Src protein–tyrosine kinase structure and regulation

The α2 isoform Na,K-ATPase modulates contraction of rat mesenteric small artery via cSrc-dependent Ca2+ sensitization

AIMS

The Na,K-ATPase is involved in a large number of regulatory activities including cSrc-dependent signalling. Upon inhibition of the Na,K-ATPase with ouabain, cSrc activation is shown to occur in many cell types. This study tests the hypothesis that acute potentiation of agonist-induced contraction by ouabain is mediated through Na,K-ATPase-cSrc signalling-dependent sensitization of vascular smooth muscle cells to Ca²⁺ .

METHODS

Agonist-induced rat mesenteric small artery contraction was examined in vitro under isometric conditions and in vivo in anaesthetized rats. Arterial wall tension and [Ca²⁺ ]_i in vascular smooth muscle cells were measured simultaneously. Changes in cSrc and myosin phosphatase targeting protein 1 (MYPT1) phosphorylation were analysed by Western blot. Protein expression was examined with immunohistochemistry. The α1 and α2 isoforms of the Na,K-ATPase were transiently downregulated by siRNA transfection in vivo.

RESULTS

Ten micromolar ouabain, but not digoxin, potentiated contraction to noradrenaline. This effect was not endothelium-dependent. Ouabain sensitized smooth muscle cells to Ca²⁺ , and this was associated with increased phosphorylation of cSrc and MYPT1. Inhibition of tyrosine kinase by genistein, PP2 or pNaKtide abolished the potentiating effect of ouabain on arterial contraction and Ca²⁺ sensitization. Downregulation of the Na,K-ATPase α2 isoform made arterial contraction insensitive to ouabain and tyrosine kinase inhibition.

CONCLUSION

Data suggest that micromolar ouabain potentiates agonist-induced contraction of rat mesenteric small artery via Na,K-ATPase-dependent cSrc activation, which increases Ca²⁺ sensitization of vascular smooth muscle cells by MYPT1 phosphorylation. This mechanism may be critical for acute control of vascular tone.

EGF Regulates the Interaction of Tks4 with Src through Its SH2 and SH3 Domains

The nonreceptor tyrosine kinase Src is a central component of the epidermal growth factor (EGF) signaling pathway. Our group recently showed that the Frank-ter Haar syndrome protein Tks4 (tyrosine kinase substrate with four Src homology 3 domains) is also involved in EGF signaling. Here we demonstrate that Tks4 and Src bind directly to each other and elucidate the details of the molecular mechanism of this complex formation. Results of GST pull-down and fluorescence polarization assays show that both a proline-rich SH3 binding motif (PSRPLPDAP, residues 466-474) and an adjacent phosphotyrosine-containing SH2 binding motif (pYEEI, residues 508-511) in Tks4 are responsible for Src binding. These motifs interact with the SH3 and SH2 domains of Src, respectively, leading to a synergistic enhancement of binding strength and a highly stable, "bidentate"-type of interaction. In agreement with these results, we found that the association of Src with Tks4 is permanent and the complex lasts at least 3 h in living cells. We conclude that the interaction of Tks4 with Src may result in the long term stabilization of the kinase in its active conformation, leading to prolonged Src activity following EGF stimulation.

Identification of phosphorylation sites and binding pockets for modulation of NaV 1.5 channel by Fyn tyrosine kinase

Cardiac sodium channel Na_V 1.5 is the predominant form of sodium channels in cardiomyocytes, which exists as a macromolecular complex and interacts with multiple protein partners. Fyn kinase is one of the interacting proteins which colocalize, phosphorylate and modulate the Na_V 1.5 channel. To elaborate this interaction we created expression vectors for the N-terminal, intracellular loop, and C-terminal regions of the Na_V 1.5 channel, to express in HEK-293 cells. By co-immunoprecipitation and anti-phosphotyrosine blotting, we identified proline-rich binding sites for Fyn kinase in the N-terminal, IC-loop_i-ii and C-terminal. After binding, Fyn kinase phosphorylates tyrosine residues present in the N- and C-terminal, which produce a depolarizing shift of 7 mV in fast inactivation. The functional relevance of these binding and phosphorylation sites was further underpinned by creating full length mutants masking these sites sequentially. An activation and inactivation curves were recorded with or without co-expressed Fyn kinase which indicates that phosphorylation of tyrosine residues at positions 68, 87, 112 in the N-terminal and at positions 1811 and 1889 in the C-terminal creates a depolarizing shift in fast inactivation of Na_V 1.5 channel.

Src kinase activation by nitric oxide promotes resistance to anoikis in tumour cell lines

Tumour progression involves the establishment of tumour metastases at distant sites. Resistance to anoikis, a form of cell death that occurs when cells lose contact with the extracellular matrix and with neighbouring cells, is essential for metastases. NO has been associated with anoikis. NO treated HeLa cells and murine melanoma cells in suspension triggered a nitric oxide (NO)-Src kinase signalling circuitry that enabled resistance to anoikis. Two NO donors, sodium nitroprusside (SNP) (500 µM) and DETANO (125 µM), protected against cell death derived from detachment of a growth permissive surface (experimental anoikis). Under conditions of NO-mediated Src activation the following were observed: (a) down-regulation of the pro-apoptotic proteins Bim and cleaved caspase-3 and the cell surface protein, E-cadherin, (b) up-regulation of caveolin-1, and (c) the dissociation of cell aggregates formed when cells are detached from a growth permissive surface. Efficiency of reattachment of tumour cells in suspension and treated with different concentrations of an NO donor, was dependent on the NO concentration. These findings indicate that NO-activated Src kinase triggers a signalling circuitry that provides resistance to anoikis, and allows for metastases.

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.

Apelin-13 Is an Early Promoter of Cytoskeleton and Tight Junction in Diabetic Macular Edema via PI-3K/Akt and MAPK/Erk Signaling Pathways

Diabetic macular edema is major cause of vision loss associated with diabetic retinopathy. Breakdown of blood-retinal barrier, especially inner BRB, is an early event in pathogenesis of DR. Apelin, an endogenous ligand of APJ, mediates angiogenesis and is involved in the development of DR. The present study aimed to investigate effects and mechanism of apelin-13 in vascular permeability during DME. We verified apelin-13 was upregulated in DME patients' vitreous. High glucose incubation led to a progressive increase of apelin-13, APJ, cytoskeleton, and tight junction proteins, including VE-Cadherin, FAK, Src, ZO-1, and occludin. Apelin-13 promoted HRMEC proliferation and migration and phosphorylation of both cytoskeleton and tight junction under both normal and high glucose conditions. Besides, apelin-13 activated PI-3K/Akt and MAPK/Erk signaling pathways, including PLCγ1, p38, Akt, and Erk both in HRMEC and in C57BL/6 mice. Meanwhile, F13A performed opposite effects compared with apelin-13. In in vivo study, apelin-13 was also upregulated in retina of db/db mice. Taken together, apelin-13 increased biologic activity of HRMEC, as well as expression of both cytoskeleton and tight junction in DME via PI-3K/Akt and MAPK/Erk signaling pathways. Apelin-13 as an early promoter of vascular permeability may offer a new perspective strategy in early treatment of DR.

