Activated Src abrogates the Myc requirement for the G0/G1 transition but not for the G1/S transition

Newly synthesized 6-substituted piperazine/phenyl-9-cyclopentyl containing purine nucleobase analogs act as potent anticancer agents and induce apoptosis via inhibiting Src in hepatocellular carcinoma cells

Newly synthesized 6-substituted piperazine/phenyl-9-cyclopentyl-containing purine nucleobase analogs were tested for their in vitro anticancer activity against human cancer cells. Compounds 15, 17-24, 49, and 56 with IC50 values less than 10 μM were selected for further examination on an enlarged panel of liver cancer cell lines. Experiments revealed that compound 19 utilizes its high cytotoxic potential (IC50 < 5 μM) to induce apoptosis in vitro. Compound 19 displayed a KINOMEscan selectivity score S35 of 0.02 and S10 of 0.01 and demonstrated a significant selectivity against anaplastic lymphoma kinase (ALK) and Bruton's tyrosine kinase (BTK) over other kinases. Compounds 19, 21, 22, 23, and 56 complexed with ALK, BTK, and (discoidin domain-containing receptor 2) DDR2 were analyzed structurally for binding site interactions and binding affinities via molecular docking and molecular dynamics simulations. Compounds 19 and 56 displayed similar interactions with the activation loop of the kinases, while only compound 19 reached toward the multiple subsites of the active site. Cell cycle and signaling pathway analyses exhibited that compound 19 decreases phosho-Src, phospho-Rb, cyclin E, and cdk2 levels in liver cancer cells, eventually inducing apoptosis.

EZH2 Cooperates with BRD4-NUT to Drive NUT Carcinoma Growth by Silencing Key Tumor Suppressor Genes

NUT carcinoma is an aggressive carcinoma driven by the BRD4-NUT fusion oncoprotein, which activates chromatin to promote expression of progrowth genes. BET bromodomain inhibitors (BETi) are a promising treatment for NUT carcinoma that can impede BRD4-NUT's ability to activate genes, but the efficacy of BETi as monotherapy is limited. Here, we demonstrated that enhancer of zeste homolog 2 (EZH2), which silences genes through establishment of repressive chromatin, is a dependency in NUT carcinoma. Inhibition of EZH2 with the clinical compound tazemetostat potently blocked growth of NUT carcinoma cells. Epigenetic and transcriptomic analysis revealed that tazemetostat reversed the EZH2-specific H3K27me3 silencing mark and restored expression of multiple tumor suppressor genes while having no effect on key oncogenic BRD4-NUT-regulated genes. Indeed, H3K27me3 and H3K27ac domains were found to be mutually exclusive in NUT carcinoma cells. CDKN2A was identified as the only gene among all tazemetostat-derepressed genes to confer resistance to tazemetostat in a CRISPR-Cas9 screen. Combined inhibition of EZH2 and BET synergized to downregulate cell proliferation genes, resulting in more pronounced growth arrest and differentiation than either inhibitor alone. In preclinical models, combined tazemetostat and BETi synergistically blocked tumor growth and prolonged survival of NUT carcinoma-xenografted mice, with complete remission without relapse in one cohort. Identification of EZH2 as a dependency in NUT carcinoma substantiates the reliance of NUT carcinoma tumor cells on epigenetic dysregulation of functionally opposite, yet highly complementary, chromatin regulatory pathways to maintain NUT carcinoma growth.

SIGNIFICANCE

Repression of tumor suppressor genes, including CDKN2A, by EZH2 provides a mechanistic rationale for combining EZH2 and BET inhibitors for the clinical treatment of NUT carcinoma. See related commentary by Kazansky and Kentsis, p. 3827.

EZH2 synergizes with BRD4-NUT to drive NUT carcinoma growth through silencing of key tumor suppressor genes

NUT carcinoma (NC) is an aggressive carcinoma driven by the BRD4-NUT fusion oncoprotein, which activates chromatin to promote expression of pro-growth genes. BET bromodomain inhibitors (BETi) impede BRD4-NUT's ability to activate genes and are thus a promising treatment but limited as monotherapy. The role of gene repression in NC is unknown. Here, we demonstrate that EZH2, which silences genes through establishment of repressive chromatin, is a dependency in NC. Inhibition of EZH2 with the clinical compound tazemetostat (taz) potently blocked growth of NC cells. Epigenetic and transcriptomic analysis revealed that taz reversed the EZH2-specific H3K27me3 silencing mark, and restored expression of multiple tumor suppressor genes while having no effect on key oncogenic BRD4- NUT-regulated genes. CDKN2A was identified as the only gene amongst all taz-derepressed genes to confer resistance to taz in a CRISPR-Cas9 screen. Combined EZH2 inhibition and BET inhibition synergized to downregulate cell proliferation genes resulting in more pronounced growth arrest and differentiation than either inhibitor alone. In pre-clinical models, combined taz and BETi synergistically blocked growth and prolonged survival of NC-xenografted mice, with all mice cured in one cohort.

STATEMENT OF SIGNIFICANCE

Identification of EZH2 as a dependency in NC substantiates the reliance of NC tumor cells on epigenetic dysregulation of functionally opposite, yet highly complementary chromatin regulatory pathways to maintain NC growth. In particular, repression of CDKN2A expression by EZH2 provides a mechanistic rationale for combining EZH2i with BETi for the clinical treatment of NC.

microRNA-205 in prostate cancer: Overview to clinical translation

Prostate cancer (PrCa) is the most common type of cancer among men in the United States. The metastatic and advanced PrCa develops drug resistance to current regimens which accounts for the poor management. microRNAs (miRNAs) have been well-documented for their diagnostic, prognostic, and therapeutic roles in various human cancers. Recent literature confirmed that microRNA-205 (miR-205) has been established as one of the tumor suppressors in PrCa. miR-205 regulates number of cellular functions, such as proliferation, invasion, migration/metastasis, and apoptosis. It is also evident that miR-205 can serve as a key biomarker in diagnostic, prognostic, and therapy of PrCa. Therefore, in this review, we will provide an overview of tumor suppressive role of miR-205 in PrCa. This work also outlines miR-205's specific role in targeted mechanisms for chemosensitization and radiosensitization in PrCa. A facile approach of delivery paths for successful clinical translation is documented. Together, all these studies provide a novel insight of miR-205 as an adjuvant agent for reducing the widening gaps in clinical outcome of PrCa patients.

