Protein tyrosine kinase 6 protects cells from anoikis by directly phosphorylating focal adhesion kinase and activating AKT

Targeted delivery of FAK siRNA by engineered exosomes to reverse cetuximab resistance via activating paraptosis in colon cancer

BACKGROUND

Cetuximab is extensively used in the treatment of metastatic colorectal cancer (mCRC). However, resistance poses a significant challenge to successful therapy. Recently, paraptosis, a non-classical programmed cell death, has garnered increased attention for its potential application value in antitumor treatments. We aimed to identify the essential pathways and signaling molecules involved in paraptosis inhibition and select them as therapeutic targets in cetuximab resistance. Additionally, engineered exosome technology is used as a drug delivery system with both targeted and effector properties.

RESULTS

By comparing the differential expression of paraptosis-related genes between drug-resistant colon cancer cells and sensitive cells, it was observed that the paraptosis level induced by cetuximab was significantly downregulated in drug-resistant cells. Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis identified the focal adhesion kinase (FAK) signaling pathway as a key pathway involved in the suppression of paraptosis. The biological function of FAK in cetuximab-resistant cells was investigated through cell morphology observation, CCK-8 assay, colony formation assay, RT-qPCR, Western Blot, and loss-of-function experiments. The results showed that the FAK signaling pathway was significantly upregulated in cetuximab-resistant colon cancer cells, and siRNA interference targeting FAK could notably inhibit cell proliferation while upregulating the paraptosis level. Based on this, engineered colon cancer cells targeted and FAK siRNA loaded exosomes (CT-Exo-siFAK1) were constructed. In vitro experiments, CT-Exo-siFAK1 could effectively activate paraptosis and inhibit the proliferation of drug-resistant colon cancer cells. In vivo experiments also confirmed that CT-Exo-siFAK1 significantly suppressed tumor growth and metastasis while upregulating the paraptosis level.

CONCLUSION

This study suggests that FAK signaling pathway-mediated inhibition of paraptosis levels is crucial in the sensitivity of cetuximab targeted therapy in colon cancer, and the use of engineered exosomes to deliver FAK siRNA may be an effective strategy to reverse cetuximab resistance.

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 confers tamoxifen-resistance in breast cancer via regulation of tyrosine phosphorylation of CDK1

Tamoxifen (Tam) has been the first-line therapy for estrogen receptor-positive breast cancer since its FDA-approval in 1998. Tam-resistance, however, presents a challenge and the mechanisms that drive it have yet to be fully elucidated. The non-receptor tyrosine kinase BRK/PTK6 is a promising candidate as previous research has shown that BRK knockdown resensitizes Tam-resistant breast cancer cells to the drug. However, the specific mechanisms that drive its importance to resistance remain to be investigated. Here, we investigate the role and mechanism of action of BRK in Tam-resistant (TamR), ER+, and T47D breast cancer cells using phosphopeptide enrichment and high throughput phopshoproteomics analysis. We conducted BRK-specific shRNA knockdown in TamR T47D cells and compared phosphopeptides identified in these cells with their Tam-resistant counterpart and parental, Tam-sensitive cells (Par). A total of 6492 STY phosphosites were identified. Of these sites, 3739 high-confidence pST sites and 118 high-confidence pY sites were analyzed for significant changes in phosphorylation levels to identify pathways that were differentially regulated in TamR versus Par and to investigate changes in these pathways when BRK is knocked down in TamR. We observed and validated increased CDK1 phosphorylation at Y15 in TamR cells compared to BRK-depleted TamR cells. Our data suggest that BRK is a potential Y15-directed CDK1 regulatory kinase in Tam-resistant breast cancer.

Cell-Cell Interaction-Mediated Signaling in the Testis Induces Reproductive Dysfunction—Lesson from the Toxicant/Pharmaceutical Models

Emerging evidence has shown that cell-cell interactions between testicular cells, in particular at the Sertoli cell-cell and Sertoli-germ cell interface, are crucial to support spermatogenesis. The unique ultrastructures that support cell-cell interactions in the testis are the basal ES (ectoplasmic specialization) and the apical ES. The basal ES is found between adjacent Sertoli cells near the basement membrane that also constitute the blood-testis barrier (BTB). The apical ES is restrictively expressed at the Sertoli-spermatid contact site in the apical (adluminal) compartment of the seminiferous epithelium. These ultrastructures are present in both rodent and human testes, but the majority of studies found in the literature were done in rodent testes. As such, our discussion herein, unless otherwise specified, is focused on studies in testes of adult rats. Studies have shown that the testicular cell-cell interactions crucial to support spermatogenesis are mediated through distinctive signaling proteins and pathways, most notably involving FAK, Akt1/2 and Cdc42 GTPase. Thus, manipulation of some of these signaling proteins, such as FAK, through the use of phosphomimetic mutants for overexpression in Sertoli cell epithelium in vitro or in the testis in vivo, making FAK either constitutively active or inactive, we can modify the outcome of spermatogenesis. For instance, using the toxicant-induced Sertoli cell or testis injury in rats as study models, we can either block or rescue toxicant-induced infertility through overexpression of p-FAK-Y397 or p-FAK-Y407 (and their mutants), including the use of specific activator(s) of the involved signaling proteins against pAkt1/2. These findings thus illustrate that a potential therapeutic approach can be developed to manage toxicant-induced male reproductive dysfunction. In this review, we critically evaluate these recent findings, highlighting the direction for future investigations by bringing the laboratory-based research through a translation path to clinical investigations.

