in silver pomfret (Pampus argenteus) involved in different exogenous stresses

Members of the Signal Transducer and Activator of Transcription (STAT) family function pivotally as transcriptional activators integral to the modulation of inflammatory responses. The aquaculture of silver pomfret is frequently compromised by the imposition of exogenous stressors, which include thermal fluctuations, notably low-temperatures, diminished oxygen levels, and the onslaught of bacterial pathogens. Notwithstanding the critical impact of these stressors, the scientific literature presents a notable gap in our understanding of the STAT pathway's role in the silver pomfret's adaptive response mechanisms. To address this lacuna, we identified stat genes in the silver pomfret-denominated as Pastat1, Pastat2, Pastat3, Pastat4, and Pastat5-through a thorough and systematic bioinformatics analysis. Further scrutiny of the gene configurations and constituent motifs has elucidated that STAT proteins possess analogous structural frameworks and exhibit significant evolutionary preservation. Subsequently, the expression patterns of five stat genes were verified by RT-qPCR in twelve different tissues and four growth periods in healthy fish, showing that the expression of Pastat genes was temporally and spatially specific, with most of the stat genes expressed at higher levels in the spleen, following muscle, gill, and liver. Transcriptomic analysis of exposure to exogenous stressors, specifically formaldehyde and low-temperature conditions, elucidated that Pastat1 and Pastat2 genes exhibited a heightened sensitivity to these environmental challenges. RT-qPCR assays demonstrated a marked alteration in the expression profiles of jak1 and Pastat gene suites in PaS upon prolonged bacterial infection subsequent to these exogenous insults. Moreover, the gene expression of the downstream effectors involved in innate immunity and apoptosis displayed marked deviations. This study additionally elucidated the Pastat gene family's role in modulating the innate immune response and apoptotic regulation within the silver pomfret during exogenous stressors and subsequent pathogenic incursions.

Src-Dependent NM2A Tyrosine Phosphorylation Regulates Actomyosin Remodeling

Non-muscle myosin 2A (NM2A) is a key cytoskeletal enzyme that, along with actin, assembles into actomyosin filaments inside cells. NM2A is fundamental for cell adhesion and motility, playing important functions in different stages of development and during the progression of viral and bacterial infections. Phosphorylation events regulate the activity and the cellular localization of NM2A. We previously identified the tyrosine phosphorylation of residue 158 (pTyr¹⁵⁸) in the motor domain of the NM2A heavy chain. This phosphorylation can be promoted by Listeria monocytogenes infection of epithelial cells and is dependent on Src kinase; however, its molecular role is unknown. Here, we show that the status of pTyr¹⁵⁸ defines cytoskeletal organization, affects the assembly/disassembly of focal adhesions, and interferes with cell migration. Cells overexpressing a non-phosphorylatable NM2A variant or expressing reduced levels of Src kinase display increased stress fibers and larger focal adhesions, suggesting an altered contraction status consistent with the increased NM2A activity that we also observed. We propose NM2A pTyr¹⁵⁸ as a novel layer of regulation of actomyosin cytoskeleton organization.

Maturation of a postsynaptic domain: Role of small Rho GTPases in organising nicotinic acetylcholine receptor aggregates at the vertebrate neuromuscular junction

The neuromuscular junction (NMJ) is the peripheral synapse formed between a motor axon and a skeletal muscle fibre that allows muscle contraction and the coordinated movement in many species. A main hallmark of the mature NMJ is the assembly of nicotinic acetylcholine receptor (nAChR) aggregates in the muscle postsynaptic domain, that distributes in perfect apposition to presynaptic motor terminals. To assemble its unique functional architecture, initial embryonic NMJs undergo an early postnatal maturation process characterised by the transformation of homogenous nAChR-containing plaques to elaborate and branched pretzel-like structures. In spite of a detailed morphological characterisation, the molecular mechanisms controlling the intracellular scaffolding that organises a postsynaptic domain at the mature NMJ have not been fully elucidated. In this review, we integrate evidence of key processes and molecules that have shed light on our current understanding of the NMJ maturation process. On the one hand, we consider in vitro studies revealing the potential role of podosome-like structures to define discrete low nAChR-containing regions to consolidate a plaque-to-pretzel transition at the NMJ. On the other hand, we focus on in vitro and in vivo evidence demonstrating that members of the Ras homologous (Rho) protein family of small GTPases (small Rho GTPases) play indispensable roles on NMJ maturation by regulating the stability of nAChR aggregates. We combine this evidence to propose that small Rho GTPases are key players in the assembly of podosome-like structures that drive the postsynaptic maturation of vertebrate NMJs.

Dual-target Janus kinase (JAK) inhibitors: Comprehensive review on the JAK-based strategies for treating solid or hematological malignancies and immune-related diseases

Janus kinases (JAKs) are the non-receptor tyrosine kinases covering JAK1, JAK2, JAK3, and TYK2 which regulate signal transductions of hematopoietic cytokines and growth factors to play essential roles in cell growth, survival, and development. Dysregulated JAK activity leading to a constitutively activated signal transducers and activators of transcription (STAT) is strongly associated with immune-related diseases and cancers. Targeting JAK to interfere the signaling of JAK/STAT pathway has achieved quite success in the treatment of these diseases. However, inadequate clinical response and serious adverse events come along by the treatment of monotherapy of JAK inhibitors. With better and deeper understanding of JAK/STAT pathway in the pathogenesis of diseases, researchers start to show huge interest in combining inhibition of JAK and other oncogenic targets to realize a broader regulation on pathological processes to block disease development and progression, which has hastened extensive research of dual JAK inhibitors over the past decades. Until now, studies of dual JAK inhibitors have added BTK, SYK, FLT3, HDAC, Src, and Aurora kinases to the overall inhibitory profile and demonstrated significant advantage and superiority over single-target inhibitors. In this review, we elucidated the possible mechanism of synergic effects caused by dual JAK inhibitors and briefly describe the development of these agents.

Effects of Electrical Stimulation on the Signal Transduction-Related Proteins, c-Src and Focal Adhesion Kinase, in Fibroblasts

Electrical stimulation of the skin and muscles, e.g., in the fields of rehabilitation medicine and acupuncture, is known to locally increase blood flow and metabolism, and thus have beneficial health effects. However, little is known about the changes in cellular morphology or regulation of the localization of specific proteins in response to electrical stimuli. The present study was performed to examine the effects of electrical stimulation on the cytoskeletal system of cultured fibroblasts. Following application of electrical stimulation to cultured fibroblastic cells for a period of about 2 h, the stress fibers in the cells became thicker and the cells showed a contracted appearance. Cells were subjected to periodic electrical stimulation for 0 (unstimulated control), 2, 5, or 20 h. The stress fibers showed an increase in thickness within 2 h, and became gradually thicker until 20 h. In addition, the focal adhesions and stress fibers were enlarged after 2 h of continuous stimulation, and both stress fibers and focal adhesions became larger and thicker after 20 h of periodic stimulation. Cells showed increased staining of focal adhesions with anti-phosphotyrosine antibody (PY-20) after electrical stimulation. Cells also showed increased staining of tyrosine-phosphorylated focal adhesion kinase (FAK) (pY397) and tyrosine-phosphorylated c-Src (pY418), indicating that electrical stimulation affected signal transduction-related proteins.

