Role of Src expression and activation in human cancer

The role of mammalian ribonucleases (RNases) in cancer

Ribonucleases (RNases) are a group of enzymes that cleave RNAs at phosphodiester bonds resulting in remarkably diverse biological consequences. This review focuses on mammalian RNases that are capable of, or potentially capable of, cleaving messenger RNA (mRNA) as well as other RNAs in cells and play roles in the development of human cancers. The aims of this review are to provide an overview of the roles of currently known mammalian RNases, and the evidence that associate them as regulators of tumor development. The roles of these RNases as oncoproteins and/or tumor suppressors in influencing cell growth, apoptosis, angiogenesis, and other cellular hallmarks of cancer will be presented and discussed. The RNases under discussion include RNases from the conventional mRNA decay pathways, RNases that are activated under cellular stress, RNases from the miRNA pathway, and RNases with multifunctional activity.

Improving the prognosis for patients with glioblastoma: the rationale for targeting Src

Glioblastoma is the most common and aggressive form of primary brain tumor. The prognosis for patients diagnosed with glioblastoma is poor, with a median survival of 12-14 months and a 5-year survival rate of <5%. The upfront standard treatment for patients with newly diagnosed glioblastoma, consisting of surgery followed by chemotherapy combined with radiotherapy, provides only short-term survival benefits. Recurrent glioblastoma is an extremely challenging therapeutic setting because of the aggressive and resistant nature of the tumor. A set of key molecular targets in oncology is the Src family of non-receptor protein kinases. Dysregulated signaling via the Src kinases has been shown to underlie glioma-related proliferation, angiogenesis, migration, and survival. Here we review the biologic role of Src in malignant glioma and discuss key preclinical studies demonstrating the potential utility of inhibiting Src in glioma. Proof of clinical benefit is forthcoming from the first clinical studies involving the newest generation of small molecule Src inhibitors currently in clinical trials for recurrent glioblastoma. Blocking Src alone will likely not translate into a significant clinical benefit; thus, strategies for combining Src inhibitors with potential synergistic therapeutic modalities will be discussed. This review will focus on dasatinib, the most advanced Src inhibitor being tested in glioblastoma, which is currently in phase I/II trials in this setting.

Phosphorylation of the SRC epithelial substrate Trask is tightly regulated in normal epithelia but widespread in many human epithelial cancers

PURPOSE

The frequently elevated activities of the c-src and c-yes products in human epithelial tumors suggest that these activated tyrosine kinases have tumorigenic functions analogous to the v-src and v-yes oncogene products. Studies of v-src-transformed fibroblasts have identified many of the effectors of this potent oncogene; however, because c-src and c-yes lack the mutational and promiscuous activities of their retroviral oncogene homologues, their presumptive tumorigenic functions in human epithelial tumors are more subtle, less well-defined, and await identification of possible effectors more directly relevant to epithelial cells.

EXPERIMENTAL DESIGN

We recently identified a transmembrane glycoprotein named Trask that is expressed in epithelial tissues but not fibroblasts and is phosphorylated by SRC kinases in mitotic epithelial cells. In this study, we have surveyed the expression and phosphorylation of Trask in many human epithelial cancer cell lines and surgical tissues and tumors.

RESULTS

Trask is widely expressed in human epithelial tissues, but its phosphorylation is tightly regulated and restricted to detached mitotic cells or cells undergoing physiologic shedding. However, abberant Trask phosphorylation is seen in many epithelial tumors from all stages including preinvasive, invasive, and metastatic tumors. Trask phosphorylation requires SRC kinases, and is also aberrantly hyperphosphorylated in the SRC-activated PyMT mouse epithelial tumors and dephosphorylated by the SRC inhibitor treatment of these tumors.

CONCLUSIONS

The widespread phosphorylation of Trask in many human epithlelial cancers identifies a new potential effector of SRC kinases in human epithelial tumorigenesis.

Inhibition of Src family kinases with dasatinib blocks migration and invasion of human melanoma cells

Src family kinases (SFK) are involved in regulating a multitude of biological processes, including cell adhesion, migration, proliferation, and survival, depending on the cellular context. Therefore, although SFKs are currently being investigated as potential targets for treatment strategies in various cancers, the biological responses to inhibition of SFK signaling in any given tumor type are not predictable. Dasatinib (BMS-354825) is a dual Src/Abl kinase inhibitor with potent antiproliferative activity against hematologic malignancies harboring activated BCR-ABL. In this study, we show that dasatinib blocks migration and invasion of human melanoma cells without affecting proliferation and survival. Moreover, dasatinib completely inhibits SFK kinase activity at low nanomolar concentrations in all eight human melanoma cell lines investigated. In addition, two known downstream targets of SFKs, focal adhesion kinase and Crk-associated substrate (p130(CAS)), are inhibited with similar concentrations and kinetics. Consistent with inhibition of these signaling pathways and invasion, dasatinib down-regulates expression of matrix metalloproteinase-9. We also provide evidence that dasatinib directly inhibits kinase activity of the EphA2 receptor tyrosine kinase, which is overexpressed and/or overactive in many solid tumors, including melanoma. Thus, SFKs and downstream signaling are implicated as having key roles in migration and invasion of melanoma cells.

Current and future treatments of bone metastases

Bone metastases contribute to a significant degree of morbidity in patients with common cancers through the development of skeletal related events (SRE) such as bone pain and pathological fracture. Traditional therapy has relied on surgical removal of lesions and, with the advent of adjuvant therapies, has been combined with radiotherapy, chemotherapy, and more recently osteoclast inhibiting agents like bisphosphonates. Although these therapeutic combinations can achieve a degree of local control, and rarely cure, across the vast majority of metastatic cancers they provide only palliation. Newer molecular agents currently under investigation, combined with innovations in surgery and radiation therapy offer a more targeted approach to bone metastasis. These utilise our understanding of key steps in the metastatic cascade including chemotactic attraction to bone, secretion of proteases, the cancer supporting microenvironment of bone matrix and the RANK-RANKL interaction for osteoclast activation. Direct inhibition of metastasis progression and osteolysis with less reliance on cytotoxic agents and invasive therapy should result in improved metastatic control, longer survival and less overall morbidity.