MAP9 Exhibits Protumor Activities and Immune Escape toward Bladder Cancer by Mediating TGF-β1 Pathway

To investigate more potential targets for the treatment of human bladder cancer, quantitative reverse transcription polymerase chain reaction (qRT-PCR) and high-content screening (HCS) analysis were performed, and microtubule-associated protein 9 (MAP9), which had the strongest proliferation inhibition from 809 downregulated genes, has been selected. MAP9 is responsible for bipolar spindle assembly and is involved in the progression of many types of tumors; however, its role in bladder cancer (BC) remains unknown. Expressive levels of MAP9 in BC tissues were determined through immunohistochemistry, and the clinical significance of MAP9 in BC was analyzed. Short hairpin ribonucleic acid- (ShRNA-) MAP9 was used to construct stable MAP9 knockdown BC cell lines. The proliferative abilities of MAP9 were measured through assays in vivo and in vitro, and the migrated and invasive abilities of MAP9 were analyzed via in vitro experiments. Quantitative reverse transcription PCR, western blotting, coimmunoprecipitation (Co-IP), and rescue assays were used to identify downstream targets of MAP9. MAP9 expression increased in the tumor tissues, and its increased level was negatively correlated with prognosis. Further, the loss of MAP9 caused decreased BC cell proliferation via inducing the growth 1/synthesis (G1/S) cell cycle arrest in vitro and slowed tumor growth in vivo. In addition, MAP9 silencing attenuated BC cell migration and invasion. Moreover, we found that the growth 1/synthesis (G1/S) cell cycle-related genes and the epithelial mesenchymal transition (EMT) marker levels decreased after silencing MAP9. Finally, we found that the transforming growth factor beta 1 (TGF-β1) pathway is activated as a mediator for MAP9 to regulate genes related to the G1/S cell cycle and EMT. MAP9 promotes BC progression and immune escape activity through the TGF-β1 pathway and is a potential novel target for therapies of BC.

Immortalization of chicken embryonic liver-derived cell line by stable expression of hMRP18S-2 for serotype 4 fowl adenovirus propagation

Inclusion body hepatitis and hydropericardium-hepatitis syndrome caused by serotype 4 fowl adenovirus (FAdV-4) have emerged in China since 2013. FAdV is usually propagated in primary chicken embryonic liver cells or embryo yolk sac. The aim of this work was to develop an immortalized CEL cell line by stable expression of human mitochondrial ribosomal protein 18S-2, named CEL-hMRP18S-2 cells, for the propagation of FAdV-4. The maximum cell density of CEL-hMRP18S-2 cells could reach 2.65 × 10⁶ cells/ml in four-days culture. According to the mRNA levels of cell-cycle related genes in CEL-hMRP18S-2 cells tested by qRT-PCR, we speculated that the transformation of hMRP18S-2 into CEL cells caused the functional inactivation of p53 and the significant down-regulation of p15^INK4b might cause the hyperphosphorylated form of Rb, releasing E2F-1 factor and enhancing the E2F-dependent transcription for cell cycle progression. It was suspected that the up-regulated c-Myc mRNA level at the initial period of immortalization might prompt transformed cells through the G0-G1 checkpoint. The normal CPE was observed in CEL-hMRP18S-2 cells infected by FAdV-4 and microcarrier suspension culture performed for FAdV-4 propagation with 9.0 lgTCID₅₀/ml suggested that CEL-hMRP18S-2 cells could be a useful continuous cell line for isolation of wild FAdV and production of FAdV-inactivated vaccine.

βArrestin2 Mediates Renal Cell Carcinoma Tumor Growth

Renal Cell Carcinoma (RCC) is one of the most lethal urological cancers worldwide. The disease does not present early clinical symptoms and is commonly diagnosed at an advanced stage. Limited molecular drivers have been identified for RCC, resulting in the lack of effective treatment for patients with progressive disease. Ubiquitous βArrestin2 (βArr2) is well established for its function in the desensitization and trafficking of G protein-coupled receptors. More recently, βArr2 has been implicated in the regulation of fundamental cellular functions, including proliferation and invasion. We used bioinformatic and genetic approaches to determine role of βArr2 in RCC tumor growth. Analysis of published human datasets shows that ARRB2 (gene encoding βArr2) expression is increased in RCC tumor compared to normal tissue and that high levels of ARRB2 correlate with worse patient survival. Experimentally, we show that knockout of ARRB2 decreases rate of RCC cell proliferation and migration in vitro and xenograft tumor growth in animals. Mechanistically, βArr2 regulates c-Src activity, Cyclin A expression and cell cycle progression that are involved in tumor growth. These results show that βArr2 is a critical regulator of RCC tumor growth and suggest its utility as a potential marker and drug target to treat advanced disease.

Targeting MYC in multiple myeloma

Multiple myeloma (MM) is a plasma cell tumor marked by clonal evolution and preceded by a premalignant stage, which progresses via molecular pathway deregulation, including MYC activation. This activation relates to translocation or gain of the MYC locus and deregulation of upstream pathways such as IRF4, DIS3/LIN28B/let-7, or MAPK. Precision medicine is an approach to predict more accurately which treatment strategies for a particular disease will work in which groups of patients, in contrast to a "one-size-fits-all" approach. The knowledge of mechanisms responsible for MYC deregulation in MM enables identification of vulnerabilities and therapeutic targets in MYC-driven tumors. MYC can be targeted directly or indirectly, by interacting with several of its functions in cancer. Several such therapeutic strategies are evaluated in clinical trials in MM. In this review, we describe the mechanism of MYC activation in MM, the role of MYC in cancer progression, and the therapeutic options to targeting MYC.

