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.

Identification of ovarian high-grade serous carcinoma cell lines that show estrogen-sensitive growth as xenografts in immunocompromised mice.

Ovarian cancer remains a significant challenge in women worldwide. Tumors of the high-grade serous carcinoma (HGSC) type represent the most common form of the disease. Development of new therapies for HGSC has been hampered by a paucity of preclinical models in which new drugs could be tested for target engagement and anti-tumor efficacy. Here, we systematically assessed in vivo growth of ovarian cancer cells, including six validated HGSC cell lines, in highly immunocompromised NSG mice by varying the injection site. We found that, with the exception of OVCAR3, HGSC cell lines COV318, COV362, KURAMOCHI, OVCAR4, and OVSAHO, generally demonstrate poor growth as either subcutaneous or intraperitoneal xenografts. Intrabursal injections performed with KURAMOCHI and COV362 cells did not improve tumor growth in vivo. Additional analysis revealed that OVSAHO and COV362 express moderate levels of estrogen receptor (ERα), which translated into improved growth of xenografts in the presence of 17ß-Estradiol. Surprisingly, we also found that the growth of the widely used non-HGSC ovarian cell line SKOV3 could be significantly improved by estrogen supplementation. By describing successful establishment of estrogen-sensitive HGSC xenograft models, OVSAHO and COV362, this work will enable testing of novel therapies for this aggressive form of ovarian cancer.

In Vitro Assays for Endothelial Cell Functions Required for Angiogenesis: Proliferation, Motility, Tubular Differentiation, and Matrix Proteolysis.

This chapter deconstructs the process of angiogenesis into its component parts in order to provide simple assays to measure discrete endothelial cell functions. The techniques described will be suitable for studying stimulators and/or inhibitors of angiogenesis and determining which aspect of the process is modulated. The assays are designed to be robust and straightforward, using human umbilical vein endothelial cells, but with an option to use other sources such as microvascular endothelial cells from various tissues or lymphatic endothelial cells. It must be appreciated that such reductionist approaches cannot cover the complexity of the angiogenic process as a whole, incorporating as it does a myriad of positive and negative signals, three-dimensional interactions with host tissues and many accessory cells including fibroblasts, macrophages, pericytes and platelets. The extent to which in vitro assays predict physiological or pathological processes in vivo (e.g., wound healing, tumor angiogenesis) or surrogate techniques such as the use of Matrigel™ plugs, sponge implants, corneal assays etc remains to be determined.

PAK4 promotes kinase-independent stabilization of RhoU to modulate cell adhesion.

P21-activated kinase 4 (PAK4) is a Cdc42 effector protein thought to regulate cell adhesion disassembly in a kinase-dependent manner. We found that PAK4 expression is significantly higher in high-grade human breast cancer patient samples, whereas depletion of PAK4 modifies cell adhesion dynamics of breast cancer cells. Surprisingly, systematic analysis of PAK4 functionality revealed that PAK4-driven adhesion turnover is neither dependent on Cdc42 binding nor kinase activity. Rather, reduced expression of PAK4 leads to a concomitant loss of RhoU expression. We report that RhoU is targeted for ubiquitination by the Rab40A-Cullin 5 complex and demonstrate that PAK4 protects RhoU from ubiquitination in a kinase-independent manner. Overexpression of RhoU rescues the PAK4 depletion phenotype, whereas loss of RhoU expression reduces cell adhesion turnover and migration. These data support a new kinase-independent mechanism for PAK4 function, where an important role of PAK4 in cellular adhesions is to stabilize RhoU protein levels. Thus, PAK4 and RhoU cooperate to drive adhesion turnover and promote cell migration.

Discovery of potent, orally bioavailable, small-molecule inhibitors of WNT signaling from a cell-based pathway screen.