Dasatinib-induced autophagy is enhanced in combination with temozolomide in glioma

Glioblastoma is defined by its aggressive invasion, microvascular proliferation, and central necrosis. BMS-354825 (dasatinib) is an ATP-competitive small-molecule inhibitor effective in treating drug-resistant tumors with mutant BCR-ABL, KIT, and epidermal growth factor receptor by blocking tyrosine phosphorylation sites that are critical in tumorigenesis. In studying the action of dasatinib in human glioblastoma, we found that levels of phospho-SRC, AKT, and ribosomal protein S6 were decreased in cell lines treated with low nanomolar concentrations of dasatinib at baseline and following stimulation with epidermal growth factor. Furthermore, an increased sensitivity to dasatinib was noted in glioma cells with functional PTEN. Reduction of invasive potential was observed in vitro at concentrations well below the IC(50) of dasatinib, which was corroborated by immunofluorescence staining showing disruption of paxillin localization to focal adhesions and decreases in focal adhesion kinase autophosphorylation. Cell cycle analysis revealed minimal G(1) arrest but a significant increase in autophagic cell death in glioma cells treated with dasatinib as assessed by acridine orange staining and a concomitant increase in light chain 3 expression and processing. Combination treatment of glioma cells with dasatinib and temozolomide resulted in a significant increase in cell cycle disruption and autophagic cell death. Dasatinib in combination with temozolomide more effectively increased the therapeutic efficacy of temozolomide than when dasatinib was combined with carboplatin or irinotecan. These results strongly support the clinical use of dasatinib in the treatment of glioblastoma and provide a rationale for combination therapy with dasatinib and temozolomide.

Saracatinib Impairs Head and Neck Squamous Cell Carcinoma Invasion by Disrupting Invadopodia Function

Elevated Src kinase activity is linked to the progression of solid tumors, including head and neck squamous cell carcinoma (HNSCC). Src regulates HNSCC proliferation and tumor invasion, with the Src-targeted small molecule inhibitor saracatinib displaying potent anti-invasive effects in preclinical studies. However, the pro-invasive cellular mechanism(s) perturbed by saracatinib are unclear. The anti-proliferative and anti-invasive effects of saracatinib on HNSCC cell lines were therefore investigated in pre-clinical cell and mouse model systems. Saracatinib treatment inhibited growth, cell cycle progression and transwell Matrigel invasion in HNSCC cell lines. Dose-dependent decreases in Src activation and phosphorylation of the invasion-associated substrates focal adhesion kinase, p130 CAS and cortactin were also observed. While saracatinib did not significantly impact HNSCC tumor growth in a mouse orthotopic model of tongue squamous cell carcinoma, impaired perineural invasion and cervical lymph node metastasis was observed. Accordingly, saracatinib treatment displayed a dose-dependent inhibitory effect on invadopodia formation, extracellular matrix degradation and matrix metalloprotease 9 activation. These results suggest that inhibition of Src kinase by saracatinib impairs the pro-invasive activity of HNSCC by inhibiting Src substrate phosphorylation important for invadopodia formation and associated matrix metalloprotease activity.

N-acetyl-l-cysteine fosters inactivation and transfer to endolysosomes of c-Src

The non-receptor-protein tyrosine kinase c-Src is overexpressed and activated in a large number of human cancers, in which it is associated with tumor development and progression. Canonical regulation takes place by means of an alternative phosphorylation of tyrosine residues -- Tyr419 for activation and Tyr530 for inactivation. An independent redox regulation mechanism, involving cysteine residues, has also been proposed, in which oxidation activates the enzyme. Here we present a kinetic analysis of the effect of N-acetyl-l-cysteine (NAC) on c-Src, demonstrating that reduction reverts the oxidation-driven activation. In cancer cells, we show that NAC treatment produces an increase in specifically labeled reduced thiols of c-Src cysteines, thus confirming a redox transition. In addition to a decrease in Tyr419 phosphorylation, this leads to a massive shift of c-Src from plasma membranes -- where its active form is located -- to endolysosomal compartments. With the objective of deciphering the complex issue of c-Src regulation and of devising new strategies to revert its activation in cancers, redox regulation thus emerges as a promising area for study.

A variant affecting a putative miRNA target site in estrogen receptor (ESR) 1 is associated with breast cancer risk in premenopausal women

MicroRNAs (miRNAs) negatively regulate expression of target transcripts by hybridization to complementary sites of their messenger RNA targets. Chen et al. have described several putative functional single nucleotide polymorphisms (SNPs) in miRNA target sites. Here, we selected 11 miRNA target site SNPs located in 3' untranslated regions of genes involved in cancer and breast cancer to analyze their impact on breast cancer risk using a large familial study population. Whereas no association was observed for 10 SNPs, a significant association was revealed for the variant affecting a miRNA target site in the estrogen receptor (ESR) 1. Age stratification showed that the association was stronger in premenopausal women [C versus T: odds ratio (OR) = 0.60, confidence interval (CI) = 0.41-0.89, P = 0.010]. Furthermore, the effect was stronger in high-risk familial cases (C versus T: OR = 0.42, CI = 0.25-0.71, P = 0.0009). Clinical studies have shown that elimination of ESR1 significantly reduces breast cancer risk. Thus, therapies that inhibit ESR1 are used for breast cancer treatment. According to in silico analysis, ESR1_rs2747648 affects the binding capacity of miR-453, which is stronger when the C allele is present. In contrast, the T allele attenuates the binding of miR-453, which might lead to a reduced miRNA-mediated ESR1 repression, in consequence higher ESR1 protein levels and an increased breast cancer risk. Thus, the breast cancer protective effect observed for the C allele in premenopausal women is biologically reasonable. The analysis of large study populations in multicentre collaboration will be needed to verify the association and answer questions regarding the possible impact of this variant on therapeutic and clinical outcome.

