An Intramolecular Interaction between SH2-Kinase Linker and Kinase Domain Is Essential for the Catalytic Activity of Protein-tyrosine Kinase-6

In Silico Screening of Multi-Domain Targeted Inhibitors for PTK6: A Strategy Integrating Drug Repurposing and Consensus Docking

Protein tyrosine kinase 6 (PTK6), also known as breast tumor kinase (BRK), serves as a non-receptor intracellular tyrosine kinase within the Src kinases family. Structurally resembling other Src kinases, PTK6 possesses an Src homology 3 (SH3) domain, an Src homology 2 (SH2) domain, and a tyrosine kinase domain (SH1). While considerable efforts have been dedicated to designing PTK6 inhibitors targeting the SH1 domain, which is responsible for kinase activity in various pathways, it has been observed that solely inhibiting the SH1 domain does not effectively suppress PTK6 activity. Subsequent investigations have revealed the involvement of SH2 and SH3 domains in intramolecular and substrate binding interactions, which are crucial for PTK6 function. Consequently, the identification of PTK6 inhibitors targeting not only the SH1 domain but also the SH2 and SH3 domains becomes imperative. Through an in silico structural-based virtual screening approach, incorporating drug repurposing and a consensus docking approach, we have successfully identified four potential ligands capable of concurrently inhibiting the tyrosine kinase domain and SH2/SH3 domains of PT6K simultaneously. This finding suggests potential pathways for therapeutic interventions in PTK6 inhibition.

Brk/PTK6 and Involucrin Expression May Predict Breast Cancer Cell Responses to Vitamin D3

The process of human embryonic mammary development gives rise to the structures in which mammary cells share a developmental lineage with skin epithelial cells such as keratinocytes. As some breast carcinomas have previously been shown to express high levels of involucrin, a marker of keratinocyte differentiation, we hypothesised that some breast tumours may de-differentiate to a keratinocyte-derived 'evolutionary history'. To confirm our hypothesis, we investigated the frequency of involucrin expression along with that of Brk, a tyrosine kinase expressed in up to 86% of breast carcinomas whose normal expression patterns are restricted to differentiating epithelial cells, most notably those in the skin (keratinocytes) and the gastrointestinal tract. We found that involucrin, a keratinocyte differentiation marker, was expressed in a high proportion (78%) of breast carcinoma samples and cell lines. Interestingly, tumour samples found to express high levels of involucrin were also shown to express Brk. 1,25-dihydroxyvitamin D3, a known differentiation agent and potential anti-cancer agent, decreased proliferation in the breast cancer cell lines that expressed both involucrin and Brk, whereas the Brk/involucrin negative cell lines tested were less susceptible. In addition, responses to 1,25-dihydroxyvitamin D3 were not correlated with vitamin D receptor expression. These data contribute to the growing body of evidence suggesting that cellular responses to 1,25-dihydroxyvitamin D3 are potentially independent of vitamin D receptor status and provide an insight into potential markers, such as Brk and/or involucrin that could predict therapeutic responses to 1,25-dihydroxyvitamin D3.

Putting the BRK on breast cancer: From molecular target to therapeutics

BReast tumor Kinase (BRK, also known as PTK6) is a non-receptor tyrosine kinase that is highly expressed in breast carcinomas while having low expression in the normal mammary gland, which hints at the oncogenic nature of this kinase in breast cancer. In the past twenty-six years since the discovery of BRK, an increasing number of studies have strived to understand the cellular roles of BRK in breast cancer. Since then, BRK has been found both in vitro and in vivo to activate a multitude of oncoproteins to promote cell proliferation, metastasis, and cancer development. The compelling evidence concerning the oncogenic roles of BRK has also led, since then, to the rapid and exponential development of inhibitors against BRK. This review highlights recent advances in BRK biology in contributing to the “hallmarks of cancer”, as well as BRK's therapeutic significance. Importantly, this review consolidates all known inhibitors of BRK activity and highlights the connection between drug action and BRK-mediated effects. Despite the volume of inhibitors designed against BRK, none have progressed into clinical phase. Understanding the successes and challenges of these inhibitor developments are crucial for the future improvements of new inhibitors that can be clinically relevant.

