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

A PROTAC degrader suppresses oncogenic functions of PTK6 inducing apoptosis of breast cancer cells

Protein tyrosine kinase 6 (PTK6), a non-receptor tyrosine kinase, is an oncogenic driver in many tumor types. However, agents that therapeutically target PTK6 are lacking. Although several PTK6 kinase inhibitors have been developed, none have been clinically translated, which may be due to kinase-independent functions that compromise their efficacy. PTK6 kinase inhibitor treatment phenocopies some, but not all effects of PTK6 downregulation. PTK6 downregulation inhibits growth of breast cancer cells, but treatment with PTK6 kinase inhibitor does not. To chemically downregulate PTK6, we designed a PROTAC, MS105, which potently and specifically degrades PTK6. Treatment with MS105, but not PTK6 kinase inhibitor, inhibits growth and induces apoptosis of breast cancer cells, phenocopying the effects of PTK6 (short hairpin RNA) shRNA/CRISPR. In contrast, both MS105 and PTK6 kinase inhibitor effectively inhibit breast cancer cell migration, supporting the differing kinase dependencies of PTK6's oncogenic functions. Our studies support PTK6 degraders as a preferred approach to targeting PTK6 in cancer.

PTK6: An emerging biomarker for prognosis and immunotherapeutic response in clear cell renal carcinoma (KIRC)

Kidney renal clear cell carcinoma (KIRC), one of the most prevalent form of kidney carcinoma, is highly aggressive cancer known for significant immune infiltration and high mortality rates. The absence of sensitivity to traditional therapy has spurred the search for new treatments. Protein Tyrosine Kinase 6 (PTK6) is implicated in promoting cancer growth, spread, and metastasis. Our review of The Cancer Genome Atlas database revealed PTK6 overexpression in KIRC, though its specific role in this cancer type was unclear. We investigated PTK6's cancer-promoting roles in KIRC using the database and confirmed our findings with patient-derived tissues. Our analysis showed that elevated PTK6 expression is linked to worse outcomes and higher levels of immune infiltration. It also correlates positively with neo-antigens (NEO) and DNA ploidy changes in KIRC. This research delves into PTK6's role in KIRC development, suggesting PTK6 as a possible biomarker for prognosis and treatment in KIRC.

MARCH2, a Novel Oncogene-regulated SNAIL E3 Ligase, Suppresses Triple-negative Breast Cancer Metastases

Epithelial-mesenchymal transition (EMT) in cancer promotes metastasis and chemotherapy resistance. A subset of triple-negative breast cancer (TNBC) exhibits a mesenchymal gene signature that is associated with poor patient outcomes. We previously identified PTK6 tyrosine kinase as an oncogenic driver of EMT in a subset of TNBC. PTK6 induces EMT by stabilizing SNAIL, a key EMT-initiating transcriptional factor. Inhibition of PTK6 activity reverses mesenchymal features of TNBC cells and suppresses their metastases by promoting SNAIL degradation via a novel mechanism. In the current study, we identify membrane-associated RING-CH2 (MARCH2) as a novel PTK6-regulated E3 ligase that promotes the ubiquitination and degradation of SNAIL protein. The MARCH2 RING domain is critical for SNAIL ubiquitination and subsequent degradation. PTK6 inhibition promotes the interaction of MARCH2 with SNAIL. Overexpression of MARCH2 exhibits tumor suppressive properties and phenocopies the effects of SNAIL downregulation and PTK6 inhibition in TNBC cells, such as inhibition of migration, anoikis resistance, and metastasis. Consistent with this, higher levels of MARCH2 expression in breast and other cancers are associated with better prognosis. We have identified MARCH2 as a novel SNAIL E3 ligase that regulates EMT and metastases of mesenchymal TNBC.

SIGNIFICANCE

EMT is a process directly linked to drug resistance and metastasis of cancer cells. We identified MARCH2 as a novel regulator of SNAIL, a key EMT driver, that promotes SNAIL ubiquitination and degradation in TNBC cells. MARCH2 is oncogene regulated and inhibits growth and metastasis of TNBC. These insights could contribute to novel strategies to therapeutically target TNBC.

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

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

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.

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.

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.