Knockdown of microRNA-214-3p Promotes Tumor Growth and Epithelial-Mesenchymal Transition in Prostate Cancer

Simple Summary

Prostate Cancer is the second leading cause of cancer-related deaths in the United States. In this study, we analyzed a molecule known as a microRNA, which regulates the expression of genes. microRNAs are involved in processes related to cancer onset and progression. Abnormal expression of microRNAs can promote prostate cancer. This study showed that knockdown of microRNA miR-214-3p enhanced the progression and of prostate cancer. In addition, miR-214 regulated the expression of many genes. These results are useful to better understand the function of miR-214-3p in prostate cancer and can be a useful target in the treatment of the disease.

Abstract

Abnormal expression of microRNA miR-214-3p (miR-214) is associated with multiple cancers. In this study, we assessed the effects of CRISPR/Cas9 mediated miR-214 depletion in prostate cancer (PCa) cells and the underlying mechanisms. Knockdown of miR-214 promoted PCa cell proliferation, invasion, migration, epithelial-mesenchymal transition (EMT), and increased resistance to anoikis, a key feature of PCa cells that undergo metastasis. The reintroduction of miR-214 in miR-214 knockdown cells reversed these effects and significantly suppressed cell proliferation, migration, and invasion. These in vitro studies are consistent with the role of miR-214 as a tumor suppressor. Moreover, miR-214 knockout increased tumor growth in PCa xenografts in nude mice supporting its anti-oncogenic role in PCa. Knockdown of miR-214 increased the expression of its target protein, Protein Tyrosine Kinase 6 (PTK6), a kinase shown to promote oncogenic signaling and tumorigenesis in PCa. In addition, miR-214 modulated EMT as exhibited by differential regulation of E-Cadherin, N-Cadherin, and Vimentin both in vitro and in vivo. RNA-seq analysis of miR-214 knockdown cells revealed altered gene expression related to PCa tumor growth pathways, including EMT and metastasis. Collectively, our findings reveal that miR-214 is a key regulator of PCa oncogenesis and is a potential novel therapeutic target for the treatment of the disease.

Drug Discovery Targeting Focal Adhesion Kinase (FAK) as a Promising Cancer Therapy

FAK is a nonreceptor intracellular tyrosine kinase which plays an important biological function. Many studies have found that FAK is overexpressed in many human cancer cell lines, which promotes tumor cell growth by controlling cell adhesion, migration, proliferation, and survival. Therefore, targeting FAK is considered to be a promising cancer therapy with small molecules. Many FAK inhibitors have been reported as anticancer agents with various mechanisms. Currently, six FAK inhibitors, including GSK-2256098 (Phase I), VS-6063 (Phase II), CEP-37440 (Phase I), VS-6062 (Phase I), VS-4718 (Phase I), and BI-853520 (Phase I) are undergoing clinical trials in different phases. Up to now, there have been many novel FAK inhibitors with anticancer activity reported by different research groups. In addition, FAK degraders have been successfully developed through “proteolysis targeting chimera” (PROTAC) technology, opening up a new way for FAK-targeted therapy. In this paper, the structure and biological function of FAK are reviewed, and we summarize the design, chemical types, and activity of FAK inhibitors according to the development of FAK drugs, which provided the reference for the discovery of new anticancer agents.

The Seminiferous Epithelial Cycle of Spermatogenesis: Role of Non-receptor Tyrosine Kinases

Non-receptor tyrosine kinases (NRTKs) are implicated in various biological processes including cell proliferation, differentiation, survival, and apoptosis, as well as cell adhesion and movement. NRTKs are expressed in all mammals and in different cell types, with extraordinarily high expression in the testis. Their association with the plasma membrane and dynamic subcellular localization are crucial parameters in their activation and function. Many NRTKs are found in endosomal protein trafficking pathways, which suggests a novel mechanism to regulate the timely junction restructuring in the mammalian testis to facilitate spermiation and germ cell transport across the seminiferous epithelium.

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.

