Use of RNA interference to validate Brk as a novel therapeutic target in breast cancer: Brk promotes breast carcinoma cell proliferation

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.

Breast Tumour Kinase (Brk/PTK6) Contributes to Breast Tumour Xenograft Growth and Modulates Chemotherapeutic Responses In Vitro

Breast tumour kinase (Brk/PTK6) is overexpressed in up to 86% of breast cancers and is associated with poorer patient outcomes. It is considered a potential therapeutic target in breast cancer, even though the full spectrum of its kinase activity is not known. This study investigated the role of the kinase domain in promoting tumour growth and its potential in sensitising triple negative breast cancer cells to standard of care chemotherapy. Triple negative human xenograft models revealed that both kinase-inactive and wild-type Brk promoted xenograft growth. Suppression of Brk activity in cells subsequently co-treated with the chemotherapy agents doxorubicin or paclitaxel resulted in an increased cell sensitivity to these agents. In triple negative breast cancer cell lines, the inhibition of Brk kinase activity augmented the effects of doxorubicin or paclitaxel. High expression of the alternatively spliced isoform, ALT-PTK6, resulted in improved patient outcomes. Our study is the first to show a role for kinase-inactive Brk in human breast tumour xenograft growth; therefore, it is unlikely that kinase inhibition of Brk, in isolation, would halt tumour growth in vivo. Breast cancer cell responses to chemotherapy in vitro were kinase-dependent, indicating that treatment with kinase inhibitors could be a fruitful avenue for combinatorial treatment. Of particular prognostic value is the ratio of ALT-PTK6:Brk expression in predicating patient outcomes.

Strategic Developments & Future Perspective on Gene Therapy for Breast Cancer: Role of mTOR and Brk/ PTK6 as Molecular Targets

Breast cancer is a serious health issue and a major concern in biomedical research. Alteration in major signaling (viz. PI3K-AKT-mTOR, Ras-Raf-MEK-Erk, NF-kB, cyclin D1, JAK-STAT, Wnt, Notch, Hedgehog signaling and apoptotic pathway) contributes to the development of major subtypes of mammary carcinoma such as HER2 positive, TNBC, luminal A and B and normal-like breast cancer. Further, mutation and expression parameters of different genes involved in the growth and development of cells play an important role in the progress of different types of carcinoma, making gene therapy an emerging new therapeutic approach for the management of life-threatening diseases like cancer. The genetic targets (oncogenes and tumor suppressor genes) play a major role in the formation of a tumor. Brk/PTK6 and mTOR are two central molecules that are involved in the regulation of numerous signaling related to cell growth, proliferation, angiogenesis, survival, invasion, metastasis, apoptosis, and autophagy. Since these two proteins are highly upregulated in mammary carcinogenesis, this can be used as targeted genes for the treatment of breast cancer. However, not much work has been done on them. This review highlights the therapeutic significance of Brk and mTOR and their associated signaling in mammary carcinogenesis, which may provide a strategy to develop gene therapy for breast cancer management.

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.

The Clinical Kinase Index: A Method to Prioritize Understudied Kinases as Drug Targets for the Treatment of Cancer

The approval of the first kinase inhibitor, Gleevec, ushered in a paradigm shift for oncological treatment-the use of genomic data for targeted, efficacious therapies. Since then, over 48 additional small-molecule kinase inhibitors have been approved, solidifying the case for kinases as a highly druggable and attractive target class. Despite the role deregulated kinase activity plays in cancer, only 8% of the kinome has been effectively "drugged." Moreover, 24% of the 634 human kinases are understudied. We have developed a comprehensive scoring system that utilizes differential gene expression, pathological parameters, overall survival, and mutational hotspot analysis to rank and prioritize clinically relevant kinases across 17 solid tumor cancers from The Cancer Genome Atlas. We have developed the clinical kinase index (CKI) app (http://cki.ccs.miami.edu) to facilitate interactive analysis of all kinases in each cancer. Collectively, we report that understudied kinases have potential clinical value as biomarkers or drug targets that warrant further study.

Targeting protein tyrosine kinase 6 in cancer

Protein tyrosine kinase 6 (PTK6) is the most well studied member of the PTK6 family of intracellular tyrosine kinases. While it is expressed at highest levels in differentiated cells in the regenerating epithelial linings of the gastrointestinal tract and skin, induction and activation of PTK6 is detected in several cancers, including breast and prostate cancer where high PTK6 expression correlates with worse outcome. PTK6 expression is regulated by hypoxia and cell stress, and its kinase activity is induced by several growth factor receptors implicated in cancer including members of the ERBB family, IGFR1 and MET. Activation of PTK6 at the plasma membrane has been associated with the epithelial mesenchymal transition and tumor metastasis. Several lines of evidence indicate that PTK6 has context dependent functions that depend on cell type, intracellular localization and kinase activation. Systemic disruption of PTK6 has been shown to reduce tumorigenesis in mouse models of breast and prostate cancer, and more recently small molecule inhibitors of PTK6 have exhibited efficacy in inhibiting tumor growth in animal models. Here we review data that suggest targeting PTK6 may have beneficial therapeutic outcomes in some cancers.

Genomic landscape of metastatic breast cancer identifies preferentially dysregulated pathways and targets

Nearly all breast cancer deaths result from metastatic disease. Despite this, the genomic events that drive metastatic recurrence are poorly understood. We performed whole-exome and shallow whole-genome sequencing to identify genes and pathways preferentially mutated or copy-number altered in metastases compared with the paired primary tumors from which they arose. Seven genes were preferentially mutated in metastases - MYLK, PEAK1, SLC2A4RG, EVC2, XIRP2, PALB2, and ESR1 - 5 of which are not significantly mutated in any type of human primary cancer. Four regions were preferentially copy-number altered: loss of STK11 and CDKN2A/B, as well as gain of PTK6 and the membrane-bound progesterone receptor, PAQR8. PAQR8 gain was mutually exclusive with mutations in the nuclear estrogen and progesterone receptors, suggesting a role in treatment resistance. Several pathways were preferentially mutated or altered in metastases, including mTOR, CDK/RB, cAMP/PKA, WNT, HKMT, and focal adhesion. Immunohistochemical analyses revealed that metastases preferentially inactivate pRB, upregulate the mTORC1 and WNT signaling pathways, and exhibit nuclear localization of activated PKA. Our findings identify multiple therapeutic targets in metastatic recurrence that are not significantly mutated in primary cancers, implicate membrane progesterone signaling and nuclear PKA in metastatic recurrence, and provide genomic bases for the efficacy of mTORC1, CDK4/6, and PARP inhibitors in metastatic breast cancer.

p190RhoGAPs, the ARHGAP35- and ARHGAP5-Encoded Proteins, in Health and Disease

Small guanosine triphosphatases (GTPases) gathered in the Rat sarcoma (Ras) superfamily represent a large family of proteins involved in several key cellular mechanisms. Within the Ras superfamily, the Ras homolog (Rho) family is specialized in the regulation of actin cytoskeleton-based mechanisms. These proteins switch between an active and an inactive state, resulting in subsequent inhibiting or activating downstream signals, leading finally to regulation of actin-based processes. The On/Off status of Rho GTPases implicates two subsets of regulators: GEFs (guanine nucleotide exchange factors), which favor the active GTP (guanosine triphosphate) status of the GTPase and GAPs (GTPase activating proteins), which inhibit the GTPase by enhancing the GTP hydrolysis. In humans, the 20 identified Rho GTPases are regulated by over 70 GAP proteins suggesting a complex, but well-defined, spatio-temporal implication of these GAPs. Among the quite large number of RhoGAPs, we focus on p190RhoGAP, which is known as the main negative regulator of RhoA, but not exclusively. Two isoforms, p190A and p190B, are encoded by ARHGAP35 and ARHGAP5 genes, respectively. We describe here the function of each of these isoforms in physiological processes and sum up findings on their role in pathological conditions such as neurological disorders and cancers.