Pharmacological inhibition of Src kinase protects against acute kidney injury in a murine model of renal ischemia/reperfusion

Activation of Src kinase has been implicated in the pathogenesis of acute brain, liver, and lung injury. However, the role of Src in acute kidney injury (AKI) remains unestablished. To address this, we evaluated the effects of Src inhibition on renal dysfunction and pathological changes in a murine model of AKI induced by ischemia/reperfusion (I/R). I/R injury to the kidney resulted in increased Src phosphorylation at tyrosine 416 (activation). Administration of PP1, a highly selective Src inhibitor, blocked Src phosphorylation, improved renal function and ameliorated renal pathological damage. PP1 treatment also suppressed renal expression of neutrophil gelatinase-associated lipocalin and reduced apoptosis in the injured kidney. Moreover, Src inhibition prevented downregulation of several adherens and tight junction proteins, including E-cadherin, ZO-1, and claudins-1/−4 in the kidney after I/R injury as well as in cultured renal proximal tubular cells following oxidative stress. Finally, PP1 inhibited I/R–induced renal expression of matrix metalloproteinase-2 and -9, phosphorylation of extracellular signal–regulated kinases1/2, signal transducer and activator of transcription-3, and nuclear factor-κB, and the infiltration of macrophages into the kidney. These data indicate that Src is a pivotal mediator of renal epithelial injury and that its inhibition may have a therapeutic potential to treat AKI.

Extracellular Phosphorylation of TIMP-2 by Secreted c-Src Tyrosine Kinase Controls MMP-2 Activity

The tissue inhibitor of metalloproteinases 2 (TIMP-2) is a specific endogenous inhibitor of matrix metalloproteinase 2 (MMP-2), which is a key enzyme that degrades the extracellular matrix and promotes tumor cell invasion. Although the TIMP-2:MMP-2 complex controls proteolysis, the signaling mechanism by which the two proteins associate in the extracellular space remains unidentified. Here we report that TIMP-2 is phosphorylated outside the cell by secreted c-Src tyrosine kinase. As a consequence, phosphorylation at Y90 significantly enhances TIMP-2 potency as an MMP-2 inhibitor and weakens the catalytic action of the active enzyme. TIMP-2 phosphorylation also appears to be essential for its interaction with the latent enzyme proMMP-2 in vivo. Absence of the kinase or non-phosphorylatable Y90 abolishes TIMP-2 binding to the latent enzyme, ultimately hampering proMMP-2 activation. Together, TIMP-2 phosphorylation by secreted c-Src represents a critical extracellular regulatory mechanism that controls the proteolytic function of MMP-2.

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c-Src tyrosine kinase phosphorylates TIMP-2

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Secreted c-Src phosphorylates TIMP-2 extracellularly

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TIMP-2 Y90 phosphorylation promotes extracellular interaction with proMMP-2

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Tyrosine phosphorylation of TIMP-2 regulates proMMP-2 processing and MMP-2 activity

Lyn, Lupus, and (B) Lymphocytes, a Lesson on the Critical Balance of Kinase Signaling in Immunity

Systemic lupus erythematosus (SLE) is a progressive autoimmune disease characterized by increased sensitivity to self-antigens, auto-antibody production, and systemic inflammation. B cells have been implicated in disease progression and as such represent an attractive therapeutic target. Lyn is a Src family tyrosine kinase that plays a major role in regulating signaling pathways within B cells as well as other hematopoietic cells. Its role in initiating negative signaling cascades is especially critical as exemplified by Lyn-/- mice developing an SLE-like disease with plasma cell hyperplasia, underscoring the importance of tightly regulating signaling within B cells. This review highlights recent advances in our understanding of the function of the Src family tyrosine kinase Lyn in B lymphocytes and its contribution to positive and negative signaling pathways that are dysregulated in autoimmunity.

Advances in studies of tyrosine kinase inhibitors and their acquired resistance

Protein tyrosine kinase (PTK) is one of the major signaling enzymes in the process of cell signal transduction, which catalyzes the transfer of ATP-γ-phosphate to the tyrosine residues of the substrate protein, making it phosphorylation, regulating cell growth, differentiation, death and a series of physiological and biochemical processes. Abnormal expression of PTK usually leads to cell proliferation disorders, and is closely related to tumor invasion, metastasis and tumor angiogenesis. At present, a variety of PTKs have been used as targets in the screening of anti-tumor drugs. Tyrosine kinase inhibitors (TKIs) compete with ATP for the ATP binding site of PTK and reduce tyrosine kinase phosphorylation, thereby inhibiting cancer cell proliferation. TKI has made great progress in the treatment of cancer, but the attendant acquired acquired resistance is still inevitable, restricting the treatment of cancer. In this paper, we summarize the role of PTK in cancer, TKI treatment of tumor pathways and TKI acquired resistance mechanisms, which provide some reference for further research on TKI treatment of tumors.

MeHg affects the activation of FAK, Src, Rac1 and Cdc42, critical proteins for cell movement in PDGF-stimulated SH-SY5Y neuroblastoma cells

Methylmercury (MeHg) is an environmental neurotoxicant that inhibits neuronal migration. This process requires several cyclic steps involving the formation of membrane protrusions (lamellipodia and filopodia) and focal adhesion turnover. FAK and Src are critical proteins that regulate both processes. The FAK-Src complex promotes the activation of Rac1 and Cdc42, two GTPases involved in the remodeling of the actin cytoskeletal network. Here, we studied the effect of MeHg (1, 10, 100, 500 and 1000nM) on cell migration, the formation of cell protrusions, focal adhesion location and the activation of FAK, Src, Rac1 and Cdc42 using the SH-SY5Y neuroblastoma cell line stimulated with PDGF-BB (PDGF). The data show that MeHg (1-500nM) inhibited PDGF-stimulated cell migration. In PDGF-stimulated cells, MeHg (100-1000nM) decreased protrusions and increased the size of the p-FAK^Y397 clusters. MeHg also inhibited PDGF-induced FAK and Src activation and, at 100nM, MeHg inhibited the activation of Rac1 and Cdc42. Altogether, the findings show that low concentrations of MeHg inhibit SH-SY5Y cell migration by disrupting the activation and disassembly of FAK. This negatively affects the activation of Src, Rac1 and Cdc42, all of which are critical proteins for the regulation of cell movement. These effects could be related to the MeHg-mediated inhibition of PDGF-induced formation of lamellipodia and filopodia, focal adhesion disassembly and PDGF-induced movement.