Effects of silver and gold nanoparticles phytosynthesized with Cornus mas extract on oral dysplastic human cells

Background: Oral cancer is highly aggressive due to difficult diagnosis, therapy resistance and increasing frequency; thus finding prevention therapies is very important. Aim: This study evaluates the use of gold and silver nanoparticles (NPs), phyto-synthesized with Cornus mas extract against oral dysplastic lesions. Methods: NPs were characterized by UV-Vis, Fourier-transform infrared spectroscopy, transmission electron microscopy, x-ray diffraction and laser Doppler microelectrophoresis. Biological testing employed two human oral cell lines: gingival fibroblasts and dysplastic keratinocytes and evaluated viability, cell death mechanisms and cellular uptake. Results: NPs induced selective toxic effects against dysplastic cells. p53/BAX/BCL2 activation and PI3K/AKT inhibition led to cell death through necrosis and apoptosis. NPs also induced antioxidant and anti-inflammatory effects. Conclusion: NPs of gold and silver showed promising beneficial effects in the therapy of oral dysplasia.

Targeting protein tyrosine kinase 6 in cancer

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

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

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

Protein tyrosine kinase 6 signaling in prostate cancer

More than 25 years have passed since the discovery of protein tyrosine kinase 6 (PTK6), a non-receptor tyrosine kinase distantly related to SRC family kinases. Since then, a variety of data suggest that PTK6 promotes oncogenic signaling and tumorigenesis, generally dependent on its kinase activity. Increased PTK6 expression, activation at the plasma membrane and altered intracellular localization have been discovered in prostate cancers. While PTK6 is localized to nuclei of epithelial cells in normal prostate, it is relocalized and activated at the plasma membrane in prostate tumors. Active PTK6 interacts with and directly phosphorylates AKT, FAK and BCAR1 to promote oncogenic signaling. Furthermore, PTK6 can enhance the epithelial mesenchymal transition by inhibiting E-cadherin expression and inducing expression of the mesenchymal markers vimentin, SLUG and ZEB1. Several lines of evidence suggest that PTK6 plays a role in Pten null prostate tumors. PTEN targets activating phosphorylation of PTK6 and loss of PTEN subsequently leads to PTK6 activation. Different studies provide compelling evidence as to why PTK6 is a potential therapeutic target in prostate cancer. Here, we briefly review the advances and significance of PTK6 in prostate cancer.

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.

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.

Molecular mechanisms of cell death: recommendations of the Nomenclature Committee on Cell Death 2018

Over the past decade, the Nomenclature Committee on Cell Death (NCCD) has formulated guidelines for the definition and interpretation of cell death from morphological, biochemical, and functional perspectives. Since the field continues to expand and novel mechanisms that orchestrate multiple cell death pathways are unveiled, we propose an updated classification of cell death subroutines focusing on mechanistic and essential (as opposed to correlative and dispensable) aspects of the process. As we provide molecularly oriented definitions of terms including intrinsic apoptosis, extrinsic apoptosis, mitochondrial permeability transition (MPT)-driven necrosis, necroptosis, ferroptosis, pyroptosis, parthanatos, entotic cell death, NETotic cell death, lysosome-dependent cell death, autophagy-dependent cell death, immunogenic cell death, cellular senescence, and mitotic catastrophe, we discuss the utility of neologisms that refer to highly specialized instances of these processes. The mission of the NCCD is to provide a widely accepted nomenclature on cell death in support of the continued development of the field.

The oleocanthal-based homovanillyl sinapate as a novel c-Met inhibitor

The hepatocyte growth factor (HGF)/mesenchymal-epithelial transition factor (c-Met) signaling axis has gained considerable attention as an attractive molecular target for therapeutic blockade of cancer. Inspired by the chemical structure of S (-)-oleocanthal, a natural secoiridoid from extra-virgin olive oil with documented anticancer activity against c-Met-dependent malignancies, the research presented herein reports on the discovery of the novel olive-derived homovanillyl sinapate (HVS) as a promising c-Met inhibitor. HVS was distinguished for its remarkable potency against wild-type c-Met and its oncogenic variant in cell-free assays and confirmed by in silico docking studies. Furthermore, HVS substantially impaired the c-Met-mediated growth across a broad spectrum of breast cancer cells, while similar treatment doses had no effect on the non-tumorigenic mammary epithelial cell growth. In addition, HVS caused a dose-dependent inhibition of HGF-induced, but not epidermal growth factor (EGF)-induced, cell scattering in addition to HGF-mediated migration, invasion, and 3-dimensional (3D) proliferation of tumor cell spheroids. HVS treatment effects were mediated via inhibition of ligand-mediated c-Met activation and its downstream mitogenic signaling and blocking molecular mediators involved in cellular motility across different cellular contexts. An interesting feature of HVS is its good selectivity for c-Met and Abelson murine leukemia viral oncogene homolog 1 (ABL1) when profiled against a panel of kinases. Docking studies revealed interactions likely to impart high dual affinity for both ABL1 and c-Met kinases. HVS markedly reduced tumor growth, showed excellent pharmacodynamics, and suppressed cell proliferation and microvessel density in an orthotopic model of triple negative breast cancer. Collectively, the present findings suggested that the oleocanthal-based HVS is a promising c-Met inhibitor lead entity with excellent therapeutic potential to control malignancies with aberrant c-Met activity.

