Macrophage-derived nitric oxide inhibits the proliferation of activated T helper cells and is induced during antigenic stimulation of resting T cells

To examine how macrophage-derived nitric oxide (NO) affects T helper (Th) cell activity, T cell clones representing Th1 and Th2 subsets were activated before exposure to stimulated peritoneal macrophages or microglia. Both Th subsets were similarly sensitive to inhibition by NO, indicating that macrophage-derived NO regulates the proliferation of activated Th1 and Th2 cells equally well. Since IFN-gamma production remained intact in NO-treated Th1 cells, we studied whether NO was produced during antigen-specific activation of Th1 cells by unstimulated macrophages. Indeed, T cell proliferation only occurred when a NO synthase inhibitor was included, while IFN-gamma was essential for the induction of NO. These studies demonstrate that macrophages produce NO following antigen presentation to Th1 cells and that macrophage-derived NO inhibits Th1 and Th2 cell proliferation without inhibiting cytokine production.     

Modification of mouse hemopoietic cell proliferation in vivo by a hemoregulatory pentapeptide (HP 5b). Relation to circadian rhythms

The effects on murine hemopoietic cell proliferation of hemoregulatory peptide 5b, a synthetic factor which inhibits granulopoiesis, have been studied in vivo by 3H-thymidine labeling, cell cycle analyses by flow cytometry and total cell counts per femur. Single injections of the peptide seemed to obliterate the normal circadian rhythm of bone marrow cell proliferation. By autoradiography, the main effect was an inhibition of myelopoietic cell proliferation, although erythropoiesis was also affected. This occurred in two ways: a) an immediate effect observed in the first 24 h after a single injection, and b) a delayed effect observed 6-12 days after a week of peptide infusion which could be a result of stem cell inhibition. A secondary accumulation of cells with G2 phase DNA content was seen regularly. Thus the hemoregulatory peptide seems to influence hemopoietic cell proliferation and has its main effect on myelopoiesis. However, inhibition is less marked than observed previously in stem cells.     

Regulation of B cell maturation and differentiation. I. Suppression of pokeweed mitogen-induced B cell differentiation by tumor necrosis factor (TNF)

Growth and differentiation of B cells into Ig-secreting plasma cells is regulated by both T cells and macrophages and/or their secreted factors. Although the regulatory role of various cell-derived factors has been examined, the involvement of the macrophage-derived factor, TNF, in human B cell growth and differentiation has not yet been investigated. In the present study we examine the role of rTNF in polyclonal B cell response of human PBL induced by PWM. The addition of rTNF at the initiation of the culture resulted in the dose-dependent inhibition of the generation of both IgG and IgM PFC. Inhibition of PFC development followed the same dose response as rTNF-mediated cytotoxicity against a TNF-sensitive tumor target. The mechanism of rTNF-mediated suppression was examined in different experimental systems. Recombinant TNF did not affect the viability or proliferation of either the T cell or B cell subpopulations, suggesting that TNF does not mediate its suppressive effect by cytotoxic mechanisms. Kinetic studies in which rTNF was added at different times after initiation of culture indicated that inhibition can be observed as late as 4 days of culture and suggested that TNF acts at a late phase of the growth and differentiation pathway of B cells. In further studies we examined the cellular level of TNF-mediated suppression. The addition of rTNF to supernatants containing helper factors and enriched B cells resulted in no inhibition, suggesting that TNF does not act at the B cell level. This was confirmed by demonstrating that rTNF does not inhibit spontaneous PFC development by the CESS B cell line. The effect of TNF on T cell subpopulations was examined by using normal or irradiated T cells, which inactivate suppressor cells. Addition of rTNF to B cells combined with either T cell population suppressed both IgG and IgM PFC development, indicating that the target cell for suppression is the T helper cell but not ruling out an effect on macrophages or the T suppressor cells. Combined, the observed results demonstrate that rTNF suppresses PWM-induced B cell differentiation without affecting B cell proliferation. TNF appears to mediate the suppression by acting directly on T helper cells or else by regulating the production of factors controlling T cell activation and lymphokine secretion.     

Stimulation of rat mesangial cell proliferation by macrophage interleukin 1

Conditioned media from LPS-activated rat peritoneal macrophages enhanced the proliferation rates of cultured rat glomerular mesangial cells. This macrophage-derived activity extensively co-purified with interleukin 1 (IL 1) activity through sequential ammonium sulfate precipitation, S-200 gel chromatography, DEAE-cellulose anion exchange chromatography, and phenyl-Sepharose chromatography. In addition, the macrophage-derived factor was heat-labile (80 degrees C) and inactivated by phenylglyoxal, thus allowing tentative identification as IL 1. Macrophage supernatants and purified IL 1 enhanced the proliferative rates of mesangial cells only in the presence of serum; the use of platelet-poor plasma or serum depleted of platelet-derived growth factor was without effect. IL 1 acted to increase the percentage of cycling cells, without a change in the length of the individual cell cycle times. These findings provide a potential mechanism whereby activated macrophages, in combination with platelet factors, enhance mesangial cell proliferation. Such processes may contribute to the mesangial hypercellularity frequently found in immune-mediated glomerulonephritis.     

Regulation of blood cell diferentiation.