Down regulation of the long non-coding RNA PCAT-1 induced growth arrest and apoptosis of colorectal cancer cells

AIMS

The long non-coding RNA (lncRNA) was reported to be involved in the progress of various cancers, however, its effect in colorectal cancer (CRC) remains unknown. The goal of the present study is to investigate the function role of lncRNA PCAT-1 in colorectal cancer.

MAIN METHODS

The expression of lncRNA PCAT-1 in four CRC cell lines was measured by real-time PCR, and two lncRNA PCAT-1 high expression cell lines were selected. LncRNA PCAT-1 in these two CRC cell lines was down-regulated by shRNA, and the stable transfected cells were established. Functional involvement of lncRNA PCAT-1 in proliferation and apoptosis of the two CRC cells were evaluated in vitro. Mover, the effect of lncRNA PCAT-1 in tumor proliferation was also evaluated in CRC cell xenograft.

KEY FINDINGS

The results showed that down-regulation of lncRNA PCAT-1 in CRC cells inhibited proliferation, blocked cell cycle transition, and suppressed the expression of cyclins and c-myc. The apoptosis cell proportion was elevated with increased expression of pro-apoptotic proteins and decreased anti-apoptotic proteins in lncRNA PCAT-1 knock down cells. Forced over-expression of c-myc in PCAT-1 down-regulated CRC cells increased the level of cyclins. The xenograft growth in lncRNA PCAT-1 down-regulated cells was significantly inhibited along with the reduced proliferative cells.

SIGNIFICANCE

Our study revealed a tumorigenic effect of lncRNA PCAT-1 in CRC cells, and this effect is partly dependent on the inhibition of c-myc.

Mitochondrial ribosomal protein S18-2 evokes chromosomal instability and transforms primary rat skin fibroblasts

We have shown earlier that overexpression of the human mitochondrial ribosomal protein MRPS18-2 (S18-2) led to immortalization of primary rat embryonic fibroblasts. The derived cells expressed the embryonic stem cell markers, and cellular pathways that control cell proliferation, oxidative phosphorylation, cellular respiration, and other redox reactions were activated in the immortalized cells.Here we report that, upon overexpression of S18-2 protein, primary rat skin fibroblasts underwent cell transformation. Cells passed more than 300 population doublings, and two out of three tested clones gave rise to tumors in experimental animals. Transformed cells showed anchorage-independent growth and loss of contact inhibition; they expressed epithelial markers, such as E-cadherin and β-catenin. Transformed cells showed increased telomerase activity, disturbance of the cell cycle, and chromosomal instability. Taken together, our data suggest that S18-2 is a newly identified oncoprotein that may be involved in cancerogenesis.

Effects of LG268 on Cell Proliferation and Apoptosis of NB4 Cells

AIMS

To investigate the effect of LG100268 (LG268) on cell proliferation and apoptosis in NB4 cells.

METHODS

NB4 cells were treated with LG268 for 24 h or 48 h. The effect of LG268 on cell proliferation was assessed by the CCK-8 assay and colony-forming assay. Apoptosis and cell cycle were evaluated by flow cytometry. The protein expression levels of Survivin, PARP, c-Myc, cyclin D1, ERK, p-ERK, p38 MAPK, and p- p38 MAPK were detected by western blot.

RESULTS

We found that LG268 inhibited the proliferation of NB4 cells in a dose-dependent manner. Flow cytometry analysis showed that LG268 accelerated apoptosis in NB4 cells in a time- dependent manner and that LG268 treatment led to cell cycle arrest at G0/G1 phase. Moreover, LG268 significantly decreased the protein levels of Survivin, c-Myc, and cyclinD1. Cleaved PARP was observed in the LG268 treatment group but not in the control group. In addition, LG268 increased the phosphorylation level of p38 MAPK and decreased the phosphorylation level of ERK.

CONCLUSIONS

LG268 inhibited cell proliferation and promoted cell apoptosis in NB4 cells.

SRC inhibition represents a potential therapeutic strategy in liposarcoma

Liposarcomas (LS) are the most common malignant mesenchymal tumors, with an overall long-term mortality rate of 60%. LS comprise three major subtypes, i.e., well-differentiated/dedifferentiated liposarcoma (WDLS/DDLS), myxoid/round cell liposarcoma (MLS) and pleomorphic liposarcoma (PLS). Aiming at the preclinical identification of novel therapeutic options, we here investigate the functional significance of SRC in primary human LS and in LS-derived cell lines. Immunohistochemical and Western blot analyses reveal relevant levels of activated p-(Tyr416)-SRC in LS of the different subtypes with particular activation in MLS and PLS. Dysregulation of the SRC modifiers CSK and PTP1B was excluded as major reason for the activation of the kinase. Consistent siRNA-mediated knockdown of SRC or inhibition by the SRC inhibitor Dasatinib led to decreased proliferation of LS cell lines of the different subtypes, with MLS cells reacting particularly sensitive in MTT assays. Flow cytometric analyses revealed that this effect was due to a significant decrease in mitotic activity and an induction of apoptosis. SRC inhibition by Dasatinib resulted in dephosphorylation of SRC itself, its interacting partners FAK and IGF-IR as well as its downstream target AKT. Consistent with a particular role of SRC in cell motility, Dasatinib reduced the migratory and invasive potential of MLS cells in Boyden chamber and Matrigel chamber assays. In summary, we provide evidence that SRC activation plays an important role in LS biology and therefore represents a potential therapeutic target, particularly in MLS and PLS.

Pharmacology of Src family kinases and therapeutic implications of their modulators

Src family kinases (SFKs), the largest family of nonreceptor tyrosine kinases, include 10 members. Src was the first gene product discovered to have intrinsic protein tyrosine kinase activity. Src is widely expressed in many cell types and can have different locations within a cell; the subcellular location of Src can affect its function. Src can associate with cellular membranes, such as the plasma membrane, the perinuclear membrane, and the endosomal membrane. SFKs actions on mammalian cells are pleiotropic and include effect on cell morphology, adhesion, migration, invasion, proliferation, differentiation, and survival. SFKs at one end have been documented to play some important physiological functions; on the other end, they have been described in the pathophysiology of some disorders. In this review article, an exhaustive attempt has been made to unearth pharmacology of SFKs and therapeutic implications of SFKs modulators.