Multifaceted C-terminus of HSP70-interacting protein regulates tumorigenesis via protein quality control

C-terminus of heat shock protein 70 (HSP70)-interacting protein (CHIP) is an E3 ligase involved in a variety of protein homeostasis events implicated in diverse signaling pathways. Its involvement in varied and even opposite signaling circuits might be due to its hallmark signature of associating with molecular chaperones, including HSP90 and HSP70. Together, these proteins may be pivotal in implementing protein quality control. A curious and puzzling aspect of the function of CHIP is its capability to induce protein degradation via the proteasome- or lysosome-dependent pathways. In addition, these pathways are combined with ubiquitin-dependent or -independent pathways. This review focuses on the role of CHIP in the development or suppression of tumorigenesis. CHIP can act as a tumor suppressor by downregulating various oncogenes. CHIP also displays an oncogenic feature involving the inhibition of diverse tumor suppressors, including proteins related to intrinsic and extrinsic apoptotic pathways. The ability of CHIP to exhibit dual roles in determining the fate of cells has not been studied analytically. However, its association with various proteins involved in protein quality control might play a major role. In this review, the mechanistic roles of CHIP in tumor formation based on the regulation of diverse proteins are discussed.

Small molecule inhibitors reveal PTK6 kinase is not an oncogenic driver in breast cancers

Protein tyrosine kinase 6 (PTK6, or BRK) is aberrantly expressed in breast cancers, and emerging as an oncogene that promotes tumor cell proliferation, migration and evasion. Both kinase-dependent and -independent functions of PTK6 in driving tumor growth have been described, therefore targeting PTK6 kinase activity by small molecule inhibitors as a therapeutic approach to treat cancers remains to be validated. In this study, we identified novel, potent and selective PTK6 kinase inhibitors as a means to investigate the role of PTK6 kinase activity in breast tumorigenesis. We report here the crystal structures of apo-PTK6 and inhibitor-bound PTK6 complexes, providing the structural basis for small molecule interaction with PTK6. The kinase inhibitors moderately suppress tumor cell growth in 2D and 3D cell cultures. However, the tumor cell growth inhibition shows neither correlation with the PTK6 kinase activity inhibition, nor the total or activated PTK6 protein levels in tumor cells, suggesting that the tumor cell growth is independent of PTK6 kinase activity. Furthermore, in engineered breast tumor cells overexpressing PTK6, the inhibition of PTK6 kinase activity does not parallel the inhibition of tumor cell growth with a >500-fold shift in compound potencies (IC₅₀ values). Overall, these findings suggest that the kinase activity of PTK6 does not play a significant role in tumorigenesis, thus providing important evidence against PTK6 kinase as a potential therapeutic target for breast cancer treatment.

PTK6 Localized at the Plasma Membrane Promotes Cell Proliferation and MigratiOn Through Phosphorylation of Eps8

Protein tyrosine kinase 6 (PTK6; also known as Brk) is closely related to the Src family kinases, but lacks a membrane-targeting myristoylation signal. Sublocalization of PTK6 at the plasma membrane enhances its oncogenic potential. To understand the mechanism(s) underlying the oncogenic property of plasma---membrane-associated PTK6, proteins phosphorylated by membrane-targeted myristoylated PTK6 (Myr-PTK6) were enriched and analyzed using a proteomics approach. Eps8 which was identified by this method is phosphorylated by Myr-PTK6 in HEK293 cells. Mouse Eps8 expressed in HEK293 cells is phosphorylated by Myr-PTK6 at residues Tyr497, Tyr524, and Tyr534. Compared to wild-type Eps8 (Eps8 WT), the phosphorylation-defective 3YF mutant (Eps8 3YF) reverts the increased proliferation, migration, and phosphorylation of ERK and FAK mediated by Eps8 WT in HEK293 cells overexpressing PTK6. PTK6 knockdown in T-47D breast cancer cells decreased EGF-induced phosphorylation of Eps8. Endogenous PTK6 phosphorylates ectopically expressed Eps8 WT, but not Eps8 3YF mutant, in EGF-stimulated T-47D cells. The EGF-induced Eps8 phosphorylation enhances activation of ERK and FAK, cell adhesion, and anchorage-independent colony formation in T-47D cells, but not in the PTK6-knokdown T-47D cells. These results indicate that plasma-membrane-associated PTK6 phosphorylates Eps8, which promotes cell proliferation, adhesion, and migration and, thus, tumorigenesis. J. Cell. Biochem. 118: 2887-2895, 2017. © 2017 Wiley Periodicals, Inc.

