A truncated fragment of Src protein kinase generated by calpain-mediated cleavage is a mediator of neuronal death in excitotoxicity.

Excitotoxicity resulting from overstimulation of glutamate receptors is a major cause of neuronal death in cerebral ischemic stroke. The overstimulated ionotropic glutamate receptors exert their neurotoxic effects in part by overactivation of calpains, which induce neuronal death by catalyzing limited proteolysis of specific cellular proteins. Here, we report that in cultured cortical neurons and in vivo in a rat model of focal ischemic stroke, the tyrosine kinase Src is cleaved by calpains at a site in the N-terminal unique domain. This generates a truncated Src fragment of ~52 kDa, which we localized predominantly to the cytosol. A cell membrane-permeable fusion peptide derived from the unique domain of Src prevents calpain from cleaving Src in neurons and protects against excitotoxic neuronal death. To explore the role of the truncated Src fragment in neuronal death, we expressed a recombinant truncated Src fragment in cultured neurons and examined how it affects neuronal survival. Expression of this fragment, which lacks the myristoylation motif and unique domain, was sufficient to induce neuronal death. Furthermore, inactivation of the prosurvival kinase Akt is a key step in its neurotoxic signaling pathway. Because Src maintains neuronal survival, our results implicate calpain cleavage as a molecular switch converting Src from a promoter of cell survival to a mediator of neuronal death in excitotoxicity. Besides unveiling a new pathological action of Src, our discovery of the neurotoxic action of the truncated Src fragment suggests new therapeutic strategies with the potential to minimize brain damage in ischemic stroke.

Inhibition of Lck: evidence for a novel natural Src family kinase inhibitor.

Src family kinase (SFK) is a family of protein tyrosine kinases that play important roles in the development of various cancers. Here, we showed that a naturally occurring inhibitory factor of SFK can be extracted from the rat brain. This inhibitor strongly suppressed the activity of SFKs including Lck and Fyn. It did not inhibit other protein tyrosine kinases including Wee1 or serine/threonine kinases Mst2, Cdk5/p25, Cdk5/p35, and Cdk2/cyclin A. The inhibitor was not an ATPase, a phosphatase that dephosphorylates substrates of the SFK reaction, or a protease that degrades SFKs. Activity of mutant Lck with C-terminal tyrosine substituted with phenylalanine was also suppressed by the inhibitor to a similar extent of wild-type Lck, indicating that the inhibitor was not CSK. Gel filtration chromatography indicated that the molecular size of the prevalent form of this inhibitor was approximately 44 kDa.

SRC is dephosphorylated at tyrosine 530 in human colon carcinomas.

OBJECTIVE: Src is a protein tyrosine kinase that plays important roles in cancer development, and Src kinase activity has been found to be elevated in several types of cancers. However, the cause of the elevation of Src kinase activity in the majority of human colon carcinomas is still largely unknown. We aim at finding the cause of elevated Src kinase activity in human colon carcinomas. METHODS: We employed normal colon epithelial FHC cells and examined Src activation in human colon carcinoma specimens from 8 patients. Protein expression levels were determined by Western blotting, and the activity of Src kinase by kinase assay. RESULTS: Actin levels were different between tumor and normal tissues, demonstrating the complexities and inhomogeneities of the tissue samples. Src kinase activities were increased in the majority of the colon carcinomas as compared with normal colon epithelial cells (range 13-29). Src protein levels were reduced in the colon carcinomas. Src Y530 phosphorylation levels were reduced to a higher extent than protein levels in the carcinomas. CONCLUSION: The results suggest that Src specific activities were highly increased in human colon carcinomas; phosphorylation at Src Y530 was reduced, contributing to the highly elevated Src specific activity and Src kinase activity.

CSK-homologous kinase (CHK/MATK) is a potential colorectal cancer tumour suppressor gene epigenetically silenced by promoter methylation.