MYC and the control of DNA replication

The MYC oncogene is a multifunctional protein that is aberrantly expressed in a significant fraction of tumors from diverse tissue origins. Because of its multifunctional nature, it has been difficult to delineate the exact contributions of MYC's diverse roles to tumorigenesis. Here, we review the normal role of MYC in regulating DNA replication as well as its ability to generate DNA replication stress when overexpressed. Finally, we discuss the possible mechanisms by which replication stress induced by aberrant MYC expression could contribute to genomic instability and cancer.

AKN-028 induces cell cycle arrest, downregulation of Myc associated genes and dose dependent reduction of tyrosine kinase activity in acute myeloid leukemia

AKN-028 is a novel tyrosine kinase inhibitor with preclinical activity in acute myeloid leukemia (AML), presently undergoing investigation in a phase I/II study. It is a potent inhibitor of the FMS-like kinase 3 (FLT3) but shows in vitro activity in a wide range of AML samples. In the present study, we have characterized the effects of AKN-028 on AML cells in more detail. AKN-028 induced a dose-dependent G0/1 arrest in AML cell line MV4-11. Treatment with AKN-028 caused significantly altered gene expression in all AML cell types tested (430 downregulated, 280 upregulated transcripts). Subsequent gene set enrichment analysis revealed enrichment of genes associated with the proto-oncogene and cell cycle regulator c-Myc among the downregulated genes in both AKN-028 and midostaurin treated cells. Kinase activity profiling in AML cell lines and primary AML samples showed that tyrosine kinase activity, but not serine/threonine kinase activity, was inhibited by AKN-028 in a dose dependent manner in all samples tested, reaching approximately the same level of kinase activity. Cells sensitive to AKN-028 showed a higher overall tyrosine kinase activity than more resistant ones, whereas serine/threonine kinase activity was similar for all primary AML samples. In summary, AKN-028 induces cell cycle arrest in AML cells, downregulates Myc-associated genes and affect several signaling pathways. AML cells with high global tyrosine kinase activity seem to be more sensitive to the cytotoxic effect of AKN-028 in vitro.

miR-205 is frequently downregulated in prostate cancer and acts as a tumor suppressor by inhibiting tumor growth

The purpose of this study was to elucidate the molecular mechanisms of microRNA-205 (miR-205) as a tumor suppressor in prostate cancer (PCa). In the present study, microRNA microarray analysis suggested that the expression of miR-205 was significantly decreased in advanced PCa compared with early PCa. Real-time PCR analysis also indicated that miR-205 expression was significantly decreased in PCa tissues compared with non-cancerous tissues. Moreover, the expression of miR-205 has been demonstrated to be associated with the clinicopathological stage and total/free prostate-specific antigen (PSA) level of PCa. Functional analyses showed that both the overexpression of miR-205 and the knockdown of c-SRC in PCa cell lines could inhibit cell growth, colony formation, migration, invasion and the cell cycle as well as induce cell apoptosis in vitro. Furthermore, over-expressing miR-205 reduced tumorigenicity in vivo. Through a luciferase activity assay and Western blotting, c-SRC was identified as a target of miR-205 in cells. The overexpression of miR-205 suppressed c-SRC and its downstream signaling molecules, including FAK, p-FAK, ERK1/2 and p-ERK1/2, and attenuated cell proliferation, invasion and tumor growth.

miR-23b represses proto-oncogene Src kinase and functions as methylation-silenced tumor suppressor with diagnostic and prognostic significance in prostate cancer

The miRNAs have great potential as biomarkers and therapeutic agents owing to their ability to control multiple genes and potential to influence cellular behavior. Here, we identified that miR-23b is a methylation-silenced tumor suppressor in prostate cancer. We showed that miR-23b expression is controlled by promoter methylation and has great promise as a diagnostic and prognostic biomarker in prostate cancer. High levels of miR-23b expression are positively correlated with higher overall and recurrence-free survival in patients with prostate cancer. Furthermore, we elucidated the tumor suppressor role of miR-23b using in vitro and in vivo models. We showed that proto-oncogene Src kinase and Akt are direct targets of miR-23b. Increased expression of miR-23b inhibited proliferation, colony formation, migration/invasion, and triggered G(0)-G(1) cell-cycle arrest and apoptosis in prostate cancer. Overexpression of miR-23b inhibited epithelial-to-mesenchymal transition (EMT) causing a decline in mesenchymal markers Vimentin and Snail and increasing the epithelial marker, E-cadherin. Depletion of Src by RNA interference conferred similar functional effects as that of miR-23b reconstitution. miR-23b expression caused a dramatic decrease in tumor growth in nude mice and attenuated Src expression in excised tumors compared with a control miR. These findings suggest that miR-23b is a methylation-silenced tumor suppressor that may be a useful biomarker in prostate cancer. Loss of miR-23b may confer proliferative advantage and promote prostate cancer migration and invasion, and reexpression of miR-23b may contribute to the epigenetic therapy for prostate cancer.