Phosphorylated c-Src in the nucleus is associated with improved patient outcome in ER-positive breast cancer

Elevated c-Src protein expression has been shown in breast cancer and in vitro evidence suggests a role in endocrine resistance. To investigate whether c-Src is involved in endocrine resistance, we examined the expression of both total and activated c-Src in human breast cancer specimens from a cohort of oestrogen receptor (ER)-positive tamoxifen-treated breast cancer patients. Tissue microarray technology was employed to analyse 262 tumour specimens taken before tamoxifen treatment. Immunohistochemistry using total c-Src and activated c-Src antibodies was performed. Kaplan–Meier survival curves were constructed and log-rank test were performed. High level of nuclear activated Src was significantly associated with improved overall survival (P=0.047) and lower recurrence rates on tamoxifen (P=0.02). Improved patient outcome was only seen with activated Src in the nucleus. Nuclear activated Src expression was significantly associated with node-negative disease and a lower NPI (P<0.05). On subgroup analysis, only ER-positive/progesterone receptor (PgR)-positive tumours were associated with improved survival (P=0.004). This shows that c-Src activity is increased in breast cancer and that activated Src within the nucleus of ER-positive tumours predicts an improved outcome. In ER/PgR-positive disease, activated Src kinase does not appear to be involved in de novo endocrine resistance. Further study is required in ER-negative breast cancer as this may represent a cohort in which it is associated with poor outcome.

mda-9/Syntenin promotes metastasis in human melanoma cells by activating c-Src

The scaffold PDZ-domain containing protein mda-9/syntenin functions as a positive regulator of cancer cell progression in human melanoma and other tumors. mda-9/Syntenin regulates cell motility and invasion by altering defined biochemical and signaling pathways, including focal adhesion kinase (FAK), p38 mitogen-activated protein kinase (MAPK) and NF-kappaB, but precisely how mda-9/syntenin organizes these multiprotein signaling complexes is not well understood. Using a clinically relevant human melanoma model, we demonstrate that mda-9/syntenin physically interacts with c-Src and this communication correlates with an increase in FAK/c-Src complex formation and c-Src activation. Inhibiting mda-9/syntenin, using an adenovirus expressing antisense mda-9/syntenin or addition of c-Src siRNA, suppresses melanoma cell migration, anchorage-independent growth, and spontaneous tumor cell dissemination in vivo in a human melanoma animal metastasis model. These data are compatible with a model wherein interaction of MDA-9/syntenin with c-Src promotes the formation of an active FAK/c-Src signaling complex, leading to enhanced tumor cell invasion and metastatic spread. These provocative findings highlight mda-9/syntenin and its interacting partners as promising therapeutic targets for intervention of metastasis.

Src family kinases as mediators of endothelial permeability: effects on inflammation and metastasis

Src family kinases (SFKs) are signaling enzymes that have long been recognized to regulate critical cellular processes such as proliferation, survival, migration, and metastasis. Recently, considerable work has elucidated mechanisms by which SFKs regulate normal and pathologic processes in vascular biology, including endothelial cell proliferation and permeability. Further, when inappropriately activated, SFKs promote pathologic inflammatory processes and tumor metastasis, in part through their effects on the regulation of endothelial monolayer permeability. In this review, we discuss the roles of aberrantly activated SFKs in mediating endothelial permeability in the context of inflammatory states and tumor cell metastasis. We further summarize recent efforts to translate Src-specific inhibitors into therapy for systemic inflammatory conditions and numerous solid organ cancers.

Requirement of inducible nitric-oxide synthase in lipopolysaccharide-mediated Src induction and macrophage migration

Previously, we have demonstrated the induction of Src in lipopolysaccharide (LPS)-stimulated macrophages. In this study, we observed that pharmacological blockade or knockout of inducible nitric-oxide synthase (iNOS) reduced LPS-mediated Src induction and macrophage migration. Either SNAP (a NO donor) or 8-Br-cGMP (a cGMP analogue) could rescue these defects in iNOS-null macrophages, which indicated the participation of NO/cGMP in LPS-elicited Src expression and mobilization. In addition, Src family kinase (SFK)-specific inhibitor, PP2, inhibited SNAP- and 8-Br-cGMP-evoked motility implicating the involvement of SFKs downstream of NO/cGMP. Analysis of the expression of SFKs indicated LPS dramatically induced Src, which could be attributable to the increased level of the src transcript. Attenuation of Src by src-specific siRNA reduced LPS- and SNAP-evoked mobilization in Raw264.7 macrophages, and reintroduction of avian Src could rescue their motility. Furthermore, LPS-mediated Src induction led to increased FAK Pi-Tyr-397 and Pi-Tyr-861, which was also iNOS-dependent. With these findings, we concluded that iNOS was important for LPS-mediated macrophage locomotion and Src was a critical player in this process.

Design, synthesis and evaluation of new 6-substituted-5-benzyloxy-4-oxo-4H-pyran-2-carboxamides as potential Src inhibitors

Src family kinases (SFKs) are nonreceptor tyrosine kinases that are reported to be critical for cancer progression. Inhibiting the catalytic activity of these proteins has become one of the major therapeutic concepts in contemporary drug discovery. We report here the design and the synthesis of novel 6-substituted-5-benzyloxy-4-oxo-4H-pyran-2-carboxamides as potential inhibitors of Src kinase. The synthesis of these derivatives and the preliminary results of biological activity will be discussed.

Src kinase induces tumor formation in the c-SRC C57BL/6 mouse

Src kinase has been linked as a causative agent in the progression of a number of cancers including colon, breast, lung and melanoma. Src protein and activity levels are increased in colorectal cancer and liver metastases arising secondary to colon cancer. However, although Src protein is increased in colon cancer as early as the adenomatous polyp stage, a role for Src in carcinogenesis has not been established. We developed the c-SRC transgenic mouse in the C57BL/6 strain to address the issue of carcinogenesis in cells with high levels of Src expression. The transgene was constructed with the human c-SRC gene downstream of the mouse metallothionein promoter to create zinc inducible gene expression. In these C57BL/6 mice, Src protein was increased in a number of tissues both with and without zinc induction. No additional carcinogenic agent was administered. After 20 months, mice were assessed for tumor development in the liver and GI tract, as well as other organs. Of the mice with the transgene, 15% developed tumors in the liver while no tumors were detected in wild type C57BL/6 mice. A further study was conducted by crossing c-SRC C57BL/6 mice with p21 nullizygous mice to determine the effect of oncogene expression combined with inactivation of the tumor suppressor gene, p21. Addition of the c-SRC transgene to the p21-/- background increased tumor formation almost 3-fold, while it increased metastasis 6-fold. The data from our study show, for the first time, that Src kinase may play a role in carcinogenesis.