WNT signaling is frequently deregulated in malignancy, particularly in colon cancer, and plays a key role in the generation and maintenance of cancer stem cells. We report the discovery and optimization of a 3,4,5-trisubstituted pyridine 9 using a high-throughput cell-based reporter assay of WNT pathway activity. We demonstrate a twisted conformation about the pyridine-piperidine bond of 9 by small-molecule X-ray crystallography. Medicinal chemistry optimization to maintain this twisted conformation, cognisant of physicochemical properties likely to maintain good cell permeability, led to 74 (CCT251545), a potent small-molecule inhibitor of WNT signaling with good oral pharmacokinetics. We demonstrate inhibition of WNT pathway activity in a solid human tumor xenograft model with evidence for tumor growth inhibition following oral dosing. This work provides a successful example of hypothesis-driven medicinal chemistry optimization from a singleton hit against a cell-based pathway assay without knowledge of the biochemical target.

A cyclic peptide inhibitor of HIF-1 heterodimerization that inhibits hypoxia signaling in cancer cells.

Hypoxia inducible factor-1 (HIF-1) is a heterodimeric transcription factor that acts as the master regulator of cellular response to reduced oxygen levels, thus playing a key role in the adaptation, survival, and progression of tumors. Here we report cyclo-CLLFVY, identified from a library of 3.2 million cyclic hexapeptides using a genetically encoded high-throughput screening platform, as an inhibitor of the HIF-1α/HIF-1ß protein-protein interaction in vitro and in cells. The identified compound inhibits HIF-1 dimerization and transcription activity by binding to the PAS-B domain of HIF-1α, reducing HIF-1-mediated hypoxia response signaling in a variety of cell lines, without affecting the function of the closely related HIF-2 isoform. The reported cyclic peptide demonstrates the utility of our high-throughput screening platform for the identification of protein-protein interaction inhibitors, and forms the starting point for the development of HIF-1 targeted cancer therapeutics.

Advances in establishment and analysis of three-dimensional tumor spheroid-based functional assays for target validation and drug evaluation.

BACKGROUND: There is overwhelming evidence that in vitro three-dimensional tumor cell cultures more accurately reflect the complex in vivo microenvironment than simple two-dimensional cell monolayers, not least with respect to gene expression profiles, signaling pathway activity and drug sensitivity. However, most currently available three-dimensional techniques are time consuming and/or lack reproducibility; thus standardized and rapid protocols are urgently needed. RESULTS: To address this requirement, we have developed a versatile toolkit of reproducible three-dimensional tumor spheroid models for dynamic, automated, quantitative imaging and analysis that are compatible with routine high-throughput preclinical studies. Not only do these microplate methods measure three-dimensional tumor growth, but they have also been significantly enhanced to facilitate a range of functional assays exemplifying additional key hallmarks of cancer, namely cell motility and matrix invasion. Moreover, mutual tissue invasion and angiogenesis is accommodated by coculturing tumor spheroids with murine embryoid bodies within which angiogenic differentiation occurs. Highly malignant human tumor cells were selected to exemplify therapeutic effects of three specific molecularly-targeted agents: PI-103 (phosphatidylinositol-3-kinase (PI3K)-mammalian target of rapamycin (mTOR) inhibitor), 17-N-allylamino-17-demethoxygeldanamycin (17-AAG) (heat shock protein 90 (HSP90) inhibitor) and CCT130234 (in-house phospholipase C (PLC)γ inhibitor). Fully automated analysis using a Celigo cytometer was validated for tumor spheroid growth and invasion against standard image analysis techniques, with excellent reproducibility and significantly increased throughput. In addition, we discovered key differential sensitivities to targeted agents between two-dimensional and three-dimensional cultures, and also demonstrated enhanced potency of some agents against cell migration/invasion compared with proliferation, suggesting their preferential utility in metastatic disease. CONCLUSIONS: We have established and validated a suite of highly reproducible tumor microplate three-dimensional functional assays to enhance the biological relevance of early preclinical cancer studies. We believe these assays will increase the translational predictive value of in vitro drug evaluation studies and reduce the need for in vivo studies by more effective triaging of compounds.

