A role for the epithelial-cell-specific tyrosine kinase Sik during keratinocyte differentiation

Therapeutic Potential of Protein Tyrosine Kinase 6 in Colorectal Cancer

PTK6, a non-receptor tyrosine kinase, modulates the pathogenesis of breast and prostate cancers and is recognized as a biomarker of breast cancer prognosis. There are over 30 known substrates of PTK6, including signal transducers, transcription factors, and RNA-binding proteins. Many of these substrates are known drivers of other cancer types, such as colorectal cancer. Colon and rectal tumors also express higher levels of PTK6 than the normal intestine suggesting a potential role in tumorigenesis. However, the importance of PTK6 in colorectal cancer remains unclear. PTK6 inhibitors such as XMU-MP-2 and Tilfrinib have demonstrated potency and selectivity in breast cancer cells when used in combination with chemotherapy, indicating the potential for PTK6 targeted therapy in cancer. However, most of these inhibitors are yet to be tested in other cancer types. Here, we discuss the current understanding of the function of PTK6 in normal intestinal cells compared with colorectal cancer cells. We review existing PTK6 targeting therapeutics and explore the possibility of PTK6 inhibitory therapy for colorectal cancer.

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.

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.

Vemurafenib Inhibits Active PTK6 in PTEN-null Prostate Tumor Cells

Protein tyrosine kinase 6 (PTK6, also called BRK) is overexpressed and activated in human prostate cancer. Loss of the tumor suppressor PTEN, a frequent event in prostate cancer, leads to PTK6 activation at the plasma membrane and its oncogenic signaling. The small molecule inhibitor vemurafenib, also known as PLX4032, and its tool analog PLX4720 were designed to inhibit constitutively active BRAF V600E, yet they also have potent effects against PTK6. Vemurafenib is used in the treatment of metastatic melanoma, but its efficacy in prostate cancer has not been assessed. When activated at the plasma membrane, PTK6 promotes signaling through FAK, EGFR, and ERK1/2, and we show this can be blocked by vemurafenib. In addition, PTK6-mediated cell growth, migration, and invasion are inhibited upon vemurafenib administration. Using a flank xenograft model, vemurafenib treatment reduced tumor burden. Using saturation transfer difference NMR and molecular docking, we demonstrate that vemurafenib binds in the active site of PTK6, inhibiting its activation. These structural studies provide insight into the PTK6-vemurafenib complex, which can be utilized for further refinement chemistry, whereas functional studies demonstrate that active PTK6 is a viable drug target in prostate cancer.

PTEN is a protein phosphatase that targets active PTK6 and inhibits PTK6 oncogenic signaling in prostate cancer

PTEN activity is often lost in prostate cancer. We show that the tyrosine kinase PTK6 (BRK) is a PTEN substrate. Phosphorylation of PTK6 tyrosine 342 (PY342) promotes activation, while phosphorylation of tyrosine 447 (PY447) regulates auto-inhibition. Introduction of PTEN into a PTEN null prostate cancer cell line leads to dephosphorylation of PY342 but not PY447 and PTK6 inhibition. Conversely, PTEN knockdown promotes PTK6 activation in PTEN positive cells. Using a variety of PTEN mutant constructs, we show that protein phosphatase activity of PTEN targets PTK6, with efficiency similar to PTP1B, a phosphatase that directly dephosphorylates PTK6 Y342. Conditional disruption of Pten in the mouse prostate leads to tumorigenesis and increased phosphorylation of PTK6 Y342, and disruption of Ptk6 impairs tumorigenesis. In human prostate tumor tissue microarrays, loss of PTEN correlates with increased PTK6 PY342 and poor outcome. These data suggest PTK6 activation promotes invasive prostate cancer induced by PTEN loss.

PTEN is often lost in prostate cancer. In this study, the authors show that PTEN can act as a protein phosphatase that targets active PTK6, thereby regulating its oncogenic signaling in prostate cancer progression.

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.

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.

Brk/Protein tyrosine kinase 6 phosphorylates p27KIP1, regulating the activity of cyclin D-cyclin-dependent kinase 4

Cyclin D and cyclin-dependent kinase 4 (cdk4) are overexpressed in a variety of tumors, but their levels are not accurate indicators of oncogenic activity because an accessory factor such as p27(Kip1) is required to assemble this unstable dimer. Additionally, tyrosine (Y) phosphorylation of p27 (pY88) is required to activate cdk4, acting as an "on/off switch." We identified two SH3 recruitment domains within p27 that modulate pY88, thereby modulating cdk4 activity. Via an SH3-PXXP interaction screen, we identified Brk (breast tumor-related kinase) as a high-affinity p27 kinase. Modulation of Brk in breast cancer cells modulates pY88 and increases resistance to the cdk4 inhibitor PD 0332991. An alternatively spliced form of Brk (Alt Brk) which contains its SH3 domain blocks pY88 and acts as an endogenous cdk4 inhibitor, identifying a potentially targetable regulatory region within p27. Brk is overexpressed in 60% of breast carcinomas, suggesting that this facilitates cell cycle progression by modulating cdk4 through p27 Y phosphorylation. p27 has been considered a tumor suppressor, but our data strengthen the idea that it should also be considered an oncoprotein, responsible for cyclin D-cdk4 activity.