Human Breast Cancer Stem Cells Have Significantly Higher Rate of Clathrin-Independent and Caveolin-Independent Endocytosis than the Differentiated Breast Cancer Cells

Breast Cancer Stem (BCS) cells play critical roles in self-renewal, Multi Drug Resistance (MDR), differentiation and generation of secondary tumors. Conventional chemotherapy may efficiently kill the bulk of differentiated drug sensitive breast cancer cells, but not the MDR self-renewable BCS cells, leading to enrichment of the MDR BCS cells. In order to target the MDR BCS cells, we have isolated: 1) BCS cells from either breast cancer cell lines or fresh breast cancer specimens; 2) ATP binding cassette (ABC) transporter group G number 2 (ABCG2)-specific aptamers; and 3) BCS cell-binding aptamers. Interestingly, ABCG2-specific aptamers labeled the membrane surface of the ABCG2-expressing baby hamster kidney (BHK) cells, but stained whole cells of the BCS cells derived from mammospheres, implying that BCS cells might have much higher rate of endocytosis than the ABCG2-expressing BHK cells. In addition, 5D3, a monoclonal antibody that recognizes the extracellular loops of ABCG2 protein, also stained whole BCS cells. Furthermore, BCS cell-binding aptamers stained whole BCS cells, but not the differentiated breast cancer MCF-7 cells. All these results support above conclusion that BCS cells might have high rate of endocytosis. Further experiments performed with aptamers and human transferrin or lactosylceramide showed that BCS cells do have much higher endocytosis rate than the differentiated breast cancer cells. Interestingly, clathrin dependent endocytosis inhibitors, such as monodansylcadaverine or sucrose, or caveolin-dependent endocytosis inhibitors, such as methyl-β-cyclodextrin or genistein, can inhibit the internalization of transferrin or lactosylceramide into the differentiated breast cancer cells, but cannot block the internalization of these compounds into the BCS cells, suggesting that BCS cells undergo clathrin-independent and caveolin-independent endocytosis. Taken together, our data suggest that BCS cells have high rate of endocytosis and open the possibilities for delivering therapeutic agents directly into the MDR BCS cells with aptamer-coated liposomes.

MicroRNA-205 inhibits Src-mediated oncogenic pathways in renal cancer

The Src family of protein kinases (SFK) plays key roles in regulating fundamental cellular processes, including cell growth, differentiation, cell shape, migration, and survival, and specialized cell signals in various malignancies. The pleiotropic functions of SFKs in cancer make them promising targets for intervention. Here, we sought to investigate the role of microRNA-205 (miR-205) in inhibition of Src-mediated oncogenic pathways in renal cancer. We report that expression of miR-205 was significantly suppressed in renal cancer cell lines and tumors when compared with normal tissues and a nonmalignant cell line and is correlated inversely with the expression of SFKs. miR-205 significantly suppressed the luciferase activity of reporter plasmids containing the 3'-UTR (untranslated region) sequences complementary to either Src, Lyn, or Yes, which was abolished by mutations in these 3'-UTR regions. Overexpression of miR-205 in A498 cells reduced Src, Lyn, and Yes expression, both at mRNA and protein levels. Proliferation of renal cancer cells was suppressed by miR-205, mediated by the phospho-Src-regulated ERK1/2 pathway. Cell motility factor FAK (focal adhesion kinase) and STAT3 activation were also inhibited by miR-205. Transient and stable overexpression of miR-205 in A498 cells resulted in induction of G₀/G₁ cell-cycle arrest and apoptosis, as indicated by decreased levels of cyclin D1 and c-Myc, suppressed cell proliferation, colony formation, migration, and invasion in renal cancer cells. miR-205 also inhibited tumor cell growth in vivo. This is the first study showing that miR-205 inhibits proto-oncogenic SFKs, indicating a therapeutic potential of miR-205 in the treatment of renal cancer.

SRC: a century of science brought to the clinic

The SRC family kinases are the largest family of nonreceptor tyrosine kinases and one of the best-studied targets for cancer therapy. SRC, arguably the oldest oncogene, has been implicated in pathways regulating proliferation, angiogenesis, invasion and metastasis, and bone metabolism. More recently, researchers have proposed that the transforming ability of SRC is linked to its ability to activate key signaling molecules in these pathways, rather than through direct activity. It has been hypothesized that blocking SRC activation may inhibit these pathways, resulting in antitumor activity. However, successfully targeting SRC in a clinical setting remains a challenge, and SRC inhibitors have only recently begun to move through clinical development. Preclinical studies have identified specific molecular "subgroups" and histologies that may be more sensitive to SRC inhibition. In addition, other studies have demonstrated synergistic interactions between SRC inhibitors and other targeted therapies and cytotoxics. In this review, we summarize SRC biology and how it has been applied to the clinical development of SRC inhibitors. The status of SRC inhibitors, including dasatinib, saracatinib, and bosutinib, which are in phase 1, 2, and 3 trials, is highlighted.

p27Kip1 mediates addiction of ovarian cancer cells to MYCC (c-MYC) and their dependence on MYC paralogs

The MYCC (c-MYC) gene is amplified in 30-60% of human ovarian cancers. We assessed the functional significance of MYCC amplification by siRNA inhibition of MYCC or MYC paralogs in a panel of ovarian cancer cell lines expressing varying levels of MYCC. Inactivation of MYCC inhibited cell proliferation and induced replicative senescence only in lines with amplified MYCC, indicating that these cells are addicted to continued MYCC overexpression. In contrast, siRNA knockdown of all three MYC isoforms inhibited proliferation of MYCC non-amplified ovarian cancer cells without inducing replicative senescence, and did not inhibit the proliferation of telomerase-immortalized ovarian surface epithelial cells. The arrest induced by MYCC knockdown was accompanied by an increase in the level of the Cdk inhibitor p27(Kip1) and a decrease in cyclin A expression and Cdk2 activity, and could be reversed by RNAi knockdown of p27(Kip1) or Rb, or by overexpression of cyclin A/Cdk2. The arrest induced by knockdown of all three MYC isoforms could similarly be reversed by p27(Kip1) knockdown. Our findings indicate that the addiction of MYCC-amplified ovarian cancer cells to MYCC differs from the dependence of MYCC non-amplified cancer cells on MYC paralogs, but both are mediated, at least in part, by p27(Kip1). They also suggest that growth of ovarian cancers may be blocked by inhibition of MYCC or MYC paralogs.