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.

Discovery of a series of 1,3,4-oxadiazole-2(3H)-thione derivatives containing piperazine skeleton as potential FAK inhibitors

Focal adhesion kinase (FAK) is an important drug target that plays a fundamental role in mediating signal transduction system. We report herein the discovery of a novel class of 1,3,4-oxadiazole-2(3H)-thione derivatives containing piperazine skeleton with improved potency toward FAK. All of the 17 new synthesized compounds were assayed for the anticancer activities against four cancer cells, HepG2, Hela, SW116 and BGC823. Because of the combination of 1,4-benzodioxan, 1,3,4-oxadiazole and piperazine ring, most of them exhibited remarkable antitumor activities. Notably, compound 5m showed the most potent biological activities (IC₅₀=5.78μM for HepG2, and IC₅₀=47.15μM for SW1116), and its anti-FAK inhibitory activity (IC₅₀=0.78μM) was also the best. Computational docking studies also showed that compound 5m has interaction with FAK key residues in the active site.

PTK2-mediated degradation of ATG3 impedes cancer cells susceptible to DNA damage treatment

ATG3 (autophagy-related 3) is an E2-like enzyme essential for autophagy; however, it is unknown whether it has an autophagy-independent function. Here, we report that ATG3 is a relatively stable protein in unstressed cells, but it is degraded in response to DNA-damaging agents such as etoposide or cisplatin. With mass spectrometry and a mutagenesis assay, phosphorylation of tyrosine 203 of ATG3 was identified to be a critical modification for its degradation, which was further confirmed by manipulating ATG3Y203E (phosphorylation mimic) or ATG3Y203F (phosphorylation-incompetent) in Atg3 knockout MEFs. In addition, by using a generated phospho-specific antibody we showed that phosphorylation of Y203 significantly increased upon etoposide treatment. With a specific inhibitor or siRNA, PTK2 (protein tyrosine kinase 2) was confirmed to catalyze the phosphorylation of ATG3 at Y203. Furthermore, a newly identified function of ATG3 was recognized to be associated with the promotion of DNA damage-induced mitotic catastrophe, in which ATG3 interferes with the function of BAG3, a crucial protein in the mitotic process, by binding. Finally, PTK2 inhibition-induced sustained levels of ATG3 were able to sensitize cancer cells to DNA-damaging agents. Our findings strengthen the notion that targeting PTK2 in combination with DNA-damaging agents is a novel strategy for cancer therapy.

Protein Kinase Activity Decreases with Higher Braak Stages of Alzheimer's Disease Pathology

Alzheimer’s disease (AD) is characterized by a long pre-clinical phase (20–30 years), during which significant brain pathology manifests itself. Disease mechanisms associated with pathological hallmarks remain elusive. Most processes associated with AD pathogenesis, such as inflammation, synaptic dysfunction, and hyper-phosphorylation of tau are dependent on protein kinase activity. The objective of this study was to determine the involvement of protein kinases in AD pathogenesis. Protein kinase activity was determined in postmortem hippocampal brain tissue of 60 patients at various stages of AD and 40 non-demented controls (Braak stages 0-VI) using a peptide-based microarray platform. We observed an overall decrease of protein kinase activity that correlated with disease progression. The phosphorylation of 96.7% of the serine/threonine peptides and 37.5% of the tyrosine peptides on the microarray decreased significantly with increased Braak stage (p-value <0.01). Decreased activity was evident at pre-clinical stages of AD pathology (Braak I-II). Increased phosphorylation was not observed for any peptide. STRING analysis in combination with pathway analysis and identification of kinases responsible for peptide phosphorylation showed the interactions between well-known proteins in AD pathology, including the Ephrin-receptor A1 (EphA1), a risk gene for AD, and sarcoma tyrosine kinase (Src), which is involved in memory formation. Additionally, kinases that have not previously been associated with AD were identified, e.g., protein tyrosine kinase 6 (PTK6/BRK), feline sarcoma oncogene kinase (FES), and fyn-associated tyrosine kinase (FRK). The identified protein kinases are new biomarkers and potential drug targets for early (pre-clinical) intervention.

SMARCE1 regulates metastatic potential of breast cancer cells through the HIF1A/PTK2 pathway

Background

While aberrant activation of the chromatin-remodeling SWI/SNF complexes has been associated with cancer development and progression, the role of each subunit in tumor cells is poorly defined. This study is aimed to characterize the role of SMARCE1/BAF57 in regulating metastasis of breast cancer cells.