Protein Tyrosine Kinase 6 Regulates UVB-Induced Signaling and Tumorigenesis in Mouse Skin

Protein Tyrosine Kinase 6 (PTK6, also called BRK) is an intracellular tyrosine kinase expressed in the epithelial linings of the gastrointestinal tract and skin, where it is expressed in nondividing differentiated cells. We found PTK6 expression increases in the epidermis following UVB treatment. To evaluate the roles of PTK6 in the skin following UVB-induced damage, we exposed back skin of Ptk6 +/+ and Ptk6−/− SENCAR mice to incremental doses of UVB for thirty weeks. Wild type mice were more sensitive to UVB and exhibited increased inflammation and greater activation of STAT3 than Ptk6−/− mice. Disruption of Ptk6 did not have an impact on proliferation, although PTK6 was expressed and activated in basal epithelial cells in wild type mice following UVB treatment. However, wild type mice exhibited shortened tumor latency and increased tumor load compared with Ptk6−/− mice, and STAT3 activation was increased in these tumors. PTK6 activation was detected in UVB-induced tumors, and this correlated with increased activating phosphorylation of FAK and BCAR1. Activation of PTK6 was also detected in human squamous cell carcinomas of the skin. Although PTK6 plays roles in normal differentiation, it also contributes to UVB induced injury and tumorigenesis in vivo.

Downregulated expression of PTK6 is correlated with poor survival in esophageal squamous cell carcinoma

To investigate the clinical prognostic value of protein tyrosine kinase 6 (PTK6) in patients with esophageal squamous cell carcinoma (ESCC), quantitative RT-PCR and Western blotting were utilized to measure the mRNA and protein expression levels of PTK6 in 29 and eight pairs of ESCC and peritumoral normal esophageal tissues, respectively. Furthermore, the expression of PTK6 protein in 210 ESCCs was examined with immunohistochemistry (IHC), and its clinical value was analyzed using Kaplan-Meier plots and the Cox proportional hazards regression model. The results found that the expression levels of both PTK6 mRNA and protein in ESCC tissues were significantly lower than those in peritumoral normal esophageal tissues. Regarding the IHC analysis of ESCC, the cytoplasmic expression of PTK6 was significantly correlated with tumor grade (P < 0.001). Compared with patients with low PTK6 expression, ESCC patients with overexpression of PTK6 displayed preferable disease-free survival (DFS) and overall survival (OS) (P < 0.001 and P = 0.001, respectively), especially in stage II disease (P = 0.002 and P = 0.021, respectively). PTK6 was evaluated as an independent prognostic factor for ESCC using multivariate Cox regression analysis. All data demonstrated that the expression level of PTK6 is an independent prognostic factor in ESCCs. Low expression of PTK6 is correlated with poor DFS and OS in ESCCs.

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.

PTK6 activation at the membrane regulates epithelial-mesenchymal transition in prostate cancer

The intracellular tyrosine kinase protein tyrosine kinase 6 (PTK6) lacks a membrane-targeting SH4 domain and localizes to the nuclei of normal prostate epithelial cells. However, PTK6 translocates from the nucleus to the cytoplasm in human prostate tumor cells. Here, we show that while PTK6 is located primarily within the cytoplasm, the pool of active PTK6 in prostate cancer cells localizes to membranes. Ectopic expression of membrane-targeted active PTK6 promoted epithelial-mesenchymal transition in part by enhancing activation of AKT, thereby stimulating cancer cell migration and metastases in xenograft models of prostate cancer. Conversely, siRNA-mediated silencing of endogenous PTK6 promoted an epithelial phenotype and impaired tumor xenograft growth. In mice, PTEN deficiency caused endogenous active PTK6 to localize at membranes in association with decreased E-cadherin expression. Active PTK6 was detected at membranes in some high-grade human prostate tumors, and PTK6 and E-cadherin expression levels were inversely correlated in human prostate cancers. In addition, high levels of PTK6 expression predicted poor prognosis in patients with prostate cancer. Our findings reveal novel functions for PTK6 in the pathophysiology of prostate cancer, and they define this kinase as a candidate therapeutic target. Cancer Res; 73(17); 5426-37. ©2013 AACR.

Protein tyrosine kinase 6 is associated with nasopharyngeal carcinoma poor prognosis and metastasis

Background

The aim of this study was to analyze the expression of protein tyrosine kinase 6 (PTK6) in nasopharyngeal carcinoma (NPC) samples, and to identify whether PTK6 can serve as a biomarker for the diagnosis and prognosis of NPC.

Methods

We used quantitative RT-PCR and Western blotting analysis to detect mRNA and protein expression of PTK6 in NPC cell lines and immortalized nasopharyngeal epithelial cell lines. 31 NPC and 16 non-tumorous nasopharyngeal mucosa biopsies were collected to detect the difference in the expression of mRNA level of PTK6 by quantitative RT-PCR. We also collected 178 NPC and 10 normal nasopharyngeal epithelial cases with clinical follow-up data to investigate the expression of PTK6 by immunohistochemistry staining (IHC). PTK6 overexpression on cell growth and colony formation ability were measured by the method of cell proliferation assay and colony formation assay.

Results

The expression of PTK6 was higher in most of NPC cell lines at both mRNA and protein levels than in immortalized nasopharyngeal epithelial cell lines (NPECs) induced by Bmi-1 (Bmi-1/NPEC1, and Bmi-1/NPEC2). The mRNA level of PTK6 was high in NPC biopsies compared to non-tumorous nasopharyngeal mucosa biopsies. IHC results showed the expression of PTK6 was significantly correlated to tumor size (P<0.001), clinical stage (P<0.001), and metastasis (P=0.016). The patients with high-expression of PTK6 had a significantly poor prognosis compared to those of low-expression (47.8% versus 80.0%, P<0.001), especially in the patients at the advanced stages (42.2% versus 79.1%, P<0.001). Multivariate analysis indicated that the level of PTK6 expression was an independent prognostic factor for the overall survival of patients with NPC (P <0.001). Overexpression of PTK6 in HNE1 cells enhanced the ability of cell proliferation and colony formation.

Conclusions

Our results suggest that high-expression of PTK6 is an independent factor for NPC patients and it might serve as a potential prognostic biomarker for patients with NPC.