Targeting epidermal growth factor receptor and SRC pathways in head and neck cancer

Epidermal growth factor receptor (EGFR), a member of the ErbB family of receptor tyrosine kinases (RTKs), is highly expressed in head and neck squamous cell carcinoma (HNSCC) where increased EGFR expression levels in tumors are associated with decreased survival. HNSCC patient responses to EGFR-targeted monotherapies in clinical trials, though significant, have been limited. Tumor signaling pathway components that work in cooperation with EGFR or provide compensation for the loss of EGFR-initiated signaling will be ideal targets for therapies to be used in combination with EGFR-targeted agents. Based on the current understanding of molecular signaling pathways and available agents, ErbB family-targeted and Src family-targeted agents represent strategies for further exploration. Here, we discuss agents targeting ErbB and Src family kinases in clinical development, provide an overview of completed and ongoing clinical trials, and outline a molecular rationale for combining ErbB- and Src-targeted therapeutics.

Naturally occurring homoisoflavonoids function as potent protein tyrosine kinase inhibitors by c-Src-based high-throughput screening

Protein tyrosine kinase (PTK) inhibitors represent emerging therapeutics for cancer chemoprevention. In our study, hematoxylin (26) was identified as one of the most remarkable c-Src inhibitors in an orthogonal compound-mixing library (32200 compounds) by using an ELISA-based automated high-throughput screening (HTS) strategy. Interestingly, hematoxylin was found to be an ATP competitive broad-spectrum PTK inhibitor in vitro, with IC50 values ranging from nanomolar to micromolar level. Further studies showed that such inhibition was associated with the PTK phosphorylation and subsequent downstream signaling pathways. The structure-activity relationship assessment of the PTK inhibitory potency of hematoxylin analogues isolated from Heamatoxylon campechianum was in good agreement with the result of concurrent molecular docking simulation: the catechol moiety in ring A and the hematoxylin-like three-dimensional structure were essential for c-Src-targeted activities. Hematoxylin and its natural analogues were substantially validated to function as a new class of PTK inhibitors.

The lipid raft-anchored adaptor protein Cbp controls the oncogenic potential of c-Src

The tyrosine kinase c-Src is upregulated in various human cancers irrespective of its negative regulator Csk, but the regulatory mechanisms remain unclear. Here, we show that a lipid raft-anchored Csk adaptor, Cbp/PAG, is directly involved in controlling the oncogenicity of c-Src. Using Csk-deficient cells that can be transformed by c-Src overexpression, we found that Cbp expression is markedly downregulated by c-Src activation and re-expression of Cbp efficiently suppresses c-Src transformation as well as tumorigenesis. Cbp-deficient cells are more susceptible to v-Src transformation than their parental cells. Upon phosphorylation, Cbp specifically binds to activated c-Src and sequesters it in lipid rafts, resulting in an efficient suppression of c-Src function independent of Csk. In some human cancer cells and tumors, Cbp is downregulated and the introduction of Cbp significantly suppresses tumorigenesis. These findings indicate a potential role for Cbp as a suppressor of c-Src-mediated tumor progression.

Coordinate suppression of Sdpr and Fhl1 expression in tumors of the breast, kidney, and prostate

The Src tyrosine kinase associates with the focal adhesion adaptor protein Cas (Crk-associated substrate) to suppress the expression of potential tumor suppressor genes. For example, Src utilizes Cas to suppress the expression of the LIM-only protein Fhl1 (four and a half LIM domains 1), in order to promote non-anchored tumor-cell growth and migration. Here, we report that the promoter region of the Fhl1 gene was methylated more in Src-transformed cells than non-transformed cells. In addition, global expression analysis indicates that Fhl1 induced expression of serum deprivation response factor (Sdpr) in Src-transformed cells. Moreover, Fhl1 and Sdpr was expressed in approximately 87% and 40% of samples obtained from non-transformed breast, 100% of samples obtained from non-transformed kidney, and over 60% of samples obtained from non-transformed prostate. In contrast, Fhl1 and Sdpr was detected in approximately 40% and 7% of matched samples from mammary carcinoma, less than 11% of matched samples from kidney carcinoma, and in less than 22% of matched samples from prostate carcinoma. These data indicate that Fhl1 and Sdpr expression was significantly reduced in tumors of the breast (P < 0.02 and P < 0.001), kidney (P < 0.01), and prostate (P < 0.05). In addition, although Src can activate mitogen-activated protein kinase (MAPK) to promote tumor-cell growth, our data indicate that Src did not rely on MAPK activity to suppress the expression of Fhl1 and Sdpr in transformed cells. Thus, Src induced methylation of the promoter region of the Fhl1 gene; Src suppressed Fhl1 and Sdpr expression independent of mitogen-activated protein kinase (MAPK) activity; Fhl1 induced the expression of Sdpr in Src-transformed cells; and Fhl1 and Sdpr expression was suppressed in tumors of the breast, kidney, and prostate.

Functional dissection of transformation by c-Src and v-Src

The c-src proto-oncogene product, c-Src, is frequently over-expressed and activated in various human malignant cancers, implicating a role for c-Src in cancer progression. To verify the role of c-Src, we analyzed the transforming ability of c-Src in mouse embryonic fibroblasts that lack Csk, a negative regulator of Src family kinases. Although Csk deficiency is not sufficient for cell transformation, c-Src over-expression induced characteristic transformed phenotypes including anchorage-independent growth and tumorigenecity. These phenotypes were dose-dependently inhibited by the re-expression of Csk, indicating that there is a certain threshold for c-Src transformation, which is determined by the c-Src : Csk ratio. In contrast to v-Src, c-Src induced the phosphorylation of a limited number of cellular proteins and elicited a restricted change in gene expression profiles. The activation of some critical targets for v-Src transformation, such as STAT3, was not significantly induced by c-Src transformation. Several genes that are involved in cancer progression, that is, cyclin D1 and HIF-1alpha, were induced by v-Src, but not by c-Src. Furthermore, v-Src tumors exhibited aggressive growth and extensive angiogenesis, while c-Src tumors grew more slowly accompanied by the induction of hematomas. These findings demonstrate that c-Src has the potential to induce cell transformation, but it requires coordination with an additional pathway(s) to promote tumor progression in vivo.