Eph/Ephrin-Based Protein Complexes: The Importance of cis Interactions in Guiding Cellular Processes

Although intracellular signal transduction is generally represented as a linear process that transmits stimuli from the exterior of a cell to the interior via a transmembrane receptor, interactions with additional membrane-associated proteins are often critical to its success. These molecules play a pivotal role in mediating signaling via the formation of complexes in cis (within the same membrane) with primary effectors, particularly in the context of tumorigenesis. Such secondary effectors may act to promote successful signaling by mediating receptor-ligand binding, recruitment of molecular partners for the formation of multiprotein complexes, or differential signaling outcomes. One signaling family whose contact-mediated activity is frequently modulated by lateral interactions at the cell surface is Eph/ephrin (EphA and EphB receptor tyrosine kinases and their ligands ephrin-As and ephrin-Bs). Through heterotypic interactions in cis, these molecules can promote a diverse range of cellular activities, including some that are mutually exclusive (cell proliferation and cell differentiation, or adhesion and migration). Due to their broad expression in most tissues and their promiscuous binding within and across classes, the cellular response to Eph:ephrin interaction is highly variable between cell types and is dependent on the cellular context in which binding occurs. In this review, we will discuss interactions between molecules in cis at the cell membrane, with emphasis on their role in modulating Eph/ephrin signaling.

Impairment of Autophagic Flux Participates in the Antitumor Effects of TAT-Cx43266-283 in Glioblastoma Stem Cells

Simple Summary

Autophagy is a process in which the cell recycles components that are not needed at that moment and uses the resulting elements to satisfy more urgent needs. Depending on the specific context, this can be beneficial or detrimental for tumor development. We found that in glioblastoma, the most lethal brain tumor, autophagy is upregulated and contributes to glioblastoma stem cell survival under starvation. Importantly, the antitumor peptide TAT-Cx43_266-283 blocks autophagy flux, contributing to the death of glioblastoma stem cells. This peptide induces glioblastoma stem cell death in nutrient-deprived and complete environments, while the effect of other unsuccessful drugs for glioblastoma depends on nutrient context, supporting the potential of TAT-Cx43_266-283 as a treatment to improve the lives of glioblastoma patients.

Abstract

Autophagy is a physiological process by which various damaged or non-essential cytosolic components are recycled, contributing to cell survival under stress conditions. In cancer, autophagy can have antitumor or protumor effects depending on the developmental stage. Here, we use Western blotting, immunochemistry, and transmission electron microscopy to demonstrate that the antitumor peptide TAT-Cx43_266-283, a c-Src inhibitor, blocks autophagic flux in glioblastoma stem cells (GSCs) under basal and nutrient-deprived conditions. Upon nutrient deprivation, GSCs acquired a dormant-like phenotype that was disrupted by inhibition of autophagy with TAT-Cx43_266-283 or chloroquine (a classic autophagy inhibitor), leading to GSC death. Remarkably, dasatinib, a clinically available c-Src inhibitor, could not replicate TAT-Cx43_266-283 effect on dormant GSCs, revealing for the first time the possible involvement of pathways other than c-Src in TAT-Cx43_266-283 effect. TAT-Cx43_266-283 exerts an antitumor effect both in nutrient-complete and nutrient-deprived environments, which constitutes an advantage over chloroquine and dasatinib, whose effects depend on nutrient environment. Finally, our analysis of the levels of autophagy-related proteins in healthy and glioma donors suggests that autophagy is upregulated in glioblastoma, further supporting the interest in inhibiting this process in the most aggressive brain tumor and the potential use of TAT-Cx43_266-283 as a therapy for this type of cancer.

The tumor suppressor LATS2 reduces v-Src-induced membrane blebs in a kinase activity-independent manner

When cells with excess DNA, such as tetraploid cells, undergo cell division, it can contribute to cellular transformation via asymmetrical chromosome segregation-generated genetic diversity. Cell cycle progression of tetraploid cells is suppressed by large tumor suppressor 2 (LATS2) kinase-induced inhibitory phosphorylation of the transcriptional coactivator Yes-associated protein (YAP). We recently reported that the oncogene v-Src induces tetraploidy and promotes cell cycle progression of tetraploid cells by suppressing LATS2 activity. We explore here the mechanism by which v-Src suppresses LATS2 activity and the role of LATS2 in v-Src-expressing cells. LATS2 was directly phosphorylated by v-Src and the proto-oncogene c-Src, resulting in decreased LATS2 kinase activity. This kinase-deficient LATS2 accumulated in a YAP transcriptional activity-dependent manner, and knockdown of either LATS2 or the LATS2-binding partner moesin-ezrin-radixin-like protein (Merlin) accelerated v-Src-induced membrane bleb formation. Upon v-Src expression, the interaction of Merlin with LATS2 was increased possibly due to a decrease in Merlin phosphorylation at Ser518, the dephosphorylation of which is required for the open conformation of Merlin and interaction with LATS2. LATS2 was colocalized with Merlin at the plasma membrane in a manner that depends on the Merlin-binding region of LATS2. The bleb formation in v-Src-expressing and LATS2-knockdown cells was rescued by the reexpression of wild-type or kinase-dead LATS2 but not the LATS2 mutant lacking the Merlin-binding region. These results suggest that the kinase-deficient LATS2 plays a role with Merlin at the plasma membrane in the maintenance of cortical rigidity in v-Src-expressing cells, which may cause tumor suppression.

What Animal Cancers teach us about Human Biology

Cancers in animals present a large, underutilized reservoir of biomedical information with critical implication for human oncology and medicine in general. Discussing two distinct areas of tumour biology in non-human hosts, we highlight the importance of these findings for our current understanding of cancer, before proposing a coordinated strategy to harvest biomedical information from non-human resources and translate it into a clinical setting. First, infectious cancers that can be transmitted as allografts between individual hosts, have been identified in four distinct, unrelated groups, dogs, Tasmanian devils, Syrian hamsters and, surprisingly, marine bivalves. These malignancies might hold the key to improving our understanding of the interaction between tumour cell and immune system and, thus, allow us to devise novel treatment strategies that enhance anti-cancer immunosurveillance, as well as suggesting more effective organ and stem cell transplantation strategies. The existence of these malignancies also highlights the need for increased scrutiny when considering the existence of infectious cancers in humans. Second, it has long been understood that no linear relationship exists between the number of cells within an organism and the cancer incidence rate. To resolve what is known as Peto's Paradox, additional anticancer strategies within different species have to be postulated. These naturally occurring idiosyncrasies to avoid carcinogenesis represent novel potential therapeutic strategies.

Regulation of Fibroblast Cell Polarity by Src Tyrosine Kinase

Src protein tyrosine kinases (SFKs) are a family of nonreceptor tyrosine kinases that are localized beneath the plasma membrane and are activated during cell adhesion, migration, and elongation. Due to their involvement in the activation of signal transduction cascades, SFKs have been suggested to play important roles in the determination of cell polarity during cell extension and elongation. However, the mechanism underlying Src-mediated polarity formation remains unclear. The present study was performed to investigate the mechanisms underlying Src-induced cell polarity formation and cell elongation using Src knockout fibroblasts (SYFs) together with an inhibitor of Src. Normal and Src knockout fibroblasts were also transfected with a wild-type c-Src, dominant negative c-Src, or constitutively active c-Src gene to analyze the changes in cell morphology. SYF cells cultured on a glass substrate elongated symmetrically into spindle-shaped cells, with the formation of focal adhesions at both ends of the cells. When normal fibroblasts were treated with Src Inhibitor No. 5, a selective inhibitor of Src tyrosine kinases, they elongated into symmetrical spindle-shaped cells, similar to SYF cells. These results suggest that cell polarity during extension and elongation may be regulated by SFKs and that the expression and regulation of Src are important for the formation of polarity during cell elongation.

