Discovery of 4-anilino α-carbolines as novel Brk inhibitors

Prognostic impact of PTK6 expression in triple negative breast cancer

BACKGROUND

The aim of this study was to investigate the expression of PTK6 in different groups of triple negative breast cancer and its impact on prognosis.

METHODS

Retrospective study of a total of 209 surgical specimens of breast cancer were identified by IHC or FISH methods as triple negative,and divided into a lymph node metastasis positive (LNM +)group (n = 102) and a lymph node metastasis negative(LNM-) group (n = 107) according to the lymph node status of the surgical specimen. PTK6 expression was detected by IHC technique in all surgical specimens. PTK6 expression and clinicopathological features was explored by Chi-square test. The prognosis of different groups of patients was analyzed by Kaplan-Meier survival analysis and COX analysis.

RESULTS

The incidence of PTK6 expression in the LNM + group (78.4%) was significantly higher than in the LNM- group (28%). Clinicopathological analysis showed that PTK6 expression in the LNM + group was negatively correlated with the 5-year survival of patients. Kaplan-Meier analysis showed that only PTK6 expression in the LNM + group was negatively correlated with OS and DFS. COX analysis also showed that PTK6 expression and N stage were independent prognostic factors for DFS in the LNM + group. No correlation was observed between HER2 and PTK6 expression in any of the groups.

CONCLUSIONS

This study suggests that PTK6 promotes tumor development and was associated with poor prognosis in the LNM + group of triple negative breast cancer. Inhibition of PTK6 may be a new approach for the treatment of triple negative breast cancer patients, especially those with metastasis.

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.

Comprehensive review of α-carboline alkaloids: Natural products, updated synthesis, and biological activities

α-carboline (9H-pyrido[2,3-b]indole), contains a pyridine ring fused with an indole backbone, is a promising scaffold for medicinal chemistry. In recent decades, accumulating evidence shows that α-carboline natural products and their derivatives possess diverse bioactivities. However, hitherto, there is no comprehensive review to systematically summarize this important class of alkaloids. In this perspective, this paper represents the first review to provide a comprehensive description of α-carbolines including natural products, updated literature of synthesis, and their diverse biological activities. Their biological activities including antitumor, anti-microbial, anti-Alzheimer’s disease, anti-atherosclerosis, and antioxidant activities were hilighted. And the targets and the main structure activity relationships (SARs) will be presented. Finally, challenges and future directions of this class of compounds will be discussed. This review will be helpful in understanding and encouraging further exploration for this group of alkaloids.

Discovery of novel α-carboline derivatives as glycogen synthase kinase-3β inhibitors for the treatment of Alzheimer's disease

Alzheimer's disease (AD) is a chronic and progressive neurodegenerative disease, characterized by irreversible cognitive impairment, memory loss, and behavioral disturbances, ultimately resulting in death. The critical roles of glycogen synthase kinase-3β (GSK-3β) in tau pathology have also received considerable attention. Based on molecular docking studies, a series of novel α-carboline derivatives were designed, synthesized, and evaluated as GSK-3β inhibitors for their various biological activities. Among them, compound ZCH-9 showed the most potent inhibitory activity against GSK-3β, with an IC₅₀ value of 1.71 ± 0.09 µM. The cytotoxicity assay showed that ZCH-9 had low cytotoxicity toward the cell lines SH-SY5Y, HepG2, and HL-7702. Moreover, Western blot analysis indicated that ZCH-9 effectively inhibited hyperphosphorylation of the tau protein in okadaic acid-treated SH-SY5Y cells. The binding mode between ZCH-9 and GSK-3β was analyzed and further clarified throughout the molecular dynamics simulations. In general, these results suggested that the α-carboline-based small-molecule compounds could serve as potential candidates targeting GSK-3β for the treatment of AD.

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

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

TRAF3 in T Cells Restrains Negative Regulators of LAT to Promote TCR/CD28 Signaling

The adaptor protein TNFR-associated factor 3 (TRAF3) is required for in vivo T cell effector functions and for normal TCR/CD28 signaling. TRAF3-mediated enhancement of TCR function requires engagement of both CD3 and CD28, but the molecular mechanisms underlying how TRAF3 interacts with and impacts TCR/CD28-mediated complexes to enhance their signaling remains an important knowledge gap. We investigated how TRAF3 is recruited to, and regulates, CD28 as a TCR costimulator. Direct association with known signaling motifs in CD28 was dispensable for TRAF3 recruitment; rather, TRAF3 associated with the CD28-interacting protein linker of activated T cells (LAT) in human and mouse T cells. TRAF3-LAT association required the TRAF3 TRAF-C domain and a newly identified TRAF2/3 binding motif in LAT. TRAF3 inhibited function of the LAT-associated negative regulatory protein Dok1, which is phosphorylated at an inhibitory tyrosine residue by the tyrosine kinase breast tumor kinase (Brk/PTK6). TRAF3 regulated Brk activation in T cells, limiting the association of protein tyrosine phosphatase 1B (PTP1B) with the LAT complex. In TRAF3-deficient cells, LAT complex-associated PTP1B was associated with dephosphorylation of Brk at an activating tyrosine residue, potentially reducing its ability to inhibit Dok1. Consistent with these findings, inhibiting PTP1B activity in TRAF3-deficient T cells rescued basal and TCR/CD28-mediated activation of Src family kinases. These results reveal a new mechanism for promotion of TCR/CD28-mediated signaling through restraint of negative regulation of LAT by TRAF3, enhancing the understanding of regulation of the TCR complex.

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.

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.

Dual Inhibition of CDK4 and CDK2 via Targeting p27 Tyrosine Phosphorylation Induces a Potent and Durable Response in Breast Cancer Cells

Cyclin-dependent kinase 4/6 (CDK4/6)-specific inhibitors, such as palbociclib, have shown clinical efficacy, but primary or secondary resistance has emerged as a problem. To develop more effective therapeutic approaches, investigation is needed into the mechanisms of resistance or adaption. Here, it is demonstrated that CDK2 compensates for loss of CDK4 activity to rescue palbociclib-arrested breast cancer cells, suggesting that inhibition of both kinases is required to achieve durable response. In addition, a novel strategy is described to inhibit tyrosine phosphorylation of p27Kip1 (CDKN1B) and simultaneously inhibit both CDK2 and CDK4. p27Kip1 is a required assembly factor for cyclin-CDK4 complexes, but it must be phosphorylated on residue Y88 to open or activate the complex. The Brk-SH3 peptide, ALT, blocks p27 Y88 phosphorylation, inhibiting CDK4. Nonphosphorylated p27 is no longer a target for ubiquitin-mediated degradation and this stabilized p27 now also inhibits CDK2 activity. Thus, ALT induction inhibits both the kinase that drives proliferation (CDK4) and the kinase that mediates resistance (CDK2), causing a potent and long-lasting cell-cycle arrest. ALT arrests growth of all breast cancer subgroups and synergizes with palbociclib to increase cellular senescence and to cause tumor regression in breast cancer xenograft models. The use of ALT demonstrates that both CDK4 and CDK2 need to be inhibited if long-term efficacy is to be achieved and represents a novel modality to inhibit breast cancer cells.Implications: Modulating tyrosine phosphorylation of p27 impacts both proliferative (CDK4) and resistance (CDK2) mechanisms in breast cancer and suggests that phospho-p27 status may serve as a biomarker for patients that are responsive to CDK4/6 inhibition. Mol Cancer Res; 16(3); 361-77. ©2018 AACR.

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.

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.