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 regulates growth and survival of endocrine therapy-resistant ER+ breast cancer cells

The non-receptor tyrosine kinase, PTK6/BRK, is highly expressed in multiple tumor types, including prostate, ovarian, and breast cancers, and regulates oncogenic phenotypes such as proliferation, migration, and survival. PTK6 inhibition also overcomes targeted therapy resistance of HER2+ breast cancer. Although PTK6 is highly expressed in ER+ Luminal breast cancers, the role of PTK6 in this subtype has not been elucidated. In this study, we investigated the functions of PTK6 in ER+ Luminal breast cancer cells, including those that are relatively resistant to estrogen deprivation or targeted endocrine therapies used in the treatment of ER+ cancers. Enhanced expression of PTK6 in ER+ breast cancer cells enhances growth of ER+ breast cancer cells, including tamoxifen-treated cells. Downregulation of PTK6 in ER+ breast cancer cells, including those resistant to tamoxifen, fulvestrant, and estrogen deprivation, induces apoptosis, as evidenced by increased levels of cleaved PARP, and an increase in the AnnexinV+ population. PTK6 downregulation impairs growth of these cells in 3D Matrigel^TM cultures, and virtually abrogates primary tumor growth of both tamoxifen-sensitive and resistant MCF-7 xenografts. Finally, we show that p38 MAPK activation is critical for PTK6 downregulation-induced apoptosis, a mechanism that we previously reported for survival of HER2+ breast cancer cells, highlighting conserved mechanisms of survival regulation by PTK6 across breast cancer subtypes. In conclusion, our studies elucidate critical functions of PTK6 in ER+ Luminal breast cancers and support PTK6 as an attractive therapeutic target for ER+ breast cancers.

Drugs that target a tumor-promoting enzyme called protein tyrosine kinase 6 (PTK6) could help treat hormone-receptor positive breast cancer. A team led by Hanna Irie from the Icahn School of Medicine at Mount Sinai in New York, NY, USA, investigated the role of PTK6, also known as breast tumor kinase, in breast cancer cells that grow in response to the hormone estrogen. They boosted the enzyme’s expression in estrogen receptor-expressing breast cancer cells and saw enhanced growth. Conversely, downregulating PTK6 levels led to cell death, including in tumor cells that were resistant to tamoxifen
 and other therapies commonly used to treat estrogen-receptor positive breast cancer. The researchers showed that this effect was dependent on the activation of a signaling pathway previously found to be important in another subtype of breast cancer, highlighting a conserved mechanism of cell survival regulation by PTK6.

Overexpression of PTK6 predicts poor prognosis in bladder cancer patients

Protein tyrosine kinase 6 (PTK6) is a non-receptor tyrosine kinase and works as an oncogene in various cancers. Recently, PTK6 has been used as a therapeutic target for breast cancer patients in a clinical study. However, the prognostic value of PTK6 in bladder cancer (BC) remains vague. Therefore, we retrieved 3 independent investigations of Oncomine database and found that PTK6 is highly expressed in BC tissues compared with corresponding normal controls. Similar results were also observed in clinical specimens at both mRNA and protein levels. Immunohistochemical analysis indicated that PTK6 overexpression was highly related to the T classification, N classification, grade, recurrence, and poor prognosis of BC patients. Furthermore, we demonstrated that when PTK6 expression was knocked down by siRNAs, cell proliferation and migration were considerably inhibited in BC cell lines T24 and EJ. By these approaches, we are intended to elucidate PTK6 may be a reliable therapeutic target in BC and might benefit from PTK6 inhibitors in the future.

Structural Basis for the Non-catalytic Functions of Protein Kinases

Protein kinases are known primarily for their ability to phosphorylate protein substrates, which constitutes an essential biological process. Recently, compelling evidence has accumulated that the functions of many protein kinases extend beyond phosphorylation and include an impressive spectrum of non-catalytic roles, such as scaffolding, allosteric regulation, or even protein-DNA interactions. How the conserved kinase fold shared by all metazoan protein kinases can accomplish these diverse tasks in a specific and regulated manner is poorly understood. In this review, we analyze the molecular mechanisms supporting phosphorylation-independent signaling by kinases and attempt to identify common and unique structural characteristics that enable kinases to perform non-catalytic functions. We also discuss how post-translational modifications, protein-protein interactions, and small molecules modulate these non-canonical kinase functions. Finally, we highlight current efforts in the targeted design of small-molecule modulators of non-catalytic kinase functions, a new pharmacological challenge for which structural considerations are more important than ever.

Signal transducer and activator of transcription 3 regulation by novel binding partners

Signal transducers and activators of transcription (STATs) mediate essential signals for various biological processes, including immune responses, hematopoiesis, and neurogenesis. STAT3, for example, is involved in the pathogenesis of various human diseases, including cancers, autoimmune and inflammatory disorders. STAT3 activation is therefore tightly regulated at multiple levels to prevent these pathological conditions. A number of proteins have been reported to associate with STAT3 and regulate its activity. These STAT3-interacting proteins function to modulate STAT3-mediated signaling at various steps and mediate the crosstalk of STAT3 with other cellular signaling pathways. This article reviews the roles of novel STAT3 binding partners such as DAXX, zipper-interacting protein kinase, Krüppel-associated box-associated protein 1, Y14, PDZ and LIM domain 2 and signal transducing adaptor protein-2, in the regulation of STAT3-mediated signaling.

PTK6/BRK is expressed in the normal mammary gland and activated at the plasma membrane in breast tumors

Protein Tyrosine kinase 6 (PTK6/BRK) is overexpressed in the majority of human breast tumors and breast tumor cell lines. It is also expressed in normal epithelial linings of the gastrointestinal tract, skin, and prostate. To date, expression of PTK6 has not been extensively examined in the normal human mammary gland. We detected PTK6 mRNA and protein expression in the immortalized normal MCF-10A human mammary gland epithelial cell line, and examined PTK6 expression and activation in a normal human breast tissue microarray, as well as in human breast tumors. Phosphorylation of tyrosine residue 342 in the PTK6 activation loop corresponds with its activation. Similar to findings in the prostate, we detect nuclear and cytoplasmic PTK6 in normal mammary gland epithelial cells, but no phosphorylation of tyrosine residue 342. However, in human breast tumors, striking PTK6 expression and phosphorylation of tyrosine 342 is observed at the plasma membrane. PTK6 is expressed in the normal human mammary gland, but does not appear to be active and may have kinase-independent functions that are distinct from its cancer promoting activities at the membrane. Understanding consequences of PTK6 activation at the plasma membrane may have implications for developing novel targeted therapies against this kinase.