Context-specific protein tyrosine kinase 6 (PTK6) signalling in prostate cancer

BACKGROUND

Protein tyrosine kinase 6 (PTK6) is an intracellular tyrosine kinase that is distantly related to SRC family kinases. PTK6 is nuclear in normal prostate epithelia, but nuclear localization is lost in prostate tumours. Increased expression of PTK6 is detected in human prostate cancer, especially at metastatic stages, and in other types of cancers, including breast, colon, head and neck cancers, and serous carcinoma of the ovary.

MATERIALS AND METHODS

Potential novel substrates of PTK6 identified by mass spectrometry were validated in vitro. The significance of PTK6-induced phosphorylation of these substrates was addressed using human prostate cell lines by knockdown of endogenous PTK6 or overexpression of targeted PTK6 to different intracellular compartments.

RESULTS

We identified AKT, p130CAS and focal adhesion kinase (FAK) as novel PTK6 substrates and demonstrated their roles in promoting cell proliferation, migration and resistance to anoikis. In prostate cancer cells, active PTK6 is primarily associated with membrane compartments, although the majority of total PTK6 is localized within the cytoplasm. Ectopic expression of membrane-targeted PTK6 transforms immortalized fibroblasts. Knockdown of endogenous cytoplasmic PTK6 in PC3 prostate cancer cells impairs proliferation, migration and anoikis resistance. However, re-introduction of PTK6 into the nucleus significantly decreases cell proliferation, suggesting context-specific functions for nuclear PTK6.

CONCLUSIONS

In human prostate cancer, elevated PTK6 expression, translocation of PTK6 from the nucleus to the cytoplasm and its activation at the plasma membrane contribute to increased phosphorylation and activation of its substrates such as AKT, p130CAS and FAK, thereby promoting prostate cancer progression.

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.

Mechanisms of HGF/Met signaling to Brk and Sam68 in breast cancer progression

Signal transduction pathways downstream of receptor tyrosine kinases (RTKs) are often deregulated during oncogenesis, tumor progression, and metastasis. In particular, the peptide growth factor hormone, hepatocyte growth factor (HGF), and its specific receptor, Met tyrosine kinase, regulate cancer cell migration, thereby conferring an aggressive phenotype (Nakamura et al., J Clin Invest 106(12):1511-1519, 2000; Huh et al., Proc Natl Acad Sci U S A 101:4477-4482, 2004). Additionally, overexpression of Met is associated with enhanced invasiveness of breast cancer cells (Edakuni et al., Pathol Int 51(3):172-178, 2001; Jin et al., Cancer 79(4):749-760, 1997; Tuck et al., Am J Pathol 148(1):225-232, 1996). Here, we review the regulation of recently identified novel downstream mediators of HGF/Met signaling, Breast tumor kinase (Brk/PTK6), and Src-associated substrate during mitosis of 68 kDa (Sam68), and discuss their relevance to mechanisms of breast cancer progression.

Protein-tyrosine kinase 6 promotes peripheral adhesion complex formation and cell migration by phosphorylating p130 CRK-associated substrate

Protein-tyrosine kinase 6 (PTK6) is a non-myristoylated intracellular tyrosine kinase evolutionarily related to Src kinases. Aberrant PTK6 expression and intracellular localization have been detected in human prostate tumors. In the PC3 prostate cancer cell line, the pool of endogenous activated PTK6, which is phosphorylated on tyrosine residue 342, is localized at the membrane. Expression of ectopic membrane-targeted PTK6 led to dramatic morphology changes and formation of peripheral adhesion complexes in PC3 cells. Peripheral adhesion complex formation was dependent upon PTK6 kinase activity. We demonstrated that p130 CRK-associated substrate (p130CAS) is a novel direct substrate of PTK6, and it works as a crucial adapter protein in inducing peripheral adhesion complexes. Activation of ERK5 downstream of p130CAS was indispensable for this process. Knockdown of endogenous PTK6 led to reduced cell migration and p130CAS phosphorylation, whereas knockdown of p130CAS attenuated oncogenic signaling induced by membrane-targeted PTK6, including ERK5 and AKT activation. Expression of membrane-targeted PTK6 promoted cell migration, which could be impaired by knockdown of p130CAS or ERK5. Our study reveals a novel function for PTK6 at the plasma membrane and suggests that the PTK6-p130CAS-ERK5 signaling cascade plays an important role in cancer cell migration and invasion.

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.

The alternative splice variant of protein tyrosine kinase 6 negatively regulates growth and enhances PTK6-mediated inhibition of β-catenin

Protein tyrosine kinase 6 (PTK6), also called breast tumor kinase (BRK), is expressed in epithelial cells of various tissues including the prostate. Previously it was shown that PTK6 is localized to epithelial cell nuclei in normal prostate, but becomes cytoplasmic in human prostate tumors. PTK6 is also primarily cytoplasmic in the PC3 prostate adenocarcinoma cell line. Sequencing revealed expression of wild type full-length PTK6 transcripts in addition to an alternative transcript lacking exon 2 in PC3 cells. The alternative transcript encodes a 134 amino acid protein, referred to here as ALT-PTK6, which shares the first 77 amino acid residues including the SH3 domain with full length PTK6. RT-PCR was used to show that ALT-PTK6 is coexpressed with full length PTK6 in established human prostate and colon cell lines, as well as in primary cell lines derived from human prostate tissue and tumors. Although interaction between full-length PTK6 and ALT-PTK6 was not detected, ALT-PTK6 associates with the known PTK6 substrates Sam68 and β-catenin in GST pull-down assays. Coexpression of PTK6 and ALT-PTK6 led to suppression of PTK6 activity and reduced association of PTK6 with tyrosine phosphorylated proteins. While ALT-PTK6 alone did not influence β-catenin/TCF transcriptional activity in a luciferase reporter assay, it enhanced PTK6-mediated inhibition of β-catenin/TCF transcription by promoting PTK6 nuclear functions. Ectopic expression of ALT-PTK6 led to reduced expression of the β-catenin/TCF targets Cyclin D1 and c-Myc in PC3 cells. Expression of tetracycline-inducible ALT-PTK6 blocked the proliferation and colony formation of PC3 cells. Our findings suggest that ALT-PTK6 is able to negatively regulate growth and modulate PTK6 activity, protein-protein associations and/or subcellular localization. Fully understanding functions of ALT-PTK6 and its impact on PTK6 signaling will be critical for development of therapeutic strategies that target PTK6 in cancer.