BRK phosphorylates SMAD4 for proteasomal degradation and inhibits tumor suppressor FRK to control SNAIL, SLUG, and metastatic potential

The tumor-suppressing function of SMAD4 is frequently subverted during mammary tumorigenesis, leading to cancer growth, invasion, and metastasis. A long-standing concept is that SMAD4 is not regulated by phosphorylation but ubiquitination. Our search for signaling pathways regulated by breast tumor kinase (BRK), a nonreceptor protein tyrosine kinase that is up-regulated in ~80% of invasive ductal breast tumors, led us to find that BRK competitively binds and phosphorylates SMAD4 and regulates transforming growth factor-β/SMAD4 signaling pathway. A constitutively active BRK (BRK-Y447F) phosphorylates SMAD4, resulting in its recognition by the ubiquitin-proteasome system, which accelerates SMAD4 degradation. Activated BRK-mediated degradation of SMAD4 is associated with the repression of tumor suppressor gene FRK and increased expression of mesenchymal markers, SNAIL, and SLUG. Thus, our data suggest that combination therapies targeting activated BRK signaling may have synergized the benefits in the treatment of SMAD4 repressed cancers.

Taxol Induces Brk-dependent Prosurvival Phenotypes in TNBC Cells through an AhR/GR/HIF-driven Signaling Axis

The metastatic cascade is a complex process that requires cancer cells to survive despite conditions of high physiologic stress. Previously, cooperation between the glucocorticoid receptor (GR) and hypoxia-inducible factors (HIF) was reported as a point of convergence for host and cellular stress signaling. These studies indicated p38 MAPK-dependent phosphorylation of GR on Ser134 and subsequent p-GR/HIF-dependent induction of breast tumor kinase (PTK6/Brk), as a mediator of aggressive cancer phenotypes. Herein, p-Ser134 GR was quantified in human primary breast tumors (n = 281) and the levels of p-GR were increased in triple-negative breast cancer (TNBC) relative to luminal breast cancer. Brk was robustly induced following exposure of TNBC model systems to chemotherapeutic agents (Taxol or 5-fluorouracil) and growth in suspension [ultra-low attachment (ULA)]. Notably, both Taxol and ULA resulted in upregulation of the Aryl hydrocarbon receptor (AhR), a known mediator of cancer prosurvival phenotypes. Mechanistically, AhR and GR copurified and following chemotherapy and ULA, these factors assembled at the Brk promoter and induced Brk expression in an HIF-dependent manner. Furthermore, Brk expression was upregulated in Taxol-resistant breast cancer (MCF-7) models. Ultimately, Brk was critical for TNBC cell proliferation and survival during Taxol treatment and in the context of ULA as well as for basal cancer cell migration, acquired biological phenotypes that enable cancer cells to successfully complete the metastatic cascade. These studies nominate AhR as a p-GR binding partner and reveal ways to target epigenetic events such as adaptive and stress-induced acquisition of cancer skill sets required for metastatic cancer spread.Implication: Breast cancer cells enlist intracellular stress response pathways that evade chemotherapy by increasing cancer cell survival and promoting migratory phenotypes. Mol Cancer Res; 16(11); 1761-72. ©2018 AACR.

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.

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

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

Understanding the cellular roles of Fyn-related kinase (FRK): implications in cancer biology

The non-receptor tyrosine kinase Fyn-related kinase (FRK) is a member of the BRK family kinases (BFKs) and is distantly related to the Src family kinases (SFKs). FRK was first discovered in 1993, and studies pursued thereafter attributed a potential tumour-suppressive function to the enzyme. In recent years, however, further functional characterization of the tyrosine kinase in diverse cancer types suggests that FRK may potentially play an oncogenic role as well. Specifically, while ectopic expression of FRK suppresses cell proliferation and migration in breast and brain cancers, knockdown or catalytic inhibition of FRK suppresses these cellular processes in pancreatic and liver cancer. Such functional paradox is therefore evidently exhibited in a tissue-specific context. This review sheds light on the recent developments emerged from investigations on FRK which include: (a) a review of the expression pattern of the protein in mammalian cells/tissues, (b) underlying genomic perturbations and (c) a mechanistic function of the enzyme across different cellular environments. Given its functional heterogeneity observed across different cancers, we also discuss the therapeutic significance of FRK.