Brk protects breast cancer cells from autophagic cell death induced by loss of anchorage.

Brk, a tyrosine kinase expressed in a majority of breast tumors, but not normal mammary tissue, promotes breast carcinoma cell proliferation. Normal epithelial cells are dependent on cell-cell or cell-matrix interactions for survival and undergo apoptosis after disruption of these interactions. Tumor cells are less sensitive to the induction of apoptosis and are predicted to have the potential to disseminate. We investigated whether Brk has further roles in breast tumor progression by relating its expression to tumor grade and demonstrating its role in the regulation of carcinoma cell survival under non-adherent conditions. Brk expression was determined by reverse transcription PCR on RNA extracted from surgical samples of human breast cancers. Breast carcinoma cell survival in suspension culture was examined when Brk protein levels were suppressed by RNA interference. Additionally, the effect of experimentally overexpressing Brk in otherwise Brk-negative breast carcinoma cells was assessed. Brk mRNA expression was notably higher in grade 3 breast tumors, as compared with lower tumor grades. In suspension culture, Brk suppression increased the rate of cell death, as compared with controls, and this cell death program exhibited characteristics of autophagy but not of apoptosis. Conversely, experimental expression of Brk in Brk-negative cells increased cell survival whereas kinase-inactive Brk did not. Therefore, Brk enhances breast carcinoma cell survival in suspension, suggesting a role for Brk in supporting breast cancer cell dissemination.

The identification of novel PLC-gamma inhibitors using virtual high throughput screening.

Phospholipase C-gamma (PLC-gamma) has been identified as a possible biological target for anticancer drug therapy but suitable inhibitors are lacking. Therefore, in order to identify active compounds (hits) virtual high throughput screening was performed. The crystal structure of the PLC-delta isoform was used as a model docking scaffold since no crystallographic data are available on its gamma counterpart. A pilot screen was performed using approximately 9.2x10(4) compounds, where the robustness of the methodology was tested. This was followed by the main screening effort where approximately 4.4x10(5) compounds were used. In both cases, plausible compounds were identified (virtual hits) and a selection of these was experimentally tested. The most potent compounds were in the single digit micro-molar range as determined from the biochemical (Flashplate) assay. This translated into approximately 15 microM in a functional assay in cells. About 30% of the virtual hits showed activity against PLC-gamma (IC(50)<50 microM).

In vitro assays for endothelial cell functions related to angiogenesis: proliferation, motility, tubular differentiation, and proteolysis.

This chapter covers the breakdown of the process of angiogenesis into simple assays to measure discrete endothelial cell functions. The techniques described are suitable for studying stimulators or inhibitors of angiogenesis and determining which aspect of the process is modulated. The procedures outlined are robust and straightforward but cannot cover the complexity of the angiogenic process as a whole, incorporating as it does myriad positive and negative signals, three-dimensional interactions with host tissues and many accessory cells, including fibroblasts, macrophages, pericytes, and platelets. The extent to which in vitro assays predict responses in vivo (e.g., wound healing, tumor angiogenesis, or surrogate techniques such as Matrigel plugs, sponge implants, corneal assays, etc.) remains to be determined.

Syk tyrosine kinase is linked to cell motility and progression in squamous cell carcinomas of the head and neck.