Insight into the Selectivity of the G7-18NATE Inhibitor Peptide for the Grb7-SH2 Domain Target

Growth factor receptor bound protein 7 (Grb7) is an adaptor protein with established roles in the progression of both breast and pancreatic cancers. Through its C-terminal SH2 domain, Grb7 binds to phosphorylated tyrosine kinases to promote proliferative and migratory signaling. Here, we investigated the molecular basis for the specificity of a Grb7 SH2-domain targeted peptide inhibitor. We identified that arginine 462 in the BC loop is unique to Grb7 compared to Grb2, another SH2 domain bearing protein that shares the same consensus binding motif as Grb7. Using surface plasmon resonance we demonstrated that Grb7-SH2 binding to G7-18NATE is reduced 3.3-fold when the arginine is mutated to the corresponding Grb2 amino acid. The reverse mutation in Grb2-SH2 (serine to arginine), however, was insufficient to restore binding of G7-18NATE to Grb2-SH2. Further, using a microarray, we confirmed that G7-18NATE is specific for Grb7 over a panel of 79 SH2 domains, and identified that leucine at the βD6 position may also be a requirement for Grb7-SH2 binding. This study provides insight into the specificity defining features of Grb7 for the inhibitor molecule G7-18NATE, that will assist in the development of improved Grb7 targeted inhibitors.

Cryopreserved platelets demonstrate reduced activation responses and impaired signaling after agonist stimulation

BACKGROUND

Room temperature-stored (20-24°C) platelets (PLTs) have a shelf life of 5 days, making it logistically challenging to supply remote medical centers with PLT products. Cryopreservation of PLTs in dimethyl sulfoxide (DMSO) and storage at -80°C enables an extended shelf life up to 2 years. Although cryopreserved PLTs have been widely characterized under resting conditions, their ability to undergo agonist-induced activation is yet to be fully explored.

STUDY DESIGN AND METHODS

Buffy coat PLTs were cryopreserved at -80°C with 5% to 6% DMSO and sampled before freezing and after thawing. PLTs were analyzed under resting conditions and after agonist stimulation with adenosine diphosphate, collagen, or thrombin receptor-activating peptide-6. The expression of activation markers, microparticle formation, and calcium mobilization were analyzed by flow cytometry. Soluble PLT proteins present in the PLT supernatant were examined by enzyme-linked immunosorbent assay. Protein phosphorylation was investigated with Western blotting.

RESULTS

After cryopreservation, PLTs displayed increased surface activation markers and higher basal calcium levels. Cryopreserved PLTs demonstrated diminished aggregation responses. Additionally, cryopreserved PLTs showed a limited ability to become activated (as measured by CD62P and phosphatidylserine exposure and cytokine release) after agonist stimulation. A reduction in the abundance and phosphorylation of key signaling proteins (Akt, Src, Lyn, ERK, and p38) was seen in cryopreserved PLTs.

CONCLUSIONS

Cryopreservation of PLTs induces dramatic changes to the basal PLT phenotype and renders them largely nonresponsive to agonist stimulation, likely due to the alterations in signal transduction. Therefore, further efforts are required to understand how cryopreserved PLTs achieve their hemostatic effect once transfused.

Characterization of a novel non-receptor tyrosine kinase Src from Litopenaeus vannamei and its response to white spot syndrome virus infection

Src family kinases (SFKs), a class of non-receptor tyrosine kinases, mediate a wide aspect of cellular signaling pathways that regulate cell proliferation, differentiation, motility and survival. In this study, we identified and characterized for the first time a novel SFK homologue from Litopenaeus vannamei (designated as LvSrc). Sequence analysis showed that LvSrc had a high homology with the identified SFKs, especially those from invertebrates. LvSrc contained the conserved SH3, SH2 and tyrosine kinase domains, as well as the potential phosphorylation and lipid modification sites. Immunofluorescence analysis demonstrated that LvSrc was mostly localized at the plasma membrane and partly resided in the perinuclear vesicle and nucleus or whole cell. Infection with white spot syndrome virus (WSSV) could up-regulate the transcription and expression levels of LvSrc and further induced its phosphorylation, suggesting that LvSrc was implicated in WSSV infection. Furthermore, our co-immunoprecipitation result confirmed the interaction between Src and focal adhesion kinase (FAK) in shrimp, while the phosphorylation of FAK was markedly enhanced by co-expression with LvSrc. In sum, our studies suggested that LvSrc might act in the FAK-regulated signaling pathway during WSSV infection, which would give us a better insight in understanding the role of SKFs in host-virus interactions in crustaceans.

Spatial cycles mediated by UNC119 solubilisation maintain Src family kinases plasma membrane localisation

The peripheral membrane proto-oncogene Src family protein tyrosine kinases relay growth factor signals to the cytoplasm of mammalian cells. We unravel the spatial cycles of solubilisation, trapping on perinuclear membrane compartments and vesicular transport that counter entropic equilibration to endomembranes for maintaining the enrichment and activity of Src family protein tyrosine kinases at the plasma membrane. The solubilising factor UNC119 sequesters myristoylated Src family protein tyrosine kinases from the cytoplasm, enhancing their diffusion to effectively release Src family protein tyrosine kinases on the recycling endosome by localised Arl2/3 activity. Src is then trapped on the recycling endosome via electrostatic interactions, whereas Fyn is quickly released to be kinetically trapped on the Golgi by palmitoyl acyl-transferase activity. Vesicular trafficking from these compartments restores enrichment of the Src family protein tyrosine kinases to the plasma membrane. Interference with these spatial cycles by UNC119 knockdown disrupts Src family protein tyrosine kinase localisation and signalling activity, indicating that UNC119 could be a drug target to affect oncogenic Src family protein tyrosine kinase signalling.

The peripheral membrane proto-oncogene Src family protein tyrosine kinases (SFKs) transmit growth factor signals to the cytoplasm. Here the authors show that the solubilising factor UNC119 sequesters myristoylated SFKs to maintain its enrichment at the plasma membrane to enable signal transduction.

Prognostic relevance of Src activation in stage II-III colon cancer

Src belongs to a family of cytoplasmic tyrosine kinases that play a key role in tumor initiation and progression. Src activation has been associated with a more aggressive neoplastic phenotype and induces resistance to platinum agents in preclinical models. The aim of our study was to assess the prognostic and/or predictive value of Src activation in patients with stage II-III colon cancer. pSrc expression was assessed in paraffin-embedded tumor samples by immunohistochemistry (phospho-Y418, ab4816; Abcam). Cases were classified by staining intensity in 4 categories: no staining (0), weak (1+), moderate (2+), and intense (3+) staining. A total of 487 patients were evaluated (240 stage II, 247 stage III), of whom 298 (61%) had received adjuvant chemotherapy. Staining was absent in 78 (16%), weak in 262 (54%), moderate in 103 (21%), and intense in 44 (9%). High pSrc expression was significantly associated with decreased 5-year disease-free survival (39% versus 63% for patients with high versus low pSrc expression; hazard ratio, 0.56; P=.005) and overall survival (58% versus 74%; hazard ratio, 0.55; P=.02). Multivariate analysis confirmed pSrc expression as a significant prognostic factor both for disease-free survival and overall survival, independent of age, sex, tumor stage, bowel obstruction/perforation, or adjuvant chemotherapy. These findings illustrate the relevance of Src activation in colon cancer biology, conferring a poor prognosis to patients with early stage colon cancer regardless of adjuvant chemotherapy. Our findings may help improve prognostic stratification of patients for clinical decisions and open new avenues for potential pharmacologic manipulation that may eventually improve patients' outcomes.