The associated pyrazolopyrimidines PP1 and PP2 inhibit protein tyrosine kinase 6 activity and suppress breast cancer cell proliferation

Protein tyrosine kinase (PTK)6, also known as breast tumor kinase, is a non-receptor tyrosine kinase. It is closely associated with, but evolutionarily distinct from, the Src family members. PTK6 has a role in proliferation, migration and invasion in various cancers, and therefore has been suggested as a potentially valuable therapeutic target. In an attempt to develop PTK6 inhibitors, chemicals known to inhibit various kinases were screened for their ability to inhibit PTK6. Pyrazolopyrimidine (PP)1, PP2 and a lymphocyte-specific protein tyrosine kinase inhibitor strongly inhibited the catalytic activity of PTK6 in vitro. These chemicals suppressed the phosphorylation of PTK6 substrate proteins, including signal transducer and activator of transcription 3, in human embryonic kidney (HEK) 293 cells expressing hyperactive PTK6. They also expressed selectivity towards PTK6 over other PTK members in HEK 293 cells. PP1 and PP2 specifically inhibited the PTK6-dependent proliferation of human breast carcinoma T-47D cells. PP1 and PP2 were more selective for PTK6 than for Src family kinases, and may be useful for the treatment of PTK6-positive malignant diseases such as breast cancer.

Growth-stimulatory activity of TIMP-2 is mediated through c-Src activation followed by activation of FAK, PI3-kinase/AKT, and ERK1/2 independent of MMP inhibition in lung adenocarcinoma cells

Tissue inhibitors of metalloproteinases (TIMPs) control extracellular matrix (ECM) homeostasis by inhibiting the activity of matrix metalloproteinases (MMPs), which are associated with ECM turnover. Recent studies have revealed that TIMPs are implicated in tumorigenesis in both MMP-dependent and MMP-independent manners. We examined a mechanism by which TIMP-2 stimulated lung adenocarcinoma cell proliferation, independent of MMP inhibition. The stimulation of growth by TIMP-2 in A549 cells required c-Src kinase activation. c-Src kinase activity, induced by TIMP-2, concomitantly increased FAK, phosphoinositide 3-kinase (PI3-kinase)/AKT, and ERK1/2 activation. Selective knockdown of integrin α3β1, known as a TIMP-2 receptor, did not significantly change TIMP-2 growth promoting activity. Furthermore, we showed that high TIMP-2 expression in lung adenocarcinomas is associated with a worse prognosis from multiple cohorts, especially for stage I lung adenocarcinoma. Through integrated analysis of The Cancer Genome Atlas data, TIMP-2 expression was significantly associated with the alteration of driving genes, c-Src activation, and PI3-kinase/AKT pathway activation. Taken together, our results demonstrate that TIMP-2 stimulates lung adenocarcinoma cell proliferation through c-Src, FAK, PI3-kinase/AKT, and ERK1/2 pathway activation in an MMP-independent manner.

Tracing the footprints of the breast cancer oncogene BRK - Past till present

Twenty years have passed since the non-receptor tyrosine kinase, Breast tumor kinase (BRK) was cloned. While BRK is evolutionarily related to the Src family kinases it forms its own distinct sub-family referred here to as the BRK family kinases. The detection of BRK in over 60% of breast carcinomas two decades ago and more remarkably, its absence in the normal mammary gland attributed to its recognition as a mammary gland-specific potent oncogene and led BRK researchers on a wild chase to characterize the role of the enzyme in breast cancer. Where has this chase led us? An increasing number of studies have been focused on understanding the cellular roles of BRK in breast carcinoma and normal tissues. A majority of such studies have proposed an oncogenic function of BRK in breast cancers. Thus far, the vast evidence gathered highlights a regulatory role of BRK in critical cellular processes driving tumor formation such as cell proliferation, migration and metastasis. Functional characterization of BRK has identified several signaling proteins that work in concert with the enzyme to sustain such a malignant phenotype. As such targeting the non-receptor tyrosine kinase has been proposed as an attractive approach towards therapeutic intervention. Yet much remains to be explored about (a) the discrepant expression levels of BRK in cancer versus normal conditions, (b) the dependence on the enzymatic activity of BRK to promote oncogenesis and (c) an understanding of the normal physiological roles of the enzyme. This review outlines the advances made towards understanding the cellular and physiological roles of BRK, the mechanisms of action of the protein and its therapeutic significance, in the context of breast cancer.