ERK5 promotes Src-induced podosome formation by limiting Rho activation

Increased Src activity, often associated with tumorigenesis, leads to the formation of invasive adhesions termed podosomes. Podosome formation requires the function of Rho family guanosine triphosphatases and reorganization of the actin cytoskeleton. In addition, Src induces changes in gene expression required for transformation, in part by activating mitogen-activated protein kinase (MAPK) signaling pathways. We sought to determine whether MAPK signaling regulates podosome formation. Unlike extracellular signal–regulated kinase 1/2 (ERK1/2), ERK5 is constitutively activated in Src-transformed fibroblasts. ERK5-deficient cells expressing v-Src exhibited increased RhoA activation and signaling, which lead to cellular retraction and an inability to form podosomes or induce invasion. Addition of the Rho-kinase inhibitor Y27632 to ERK5-deficient cells expressing v-Src led to cellular extension and restored podosome formation. In Src-transformed cells, ERK5 induced the expression of a Rho GTPase-activating protein (RhoGAP), RhoGAP7/DLC-1, via activation of the transcription factor myocyte enhancing factor 2C, and RhoGAP7 expression restored podosome formation in ERK5-deficient cells. We conclude that ERK5 promotes Src-induced podosome formation by inducing RhoGAP7 and thereby limiting Rho activation.

Initial testing of dasatinib by the pediatric preclinical testing program

BACKGROUND

Dasatinib, a dual inhibitor of the src and abl tyrosine kinases, was recently approved by the Federal Drug Administration for the treatment of imatinib mesylate-resistant chronic myeloid leukemia.

PROCEDURES

Dasatinib was tested against the Pediatric Preclinical Testing Program (PPTP) in vitro panel at concentrations ranging from 0.1 nM to 1.0 microM and was tested in vivo at a dose of 50 mg/kg administered orally twice daily 5 days per week for 4 weeks for the solid tumor xenografts and once daily for the acute lymphoblastic leukemia (ALL) xenografts.

RESULTS

Dasatinib was selectively active against the cell lines of the PPTP in vitro panel, reaching an IC(50) in 6 of the 22 lines. The most sensitive were the AML line Kasumi-1, which has a gain-of-function c-Kit mutation (Asn822Lys), and the rhabdoid tumor line CHLA-266 (IC(50) approximately 10 nM for each). In the in vivo panel, dasatinib induced significant differences in EFS distribution in 8 of 32 (25%) solid tumor models and 3 of 7 ALL models. Using the time to event activity measure, dasatinib had intermediate activity against 1 of 27 (4%) evaluable solid tumor xenografts and 3 of 7 ALL xenografts. One xenograft in the ALL panel, a Philadelphia chromosome positive (Ph(+)) ALL xenograft, demonstrated a complete response.

CONCLUSIONS

Dasatinib was active at low nanomolar concentrations against a small subset of the PPTP's in vitro panel. Dasatinib had limited in vivo activity against the PPTP solid tumor xenografts, but was highly active against a Ph(+) ALL xenograft and also had anti-leukemia activity against two other xenografts.

A RNA interference screen identifies the protein phosphatase 2A subunit PR55gamma as a stress-sensitive inhibitor of c-SRC

Protein Phosphatase type 2A (PP2A) represents a family of holoenzyme complexes with diverse biological activities. Specific holoenzyme complexes are thought to be deregulated during oncogenic transformation and oncogene-induced signaling. Since most studies on the role of this phosphatase family have relied on the use of generic PP2A inhibitors, the contribution of individual PP2A holoenzyme complexes in PP2A-controlled signaling pathways is largely unclear. To gain insight into this, we have constructed a set of shRNA vectors targeting the individual PP2A regulatory subunits for suppression by RNA interference. Here, we identify PR55γ and PR55δ as inhibitors of c-Jun NH₂-terminal kinase (JNK) activation by UV irradiation. We show that PR55γ binds c-SRC and modulates the phosphorylation of serine 12 of c-SRC, a residue we demonstrate to be required for JNK activation by c-SRC. We also find that the physical interaction between PR55γ and c-SRC is sensitive to UV irradiation. Our data reveal a novel mechanism of c-SRC regulation whereby in response to stress c-SRC activity is regulated, at least in part, through loss of the interaction with its inhibitor, PR55γ.

Protein Phosphatase type 2A (PP2A) represent a family of holoenzyme complexes involved in wide range of activities such as growth, differentiation, and cell death. The PP2A holoenzyme complex is made up of a catalytic, a structural, and one of various “B” subunits. These “B” subunits are thought to provide the substrate specificity required for PP2A activity. Previous work on PP2A has mostly been derived by inhibiting the catalytic subunit through chemical inhibition, as such inhibiting all of the pathways associated with PP2A. To identify individual “B” subunits involved in specific cellular processes we have generated a “B” subunit gene knockdown library, which allows us to inhibit each of the known “B” subunits individually. One of the many pathways regulated by PP2A is the c-Jun NH₂-terminal kinase (JNK) kinase pathway, which, depending on stimulus, can affect either cell survival or cell proliferation. Here we report that the “B” subunit PR55γ acts as a negative regulator of JNK activity and cell death. We show that PR55γ influences JNK activity by inhibiting one of its upstream regulators, the proto-oncogene c-SRC, through dephosphorylation at one of the key residues on c-SRC, a site we show to be critical for c-SRC activation following cell stress. Overall our work describes the novel function of a specific PP2A subunit involved in cell survival and identifies a novel mechanism of c-SRC regulation.

The involvement of the tyrosine kinase c-Src in the regulation of reactive oxygen species generation mediated by NADPH oxidase-1

NADPH oxidase (Nox) family enzymes are one of the main sources of cellular reactive oxygen species (ROS), which have been shown to function as second messenger molecules. To date, seven members of this family have been reported, including Nox1-5 and Duox1 and -2. With the exception of Nox2, the regulation of the Nox enzymes is still poorly understood. Nox1 is highly expressed in the colon, and it requires two cytosolic regulators, NoxO1 and NoxA1, as well as the binding of Rac1 GTPase, for its activity. In this study, we investigate the role of the tyrosine kinase c-Src in the regulation of ROS formation by Nox1. We show that c-Src induces Nox1-mediated ROS generation in the HT29 human colon carcinoma cell line through a Rac-dependent mechanism. Treatment of HT29 cells with the Src inhibitor PP2, expression of a kinase-inactive form of c-Src, and c-Src depletion by small interfering RNA (siRNA) reduce both ROS generation and the levels of active Rac1. This is associated with decreased Src-mediated phosphorylation and activation of the Rac1-guanine nucleotide exchange factor Vav2. Consistent with this, Vav2 siRNA that specifically reduces endogenous Vav2 protein is able to dramatically decrease Nox1-dependent ROS generation and abolish c-Src-induced Nox1 activity. Together, these results establish c-Src as an important regulator of Nox1 activity, and they may provide insight into the mechanisms of tumor formation in colon cancers.