Dynamic, structural and thermodynamic basis of insulin-like growth factor 1 kinase allostery mediated by activation loop phosphorylation

Despite the importance of kinases' catalytic activity regulation in cell signaling, detailed mechanisms underlying their activity regulation are poorly understood. Herein, using insulin-like growth factor 1 receptor kinase (IGF-1RK) as a model, the mechanisms of kinase regulation by its activation loop (A-loop) phosphorylation were investigated through molecular dynamics (MD) and alchemical free energy simulations. Analyses of the simulation results and free energy landscapes determined for the entire catalytic cycle of the kinase revealed that A-loop phosphorylation affects each step in the IGF-1RK catalytic cycle, including conformational change, substrate binding/product release and catalytic phosphoryl transfer. Specifically, the conformational equilibrium of the kinase is shifted by 13.2 kcal mol-1 to favor the active conformation after A-loop phosphorylation, which increases substrate binding affinity of the activated kinase. This free energy shift is achieved primarily via destabilization of the inactive conformation. The free energy of the catalytic reaction is also changed by 3.3 kcal mol-1 after the phosphorylation and in the end, facilitates product release. Analyses of MD simulations showed that A-loop phosphorylation produces these energetic effects by perturbing the side chain interactions around each A-loop tyrosine. These interaction changes are propagated to the remainder of the kinase to modify the orientations and dynamics of the αC-helix and A-loop, and together yield the observed free energy changes. Since many protein kinases share similar interactions identified in this work, the mechanisms of kinase allostery and catalysis unraveled here can be applicable to them.

Nobiletin Inhibits Angiogenesis by Regulating Src/FAK/STAT3-Mediated Signaling through PXN in ER⁺ Breast Cancer Cells

Tumor angiogenesis is one of the major hallmarks of tumor progression. Nobiletin is a natural flavonoid isolated from citrus peel that has anti-angiogenic activity. Steroid receptor coactivator (Src) is an intracellular tyrosine kinase so that focal adhesion kinase (FAK) binds to Src to play a role in tumor angiogenesis. Signal transducer and activator of transcription 3 (STAT3) is a marker for tumor angiogenesis which interacts with Src. Paxillin (PXN) acts as a downstream target for both FAK and STAT3. The main goal of this study was to assess inhibition of tumor angiogenesis by nobiletin in estrogen receptor positive (ER⁺) breast cancer cells via Src, FAK, and STAT3-mediated signaling through PXN. Treatment with nobiletin in MCF-7 and T47D breast cancer cells inhibited angiogenesis markers, based on western blotting and RT-PCR. Validation of in vitro angiogenesis in the human umbilical vein endothelial cells (HUVEC) endothelial cell line proved the anti-angiogenic activity of nobiletin. Electrophoretic mobility shift assay and the ChIP assay showed that nobiletin inhibits STAT3/DNA binding activity and STAT3 binding to a novel binding site of the PXN gene promoter. We also investigated the migration and invasive ability of nobiletin in ER⁺ cells. Nobiletin inhibited tumor angiogenesis by regulating Src, FAK, and STAT3 signaling through PXN in ER⁺ breast cancer cells.

Na-K-ATPase regulates intercellular communication in the vascular wall via cSrc kinase-dependent connexin43 phosphorylation

Communication between vascular smooth muscle cells (VSMCs) is dependent on gap junctions and is regulated by the Na-K-ATPase. The Na-K-ATPase is therefore important for synchronized VSMC oscillatory activity, i.e., vasomotion. The signaling between the Na-K-ATPase and gap junctions is unknown. We tested here the hypothesis that this signaling involves cSrc kinase. Intercellular communication was assessed by membrane capacitance measurements of electrically coupled VSMCs. Vasomotion in isometric myograph, input resistance, and synchronized [Ca2+]i transients were used as readout for intercellular coupling in rat mesenteric small arteries in vitro. Phosphorylation of cSrc kinase and connexin43 (Cx43) were semiquantified by Western blotting. Micromole concentration of ouabain reduced the amplitude of norepinephrine-induced vasomotion and desynchronized Ca2+ transients in VSMC in the arterial wall. Ouabain also increased input resistance in the arterial wall. These effects of ouabain were antagonized by inhibition of tyrosine phosphorylation with genistein, PP2, and by an inhibitor of the Na-K-ATPase-dependent cSrc activation, pNaKtide. Moreover, inhibition of cSrc phosphorylation increased vasomotion amplitude and decreased the resistance between cells in the vascular wall. Ouabain inhibited the electrical coupling between A7r5 cells, but pNaKtide restored the electrical coupling. Ouabain increased cSrc autophosphorylation of tyrosine 418 (Y418) required for full catalytic activity whereas pNaKtide antagonized it. This cSrc activation was associated with Cx43 phosphorylation of tyrosine 265 (Y265). Our findings demonstrate that Na-K-ATPase regulates intercellular communication in the vascular wall via cSrc-dependent Cx43 tyrosine phosphorylation.

Hsp90 dependence of a kinase is determined by its conformational landscape

Heat shock protein 90 (Hsp90) is an abundant molecular chaperone, involved in the folding and activation of 60% of the human kinome. The oncogenic tyrosine kinase v-Src is one of the most stringent client proteins of Hsp90, whereas its almost identical homolog c-Src is only weakly affected by the chaperone. Here, we perform atomistic molecular simulations and in vitro kinase assays to explore the mechanistic differences in the activation of v-Src and c-Src. While activation in c-Src is strictly controlled by ATP-binding and phosphorylation, we find that activating conformational transitions are spontaneously sampled in Hsp90-dependent Src mutants. Phosphorylation results in an enrichment of the active conformation and in an increased affinity for Hsp90. Thus, the conformational landscape of the mutated kinase is reshaped by a broken “control switch”, resulting in perturbations of long-range electrostatics, higher activity and increased Hsp90-dependence.