Breast tumor kinase/protein tyrosine kinase 6 (Brk/PTK6) activity in normal and neoplastic biliary epithelia

BACKGROUND & AIMS

Breast tumor kinase (BRK) augments proliferation and promotes cell survival in breast cancers via interactions with SH2 and SH3 ligand-containing proteins, such as receptor tyrosine kinases (RTK; e.g. EGFR, ErbB2/neu). Since RTK contribute to cholangiocarcinoma (CC) evolution we probed BRK protein expression and function in normal and CC livers.

METHODS

Immunohistochemical staining of normal livers and CC (n=93) in a tissue microarray and three CC and an immortalized human cholangiocyte cell lines (real-time PCR, Western blotting, siRNA) were used to study the functional relationships between BRK, EGFR, ErbB2, SAM68, and SPRR2a.

RESULTS

BRK protein was expressed in normal human intrahepatic bile ducts; all CC cell lines and a majority of CC showed strong BRK protein expression. Multiplex immunostaining/tissue cytometry and immunoprecipitation studies showed: 1) BRK co-localized with EGFR and ErbB2/neu; 2) BRK(high)/EGFR(high)-co-expressing CC cells had significantly higher Ki67 labeling and; 3) stronger BRK protein expression was seen in perihilar and distal CC than intrahepatic CC and directly correlated with CC differentiation. In cell lines, BRK expression augmented proliferation in response to exogenous EGF, whereas BRK siRNA significantly reduced growth. The SH3 ligand-containing, SPRR2A activated pTyr342 BRK, which in turn, phosphorylated SAM68, causing nuclear localization and increased cell proliferation similar to observations in breast cancers.

CONCLUSION

BRK expression in a majority of CC can interact with RTK, augmenting growth and interfering with proliferation inhibitors (SAM68). Therapeutically targeting BRK function (in addition to RTK) should be of benefit for CC treatment.

PTK6 inhibition promotes apoptosis of Lapatinib-resistant Her2(+) breast cancer cells by inducing Bim

Introduction

Protein tyrosine kinase 6 (PTK6) is a non-receptor tyrosine kinase that is highly expressed in Human Epidermal Growth Factor 2⁺ (Her2⁺) breast cancers. Overexpression of PTK6 enhances anchorage-independent survival, proliferation, and migration of breast cancer cells. We hypothesized that PTK6 inhibition is an effective strategy to inhibit growth and survival of Her2⁺ breast cancer cells, including those that are relatively resistant to Lapatinib, a targeted therapy for Her2⁺ breast cancer, either intrinsically or acquired after continuous drug exposure.

Methods

To determine the effects of PTK6 inhibition on Lapatinib-resistant Her2⁺ breast cancer cell lines (UACC893R1 and MDA-MB-453), we used short hairpin ribonucleic acid (shRNA) vectors to downregulate PTK6 expression. We determined the effects of PTK6 downregulation on growth and survival in vitro and in vivo, as well as the mechanisms responsible for these effects.

Results

Lapatinib treatment of “sensitive” Her2⁺ cells induces apoptotic cell death and enhances transcript and protein levels of Bim, a pro-apoptotic Bcl2 family member. In contrast, treatment of relatively “resistant” Her2⁺ cells fails to induce Bim or enhance levels of cleaved, poly-ADP ribose polymerase (PARP). Downregulation of PTK6 expression in these “resistant” cells enhances Bim expression, resulting in apoptotic cell death. PTK6 downregulation impairs growth of these cells in in vitro 3-D Matrigel^TM cultures, and also inhibits growth of Her2⁺ primary tumor xenografts. Bim expression is critical for apoptosis induced by PTK6 downregulation, as co-expression of Bim shRNA rescued these cells from PTK6 shRNA-induced death. The regulation of Bim by PTK6 is not via changes in Erk/MAPK or Akt signaling, two pathways known to regulate Bim expression. Rather, PTK6 downregulation activates p38, and pharmacological inhibition of p38 activity prevents PTK6 shRNA-induced Bim expression and partially rescues cells from apoptosis.

Conclusions

PTK6 downregulation induces apoptosis of Lapatinib-resistant Her2⁺ breast cancer cells by enhancing Bim expression via p38 activation. As Bim expression is a critical biomarker for response to many targeted therapies, PTK6 inhibition may offer a therapeutic approach to treating patients with Her2 targeted therapy-resistant breast cancers.

Electronic supplementary material

The online version of this article (doi:10.1186/s13058-015-0594-z) contains supplementary material, which is available to authorized users.

PTK6 Potentiates Gemcitabine-Induced Apoptosis by Prolonging S-phase and Enhancing DNA Damage in Pancreatic Cancer

Protein Tyrosine Kinase 6 (PTK6) is a non-receptor-type tyrosine kinase known to be expressed in various cancers, including pancreatic cancer. The role of PTK6 in cancer chemoresistance remains unclear. Therefore, it was hypothesized that PTK6 mechanistically regulates gemcitabine resistance in pancreatic cancer. Gemcitabine treatment stimulated endogenous PTK6 overexpression in MIAPaCa2 and Panc1 cells. PTK6 gene silencing increased cell survival after gemcitabine treatment and decreased apoptosis, whereas PTK6 overexpression decreased cell survival and increased apoptosis. Selection for gemcitabine resistance revealed substantially lower PTK6 expression in the gemcitabine-resistant subclones compared with the parental lines, while restoring PTK6 rescued gemcitabine sensitivity. Gemcitabine induced phosphorylation of H2AX (γ-H2AX) and ataxia-telangiectasia mutated kinase (pATM), specific markers for DNA double-strand breaks. Both gemcitabine-induced phosphorylation of H2AX and ATM were reduced by PTK6 knockdown and increased by PTK6 overexpression. PTK6 overexpression also increased the S-phase fraction 48 hours after gemcitabine treatment. Although gemcitabine activated both caspase-8 (CASP8) and caspase-9 (CASP9), the effect of PTK6 on gemcitabine-induced apoptosis required CASP8 but not CASP9. In mouse xenografts, PTK6 overexpression in subcutaneous tumors attenuated tumor growth after gemcitabine treatment. In conclusion, PTK6 prolongs S-phase and increases the ability of gemcitabine to cause DNA damage in vitro and in vivo.

IMPLICATIONS

PTK6 affects cell cycle and DNA damage, thus making it an important therapeutic target to improve the outcomes of patients with pancreatic cancer.

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.

Cancer-Associated Mutations in Breast Tumor Kinase/PTK6 Differentially Affect Enzyme Activity and Substrate Recognition

Brk (breast tumor kinase, also known as PTK6) is a nonreceptor tyrosine kinase that is aberrantly expressed in several cancers and promotes cell proliferation and transformation. Genome sequencing studies have revealed a number of cancer-associated somatic mutations in the Brk gene; however, their effect on Brk activity has not been examined. We analyzed a panel of cancer-associated mutations and determined that several of the mutations activate Brk, while two eliminated enzymatic activity. Three of the mutations (L16F, R131L, and P450L) are located in important regulatory domains of Brk (the SH3, SH2 domains, and C-terminal tail, respectively). Biochemical data suggest that they activate Brk by disrupting intramolecular interactions that normally maintain Brk in an autoinhibited conformation. We also observed differential effects on recognition and phosphorylation of substrates, suggesting that the mutations can influence downstream Brk signaling by multiple mechanisms.

