PH006, a novel and selective Src kinase inhibitor, suppresses human breast cancer growth and metastasis in vitro and in vivo

Intramitochondrial Src kinase links mitochondrial dysfunctions and aggressiveness of breast cancer cells

High levels and activity of Src kinase are common among breast cancer subtypes, and several inhibitors of the kinase are currently tested in clinical trials. Alterations in mitochondrial activity is also observed among the different types of breast cancer. Src kinase is localized in several subcellular compartments, including mitochondria where it targets several proteins to modulate the activity of the organelle. Although the subcellular localization of other oncogenes modulates the potency of known treatments, nothing is known about the specific role of intra-mitochondrial Src (mtSrc) in breast cancer. The aim of this work was to determine whether mtSrc kinase has specific impact on breast cancer cells. We first observed that activity of mtSrc is higher in breast cancer cells of the triple negative subtype. Over-expression of Src specifically targeted to mitochondria reduced mtDNA levels, mitochondrial membrane potential and cellular respiration. These alterations of mitochondrial functions led to lower cellular viability, shorter cell cycle and increased invasive capacity. Proteomic analyses revealed that mtSrc targets the mitochondrial single-stranded DNA-binding protein, a regulator of mtDNA replication. Our findings suggest that mtSrc promotes aggressiveness of breast cancer cells via phosphorylation of mitochondrial single-stranded DNA-binding protein leading to reduced mtDNA levels and mitochondrial activity. This study highlights the importance of considering the subcellular localization of Src kinase in the development of potent therapy for breast cancer.

SRC tyrosine kinase activates the YAP/TAZ axis and thereby drives tumor growth and metastasis

When properly employed, targeted therapies are effective cancer treatments. However, the development of such therapies requires the identification of targetable drivers of cancer development and metastasis. The expression and nuclear localization of the transcriptional coactivators Yes-associated protein (YAP) and transcriptional co-activator with PDZ-binding motif (TAZ) are increased in many human cancers, and experimental evidence indicates that aberrant YAP or TAZ activation drives tumor formation and metastasis. Although these findings make YAP and TAZ appealing therapeutic targets, both have important functions in adult tissues, so directly targeting them could cause adverse effects. The identification of pathways active in cancer cells and required for YAP/TAZ activity could provide a way to inhibit YAP and TAZ. Here, we show that SRC proto-oncogene, nonreceptor tyrosine kinase (SRC) is an important driver of YAP/TAZ activity in human breast cancer and melanoma cells. SRC activation increased YAP/TAZ activity and the expression of YAP/TAZ-regulated genes. In contrast, SRC inhibition or knockdown repressed both YAP/TAZ activity and the expression of YAP/TAZ-regulated genes. We also show that SRC increases the activity of YAP and TAZ by repressing large tumor suppressor homolog (LATS), and we identify the GTPase-activating protein GIT ArfGAP 1 (GIT1) as an SRC effector that regulates both YAP and TAZ. Importantly, we demonstrate that SRC-mediated YAP/TAZ activity promotes tumor growth and enhances metastasis and that SRC-dependent tumor progression depends, at least in part, on YAP and TAZ. Our findings suggest that therapies targeting SRC could help manage some YAP/TAZ-dependent cancers.

YAP/TAZ Activation as a Target for Treating Metastatic Cancer

Yes-Associated Protein (YAP) and Transcriptional Co-activator with PDZ-binding Motif (TAZ) have both emerged as important drivers of cancer progression and metastasis. YAP and TAZ are often upregulated or nuclear localized in aggressive human cancers. There is abundant experimental evidence demonstrating that YAP or TAZ activation promotes cancer formation, tumor progression, and metastasis. In this review we summarize the evidence linking YAP/TAZ activation to metastasis, and discuss the roles of YAP and TAZ during each step of the metastatic cascade. Collectively, this evidence strongly suggests that inappropriate YAP or TAZ activity plays a causal role in cancer, and that targeting aberrant YAP/TAZ activation is a promising strategy for the treatment of metastatic disease. To this end, we also discuss several potential strategies for inhibiting YAP/TAZ activation in cancer and the challenges each strategy poses.

