Protein arginine methyltransferase 3-induced metabolic reprogramming is a vulnerable target of pancreatic cancer.

BACKGROUND: The biological function of protein arginine methyltransferase 3 (PRMT3) is not well known because very few physiological substrates of this methyltransferase have been identified to date. METHODS: The clinical significance of PRMT3 in pancreatic cancer was studied by database analysis. The PRMT3 protein level of human pancreatic tumors was detected by immunoblotting and immunohistochemical staining. PRMT3-associated proteins and the methylation sites on the proteins were investigated using mass spectrometry. Seahorse Bioscience analyzed the metabolic reprogramming. Combination index analysis and xenograft animal model were conducted to explore the effects of combination of inhibitors of glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and oxidative phosphorylation on tumor growth. RESULTS: We found that the expression of PRMT3 is upregulated in pancreatic cancer, and its expression is associated with poor survival. We identified GAPDH as a PRMT3-binding protein and demonstrated that GAPDH is methylated at R248 by PRMT3 in vivo. The methylation of GAPDH by PRMT3 enhanced its catalytic activity while the mutation of R248 abolished the effect. In cells, PRMT3 overexpression triggered metabolic reprogramming and enhanced glycolysis and mitochondrial respiration simultaneously in a GAPDH-dependent manner. PRMT3-overexpressing cancer cells were addicted to GAPDH-mediated metabolism and sensitive to the inhibition of GAPDH and mitochondrial respiration. The combination of inhibitors of GAPDH and oxidative phosphorylation induced a synergistic inhibition on cellular growth in vitro and in vivo. CONCLUSION: Our results suggest that PRMT3 mediates metabolic reprogramming and cellular proliferation through methylating R248 of GAPDH, and double blockade of GAPDH and mitochondrial respiration could be a novel strategy for the treatment of PRMT3-overexpressing pancreatic cancer.

Alteration of Epigenetic Modifiers in Pancreatic Cancer and Its Clinical Implication.

The incidence of pancreatic cancer has considerably increased in the past decade. Pancreatic cancer has the worst prognosis among the cancers of the digestive tract because the pancreas is located in the posterior abdominal cavity, and most patients do not show clinical symptoms for early detection. Approximately 55% of all patients are diagnosed with pancreatic cancer only after the tumors metastasize. Therefore, identifying useful biomarkers for early diagnosis and screening high-risk groups are important to improve pancreatic cancer therapy. Recent emerging evidence has suggested that genetic and epigenetic alterations play a crucial role in the molecular aspects of pancreatic tumorigenesis. Here, we summarize recent progress in our understanding of the epigenetic alterations in pancreatic cancer and propose potential synthetic lethal strategies to target these genetic defects to treat this deadly disease.

Two Birds, One Stone: Double Hits on Tumor Growth and Lymphangiogenesis by Targeting Vascular Endothelial Growth Factor Receptor 3.

Vascular endothelial growth factor receptor 3 (VEGFR3) has been known for its involvement in tumor-associated lymphangiogenesis and lymphatic metastasis. The VEGFR3 signaling is stimulated by its main cognate ligand, vascular endothelial growth factor C (VEGF-C), which in turn promotes tumor progression. Activation of VEGF-C/VEGFR3 signaling in lymphatic endothelial cells (LECs) was shown to enhance the proliferation of LECs and the formation of lymphatic vessels, leading to increased lymphatic metastasis of tumor cells. In the past decade, the expression and pathological roles of VEGFR3 in tumor cells have been described. Moreover, the VEGF-C/VEGFR3 axis has been implicated in regulating immune tolerance and suppression. Therefore, the inhibition of the VEGF-C/VEGFR3 axis has emerged as an important therapeutic strategy for the treatment of cancer. In this review, we discuss the current findings related to VEGF-C/VEGFR3 signaling in cancer progression and recent advances in the development of therapeutic drugs targeting VEGF-C/VEGFR3.

Protein Arginine Methyltransferase 3 Enhances Chemoresistance in Pancreatic Cancer by Methylating hnRNPA1 to Increase ABCG2 Expression.