Epithelial to mesenchymal transition in head and neck squamous carcinoma: association of Src activation with E-cadherin down-regulation, vimentin expression, and aggressive tumor features

BACKGROUND

Epithelial-mesenchymal transformations (EMT) are critical for the invasion, progression, and metastasis of epithelial carcinogenesis. The role of EMT in head and neck squamous carcinoma (HNSC) tumorigenesis remains unexplored. In the current study, the expressions of several factors associated with the induction of EMT in HNSC cell lines and tumor specimens were investigated to define their functional and pathologic role in HNSC.

METHODS

Eleven HNSC cell lines and 50 primary tumor tissue specimens formed the materials of this study. Western blot analysis as well as immunohistochemical, and functional techniques were used to assess the status of activated Src (p-Src), E-cadherin, and vimentin in both cell lines and tumor tissues and the results were correlated with patients' clinicopathologic parameters.

RESULTS

The results demonstrated the inverse expression of p-Src and E-cadherin in the majority of cell lines and in primary tumor tissues compared with normal squamous mucosa. Elevated levels of p-Src were accompanied by down-regulation of E-cadherin and the expression of vimentin in epithelial tumor cells. In vitro inhibition of Src led to E-cadherin reexpression and increased cell contact in squamous carcinoma cell lines. Immunophenotypic analysis of these markers in primary tumor tissues demonstrated a significant correlation between increased p-Src, decreased E-cadherin, and vimentin expression and aggressive tumor features including penetrating invasive fronts, high-grade sarcomatoid transformation, and lymph node metastasis.

CONCLUSIONS

The results of the current study indicate that Src and E-cadherin may play an important role in EMT, invasion, and aggressive clinicopathologic features of HNSC. These proteins may be targeted for the therapeutic intervention of patients with HNSC.

Transcriptional changes induced by bovine papillomavirus type 1 in equine fibroblasts

Bovine papillomavirus type 1 (BPV-1) and, less commonly, BPV-2 are associated with the pathogenesis of common equine skin tumors termed sarcoids. In an attempt to understand the mechanisms by which BPV-1 induces sarcoids, we used gene expression profiling as a screening tool to identify candidate genes implicated in disease pathogenesis. Gene expression profiles of equine fibroblasts transformed by BPV-1 experimentally or from explanted tumors were compared with those of control equine fibroblasts to identify genes associated with expression of BPV-1. Analysis of the microarray data identified 81 probe sets that were significantly (P < 0.01) differentially expressed between the BPV-1-transformed and control cell lines. Expression of several deregulated genes, including MMP-1, CXCL5, FRA-1, NKG7, TLR4, and the gene encoding the major histocompatibility complex class I (MHC-I) protein, was confirmed using other BPV-1-transformed cell lines. Furthermore, expression of these genes was examined using a panel of 10 sarcoids. Increased expression of MMP-1, CXCL5, FRA-1, and NKG7 was detected in a subset of tumors, and TLR4 and MHC I showed robust down-regulation in all tumors. Deregulated expression was confirmed at the protein level for MMP-1 and MHC-I. The present report identifies genes modulated by BPV-1 transformation and will help identify the molecular mechanisms involved in disease pathogenesis.

Src inhibitors in early breast cancer: a methodology, feasibility and variability study

Early clinical trials of anticancer agents may be enriched by robust biomarkers of activity. Surrogate measures used in trials of cytotoxic agents, such as tumor size regression, may not be informative when investigating targeted agents that act principally to inhibit invasion or proliferation. This study aimed to determine the validity of invasion-related biomarkers of activity for AZD0530, a potent Src inhibitor currently in clinical development. Focal adhesion kinase (FAK) and paxillin are downstream phosphorylation substrates of Src and mediate tumor cell adhesion and invasiveness. These were therefore selected as biologically relevant markers of Src inhibition. Early breast cancer was chosen as a model as multiple samples can be collected during standard treatment and there is an intervening period in which experimental intervention can be applied. Tumor tissue was collected from diagnostic core biopsies and subsequent surgical tumor excision samples in 29 women with early breast cancer attending a single center. Protein levels were assessed quantitatively by Luminex and qualitatively by immunohistochemistry. AZD0530 inhibited tumor growth in a manner independent of dose and inhibited phosphorylation of FAK and paxillin in a dose-dependent manner in a Calu-6 xenograft model. In the clinical study, agreement of within-visit and also of between-visit measurements was high and the estimated number of patients required to detect a drug effect would be low enough to allow use of these markers as endpoints in future dose selection studies.

Signaling properties of a non-metazoan Src kinase and the evolutionary history of Src negative regulation

Choanoflagellates, unicellular organisms that are closely related to metazoans, possess cell adhesion and signaling proteins previously thought to be unique to animals, suggesting that these components may have played roles in the evolution of metazoan multicellularity. We have cloned, expressed, and purified the nonreceptor tyrosine kinase MbSrc1 from the choanoflagellate Monosiga brevicollis. The kinase has the same domain arrangement as mammalian Src kinases, and we find that the individual Src homology 3 (SH3), SH2, and catalytic domains have similar functions to their mammalian counterparts. In contrast to mammalian c-Src, the SH2 and catalytic domains of MbSrc1 do not appear to be functionally coupled. We cloned and expressed the M. brevicollis homolog of c-Src C-terminal kinase (MbCsk) and showed that it phosphorylates the C terminus of MbSrc1, yet this phosphorylation does not inhibit MbSrc to the same degree seen in the mammalian Src/Csk pair. Thus, Src autoinhibition likely evolved more recently within the metazoan lineage, and it may have played a role in the establishment of intercellular signaling in metazoans.