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.

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.

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.

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.

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.

Identification of beta-catenin as a target of the intracellular tyrosine kinase PTK6

Disruption of the gene encoding protein tyrosine kinase 6 (PTK6) leads to increased growth, impaired enterocyte differentiation and higher levels of nuclear beta-catenin in the mouse small intestine. Here, we demonstrate that PTK6 associates with nuclear and cytoplasmic beta-catenin and inhibits beta-catenin- and T-cell factor (TCF)-mediated transcription. PTK6 directly phosphorylates beta-catenin on Tyr64, Tyr142, Tyr331 and/or Tyr333, with the predominant site being Tyr64. However, mutation of these sites does not abrogate the ability of PTK6 to inhibit beta-catenin transcriptional activity. Outcomes of PTK6-mediated regulation appear to be dependent on its intracellular localization. In the SW620 colorectal adenocarcinoma cell line, nuclear-targeted PTK6 negatively regulates endogenous beta-catenin/TCF transcriptional activity, whereas membrane-targeted PTK6 enhances beta-catenin/TCF regulated transcription. Levels of TCF4 and the transcriptional co-repressor TLE/Groucho increase in SW620 cells expressing nuclear-targeted PTK6. Knockdown of PTK6 in SW620 cells leads to increased beta-catenin/TCF transcriptional activity and increased expression of beta-catenin/TCF target genes Myc and Survivin. Ptk6-null BAT-GAL mice, containing a beta-catenin-activated LacZ reporter transgene, have increased levels of beta-galactosidase expression in the gastrointestinal tract. The ability of PTK6 to negatively regulate beta-catenin/TCF transcription by modulating levels of TCF4 and TLE/Groucho could contribute to its growth-inhibitory activities in vivo.

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.

Induction of protein tyrosine kinase 6 in mouse intestinal crypt epithelial cells promotes DNA damage-induced apoptosis

BACKGROUND & AIMS

Protein tyrosine kinase 6 (PTK6) is expressed in epithelial linings of the gastrointestinal tract. PTK6 sensitizes the nontransformed Rat1a fibroblast cell line to apoptotic stimuli. The aim of this study was to determine if PTK6 regulates apoptosis in vivo after DNA damage in the small intestine.

METHODS

Wild-type and Ptk6(-/-) mice were subjected to gamma-irradiation; intestinal tissues were collected, protein was isolated, and samples were fixed for immunohistochemical analyses at 0, 6, and 72 hours after the mice were irradiated. Expression of PTK6 was examined in the small intestine before and after irradiation. Apoptosis and proliferation were compared between wild-type and Ptk6(-/-) mice. Expression and activation of prosurvival signaling proteins were assessed.

RESULTS

Irradiation induced PTK6 in crypt epithelial cells of the small intestine in wild-type mice. Induction of PTK6 corresponded with DNA damage-induced apoptosis in the wild-type small intestine. Following irradiation, the apoptotic response was impaired in the intestinal crypts of Ptk6(-/-) mice. Increased activation of AKT and extracellular signal-regulated kinase (ERK)1/2 and increased inhibitory phosphorylation of the proapoptotic protein BAD were detected in Ptk6(-/-) mice after irradiation. In response to the induction of apoptosis, compensatory proliferation increased in the small intestines of wild-type mice but not in Ptk6(-/-) mice at 6 hours after irradiation.

CONCLUSIONS

PTK6 is a stress-induced kinase that promotes apoptosis by inhibiting prosurvival signaling. After DNA damage, induction of PTK6 is required for efficient apoptosis and inhibition of AKT and ERK1/2.

Breast tumor kinase (protein tyrosine kinase 6) regulates heregulin-induced activation of ERK5 and p38 MAP kinases in breast cancer cells

Total tyrosine kinase activity is often elevated in both cytosolic and membrane fractions of malignant breast tissue and correlates with a decrease in disease-free survival. Breast tumor kinase (Brk; protein tyrosine kinase 6) is a soluble tyrosine kinase that was cloned from a metastatic breast tumor and found to be overexpressed in a majority of breast tumors. Herein, we show that Brk is overexpressed in 86% of invasive ductal breast tumors and coexpressed with ErbB family members in breast cancer cell lines. Additionally, the ErbB ligand, heregulin, activates Brk kinase activity. Knockdown of Brk by stable expression of short hairpin RNA (shRNA) in T47D breast cancer cells decreases proliferation and blocks epidermal growth factor (EGF)- and heregulin-induced activation of Rac GTPase, extracellular signal-regulated kinase (ERK) 5, and p38 mitogen-activated protein kinase (MAPK) but not Akt, ERK1/2, or c-Jun NH(2)-terminal kinase. Furthermore, EGF- and heregulin-induced cyclin D1 expression is dependent on p38 signaling and inhibited by Brk shRNA knockdown. The myocyte enhancer factor 2 transcription factor target of p38 MAPK and ERK5 signaling is also sensitive to altered Brk expression. Finally, heregulin-induced migration of T47D cells requires p38 MAPK activity and is blocked by Brk knockdown. These results place Brk in a novel signaling pathway downstream of ErbB receptors and upstream of Rac, p38 MAPK, and ERK5 and establish the ErbB-Brk-Rac-p38 MAPK pathway as a critical mediator of breast cancer cell migration.