Cyr61 as mediator of Src signaling in triple negative breast cancer cells

SFKs are involved in tumorigenesis and metastasis. Here we analyzed c-Src contribution to initial steps of metastasis by tetracycline-dependent expression of a specific shRNA-c-Src, which suppressed c-Src mRNA and protein levels in metastatic MDA-MB-231 cells. c-Src suppression did not alter cell proliferation or survival, but it significantly reduced anchorage-independent growth. Concomitantly with diminished tyrosine-phosphorylation/activation of Fak, caveolin-1, paxillin and p130CAS, c-Src depletion also inhibited cellular migration, invasion and transendothelial migration. Quantitative proteomic analyses of the secretome showed that Cyr61 levels, which were detected in the exosomal fraction, were diminished upon shRNA-c-Src expression. In contrast, Cyr61 expression was unaltered inside cells. Cyr61 partially colocalized with cis-Golgi gp74 marker and with exosomal marker CD63, but c-Src depletion did not alter their cellular distribution. In SUM159PT cells, transient c-Src suppression also reduced secreted exosomal Cyr61 levels. Furthermore, conditional expression of a c-Src dominant negative mutant (SrcDN, c-Src-K295M/Y527F) in MDA-MB-231 and in SUM159PT diminished secreted Cyr61 as well. Cyr61 transient suppression in MDA-MB-231 inhibited invasion and transendothelial migration. Finally, in both MDA-MB-231 and SUM159PT, a neutralizing Cyr61 antibody restrained migration. Collectively, these results suggest that c-Src regulates secreted proteins, including the exosomal Cyr61, which are involved in modulating the metastatic potential of triple negative breast cancer cells.

Meaningful prevention of breast cancer metastasis: candidate therapeutics, preclinical validation, and clinical trial concerns

The development of drugs to treat breast and other cancers proceeds through phase I dose finding, phase II efficacy, and phase III comparative studies in the metastatic setting, only then asking if metastasis can be prevented in adjuvant trials. Compounds without overt cytotoxic activity, such as those developed to inhibit metastatic colonization, will likely fail to shrink established lesions in the metastatic setting and never be tested in a metastasis prevention scenario where they were preclinically validated. We and others have proposed phase II primary and secondary metastasis prevention studies to address this need. Herein, we have asked whether preclinical metastasis prevention data agrees with the positive adjuvant setting trials. The data are limited but complimentary. We also review fundamental pathways involved in metastasis, including Src, integrins, focal adhesion kinase (FAK), and fibrosis, for their clinical progress to date and potential for metastasis prevention. Issues of inadequate preclinical validation and clinical toxicity profiles are discussed.

Dysregulated activation of c-Src in gestational trophoblastic disease contributes to its aggressive progression

INTRODUCTION

Gestational trophoblastic disease (GTD) is a heterogeneous group of pregnancy-related disorders. Hydatidiform mole (HM) is the most common type of GTD, whereas gestational choriocarcinoma is the most aggressive. Non-receptor tyrosine kinase c-Src contributes to the transformation to a malignant phenotype in various cancers. However, the role of c-Src in the pathogenesis of GTD remains largely unknown.

METHODS

The expression level of phosphorylated c-Src was determined by immunohistochemistry and Western blotting assay. JAR and JEG-3 cells were treated with hCG, specific c-Src inhibitor saracatinib and PP2, and PKA specific inhibitor, PKI. Cell growth rate and cell migration/invasion ability was determined by cell proliferation and transwell assays respectively.

RESULTS

c-Src was highly activated in HM tissues and choriocarcinoma cells (JAR and JEG-3). c-Src was activated by hCG in a time and concentration-dependent manner, which was abrogated by specific c-Src and PKA inhibitors. Inhibition of c-Src activity in JAR and JEG-3 cells by saracatinib leaded to a decrease in the rate of cell growth and cell migration/invasion ability. Furthermore, inhibition of c-Src phosphorylation induced cell cycle arrest and reduced expressions of cyclin A2, cyclin B1, cyclin E1, FOXD3 and NANOG. Moreover, inhibition of c-Src activity resulted in decreased p-FAK(Tyr397) phosphorylation.

DISCUSSION AND CONCLUSION

Our findings indicate an important role of c-Src in the pathogenesis of GTD, and we propose that c-Src inhibitors are potential adjuvant chemotherapeutic drugs for the treatment of GTD.