Syk, a non-receptor tyrosine kinase, is an important component of immunoreceptor signaling in hematopoietic cells. It has been implicated in key regulatory pathways including phosphoinositide 3-kinase and phospholipase Cgamma (PLCgamma) activation in B cells and integrin signaling in platelets and bronchial epithelial cells. Recently, potential roles in cancer have been reported. In breast cancers, reduced Syk expression was associated with invasion, and its overexpression in cell lines was shown to inhibit cell motility. In contrast, Syk has been shown to mediate chemomigration in nasopharyngeal carcinoma cells. Its role in squamous cell carcinomas of the head and neck (SCCHN) has not yet been investigated. Syk mRNA and protein expression was detected in 6 of 10 SCCHN cell lines. When Syk was transfected into Syk-negative cells (SIHN-011A), chemomigration was enhanced in vitro and this was associated with activation of PLCgamma1. Conversely, abrogation of Syk activity by pharmacologic inhibition or small interfering RNA in HN6 cells with high levels of endogenous expression inhibited migration, haptotaxis, and engagement with matrix proteins; this was accompanied by decreased levels of phosphorylated AKT. Similar effects were seen in Syk-positive CAL 27 cells but not in Syk-negative SIHN-011A cells. Immunoprecipitation suggested co-association of Syk with epidermal growth factor receptor and GRB-2. Syk expression in SCCHN patient tissues was examined by semiquantitative real-time PCR (n = 45) and immunohistochemistry (n = 38) in two independent cohorts. Higher levels of Syk expression were observed in tumors and lymph node metastases relative to normal tissues. High Syk expression significantly correlated with worse survival and may be of prognostic value in SCCHN due to its potential role in cell migration and invasion.

Cell migration/invasion assays and their application in cancer drug discovery.

Invasive capacity is the single most important trait that distinguishes benign from malignant lesions. Tumour cells, during intravasation and extravasation of blood and lymphatic channels and when establishing colonies at secondary sites, must move through tissue boundaries that normal adult cells (other than, for example activated leukocytes) do not cross. Similar mechanisms are also utilised by activated endothelial cells during the generation of new blood vessels that enable the sustained growth and dissemination of tumours. It is now increasingly recognised that these processes--cell motility and invasion--might provide a rich source of novel targets for cancer therapy and that appropriate inhibitors may restrain both metastasis and neoangiogenesis. This new paradigm demands screening assays that can rapidly and quantitatively measure cell movement and the ability to traverse physiological barriers. We also need to consider whether simple reductionist in vitro approaches can reliably model the complexity of in vivo tumour invasion/neoangiogenesis. There are both opportunities and challenges ahead in developing a balanced portfolio of assays that will be able to evaluate accurately and finally deliver novel anti-invasive agents with therapeutic potential for clinical use.

ErbB2 overexpression in an ovarian cancer cell line confers sensitivity to the HSP90 inhibitor geldanamycin.

ErbB2 is overexpressed in 25-30% of breast and ovarian cancers, correlates with poor prognosis and lower survival and has also been associated with chemoresistance. We have established an isogenic pair of human ovarian cells that differ only in the expression of erbB2 protein in order to elucidate the role of the protein in determining cellular sensitivity to various drugs and agents. These included cisplatin and paclitaxel, the main drugs used in the treatment of ovarian cancer, and also various signal transduction inhibitors affecting the ras and P13K pathways. Transfection of erbB2 resulted in cells stably overexpressing the protein and showing increased motility compared to the empty vector control cells. In cells overexpressing erbB2, the most notable effect on chemosensitivity was that of significantly increased (5-fold) sensitivity to the heat shock protein 90 (HSP90) molecular chaperone inhibitor geldanamycin. In contrast, erbB2-overexpressing cells showed statistically significant resistance to cisplatin, the P13K inhibitor LY294002 and the tyrosine kinase inhibitor emodin. No significant difference in growth inhibition was observed after exposure to paclitaxel, two additional HSP90 inhibitors radicicol and 17AAG, the cyclin-dependent kinase inhibitor flavopiridol, the epidermal growth factor receptor (EGFR) tyrosine kinase inhibitor PD153035, the mek inhibitor U0126 or the famesyl transferase inhibitor R115777. Exposure of cells to geldanamycin, 17AAG, emodin, LY294002 and cisplatin led to depletion of erbB2 in the transfected cells. These data suggest that erbB2 status in ovarian cancr may contribute to chemosensitivity, in some cases leading to increased sensitivity (as with geldanamycin) but in other cases leading to resistance (as with cisplatin).