Methods

Genetic approaches and chemical inhibitors were used to manipulate the activities of SMARCE1 and its downstream targets in multiple breast cancer cell lines. Xenograft mouse models were used to analyze the role of SMARCE1 in lung metastasis in vivo. Nonadherent culture conditions were used to elucidate the role of SMARCE1 in regulating anoikis. Chromatin immunoprecipitation (ChIP), immunoprecipitation, and immunoblotting assays were designed to dissect the mechanism of action of SMARCE1. Public databases were used to investigate the relationship between SMARCE1 deregulation and breast cancer prognosis.

Results

SMARCE1 knockdown reduced lung metastasis of breast cancer cells and sensitized tumor cells to anoikis. In response to loss of attachment, SMARCE1 interacted with and potentiated transcriptional activity of HIF1A, resulting in rapid PTK2 activation. Both HIF1A and PTK2 were indispensable for SMARCE1-mediated protection against anoikis by promoting activation of ERK and AKT pathways while suppressing the expression of pro-apoptotic BIM protein. Expression data analysis of a large cohort of human breast tumors revealed that high expression of SMARCE1 or PTK2 is associated with poor prognosis and tumor relapse, and PTK2 expression is positively correlated with SMARCE1 expression in basal-like and luminal B subtypes of breast tumors.

Conclusions

SMARCE1 plays an essential role in breast cancer metastasis by protecting cells against anoikis through the HIF1A/PTK2 pathway. SMARCE1-mediated PTK2 activation likely plays a key role in promoting metastasis of basal-like and luminal B subtype of breast tumors.

TNFα/IFNγ Mediated Intestinal Epithelial Barrier Dysfunction Is Attenuated by MicroRNA-93 Downregulation of PTK6 in Mouse Colonic Epithelial Cells

Since inflammatory bowel diseases (IBD) represent significant morbidity and mortality in the US, the need for defining novel drug targets and inflammatory mechanisms would be of considerable benefit. Although protein tyrosine kinase 6 (PTK6, also known as breast tumor kinase BRK) has been primarily studied in an oncogenic context, it was noted that PTK6 null mice exhibited significantly enhanced colonic epithelial barrier function. Considering that the inflammatory functions of PTK6 have not yet been explored, we hypothesized that cytokines responsible for mediating IBD, such as TNFα/IFNγ, may solicit the action of PTK6 to alter barrier function. After first assessing critical mediators of TNFα/IFNγ driven epithelial barrier dysfunction, we further explored the possibility of PTK6 in this inflammatory context. In this report, we showed that PTK6 siRNA and PTK6 null young adult mouse colonic epithelial cells (YAMC) exhibited significant attenuation of TNFα/IFNγ induced barrier dysfunction as measured by electric cell-substrate impedance sensing (ECIS) assay and permeability assays. In addition, PTK6 null cells transfected with PTK6 cDNA displayed restored barrier dysfunction in response to TNFα/IFNγ, while the cells transfected with vector alone showed similar attenuation of barrier dysfunction. Furthermore, using subcellular fractionation and immunocytochemistry experiments, we found that PTK6 plays a role in FoxO1 nuclear accumulation leading to down-regulation of claudin-3, a tight junction protein. Moreover, we searched for relevant miRNA candidates putative for targeting PTK6 in order to identify and assess the impact of microRNA that target PTK6 with respect to TNFα/IFNγ induced barrier dysfunction. Subsequently, we assayed likely targets and determined their effectiveness in attenuating PTK6 expression as well as cytokine induced barrier dysfunction. Results showed that miR-93 reduced PTK6 expression and attenuated TNFα/IFNγ imposed decrease in transepithelial electrical resistance (TER), as well as excluded FoxO1 from the nucleus. Our results indicate that PTK6 may act as a novel mediator of intestinal epithelial permeability during inflammatory injury, and miR-93 may protect intestinal epithelial barrier function, at least in part, by targeting PTK6.

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.

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.

The virus-induced protein APOBEC3G inhibits anoikis by activation of Akt kinase in pancreatic cancer cells

Pancreatic cancer is one of the more common cancers with a poor prognosis. Some varieties of cancer are related to virus infection. As a virus-induced protein, APOBEC3G (A3G) presents extensive anti-virus ability, but the role of A3G in pancreatic cancer was previously unknown. The expression of A3G in pancreatic cancer was examined using TaqMan real-time qPCR, immunohistochemical and immunofluorescent staining. Subsequently, the role of A3G in pancreatic cancer was evaluated in vivo using the tumor xenograft model. Anoikis was detected by colony formation assay and flow cytometry in vitro. The Akt kinase activity and target protein PTEN were examined by co-immunoprecipitation and immunoblot. The virus-induced protein A3G was significantly up-regulated in pancreatic cancer, and the up-regulation of A3G promoted xenograft tumor formation. A3G inactivated PTEN by binding to the C2 tensin-type and PDZ domains, thereby inducing anoikis resistance through Akt activation. Our results demonstrate that the up-regulation of A3G in pancreatic cancer cells induces anoikis resistance, and they provide novel insight into the mechanism by which A3G affects the malignant behavior of pancreatic cancer cells.