Pancreatic cancer is poorly responsive to chemotherapy due to intrinsic or acquired resistance. Our previous study showed that epigenetic modifying enzymes including protein arginine methyltransferase 3 (PRMT3) are dysregulated in gemcitabine (GEM)-resistant pancreatic cancer cells. Here, we attempt to elucidate the role of PRMT3 in chemoresistance. Overexpression of PRMT3 led to increased resistance to GEM in pancreatic cancer cells, whereas reduction of PRMT3 restored GEM sensitivity in resistant cells. We identified a novel PRMT3 target, ATP-binding cassette subfamily G member 2 (ABCG2), which is known to play a critical role in drug resistance. PRMT3 overexpression upregulated ABCG2 expression by increasing its mRNA stability. Mass spectrometric analysis identified hnRNPA1 as a PRMT3 interacting protein, and methylation of hnRNPA1 at R31 by PRMT3 in vivo and in vitro. The expression of methylation-deficient hnRNPA1-R31K mutant reduced the RNA binding activity of hnRNPA1 and the expression of ABCG2 mRNA. Taken together, this provides the first evidence that PRMT3 methylates the RNA recognition motif (RRM) of hnRNPA1 and promotes the binding between hnRNPA1 and ABCG2 to enhance drug resistance. Inhibition of PRMT3 could be a novel strategy for the treatment of GEM-resistant pancreatic cancer.

Pyruvate kinase M2 fuels multiple aspects of cancer cells: from cellular metabolism, transcriptional regulation to extracellular signaling.

Originally identified as a metabolic enzyme that catalyzes the transfer of a phosphate group from phosphoenolpyruvate (PEP) to ADP in the glycolytic pathway, pyruvate kinase M2-type (PKM2) has been shown to exhibit novel biological activities in the nucleus and outside the cells. Although cell-based studies reveal new non-canonical functions of PKM2 in gene transcription, epigenetic modulation and cell cycle progression, the importance of these non-canonical functions in PKM2-mediated tumorigenesis is still under debate because studies in genetically modified mice do not consistently echo the findings observed in cultured cancer cells. In addition to regulation of gene expression, the existence of PKM2 in exosomes opens a new venue to study the potential role of this glycolytic enzyme in cell-cell communication and extracellular signal initiation. In this review, we briefly summarize current understanding of PKM2 in metabolic switch and gene regulation. We will then emphasize recent progress of PKM2 in extracellular signaling and tumor microenvironment reprogramming. Finally, the discrepancy of some PKM2's functions in vitro and in vivo, and the application of PKM2 in cancer detection and treatment will be discussed.

Orchestration of H3K27 methylation: mechanisms and therapeutic implication.

Histone proteins constitute the core component of the nucleosome, the basic unit of chromatin. Chemical modifications of histone proteins affect their interaction with genomic DNA, the accessibility of recognized proteins, and the recruitment of enzymatic complexes to activate or diminish specific transcriptional programs to modulate cellular response to extracellular stimuli or insults. Methylation of histone proteins was demonstrated 50 years ago; however, the biological significance of each methylated residue and the integration between these histone markers are still under intensive investigation. Methylation of histone H3 on lysine 27 (H3K27) is frequently found in the heterochromatin and conceives a repressive marker that is linked with gene silencing. The identification of enzymes that add or erase the methyl group of H3K27 provides novel insights as to how this histone marker is dynamically controlled under different circumstances. Here we summarize the methyltransferases and demethylases involved in the methylation of H3K27 and show the new evidence by which the H3K27 methylation can be established via an alternative mechanism. Finally, the progress of drug development targeting H3K27 methylation-modifying enzymes and their potential application in cancer therapy are discussed.

The histone methyltransferase G9a as a therapeutic target to override gemcitabine resistance in pancreatic cancer.