Hyperactivation of SRC-family protein kinases (SFKs) contributes to the initiation and progression of human colorectal cancer (CRC). Since oncogenic mutations of SFK genes are rare in human CRC, we investigated if SFK hyperactivation is linked to dysregulation of their upstream inhibitors, C-terminal SRC kinase (CSK) and its homolog CSK-homologous kinase (CHK/MATK). We demonstrate that expression of CHK/MATK but not CSK was significantly downregulated in CRC cell lines and primary tumours compared to normal colonic tissue. Investigation of the mechanism by which CHK/MATK expression is down-regulated in CRC cells uncovered hypermethylation of the CHK/MATK promoter in CRC cell lines and primary tumours. Promoter methylation of CHK/MATK was also observed in several other tumour types. Consistent with epigenetic silencing of CHK/MATK, genetic deletion or pharmacological inhibition of DNA methyltransferases increased CHK/MATK mRNA expression in CHK/MATK-methylated colon cancer cell lines. SFKs were hyperactivated in CHK/MATK-methylated CRC cells despite expressing enzymatically active CSK, suggesting loss of CHK/MATK contributes to SFK hyperactivation. Re-expression of CHK/MATK in CRC cell lines led to reduction in SFK activity via a non-catalytic mechanism, a reduction in anchorage-independent growth, cell proliferation and migration in vitro, and a reduction in tumour growth and metastasis in a zebrafish embryo xenotransplantation model in vivo, collectively identifying CHK/MATK as a novel putative tumour suppressor gene in CRC. Furthermore, our discovery that CHK/MATK hypermethylation occurs in the majority of tumours warrants its further investigation as a diagnostic marker of CRC.

Delivery of gene targeting siRNAs to breast cancer cells using a multifunctional peptide complex that promotes both targeted delivery and endosomal release.

RNA interference has been used to dissect the importance of individual gene products in various human disease processes, including cancer. Small-interfering RNA, or siRNA, is one of the tools utilized in this regard, but specially-designed delivery agents are required to allow the siRNA to gain optimal access to the cell interior. Our laboratory has utilized two different siRNA-binding delivery peptides containing a polyarginine core, and modified by myristoylation and targeting motifs (iRGD or Lyp-1). A third peptide was designed to assist with endosomal release. Various ratios of the peptides and siRNA were combined and assayed for the ability to form stable complexes, and optimized ratios were determined. The complexes were found to form particles, with the majority having a diameter of 100-300 nm, as visualized by electron microscopy. These siRNA complexes have enhanced protection from nucleases present in serum, as compared to "naked" unprotected siRNA. The particles were internalized by the cells and could be detected in the cell cytoplasm by confocal fluorescence microscopy. In functional assays, peptide/siRNA complexes were shown to cause the knock down of corresponding targeted proteins. The peptide with the LyP-1 targeting motif was more effective at knockdown in MDA-MB-231 breast cancer cells than the peptide with the iRGD motif. Inclusion of the endosomal release peptide in the complexes greatly enhanced the peptide/siRNA effects. Peptide/siRNA complexes simultaneously targeting Stat3 and c-Myc caused a marked reduction in anchorage-independent growth, a property correlated with tumorigenicity. This study demonstrates the ability of a peptide-based siRNA-delivery system to deliver siRNA into breast cancer cells and cause both protein knockdown and suppression of the malignant phenotype. Such peptide complexes are likely to become highly useful siRNA-delivery vehicles for the characterization, and potentially for the treatment, of human cancer.

The RNA binding protein Sam68 is acetylated in tumor cell lines, and its acetylation correlates with enhanced RNA binding activity.

Sam68 (Src-associated in mitosis; 68 kDa) is a member of the STAR (signal transduction and activation of RNA) family of KH domain-containing RNA binding proteins. Accumulating evidence suggests that it plays an important role in cell cycle control. Tyrosine phosphorylation by Src family kinases and breast tumor kinase can negatively regulate its RNA binding activity. To date, there are no reports of a factor, such as a phosphatase, which can positively regulate Sam68 association with RNA. Acetylation is a reversible post-translational modification known to influence the activity of DNA binding proteins. However, acetylation of a cellular RNA binding protein as a mechanism for regulating its activity has not yet been reported. Here we demonstrate Sam68 to be acetylated in vivo. A screen of several human mammary epithelial cell lines revealed variations in Sam68 acetylation. Interestingly, the highest level of acetylation was found in tumorigenic breast cancer cell lines. The screen also showed a positive correlation between Sam68 acetylation and its ability to bind RNA. The acetyltransferase CBP was shown to acetylate Sam68 and enhance its binding to poly(U) RNA. These results suggest that Sam68 association with RNA substrates may be positively regulated by acetylation, and that enhanced acetylation and RNA binding activity of Sam68 may play a role in tumor cell proliferation.