Protein tyrosine kinase 6 negatively regulates growth and promotes enterocyte differentiation in the small intestine

Protein tyrosine kinase 6 (PTK6) (also called Brk or Sik) is an intracellular tyrosine kinase that is expressed in breast cancer and normal epithelial linings. In adult mice, PTK6 expression is high in villus epithelial cells of the small intestine. To explore functions of PTK6, we disrupted the mouse Ptk6 gene. We detected longer villi, an expanded zone of PCNA expression, and increased bromodeoxyuridine incorporation in the PTK6-deficient small intestine. Although differentiation of major epithelial cell types occurred, there was a marked delay in expression of intestinal fatty acid binding protein, suggesting a role for PTK6 in enterocyte differentiation. However, fat absorption was comparable in wild-type and Ptk6-/- mice. It was previously shown that the serine threonine kinase Akt is a substrate of PTK6 and that PTK6-mediated phosphorylation of Akt on tyrosine resulted in inhibition of Akt activity. Consistent with these findings, we detected increased Akt activity and nuclear beta-catenin in intestines of PTK6-deficient mice and decreased nuclear localization of the Akt substrate FoxO1 in villus epithelial cells. PTK6 contributes to maintenance of tissue homeostasis through negative regulation of Akt in the small intestine and is associated with cell cycle exit and differentiation in normal intestinal epithelial cells.

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.

Role of breast tumour kinase in the in vitro differentiation of HaCaT cells

BACKGROUND

Breast tumour kinase (BRK) is a newly identified non-receptor protein tyrosine kinase from a metastatic breast tumour. Its biological functions are still under extensive investigation. The mouse homologue Sik (Src-related intestinal kinase) has been implicated in mouse keratinocyte differentiation; however, not much is known about the functions of BRK in human cutaneous biology.

OBJECTIVES

Using HaCaT cells as an experimental model, to explore the mutual relationships between BRK and differentiation of human keratinocytes.

METHODS

Archival paraffin blocks of normal and pathological skin were retrieved for examining the in vivo distribution of BRK. Its expression and subcellular localization were examined via indirect immunofluorescence, and quantitative changes were analysed by Northern and Western blots. The kinase activity of BRK was determined by its autophosphorylation and phosphorylation of exogenous substrate in the in vitro kinase assay. Using a retroviral infection method, we established stably transfected HaCaT cells expressing vector, wild-type BRK or a kinase-defective mutant (K219M). Expression of the differentiation marker keratin 10 (K10) was compared among these cells using semiquantitative reverse transcription-polymerase chain reaction. Results Histochemical examination showed that BRK was expressed exclusively in suprabasal keratinocytes. Its distribution was both cytoplasmic and intranuclear. An enhanced regional suprabasal expression pattern was observed in the confluent areas of cell cultures. The expression of BRK transcript and protein was up-regulated in prolonged confluence culture in a serum-dependent manner. Its kinase activity was activated shortly after the addition of calcium and ionomycin and returned to the basal level within 30 min. Overexpression of wild-type BRK moderately promoted the expression of K10 transcript while the kinase-defective BRK mutant exerted a prominent suppressive effect.

CONCLUSIONS

The in vivo distribution of BRK and its up-regulation during in vitro differentiation of HaCaT cells, together with the activation of its kinase activity by calcium/ionomycin and its influence on K10 expression, all indicate a role for BRK in the complex process of keratinocyte differentiation.

Tyrosine phosphorylation of sam68 by breast tumor kinase regulates intranuclear localization and cell cycle progression

The breast tumor kinase (BRK) is a growth promoting non-receptor tyrosine kinase overexpressed in the majority of human breast tumors. BRK is known to potentiate the epidermal growth factor (EGF) response in these cells. Although BRK is known to phosphorylate the RNA-binding protein Sam68, the specific tyrosines phosphorylated and the exact role of this phosphorylation remains unknown. Herein, we have generated Sam68 phospho-specific antibodies against C-terminal phosphorylated tyrosine residues within the Sam68 nuclear localization signal. We show that BRK phosphorylates Sam68 on all three tyrosines in the nuclear localization signal. By indirect immunofluorescence we observed that BRK and EGF treatment not only phosphorylates Sam68 but also induces its relocalization. Tyrosine 440 was identified as a principal modulator of Sam68 localization and this site was phosphorylated in response to EGF treatment in human breast tumor cell lines. Moreover, this phosphorylation event was inhibited by BRK small interfering RNA treatment, consistent with Sam68 being a physiological substrate of BRK downstream of the EGF receptor in breast cancer cells. Finally, we observed that Sam68 suppressed BRK-induced cell proliferation, suggesting that Sam68 does indeed contain anti-proliferative properties that may be neutralized in breast cancer cells by phosphorylation.

Interaction between Brk kinase and insulin receptor substrate-4

Breast tumor kinase (Brk) is a member of the Frk family of nonreceptor tyrosine kinases that is overexpressed in a high percentage of human breast tumors. The downstream substrates and effectors of Brk remain largely unidentified. In this study, we carried out immunoprecipitation and mass spectrometry experiments to identify new Brk binding partners. One interacting protein was insulin receptor substrate 4 (IRS-4), a member of the IRS family. We confirmed that Brk associates with IRS-4 in resting and insulin-like growth factor 1 (IGF-1)-stimulated HEK 293 cells. The SH3 and SH2 domains of Brk are both involved in the association. The tyrosine phosphorylation of Brk increases after stimulation with IGF-1, and in MCF-7 breast cancer cells we show that the presence of IRS-4 enhances this effect. Finally, we demonstrate that endogenous Brk and IRS-4 interact in A431 human epidermoid carcinoma cells.