Gemcitabine (GEM) resistance is a critical issue for pancreatic cancer treatment. The involvement of epigenetic modification in GEM resistance is still unclear. We established a GEM-resistant subline PANC-1-R from the parental PANC-1 pancreatic cancer cells and found the elevation of various chromatin-modifying enzymes including G9a in GEM-resistant cells. Ectopic expression of G9a in PANC-1 cells increased GEM resistance while inactivation of G9a in PANC-1-R cells reduced it. Challenge of PANC-1 cells with GEM increased the expression of stemness markers including CD133, nestin and Lgr5 and promoted sphere forming activity suggesting chemotherapy enriched cancer cells with stem-like properties. Inhibition of G9a in PANC-1-R cells reduced stemness and invasiveness and sensitized the cells to GEM. We revealed interleukin-8 (IL-8) is a downstream effector of G9a to increase GEM resistance. G9a-overexpressing PANC-1-R cells exhibited autocrine IL-8/CXCR1/2 stimulation to increase GEM resistance which could be decreased by anti-IL-8 antibody and G9a inhibitor. IL-8 released by cancer cells also activated pancreatic stellate cell (PSC) to increase GEM resistance. In orthotopic animal model, GEM could not suppress tumor growth of PANC-1-R cells and eventually promoted tumor metastasis. Combination with G9a inhibitor and GEM reduced tumor growth, metastasis, IL-8 expression and PSC activation in animals. Finally, we showed that overexpression of G9a correlated with poor survival and early recurrence in pancreatic cancer patients. Collectively, our results suggest G9a is a therapeutic target to override GEM resistance in the treatment of pancreatic cancer.

Extracellular PKM2 induces cancer proliferation by activating the EGFR signaling pathway.

Pyruvate kinase is a key enzyme in the glycolytic pathway that converts phosphoenolpyruvate to pyruvate, and the M2 isoform of pyruvate kinase (PKM2) is associated with cancer. PKM2 has been reported to function independently of its pyruvate kinase activity, which is crucial for cancer cell proliferation. Moreover, there is growing evidence indicating that dimeric PKM2 is released from tumor cells into the circulation of cancer patients. However, the role of secreted PKM2 in cancer is not well understood. Here, we found that the phosphorylation level of epidermal growth factor receptor (EGFR) significantly increased upon the exposure of cells to the recombinant PKM2 protein. In addition, secreted PKM2 induces EGFR phosphorylation and activates the EGFR downstream signaling in triple-negative breast cancer cells. In contrast, knocking down PKM2 decreased EGFR phosphorylation. Moreover, expression of R399E mutant PKM2, which has been reported to preferentially form a dimer, enhanced EGFR phosphorylation, cellular transformation, and cell proliferation more strongly than the wild-type PKM2. Thus, our study revealed a novel function of extracellular PKM2 in the promoting cancer cell proliferation through EGFR activation.

Blocking c-Met-mediated PARP1 phosphorylation enhances anti-tumor effects of PARP inhibitors.

Poly (ADP-ribose) polymerase (PARP) inhibitors have emerged as promising therapeutics for many diseases, including cancer, in clinical trials. One PARP inhibitor, olaparib (Lynparza, AstraZeneca), was recently approved by the FDA to treat ovarian cancer with mutations in BRCA genes. BRCA1 and BRCA2 have essential roles in repairing DNA double-strand breaks, and a deficiency of BRCA proteins sensitizes cancer cells to PARP inhibition. Here we show that the receptor tyrosine kinase c-Met associates with and phosphorylates PARP1 at Tyr907 (PARP1 pTyr907 or pY907). PARP1 pY907 increases PARP1 enzymatic activity and reduces binding to a PARP inhibitor, thereby rendering cancer cells resistant to PARP inhibition. The combination of c-Met and PARP1 inhibitors synergized to suppress the growth of breast cancer cells in vitro and xenograft tumor models, and we observed similar synergistic effects in a lung cancer xenograft tumor model. These results suggest that the abundance of PARP1 pY907 may predict tumor resistance to PARP inhibitors, and that treatment with a combination of c-Met and PARP inhibitors may benefit patients whose tumors show high c-Met expression and who do not respond to PARP inhibition alone.

G9a orchestrates PCL3 and KDM7A to promote histone H3K27 methylation.