Src kinase becomes preferentially associated with the VEGFR, KDR/Flk-1, following VEGF stimulation of vascular endothelial cells.

BACKGROUND: The cytoplasmic tyrosine kinase, Src, has been found to play a crucial role in VEGF (vascular endothelial growth factor) - dependent vascular permeability involved in angiogenesis. The two main VEGFRs present on vascular endothelial cells are KDR/Flk-1 (kinase insert domain-containing receptor/fetal liver kinase-1) and Flt-1 (Fms-like tyrosine kinase-1). However, to date, it has not been determined which VEGF receptor (VEGFR) is involved in binding to and activating Src kinase following VEGF stimulation of the receptors. RESULTS: In this report, we demonstrate that Src preferentially associates with KDR/Flk-1 rather than Flt-1 in human umbilical vein endothelial cells (HUVECs), and that VEGF stimulation resulted in an increase of Src activity associated with activated KDR/Flk-1. These findings were determined through immunoprecipitation-kinase experiments and coimmunoprecipitation studies, and were further confirmed by GST-pull-down assays and Far Western studies. However, Fyn and Yes, unlike Src, were found to associate preferentially with Flt-1. CONCLUSIONS: Thus, Src preferentially associates with KDR/Flk-1, rather than with Flt-1, upon VEGF stimulation in endothelial cells. Our findings further highlight the potential significance of upregulated KDR/Flk-1-associated Src activity in the process of angiogenesis, and help to elucidate more clearly the specific roles and mechanisms involving Src family tyrosine kinase in VEGF-stimulated signal transduction events.

Src regulates the activity of the ING1 tumor suppressor.

The INhibitor of Growth 1 (ING1) is stoichiometric member of histone deacetylase (HDAC) complexes and functions as an epigenetic regulator and a type II tumor suppressor. It impacts cell growth, aging, apoptosis, and DNA repair, by affecting chromatin conformation and gene expression. Down regulation and mislocalization of ING1 have been reported in diverse tumor types and Ser/Thr phosphorylation has been implicated in both of these processes. Here we demonstrate that both in vitro and in vivo, the tyrosine kinase Src is able to physically associate with, and phosphorylate ING1, which results in a nuclear to cytoplasmic relocalization of ING1 in cells and a decrease of ING1 stability. Functionally, Src antagonizes the ability of ING1 to induce apoptosis, most likely through relocalization of ING1 and down regulation of ING1 levels. These effects were due to both kinase-dependent and kinase-independent properties of Src, and were most apparent at elevated levels of Src expression. These findings suggest that Src may play a major role in regulating ING1 levels during tumorigenesis in those cancers in which high levels of Src expression or activity are present. These data represent the first report of tyrosine kinase-mediated regulation of ING1 levels and suggest that kinase activation can impact chromatin structure through the ING1 epigenetic regulator.

Simultaneous siRNA targeting of Src and downstream signaling molecules inhibit tumor formation and metastasis of a human model breast cancer cell line.

BACKGROUND: Src and signaling molecules downstream of Src, including signal transducer and activator of transcription 3 (Stat3) and cMyc, have been implicated in the development, maintenance and/or progression of several types of human cancers, including breast cancer. Here we report the ability of siRNA-mediated Src knock-down alone, and simultaneous knock-down of Src and Stat3 and/or cMyc to inhibit the neoplastic phenotype of a highly metastatic human model breast cancer cell line, MDA-MB-435S, a widely used model for breast cancer research. METHODOLOGY/RESULTS: Src and its downstream signaling partners were specifically targeted and knocked-down using siRNA. Changes in the growth properties of the cultured cancer cells/tumors were documented using assays that included anchorage-dependent and -independent (in soft agar) cell growth, apoptosis, and both primary and metastatic tumor growth in the mouse tumor model. siRNA-mediated Src knock-down alone, and simultaneous knock-down of Src and Stat3 and/or cMyc inhibited the neoplastic phenotype of a highly metastatic human model breast cancer cell line, MDA-MB-435S. This knock-down resulted in reduced growth in monolayer and soft agar cultures, and a reduced ability to form primary tumors in NOD/SCID mice. In addition, direct intra-tumoral injection of siRNAs targeting these signaling molecules resulted in a substantial inhibition of tumor metastases as well as of primary tumor growth. Simultaneous knock-down of Src and Stat3, and/or Myc exhibited the greatest effects resulting in substantial inhibition of primary tumor growth and metastasis. CONCLUSIONS/SIGNIFICANCE: These findings demonstrate the effectiveness of simultaneous targeting of Src and the downstream signaling partners Stat3 and/or cMyc to inhibit the growth and oncogenic properties of a human cancer cell line. This knowledge may be very useful in the development of future therapeutic approaches involving targeting of specific genes products involved in tumor growth and metastasis.