Signature of positive selection of PTK6 gene in East Asian populations: a cross talk for Helicobacter pylori invasion and gastric cancer endemicity

Analysis of natural selection events is an attractive strategy for identification of functional variants shaped by gene-environmental interactions and human adaptation. Here, we identified PTK6, a Src-related tyrosine kinase gene, underlying positive selection in East Asian populations. Interestingly, PTK6 variant showed significant correlation with gastric cancer incidences which was the highest in East Asian populations. The high prevalence of gastric cancer in East Asians was also believed to be strongly affected by Helicobacter pylori infection and dietary habit. Therefore, we speculated a competitive interaction of cancer-associated molecules for activation/reduction, where PTK6 likely plays a role through CagA-driven signaling pathway after H. pylori infection. This hypothesis was also supported by our gene expression analysis and the dating of the selective event which was estimated to be ~16,500 years ago, much later than H. pylori invasion in human 50,000 years ago. Establishment of cross talk between PTK6 and CagA by functional studies may further elucidate the underlying biology of H. pylori-mediated gastric cancer.

Discovery of (E)-5-(benzylideneamino)-1H-benzo[d]imidazol-2(3H)-one derivatives as inhibitors for PTK6

A lead compound 1, which inhibits the catalytic activity of PTK6, was selected from a chemical library. Derivatives of compound 1 were synthesized and analyzed for inhibitory activity against PTK6 in vitro and at the cellular level. Selected compounds were analyzed for cytotoxicity in human foreskin fibroblasts using MTT assays and for selectivity towards PTK members in HEK 293 cells. Compounds 20 (in vitro IC50=0.12μM) and 21 (in vitro IC50=0.52μM) showed little cytotoxicity, excellent inhibition of PTK6 in vitro and at the cellular level, and selectivity for PTK6. Compounds 20 and 21 inhibited phosphorylation of specific PTK6 substrates in HEK293 cells. Thus, we have identified novel PTK6 inhibitors that may be used as treatments for PTK6-positive carcinomas, including breast cancer.

The presence of outer arm fucose residues on the N-glycans of tissue inhibitor of metalloproteinases-1 reduces its activity

Tissue inhibitor of metalloproteinases-1 (TIMP-1) inhibits matrix metalloproteinases (MMPs) by binding at a 1:1 stoichiometry. Here we have shown the involvement of N-glycosylation in the MMP inhibitory ability of TIMP-1. TIMP-1, purified from HEK 293 cells overexpressing TIMP-1 (293 TIMP-1), showed less binding and inhibitory abilities to MMPs than TIMP-1 purified from fibroblasts or SF9 insect cells infected with TIMP-1 baculovirus. Following deglycosylation of TIMP-1, all forms of TIMP-1 showed similar levels of MMP binding and inhibition, suggesting that glycosylation is involved in the regulation of these TIMP-1 activities. Analysis of the N-glycan structures showed that SF9 TIMP-1 has the simplest N-glycan structures, followed by fibroblast TIMP-1 and 293 TIMP-1, in order of increasing complexity in their N-glycan structures. Further analyses showed that cleavage of outer arm fucose residues from the N-glycans of 293 TIMP-1 or knockdown of both FUT4 and FUT7 (which encode for fucosyltransferases that add outer arm fucose residues to N-glycans) enhanced the MMP-binding and catalytic abilities of 293 TIMP-1, bringing them up to the levels of the other TIMP-1. These results demonstrate that the ability of TIMP-1 to inhibit MMPs is at least in part regulated by outer arm fucosylation of its N-glycans.