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

Protein-tyrosine kinase-6 (PTK6, also known as Brk) is a non-receptor tyrosine kinase that contains SH3, SH2, and catalytic (Kinase) domains. We have identified an intramolecular interaction between the linker (Linker) region connecting the SH2 and Kinase domains and the Kinase domain. Residue Trp-184 within the Linker region is essential for the Linker-Kinase interaction but not for the Linker-SH3 interaction. A recombinant PTK6 Kinase domain connected to the Linker region had catalytic activity in terms of autophosphorylation, phosphorylation of a PTK6 substrate, BKS, and phosphorylation of an oligopeptide substrate, whereas the Kinase domain itself, or one connected to a Linker region containing a W184A substitution, did not. The introduction of the W184A mutation into PTK6 also abrogated autophosphorylation and phosphorylation of another PTK6 substrate, Sam68, as well as phosphorylation of intracellular proteins. It also abolished the ability of PTK6 to promote proliferation and prevent apoptosis of HEK 293 cells, as well as to permit anchorage-independent colony formation. Therefore, unlike Src family members, in which the Linker-Kinase interaction inhibits catalytic activity, in PTK6 this interaction has an essential positive role.

Notch signaling in the integrated control of keratinocyte growth/differentiation and tumor suppression

Oncogenesis is closely linked to abnormalities in cell differentiation. Notch signaling provides an important form of intercellular communication involved in cell fate determination, stem cell potential and differentiation. Here we review the role of this pathway in the integrated growth/differentiation control of the keratinocyte cell type, and the maintenance of normal skin homeostasis. In parallel with the pro-differentiation function of Notch1 in keratinocytes, we discuss recent evidence pointing to a tumor suppressor function of this gene in both mouse skin and human cervical carcinogenesis. The possibility that Notch signaling elicits signals with a duality of growth positive and negative function will be discussed.

Differential expression of the non-receptor tyrosine kinase BRK in oral squamous cell carcinoma and normal oral epithelium

BRK is a non-receptor tyrosine kinase whose functional role is poorly understood. Although it is an epithelial specific kinase, its expression appears to be tissue specific. To date, little is known about BRK expression in human oral epithelium. We investigated expression of BRK in human oral squamous cell carcinomas (OSCC) and normal oral epithelium (NOE) using immunohistochemistry, laser confocal microscopy and Western blotting. The subcellular localization of BRK was identified by confocal microscopy and Western blotting of nuclear and cytoplasmic extracts from these cells. The results indicate that NOE express higher levels of BRK compared with OSCC cells. In NOE and moderately differentiated OSCC cells, BRK was localized in the nucleus and cytoplasm. However, in poorly differentiated OSCC cells, BRK was localized in perinuclear regions. These results suggest that BRK expression differs in normal and OSCC which may reflect a possible functional involvement in OSCC.

Srcasm modulates EGF and Src-kinase signaling in keratinocytes

The Src-activating and signaling molecule (Srcasm) is a recently described activator and substrate of Src-family tyrosine kinases (SFKs). When phosphorylated at specific tyrosines, Srcasm associates with Grb2 and p85, the regulatory subunit of phosphoinositide 3-kinase; however, little is known about the role of Srcasm in cellular signaling. Data presented here demonstrate that epidermal growth factor (EGF) receptor ligands promote the tyrosine phosphorylation of endogenous and adenovirally transduced Srcasm in keratinocytes, and that increased levels of Srcasm activate endogenous SFKs, with a preference for Fyn and Src. In addition, Srcasm potentiates EGF-dependent signals transmitted by SFKs in keratinocytes. Tyrosine phosphorylation of Srcasm is dependent on growth factors and the activity of EGFR and SFKs. Increased Srcasm expression enhances p44/42 mitogen-activated protein kinase activity and Elk-1-dependent transcriptional events. Elevated Srcasm levels inhibit keratinocyte proliferation while promoting specific aspects of keratinocyte differentiation. Lastly, Srcasm levels are decreased in human cutaneous neoplasia. Collectively, these data demonstrate that Srcasm plays a role in linking EGF receptor- and SFK-dependent signaling to differentiation in keratinocytes.

Regulated association of protein kinase B/Akt with breast tumor kinase

Increased protein-tyrosine kinase activity is a prognostic indicator of decreased disease-free survival in patients with advanced breast tumors. Breast tumor kinase (Brk) is a soluble protein-tyrosine kinase overexpressed in the majority of breast cancers and also in normal skin and gut epithelium, but not in normal breast epithelial cells. Herein, we show that Brk interacts with protein kinase B/Akt, a serine/threonine kinase involved in cell growth and survival. Epidermal growth factor (EGF) treatment of human keratinocytes or Brk-transfected COS-1 cells leads to the dissociation of the Brk.Akt complex, whereas a constitutively active Brk mutant containing a point mutation at Tyr-447 (YF-Brk) failed to dissociate from Akt upon EGF treatment. In addition, Brk.Akt dissociation was blocked by the inhibition of phosphatidylinositol 3-kinase. Similar to ectopic Brk, endogenous Brk in T47D breast cancer cells was less phosphorylated upon EGF treatment, but it remained constitutively associated with Akt in the presence of EGF. Overexpression of wild-type (wt)-Brk, kinase-inactive (KM)-Brk, or YF-Brk increased the Tyr phosphorylation of multiple signaling molecules including EGF receptor. However, only wt- and YF-Brk, but not KM-Brk, induced phosphorylation of Akt and inhibited the kinase activity of Akt in unstimulated cells. Similarly, overexpression of wt- or YF-, but not KM-Brk, blocked the phosphorylation of the forkhead transcription factor, a downstream Akt target. These results suggest that Brk may function as a signaling molecule whose kinase activity normally limits the activity of Akt in unstimulated cells. Additionally, these results suggest that in breast cancer cells Brk behaves similarly to a constitutively active Brk mutant (YF-Brk) and associates with tyrosine-phosphorylated proteins in deregulated signaling complexes. Together these data provide clues to the possible proto-oncogenic and oncogenic functions of Brk.