Methylation of histone H3-lysine 9 (H3K9) and H3K27 by the methyltransferase G9a and polycomb repressive complex 2 (PRC2) inhibits transcription of target genes. A crosstalk between G9a and PRC2 via direct physical interaction has been shown recently. Here, we demonstrate an alternative mechanism by which G9a promotes H3K27 methylation. Overexpression of G9a increases both H3K9 and H3K27 methylation, reduces E-cadherin expression, and induces epithelial-mesenchymal transition in PANC-1 pancreatic cancer cells. Conversely, the depletion of G9a or ectopic expression of methyltransferase-dead G9a in G9a-overexpressing gemcitabine-resistant PANC-1-R cells exhibits opposite effects. G9a promotes H3K27 methylation of the E-cadherin promoter by upregulating PCL3 to increase PRC2 promoter recruitment and by downregulating the H3K27 demethylase KDM7A to silence E-cadherin gene. The depletion of PCL3 or overexpression of KDM7A elevated expression of E-cadherin in PANC-1-R cells while ectopic expression of PCL3 or knockdown of KDM7A downregulated E-cadherin in PANC-1 cells. Collectively, we provide evidence that G9a orchestrates the dynamic balance within histone-modifying enzymes to regulate H3K27 methylation and gene expression.

RECK impedes DNA repair by inhibiting the erbB/JAB1/Rad51 signaling axis and enhances chemosensitivity of breast cancer cells.

The reversion-inducing cysteine-rich protein with kazal motif (RECK) is an endogenous matrix metalloproteinase (MMP) inhibitor and a tumor suppressor. Its expression is dramatically down-regulated in human cancers. Our recent results suggest a novel MMP-independent anti-cancer activity of RECK by inhibiting the erbB signaling. Activation of the erbB signaling is associated with chemotherapeutic resistance, however, whether RECK could modulate drug sensitivity is still unknown. Here we demonstrated that expression of RECK induced the activation of ATM and ATR pathways, and the formation of γ-H2AX foci in breast cancer cells. RECK inhibited the erbB signaling and attenuated the expression of the downstream molecules Jun activation domain-binding protein 1 (JAB1) and the DNA repair protein RAD51 to impede DNA repair and to increase drug sensitivity. Treatment of epidermal growth factor or over-expression of HER-2 effectively reversed the inhibitory effect of RECK. In addition, ectopic expression of JAB1 counteracted RECK-induced RAD51 reduction and drug sensitization. Our results elucidate a novel function of RECK to modulate DNA damage response and drug resistance by inhibiting the erbB/Jab1/RAD51 signaling axis. Restoration of RECK expression in breast cancer cells may increase sensitivity to chemotherapeutic agents.

The ZNF304-integrin axis protects against anoikis in cancer.

Ovarian cancer (OC) is a highly metastatic disease, but no effective strategies to target this process are currently available. Here, an integrative computational analysis of the Cancer Genome Atlas OC data set and experimental validation identifies a zinc finger transcription factor ZNF304 associated with OC metastasis. High tumoral ZNF304 expression is associated with poor overall survival in OC patients. Through reverse phase protein array analysis, we demonstrate that ZNF304 promotes multiple proto-oncogenic pathways important for cell survival, migration and invasion. ZNF304 transcriptionally regulates ß1 integrin, which subsequently regulates Src/focal adhesion kinase and paxillin and prevents anoikis. In vivo delivery of ZNF304 siRNA by a dual assembly nanoparticle leads to sustained gene silencing for 14 days, increased anoikis and reduced tumour growth in orthotopic mouse models of OC. Taken together, ZNF304 is a transcriptional regulator of ß1 integrin, promotes cancer cell survival and protects against anoikis in OC.

Tyrosine 370 phosphorylation of ATM positively regulates DNA damage response.