The von Hippel-Lindau Tumor Suppressor Protein Is Destabilized by Src: Implications for Tumor Angiogenesis and Progression.

The von Hippel-Lindau tumor suppressor protein (VHL), when mutated and inactivated, has been associated with renal and CNS cancer development. VHL normally plays an important role in targeting for degradation of the HIF-1α (hypoxia inducible factor-1α) transcription factor, a primary positive regulator of vascular endothelial growth factor (VEGF) production. In this report we demonstrate that VHL destabilization can be induced by Src kinase and may be involved in other cancers, including breast cancer. We have found that elevated Src can trigger a drastic reduction in VHL stability even under normoxic conditions, through phosphorylation of VHL tyrosine residue 185, leading to ubiquitination and proteasome-mediated degradation of VHL. The Src-induced degradation of VHL protein leads to increased HIF-1α levels and transcriptional activity and increased VEGF production. In this manner, Src regulation of VHL protein stability may play an important role in promoting VEGF expression, tumor angiogenesis, and cancer progression.

REAP: A two minute cell fractionation method.

BACKGROUND: The translocation or shuttling of proteins between the nucleus and cytoplasm (nucleocytoplasmic transport [NCPT]) is often a rapid event following stimulation with growth factors or in response to stress or other experimental manipulations. Commonly used methods to separate nuclei from cytoplasm employ lengthy steps such as density gradient centrifugation which exposes cells to non-physiological hyperosmotic conditions for extended time periods resulting in varying degrees of leakage between the nucleus and cytoplasm. To help maintain and quantify nuclear:cytoplasmic ratios of proteins, agents such as leptomycin B have been employed to be able to better analyze NCPT by inhibiting nuclear export. To track NCPT in the absence of these experimental manipulations that could introduce unknown artefacts, we have developed a rapid method that appears to produce pure nuclear and cytoplasmic fractions, suitable for obtaining accurate estimates of the nuclear:cytoplasmic ratios of proteins known to undergo NCPT. FINDINGS: We have developed a Rapid, Efficient And Practical (REAP) method for subcellular fractionation of primary and transformed human cells in culture. The REAP method is a two minute non-ionic detergent-based purification technique requiring only a table top centrifuge, micro-pipette and micro-centrifuge tubes. This inexpensive method has proven to efficiently separate nuclear from cytoplasmic proteins as estimated by no detectible cross-contamination of the nucleoporin and lamin A nuclear markers or the pyruvate kinase and tubulin cytoplasmic markers. REAP fractions also mirrored TNFα induced NF-κB NCPT observed in parallel by indirect immunofluorescence. CONCLUSIONS: This method drastically reduces the time needed for subcellular fractionation, eliminates detectable protein degradation and maintains protein interactions. The simplicity, brevity and efficiency of this procedure allows for tracking ephemeral changes in subcellular relocalization of proteins while maintaining protein integrity and protein complex interactions.

PTP1B contributes to the oncogenic properties of colon cancer cells through Src activation.

Src-specific activity has been reported to be elevated in a high percentage of colon cancer cell lines and tumors, but the underlying mechanisms are largely unknown. In this study, we report that, in the seven cancer cell lines tested, Src-specific activity was elevated (5.2- to 18.7-fold) relative to normal colon cells (FHC). This activation of Src correlated with reduced phosphorylation at Y530 of Src, whereas there was no significant change in the level of phosphorylation at Y419. The membrane tyrosine phosphatase activity for a Src family-specific phosphopeptide substrate FCP (Fyn COOH-terminal peptide phosphorylated by Csk) was greatly increased in the cancer cells and was attributed to PTP1B in most of the cell lines. Membrane PTP1B protein levels were also greatly increased. Overexpression of PTP1B increased Src specific activity in colon cancer cells by reducing phosphorylation at Y530 of Src. It also increased anchorage-independent cell growth and this increase was blocked by the Src inhibitor PP2 and Src small interfering RNA (siRNA). Down-regulating PTP1B activity by PTP1B inhibitor CinnGEL 2Me or knocking down PTP1B using siRNA also reduced Src kinase activity and colony formation ability of colon cancer cells. PTP1B siRNA reduced tumor growth in nonobese diabetic/severe combined immunodeficient mice. This study suggests that (a) PTP1B can act as an important activator of Src in colon cancer cells via dephosphorylation at Y530 of Src and (b) elevated levels of PTP1B can increase tumorigenicity of colon cancer cells by activating Src.