Stabilizers of the Max homodimer identified in virtual ligand screening inhibit Myc function

Many human cancers show constitutive or amplified expression of the transcriptional regulator and oncoprotein Myc, making Myc a potential target for therapeutic intervention. Here we report the down-regulation of Myc activity by reducing the availability of Max, the essential dimerization partner of Myc. Max is expressed constitutively and can form unstable homodimers. We have isolated stabilizers of the Max homodimer by applying virtual ligand screening (VLS) to identify specific binding pockets for small molecule interactors. Candidate compounds found by VLS were screened by fluorescence resonance energy transfer, and from these screens emerged a potent, specific stabilizer of the Max homodimer. In vitro binding assays demonstrated that the stabilizer enhances the formation of the Max-Max homodimer and interferes with the heterodimerization of Myc and Max in a dose-dependent manner. Furthermore, this compound interferes with Myc-induced oncogenic transformation, Myc-dependent cell growth, and Myc-mediated transcriptional activation. The Max-Max stabilizer can be considered a lead compound for the development of inhibitors of the Myc network.

Influence of the Circadian System on Disease Severity

The severity of many diseases varies across the day and night. For example, adverse cardiovascular incidents peak in the morning, asthma is often worse at night and temporal lobe epileptic seizures are most prevalent in the afternoon. These patterns may be due to the day/night rhythm in environment and behavior, and/or endogenous circadian rhythms in physiology. Furthermore, chronic misalignment between the endogenous circadian timing system and the behavioral cycles could be a cause of increased risk of diabetes, obesity, cardiovascular disease and certain cancers in shift workers. Here we describe the magnitude, relevance and potential biological basis of such daily changes in disease severity and of circadian/behavioral misalignment, and present how these insights may help in the development of appropriate chronotherapy.

A novel Src kinase inhibitor reduces tumour formation in a skin carcinogenesis model

The Src family tyrosine kinases are key modulators of cancer cell invasion and metastasis and a number of Src kinase inhibitors are currently in clinical development for the treatment of solid tumours. However, there is growing evidence that Src is also upregulated at very early stages of epithelial cancer development. We have investigated the role of Src in mouse skin, which is one of the most tractable models of epithelial homoeostasis and tumorigenesis. We found that Src protein expression and activity was regulated during the normal hair cycle and was increased specifically during the proliferative anagen phase and also in response to the tumour promoter 12-O-tetradecanoylphorbol-13-acetate (TPA). AZD0530, a selective Src inhibitor, prevented the TPA-induced proliferation of basal keratinocytes both in vivo and in vitro. Moreover, treatment with AZD0530 reduced papilloma formation following the well-established 7,12-dimethylbenz(a)anthracene/TPA skin carcinogenesis protocol but did not inhibit the subsequent proliferation of the papillomas. Furthermore, AZD0530 did not alter the malignant conversion of papillomas to squamous cell carcinoma suggesting a role for Src in early tumour development in the skin carcinogenesis model, rather than at later stages of tumour progression. Src expression and activity were also seen in human actinic keratoses that are hyperproliferative pre-malignant skin lesions, indicating that Src may also play a role in the early stages of human skin tumour development. Thus, Src inhibitors such as AZD0530 may therefore have chemopreventative properties in patients with hyperproliferative epidermal disorders.

Src family kinase oncogenic potential and pathways in prostate cancer as revealed by AZD0530

Prostate cancer is the most frequently diagnosed cancer in American men. We have previously demonstrated that Src mediates androgen-independent proliferation in prostate cancer. We sought to investigate the Src-mediated oncogenic pathways and tumor biology using AZD0530, a novel Src family kinase/Abl dual-kinase inhibitor that is entering phase II clinical trials. We show that while both Src and Abl are expressed in all prostate cancer cell lines, Src but not Abl is activated in the prostate. Furthermore, Src activation is inhibited by AZD0530 in a rapid and dose-dependent manner. We show that Src mediates cell proliferation in DU145 and PC3 cells at the G1 phase of cell cycle. Src inhibition resulted in decreased binding of beta-catenin to the promoters of G1 phase cell cycle regulators cyclin D1 and c-Myc. C-Myc may also be regulated at the protein level by extracellular signal-regulated kinase 1/2 and GSK3beta. Cell motility factors focal adhesion kinase, p130CAS and paxillin activation in DU145 and PC3 cells were also inhibited. Administration of AZD0530 in mice reduced orthotopic DU145 xenograft growth by 45%. We have further delineated the Src-mediated oncogenic growth and migration pathways in prostate cancer and established mechanistic rationale for Src inhibition as novel therapy in the treatment of prostate cancer.

Placental rescue reveals a sole requirement for c-Myc in embryonic erythroblast survival and hematopoietic stem cell function

The c-Myc protein has been implicated in playing a pivotal role in regulating the expression of a large number of genes involved in many aspects of cellular function. Consistent with this view, embryos lacking the c-myc gene exhibit severe developmental defects and die before midgestation. Here, we show that Sox2Cre-mediated deletion of the conditional c-myc(flox) allele specifically in the epiblast (hence trophoectoderm and primitive endoderm structures are wild type) rescues the majority of developmental abnormalities previously characterized in c-myc knockout embryos, indicating that they are secondary defects and arise as a result of placental insufficiency. Epiblast-restricted c-Myc-null embryos appear morphologically normal and do not exhibit any obvious proliferation defects. Nonetheless, these embryos are severely anemic and die before E12. c-Myc-deficient embryos exhibit fetal liver hypoplasia, apoptosis of erythrocyte precursors and functionally defective definitive hematopoietic stem/progenitor cells. Specific deletion of c-myc(flox) in hemogenic or hepatocytic lineages validate the hematopoietic-specific requirement of c-Myc in the embryo proper and provide in vivo evidence to support a synergism between hematopoietic and liver development. Our results reveal for the first time that physiological levels of c-Myc are essential for cell survival and demonstrate that, in contrast to most other embryonic lineages, erythroblasts and hematopoietic stem/progenitor cells are particularly dependent on c-Myc function.