Evolution of breast cancer therapeutics: Breast tumour kinase’s role in breast cancer and hope for breast tumour kinase targeted therapy

There have been significant improvements in the detection and treatment of breast cancer in recent decades. However, there is still a need to develop more effective therapeutic techniques that are patient specific with reduced toxicity leading to further increases in patients’ overall survival; the ongoing progress in understanding recurrence, resistant and spread also needs to be maintained. Better understanding of breast cancer pathology, molecular biology and progression as well as identification of some of the underlying factors involved in breast cancer tumourgenesis and metastasis has led to the identification of novel therapeutic targets. Over a number of years interest has risen in breast tumour kinase (Brk) also known as protein tyrosine kinase 6; the research field has grown and Brk has been described as a desirable therapeutic target in relation to tyrosine kinase inhibition as well as disruption of its kinase independent activity. This review will outline the current “state of play” with respect to targeted therapy for breast cancer, as well as discussing Brk’s role in the processes underlying tumour development and metastasis and its potential as a therapeutic target in breast cancer.

PTK6 promotes cancer migration and invasion in pancreatic cancer cells dependent on ERK signaling

Protein Tyrosine Kinase 6 (PTK6) is a non-receptor type tyrosine kinase that may be involved in some cancers. However, the biological role and expression status of PTK6 in pancreatic cancer is unknown. Therefore in this study, we evaluated the functional role of PTK6 on pancreatic cancer invasion. Five pancreatic cancer cell lines expressed PTK6 at varying levels. PTK6 expression was also observed in human pancreatic adenocarcinomas. PTK6 suppression by siRNA significantly reduced both cellular migration and invasion (0.59/0.49 fold for BxPC3, 0.61/0.62 for Panc1, 0.42/0.39 for MIAPaCa2, respectively, p<0.05 for each). In contrast, forced overexpression of PTK6 by transfection of a PTK6 expression vector in Panc1 and MIAPaCa2 cells increased cellular migration and invasion (1.57/1.67 fold for Panc1, 1.44/1.57 for MIAPaCa2, respectively, p<0.05). Silencing PTK6 reduced ERK1/2 activation, but not AKT or STAT3 activation, while PTK6 overexpression increased ERK1/2 activation. U0126, a specific inhibitor of ERK1/2, completely abolished the effect of PTK6 overexpression on cellular migration and invasion. These results suggest that PTK6 regulates cellular migration and invasion in pancreatic cancer via ERK signaling. PTK6 may be a novel therapeutic target for pancreatic cancer.

Protein tyrosine kinase 6 regulates mammary gland tumorigenesis in mouse models

Protein tyrosine kinase 6 (PTK6, also called BRK) is an intracellular tyrosine kinase expressed in the majority of human breast tumors and breast cancer cell lines, but its expression has not been reported in normal mammary gland. To study functions of PTK6 in vivo, we generated and characterized several transgenic mouse lines with expression of human PTK6 under control of the mouse mammary tumor virus (MMTV) long terminal repeat. Ectopic active PTK6 was detected in luminal epithelial cells of mature transgenic mammary glands. Lines expressing the MMTV-PTK6 transgene exhibited more than a two-fold increase in mammary gland tumor formation compared with nontransgenic control animals. PTK6 activates signal transducer and activator of transcription 3 (STAT3), and active STAT3 was detected in PTK6-positive mammary gland epithelial cells. Endogenous mouse PTK6 was not detected in the normal mouse mammary gland, but it was induced in mouse mammary gland tumors of different origin, including spontaneous tumors that developed in control mice, and tumors that formed in PTK6, H-Ras, ERBB2 and PyMT transgenic models. MMTV-PTK6 and MMTV-ERBB2 transgenic mice were crossed to explore crosstalk between PTK6 and ERBB2 signaling in vivo. We found no significant increase in tumor incidence, size or metastasis in ERBB2/PTK6 double transgenic mice. Although we detected increased proliferation in ERBB2/PTK6 double transgenic tumors, an increase in apoptosis was also observed. MMTV-PTK6 clearly promotes mammary gland tumorigenesis in vivo, but its impact may be underrepresented in our transgenic models because of induction of endogenous PTK6 expression.

Protein-tyrosine phosphatase 1B antagonized signaling by insulin-like growth factor-1 receptor and kinase BRK/PTK6 in ovarian cancer cells

Ovarian cancer, which is the leading cause of death from gynecological malignancies, is a heterogeneous disease known to be associated with disruption of multiple signaling pathways. Nevertheless, little is known regarding the role of protein phosphatases in the signaling events that underlie the disease; such knowledge will be essential to gain a complete understanding of the etiology of the disease and how to treat it. We have demonstrated that protein-tyrosine phosphatase 1B (PTP1B) was underexpressed in a panel of ovarian carcinoma-derived cell lines, compared with immortalized human ovarian surface epithelial cell lines. Stable restoration of PTP1B in those cancer cell lines substantially decreased cell migration and invasion, as well as proliferation and anchorage-independent survival. Mechanistically, the pro-survival IGF-1R signaling pathway was attenuated upon ectopic expression of PTP1B. This was due to dephosphorylation by PTP1B of IGF-1R β-subunit and BRK/PTK6, an SRC-like protein-tyrosine kinase that physically and functionally interacts with the IGF-1R β-subunit. Restoration of PTP1B expression led to enhanced activation of BAD, one of the major pro-death members of the BCL-2 family, which triggered cell death through apoptosis. Conversely, inhibition of PTP1B with a small molecular inhibitor, MSI-1436, increased proliferation and migration of immortalized HOSE cell lines. These data reveal an important role for PTP1B as a negative regulator of BRK and IGF-1Rβ signaling in ovarian cancer cells.

Expression of protein tyrosine kinase 6 (PTK6) in nonsmall cell lung cancer and their clinical and prognostic significance

Aim:

The aim of the study was to validate the expression of protein tyrosine kinase 6 (PTK6) in nonsmall cell lung cancer (NSCLC), and to evaluate its clinicopathological and prognostic significance.

Methods:

We first conducted a meta-analysis on the mRNA profiling data sets of NSCLC in the Oncomine database. Then, one of the most significantly upregulated tyrosine kinase targets, PTK6, was further validated by immunohistochemistry in 104 primary NSCLC tumors. Furthermore the association between PTK6 expression, the clinical parameters, and overall survival was further analyzed.

Results:

Using the Oncomine database, we identified a list of tyrosine kinase genes related to NSCLC, among which PTK6 was the second most overexpressed gene (median rank = 915, P = 2.9 × 10⁻⁵). We further confirmed that NSCLC tumors had a higher expression level of PTK6 than normal pulmonary tissues. Moreover, high PTK6 expression correlated positively with shorter overall survival time, but not with other clinicopathological characteristics. In the multivariate Cox regression model, high PTK6 expression was demonstrated to be an independent prognostic factor for NSCLC patients.