SRC Signaling in Cancer and Tumor Microenvironment

Pioneering experiments performed by Harold Varmus and Mike Bishop in 1976 led to one of the most influential discoveries in cancer research and identified the first cancer-causing oncogene called Src. Later experimental and clinical evidence suggested that Src kinase plays a significant role in promoting tumor growth and progression and its activity is associated with poor patient survival. Thus, several Src inhibitors were developed and approved by FDA for treatment of cancer patients. Tumor microenvironment (TME) is a highly complex and dynamic milieu where significant cross-talk occurs between cancer cells and TME components, which consist of tumor-associated macrophages, fibroblasts, and other immune and vascular cells. Growth factors and chemokines activate multiple signaling cascades in TME and induce multiple kinases and pathways, including Src, leading to tumor growth, invasion/metastasis, angiogenesis, drug resistance, and progression. Here, we will systemically evaluate recent findings regarding regulation of Src and significance of targeting Src in cancer therapy.

Historical retrospective of the oncogene and new perspectives (Review)

Since its first discovery as part of the Rous sarcoma virus (RSV) genome, the c-SRC (SRC) proto-oncogene has been proved a key regulator of cancer development and progression, and thus it has been highlighted as an attractive target for anti-cancer therapeutic strategies. Though the exact mechanisms of its action are still not fully understood, SRC protein mediates crucial normal cell functions, such as cell development, proliferation and survival, and its dysregulation is considered as an oncogenic signature and a driving force for cancer initiation. In the present review, we present a flashback to the history of the Src research, while focusing on the most important milestones in the field. Moreover, we investigate the proposed regulatory mechanisms and molecules that mediate its action in order to designate putative therapeutic targets and useful prognostic and/or diagnostic tools. Furthermore, we present and discuss existing therapeutic approaches that are explored in clinical settings.

EPS8 phosphorylation by Src modulates its oncogenic functions

Background

EPS8 is a scaffolding protein that regulates proliferation, actin dynamics and receptor trafficking. Its expression is increased in cancer, enhancing mitogenesis, migration and tumorigenesis. Src phosphorylates EPS8 at four tyrosine residues, although the function is unknown. Here we investigated the pro-oncogenic role of EPS8 tyrosine phosphorylation at Src target sites in HNSCC.

Methods

Plasmids expressing EPS8 Src-mediated phosphorylation site mutants (Y485F, Y525F, Y602F, Y774F and all four combined [FFFF]) were expressed in cells containing a normal endogenous level of EPS8. In addition, cells were treated with dasatinib to inhibit Src activity. EPS8 downstream targets were evaluated by western blotting. Wound closure, proliferation, immunofluorescence and tumorgenicity assays were used to investigate the impact of phenylalanine mutations on EPS8 biological functions.

Results

FOXM1, AURKA, and AURKB were decreased in cells expressing FFFF- and Y602F-EPS8 mutants, while cells harbouring the Y485F-, Y525F- and Y774F-EPS8 mutants showed no differences compared to controls. Consistent with this, dasatinib decreased the expression of EPS8 targets. Moreover, Y602F- and FFFF-EPS8 mutants reduced mitogenesis and motility. Strikingly though, FFFF- or Y602F-EPS8 mutants actually promoted tumorigenicity compared with control cells.

Conclusions

Phosphorylation of EPS8 at Y602 is crucial for signalling to the cell cycle and may provide insight to explain reduced efficacy of dasatinib treatment.

EGFR/FAK and c-Src signalling pathways mediate the internalisation of Staphylococcus aureus by osteoblasts

Internalisation of Staphylococcus aureus in osteoblasts plays a critical role in the persistence and recurrence of osteomyelitis, the mechanisms involved in this process remain largely unknown. In the present study, evidence of internalised S. aureus in osteoblasts was found in long bone of haematogenous osteomyelitis in mice after 2 weeks of infection. Meanwhile, eliminating extracellular S. aureus by gentamicin can partially rescue bone loss, whereas the remaining intracellular S. aureus in osteoblasts may be associated with continuous bone destruction. In osteoblastic MC3T3 cells, intracellular S. aureus was detectable as early as 15 min after infection, and the internalisation rates increased with the extension of infection time. Additionally, S. aureus invasion stimulated the expression of phosphor-focal adhesion kinase (FAK), phosphor-epidermal growth factor receptor (EGFR) and phosphor-c-Src in a time-dependent way, and blocking EGFR/FAK or c-Src signalling significantly reduced the internalisation rate of S. aureus in osteoblasts. Our findings provide new insights into the mechanism of S. aureus internalisation in osteoblast and raise the potential of targeting EGFR/FAK and c-Src as adjunctive therapeutics for treating chronic S. aureus osteomyelitis.

Rac1 Signaling: From Intestinal Homeostasis to Colorectal Cancer Metastasis

The small GTPase Rac1 has been implicated in a variety of dynamic cell biological processes, including cell proliferation, cell survival, cell-cell contacts, epithelial mesenchymal transition (EMT), cell motility, and invasiveness. These processes are orchestrated through the fine tuning of Rac1 activity by upstream cell surface receptors and effectors that regulate the cycling Rac1-GDP (off state)/Rac1-GTP (on state), but also through the tuning of Rac1 accumulation, activity, and subcellular localization by post translational modifications or recruitment into molecular scaffolds. Another level of regulation involves Rac1 transcripts stability and splicing. Downstream, Rac1 initiates a series of signaling networks, including regulatory complex of actin cytoskeleton remodeling, activation of protein kinases (PAKs, MAPKs) and transcription factors (NFkB, Wnt/β-catenin/TCF, STAT3, Snail), production of reactive oxygen species (NADPH oxidase holoenzymes, mitochondrial ROS). Thus, this GTPase, its regulators, and effector systems might be involved at different steps of the neoplastic progression from dysplasia to the metastatic cascade. After briefly placing Rac1 and its effector systems in the more general context of intestinal homeostasis and in wound healing after intestinal injury, the present review mainly focuses on the several levels of Rac1 signaling pathway dysregulation in colorectal carcinogenesis, their biological significance, and their clinical impact.

Ubiquitination of Src promotes its secretion via small extracellular vesicles

Upregulation of the Src tyrosine kinase is implicated in the progression of cancer. The oncogenic potential of Src is suppressed via several negative regulation systems including degradation via the ubiquitin-proteasome pathway. Here, we show that ubiquitination of Src promotes its secretion via small extracellular vesicles (sEVs) to suppress its oncogenic potential. In MDCK cells expressing a modified Src that can be activated by hydroxytamoxifen, activated Src was transported to late endosomes/lysosomes and secreted via sEVs. The secretion of Src was suppressed by ablation of Cbl E3-ligase, suggesting the contribution of ubiquitination to this process. Activated Src was ubiquitinated at multiple sites, and Lys429 was identified as a critical site for sEV-mediated secretion. Mutation of Src at Lys429 (R429) caused resistance to ubiquitination and decreased its secretion via sEVs. The activated R429 mutant was also transported to late endosomes/lysosomes, whereas its incorporation into intraluminal vesicles was reduced. Activation of the R429 mutant induced a greater FAK activation than that of wild-type Src, thereby potentiating Src-induced invasive phenotypes, such as invadopodia formation and invasive activity. These findings demonstrate that ubiquitination of activated Src at Lys429 promotes its secretion via sEVs, suggesting a potential strategy to suppress the oncogenic function of upregulated Src.

AGO2 phosphorylation by c-Src kinase promotes tumorigenesis

Numerous studies have reported that c-Src is highly expressed with high tyrosine kinase activity in a variety of tumors. However, it remains unclear whether c-Src contributes to the miRNA pathway. Here, we report that c-Src can interact with and phosphorylate AGO2, a core component of RISC complex, at tyr 393, tyr 529 and tyr749. Mechanistically, it is confirmed that c-Src phosphorylation of AGO2 at tyr393 reduces its binding to DICER, thereby suppressing the maturation of long-loop pre-miR-192. However, the other two phosphorylation sites don’t work on this function. Significantly, Ectopic expression of wild-type AGO2, but not the three tyrosine site mutants, has an obvious tumor-promoting effect in vitro and in vivo, which function could be blocked thoroughly by treatment with c-Src kinase inhibitor, Saracatinib. Our findings identify AGO2 as c-Src target and c-Src phosphorylation of AGO2 may therefore play a potential role during tumor progress.