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

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

Kinase-Dependent and -Independent Roles for PTK6 in Colon Cancer

Disruption of the gene encoding Protein Tyrosine Kinase 6 (Ptk6) delayed differentiation and increased growth in the mouse intestine. However, Ptk6-null mice were also resistant to azoxymethane-induced colon tumorigenesis. To further explore functions of PTK6 in colon cancer, expression of epithelial and mesenchymal markers, as well as proliferation, migration, and xenograft tumor growth, was examined in human colon tumor cell lines with knockdown or overexpression of PTK6. PTK6 protein, transcript, and activation were also examined in a human colon tumor tissue array, using immunohistochemistry and qRT-PCR. Knockdown of PTK6 led to the epithelial-mesenchymal transition (EMT) in SW480 and HCT116 cells, whereas overexpression of PTK6 in SW620 cells restored an epithelial phenotype in a kinase-independent manner. PTK6 knockdown also increased xenograft tumor growth of SW480 cells, suggesting tumor suppressor functions. In clinical specimens, PTK6 expression was highest in normal differentiated epithelial cells and reduced in tumors. In contrast, overexpression of constitutively active PTK6 promoted STAT3 and ERK5 activation in colon cancer cells, and endogenous PTK6 promoted cell survival and oncogenic signaling in response to DNA-damaging treatments. These data indicate that PTK6 has complex, context-specific functions in colon cancer; PTK6 promotes the epithelial phenotype to antagonize the EMT in a kinase-independent manner, whereas activation of PTK6 promotes oncogenic signaling.

IMPLICATIONS

Understanding context-specific functions of PTK6 is important, because although it promotes cell survival and oncogenic signaling after DNA damage, expression of PTK6 in established tumors may maintain the epithelial phenotype, preventing tumor progression. Mol Cancer Res; 14(6); 563-73. ©2016 AACR.

Breast Tumor Kinase (Brk/PTK6) Is Induced by HIF, Glucocorticoid Receptor, and PELP1-Mediated Stress Signaling in Triple-Negative Breast Cancer

Cancer cells use stress response pathways to sustain their pathogenic behavior. In breast cancer, stress response-associated phenotypes are mediated by the breast tumor kinase, Brk (PTK6), via the hypoxia-inducible factors HIF-1α and HIF-2α. Given that glucocorticoid receptor (GR) is highly expressed in triple-negative breast cancer (TNBC), we investigated cross-talk between stress hormone-driven GR signaling and HIF-regulated physiologic stress. Primary TNBC tumor explants or cell lines treated with the GR ligand dexamethasone exhibited robust induction of Brk mRNA and protein that was HIF1/2-dependent. HIF and GR coassembled on the BRK promoter in response to either hypoxia or dexamethasone, indicating that Brk is a direct GR/HIF target. Notably, HIF-2α, not HIF-1α, expression was induced by GR signaling, and the important steroid receptor coactivator PELP1 was also found to be induced in a HIF-dependent manner. Mechanistic investigations showed how PELP1 interacted with GR to activate Brk expression and demonstrated that physiologic cell stress, including hypoxia, promoted phosphorylation of GR serine 134, initiating a feed-forward signaling loop that contributed significantly to Brk upregulation. Collectively, our findings linked cellular stress (HIF) and stress hormone (cortisol) signaling in TNBC, identifying the phospho-GR/HIF/PELP1 complex as a potential therapeutic target to limit Brk-driven progression and metastasis in TNBC patients.

Protein tyrosine kinase 6 promotes ERBB2-induced mammary gland tumorigenesis in the mouse

Protein tyrosine kinase 6 (PTK6) expression, activation, and amplification of the PTK6 gene have been reported in ERBB2/HER2-positive mammary gland cancers. To explore contributions of PTK6 to mammary gland tumorigenesis promoted by activated ERBB2, we crossed Ptk6^−/− mice with the mouse mammary tumor virus-ERBB2 transgenic mouse line expressing activated ERBB2 and characterized tumor development and progression. ERBB2-induced tumorigenesis was significantly delayed and diminished in mice lacking PTK6. PTK6 expression was induced in the mammary glands of ERBB2 transgenic mice before tumor development and correlated with activation of signal transducer and activator of transcription 3 (STAT3) and increased proliferation. Disruption of PTK6 impaired STAT3 activation and proliferation. Phosphorylation of the PTK6 substrates focal adhesion kinase (FAK) and breast cancer anti-estrogen resistance 1 (BCAR1; p130CAS) was decreased in Ptk6^−/− mammary gland tumors. Reduced numbers of metastases were detected in the lungs of Ptk6^−/− mice expressing activated ERBB2, compared with wild-type ERBB2 transgenic mice. PTK6 activation was detected at the edges of ERBB2-positive tumors. These data support roles for PTK6 in both ERBB2-induced mammary gland tumor initiation and metastasis, and identify STAT3, FAK, and BCAR1 as physiologically relevant PTK6 substrates in breast cancer. Including PTK6 inhibitors as part of a treatment regimen could have distinct benefits in ERBB2/HER2-positive breast cancers.

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.

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.