Conclusion:

Our results validated that PTK6 was found to be overexpressed in a proportion of NSCLC samples, and was associated with a poor prognosis, suggesting that this subgroup of NSCLC patients might benefit from PTK6 inhibitors in the future.

Low expression of PTK6/Brk predicts poor prognosis in patients with laryngeal squamous cell carcinoma

Background

Protein tyrosine kinase 6 (PTK6), also known as breast tumor kinase (Brk), was a nonreceptor tyrosine kinase containing SH3, SH2, and tyrosine kinase catalytic domains. The deregulated expression of PTK6 was observed in various human cancers. However, little was known about PTK6 expression and its clinicopathological significance in human laryngeal squamous cell carcinoma (LSCC).

Materials

PTK6 expression was evaluated in 7 pairs of surgically resectable laryngeal tissues by Western blotting and in 13 pairs of surgically resectable laryngeal tissues by reverse transcription-PCR (RT-PCR). Using immunohistochemistry, we performed a retrospective study of the PTK6 expression levels on 134 archival LSCC paraffin-embedded samples. Prognostic outcomes correlated with PTK6 were examined using Kaplan–Meier analysis and Cox proportional hazards model.

Results

The PTK6 expression level was lower in LSCC tissues than in the adjacent noncancerous epithelial laryngeal tissues by Western blots and RT-PCR. By immunohistochemical analysis, we observed high expression of PTK6 in 25 of 76 (32.9%) adjacent noncancerous epithelial laryngeal tissues and in 39 of 134 (29.1%) of LSCC, respectively. Multivariate analysis demonstrated that pN status and the expression level of PTK6 (P < 0.05) were independent and significant prognostic factors. In the primary LSCC category, median DFS (disease free survival) of high, medium and low PTK6 expression patients were 88.5 months ,74.5 months and 49.0 months (log-rank test, P = 0.002); median OS (overall survival) of high, medium and low PTK6 expression patients were 88.5 months ,76.3 months and 65.7 months (log-rank test, P = 0.002). Reduced cytoplasmic PTK6 expression in LSCC was significantly associated with late pN status (P =0.005, r = 0.27), advanced pTNM stages (III and IV) (P =0.027, r = 0.147), and poor differentiated LSCC (P <0.0001, r = 0.486). In adjacent paracancerous laryngeal epithelial samples, median DFS of high, medium and low PTK6 expression patients were 92.6 months ,75.6 months and 48.5 months (log-rank test, P = 0.020); median OS of high, medium and low PTK6 expression patients were 92.9 months ,78.9 months and 74.6 months (log-rank test, P = 0.042).

Conclusion

The present findings indicated that cytoplasmic PTK6 expression is a potential prognostic factor for survival in LSCC patients. High expression of PTK6 was associated with favorable OS and DFS in LSCC patients.

Brk/PTK6 cooperates with HER2 and Src in regulating breast cancer cell survival and epithelial-to-mesenchymal transition

Breast tumor kinase (Brk)/protein tyrosine kinase-6 (PTK-6) is a nonreceptor PTK commonly expressed at high levels in breast cancer. Brk interacts closely with members of the human epidermal growth factor receptor (HER) family in breast cancer but the functional role of this interaction remains to be determined. Here, we provide novel mechanistic insights into the role of Brk in regulating cell survival and epithelial-to-mesenchymal transition (EMT) in the context of HER2-positive breast cancer cells. Overexpression of HER2 in MCF7 breast cancer cells (MCF7HER2) led to a higher level of Brk protein and concomitantly reduced Src Y416-phosphorylation, and the cells became mesenchymal in morphology. An in vivo selection of MCF7HER2 cells in nude mice resulted in a subline, termed EMT1, that exhibited not only mesenchymal morphology but also enhanced migration potential. Compared with MCF7HER2 cells, EMT1 cells maintained a similar level of HER2 protein but had much higher level of activated HER2, and the increase in Brk protein and the decrease in Src Y416-phosphorylation were less in EMT1 cells. EMT1 cells exhibited increased sensitivity to both pharmacological inhibition of HER2 and knockdown of Brk than did MCF7HER2 cells. Knockdown of Brk induced apoptosis and partially reversed the EMT phenotype in EMT1 cells. Overexpression of a constitutively active STAT3, a known substrate of Brk, overcame Brk knockdown-induced effects in EMT1 cells. Together, our findings support a new paradigm wherein Brk plays both a complementary and a counterbalancing role in cooperating with HER2 and Src to regulate breast cancer cell survival and EMT.

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.

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 sustains activated EGFR signaling through inhibiting EGFR degradation and transactivating EGFR

Epidermal growth factor receptor (EGFR)-mediated cell signaling is critical for mammary epithelial cell growth and survival; however, targeting EGFR has shown no or only minimal therapeutic benefit in patients with breast cancer. Here, we report a novel regulatory mechanism of EGFR signaling that may explain the low response rates. We found that breast tumor kinase (Brk)/protein-tyrosine kinase 6 (PTK6), a nonreceptor protein tyrosine kinase highly expressed in most human breast tumors, interacted with EGFR and sustained ligand-induced EGFR signaling. We demonstrate that Brk inhibits ligand-induced EGFR degradation through uncoupling activated EGFR from Cbl-mediated EGFR ubiquitination. In addition, upon activation by EGFR, Brk directly phosphorylated Y845 in the EGFR kinase domain, thereby further potentiating EGFR kinase activity. Experimental elevation of Brk conferred resistance of breast cancer cells to cetuximab (an EGFR-blocking antibody)-induced inhibition of cell signaling and proliferation, whereas knockdown of Brk sensitized the cells to cetuximab by inducing apoptosis. Our findings reveal a previously unknown role of Brk in EGFR-targeted therapy.

The secret life of kinases: functions beyond catalysis

Protein phosphorylation participates in the regulation of all fundamental biological processes, and protein kinases have been intensively studied. However, while the focus was on catalytic activities, accumulating evidence suggests that non-catalytic properties of protein kinases are essential, and in some cases even sufficient for their functions. These non-catalytic functions include the scaffolding of protein complexes, the competition for protein interactions, allosteric effects on other enzymes, subcellular targeting, and DNA binding. This rich repertoire often is used to coordinate phosphorylation events and enhance the specificity of substrate phosphorylation, but also can adopt functions that do not rely on kinase activity. Here, we discuss such kinase independent functions of protein and lipid kinases focussing on kinases that play a role in the regulation of cell proliferation, differentiation, apoptosis, and motility.