Src as a potential therapeutic target in non-small-cell lung cancer

Lung cancer is the most common cause of cancer-related death, with non-small-cell lung cancer (NSCLC) accounting for 80%-85% of all cases. Although survival rates are reasonably good for patients diagnosed with very early disease, the majority of patients present with advanced disease. For these patients, palliation and improvements in quality of life are the primary goals of therapy. Although chemotherapeutic agents remain the cornerstone of first-line therapy, these agents have limited use in patients who have relapsed and have metastatic disease. Therefore, new strategies are required to improve survival and quality of life in this setting. With the substantial advances in our understanding of tumour biology, it has been possible to identify signalling pathways involved in mediating tumour growth and progression. These pathways offer targets for new biological agents such as small molecule inhibitors and monoclonal antibodies. One such target is Src, a tyrosine kinase that is involved in multiple aspects of tumorigenesis including proliferation, migration and angiogenesis. Increased levels of Src expression have been found in a range of cancers, especially breast, colorectal, prostate and lung. Preliminary preclinical data and pharmacodynamic data suggest that Src inhibition is a viable therapeutic option in the treatment of advanced NSCLC.

Apoptosis of estrogen-receptor negative breast cancer and colon cancer cell lines by PTP alpha and src RNAi

We show that siRNA-mediated suppression of protein tyrosine phosphatase alpha (PTP alpha) reduces Src activity 2 to 4-fold in breast, colon and other human cancer cell lines. Src and PTP alpha RNAi induced apoptosis in estrogen receptor (ER)-negative breast cancer and colon cancer cells, but not in immortalized noncancerous breast cells, ER-positive breast cancer cells or other cancer cell types tested. RNAi of other Src family members (Fyn and Yes) or of PTP1B, a phosphatase previously suggested to be an activator of Src in breast cancer, had no effect. Although further tests with primary tumor tissues are required, the unexpected correlation between ER status and Src/PTP alpha dependence in breast cancer cell lines may be important for planning therapeutic strategies, and the insensitivity of normal breast cells to the RNAi highlights the potential of PTP alpha, which may be easier to target than Src, as a therapeutic target in ER-negative breast cancer.

Caveolin-1-mediated expression and secretion of kallikrein 6 in colon cancer cells

Kallikreins are secreted proteases that may play a functional role and/or serve as a serum biomarker for the presence or progression of certain types of cancers. Kallikrein 6 (KLK6) has been shown to be upregulated in several types of cancers, including colon. The aims of this study were to elucidate pathways that influence KLK6 gene expression and KLK6 protein secretion in the HCT116 human colon cancer cells. Our data indicate a central role for caveolin-1 (CAV-1), the main structural protein of caveolae, in both KLK6 gene expression and protein secretion. Sucrose gradient subcellular fractionation reveals that CAV-1 and KLK6 colocalize to lipid raft domains in the plasma membrane of HCT116 cells. Furthermore, we show that CAV-1, although it does not directly interact with the KLK6 molecule, enhances KLK6 secretion from the cells. Deactivation of CAV-1, through SRC-mediated phosphorylation, decreased KLK6 secretion. We also demonstrate that, in colon cancer cells, CAV-1 increased the amount of phosphorylated AKT in cells by inhibiting the activity of the AKT-negative regulators PP1 and PP2A. This study demonstrates that proteins such as CAV-1 and AKT, which are known to be altered in colon cancer, affect KLK6 expression and KLK6 secretion.

Arsenic interferes with the signaling transduction pathway of T cell receptor activation by increasing basal and induced phosphorylation of Lck and Fyn in spleen cells

Arsenic is known to produce inhibition as well as induction of immune cells proliferative responses depending on the doses as one of its mechanisms of immunotoxicity. Here we evaluate the effect of arsenic exposure on the activation of splenic mononuclear cells (SMC) in male CD57BL6N mice. Intra-gastric exposure to arsenic (as sodium arsenite) for 30 days (1, 0.1, or 0.01 mg/kg/day), reduced the proportion of CD4+ cells and the CD4+/CD8+ ratio in the spleen, increasing the proportion of CD11b+ cells. Arsenic exposure did not modify the proportion of B cells. SMC showed an increased level of phosphorylation of lck and fyn kinases (first kinases associated to TCR complex when activated). Although normal levels of apoptosis were observed on freshly isolated SMC, an increase in apoptotic cells related with the increase in phosphorylation of lck and fyn was observed when SMC were activated with Concanavalin-A (Con-A). Arsenic exposure reduced the proliferative response of SMC to Con-A, and also reduced secretion of IL-2, IL-6, IL-12 and IFNgamma. No effect was observed on IL-4, and IL-10 secretion. The same effects were observed when SMC of exposed animals were activated with anti-CD3/CD28 antibodies for 24 h, but these effects were transitory since a recovery, up to control levels or even higher, were observed after 72 h of stimulation. This study demonstrates that repeated and prolonged exposure to arsenic alters cell populations and produces functional changes depending on the specific activation pathway, and could be related with the phosphorylation status of lck and fyn kinases.

Lyn regulates BCR-ABL and Gab2 tyrosine phosphorylation and c-Cbl protein stability in imatinib-resistant chronic myelogenous leukemia cells

Lyn kinase functions as a regulator of imatinib sensitivity in chronic myelogenous leukemia (CML) cells through an unknown mechanism. In patients who fail imatinib therapy but have no detectable BCR-ABL kinase mutation, we detected persistently activated Lyn kinase. In imatinib-resistant CML cells and patients, Lyn activation is BCR-ABL independent, it is complexed with the Gab2 and c-Cbl adapter/scaffold proteins, and it mediates persistent Gab2 and BCR-ABL tyrosine phosphorylation in the presence or absence of imatinib. Lyn silencing or inhibition is necessary to suppress Gab2 and BCR-ABL phosphorylation and to recover imatinib activity. Lyn also negatively regulates c-Cbl stability, whereas c-Cbl tyrosine phosphorylation is mediated by BCR-ABL. These results suggest that Lyn exists as a component of the BCR-ABL signaling complex and, in cells with high Lyn expression or activation, BCR-ABL kinase inhibition alone (imatinib) is not sufficient to fully disengage BCR-ABL-mediated signaling and suggests that BCR-ABL and Lyn kinase inhibition are needed to prevent or treat this form of imatinib resistance.