PTK6 promotes degradation of c-Cbl through PTK6-mediated phosphorylation

PTK6 (also known as Brk) is an intracellular tyrosine kinase which induces proliferation, anti-apoptosis, migration, and anchorage-independent growth. Herein we report that PTK6 phosphorylates and down-regulates E3 ubiquitin ligase c-Cbl. Tyr(700), Tyr(731), and Tyr(774) residues in the C-terminal domain of c-Cbl are major phosphorylation sites targeted by PTK6. The phosphorylated c-Cbl is subjected to auto-ubiquitination and degraded through the ubiquitin-proteasome pathway. These results provide evidence for a novel mechanism demonstrating the oncogenic potential of PTK6 through degradation of c-Cbl, which is an E3 ligase important in down-regulation of oncoproteins.

Hsp90 rescues PTK6 from proteasomal degradation in breast cancer cells

PTK6 [protein tyrosine kinase 6; also known as Brk (breast tumour kinase)] is a non-receptor tyrosine kinase, closely related to Src, but evolutionarily distinct, that is up-regulated in various cancers, including breast cancer. Hsp90 (heat-shock protein 90) was identified as a PTK6-interacting protein in HEK (human embryonic kidney)-293 cells overexpressing PTK6. Hsp90 interacted with the PTK6 tyrosine kinase catalytic domain, but catalytic activity was not required for the interaction. Geldanamycin, an Hsp90 inhibitor, significantly decreased the PTK6 protein level through proteasome-dependent degradation, but did not affect the level of Src. Geldanamycin treatment also decreased phosphorylation of PTK6 substrates due to reduced amounts of PTK6. Moreover, overexpression of CHIP [C-terminus of Hsc70 (heat-shock cognate 70)-interacting protein], a chaperone-dependent E3 ligase, enhanced proteosomal degradation of PTK6. Geldanamycin increased the interaction of PTK6 with CHIP, but decreased the interaction of PTK6 with Hsp90. We also found that endogenous PTK6 associated with Hsp90 and geldanamycin decreased expression of endogenous PTK6 in breast carcinoma cells. Finally, we report that silencing endogenous CHIP expression in breast carcinoma cells inhibited geldanamycin-induced PTK6 reduction. These results demonstrate that Hsp90 plays an essential role in regulating PTK6 stability and suggest that Hsp90 inhibitors may be useful as therapeutic drugs for PTK6-positive cancers, including breast cancer.

The cytosolic domain of protein-tyrosine kinase 7 (PTK7), generated from sequential cleavage by a disintegrin and metalloprotease 17 (ADAM17) and γ-secretase, enhances cell proliferation and migration in colon cancer cells

Protein-tyrosine kinase 7 (PTK7) is a member of the defective receptor protein-tyrosine kinases and is known to function as a regulator of planar cell polarity during development. Its expression is up-regulated in some cancers including colon carcinomas. A 100-kDa fragment of PTK7 was detected in the culture media from colon cancer cells and HEK293 cells. The shed fragment was named sPTK7-Ig1-7 because its molecular mass was very similar to that of the entire extracellular domain of PTK7 that contains immunoglobulin-like loops 1 to 7 (Ig1-7). The shedding of sPTK7-Ig1-7 was enhanced by treatment with phorbol 12-myristate 13-acetate. In addition to the sPTK7-Ig1-7 found in the culture medium, two C-terminal fragments of PTK7 were detected in the cell lysates: PTK7-CTF1, which includes a transmembrane segment and a cytoplasmic domain, and PTK7-CTF2, which lacks most of the transmembrane segment from PTK7-CTF1. Analysis of PTK7 processing in the presence of various protease inhibitors or after knockdown of potential proteases suggests that shedding of PTK7 into sPTK7-Ig1-7 and PTK7-CTF1 is catalyzed by ADAM17, and further cleavage of PTK7-CTF1 into PTK7-CTF2 is mediated by the γ-secretase complex. PTK7-CTF2 localizes to the nucleus and enhances proliferation, migration, and anchorage-independent colony formation. Our findings demonstrate a novel role for PTK7 in the tumorigenesis via generation of PTK7-CTF2 by sequential cleavage of ADAM17 and γ-secretase.