Detection of Brk expression in non-small cell lung cancer: clinicopathological relevance

Breast tumor kinase (Brk), also known as protein tyrosine kinase 6, is a nonreceptor tyrosine kinase containing SH3, SH2, and tyrosine kinase catalytic domains. Brk upregulation and oncogenic properties have been found in several malignant tumors, including breast, colon carcinomas, and melanomas, but the expression of Brk and its clinical significance in non-small cell lung cancer (NSCLC) remains unclear. In the current study, we examined the expression of Brk and its correlation with clinicopathological features involving p53, ki67, and E-cadherin status in NSCLC tissue using immunohistochemistry. We also used immunocytochemistry and immunofluorescent staining to examine the Brk expression and its subcellular localization in NSCLC cell lines, including LTE and H460. We further confirmed cytoplasmic and nucleus expression of Brk in LTE and H460 cells using Western blotting. The Brk expression in NSCLC cells was mainly found in cytoplasm (59/122, 48.4%) with some nucleus staining (17/122, 13.9%) with a total positive rate of 53.3% (65/122). Cytoplasmic Brk expression in NSCLC was higher than that in normal lung tissues (24/122, 19.7%) (P < 0.05). Increased cytoplasmic Brk expression in NSCLC was associated with large tumor size (≥ 3 cm), lymph node metastasis, and advanced tumor-node-metastasis (TNM) stages (III and IV) (P < 0.05). Moreover, increased cytoplasmic Brk expression was positively associated with Ki67 status in NSCLC (P < 0.05). Reduced E-cadherin expression was also found to be associated with lymph node metastasis and advanced TNM stages (III and IV) in NSCLC (P < 0.05). Brk expression was not associated with E-cadherin expression and P53 status in NSCLC (P > 0.05). The present findings indicate an increase of cytoplasmic Brk expression in NSCLC which may play a role in tumor development, including tumor expansion and lymph node metastasis in which Ki67, but not E-cadherin, and P53 status may be involved.

Involvement of STAP-2 in Brk-mediated phosphorylation and activation of STAT5 in breast cancer cells

Signal-transducing adaptor protein (STAP)-2 is a recently identified adaptor protein that contains Pleckstrin homology and Src homology 2-like domains, and is also known to be a substrate of breast tumor kinase (Brk). In a previous study, we found that STAP-2 upregulated Brk-mediated activation of signal transducer and activator of transcription (STAT) 3 in breast cancer cells. Here, we examined the involvement of STAP-2 in Brk-mediated STAT5 activation in breast cancer cells. Ectopic expression of STAP-2 induced Brk-mediated transcriptional activity of STAT5. Furthermore, STAP-2-knockdown in T47D breast cancer cells induced a marked decrease in proliferation that was as strong as that after Brk- or STAT5b-knockdown. Regarding the mechanism, the Pleckstrin homology domain of STAP-2 is likely to participate in the process by which Brk phosphorylates and activates STAT5. Taken together, our findings provide insights toward the development of novel therapeutic strategies as well as novel prognostic values in breast carcinomas.

Breast tumor kinase (Brk/PTK6) plays a role in the differentiation of primary keratinocytes

Breast Tumor Kinase (Brk/PTK6) has a relatively limited expression profile in normal tissue. Its expression is restricted to epithelial cells that are differentiating such as those in the epidermis, and Brk expression appears to be absent from proliferating cells in normal tissue. Also, there is now some evidence to suggest that Brk plays a functional role in the differentiation of the keratinocytes in the epidermis. We have, therefore, investigated the role that Brk/PTK6 plays in normal human primary keratinocytes by suppressing protein levels using RNA interference. We show that as primary human keratinocytes are induced to differentiate in vitro, Brk levels decrease. Decreasing Brk protein levels lead to an increase in the number of cells with a permeable plasma membrane, a decrease in epidermal growth factor receptor (EGFR) and a parallel increase in keratin 10 levels, but classical markers of apoptosis or terminal differentiation are not affected. We propose Brk, Keratin 10 and EGFR are co-regulated during differentiation and that manipulating Brk expression can influence the differentiation of normal primary human keratinocytes.

Deregulation of the cell cycle by breast tumor kinase (Brk)

Brk is a cytoplasmic nonreceptor tyrosine kinase that is overexpressed in breast tumors but undetectable in normal or benign mammary tissues. Brk promotes proliferation of human mammary epithelial cells and tumor growth in a mouse model, but the role of Brk in cell cycle regulation is not known. In this study, we describe the mechanism of Brk-induced deregulation of the cell cycle. We provide evidence that Brk antagonizes the transcriptional activity of the transcription factor FoxO family of proteins by inhibiting its nuclear localization. As a result, the cell cycle inhibitor p27, a FoxO target gene, is down-regulated. This event is accompanied by G1/S cell cycle progression of quiescent cells. As p27 is a key regulator of the G1/S cell cycle checkpoint, these data suggest that perturbation of p27 expression induced by Brk causes S phase entrance. Deregulation of the cell cycle is a key event in neoplasia, and thus, the mechanism presented here likely contributes to breast cancer development.

Cytoplasmic retention of protein tyrosine kinase 6 promotes growth of prostate tumor cells

Protein tyrosine kinase 6 (PTK6) is an intracellular tyrosine kinase that is nuclear in epithelial cells of the normal prostate, but cytoplasmic in prostate tumors and in the PC3 prostate tumor cell line. The impact of altered PTK6 intracellular localization in prostate tumor cells has not been extensively explored. Knockdown of endogenous cytoplasmic PTK6 resulted in decreased PC3 cell proliferation and colony formation, suggesting that cytoplasmic PTK6 stimulates oncogenic pathways. In contrast, reintroduction of PTK6 into nuclei of PC3 cells had a negative effect on growth. Enhanced tyrosine phosphorylation of the PTK6 substrate Sam68 was detected in cells expressing nuclear-targeted PTK6. We found that mechanisms regulating nuclear localization of PTK6 are intact in PC3 cells. Transiently overexpressed PTK6 readily enters the nucleus. Ectopic expression of ALT-PTK6, a catalytically inactive splice variant of PTK6, did not affect localization of endogenous PTK6 in PC3 cells. Using leptomycin B, we confirmed that cytoplasmic localization of endogenous PTK6 is not due to Crm-1/exportin-1 mediated nuclear export. In addition, overexpression of the PTK6 nuclear substrate Sam68 is not sufficient to bring PTK6 into the nucleus. While exogenous PTK6 was readily detected in the nucleus when transiently expressed at high levels, low-level expression of inducible wild type PTK6 in stable cell lines resulted in its cytoplasmic retention. Our results suggest that retention of PTK6 in the cytoplasm of prostate cancer cells disrupts its ability to regulate nuclear substrates and leads to aberrant growth. In prostate cancer, restoring PTK6 nuclear localization may have therapeutic advantages.

Interactions of STAP-2 with Brk and STAT3 participate in cell growth of human breast cancer cells

STAP-2 (signal transducing adaptor protein-2) is a recently identified adaptor protein that contains pleckstrin homology (PH) and Src homology 2-like domains, as well as a STAT3-binding motif in its C-terminal region. STAP-2 is also a substrate of breast tumor kinase (Brk). In breast cancers, Brk expression is deregulated and promotes STAT3-dependent cell proliferation. In the present study, manipulated STAP-2 expression demonstrated essential roles of STAP-2 in Brk-mediated STAT3 activation. STAP-2 interacts with both Brk and STAT3. In addition, small interfering RNA-mediated reduction of endogenous STAP-2 expression strongly decreased Brk-mediated STAT3 activation in T47D breast cancer cells. The PH domain of STAP-2 is involved in multiple steps: the binding between Brk and STAP-2, the activation and tyrosine phosphorylation of STAT3, and the activation of Brk. Notably, a STAP-2 PH-Brk fusion protein exhibited robust kinase activity and increased activation and tyrosine phosphorylation of STAT3. Finally, STAP-2 knockdown in T47D cells induced a significant decrease of proliferation, as strong as that of Brk or STAT3 knockdown. Taken together, our findings are likely to inform the development of a novel therapeutic strategy, as well as the determination of novel prognostic values, in breast carcinomas.