Quokka: a comprehensive tool for rapid and accurate prediction of kinase family-specific phosphorylation sites in the human proteome

Motivation

Kinase-regulated phosphorylation is a ubiquitous type of post-translational modification (PTM) in both eukaryotic and prokaryotic cells. Phosphorylation plays fundamental roles in many signalling pathways and biological processes, such as protein degradation and protein-protein interactions. Experimental studies have revealed that signalling defects caused by aberrant phosphorylation are highly associated with a variety of human diseases, especially cancers. In light of this, a number of computational methods aiming to accurately predict protein kinase family-specific or kinase-specific phosphorylation sites have been established, thereby facilitating phosphoproteomic data analysis.

Results

In this work, we present Quokka, a novel bioinformatics tool that allows users to rapidly and accurately identify human kinase family-regulated phosphorylation sites. Quokka was developed by using a variety of sequence scoring functions combined with an optimized logistic regression algorithm. We evaluated Quokka based on well-prepared up-to-date benchmark and independent test datasets, curated from the Phospho.ELM and UniProt databases, respectively. The independent test demonstrates that Quokka improves the prediction performance compared with state-of-the-art computational tools for phosphorylation prediction. In summary, our tool provides users with high-quality predicted human phosphorylation sites for hypothesis generation and biological validation.

Availability and implementation

The Quokka webserver and datasets are freely available at http://quokka.erc.monash.edu/.

Supplementary information

Supplementary data are available at Bioinformatics online.

A Tale of Two States: Normal and Transformed, With and Without Rigidity Sensing

For many years, major differences in morphology, motility, and mechanical characteristics have been observed between transformed cancer and normal cells. In this review, we consider these differences as linked to different states of normal and transformed cells that involve distinct mechanosensing and motility pathways. There is a strong correlation between repeated tissue healing and/or inflammation and the probability of cancer, both of which involve growth in adult tissues. Many factors are likely needed to enable growth, including the loss of rigidity sensing, but recent evidence indicates that microRNAs have important roles in causing the depletion of growth-suppressing proteins. One microRNA, miR-21, is overexpressed in many different tissues during both healing and cancer. Normal cells can become transformed by the depletion of cytoskeletal proteins that results in the loss of mechanosensing, particularly rigidity sensing. Conversely, the transformed state can be reversed by the expression of cytoskeletal proteins-without direct alteration of hormone receptor levels. In this review, we consider the different stereotypical forms of motility and mechanosensory systems. A major difference between normal and transformed cells involves a sensitivity of transformed cells to mechanical perturbations. Thus, understanding the different mechanical characteristics of transformed cells may enable new approaches to treating wound healing and cancer.

RACK1 interaction with c-Src is essential for osteoclast function

The scaffolding protein receptor for activated C-kinase 1 (RACK1) mediates receptor activator of nuclear factor κΒ ligand (RANKL)-dependent activation of p38 MAPK in osteoclast precursors; however, the role of RACK1 in mature osteoclasts is unclear. The aim of our study was to identify the interaction between RACK1 and c-Src that is critical for osteoclast function. A RACK1 mutant protein (mutations of tyrosine 228 and 246 residues to phenylalanine; RACK1 Y228F/Y246F) did not interact with c-Src. The mutant retained its ability to differentiate into osteoclasts; however, the integrity of the RANKL-mediated cytoskeleton, bone resorption activity, and phosphorylation of c-Src was significantly decreased. Importantly, lysine 152 (K152) within the Src homology 2 (SH2) domain of c-Src is involved in RACK1 binding. The c-Src K152R mutant (mutation of lysine 152 into arginine) impaired the resorption of bone by osteoclasts. These findings not only clarify the role of the RACK1-c-Src axis as a key regulator of osteoclast function but will also help to develop new antiresorption therapies to prevent bone loss-related diseases.

Targeted therapies for non-HPV-related head and neck cancer: challenges and opportunities in the context of predictive, preventive, and personalized medicine

Head and neck squamous cell carcinoma (HNSCC) develops in the mucosal lining of the upper aerodigestive tract, principally as a result of exposure to carcinogens present in tobacco products and alcohol, with oncogenic papillomaviruses also being recognized as etiological agents in a limited proportion of cases. As such, there is considerable scope for prevention of disease development and progression. However, despite multimodal approaches to treatment, tumor recurrence and metastatic disease are common problems, and clinical outcome is unsatisfactory. As our understanding of the genetics and biochemical aberrations in HNSCC has improved, so the development and use of molecularly targeted drugs to combat the disease have come to the fore. In this article, we review molecular mechanisms that alter signal transduction downstream of the epidermal growth factor receptor (EGFR) as well as those that perturb orderly cell cycle progression, such as p53 mutation, cyclin overexpression, and loss of cyclin-dependent kinase inhibitor function. We outline some of the tactics that have been employed to combat the altered biochemistry. These include blockade of the EGFR using humanized monoclonal antibodies such as cetuximab and small molecule tyrosine kinase inhibitors (TKIs) such as erlotinib/gefitinib and subsequent generations of TKIs, restoration of p53 function using MIRA compounds, and inhibition of cyclin-dependent kinase and aurora kinase activity using drugs such as palbociclib and alisertib. Knowledge of the underlying molecular mechanisms may be utilizable in order to predict disease behavior and tailor therapeutic interventions in a more personalized approach to improve clinical response. Use of liquid biopsy, omics platforms, and salivary diagnostics hold promise in this regard.

Control of Tyrosine Kinase Signalling by Small Adaptors in Colorectal Cancer

Tyrosine kinases (TKs) phosphorylate proteins on tyrosine residues as an intracellular signalling mechanism to coordinate intestinal epithelial cell communication and fate decision. Deregulation of their activity is ultimately connected with carcinogenesis. In colorectal cancer (CRC), it is still unclear how aberrant TK activities contribute to tumour formation because TK-encoding genes are not frequently mutated in this cancer. In vertebrates, several TKs are under the control of small adaptor proteins with potential important physiopathological roles. For instance, they can exert tumour suppressor functions in human cancer by targeting several components of the oncogenic TK signalling cascades. Here, we review how the Src-like adaptor protein (SLAP) and the suppressor of cytokine signalling (SOCS) adaptor proteins regulate the SRC and the Janus kinase (JAK) oncogenic pathways, respectively, and how their loss of function in the intestinal epithelium may influence tumour formation. We also discuss the potential therapeutic value of these adaptors in CRC.

Characterization of the Src-regulated kinome identifies SGK1 as a key mediator of Src-induced transformation

Despite significant progress, our understanding of how specific oncogenes transform cells is still limited and likely underestimates the complexity of downstream signalling events. To address this gap, we use mass spectrometry-based chemical proteomics to characterize the global impact of an oncogene on the expressed kinome, and then functionally annotate the regulated kinases. As an example, we identify 63 protein kinases exhibiting altered expression and/or phosphorylation in Src-transformed mammary epithelial cells. An integrated siRNA screen identifies nine kinases, including SGK1, as being essential for Src-induced transformation. Accordingly, we find that Src positively regulates SGK1 expression in triple negative breast cancer cells, which exhibit a prominent signalling network governed by Src family kinases. Furthermore, combined inhibition of Src and SGK1 reduces colony formation and xenograft growth more effectively than either treatment alone. Therefore, this approach not only provides mechanistic insights into oncogenic transformation but also aids the design of improved therapeutic strategies.