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

Introduction

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

Methods

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

Results

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

Conclusions

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

Electronic supplementary material

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

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.

Targeting Focal Adhesion Kinase and Resistance to mTOR Inhibition in Pancreatic Neuroendocrine Tumors

BACKGROUND

Focal adhesion kinase (FAK) mediates survival of normal pancreatic islets through activation of AKT. Upon malignant transformation of islet cells into pancreatic neuroendocrine tumors (PanNETs), AKT is frequently overexpressed and mutations in the AKT/mTOR pathway are detected. Because mTOR inhibitors rarely induce PanNET tumor regression, partly because of feedback activation of AKT, novel combination strategies are needed to target FAK/AKT/mTOR signaling.

METHODS

We characterized the activation of FAK in PanNETs using immunohistochemistry and Western blot analysis and tested the FAK inhibitor PF-04554878 in human PanNET cells in vitro and in vivo (at least three mice per group). In addition, we evaluated the effect of combined FAK and mTOR inhibition on PanNET viability and apoptosis. All statistical tests were two-sided.

RESULTS

We found that FAK is overexpressed and hyperphosphorylated in human PanNETs and that PF-04554878 strongly inhibited FAK (Tyr397) autophosphorylation in a dose-dependent manner. We found that PF-04554878 inhibited cell proliferation and clonogenicity and induced apoptosis in PanNET cells. Moreover, oral administration of PF-04554878 statistically significantly reduced tumor growth in a patient-derived xenograft model of PanNET (P = .02) and in a human PanNET xenograft model of peritoneal carcinomatosis (P = .03). Importantly, PF-04554878 synergized with the mTOR inhibitor everolimus by preventing feedback AKT activation.

CONCLUSIONS

We demonstrate for the first time that FAK is overexpressed in PanNETs and that inhibition of FAK activity induces apoptosis and inhibits PanNET proliferation. We found that the novel FAK inhibitor PF-04554878 synergizes with everolimus, a US Food and Drug Administration-approved agent for PanNETs. Our findings warrant the clinical investigation of combined FAK and mTOR inhibition in PanNETs.

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.

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 mediates TNFα-induced endothelial barrier dysfunction

A key event in the progression of systemic inflammation resulting from severe trauma or shock involves microvascular hyperpermeability, which leads to excessive plasma fluid and proteins accumulating in extravascular space resulting in tissue edema. The precise molecular mechanism of the hyperpermeability response is not completely understood. Protein tyrosine kinase 6 (PTK6, also known as breast tumor kinase BRK) is a non-receptor tyrosine kinase related to Src-family proteins. Although it has also been shown that PTK6 participates in regulating epithelial barrier function, the role of PTK6 in endothelial barrier function has not been reported. In this study, we hypothesized that PTK6 is (1) expressed in vascular endothelial cells, and (2) contributes to vascular endothelial hyperpermeability in response to TNFα. Results showed that PTK6 was detected in mouse endothelial cells at the level of protein and mRNA. In addition, PTK6 knockdown attenuated TNFα induced decrease in endothelial barrier function as measured by electric cell-substrate impedance sensing (ECIS) and in vitro transwell albumin-flux assays. Furthermore, we showed that TNFα treatment of endothelial cells increased active PTK6 association with p120-catenin at endothelial cell-cell junctions. Further analysis using immunocytochemistry and immunoprecipitation demonstrated that PTK6 knockdown attenuated TNFα induced VE-cadherin internalization as well as promoting its association with p120-catenin. Our study demonstrates a novel role of PTK6 in mediating endothelial barrier dysfunction.

FAK signaling in human cancer as a target for therapeutics

Focal adhesion kinase (FAK) is a key regulator of growth factor receptor- and integrin-mediated signals, governing fundamental processes in normal and cancer cells through its kinase activity and scaffolding function. Increased FAK expression and activity occurs in primary and metastatic cancers of many tissue origins, and is often associated with poor clinical outcome, highlighting FAK as a potential determinant of tumor development and metastasis. Indeed, data from cell culture and animal models of cancer provide strong lines of evidence that FAK promotes malignancy by regulating tumorigenic and metastatic potential through highly-coordinated signaling networks that orchestrate a diverse range of cellular processes, such as cell survival, proliferation, migration, invasion, epithelial-mesenchymal transition, angiogenesis and regulation of cancer stem cell activities. Such an integral role in governing malignant characteristics indicates that FAK represents a potential target for cancer therapeutics. While pharmacologic targeting of FAK scaffold function is still at an early stage of development, a number of small molecule-based FAK tyrosine kinase inhibitors are currently undergoing pre-clinical and clinical testing. In particular, PF-00562271, VS-4718 and VS-6063 show promising clinical activities in patients with selected solid cancers. Clinical testing of rationally designed FAK-targeting agents with implementation of predictive response biomarkers, such as merlin deficiency for VS-4718 in mesothelioma, may help improve clinical outcome for cancer patients. In this article, we have reviewed the current knowledge regarding FAK signaling in human cancer, and recent developments in the generation and clinical application of FAK-targeting pharmacologic agents.