The oncogene c-Myc coordinates regulation of metabolic networks to enable rapid cell cycle entry

The c-myc proto-oncogene is rapidly activated by serum and regulates genes involved in metabolism and cell cycle progression. This gene is thereby uniquely poised to coordinate both the metabolic and cell cycle regulatory events required for cell cycle entry. However, this function of Myc has not been evaluated. Using a rat fibroblast model of isogenic cell lines, myc(-/-), myc(+/-), myc(+/+) and myc(-/-) cells with an inducible c-myc transgene (mycER), we show that the Myc protein programs cells to utilize both oxidative phosphorylation and glycolysis to drive cell cycle progression. We demonstrate this coordinate regulation of metabolic networks is essential, as specific inhibitors of these pathways block Myc-induced proliferation. Metabolic events temporally correlated with cell cycle entry include increased oxygen consumption, mitochondrial function, pyruvate and lactate production, and ATP generation. Treatment of normal cells with inhibitors of oxidative phosphorylation recapitulates the myc(-/-) phenotype, resulting in impaired cell cycle entry and reduced metabolism. Combined with a kinetic expression profiling analysis of genes linked to mitochondrial function, our study indicates that Myc's ability to coordinately regulate the mitochondrial metabolic network transcriptome is required for rapid cell cycle entry. This function of Myc may underlie the pervasive presence of Myc in many human cancers.

c-myc Repression of TSC2 contributes to control of translation initiation and Myc-induced transformation

The c-myc oncogene plays a key role in cellular growth control, and translation initiation factors are among the transcriptional targets of Myc. Here, we describe a defect in translation initiation control in myc-null cells due to alterations in the mammalian target of rapamycin (mTOR) pathway. Myc loss increased sensitivity to dominant inhibition of eukaryotic translation initiation factor 4E function. Polysomal profiles of myc(-/-) cells revealed decreased translation initiation rates, which were accompanied by decreased 40S/60S ribosomal subunit ratios. Because the 40S small ribosomal subunit contains the key regulatory ribosomal protein S6 (rpS6), we considered that myc loss might affect expression of components of the mTOR signaling pathway that regulate rpS6 function. Among mTOR signaling components, Myc directly affected transcription of tuberous sclerosis 2 (TSC2), as shown by quantitative mRNA analysis and by Myc binding to its promoter in chromatin immunoprecipitation assays. Importantly, Myc acted as a strong and direct repressor for TSC2 expression because its loss increased TSC2 mRNA in myc-null and in HL60 shRNA experiments, activation of a mycER construct in myc(-/-) cells suppressed TSC2 induction in a myc box II-dependent manner, and mycER activation recruited Myc to the TSC2 promoter. The biological significance of the effect of Myc on TSC2 expression was shown by markedly reduced TSC2 mRNA levels in myc-transformed cells, stimulation of S6 kinase activity in myc-null cells by TSC2 siRNA, and decreased Myc-induced soft agar colony formation following retroviral transduction of TSC2. Together, these findings show that regulation of TSC2 can contribute to the effects of Myc on cell proliferation and neoplastic growth.

Gdnf upregulates c-Fos transcription via the Ras/Erk1/2 pathway to promote mouse spermatogonial stem cell proliferation

Glial cell line-derived neurotrophic factor (GDNF) plays a crucial role in regulating the proliferation of spermatogonial stem cells (SSC). The signaling pathways mediating the function of GDNF in SSC remain unclear. This study was designed to determine whether GDNF signals via the Ras/ERK1/2 pathway in the C18-4 cells, a mouse SSC line. The identity of this cell line was confirmed by the expression of various markers for germ cells, proliferating spermatogonia, and SSC, including GCNA1, Vasa, Dazl, PCNA, Oct-4, GFRalpha1, Ret, and Plzf. Western blot analysis revealed that GDNF activated Ret tyrosine phosphorylation. All 3 isoforms of Shc were phosphorylated upon GDNF stimulation, and GDNF induced the binding of the phosphorylated Ret to Shc and Grb2 as indicated by immunoprecipitation and Western blotting. The active Ras was induced by GDNF, which further activated ERK1/2 phosphorylation. GDNF stimulated the phosphorylation of CREB-1, ATF-1, and CREM-1, and c-fos transcription. Notably, the increase in ERK1/2 phosphorylation, c-fos transcription, bromodeoxyuridine incorporation, and metaphase counts induced by GDNF, was completely blocked by pretreatment with PD98059, a specific inhibitor for MEK1, the upstream regulator of ERK1/2. GDNF stimulation eventually upregulated cyclin A and CDK2 expression. Together, these data suggest that GDNF induces CREB/ATF-1 family member phosphorylation and c-fos transcription via the Ras/ERK1/2 pathway to promote the proliferation of SSC. Unveiling GDNF signaling cascades in SSC has important implications in providing attractive targets for male contraception as well as for the regulation of stem cell renewal vs. differentiation.

c-Myc depletion inhibits proliferation of human tumor cells at various stages of the cell cycle

A major role for c-Myc in the proliferation of normal cells is attributed to its ability to promote progression through G(1) and into S phase of the cell cycle. The absolute requirement of c-Myc for cell cycle progression in human tumor cells has not been comprehensively addressed. In the present work, we used a lentiviral-based short hairpin RNA (shRNA) expression vector to stably reduce c-Myc expression in a large number of human tumor cell lines and in three different types of normal human cells. In all cases, cell proliferation was severely inhibited, with normal cells ultimately undergoing G(0)/G(1) growth arrest. In contrast, tumor cells demonstrated a much more variable cell cycle response with cells from several lines accumulating in S or G(2)/M phases. Moreover, in some tumor lines, the phase of cell cycle arrest caused by inhibition of c-Myc could be altered by depleting tumor suppressor protein p53 or its transcriptional target p21(CIP/WAF). Our data suggest that, as in the case of normal cells, c-Myc is essential for sustaining proliferation of human tumor cells. However its rate-limiting role in cell cycle control is variable and is reliant upon the status of other cell cycle regulators.