7-cyclopentyl-5-(4-phenoxyphenyl)-7H-pyrrolo[2,3-d] pyrimidin-4-ylamine inhibits the proliferation and migration of vascular smooth muscle cells by suppressing ERK and Akt pathways

Excessive vascular smooth muscle cell (VSMC) proliferation and migration after vascular injury significantly contributes to the development of occlusive vascular disease. Therefore, inhibiting the proliferation and migration of VSMCs is a validated therapeutic modality for occlusive vascular disease such as atherosclerosis and restenosis. In the present study, we screened chemical compounds for their anti-proliferative effects on VSMCs using multiple approaches, such as MTT assays, wound healing assays, and trans-well migration assays. Our data indicate that 7-cyclopentyl-5-(4-phenoxyphenyl)-7H-pyrrolo[2,3-d] pyrimidin-4-ylamine, a lymphocyte-specific protein tyrosine kinase (Lck) inhibitor, significantly inhibited both VSMC proliferation and migration. 7-cyclopentyl-5-(4-phenoxyphenyl)-7H-pyrrolo[2,3-d]pyrimidin-4-ylamine suppresses VSMC proliferation and migration via down-regulating the protein kinase B (Akt) and extracellular signal regulated kinase (ERK) pathways, and it significantly decreased the expression of proliferating cell nuclear antigen (PCNA) and cyclin D1 and, the phosphorylation of retinoblastoma protein (pRb). Additionally, 7-cyclopentyl-5-(4-phenoxyphenyl)-7H-pyrrolo[2,3-d] pyrimidin-4-ylamine suppressed the migration of VSMCs from endothelium-removed aortic rings, as well as neointima formation following rat carotid balloon injury. The present study identified 7-cyclopentyl-5-(4-phenoxyphenyl)-7H-pyrrolo[2,3-d]pyrimidin-4-ylamine as a potent VSMC proliferation and migration inhibitor and warrants further studies to elucidate its more detailed molecular mechanisms, such as its primary target, and to further validate its in vivo efficacy as a therapeutic agent for pathologic vascular conditions, such as restenosis and atherosclerosis.

From direct to indirect lithium targets: a comprehensive review of omics data

Metal ions are critical to a wide range of biological processes.

Src-family tyrosine kinases and the Ca2+ signal

In this review, we shall describe the rich crosstalk between non-receptor Src-family kinases (SFKs) and the Ca²⁺ transient generated in activated cells by a variety of extracellular and intracellular stimuli, resulting in diverse signaling events. The exchange of information between SFKs and Ca²⁺ is reciprocal, as it flows in both directions. These kinases are main actors in pathways leading to the generation of the Ca²⁺ signal, and reciprocally, the Ca²⁺ signal modulates SFKs activity and functions. We will cover how SFKs participate in the generation of the cytosolic Ca²⁺ rise upon activation of a series of receptors and the mechanism of clearance of this Ca²⁺ signal. The role of SFKs modulating Ca²⁺-translocating channels participating in these events will be amply discussed. Finally, the role of the Ca²⁺ sensor protein calmodulin on the activity of c-Src, and potentially on other SFKs, will be outlined as well. This article is part of a Special Issue entitled: ECS Meeting edited by Claus Heizmann, Joachim Krebs and Jacques Haiech.

The molecular effect of metastasis suppressors on Src signaling and tumorigenesis: new therapeutic targets

A major problem for cancer patients is the metastasis of cancer cells from the primary tumor. This involves: (1) migration through the basement membrane; (2) dissemination via the circulatory system; and (3) invasion into a secondary site. Metastasis suppressors, by definition, inhibit metastasis at any step of the metastatic cascade. Notably, Src is a non-receptor, cytoplasmic, tyrosine kinase, which becomes aberrantly activated in many cancer-types following stimulation of plasma membrane receptors (e.g., receptor tyrosine kinases and integrins). There is evidence of a prominent role of Src in tumor progression-related events such as the epithelial–mesenchymal transition (EMT) and the development of metastasis. However, the precise molecular interactions of Src with metastasis suppressors remain unclear. Herein, we review known metastasis suppressors and summarize recent advances in understanding the mechanisms of how these proteins inhibit metastasis through modulation of Src. Particular emphasis is bestowed on the potent metastasis suppressor, N-myc downstream regulated gene 1 (NDRG1) and its interactions with the Src signaling cascade. Recent studies demonstrated a novel mechanism through which NDRG1 plays a significant role in regulating cancer cell migration by inhibiting Src activity. Moreover, we discuss the rationale for targeting metastasis suppressor genes as a sound therapeutic modality, and we review several examples from the literature where such strategies show promise. Collectively, this review summarizes the essential interactions of metastasis suppressors with Src and their effects on progression of cancer metastasis. Moreover, interesting unresolved issues regarding these proteins as well as their potential as therapeutic targets are also discussed.

Src tyrosine kinase mediates endothelin-1-induced early growth response protein-1 expression via MAP kinase-dependent pathways in vascular smooth muscle cells

We have previously demonstrated that the non-receptor protein tyrosine kinase (NR-PTK) c-Src is an upstream regulator of endothelin-1 (ET-1) and angiotensin II-induced activation of protein kinase B (PKB) signaling in vascular smooth muscle cells (VSMCs). We have also demonstrated that ET-1 potently induces the expression of the early growth response protein-1 (Egr-1), a zinc finger transcription factor that is overexpressed in models of vascular diseases, such as atherosclerosis. However, the involvement of c-Src in ET-1‑induced Egr-1 expression has not yet been investigated and its role in mitogen-activated protein kinase (MAPK) signaling remains controversial. Therefore, the aim of the present study was to examine the role of c-Src in the ET-1-induced phosphorylation of extracellular signal-regulated kinase (ERK)1/2, c-Jun N-terminal kinase (JNK) and p38 MAPK, 3 key members of the MAPK family and in the regulation of Egr-1 expression in rat aortic A10 VSMCs. ET-1 rapidly induced the phosphorylation of MAPKs, as well as the expression of Egr-1; however, treatment of the VSMCs with PP2, a specific pharmacological inhibitor of c-Src, dose-dependently reduced the phosphorylation of the 3 MAPKs and the expression of Egr-1 induced by ET-1. Furthermore, in mouse embryonic fibroblasts (MEFs) deficient in c-Src (SYF), the ET-1-induced Egr-1 expression and MAPK phosphorylation were significantly suppressed, as compared to MEFs expressing normal Src levels. These results suggest that c-Src plays a critical role in mediating ET-1-induced MAPK phosphorylation and Egr-1 expression in VSMCs.