The systemic understanding of oncogenic kinase signalling is still limited. Here, the authors combine chemical proteomics with functional screens to assess the impact of oncogenic Src on the expressed kinome and identify SGK1 as a critical mediator of Src-induced cell transformation.

Potential therapeutic targets for growth arrest of colorectal cancer cells exposed to PTHrP

Although PTHrP is implicated in several cancers, its role in chemoresistance is not fully elucidated. We found that in CRC cells, PTHrP exerts proliferative and protective effects and induces cell migration. The aim of this work was to further study the effects of PTHrP in CRC cells. Herein we evidenced, for the first time, that PTHrP induces resistance to CPT-11 in Caco-2 and HCT116 cells; although both cell lines responded to the drug through different molecular mechanisms, the chemoresistance by PTHrP in these models is mediated through ERK, which in turn is activated by PCK, Src and Akt. Moreover, continue administration of PTHrP in nude mice xenografts increased the protein levels of this MAPK and of other markers related to tumorigenic events. The understanding of the molecular mechanisms leading to ERK 1/2 activation and the study of ERK targets may facilitate the development of new therapeutic strategies for CRC treatment.

Cell cycle-dependent force transmission in cancer cells

The generation of traction forces and their transmission to the extracellular environment supports the disseminative migration of cells from a primary tumor. In cancer cells, the periodic variation of nuclear stiffness during the cell cycle provides a functional link between efficient translocation and proliferation. However, the mechanical framework completing this picture remains unexplored. Here, the Fucci2 reporter was expressed in various human epithelial cancer cells to resolve their cell cycle phase transition. The corresponding tractions were captured by a recently developed reference-free confocal traction-force microscopy platform. The combined approach was conducive to the analysis of phase-dependent force variation at the level of individual integrin contacts. Detected forces were invariably higher in the G1 and early S phases than in the ensuing late S/G2, and locally colocalized with high levels of paxillin phosphorylation. Perturbation of paxillin phosphorylation at focal adhesions, obtained through the biochemical inhibition of focal adhesion kinase (FAK) or the transfection of nonphosphorylatable or phosphomimetic paxillin mutants, significantly diminished the force transmitted to the substrate. These data demonstrate a reproducible modulation of force transmission during the cell cycle progression of cancer cells, instrumental to their invasion of dense environments. In addition, they delineate a model in which paxillin phosphorylation supports the mechanical maturation of adhesions relaying forces to the substrate.

Folic acid inhibits colorectal cancer cell migration

We recently showed that folic acid (FA) could decrease the proliferation rate of colorectal cancer cells in vitro and reduce the volume of COLO-205 tumor in vivo. Since cancer cell proliferation and migration are two major events during cancer development, we further examined whether FA could also affect the migration of colorectal cancer cells. Transwell invasion assays demonstrated that FA reduced the invasion ability of colorectal cancer cell lines, COLO-205, LoVo and HT-29. Using COLO-205 as a cell model, we further delineated the molecular mechanism underlying FA-inhibited colorectal cancer cell invasion. Western blot analyses showed that FA (10 μM) activated cSrc, ERK1/2, NFκB, and p27 at serine 10 (Ser10), and up-regulated p53, p27, and KIS protein. Subcellular fractionation illustrated that FA treatment increased cytosolic translocation of p27, formation of the p27-RhoA complex, and RhoA degradation. The FA-induced migration inhibition in COLO-205 was abolished by blockade of the cSrc or ERK1/2 activity, knockdown of p27 or KIS using the siRNA technique, or over-expression of a constitutive active RhoA cDNA. Our results suggest that FA up-regulated p27 through increasing the cSrc/ERK1/2/NFκB/p53-mediated pathway. In the nucleus, FA up-regulated KIS, which in turn increased p27 phosphorylation at serine 10 (Ser10), subsequently resulting in cytosolic translocation of p27 and forming the p27-RhoA complex, thereby causing RhoA degradation, and eventually inhibited COLO-205 cell migration. Together with our previous findings suggest that FA reduced colorectal cancer development through inhibiting colorectal cancer cell proliferation and migration.

The tyrosine kinase v-Src causes mitotic slippage by phosphorylating an inhibitory tyrosine residue of Cdk1

The nonreceptor tyrosine kinase v-Src is an oncogene first identified in Rous sarcoma virus. The oncogenic effects of v-Src have been intensively studied; however, its effects on chromosomal integrity are not fully understood. Here, using HeLa S3/v-Src cells having inducible v-Src expression, we found that v-Src causes mitotic slippage in addition to cytokinesis failure, even when the spindle assembly checkpoint is not satisfied because of the presence of microtubule-targeting agents. v-Src's effect on mitotic slippage was also observed in cells after a knockdown of C-terminal Src kinase (Csk), a protein-tyrosine kinase that inhibits Src-family kinases and was partially inhibited by PP2, an Src-family kinase inhibitor. Proteomic analysis and in vitro kinase assay revealed that v-Src phosphorylates cyclin-dependent kinase 1 (Cdk1) at Tyr-15. This phosphorylation attenuated Cdk1 kinase activity, resulting in a decrease in the phosphorylation of Cdk1 substrates. Furthermore, v-Src-induced mitotic slippage reduced the sensitivity of the cells to microtubule-targeting agents, and cells that survived the microtubule-targeting agents exhibited polyploidy. These results suggest that v-Src causes mitotic slippage by attenuating Cdk1 kinase activity via direct phosphorylation of Cdk1 at Tyr-15. On the basis of these findings, we propose a model for v-Src-induced oncogenesis, in which v-Src-promoted mitotic slippage due to Cdk1 phosphorylation generates genetic diversity via abnormal cell division of polyploid cells and also increases the tolerance of cancer cells to microtubule-targeting agents.

Cycloartane triterpenoid (23R, 24E)-23-acetoxymangiferonic acid inhibited proliferation and migration in B16-F10 melanoma via MITF downregulation caused by inhibition of both β-catenin and c-Raf-MEK1-ERK signaling axis

We recently reported that (23R, 24E)-23-acetoxymangiferonic acid (23R-AMA), a cycloartane triterpenoid isolated by activity-guided separation from a methanol extract of Garcinia sp. bark, inhibited melanin production via inhibition of tyrosinase (TYR) expression in the B16-F10 melanoma cell line. Since 23R-AMA also inhibited microphthalmia-associated transcription factor (MITF) expression, an upstream factor of TYR, these features of 23R-AMA were thought to be appropriate for development of whitening cosmetics. However, 23R-AMA exhibited growth inhibition other than inhibition of melanin production in B16-F10 cells. Therefore, we investigated biological activities of 23R-AMA in detail, focused on its application as an anti-melanoma compound. In this study, we demonstrated that 23R-AMA inhibited cell proliferation and basic FGF (bFGF)-induced migration in B16-F10 cells. Furthermore, 23R-AMA promoted ser45/thr41 phosphorylation of β-catenin and suppressed its intranuclear accumulation, which was suggested to be related to inhibition of MITF expression. The transcriptional activity of MITF is known to be regulated by phosphorylation via activated ERK. Further investigation revealed that 23R-AMA inhibited phosphorylation of c-Raf, MEK-1, and ERK, and also that of upstream molecules including FAK and c-Src. These results suggested that 23R-AMA inhibited growth and migration of B16-F10 melanoma by regulating both MITF expression and its activity. The activities of 23R-AMA reported in this study are new aspects of cycloartane triterpenoids.