Non-transcriptional control of DNA replication by c-Myc

The c-Myc proto-oncogene encodes a transcription factor that is essential for cell growth and proliferation and is broadly implicated in tumorigenesis. However, the biological functions required by c-Myc to induce oncogenesis remain elusive. Here we show that c-Myc has a direct role in the control of DNA replication. c-Myc interacts with the pre-replicative complex and localizes to early sites of DNA synthesis. Depletion of c-Myc from mammalian (human and mouse) cells as well as from Xenopus cell-free extracts, which are devoid of RNA transcription, demonstrates a non-transcriptional role for c-Myc in the initiation of DNA replication. Overexpression of c-Myc causes increased replication origin activity with subsequent DNA damage and checkpoint activation. These findings identify a critical function of c-Myc in DNA replication and suggest a novel mechanism for its normal and oncogenic functions.

Identification and functional signature of genes regulated by structurally different ABL kinase inhibitors

Dasatinib is an ATP-competitive, multi-targeted SRC and ABL kinase inhibitor that can bind BCR-ABL in both the active and inactive conformations. From a clinical standpoint, dasatinib is particularly attractive because it has been shown to induce hematologic and cytogenetic responses in imatinib-resistant chronic myeloid leukemia patients. The fact because the combination of imatinib and dasatinib shows the additive/synergistic growth inhibition on wild-type p210 BCR-ABL-expressing cells, we reasoned that these ABL kinase inhibitors might induce the different molecular pathways. To address this question, we used DNA microarrays to identify genes whose transcription was altered by imatinib and dasatinib. K562 cells were cultured with imatinib or dasatinib for 16 h, and gene expression data were obtained from three independent microarray hybridizations. Almost all of the imatinib- and dasatinib-responsive genes appeared to be similarly increased or decreased in K562 cells; however, small subsets of genes were identified as selectively altered expression by either imatinib or dasatinib. The distinct genes that are selectively modulated by dasatinib are cyclin-dependent kinase 2 (CDK2) and CDK8, which had a maximal reduction of <5-fold in microarray screen. To assess the functional importance of dasatinib regulated genes, we used RNA interference to determine whether reduction of CDK2 and CDK8 affected the growth inhibition. K562 and TF-1BCR-ABL cells, pretreated with CDK2 or CDK8 small interfering RNA, showed additive growth inhibition with imatinib, but not with dasatinib. These findings demonstrate that the additive/synergistic growth inhibition by imatinib and dasatinib may be mediated in part by CDK2 and CDK8.

Targeting c-Src kinase enhances tamoxifen's inhibitory effect on cell growth by modulating expression of cell cycle and survival proteins

PURPOSE

Many studies have implicated the non-receptor tyrosine kinase c-Src in the development and metastatic progression of many types of cancer. In breast cancer, c-Src has been proposed to mediate the actions of estrogen in cell cycle progression.

METHODS

In this study we investigated the interaction between c-Src inhibition and estrogen receptor (ER) function using the ER-positive and tamoxifen-sensitive MCF-7 breast cancer cells.

RESULTS

Pharmacological inhibition of c-Src blocked estrogen-dependent proliferation in MCF-7 cells and enhanced the inhibitory effects of tamoxifen or estrogen-deprivation on cell growth. Maximum inhibition (95%) of cell growth was obtained when tamoxifen and c-Src blockade were combined. Inhibition of c-Src kinase decreased levels of the ER targets c-Myc and cyclin D1 expression but not of Bcl-2. Nevertheless, blocking c-Src kinase in tamoxifen-treated MCF-7 cells led to apoptosis. Inhibition of c-Src kinase altered the ratio of Mcl-1 isoforms in favor of cell death whereas expression of the proapoptotic molecules Bad, Bak, and Bax was not altered. Surprisingly, blocking ER function increased the levels of Bad phosphorylation at serine 112 (BadpS112), an inactive (nonapoptotic) form of Bad. This inactivation of Bad upon ER blockade seemed to depend on c-Src function as chemical inhibition of c-Src kinase reduced BadpS112 levels in cells with impaired ER function but not in estrogen-treated cells.

CONCLUSION

These results indicate a crucial role for c-Src kinase in the survival of ER-positive breast cancer cells only when ER function is blocked. Therefore, this study suggests that targeting simultaneously c-Src and ER may effectively inhibit growth of ER-positive breast cancer.

Skin epidermis lacking the c-Myc gene is resistant to Ras-driven tumorigenesis but can reacquire sensitivity upon additional loss of the p21Cip1 gene

The target gene(s) required for Myc-mediated tumorigenesis are still elusive. Here we show that while endogenous c-Myc is surprisingly dispensable for skin homeostasis and TPA-induced hyperplasia, c-Myc-deficient epidermis is resistant to Ras-mediated DMBA/TPAinduced tumorigenesis. This is mechanistically linked to p21(Cip1), which is induced in tumors by the activated Ras-ERK pathway but repressed by c-Myc. Acute elimination of c-Myc in established tumors leads to the up-regulation of p21(Cip1), and epidermis lacking both p21(Cip1) and c-Myc reacquires normal sensitivity to DMBA/TPA-induced tumorigenesis. This identifies c-Myc-mediated repression of p21(Cip1) as a key step for Ras-driven epidermal tumorigenesis.

The role of Src in solid and hematologic malignancies

c-Src was the first protooncogene described and was among the first molecules in which tyrosine kinase activity was documented. c-Src has been defined as a common modular structure that participates in much of the crosstalk between the cytoplasmic protein tyrosine kinases and tyrosine kinase receptors. Understanding the structure and function of this important class of protein kinases and elucidating the molecular signaling events mediated by c-Src are important not only for identifying the critical pathways but also for designing new strategies to block or inhibit the action of these kinases. Despite the large amount of information available on c-Src, its precise functions in cancer remain to be elucidated. Recently, there has been renewed interest in c-Src as a molecular target for cancer therapy, and multiple c-Src inhibitors are entering clinical trials. In this review, the authors describe the function and expression of c-Src in human malignancies and the novel c-Src inhibitors and their potential applications for cancer treatment.