Constitutive activation of breast tumor kinase accelerates cell migration and tumor growth in vivo

Breast tumor kinase (BRK) is a non-receptor tyrosine kinase overexpressed in most human breast tumors, including lymph node metastases, but undetected in normal mammary tissue or in fibroadenomas. The activity of BRK-like Src family tyrosine kinase, is regulated negatively by phosphorylation of C-terminal tyrosine 447. Although the kinase that regulates BRK activation has not been identified, we and others have previously shown that BRK-Y447F is a constitutively active variant. Because BRK-Y447F significantly enhances the catalytic activity of the enzyme, we investigated the role of the constitutively active BRK variant in tumor formation and metastasis. Using stable breast cancer cell MDA-MB-231 we observed significantly enhanced rates of cell proliferation, migration and tumor formation in BRK-Y447F stable cells compared with wild-type stable cell lines. Our results indicate full activation of BRK is an essential component in the tumorigenic role of BRK.

Brk/PTK6 signaling in normal and cancer cell models

Breast tumor kinase (Brk), also termed PTK6, is known to function in cell-type and context-dependent processes governing normal differentiation. However, in tumors in which Brk is overexpressed, this unusual soluble tyrosine kinase is emerging as a mediator of cancer cell phenotypes, including increased proliferation, survival, and migration. Nuclear and cytoplasmic substrates phosphorylated by Brk include a collection of regulatory RNA-binding proteins, adaptor molecules that link Brk to signaling pathways generally associated with the activation of growth factor receptors, and Signal Transducers and Activators of Transcription (STAT) molecules that are direct regulators of gene expression. Understanding Brk-dependent regulation of these key signaling pathways and how they influence cancer cell behavior is predicted to inform the development of improved 'targeted' cancer therapies and may provide insight into ways to avoid chemo-resistance to established treatments.

PTK6 inhibits down-regulation of EGF receptor through phosphorylation of ARAP1

PTK6 (also known as Brk) is a non-receptor-tyrosine kinase containing SH3, SH2, and catalytic domains, that is expressed in more than 60% of breast carcinomas but not in normal mammary tissues. To analyze PTK6-interacting proteins, we have expressed Flag-tagged PTK6 in HEK293 cells and performed co-immunoprecipitation assays with Flag antibody-conjugated agarose. A 164-kDa protein in the precipitated fraction was identified as ARAP1 (also known as centaurin delta-2) by MALDI-TOF mass analysis. ARAP1 associated with PTK6 in an EGF/EGF receptor (EGFR)-dependent manner. In addition, the SH2 domain of PTK6, particularly the Arg(105) residue that contacts the phosphate group of the tyrosine residue, was essential for the association. Moreover, PTK6 phosphorylated residue Tyr(231) in the N-terminal domain of ARAP1. Expression of ARAP1, but not of the Y231F mutant, inhibited the down-regulation of EGFR in HEK293 cells expressing PTK6. Silencing of endogenous PTK6 expression in breast carcinoma cells decreased EGFR levels. These results demonstrate that PTK6 enhances EGFR signaling by inhibition of EGFR down-regulation through phosphorylation of ARAP1 in breast cancer cells.

Building a better understanding of the intracellular tyrosine kinase PTK6 - BRK by BRK

Protein tyrosine kinase 6 (PTK6), also referred to as breast tumor kinase BRK, is a member of a distinct family of kinases that is evolutionarily related to the SRC family of tyrosine kinases. While not expressed in the normal mammary gland, PTK6 expression is detected in a large proportion of human mammary gland tumors. In breast tumor cells, PTK6 promotes growth factor signaling and cell migration. PTK6 expression is also increased in a number of other epithelial tumors, including ovarian and colon cancer. In contrast, PTK6 is expressed in diverse normal epithelia, including the linings of the gastrointestinal tract, skin and prostate, where its expression correlates with cell cycle exit and differentiation. Disruption of the mouse Ptk6 gene leads to increased growth and impaired differentiation in the small intestine that is accompanied by increased AKT and Wnt signaling. Following total body irradiation, PTK6 expression is induced in proliferating progenitor cells of the intestine, where it plays an essential role in DNA-damage induced apoptosis. A distinguishing feature of PTK6 is its flexibility in intracellular localization, due to a lack of amino-terminal myristoylation/palmitoylation. Recently a number of substrates of PTK6 have been identified, including nuclear RNA-binding proteins and transcription factors. We discuss PTK6 signaling, its apparent conflicting roles in cancer and normal epithelia, and its potential as a therapeutic target in epithelial cancers.