Ataxia telangiectasia mutated (ATM) mediates DNA damage response by controling irradiation-induced foci formation, cell cycle checkpoint, and apoptosis. However, how upstream signaling regulates ATM is not completely understood. Here, we show that upon irradiation stimulation, ATM associates with and is phosphorylated by epidermal growth factor receptor (EGFR) at Tyr370 (Y370) at the site of DNA double-strand breaks. Depletion of endogenous EGFR impairs ATM-mediated foci formation, homologous recombination, and DNA repair. Moreover, pretreatment with an EGFR kinase inhibitor, gefitinib, blocks EGFR and ATM association, hinders CHK2 activation and subsequent foci formation, and increases radiosensitivity. Thus, we reveal a critical mechanism by which EGFR directly regulates ATM activation in DNA damage response, and our results suggest that the status of ATM Y370 phosphorylation has the potential to serve as a biomarker to stratify patients for either radiotherapy alone or in combination with EGFR inhibition.

S6K1 promotes invasiveness of breast cancer cells in a model of metastasis of triple-negative breast cancer.

Breast cancer is the second-leading cause of oncology-related death in US women. Of all invasive breast cancers, patients with tumors lacking expression of the estrogen and progesterone hormone receptors and overexpression of human epidermal growth factor receptor 2 have the poorest clinical prognosis. These referred to as triple-negative breast cancer (TNBC) represent an aggressive form of disease that is marked by early-onset metastasis, high tumor recurrence rate, and low overall survival during the first three years post-diagnosis. In this report, we discuss a novel model of early-onset TNBC metastasis to bone and lungs, derived from MDA-MB-231 cells. Breast cancer cells injected intravenously produced rapid, osteolytic metastases in long bones and spines of athymic nude mice, with concurrent metastasis to lungs, liver, and soft tissues. From the bone metastases, we developed a highly metastatic luciferase-tagged cell line variant named MDA-231-LUC Met. In this report, we demonstrate that the Akt/mTOR/S6K1 axis is hyperactivated in these cells, leading to a dramatic increase in phosphorylation of S6 ribosomal protein at Ser235/236. Lastly, we provide evidence that inhibition of the furthest downstream kinase in the mTOR pathway, S6K1, with a highly specific inhibitor PF-4708671 inhibits cell migration, and thus may provide a potent anti-metastatic adjuvant therapy approach.

Epidermal growth factor receptor potentiates MCM7-mediated DNA replication through tyrosine phosphorylation of Lyn kinase in human cancers.

Epidermal growth factor receptor (EGFR) initiates a signaling cascade that leads to DNA synthesis and cell proliferation, but its role in regulating DNA replication licensing is unclear. Here, we show that activated EGFR phosphorylates the p56 isoform of Lyn, p56(Lyn), at Y32, which then phosphorylates MCM7, a licensing factor critical for DNA replication, at Y600 to increase its association with other minichromosome maintenance complex proteins, thereby promoting DNA synthesis complex assembly and cell proliferation. Both p56(Lyn) Y32 and MCM7 Y600 phosphorylation are enhanced in proliferating cells and correlated with poor survival of breast cancer patients. These results establish a signaling cascade in which EGFR enhances MCM7 phosphorylation and DNA replication through Lyn phosphorylation in human cancer cells.

Caspase-independent cell death is involved in the negative effect of EGF receptor inhibitors on cisplatin in non-small cell lung cancer cells.

PURPOSE: Results of multiple clinical trials suggest that EGF receptor (EGFR) tyrosine kinase inhibitors (TKI) exhibit negative effects on platinum-based chemotherapy in patients with lung cancer with wild-type (WT) EGFR, but the underlying molecular mechanisms are still uncertain. Studies that identify the mechanism of how TKIs negatively affect patients with WT EGFR are important for future development of effective strategies to target lung cancer. Thus, we returned to in vitro study to investigate and determine a possible explanation for this phenomenon. EXPERIMENTAL DESIGN: We investigated the effects of TKIs and cisplatin on caspase-independent cell death (CID) and the role of CID in the efficacy of each drug and the combination. Furthermore, we studied the mechanism by which EGFR signaling pathway is involved in CID. Finally, on the basis of the identified mechanism, we tested the combinational effects of cisplatin plus suberoylanilide hydroxamic acid (SAHA) or erastin on CID. RESULTS: We found that gefitinib inhibited cisplatin-induced CID but not caspase-dependent apoptotic cell death. In WT EGFR cells, gefitinib not only inhibited CID but also failed to induce apoptosis, therefore compromising the efficacy of cisplatin. Inhibition of EGFR-ERK/AKT by gefitinib activates FOXO3a, which in turn reduces reactive oxygen species (ROS) and ROS-mediated CID. To overcome this, we showed that SAHA and erastin, the inducers of ROS-mediated CID, strongly enhanced the effect of cisplatin in WT EGFR cells. CONCLUSION: TKI-mediated inhibition of CID plays an important role in the efficacy of chemotherapy. Moreover, FOXO3a is a key factor in the negative effects of TKI by eliminating cisplatin-induced ROS.