HIF-1α-independent hypoxia-induced rapid PTK6 stabilization is associated with increased motility and invasion

PTK6/Brk is a non-receptor tyrosine kinase overexpressed in cancer. Here we demonstrate that cytosolic PTK6 is rapidly and robustly induced in response to hypoxic conditions in a HIF-1-independent manner. Furthermore, a proportion of hypoxic PTK6 subsequently re-localized to the cell membrane. We observed that the rapid stabilization of PTK6 is associated with a decrease in PTK6 ubiquitylation and we have identified c-Cbl as a putative PTK6 E3 ligase in normoxia. The consequences of hypoxia-induced PTK6 stabilization and subcellular re-localization to the plasma membrane include increased cell motility and invasion, suggesting PTK6 targeting as a therapeutic approach to reduce hypoxia-regulated metastatic potential. This could have particular significance for breast cancer patients with triple negative disease.

FAK in cancer: mechanistic findings and clinical applications

Focal adhesion kinase (FAK) is a cytoplasmic protein tyrosine kinase that is overexpressed and activated in several advanced-stage solid cancers. FAK promotes tumour progression and metastasis through effects on cancer cells, as well as stromal cells of the tumour microenvironment. The kinase-dependent and kinase-independent functions of FAK control cell movement, invasion, survival, gene expression and cancer stem cell self-renewal. Small molecule FAK inhibitors decrease tumour growth and metastasis in several preclinical models and have initial clinical activity in patients with limited adverse events. In this Review, we discuss FAK signalling effects on both tumour and stromal cell biology that provide rationale and support for future therapeutic opportunities.

Cancer cell survival during detachment from the ECM: multiple barriers to tumour progression

Epithelial cells require attachment to the extracellular matrix (ECM) for survival. However, during tumour progression and metastasis, cancerous epithelial cells must adapt to and survive in the absence of ECM. During the past 20 years, several cellular changes, including anoikis, have been shown to regulate cell viability when cells become detached from the ECM. In this Opinion article, we review in detail how cancer cells can overcome or take advantage of these specific processes. Gaining a better understanding of how cancer cells survive during detachment from the ECM will be instrumental in designing chemotherapeutic strategies that aim to eliminate ECM-detached metastatic cells.

Notch3 overexpression promotes anoikis resistance in epithelial ovarian cancer via upregulation of COL4A2

Ovarian cancer is a lethal disease with the majority of diagnosed women having distant metastases. Interestingly, although Notch3 overexpression has been correlated with poor survival in epithelial ovarian cancer (EOC), little is known about its mechanism of action. Data show that Notch3 specifically promotes anoikis resistance. In addition, data indicate a positive role for focal adhesion kinase (FAK) as well as downstream signaling kinases such as Akt and Erk 1/2 in promoting anchorage-independent growth. Mechanistically, both mRNA transcript and protein levels of type IV collagen (COL4A2) are reduced when Notch3 levels are decreased and exogenous collagen IV supplementation reverses the anoikis sensitivity. Reduction of COL4A2 expression by RNAI-mediated knockdown induces cell death. Finally, elevated Notch3 expression levels correlate with higher COL4A2 expression in human ovarian tumor specimens.

IMPLICATIONS

These data highlight type IV collagen as a novel therapeutic target for metastatic EOC. Visual Overview: http://mcr.aacrjournals.org/content/early/2014/11/25/1541-7786.MCR-14-0334/F1.large.jpg

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

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

The marine-derived sipholenol A-4-O-3',4'-dichlorobenzoate inhibits breast cancer growth and motility in vitro and in vivo through the suppression of Brk and FAK signaling

Sipholenol A is a natural sipholane triterpenoid isolated from the Red Sea sponge, Callyspongia siphonella. Previous studies showed the antimigratory and antiproliferative activities of the semisynthetic sipholenol A esters against breast cancer cell lines. This study investigated the effects of sipholenol A-4-O-3',4'-dichlorobenzoate (SPA) on the growth, migration and invasion of diverse human breast cancer cells. Results showed that SPA inhibited the growth of the human breast cancer cells, MDA-MB-231, MCF-7, BT-474 and T-47D, in a dose-dependent manner. Immunofluorescent analysis showed that SPA significantly reduced Ki-67-positive cells in MDA-MB-231 cells. Flow cytometry and Western blot analyses revealed that SPA treatment suppressed MDA-MB-231 cell growth by inducing cell cycle arrest at the G1 phase. In addition, SPA suppressed breast cancer cell migration, invasion and decreased Brk and FAK activation in a dose-dependent manner. Molecular docking study suggested a perfect fitting at the FAK's FERM domain, inhibiting the main autophosphorylation site, Y397, which was further confirmed by Western blot analysis. Most known small molecule FAK inhibitors target the kinase domain, creating several off-target side effects. The in vivo studies showed that SPA treatment suppressed breast tumor growth and Ki-67, CD31, p-Brk and p-FAK expression in orthotopic breast cancer in nude mice. In conclusion, SPA inhibited the growth, invasion and migration of breast cancer cells possibly via deactivating Brk and FAK signaling, suggesting good potential for therapeutic use to control invasive breast cancer.