Brk protects breast cancer cells from autophagic cell death induced by loss of anchorage

Brk, a tyrosine kinase expressed in a majority of breast tumors, but not normal mammary tissue, promotes breast carcinoma cell proliferation. Normal epithelial cells are dependent on cell-cell or cell-matrix interactions for survival and undergo apoptosis after disruption of these interactions. Tumor cells are less sensitive to the induction of apoptosis and are predicted to have the potential to disseminate. We investigated whether Brk has further roles in breast tumor progression by relating its expression to tumor grade and demonstrating its role in the regulation of carcinoma cell survival under non-adherent conditions. Brk expression was determined by reverse transcription PCR on RNA extracted from surgical samples of human breast cancers. Breast carcinoma cell survival in suspension culture was examined when Brk protein levels were suppressed by RNA interference. Additionally, the effect of experimentally overexpressing Brk in otherwise Brk-negative breast carcinoma cells was assessed. Brk mRNA expression was notably higher in grade 3 breast tumors, as compared with lower tumor grades. In suspension culture, Brk suppression increased the rate of cell death, as compared with controls, and this cell death program exhibited characteristics of autophagy but not of apoptosis. Conversely, experimental expression of Brk in Brk-negative cells increased cell survival whereas kinase-inactive Brk did not. Therefore, Brk enhances breast carcinoma cell survival in suspension, suggesting a role for Brk in supporting breast cancer cell dissemination.

Structural basis of the auto-inhibition mechanism of nonreceptor tyrosine kinase PTK6

Protein tyrosine kinase 6 (PTK6) is composed of SH3, SH2, and Kinase domains, with a linker region (Linker) between the SH2 and Kinase domains. Here, we report the structural basis of the SH3-Linker interaction that results in auto-inhibition of PTK6. The solution structures of the SH3 domain and SH3/Linker complex were determined by NMR spectroscopy. The structure of the SH3 domain forms a conventional beta-barrel with two beta-sheets comprised of five beta-strands. However, the molecular topology and charge distribution of PTK6-SH3 slightly differs from that of the other SH3 domains. The structure of the N-terminal Linker within the complex showed that the proline-rich region (P175-P187) of the Linker forms a compact hairpin structure through hydrophobic interactions. The structure of the SH3/Linker complex revealed intra-molecular interaction between the amino acid pairs R22/E190, W44/W184, N65/P177, and Y66/P179. Mutations in PTK6 at R22, W44, N65, and Y66 residues in the SH3 domain increased catalytic activity compared with wild-type protein, implying that specific interactions between hydrophobic residues in the proline-rich linker region and hydrophobic residues in the SH3 domain are mainly responsible for down-regulating the catalytic activity of PTK6.

Oncogenic functions of PTK6 are enhanced by its targeting to plasma membrane but abolished by its targeting to nucleus

PTK6 (also known as Brk) is an intracellular tyrosine kinase whose expression is up-regulated in several tumour types. Because localization of protein tyrosine kinases plays an important role in the development of cancers, we investigated the relationship between subcellular localization of PTK6 and its oncogenic properties. PTK6 was targeted to the plasma membrane or the nucleus of HEK 293 cells using the Src myristoylation signal (Myr) or SV40 T-antigen nuclear localization signal (NLS), respectively. The profile of cellular proteins phosphorylated by Myr-PTK6 was quite different from those phosphorylated by NLS-PTK6. Localization of PTK6 to the plasma membrane enhanced the ability of PTK6 to promote proliferation, cell survival and migration and to permit anchorage-independent colony formation. In contrast, nuclear localization of PTK6 impaired these functions. Our results demonstrate that recruitment of PTK6 to the plasma membrane is required for oncogenic function.