Bcr-Abl oncogene stimulates Jab1 expression via cooperative interaction of ß-catenin and STAT1 in chronic myeloid leukemia cells.

Jab1, a co-activator of AP-1 transcription factor and the fifth subunit of the COP9 signalosome, mediates degradation of the tumor suppressor p53 and p27(Kip1) and functions as a tumor promoter in different types of human cancer. In this study, we show that inhibition of Bcr-Abl oncogene by imatinib induces down-regulation of Jab1 in Bcr-Abl-positive K562, Ku812, and MEG01 leukemia cells suggesting Bcr-Abl may regulate Jab1 expression. Promoter deletion and mutation analysis indicate the Tcf-4/ß-catenin and STAT1 binding sites located between the -405/-223 region of the human Jab1 promoter are important for the activation of Jab1 by Bcr-Abl. Double mutation of these two sites reverses the inhibitory effect of imatinib. Chromatin immunoprecipitation assay verifies the binding of ß-catenin and STAT1 to human Jab1 promoter. Ectopic expression of dominant-negative Tcf-4 mutant significantly attenuates Jab1 expression while over-expression of ß-catenin and STAT1 cooperatively up-regulates Jab1 promoter activity and mRNA expression. Our results also demonstrate that the AKT signaling pathway is involved in the regulation of Jab1 by Bcr-Abl because the AKT inhibitor LY294002 but not the ERK inhibitor PD98059 reduces Jab1 promoter activity and mRNA expression. Taken together, our results suggest that Bcr-Abl stimulates Jab1 expression via the cooperative interaction of ß-catenin and STAT1 in leukemia cells.

The tumor suppressor RECK interferes with HER-2/Neu dimerization and attenuates its oncogenic signaling.

Our previous study demonstrates that HER-2/Neu oncogene inhibits a matrix metalloproteinase inhibitor and tumor metastasis suppressor RECK to promote metastasis. Conversely, the effect of RECK on the oncogenic function of HER-2/Neu is unknown. Ectopic expression of RECK in 293T cells and HER-2/Neu-overexpressing breast cancer cells shows that RECK and HER-2/Neu are co-localized and these two proteins can be co-immunoprecipitated. RECK inhibits HER-2/Neu receptor dimerization and autophosphorylation, which causes reduction of ERK and AKT kinase activity and down-regulation of HER-2/Neu target genes. RECK expression is reduced in 58.8% of breast cancer tissues and is associated with lymph node invasion supporting its anti-metastatic role. Collectively, we provide the first evidence that RECK can negatively regulate oncogenic activity of HER-2/Neu by inhibiting receptor dimerization.

Regulation of tumor angiogenesis by EZH2.

Although VEGF-targeted therapies are showing promise, new angiogenesis targets are needed to make additional gains. Here, we show that increased Zeste homolog 2 (EZH2) expression in either tumor cells or in tumor vasculature is predictive of poor clinical outcome. The increase in endothelial EZH2 is a direct result of VEGF stimulation by a paracrine circuit that promotes angiogenesis by methylating and silencing vasohibin1 (vash1). Ezh2 silencing in the tumor-associated endothelial cells inhibited angiogenesis mediated by reactivation of VASH1, and reduced ovarian cancer growth, which is further enhanced in combination with ezh2 silencing in tumor cells. Collectively, these data support the potential for targeting ezh2 as an important therapeutic approach.