Protein tyrosine kinase 6 regulates mammary gland tumorigenesis in mouse models

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

Interdigital cell death in the embryonic limb is associated with depletion of Reelin in the extracellular matrix

Interdigital cell death is a physiological regression process responsible for sculpturing the digits in the embryonic vertebrate limb. Changes in the intensity of this degenerative process account for the different patterns of interdigital webbing among vertebrate species. Here, we show that Reelin is present in the extracellular matrix of the interdigital mesoderm of chick and mouse embryos during the developmental stages of digit formation. Reelin is a large extracellular glycoprotein which has important functions in the developing nervous system, including neuronal survival; however, the significance of Reelin in other systems has received very little attention. We show that reelin expression becomes intensely downregulated in both the chick and mouse interdigits preceding the establishment of the areas of interdigital cell death. Furthermore, fibroblast growth factors, which are cell survival signals for the interdigital mesoderm, intensely upregulated reelin expression, while BMPs, which are proapototic signals, downregulate its expression in the interdigit. Gene silencing experiments of reelin gene or its intracellular effector Dab-1 confirmed the implication of Reelin signaling as a survival factor for the limb undifferentiated mesoderm. We found that Reelin activates canonical survival pathways in the limb mesoderm involving protein kinase B and focal adhesion kinase. Our findings support that Reelin plays a role in interdigital cell death, and suggests that anoikis (apoptosis secondary to loss of cell adhesion) may be involved in this process.

Histone deacetylase inhibitors restore toxic BH3 domain protein expression in anoikis-resistant mammary and brain cancer stem cells, thereby enhancing the response to anti-ERBB1/ERBB2 therapy

The present studies focused on defining the mechanisms by which anoikis-resistant (AR) mammary carcinoma cells can be reverted to a therapy-sensitive phenotype. AR mammary carcinoma cells had reduced expression of the toxic BH3 domain proteins BAX, BAK, NOXA, and PUMA. In AR cells expression of the protective BCL-2 family proteins BCL-XL and MCL-1 was increased. AR cells were resistant to cell killing by multiple anti-tumor cell therapies, including ERBB1/2 inhibitor + MCL-1 inhibitor treatment, and had a reduced autophagic flux response to these therapies, despite similarly exhibiting increased levels of LC3II processing. Knockdown of MCL-1 and BCL-XL caused necro-apoptosis in AR cells to a greater extent than in parental cells. Pre-treatment of anoikis-resistant cells with histone deacetylase inhibitors (HDACIs) for 24 h increased the levels of toxic BH3 domain proteins, reduced MCL-1 levels, and restored/re-sensitized the cell death response of AR tumor cells to multiple toxic therapies. In vivo, pre-treatment of AR breast tumors in the brain with valproate restored the chemo-sensitivity of the tumors and prolonged animal survival. These data argue that one mechanism to enhance the anti-tumor effect of chemotherapy could be HDACI pre-treatment.

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.

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.

Targeting protein tyrosine kinase 6 enhances apoptosis of colon cancer cells following DNA damage

Protein tyrosine kinase 6 (PTK6) is an intracellular tyrosine kinase that has distinct functions in normal epithelia and cancer. It is expressed primarily in nondividing epithelial cells in the normal intestine, where it promotes differentiation. However, after DNA damage, PTK6 is induced in proliferating progenitor cells, where it contributes to apoptosis. We examined links between PTK6 and the tumor suppressor p53 in the isogenic p53(+/+) and p53(-/-) HCT116 colon tumor cell lines. We found that p53 promotes expression of PTK6 in HCT116 cells, and short hairpin RNA-mediated knockdown of PTK6 leads to reduced induction of the cyclin-dependent kinase inhibitor p21. Knockdown of PTK6 enhances apoptosis in HCT116 cells with wild-type p53, following treatment of cells with γ-radiation, doxorubicin, or 5-fluorouracil. No differences in the activation of AKT, ERK1/2, or ERK5, known PTK6-regulated prosurvival signaling proteins, were detected. However, activity of STAT3, a PTK6 substrate, was impaired in cells with knockdown of PTK6 following DNA damage. In contrast to its role in the normal epithelium following DNA damage, PTK6 promotes survival of cancer cells with wild-type p53 by promoting p21 expression and STAT3 activation. Targeting PTK6 in combination with use of chemotherapeutic drugs or radiation may enhance death of colon tumor cells with wild-type p53.