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.

In situ quantification of HER2-protein tyrosine kinase 6 (PTK6) protein-protein complexes in paraffin sections from breast cancer tissues

Background:

Protein tyrosine kinase 6 (PTK6; breast tumour kinase) is overexpressed in up to 86% of the invasive breast cancers, and its association with the oncoprotein human epidermal growth factor receptor 2 (HER2) was shown in vitro by co-precipitation. Furthermore, expression of PTK6 in tumours is linked with the expression of HER2.

Method and results:

In this study, we used the proximity ligation assay (PLA) technique on formalin-fixed paraffin sections from eighty invasive breast carcinoma tissue specimens to locate PTK6–HER2 protein–protein complexes. Proximity ligation assay signals from protein complexes were assessed quantitatively, and expression levels showed a statistically significant association with tumour size (P=0.015) and course of the cancer disease (P=0.012).

Conclusion:

Protein tyrosine kinase 6 forms protein complexes with HER2 in primary breast cancer tissues, which can be visualised by use of the PLA technique. Human epidermal growth factor receptor 2–PTK6 complexes are of prognostic relevance.

Breast tumor kinase and extracellular signal-regulated kinase 5 mediate Met receptor signaling to cell migration in breast cancer cells

Introduction

Breast tumor kinase (Brk/protein tyrosine kinase 6 (PTK6)) is a nonreceptor, soluble tyrosine kinase overexpressed in the majority of breast tumors. Previous work has placed Brk downstream of epidermal growth factor receptor (ErbB) activation and upstream of extracellular signal-regulated kinase 5 (ERK5) and p38 mitogen-activated protein (MAP) kinases. Herein we investigate the regulation of Brk kinase activity and cell migration in response to treatment of keratinocytes (HaCaT cells) and breast cancer cell lines (MDA-MB-231 and T47D cells) with hepatocyte growth factor (HGF) and macrophage stimulating protein (MSP), peptide ligands for Met and Ron receptors, respectively.

Methods

In vitro kinase assays were performed to directly measure Brk kinase activity in response to MET and RON ligands. Transfection of Brk-targeted RNAi was used to knock down endogenous Brk or ERK5 in multiple cell lines. Kinase activities (downstream of MET signaling) were assayed by Western blotting using total and phospho-specific antibodies. Boyden chamber assays were used to measure cell migration in response to manipulation of Brk and downstream MET effectors. Rescue experiments were performed by knock down of endogenous Brk using RNAi (targeting the untranslated region (3′-UTR)) and transient transfection (re-expression) of either wild-type or kinase-inactive Brk.

Results

Brk gene silencing revealed that HGF, but not MSP, induced robust Brk-dependent cell migration. Brk and ERK5 copurified in HGF-induced protein complexes, and Brk/ERK5 complexes formed independently of Brk kinase activity. ERK5 was required for breast cancer cell but not keratinocyte cell migration, which became ERK1/2-dependent upon ERK5 knockdown. Notably, rescue experiments indicated that the kinase activity of Brk was not required for HGF-induced cell migration. Further, expression of either wild-type or kinase-inactive Brk in Brk-null MDA-MB-435 cells activated ERK5 and conferred increased HGF-induced cell migration.

Conclusions

These results have identified Brk and ERK5 as important downstream effectors of Met signaling to cell migration. Targeting ERK5 kinase activity or inhibiting the formation of Brk/ERK5 complexes may provide an additional means of blocking cell migration associated with breast cancer progression to metastasis.

Protein tyrosine kinase 6 directly phosphorylates AKT and promotes AKT activation in response to epidermal growth factor

Protein tyrosine kinase 6 (PTK6) is a nonmyristoylated Src-related intracellular tyrosine kinase. Although not expressed in the normal mammary gland, PTK6 is expressed in a majority of human breast tumors examined, and it has been linked to ErbB receptor signaling and AKT activation. Here we demonstrate that AKT is a direct substrate of PTK6 and that AKT tyrosine residues 315 and 326 are phosphorylated by PTK6. Association of PTK6 with AKT occurs through the SH3 domain of PTK6 and is enhanced through SH2 domain-mediated interactions following tyrosine phosphorylation of AKT. Using Src, Yes, and Fyn null mouse embryonic fibroblasts (SYF cells), we show that PTK6 phosphorylates AKT in a Src family kinase-independent manner. Introduction of PTK6 into SYF cells sensitized these cells to physiological levels of epidermal growth factor (EGF) and increased AKT activation. Stable introduction of active PTK6 into SYF cells also resulted in increased proliferation. Knockdown of PTK6 in the BPH-1 human prostate epithelial cell line led to decreased AKT activation in response to EGF. Our data indicate that in addition to promoting growth factor receptor-mediated activation of AKT, PTK6 can directly activate AKT to promote oncogenic signaling.

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.

RAKing in AKT: a tumor suppressor function for the intracellular tyrosine kinase FRK

The Fyn related kinase FRK, originally called RAK, is a member of a small family of intracellular Src-related tyrosine kinases that includes PTK6 and Srms. These kinases share a conserved gene structure that is distinct from that of the Src family. Expression of FRK and PTK6 was originally identified in melanoma, breast cancer cells and normal intestinal epithelium, and both FRK and PTK6 have been implicated in the regulation of epithelial cell differentiation and apoptosis. Recently FRK was reported to phosphorylate the tumor suppressor PTEN (phosphatase and tensin homolog deleted from chromosome 10), a negative regulator of phosphatidylinositol 3 kinase (PI3K) signaling and AKT activation. FRK-mediated tyrosine phosphorylation of PTEN suppressed its association with NEDD4-1, an E3 ubiquitin ligase that may target it for polyubiquitination and proteosomal degradation. As a positive regulator of PTEN, FRK suppresses AKT signaling and inhibits breast cancer cell tumorgenicity in xenograft models. Both FRK and the related tyrosine kinase PTK6 appear to have multiple context-dependent functions, including the ability to regulate AKT. Although PTK6 negatively regulates AKT signaling in normal tissues in vivo, it may enhance AKT signaling in breast cancer cells. In contrast, FRK, which is expressed in the normal mammary gland but lost in some breast tumors, has tumor suppressor functions in mammary gland cells.

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.

Overexpression of PTK6 (breast tumor kinase) protein--a prognostic factor for long-term breast cancer survival--is not due to gene amplification

In a previous retrospective study, we demonstrated the prognostic value of protein tyrosine kinase 6 (PTK6) protein expression in breast carcinomas. Here, we analyzed PTK6 gene amplification using fluorescence in situ hybridization technique in a cohort of 426 invasive breast carcinomas and compared it with PTK6 expression level as well as with the clinical outcome of patients. Forty-five percent of tumors show increased PTK6 gene copy numbers when compared to normal tissue. Most of these, however, were related to chromosome 20 polysomy (30%), while gene amplification accounted for only 15%. Only "low level" amplification of the PTK6 gene, with up to eight signals per nucleus, was found. The PTK6 cytogenetic status (normal, gene amplification, polysomy 20) was not associated with histopathological parameters or with the protein expression of HER receptors. No statistical association was identified between PTK6 gene status and expression level. Further, the PTK6 gene status does not influence the disease-free survival of patients at > or = 240 months. Based on these results, we state that the PTK6 overexpression is not essentially attributed to gene amplification, and the PTK6 protein expression-but not gene status-is of prognostic value in breast carcinomas. PTK6 protein overexpression may result from polysomy 20 in a minority of the tumors. In a marked proportion of tumors, however, the overexpression is likely to be caused by posttranscriptional regulation mechanisms.