Inhibition of Src activation with cardiotoxin III blocks migration and invasion of MDA-MB-231 cells

Cardiotoxin III (CTX III), a basic polypeptide isolated from Naja naja atra venom, has been demonstrated to display anticancer activity. Breast cancer is a highly malignant carcinoma and most deaths of breast cancer are caused by metastasis. In this study, we show that CTX III blocks migration and invasion of MDA-MB-231 breast cancer cells without affecting apoptosis or cell cycle arrest. CTX III caused significant block of Src kinase activity in MDA-MB-231 cells. Moreover, CTX III treatment was correlated with reduced phosphorylation of FAK at Tyr576, 861 and 925 sites, p130(Cas) at Tyr410, and paxillin at Tyr118. CTX III also suppressed the activation of extracellular signal-regulated kinase1/2 and phosphatidylinositol 3-kinase/Akt. Consistent with inhibition of these signaling pathways and invasion, CTX III inhibited the expression of matrix metalloproteinase-9. In addition, Src specific inhibitor PP2 caused a significant decrease in the phosphorylation of FAK, p130(Cas), paxillin, PI3K/Akt, and ERK1/2. Taken together, CTX III significantly inhibited phosphorylation of Src and downstream molecules as well as cell migration and invasion. Our findings provide evidences that CTX III inhibits Src-mediated signaling pathways involved in controlling MDA-MB-231 cell migration and invasion, suggesting that it has therapeutic potential in breast cancer treatment.

A guide to picking the most selective kinase inhibitor tool compounds for pharmacological validation of drug targets

To establish the druggability of a target, genetic validation needs to be supplemented with pharmacological validation. Pharmacological studies, especially in the kinase field, are hampered by the fact that many reference inhibitors are not fully selective for one target. Fortunately, the initial trickle of selective inhibitors released in the public domain has steadily swelled into a stream. However, rationally picking the most selective tool compound out of the increasing amounts of available inhibitors has become progressively difficult due to the lack of accurate quantitative descriptors of drug selectivity. A recently published approach, termed 'selectivity entropy', is an improved way of expressing selectivity as a single-value parameter and enables rank ordering of inhibitors. We provide a guide to select the best tool compounds for pharmacological validation experiments of candidate drug targets using selectivity entropy. In addition, we recommend which inhibitors to use for studying the biology of the 20 most investigated kinases that are clinically relevant: Abl (ABL1), AKT1, ALK, Aurora A/B, CDKs, MET, CSF1R (FMS), EGFR, FLT3, ERBB2 (HER2), IKBKB (IKK2), JAK2/3, JNK1/2/3 (MAPK8/9/10), MEK1/2, PLK1, PI3Ks, p38α (MAPK14), BRAF, SRC and VEGFR2 (KDR).

Inhibition of tumor cell growth, proliferation and migration by X-387, a novel active-site inhibitor of mTOR

The mammalian target of rapamycin (mTOR), is deregulated in about 50% of human malignancies and exists in two complexes: mTORC1 and mTORC2. Rapalogs partially inhibit mTORC1 through allosteric binding to mTORC1 and their efficacy is modest as a cancer therapy. A few mTOR kinase inhibitors that inhibit both mTORC1 and mTORC2 have been reported to possess potent anticancer activities. Herein, we designed and synthesized a series of pyrazolopyrimidine derivatives targeting mTOR kinase domain and X-387 was identified as a promising lead. X-387 selectively inhibited mTOR in an ATP-competitive manner while sparing a panel of kinases from the PIKK family. X-387 blocked mTORC1 and mTORC2-mediacted signaling pathway in cell lines with activated mTOR signaling and in rapamycin-resistant cells. Specifically, X-387 inhibited phosphorylation of AKT at T308, which is thought to be a target of PDK1 but not mTOR. Such activity was not due to inhibition of PI3K since X-387 did not inhibit translocation of AKT to the cell membrane. X-387 induced autophagy as observed for other mTOR inhibitors, while induced autophagy is pro-survival since concurrent inhibition of autophagy by 3-MA reinforced the antiproliferative activity of mTOR inhibitors. X-387 also inhibited cell motility, which is associated with decrease in activity of small GTPases such as RhoA, Rac1 and Cdc42. Taken together, X-387 is a promising compound lead targeting mTOR and with a wide spectrum anticancer activity among tumor cell lines. The data also underscores the complexity of the mTOR signaling pathways which are far from being understood.