C-terminal Src kinase-homologous kinase (CHK), a unique inhibitor inactivating multiple active conformations of Src family tyrosine kinases.

The Src family of protein kinases (SFKs) mediates mitogenic signal transduction, and constitutive SFK activation is associated with tumorigenesis. To prevent constitutive SFK activation, the catalytic activity of SFKs in normal mammalian cells is suppressed mainly by two inhibitors called C-terminal Src kinase (CSK) and CSK-homologous kinase (CHK), which inactivate SFKs by phosphorylating a consensus tyrosine near the C terminus of SFKs (Y(T)). The phosphorylated Y(T) intramolecularly binds to the SH2 domain of SFKs. This interaction, known as pY(T)/SH2 interaction, together with binding between the SH2 kinase linker and the SH3 domain of SFKs (linker/SH3 interaction) stabilizes SFKs in a "closed" inactive conformation. We previously discovered an alternative mechanism CHK employs to inhibit SFKs. This mechanism, referred to as the non-catalytic inhibitory mechanism, involves tight binding of CHK to SFKs; the binding alone is sufficient to inhibit SFKs. Herein, we constructed multiple active conformations of an SFK member, Hck, by systematically disrupting the two inhibitory interactions. We found that CHK employs the non-catalytic mechanism to inactivate these active conformations of Hck. However, CHK does not bind Hck when it adopts the inactive conformation in which both inhibitory interactions are intact. These data indicate that binding of CHK to SFKs via the non-catalytic mechanism is governed by the conformations of SFKs. Although CSK is also an inhibitor of SFKs, it does not inhibit SFKs by a similar non-catalytic mechanism. Thus, the non-catalytic inhibitory mechanism is a unique property of CHK that allows it to down-regulate multiple active conformations of SFKs.

A novel non-catalytic mechanism employed by the C-terminal Src-homologous kinase to inhibit Src-family kinase activity.

Although C-terminal Src kinase (CSK)-homologous kinase (CHK) is generally believed to inactivate Src-family tyrosine kinases (SFKs) by phosphorylating their consensus C-terminal regulatory tyrosine (Tyr(T)), exactly how CHK inactivates SFKs is not fully understood. Herein, we report that in addition to phosphorylating Tyr(T), CHK can inhibit SFKs by a novel non-catalytic mechanism. First, CHK directly binds to the SFK members Hck, Lyn, and Src to form stable protein complexes. The complex formation is mediated by a non-catalytic Tyr(T)-independent mechanism because it occurs even in the absence of ATP or when Tyr(T) of Hck is replaced by phenylalanine. Second, the non-catalytic CHK-SFK interaction alone is sufficient to inactivate SFKs by inhibiting the catalytic activity of autophosphorylated SFKs. Third, CHK and Src co-localize to specific plasma membrane microdomains of rat brain cells, suggesting that CHK is in close proximity to Src such that it can effectively inactivate Src in vivo. Fourth, native CHK.Src complex exists in rat brain, and recombinant CHK.Hck complex exists in transfected HEK293T cells, implying that CHK forms stable complexes with SFKs in vivo. Taken together, our findings suggest that CHK inactivates SFKs (i) by phosphorylating their Tyr(T) and (ii) by this novel Tyr(T)-independent mechanism involving direct binding of CHK to SFKs. It has been documented that autophosphorylated SFKs can still be active, in some cases even when their Tyr(T) is phosphorylated. Thus, the ability of the Tyr(T)-independent mechanism to suppress the activity of both non-phosphorylated and autophosphorylated SFKs represents a fail-safe measure employed by CHK to down-regulate SFK signaling under all circumstances.