The nuclear tyrosine kinase BRK/Sik phosphorylates and inhibits the RNA-binding activities of the Sam68-like mammalian proteins SLM-1 and SLM-2

Expression of the intracellular tyrosine kinase BRK/Sik is epithelial-specific and regulated during differentiation. Only a few substrates have been identified for BRK/Sik, including the KH domain containing RNA-binding protein Sam68 and the novel adaptor protein BKS. Although the physiological role of Sam68 is unknown, it has been shown to regulate mRNA transport, pre-mRNA splicing, and polyadenylation. Here we demonstrate that the Sam68-like mammalian proteins SLM-1 and SLM-2 but not the related KH domain containing heterogeneous nuclear ribonucleoprotein K are novel substrates of BRK/Sik. The expression of active BRK/Sik results in increased SLM-1 and SLM-2 phosphorylation and increased retention of BRK/Sik within the nucleus. The phosphorylation of SLM-1 and SLM-2 has functional relevance and leads to inhibition of their RNA-binding abilities. We show that SLM-1, SLM-2, and BRK/Sik have restricted patterns of expression unlike the ubiquitously expressed Sam68. Moreover, BRK/Sik, SLM-1, and Sam68 transcripts were coexpressed in the mouse gastrointestinal tract and skin, suggesting that SLM-1 and Sam68 could be physiologically relevant BRK/Sik targets in vivo. The ability of BRK/Sik to negatively regulate the RNA-binding activities of the KH domain RNA binding proteins SLM-1 and Sam68 may have an impact on the posttranscriptional regulation of epithelial cell gene expression.

Solution structure and backbone dynamics of the non-receptor protein-tyrosine kinase-6 Src homology 2 domain

Human protein-tyrosine kinase-6 (PTK6, also known as breast tumor kinase (Brk)) is a member of the non-receptor protein-tyrosine kinase family and is expressed in two-thirds of all breast tumors. To understand the structural basis of PTK6 function, we have determined the solution structure and backbone dynamics of the PTK6-Src homology 2 (SH2) domain using multidimensional NMR spectroscopy. The solution structure clearly indicates that the SH2 domain of human PTK6 contains a consensus alpha/beta-fold and a Tyr(P) peptide binding surface, which are common to other SH2 domains. However, two of the alpha-helices (alphaA and alphaB) are located on opposite faces of the central beta-sheet. In addition, the topological arrangement of a central four-stranded antiparallel beta-sheet (strands betaA, betaB, betaC, and betaD) differs from that of other Src family members. Backbone dynamics and Tyr(P) peptide titration experiments revealed that the putative ligand binding sites of the PTK6-SH2 domain undergo distinctive internal motions when compared with other regions of the protein. Surface plasmon resonance analysis showed that the Tyr(P) peptide had a dissociation constant of about 60 microm, which is substantially weaker binding than previously reported for Src family members. The solution structure together with data from the ligand binding mode of PTK6-SH2 provides insight into the molecular basis of the autoinhibitory role of PTK6.

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

Brk (PTK6) is a nonreceptor protein tyrosine kinase, which is expressed in over 60% of breast carcinoma tissue samples and breast tumour cell lines, but not normal mammary tissue or benign lesions. Since experimental Brk expression in nontransformed mammary epithelial cells enhances their mitogenic response to epidermal growth factor, it was important to determine the role Brk plays in the proliferation of breast carcinoma cells and validate it as a therapeutic target. We have used RNA interference to efficiently and specifically downregulate Brk protein levels in breast carcinoma cells, and determined that this results in a significant suppression of their proliferation. Additionally, through the expression of a kinase-inactive mutant, we have determined that Brk can mediate promotion of proliferation via a kinase-independent mechanism, potentially functioning as an 'adapter'. These data identify Brk as a novel target for antiproliferative therapy in the majority of breast cancers, and illustrate the power of RNA interference for rapidly validating candidate therapeutic targets.

Altered localization and activity of the intracellular tyrosine kinase BRK/Sik in prostate tumor cells

Breast tumor kinase (BRK) is an intracellular tyrosine kinase expressed in differentiating epithelial cells of the gastrointestinal tract and skin, and in several epithelial cancers including carcinomas of the breast and colon. We examined expression of BRK and its mouse ortholog Src-related intestinal kinase (Sik) in prostate tissues and detected it in the nuclei of normal luminal prostate epithelial cells. BRK localization was then examined in 58 human prostate biopsy samples representing various grades of prostate cancer. While nuclear localization of BRK was present in well-differentiated tumors, it was absent in poorly differentiated tumors. However localization of Sam68, a nuclear substrate of BRK/Sik, was unaltered in all prostate tumors examined. Consistent with these results, nuclear BRK was detected in the more differentiated androgen-responsive LNCaP human prostate cancer cell line that is poorly tumorigenic in host animals, but it was primarily cytoplasmic in the undifferentiated androgen-unresponsive PC3 prostate cancer cell line that forms aggressive tumors. While PC3 cells expressed higher levels of endogenous BRK than LNCaP cells, BRK was less active in these cells. Our data suggest that BRK plays a role in differentiation of prostate epithelial cells. Altered BRK localization and/or activity may provide a prognostic indicator for prostate tumor progression and be a potential target for therapeutic intervention.