The NADPH Oxidases DUOX1 and NOX2 Play Distinct Roles in Redox Regulation of Epidermal Growth Factor Receptor Signaling

The epidermal growth factor receptor (EGFR) plays a critical role in regulating airway epithelial homeostasis and responses to injury. Activation of EGFR is regulated by redox-dependent processes involving reversible cysteine oxidation by reactive oxygen species (ROS) and involves both ligand-dependent and -independent mechanisms, but the precise source(s) of ROS and the molecular mechanisms that control tyrosine kinase activity are incompletely understood. Here, we demonstrate that stimulation of EGFR activation by ATP in airway epithelial cells is closely associated with dynamic reversible oxidation of cysteine residues via sequential sulfenylation and S-glutathionylation within EGFR and the non-receptor-tyrosine kinase Src. Moreover, the intrinsic kinase activity of recombinant Src or EGFR was in both cases enhanced by H₂O₂ but not by GSSG, indicating that the intermediate sulfenylation is the activating modification. H₂O₂-induced increase in EGFR tyrosine kinase activity was not observed with the C797S variant, confirming Cys-797 as the redox-sensitive cysteine residue that regulates kinase activity. Redox-dependent regulation of EGFR activation in airway epithelial cells was found to strongly depend on activation of either the NADPH oxidase DUOX1 or the homolog NOX2, depending on the activation mechanism. Whereas DUOX1 and Src play a primary role in EGFR transactivation by wound-derived signals such as ATP, direct ligand-dependent EGFR activation primarily involves NOX2 with a secondary role for DUOX1 and Src. Collectively, our findings establish that redox-dependent EGFR kinase activation involves a dynamic and reversible cysteine oxidation mechanism and that this activation mechanism variably involves DUOX1 and NOX2.

Rhodomycin A, a novel Src-targeted compound, can suppress lung cancer cell progression via modulating Src-related pathways

Src activation is involved in cancer progression and the interplay with EGFR. Inhibition of Src activity also represses the signalling pathways regulated by EGFR. Therefore, Src has been considered a target molecule for drug development. This study aimed to identify the compounds that target Src to suppress lung cancer tumourigenesis and metastasis and investigate their underlying molecular mechanisms. Using a molecular docking approach and the National Cancer Institute (NCI) compound dataset, eight candidate compounds were selected, and we evaluated their efficacy. Among them, rhodomycin A was the most efficient at reducing the activity and expression of Src in a dose-dependent manner, which was also the case for Src-associated proteins, including EGFR, STAT3, and FAK. Furthermore, rhodomycin A significantly suppressed cancer cell proliferation, migration, invasion, and clonogenicity in vitro and tumour growth in vivo. In addition, rhodomycin A rendered gefitinib-resistant lung adenocarcinoma cells more sensitive to gefitinib treatment, implying a synergistic effect of the combination therapy. Our data also reveal that the inhibitory effect of rhodomycin A on lung cancer progression may act through suppressing the Src-related multiple signalling pathways, including PI3K, JNK, Paxillin, and p130cas. These findings will assist the development of anti-tumour drugs to treat lung cancer.

Phosphorylation of Src by phosphoinositide 3-kinase regulates beta-adrenergic receptor-mediated EGFR transactivation

β2-Adrenergic receptors (β2AR) transactivate epidermal growth factor receptors (EGFR) through formation of a β2AR-EGFR complex that requires activation of Src to mediate signaling. Here, we show that both lipid and protein kinase activities of the bifunctional phosphoinositide 3-kinase (PI3K) enzyme are required for β2AR-stimulated EGFR transactivation. Mechanistically, the generation of phosphatidylinositol (3,4,5)-tris-phosphate (PIP3) by the lipid kinase function stabilizes β2AR-EGFR complexes while the protein kinase activity of PI3K regulates Src activation by direct phosphorylation. The protein kinase activity of PI3K phosphorylates serine residue 70 on Src to enhance its activity and induce EGFR transactivation following βAR stimulation. This newly identified function for PI3K, whereby Src is a substrate for the protein kinase activity of PI3K, is of importance since Src plays a key role in pathological and physiological signaling.

PRR14 is a novel activator of the PI3K pathway promoting lung carcinogenesis

Chromosomal focal amplifications often cause an increase in gene copy number, contributing to the pathogenesis of cancer. PRR14 overexpression is associated with recurrent locus amplification in lung cancer, and it correlates with a poor prognosis. We show that increased PRR14 expression promoted and reduced PRR14 expression impeded lung cancer cell proliferation. Interestingly, PRR14 cells were markedly enlarged in size and exhibited an elevated activity of the PI3-kinase/Akt/mTOR pathway, which was associated with a heightened sensitivity to the inhibitors of PI3K and mammalian target of rapamycin (mTOR). Biochemical analysis revealed that PRR14, as a proline-rich protein, binds to the Src homology 3 (SH3) domains of GRB2 resulting in PI3K activation. Significantly, two cancer patient-derived PRR14 mutants displayed considerably augmented GRB2-binding and an enhanced ability of promoting cell proliferation. Together with the in vivo data demonstrating a strong tumor-promoting activity of PRR14 and the mutants, our work uncovered this proline-rich protein as a novel activator of the PI3K pathway that promoted tumorigenesis in lung cancer.

A synthetic isoflavone, DCMF, promotes human keratinocyte migration by activating Src/FAK signaling pathway

Flavonoids are plant secondary compounds with various pharmacological properties. We previously showed that one flavonoid, trimethoxyisoflavone (TMF), could promote wound healing by inducing keratinocyte migration. Here, we screened TMF derivatives for enhanced activity and identified one compound, 2',6 Dichloro-7-methoxyisoflavone (DCMF), as most effective at promoting migration in a scratch wound assay. Using the HaCaT keratinocyte cell line, we found DCMF treatment induced phosphorylation of both FAK and Src, and increased keratinocyte migration. DCMF-induced Src kinase could promote activation of ERK, AKT, and p38 signaling pathways, and DCMF-induced secretion of matrix metalloproteinase (MMP)-2 and MMP-9 and partial epithelial-mesenchymal transition (EMT), whereas Src inhibition abolished DCMF-induced EMT. Using an in vivo excisional wound model, we observed improved wound closure and re-epithelialization in DCMF-treated mice, as compared to controls. Collectively, our data demonstrate that DCMF induces cell migration and promotes wound healing through activation of Src/FAK, ERK, AKT, and p38 MAPK signaling.