MicroRNA-137-mediated Src oncogenic signaling promotes cancer progression

The tyrosine kinase c-Src is frequently overexpressed and activated in a wide variety of human cancers. However, the molecular mechanisms responsible for the upregulation of c-Src remain elusive. To examine whether microRNA-mediated c-Src upregulation promotes cancer progression, we screened miRNAs with complementarity to the 3'-UTR of c-Src mRNA. Among these miRNAs, down-regulation of miR-137 was tightly associated with c-Src-mediated tumor progression of human colon cancer cells/tissues. Re-expression of miR-137 in human colon cancer cells suppressed tumor growth and caused the disruption of focal contacts, suppression of cell adhesion, and invasion, although restoration of c-Src in miR-137-treated cells could not fully rescue the tumor-suppressive effect of miR-137. We found that miR-137 targets AKT2 and paxillin also and miR-137-mediated regulation of c-Src /AKT2 is crucial for controlling tumor growth, whereas that of c-Src/paxillin contributes to malignancy. miR-137 suppressed Src-related oncogenic signaling and changed the expression of miRNAs that are regulated by Src activation. miR-137 controls the expression of c-Src/AKT2/paxillin and synergistically suppresses Src oncogenic signaling evoked from focal adhesions. In various human cancers that harbor c-Src upregulation, the dysfunction of this novel mechanism would serve as a critical trigger for tumor progression.

Src Cooperates with Oncogenic Ras in Tumourigenesis via the JNK and PI3K Pathways in Drosophila epithelial Tissue

The Ras oncogene (Rat Sarcoma oncogene, a small GTPase) is a key driver of human cancer, however alone it is insufficient to produce malignancy, due to the induction of cell cycle arrest or senescence. In a Drosophila melanogaster genetic screen for genes that cooperate with oncogenic Ras (bearing the Ras^V12 mutation, or Ras^ACT), we identified the Drosophila Src (Sarcoma virus oncogene) family non-receptor tyrosine protein kinase genes, Src42A and Src64B, as promoting increased hyperplasia in a whole epithelial tissue context in the Drosophila eye. Moreover, overexpression of Src cooperated with Ras^ACT in epithelial cell clones to drive neoplastic tumourigenesis. We found that Src overexpression alone activated the Jun N-terminal Kinase (JNK) signalling pathway to promote actin cytoskeletal and cell polarity defects and drive apoptosis, whereas, in cooperation with Ras^ACT, JNK led to a loss of differentiation and an invasive phenotype. Src + Ras^ACT cooperative tumourigenesis was dependent on JNK as well as Phosphoinositide 3-Kinase (PI3K) signalling, suggesting that targeting these pathways might provide novel therapeutic opportunities in cancers dependent on Src and Ras signalling.

Discovery of Novel Dual Mechanism of Action Src Signaling and Tubulin Polymerization Inhibitors (KX2-391 and KX2-361)

The discovery of potent, peptide site directed, tyrosine kinase inhibitors has remained an elusive goal. Herein we describe the discovery of two such clinical candidates that inhibit the tyrosine kinase Src. Compound 1 is a phase 3 clinical trial candidate that is likely to provide a first in class topical treatment for actinic keratosis (AK) with good efficacy and dramatically less toxicity compared to existing standard therapy. Compound 2 is a phase 1 clinical trial candidate that is likely to provide a first in class treatment of malignant glioblastoma and induces 30% long-term complete tumor remission in animal models. The discovery strategy for these compounds iteratively utilized molecular modeling, along with the synthesis and testing of increasingly elaborated proof of concept compounds, until the final clinical candidates were arrived at. This was followed with mechanism of action (MOA) studies that revealed tubulin polymerization inhibition as the second MOA.

Network approach of the conformational change of c-Src, a tyrosine kinase, by molecular dynamics simulation

Non-receptor tyrosine kinase c-Src plays a critical role in numerous cellular signalling pathways. Activation of c-Src from its inactive to the active state involves large-scale conformational changes, and is controlled by the phosphorylation state of two major phosphorylation sites, Tyr416 and Tyr527. A detailed mechanism for the entire conformational transition of c-Src via phosphorylation control of Tyr416 and Tyr527 is still elusive. In this study, we investigated the inactive-to-active conformational change of c-Src by targeted molecular dynamics simulation. Based on the simulation, we proposed a dynamical scenario for the activation process of c-Src. A detailed study of the conformational transition pathway based on network analysis suggests that Lys321 plays a key role in the c-Src activation process.

Connexin43 recruits PTEN and Csk to inhibit c-Src activity in glioma cells and astrocytes

Connexin43 (Cx43), the major protein forming gap junctions in astrocytes, is reduced in high-grade gliomas, where its ectopic expression exerts important effects, including the inhibition of the proto-oncogene tyrosine-protein kinase Src (c-Src). In this work we aimed to investigate the mechanism responsible for this effect. The inhibition of c-Src requires phosphorylation at tyrosine 527 mediated by C-terminal Src kinase (Csk) and dephosphorylation at tyrosine 416 mediated by phosphatases, such as phosphatase and tensin homolog (PTEN). Our results showed that the antiproliferative effect of Cx43 is reduced when Csk and PTEN are silenced in glioma cells, suggesting the involvement of both enzymes. Confocal microscopy and immunoprecipitation assays confirmed that Cx43, in addition to c-Src, binds to PTEN and Csk in glioma cells transfected with Cx43 and in astrocytes. Pull-down assays showed that region 266–283 in Cx43 is sufficient to recruit c-Src, PTEN and Csk and to inhibit the oncogenic activity of c-Src. As a result of c-Src inhibition, PTEN was increased with subsequent inactivation of Akt and reduction of proliferation of human glioblastoma stem cells. We conclude that the recruitment of Csk and PTEN to the region between residues 266 and 283 within the C-terminus of Cx43 leads to c-Src inhibition.

A Ser75-to-Asp phospho-mimicking mutation in Src accelerates ageing-related loss of retinal ganglion cells in mice

Src knockout mice show no detectable abnormalities in central nervous system (CNS) post-mitotic neurons, likely reflecting functional compensation by other Src family kinases. Cdk1- or Cdk5-dependent Ser75 phosphorylation in the amino-terminal Unique domain of Src, which shares no homology with other Src family kinases, regulates the stability of active Src. To clarify the roles of Src Ser75 phosphorylation in CNS neurons, we established two types of mutant mice with mutations in Src: phospho-mimicking Ser75Asp (SD) and non-phosphorylatable Ser75Ala (SA). In ageing SD/SD mice, retinal ganglion cell (RGC) number in whole retinas was significantly lower than that in young SD/SD mice in the absence of inflammation and elevated intraocular pressure, resembling the pathogenesis of progressive optic neuropathy. By contrast, SA/SA mice and wild-type (WT) mice exhibited no age-related RGC loss. The age-related retinal RGC number reduction was greater in the peripheral rather than the mid-peripheral region of the retina in SD/SD mice. Furthermore, Rho-associated kinase activity in whole retinas of ageing SD/SD mice was significantly higher than that in young SD/SD mice. These results suggest that Src regulates RGC survival during ageing in a manner that depends on Ser75 phosphorylation.