Targeting angiogenesis in pancreatic cancer: rationale and pitfalls

INTRODUCTION

Pancreatic ductal adenocarcinoma (PDAC) is a highly aggressive cancer responsible for over 20% of deaths due to gastrointestinal malignancies. PDAC is usually diagnosed at an advanced stage which, in part, helps to explain its high resistance to chemotherapy and radiotherapy. In addition, the cancer cells in PDAC have a high propensity to metastasize and to aberrantly express several key regulators of angiogenesis and invasion. Chemotherapy has only provided a modest impact on mean survival and often induces side effects. Targeting angiogenesis alone or in combination with other modalities should be investigated to determine if it may provide for increased survival.

MATERIALS AND METHODS

This review summarizes the alterations in PDAC that play a critical role in angiogenesis and provides an overview of current and therapeutic strategies that may be useful for targeting angiogenesis in this malignancy.

Hexachlorobenzene triggers AhR translocation to the nucleus, c-Src activation and EGFR transactivation in rat liver

Hexachlorobenzene (HCB) is a widespread environmental pollutant. It has some properties that are typical for dioxin-like compounds that act mainly through the aryl hydrocarbon receptor (AhR) protein. Upon dioxin binding, the AhR translocates to the nucleus and modulates gene expression. At the same time, c-Src kinase frees from the AhR complex and thereby activates its own kinase activity, which acts as a trigger for the growth factor receptor signal transduction pathway. HCB is a weak agonist of the AhR, and the evidence that HCB toxicity is mediated via the AhR complex is limited and inconclusive. In the present study, female Wistar rats were administered HCB (1, 10 and 100mg/kg) for 30 days. Liver cytosolic AhR was translocated to the nucleus. The activity of liver microsomal c-Src increased at all assayed doses. HCB induced the association of the EGFR with c-Src and increased the phosphorylation of EGFR at tyrosine 845 (Tyr845), a known c-Src phosphorylation site. c-Src from WB-F344 cells treated with HCB exhibited increased protein levels and c-Src-pTyr416 phosphorylation than the control cells. Again HCB induced EGFR phosphorylation at Tyr845. Such an effect of HCB could not be detected when c-Src activity was blocked by PP2. All together, our data demonstrates that HCB may induce EGFR transactivation through an c-Src-dependent pathway.

Differing Src signaling levels have distinct outcomes in Drosophila

High levels of Src activity are found in a broad spectrum of cancers. The roles of Src and its negative regulator Csk have been extensively studied, although results have often proved contradictory or the relevance to whole organisms is unclear. In Drosophila, overexpression of either Src orthologue resulted in apoptotic cell death, but paradoxically, reducing dCsk activity led to over-proliferation and tissue overgrowth. Here, we show that in Drosophila epithelia in situ, the levels of Src signaling determine the cellular outcome of Src activation. Apoptotic cell death was triggered specifically at high Src signaling levels; lower levels directed antiapoptotic signals while promoting proliferation. Furthermore, our data indicate that expression of kinase-dead Src isoforms do not necessarily act as dominant-negative factors, but can instead increase Src pathway activity, most likely by titrating Csk activity away from endogenous Src. The importance of Src activity levels was emphasized when we examined oncogenic cooperation between Src and Ras: malignant overgrowth was observed specifically when high Src signaling levels were achieved. We propose a model in which low levels of Src signaling promote survival and proliferation during early stages of tumorigenesis, whereas strong Src signaling, coupled with antiapoptotic signals, directs invasive migration and metastasis during advanced tumor stages.

A redox-silent analogue of tocotrienol inhibits hypoxic adaptation of lung cancer cells

We have previously reported that a redox-silent analogue of alpha-tocotrienol (T3), 6-O-carboxypropyl-alpha-tocotrienol (T3E) shows more potential anti-carcinogenic property than T3 in a lung cancer cell (A549 cell). However, the mechanisms by which T3E exerts its potential anti-carcinogenic effect is still unclear. As tumor malignancy is associated with hypoxia adaptation, in this study, we examined whether T3E could suppress survival and invasion in A549 cells under hypoxia. Hypoxia treatment drastically-induced activation of the protein tyrosine kinase, Src, and its regulated signaling required for hypoxia adaptation of A549 tumor cells. The survival and invasion capacity of the tumor cells under hypoxia was suppressed by T3E via the inactivation of Src. More specifically, T3E-dependent inhibition of Src-induced Akt activation contributed to suppression of cell survival under hypoxia, and the reduction of fibrinolytic factors such as plasminogen activator-1(PAI-1) via the decrease of hypoxia-inducible factor-2alpha by T3E led to inhibition of hypoxic invasion. Overall these results suggest that T3E suppresses hypoxia adaptation of A549 cells by the inhibition in hypoxia-induced activation of Src signaling.

Gab2 and Src co-operate in human mammary epithelial cells to promote growth factor independence and disruption of acinar morphogenesis

The Gab2 docking protein is a target of several oncogenic protein tyrosine kinases and potentiates activation of the Ras/extracellular signal regulated kinase and phosphatidylinositol 3-kinase (PI3-kinase) pathways. Since Gab2 is phosphorylated by c-Src, and both proteins are overexpressed in breast cancers, we have determined the biological consequences of their co-expression in the immortalized human mammary epithelial cell line MCF-10A. While overexpression of c-Src did not affect acinar morphogenesis or growth factor dependence in three-dimensional culture, c-Src co-operated with Gab2 to promote epidermal growth factor (EGF)-independent acinar growth. In contrast, expression of v-Src or the activated mutant c-SrcY527F led to a spectrum of aberrant phenotypes ranging from spheroids with incomplete luminal clearance to highly disrupted, dispersed structures. Gab2 co-expression shifted the phenotypic distribution towards the dispersed phenotype, an effect not observed with a Gab2 mutant unable to bind the p85 subunit of PI3-kinase (Gab2Deltap85). In v-Src-expressing cells, Gab2, but not Gab2Deltap85, significantly decreased E-cadherin adhesive strength without altering its surface expression. Gab2 associated with E-cadherin in the presence and absence of v-Src, indicating that the ability of Gab2 to weaken the strength of cell-cell contacts may reflect enhanced activation of PI3-kinase at adherens junctions. Gab2 also increased migration and invasion of these cells in transwell assays, but these effects were p85-independent. Overall, these findings demonstrate a novel mechanism whereby Gab2 may promote metastatic spread and indicate that Gab2 may play several roles during breast cancer progression.