Urban Endocrine Disruptors Targeting Breast Cancer Proteins

Humans are exposed to a huge amount of environmental pollutants called endocrine disrupting chemicals (EDCs). These molecules interfere with the homeostasis of the body, usually through mimicking natural hormones leading to activation or blocking of their receptors. Many of these compounds have been associated with a broad range of diseases including the development or increased susceptibility to breast cancer, the most prevalent cancer in women worldwide, according to the World Health Organization. Thus, this article presents a virtual high-throughput screening (vHTS) to evaluate the affinity of proteins related to breast cancer, such as ESR1, ERBB2, PGR, BCRA1, and SHBG, among others, with EDCs from urban sources. A blind docking strategy was employed to screen each protein-ligand pair in triplicate in AutoDock Vina 2.0, using the computed binding affinities as ranking criteria. The three-dimensional structures were previously obtained from EDCs DataBank and Protein Data Bank, prepared and optimized by SYBYL X-2.0. Some of the chemicals that exhibited the best affinity scores for breast cancer proteins in each category were 1,3,7,8-tetrachlorodibenzo-p-dioxin, bisphenol A derivatives, perfluorooctanesulfonic acid, and benzo(a)pyrene, for catalase, several proteins, sex hormone-binding globulin, and cytochrome P450 1A2, respectively. An experimental validation of this approach was performed with a complex that gave a moderate binding affinity in silico, the sex hormone binding globulin (SHBG), and bisphenol A (BPA) complex. The protein was obtained using DNA recombinant technology and the physical interaction with BPA assessed through spectroscopic techniques. BPA binds on the recombinant SHBG, and this results in an increase of its α helix content. In short, this work shows the potential of several EDCs to bind breast cancer associated proteins as a tool to prioritize compounds to perform in vitro analysis to benefit the regulation or exposure prevention by the general population.

Osteoprotegerin disrupts peripheral adhesive structures of osteoclasts by modulating Pyk2 and Src activities

Osteoprotegerin has previously been shown to modulate bone mass by blocking osteoclast maturation and function. The detailed mechanisms of osteoprotegerin-induced disassembly of podosomes, disruption of adhesive structures and modulation of adhesion-related proteins in osteoclasts, however, are not well characterized. In this study, tartrate-resistant acidic phosphatase staining demonstrated that osteoprotegerin inhibited differentiation of osteoclasts. The use of scanning electron microscopy, real-time cell monitoring and confocal microscopy indicated that osteoclasts responded in a time and dose-dependent manner to osteoprotegerin treatments with retraction of peripheral adhesive structures and detachment from the extracellular substrate. Combined imaging and Western blot studies showed that osteoprotegerin induced dephosphorylation of Tyr 402 in Pyk2 and decreased its labeling in peripheral adhesion regions. osteoprotegerin induced increased intracellular labeling of Tyr 402 in Pyk2, Tyr 416 in Src, increased dephosphorylation of Tyr 527 in Src, and increased Pyk2/Src association in the central region of osteoclasts. This evidence suggests that Src may function as an adaptor protein that competes for Pyk2 and relocates it from the peripheral adhesive zone to the central region of osteoclasts in response to osteoprotegerin treatment. Osteoprotegerin may induce podosome reassembly and peripheral adhesive structure detachment by modulating phosphorylation of Pyk2 and Src and their intracellular distribution in osteoclasts.

Theoretical studies of the role of C-terminal cysteines in the process of S-nitrosylation of human Src kinases

Src tyrosine kinases are a family of non-receptor proteins that are responsible for the growth process, cellular proliferation, differentiation and survival. Lack of Src kinase control has been associated with the development of certain human cancers. This family of proteins is constituted of four domains, with SH1 being the kinase or catalytic domain. SH1 also presents three important regulatory sites. Two residues, Tyr416 and Tyr527, are responsible for important phosphorylation processes that lead to, respectively, activation and deactivation of these kinases. More recently, however, a set of four cysteine residues located near the C-terminus-Cys483, Cys487, Cys496 and Cys498-has been associated with the activation of the Src kinases through S-nitrosylation reactions. Particularly, the Cys498 has been specified as a fundamental residue when considering this regulatory mechanism. Aiming to understand the role of these four cysteines in S-nitrosylation, theoretical studies of electrostatic, steric and hydrophobic properties were performed with a sequence of 20 amino acids, enclosing the four cysteine residues under study, extracted from the PDB coordinates of the crystal obtained from the inactive state of Src kinase. Results indicate that Cys498 is buried deeply in the protein, in hydrophobic surroundings in which NO is more likely to suffer decomposition into the electrophilic intermediates known to be responsible for S-nitrosylation reactions. Electronic calculated properties, such as punctual atomic charges, electrostatic potentials and molecular orbital energy, also demonstrated the good nucleophilic potential of Cys498. Graphical Abstract Structure of Src kinase with zoomed area representing the 20 amino acids comprising the CC motif extracted from the whole protein structure. Right upper panel Electrostatic potential map, right lower panel hydrophilic map in anterior view.

3,3'-Diindolylmethane Inhibits Flt3L/GM-CSF-induced-bone Marrow-derived CD103(+) Dendritic Cell Differentiation Regulating Phosphorylation of STAT3 and STAT5

The intestinal immune system maintains oral tolerance to harmless antigens or nutrients. One mechanism of oral tolerance is mediated by regulatory T cell (Treg)s, of which differentiation is regulated by a subset of dendritic cell (DC)s, primarily CD103⁺ DCs. The aryl hydrocarbon receptor (AhR), a ligand-activated transcription factor, plays an important role in regulating immunity. The intestines are exposed to various AhR ligands, including endogenous metabolites and phytochemicals. It was previously reported that AhR activation induced tolerogenic DCs in mice or in cultures of bone marrow-derived DCs. However, given the variety of tolerogenic DCs, which type of tolerogenic DCs is regulated by AhR remains unknown. In this study, we found that AhR ligand 3,3'-diindolylmethane (DIM) inhibited the development of CD103⁺ DCs from mouse bone marrow cells stimulated with Flt3L and GM-CSF. DIM interfered with phosphorylation of STAT3 and STAT5 inhibiting the expression of genes, including Id2, E2-2, IDO-1, and Aldh1a2, which are associated with DC differentiation and functions. Finally, DIM suppressed the ability of CD103⁺ DCs to induce Foxp3⁺ Tregs.

E-cadherin and Src associate with extradesmosomal Dsg3 and modulate desmosome assembly and adhesion

Desmosomes provide strong intercellular cohesion essential for the integrity of cells and tissues exposed to continuous mechanical stress. For desmosome assembly, constitutively synthesized desmosomal cadherins translocate to the cell-cell border, cluster and mature in the presence of Ca(2+) to stable cell contacts. As adherens junctions precede the formation of desmosomes, we investigated in this study the relationship between the classical cadherin E-cadherin and the desmosomal cadherin Desmoglein 3 (Dsg3), the latter of which is indispensable for cell-cell adhesion in keratinocytes. By using autoantibodies from patients with the blistering skin disease pemphigus vulgaris (PV), we showed in loss of function studies that E-cadherin compensates for effects of desmosomal disassembly. Overexpression of E-cadherin reduced the loss of cell cohesion induced by PV autoantibodies and attenuated activation of p38 MAPK. Silencing of E-cadherin abolished the localization of Dsg3 at the membrane and resulted in a shift of Dsg3 from the cytoskeletal to the non-cytoskeletal protein pool which conforms to the notion that E-cadherin regulates desmosome assembly. Mechanistically, we identified a complex consisting of extradesmosomal Dsg3, E-cadherin, β-catenin and Src and that the stability of this complex is regulated by Src. Moreover, Dsg3 and E-cadherin are phosphorylated on tyrosine residues in a Src-dependent manner and Src activity is required for recruiting Dsg3 to the cytoskeletal pool as well as for desmosome maturation towards a Ca(2+)-insensitive state. Our data provide new insights into the role of E-cadherin and the contribution of Src signaling for formation and maintenance of desmosomal junctions.