Purification and spectroscopic characterization of the human protein tyrosine kinase-6 SH3 domain

The human protein tyrosine kinase-6 (PTK6) polypeptide that is deduced from the cDNA sequence contains a Src homology (SH) 3 domain, SH2 domain, and catalytic domain of tyrosine kinase. We initiated biochemical and NMR characterization of PTK6 SH3 domain in order to correlate the structural role of the PTK6 using circular dichroism and heteronuclear NMR techniques. The circular dichroism data suggested that the secondary structural elements of the SH3 domain are mainly composed of beta-sheet conformations. It is most stable when the pH is neutral based on the pH titration data. In addition, a number of cross peaks at the low-field area of the proton chemical shift of the NMR spectra indicated that the PTK6 SH3 domain retains a unique and folded conformation at the neutral pH condition. For other pH conditions, the SH3 domain became unstable and aggregated during NMR measurements, indicating that the structural stability is very sensitive to pH environments. Both the NMR and circular dichroism data indicate that the PTK6 SH3 domain experiences a conformational instability, even in an aqueous solution.

Characterization of mice deficient in the Src family nonreceptor tyrosine kinase Frk/rak

Frk/rak belongs to a novel family of Src kinases with epithelial tissue-specific expression. Although developmental expression patterns and functional overexpression in vitro have associated these kinases with growth suppression and differentiation, their physiological functions remain largely unknown. We therefore generated mice carrying a null mutation in iyk, the mouse homolog of Frk/rak. We report here that frk/rak(-/-) mice are viable, show similar growth rates to wild-type animals, and are fertile. Furthermore, a 2-year study of health and survival did not identify differences in the incidence and spectrum of spontaneous tumors or provide evidence of hyperplasias in frk/rak(-/-) epithelial tissues. Histological analysis of organs failed to reveal any morphological changes in epithelial tissues that normally express high levels of Frk/rak. Ultrastructural analysis of intestinal enterocytes did not identify defects in brush border morphology or structural polarization, demonstrating that Frk/rak is dispensable for intestinal cytodifferentiation. Additionally, frk/rak-null mice do not display altered sensitivity to intestinal damage induced by ionizing radiation. cDNA microarray analysis revealed an increase in c-src expression and identified subtle changes in the expression of genes regulated by thyroid hormones. Significant decreases in the circulating levels of T3 but not T4 hormone are consistent with this observation and reminiscent of euthyroid sick syndrome, a stress-associated clinical condition.

Characterization of the 5'-flanking region of the human PTK6 gene

PTK6 (also known as Brk) is a non-receptor protein tyrosine kinase, whose mRNA was expressed in the limited normal tissues such as colon and small intestine, and in breast carcinomas and breast cancer cell lines. The 813 bp region upstream from the translation initiation codon, which constitutes a functional promoter of the human PTK6 gene, was progressively deleted and fused to the luciferase reporter gene and transient expression of the resultant constructs was measured upon transfection into a breast carcinoma cell line, T-47D. Comparative analysis of luciferase activity revealed two major regions, -93 to -76 and -702 to -655, important for transcriptional regulation. The proximal -93 to -76 region was found to be essential for the function of the minimal promoter. By primer extension and PCR, it was shown that a PTK6 transcript started at the most 5' upstream is located around base -104. Therefore, the proximal -93 to -76 region is thought to function as a downstream cis-acting element. Luciferase analysis showed that the distal -702 to -655 region contained at least two cis-acting elements. Gel mobility shift assays with T-47D nuclear extract including competition analyses with consensus and mutant oligonucleotides and supershift analyses with NF-kappaB and Sp1 antibodies showed that NF-kappaB binds to the sequence from -706 to -688 and Sp1 binds to the sequence from -688 to -669. This study thus provides the first molecular insights into the transcriptional regulation of the human PTK6 gene.

'Srcasm: a novel Src activating and signaling molecule

The Src family tyrosine kinase, Fyn, can facilitate regulation of cell proliferation and differentiation. Mice with mutations in the fyn gene have defects in the brain, immune system, and epidermal differentiation. To identify molecules that may interact with Fyn in the epidermis, we performed a yeast two-hybrid interaction screen of a murine keratinocyte library. A novel adaptor-like molecule was isolated and termed Srcasm for Src activating and signaling molecule. Murine Srcasm is a 52.7-kDa protein that contains a VHS membrane association domain and a number of tyrosine motifs suggesting that it may be a substrate for Src family kinases and serve as an adaptor protein. Northern blot analysis of murine tissues demonstrates that Srcasm expression is highest in brain and kidney. In situ hybridization analysis reveals that srcasm mRNA is expressed in regions of the epidermis and hair follicle where keratinocyte differentiation occurs. In the brain, srcasm mRNA distribution correlates with that of fyn, with both being highly expressed in the hippocampal and cerebellar Purkinje neurons. Fyn can phosphorylate Srcasm, and association of these molecules relies on cooperative binding between the SH2 and SH3 domains of Fyn and corresponding canonical binding sites in Srcasm. Srcasm is capable of interacting with Grb2 and the regulatory subunit of phosphoinositide 3-kinase, p85, in a phosphorylation-dependent manner. The evidence suggests that Srcasm may help promote Src family kinase signaling in cells.

The protein tyrosine kinase family of the human genome

As the sequencing of the human genome is completed by the Human Genome Project, the analysis of this rich source of information will illuminate many areas in medicine and biology. The protein tyrosine kinases are a large multigene family with particular relevance to many human diseases, including cancer. A search of the human genome for tyrosine kinase coding elements identified several novel genes and enabled the creation of a nonredundant catalog of tyrosine kinase genes. Ninety unique kinase genes can be identified in the human genome, along with five pseudogenes. Of the 90 tyrosine kinases, 58 are receptor type, distributed into 20 subfamilies. The 32 nonreceptor tyrosine kinases can be placed in 10 subfamilies. Additionally, mouse orthologs can be identified for nearly all the human tyrosine kinases. The completion of the human tyrosine kinase family tree provides a framework for further advances in biomedical science.