Anticancer properties of sodium selenite in human glioblastoma cell cluster spheroids

Glioblastoma (GBM) is the most common type of primary tumor of the central nervous system with a poor prognosis, needing the development of new therapeutic drugs. Few studies focused on sodium selenite (SS) effects in cancer cells cultured as multicellular tumor spheroids (MCTS or 3D) closer to in vivo tumor. We investigated SS anticancer effects in three human GBM cell lines cultured in 3D: LN229, U87 (O(6)-methyguanine-DNA-methyltransferase (MGMT) negative) and T98G (MGMT positive). SS absorption was evaluated and the cytotoxicity of SS and temozolomide (TMZ), the standard drug used against GBM, were compared. SS impacts on proliferation, cell death, and invasiveness were evaluated as well as epigenetic modifications by focusing on histone deacetylase (HDAC) activity and dimethyl-histone-3-lysine-9 methylation (H3K9m2), after 24h to 72h SS exposition. SS was absorbed by spheroids and was more cytotoxic than TMZ (i.e., for LN229, the IC50 was 38 fold-more elevated for TMZ than SS, at 72h). SS induced a cell cycle arrest in the S phase and apoptosis via caspase-3. SS decreased carbonic anhydrase-9 (CA9) expression, invasion on a Matrigel matrix and modulated E- and N-Cadherin transcript expressions. SS decreased HDAC activity and modulated H3K9m2 levels. 3D model provides a relevant strategy to screen new drugs and SS is a promising drug against GBM that should now be tested in GBM animal models.

ARHGAP42 is activated by Src-mediated tyrosine phosphorylation to promote cell motility

The tyrosine kinase Src acts as a key regulator of cell motility by phosphorylating multiple protein substrates that control cytoskeletal and adhesion dynamics. In an earlier phosphotyrosine proteomics study, we identified a novel Rho-GTPase activating protein, now known as ARHGAP42, as a likely biologically relevant Src substrate. ARHGAP42 is a member of a family of RhoGAPs distinguished by tandem BAR-PH domains lying N-terminal to the GAP domain. Like other family members, ARHGAP42 acts preferentially as a GAP for RhoA. We show that Src principally phosphorylates ARHGAP42 on tyrosine 376 (Tyr-376) in the short linker between the BAR-PH and GAP domains. The expression of ARHGAP42 variants in mammalian cells was used to elucidate its regulation. We found that the BAR domain is inhibitory toward the GAP activity of ARHGAP42, such that BAR domain deletion resulted in decreased active GTP-bound RhoA and increased cell motility. With the BAR domain intact, ARHGAP42 GAP activity could be activated by phosphorylation of Tyr-376 to promote motile cell behavior. Thus, phosphorylation of ARHGAP42 Tyr-376 is revealed as a novel regulatory event by which Src can affect actin dynamics through RhoA inhibition.

P-glycoprotein attenuates DNA repair activity in multidrug-resistant cells by acting through the Cbp-Csk-Src cascade

Recent studies have demonstrated that P-glycoprotein (P-gp) expression impairs DNA interstrand cross-linking agent-induced DNA repair efficiency in multidrug-resistant (MDR) cells. To date, the detailed molecular mechanisms underlying how P-gp interferes with Src activation and subsequent DNA repair activity remain unclear. In this study, we determined that the C-terminal Src kinase-binding protein (Cbp) signaling pathway involved in the negative control of Src activation is enhanced in MDR cells. We also demonstrated that cells that ectopically express P-gp exhibit reduced activation of DNA damage response regulators, such as ATM, Chk2, Braca1 and Nbs1 and hence attenuated DNA double-strand break repair capacity and become more susceptible than vector control cells to DNA interstrand cross-linking (ICL) agents. Moreover, we demonstrated that P-gp can not only interact with Cbp and Src but also enhance the formation of inhibitory C-terminal Src kinase (Csk)-Cbp complexes that reduce phosphorylation of the Src activation residue Y416 and increase phosphorylation of the Src negative regulatory residue Y527. Notably, suppression of Cbp expression in MDR cells restores cisplatin-induced Src activation, improves DNA repair capacity, and increases resistance to ICL agents. Ectopic expression of Cbp attenuates cisplatin-induced Src activation and increases the susceptibility of cells to ICL agents. Together, the current results indicate that P-gp inhibits DNA repair activity by modulating Src activation via Cbp-Csk-Src cascade. These results suggest that DNA ICL agents are likely to have therapeutic potential against MDR cells with P-gp-overexpression.

Actin stress fiber organization promotes cell stiffening and proliferation of pre-invasive breast cancer cells

Studies of the role of actin in tumour progression have highlighted its key contribution in cell softening associated with cell invasion. Here, using a human breast cell line with conditional Src induction, we demonstrate that cells undergo a stiffening state prior to acquiring malignant features. This state is characterized by the transient accumulation of stress fibres and upregulation of Ena/VASP-like (EVL). EVL, in turn, organizes stress fibres leading to transient cell stiffening, ERK-dependent cell proliferation, as well as enhancement of Src activation and progression towards a fully transformed state. Accordingly, EVL accumulates predominantly in premalignant breast lesions and is required for Src-induced epithelial overgrowth in Drosophila. While cell softening allows for cancer cell invasion, our work reveals that stress fibre-mediated cell stiffening could drive tumour growth during premalignant stages. A careful consideration of the mechanical properties of tumour cells could therefore offer new avenues of exploration when designing cancer-targeting therapies.

When cells acquire a malignant phenotype they become less stiff and this helps migration and invasion favouring metastasis. Here the authors show that Src-driven cell transformation and transition to a less stiff state follows an event of membrane stiffening due to stress fibres accumulation.

Cytokinesis Failure Leading to Chromosome Instability in v-Src-Induced Oncogenesis

v-Src, an oncogene found in Rous sarcoma virus, is a constitutively active variant of c-Src. Activation of Src is observed frequently in colorectal and breast cancers, and is critical in tumor progression through multiple processes. However, in some experimental conditions, v-Src causes growth suppression and apoptosis. In this review, we highlight recent progress in our understanding of cytokinesis failure and the attenuation of the tetraploidy checkpoint in v-Src-expressing cells. v-Src induces cell cycle changes—such as the accumulation of the 4N cell population—and increases the number of binucleated cells, which is accompanied by an excess number of centrosomes. Time-lapse analysis of v-Src-expressing cells showed that cytokinesis failure is caused by cleavage furrow regression. Microscopic analysis revealed that v-Src induces delocalization of cytokinesis regulators including Aurora B and Mklp1. Tetraploid cell formation is one of the causes of chromosome instability; however, tetraploid cells can be eliminated at the tetraploidy checkpoint. Interestingly, v-Src weakens the tetraploidy checkpoint by inhibiting the nuclear exclusion of the transcription coactivator YAP, which is downstream of the Hippo pathway and its nuclear exclusion is critical in the tetraploidy checkpoint. We also discuss the relationship between v-Src-induced chromosome instability and growth suppression in v-Src-induced oncogenesis.

Intramembranous processing by γ-secretase regulates reverse signaling of ephrin-B2 in migration of microglia

The Eph-ephrin system plays pivotal roles in cell adhesion and migration. The receptor-like functions of the ephrin ligands allow the regulation of intracellular processes via reverse signaling. γ-Secretase mediated processing of ephrin-B has previously been linked to activation of Src, a kinase crucial for focal adhesion and podosome phosphorylation. Here, we analyzed the role of γ-secretase in the stimulation of reverse ephrin-B2 signaling in the migration of mouse embryonic stem cell derived microglia. The proteolytic generation of the ephrin-B2 intracellular domain (ICD) by γ-secretase stimulates Src and focal adhesion kinase (FAK). Inhibition of γ-secretase decreased the phosphorylation of Src and FAK, and reduced cell motility. These effects were associated with enlargement of the podosomal surface. Interestingly, expression of ephrin-B2 ICD could rescue these effects, indicating that this proteolytic fragment mediates the activation of Src and FAK, and thereby regulates podosomal dynamics in microglial cells. Together, these results identify γ-secretase as well as ephrin-B2 as regulators of microglial migration.

HECTD1 controls the protein level of IQGAP1 to regulate the dynamics of adhesive structures

Background

Cell migration including collective cell movement and individual cell migration are crucial factors in embryogenesis. During the spreading/migration of cells, several types of adhesive structures physically interacting with the extracellular matrix (ECM) or with another cell have been described and the formation and maturation of adhesion structures are coordinated, however the molecular pathways involved are still not fully understood.