Microtubule Dynamics in Neuronal Development, Plasticity, and Neurodegeneration

Neurons are the basic information-processing units of the nervous system. In fulfilling their task, they establish a structural polarity with an axon that can be over a meter long and dendrites with a complex arbor, which can harbor ten-thousands of spines. Microtubules and their associated proteins play important roles during the development of neuronal morphology, the plasticity of neurons, and neurodegenerative processes. They are dynamic structures, which can quickly adapt to changes in the environment and establish a structural scaffold with high local variations in composition and stability. This review presents a comprehensive overview about the role of microtubules and their dynamic behavior during the formation and maturation of processes and spines in the healthy brain, during aging and under neurodegenerative conditions. The review ends with a discussion of microtubule-targeted therapies as a perspective for the supportive treatment of neurodegenerative disorders.

SH2 Ligand-Like Effects of Second Cytosolic Domain of Na/K-ATPase α1 Subunit on Src Kinase

Our previous studies have suggested that the α1 Na/K-ATPase interacts with Src to form a receptor complex. In vitro binding assays indicate an interaction between second cytosolic domain (CD2) of Na/K-ATPase α1 subunit and Src SH2 domain. Since SH2 domain targets Src to specific signaling complexes, we expressed CD2 as a cytosolic protein and studied whether it could act as a Src SH2 ligand in LLC-PK1 cells. Co-immunoprecipitation analyses indicated a direct binding of CD2 to Src, consistent with the in vitro binding data. Functionally, CD2 expression increased basal Src activity, suggesting a Src SH2 ligand-like property of CD2. Consistently, we found that CD2 expression attenuated several signaling pathways where Src plays an important role. For instance, although it increased surface expression of Na/K-ATPase, it decreased ouabain-induced activation of Src and ERK by blocking the formation of Na/K-ATPase/Src complex. Moreover, it also attenuated cell attachment-induced activation of Src/FAK. Consequently, CD2 delayed cell spreading, and inhibited cell proliferation. Furthermore, these effects appear to be Src-specific because CD2 expression had no effect on EGF-induced activation of EGF receptor and ERK. Hence, the new findings indicate the importance of Na/K-ATPase/Src interaction in ouabain-induced signal transduction, and support the proposition that the CD2 peptide may be utilized as a Src SH2 ligand capable of blocking Src-dependent signaling pathways via a different mechanism from a general Src kinase inhibitor.

Cytotoxicity of the Sesquiterpene Lactones Neoambrosin and Damsin from Ambrosia maritima Against Multidrug-Resistant Cancer Cells

Multidrug resistance is a prevailing phenomenon leading to chemotherapy treatment failure in cancer patients. In the current study two known cytotoxic pseudoguaianolide sesquiterpene lactones; neoambrosin (1) and damsin (2) that circumvent MDR were identified. The two cytotoxic compounds were isolated using column chromatography, characterized using 1D and 2D NMR, MS, and compared with literature values. The isolated compounds were investigated for their cytotoxic potential using resazurin assays and thereafter confirmed with immunoblotting and in silico studies. MDR cells overexpressing ABC transporters (P-glycoprotein, BCRP, ABCB5) did not confer cross-resistance toward (1) and (2), indicating that these compounds are not appropriate substrates for any of the three ABC transporters analyzed. Resistance mechanisms investigated also included; the loss of the functions of the TP53 and the mutated EGFR. The HCT116 p53^-/- cells were sensitive to 1 but resistant to 2. It was interesting to note that resistant cells transfected with oncogenic ΔEGFR exhibited hypersensitivity CS toward (1) and (2) (degrees of resistances were 0.18 and 0.15 for (1) and (2), respectively). Immunoblotting and in silico analyses revealed that 1 and 2 silenced c-Src kinase activity. It was hypothesized that inhibition of c-Src kinase activity may explain CS in EGFR-transfected cells. In conclusion, the significant cytotoxicity of 1 and 2 against different drug-resistant tumor cell lines indicate that they may be promising candidates to treat refractory tumors.

The Src non-receptor tyrosine kinase paradigm: New insights into mammalian Sertoli cell biology

Src kinases are non-receptor tyrosine kinases that phosphorylate diverse substrates, which control processes such as cell proliferation, differentiation and survival; cell adhesion; and cell motility. c-Src, the prototypical member of this protein family, is widely expressed by several organs that include the testis. In the seminiferous epithelium of the adult rat testis, c-Src is highest at the tubule lumen during the release of mature spermatids. Other studies show that testosterone regulates spermatid adhesion to Sertoli cells via c-Src, indicating Src phosphorylates key substrates that prompt the disassembly of Sertoli cell-spermatid junctions. A more recent in vitro study reveals that c-Src participates in the internalization of proteins that constitute the blood-testis barrier, which is present between Sertoli cells, suggesting a similar mechanism of junction disassembly is at play during spermiation. In this review, we discuss recent findings on c-Src, with an emphasis on its role in spermatogenesis in the mammalian testis.

Myristoleic acid inhibits osteoclast formation and bone resorption by suppressing the RANKL activation of Src and Pyk2

Cytoskeletal changes in osteoclasts such as formation of actin ring is required for bone-resorbing activity. The tyrosine kinase Src is a key player in massive cytoskeletal change of osteoclasts, thereby in bone destruction. In order for Src to be activated, trafficking to the inner plasma membrane via myristoylation is of importance. A previous study reported that myristoleic acid derived from myristic acid, inhibited N-myristoyl-transferase, an essential enzyme for myristoylation process. This prompted us to investigate whether myristoleic acid could affect osteoclastogenesis. Indeed, we observed that myristoleic acid inhibited RANKL-induced osteoclast formation in vitro, especially, at later stages of differentiation. Myristoleic acid attenuated the tyrosine phosphorylation of c-Src and Pyk2, which associates with Src, by RANKL. When myristoleic acid was co-administered with soluble RANKL into mice, RANKL-induced bone loss was substantially prevented. Bone dissection clearly revealed that the number of multinucleated osteoclasts was significantly diminished by myristoleic acid. On the other hand, myristoleic acid treatment had little or no influence on early osteoclast differentiation markers, such as c-Fos and NFATc1, and proteins related to cytoskeletal rearrangement, including DC-STAMP, integrin αv and integrin β3 in vitro. Taken together, our data suggest that myristoleic acid is capable of blocking the formation of large multinucleated osteoclasts and bone resorption likely through suppressing activation of Src and Pyk2.