Prognostic value of protein tyrosine kinase 6 (PTK6) for long-term survival of breast cancer patients

The cytoplasmic tyrosine kinase PTK6 (BRK) shows elevated expression in approximately two-thirds of primary breast tumours, and is implicated in EGF receptor-dependent signalling and epithelial tumorigenesis. Using immunohistochemistry, we performed a retrospective study on 426 archival breast cancer samples from patients with long-term follow-up and compared the protein expression levels of PTK6, the HER receptors, Sam68 (a substrate of PTK6), and signalling proteins including MAP kinase (MAPK), phosphorylated MAPK (P-MAPK), and PTEN. We show that PTK6 expression is of significant prognostic value in the outcome of breast carcinomas. In multivariate analysis, the disease-free survival of patients of >or=240 months was directly associated with the protein expression level of PTK6 (P<or=0.001), but was also inversely associated with nodal status (P<or=0.001) and tumour size (P<or=0.01). PTK6 expression in tumour tissue significantly correlated (P<or=0.05) with the expression of PTEN, MAPK, P-MAPK, and Sam68. To investigate whether these correlations may be due to molecular interactions between PTK6 and these proteins, we used protein extracts from the T47D cell line for immunoprecipitation and western blot analysis. By this, interactions could be demonstrated between PTK6 and MAPK, P-MAPK, HER2/neu, HER3, HER4, PTEN, and Sam68. On the basis of these results, we suggest that PTK6 may serve as a future target for the development of novel treatments in breast cancer.

Signal transducer and activator of transcription 5b: a new target of breast tumor kinase/protein tyrosine kinase 6

Introduction

Signal transducers and activators of transcription (STATs) are mediators of cytokine and growth factor signaling. In recent years, STAT5b has emerged as a key regulator of tumorigenesis. STAT5b phosphorylation and activation is mediated by several kinases known to be overexpressed in breast cancer, such as epidermal growth factor receptor, HER2, and c-Src. Breast tumor kinase (Brk), also known as protein tyrosine kinase 6, is a nonreceptor tyrosine kinase expressed in more than 60% of breast cancers. Only a few substrates of the Brk tyrosine kinase have been identified, the most recent being STAT3. In the present article we investigate the potential role of Brk in the phosphorylation and activation STAT5b.

Methods

To determine whether Brk can phosphorylate STAT5b, transient transfection and in vitro kinase assays were performed. Luciferase reporter assays were used to measure Brk-induced STAT5b transcriptional activity. siRNA technology was utilized to investigate the biological significance of Brk-induced activation of STAT5b in breast cancer cell models.

Results

Phosphospecific antibodies, mutational analysis, and in vitro kinase assays demonstrated that Brk specifically mediated STAT5b phosphorylation at the activating tyrosine, Y699. Transient transfection of Brk into the Brk-negative BT-549 breast cancer cell line enhanced STAT5b transcriptional activity, as measured by a STAT5-specific luciferase reporter. Furthermore, overexpression of kinase active c-Src enhanced Brk-induced STAT5b transcriptional activity. In Brk-positive breast cancer cell lines BT-20 and SKBr3, knockdown of Brk protein or of STAT5b protein using siRNA methodology resulted in a decrease in DNA synthesis. Knockdown of Brk and STAT5b together did not further decrease DNA synthesis compared with each alone, suggesting that Brk and STAT5b converge on the same pathway, ultimately leading to cellular proliferation.

Conclusion

Our studies demonstrate that Brk phosphorylates STAT5b on Y699, leading to increased STAT5b transcriptional activity. Furthermore, analysis of DNA synthesis suggests that STAT5b and Brk are converging upon the same proproliferative signaling pathway in breast cancer cells. We propose that Brk, like other tyrosine kinases, signals downstream to STAT5b to mediate proliferation of breast cancer cells. These results further establish STAT5b as well as Brk as potential targets for breast cancer therapy.

Distinct functions of natural ADAM-15 cytoplasmic domain variants in human mammary carcinoma

Adamalysins [a disintegrin and metalloproteinase (ADAM)] are a family of cell surface transmembrane proteins that have broad biological functions encompassing proteolysis, adhesion, and cell signal regulation. We previously showed that the cytoplasmic domain of ADAM-15 interacts with Src family protein tyrosine kinases and the adaptor protein growth factor receptor binding protein 2 (Grb2). In the present study, we have cloned and characterized four alternatively spliced forms of ADAM-15, which differ only in their cytoplasmic domains. We show that the four ADAM-15 variants were differentially expressed in human mammary carcinoma tissues compared with normal breast. The expression of the individual isoforms did not correlate with age, menopausal status, tumor size or grade, nodal status, Nottingham Prognostic Index, or steroid hormone receptor status. However, higher levels of two isoforms (ADAM-15A and ADAM-5B) were associated with poorer relapse-free survival in node-negative patients, whereas elevated ADAM-15C correlated with better relapse-free survival in node-positive, but not in node-negative, patients. The expression of ADAM-15A and ADAM-15B variants in MDA-MB-435 cells had differential effects on cell morphology, with adhesion, migration, and invasion enhanced by expression of ADAM-15A, whereas ADAM-15B led to reduced adhesion. Using glutathione S-transferase pull-down assays, we showed that the cytoplasmic domains of ADAM-15A, ADAM-15B, and ADAM-15C show equivalent abilities to interact with extracellular signal-regulated kinase and the adaptor molecules Grb2 and Tks5/Fish, but associate in an isoform-specific fashion with Nck and the Src and Brk tyrosine kinases. These data indicate that selective expression of ADAM-15 variants in breast cancers could play an important role in determining tumor aggressiveness by interplay with intracellular signaling pathways.

Targeting the RNA-binding protein Sam68 as a treatment for cancer?

The contradictory properties of RNA-binding proteins (RBPs) have mystified their roles in human diseases including cancer. Are certain RBPs oncogenes or tumor suppressors? In the case of the signal transduction activator of RNA metabolism (STAR) family of hnRNP K homology (KH)-domain-containing RBPs, the dominant view with loose experimental evidence is that these proteins are tumor suppressors. However, recent developments support a pro-oncogenic role for archetypical STAR protein Sam68. Sam68-null mice are not prone to cancer, but instead display pronounced defects in mammary gland ductal development, and haploinsufficiency of Sam68 impedes mammary tumor onset and tumor multiplicity in mouse models expressing the mammary-targeted polyoma middle T antigen oncogene. These advances have increased the interest in the role of Sam68 as a positive regulator of cancer progression and position Sam68 as a viable therapeutic target. Retrospective and perspective implications of Sam68 in cancer are discussed.