Results

We generated a mouse embryonic fibroblast line (MEF) from homozygous mutant (Hectd1^R/R, Hectd1^Gt(RRC200)) mouse of the E3 ubiquitin ligase for inhibin B receptor (Hectd1). Detailed examination of cell motion on MEF cells demonstrated that loss of Hectd1 resulted in accelerated cell spreading and migration but impaired directionality of migration. In Hectd1^R/R cells paxillin and zyxin were largely mis-localized, whereas their expression levels were unchanged. In addition the formation of focal adhesions (FAs) was impaired and the focal complexes (FXs) were increased. We further identified HECTD1 as a key regulator of IQGAP1. IQGAP1 co-localized together with HECTD1 in the leading edge of cells. HECTD1 interacted with IQGAP1 and regulated its degradation through ubiquitination. Over-expression of IQGAP1 in control MEF phenocopied the spreading and migration defects of Hectd1^R/R cells. In contrast, siRNA-mediated knockdown of IQGAP1 rescued the defects in cellular movement of Hectd1^R/R cells.

Conclusions

The E3 ligase activity of Hectd1 regulates the protein level of IQGAP1 through ubiquitination and therefore mediates the dynamics of FXs including the recruitment of paxillin and actinin. IQGAP1 is one of the effectors of HECTD1.

Electronic supplementary material

The online version of this article (doi:10.1186/s12964-016-0156-8) contains supplementary material, which is available to authorized users.

The residue at position 5 of the N-terminal region of Src and Fyn modulates their myristoylation, palmitoylation, and membrane interactions

Using biophysical methods in live cells and palmitoylation mutants of Src and Fyn, we show that palmitoylation stabilizes the interactions of SFKs with the plasma membrane. Moreover, we show that the amino acid at position 5 regulates the myristoylation and palmitoylation of these proteins, and thereby their targeting to raft domains.

The interactions of Src family kinases (SFKs) with the plasma membrane are crucial for their activity. They depend on their fatty-acylated N-termini, containing N-myristate and either a polybasic cluster (in Src) or palmitoylation sites (e.g., Fyn). To investigate the roles of these moieties in SFK membrane association, we used fluorescence recovery after photobleaching beam-size analysis to study the membrane interactions of c-Src-GFP (green fluorescent protein) or Fyn-GFP fatty-acylation mutants. Our studies showed for the first time that the membrane association of Fyn is more stable than that of Src, an effect lost in a Fyn mutant lacking the palmitoylation sites. Unexpectedly, Src-S3C/S6C (containing cysteines at positions 3/6, which are palmitoylated in Fyn) exhibited fast cytoplasmic diffusion insensitive to palmitoylation inhibitors, suggesting defective fatty acylation. Further replacement of the charged Lys-5 by neutral Gln to resemble Fyn (Src-S3C/S6C/K5Q) restored Fyn-like membrane interactions, indicating that Lys-5 in the context of Src-S3C/S6C interferes with its myristoylation/palmitoylation. This was validated by direct myristoylation and palmitoylation studies, which indicated that the residue at position 5 regulates the membrane interactions of Src versus Fyn. Moreover, the palmitoylation levels correlated with targeting to detergent-resistant membranes (rafts) and to caveolin-1. Palmitoylation-dependent preferential containment of Fyn in rafts may contribute to its lower transformation potential.

The Rho guanine nucleotide exchange factor ARHGEF5 promotes tumor malignancy via epithelial-mesenchymal transition

Epithelial tumor cells often acquire malignant properties, such as invasion/metastasis and uncontrolled cell growth, by undergoing epithelial–mesenchymal transition (EMT). However, the mechanisms by which EMT contributes to malignant progression remain elusive. Here we show that the Rho guanine nucleotide exchange factor (GEF) ARHGEF5 promotes tumor malignancy in a manner dependent on EMT status. We previously identified ARHGEF5, a member of the Dbl family of GEFs, as a multifunctional mediator of Src-induced cell invasion and tumor growth. In the present study, ARHGEF5 was upregulated during tumor growth factor-β-induced EMT in human epithelial MCF10A cells, and promoted cell migration by activating the Rho-ROCK pathway. ARHGEF5 was necessary for the invasive and in vivo metastatic activity of human colorectal cancer HCT116 cells. These findings underscore the crucial role of ARHGEF5 in cell migration and invasion/metastasis. An in vivo tumorigenesis assay revealed that ARHGEF5 had the potential to promote tumor growth via the phosphatidylinositol 3-kinase (PI3K) pathway. However, ARHGEF5 was not required for tumor growth in epithelial-like human colorectal cancer HCT116 and HT29 cells, whereas the growth of mesenchymal-like SW480 and SW620 cells depended on ARHGEF5. Induction of EMT by tumor necrosis factor-α or Slug in HCT116 cells resulted in the dependence of tumor growth on ARHGEF5. In these mesenchymal-like cells, Akt was activated via ARHGEF5 and its activity was required for tumor growth. Analysis of a transcriptome data set revealed that the combination of ARHGEF5 upregulation and E-cadherin downregulation or Snail upregulation was significantly correlated with poor prognosis in patients with colorectal cancers. Taken together, our findings suggest that EMT-induced ARHGEF5 activation contributes to the progression of tumor malignancy. ARHGEF5 may serve as a potential therapeutic target in a subset of malignant tumors that have undergone EMT.

Effect of saracatinib on pulmonary metastases from hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is the third leading cause of cancer-related death worldwide. Src is involved in multiple processes of cancer metastasis; however, its significance in HCC is not well defined. In the present study, overexpression of Src phosphorylation (Y416) was observed in the highly metastatic MHCC97H cell line; additionally, through inhibition of Src kinase activation, HCC cell proliferation, migration, invasion and colony formation were significantly reduced in vitro. Tumour growth was not affected in the orthotopic xenograft HCC model, but the metastasic potential was inhibited as revealed by reduced lung metastasic foci after administration of saracatinib. Phosphorylation level of Src pathway signalling molecules, such as Src, FAK and Stat3, were also reduced in vitro and in vivo, as a result of the anti-metastasic effects caused by saracatinib treatment. In conclusion, we demonstrated the pro-metastasic role of Src in HCC, and further experiments suggest the use of the Src inhibitor in combination with cytotoxic agents and other anticancer treatments to improve HCC prognosis.

Targeting cancer by binding iron: Dissecting cellular signaling pathways

Newer and more potent therapies are urgently needed to effectively treat advanced cancers that have developed resistance and metastasized. One such strategy is to target cancer cell iron metabolism, which is altered compared to normal cells and may facilitate their rapid proliferation. This is supported by studies reporting the anti-neoplastic activities of the clinically available iron chelators, desferrioxamine and deferasirox. More recently, ligands of the di-2-pyridylketone thiosemicarbazone (DpT) class have demonstrated potent and selective anti-proliferative activity across multiple cancer-types in vivo, fueling studies aimed at dissecting their molecular mechanisms of action. In the past five years alone, significant advances have been made in understanding how chelators not only modulate cellular iron metabolism, but also multiple signaling pathways implicated in tumor progression and metastasis. Herein, we discuss recent research on the targeting of iron in cancer cells, with a focus on the novel and potent DpT ligands. Several key studies have revealed that iron chelation can target the AKT, ERK, JNK, p38, STAT3, TGF-β, Wnt and autophagic pathways to subsequently inhibit cellular proliferation, the epithelial-mesenchymal transition (EMT) and metastasis. These developments emphasize that these novel therapies could be utilized clinically to effectively target cancer.