Molecular dissection of the interaction between the SH3 domain and the SH2-Kinase Linker region in PTK6

PTK6 (also known as Brk) is an intracellular tyrosine kinase that contains SH3, SH2, and tyrosine kinase catalytic (Kinase) domains. The SH3 domain of PTK6 interacts with the N-terminal half of the linker (Linker) region between the SH2 and Kinase domains. Site-directed mutagenesis and surface plasmon resonance studies showed that a tryptophan residue (Trp44) in the SH3 domain and proline residues in the Linker region, in the order of Pro177, Pro175, and Pro179, contribute to the interaction. The three-dimensional modeled structure of the SH3-Linker complex was in agreement with the biochemical data. Disruption of the intramolecular interaction between the SH3 domain and the Linker region by mutation of Trp44, Pro175, Pro177, and Pro179 markedly increased the catalytic activity of PTK6 in HEK 293 cells. These results demonstrate that Trp44 in the SH3 domain and Pro177, Pro175, and Pro179 in the N-terminal half of the Linker region play important roles in the SH3-Linker interaction to maintain the protein in an inactive conformation along with the phosphorylated Tyr447-SH2 interaction.

Expression and oncogenic role of Brk (PTK6/Sik) protein tyrosine kinase in lymphocytes

Tyrosine kinases play a fundamental role in cell proliferation, survival, adhesion, and motility and have also been shown to mediate malignant cell transformation. Here we describe constitutive expression of the protein tyrosine kinase Brk in a large proportion of cutaneous T-cell lymphomas and other transformed T- and B-cell populations. The kinase is expressed in the nuclear localization and activated state. Brk expression was also induced in normal T cells on their activation. Introduced expression of the Brk gene resulted in markedly diminished cytokine and growth factor dependence of transfected BaF3 lymphocytes in regard to their in vitro proliferation and survival. Brk also conferred in vivo oncogenicity on the BaF3 cells. siRNA-mediated inhibition of the endogenous Brk in malignant T cells diminished their growth and survival capacity. These findings document inducible expression of Brk in normal T lymphocytes and persistent expression of the activated kinase in malignant T and B cells. Furthermore, our results indicate that Brk may play a key role in lymphomagenesis, hence identifying the kinase as a potential therapeutic target in lymphomas.

Identification of STAT3 as a specific substrate of breast tumor kinase

Breast tumor kinase (Brk) is a non-receptor tyrosine kinase distantly related to the Src family kinase. It is expressed in more than 60% of breast tumors, but the biological role of this kinase remains to be determined. Only a limited number of substates have been identified for Brk, and the link of Brk to tumorigenesis remains largely unknown. In this study, we provide evidence that the signal transducer and activator of transcription 3, STAT3, is a physiological target of Brk. Activation of STAT3 previously has been linked to oncogenesis, and results in this study demonstrate that STAT3 is tyrosine phosphorylated and transcriptionally activated in cells expressing endogenous Brk. Signal transducer and activator of transcription 3 is specifically targeted since other STAT members are not responsive to Brk expression. Signal transducer and activator of transcription 3 activation requires the catalytic activity of Brk, and expression of both STAT3 and Brk stimulate cellular proliferation. In addition, we have identified a negative regulator of Brk, the suppressor of cytokine signaling, SOCS3. The SOCS3 protein is known to block signaling mediated by cytokine receptors, and here we find that SOCS3 is able to repress the activity of the Brk non-receptor tyrosine kinase.

Survey of the year 2005 commercial optical biosensor literature

We identified 1113 articles (103 reviews, 1010 primary research articles) published in 2005 that describe experiments performed using commercially available optical biosensors. While this number of publications is impressive, we find that the quality of the biosensor work in these articles is often pretty poor. It is a little disappointing that there appears to be only a small set of researchers who know how to properly perform, analyze, and present biosensor data. To help focus the field, we spotlight work published by 10 research groups that exemplify the quality of data one should expect to see from a biosensor experiment. Also, in an effort to raise awareness of the common problems in the biosensor field, we provide side-by-side examples of good and bad data sets from the 2005 literature.