The hemopoietic (blood forming) system contains pluripotent stem cells able to give rise to a variety of differentiated progeny, including erythrocytes, granulocytes, megakaryocytes, monocytes, macrophages, and possible other cell types. Although a good deal is known about cell lineage relationships in the hemopoietic system, only limited information is available about the mechanisms regulating the proliferation and differentiation of the stem cells and their progeny. An approach to this latter problem has been provided by the develoment of new techniques for the cultivation of hemopoietic cells in short-term cultures. In such cultures, the proliferation and differentiation of hemopoietic cells can be studied under controlled conditions. Two areas of investigation show particular promise: elucidation of the role of the cell surface membrane in regulation; and the possible development, through a detailed investigation of the properties of leukoviruses, of new methods for the genetic analysis of hemopoietic cells.     

BET bromodomain proteins are required for glioblastoma cell proliferation

Epigenetic proteins have recently emerged as novel anticancer targets. Among these, bromodomain and extra terminal domain (BET) proteins recognize lysine-acetylated histones, thereby regulating gene expression. Newly described small molecules that inhibit BET proteins BRD2, BRD3, and BRD4 reduce proliferation of NUT (nuclear protein in testis)-midline carcinoma, multiple myeloma, and leukemia cells in vitro and in vivo. These findings prompted us to determine whether BET proteins may be therapeutic targets in the most common primary adult brain tumor, glioblastoma (GBM). We performed NanoString analysis of GBM tumor samples and controls to identify novel therapeutic targets. Several cell proliferation assays of GBM cell lines and stem cells were used to analyze the efficacy of the drug I-BET151 relative to temozolomide (TMZ) or cell cycle inhibitors. Lastly, we performed xenograft experiments to determine the efficacy of I-BET151 in vivo. We demonstrate that BRD2 and BRD4 RNA are significantly overexpressed in GBM, suggesting that BET protein inhibition may be an effective means of reducing GBM cell proliferation. Disruption of BRD4 expression in glioblastoma cells reduced cell cycle progression. Similarly, treatment with the BET protein inhibitor I-BET151 reduced GBM cell proliferation in vitro and in vivo. I-BET151 treatment enriched cells at the G1/S cell cycle transition. Importantly, I-BET151 is as potent at inhibiting GBM cell proliferation as TMZ, the current chemotherapy treatment administered to GBM patients. Since I-BET151 inhibits GBM cell proliferation by arresting cell cycle progression, we propose that BET protein inhibition may be a viable therapeutic option for GBM patients suffering from TMZ resistant tumors.     

苦参活性成分的抗肿瘤作用机制研究进展

苦参是中国传统的植物药,具有清热燥湿等多种作用,广泛地应用于抗肿瘤研究,其活性成分能够通过细胞周期阻滞抑制肿瘤细胞的增殖、诱导肿瘤细胞分化、通过细胞周期阻滞、Fas/Fasl和线粒体途径诱导肿瘤细胞凋亡,通过降低VEGF等的表达抑制肿瘤血管生成和内皮细胞增殖,抑制肿瘤侵袭和转移,通过抑制端粒酶活性、逆转多药耐药、调节免疫耐受等辅助治疗肿瘤。通过收集、分析和整理最近几年涉及苦参活性成分抗肿瘤作用的文献,综述其抗肿瘤作用机制,为临床应用苦参治疗肿瘤提供参考。     

Adult Stromal Cells Derived from Human Adipose Tissue Provoke Pancreatic Cancer Cell Death both In Vitro and In Vivo

Background

Normal tissue homeostasis is maintained by dynamic interactions between epithelial cells and their microenvironment. Disrupting this homeostasis can induce aberrant cell proliferation, adhesion, function and migration that might promote malignant behavior. Indeed, aberrant stromal-epithelial interactions contribute to pancreatic ductal adenocarcinoma (PDAC) spread and metastasis, and this raises the possibility that novel stroma-targeted therapies represent additional approaches for combating this malignant disease. The aim of the present study was to determine the effect of human stromal cells derived from adipose tissue (ADSC) on pancreatic tumor cell proliferation.

Principal Findings

Co-culturing pancreatic tumor cells with ADSC and ADSC-conditioned medium sampled from different donors inhibited cancer cell viability and proliferation. ADSC-mediated inhibitory effect was further extended to other epithelial cancer-derived cell lines (liver, colon, prostate). ADSC conditioned medium induced cancer cell necrosis following G1-phase arrest, without evidence of apoptosis. In vivo, a single intra-tumoral injection of ADSC in a model of pancreatic adenocarcinoma induced a strong and long-lasting inhibition of tumor growth.

Conclusion

These data indicate that ADSC strongly inhibit PDAC proliferation, both in vitro and in vivo and induce tumor cell death by altering cell cycle progression. Therefore, ADSC may constitute a potential cell-based therapeutic alternative for the treatment of PDAC for which no effective cure is available.     

Suppressor factor from tumor-allosensitized spleen cells--its effect on in vitro proliferation of tumor cells and in vivo skin allograft survival

C57BL/6 mice are sensitized ip with allogeneic P-815 mastocytoma cells. Fifteen days later the spleen cells of the sensitized mice are used in the production of suppressor factor or treated with mitomycin and used as suppressor cells. Sensitized spleen cells incubated with the specific alloantigen (DBA/2 m-treated spleen cells) release suppressor factor (SF)² which inhibits cell proliferation in mixed lymphocyte culture (MLC) as well as the in vitro generation of cytotoxic cells (CML). SF is most effective when added eary during MLC. SF also inhibits mitogen responsiveness of normal spleen cells. In addition to inhibiting lymphocyte function in vitro, suppressor cells as well as SF inhibit the in vitro proliferation of tumor cells. This inhibition is specific for the tumor to which the suppressor cells are induced. The inhibition of tumor cell proliferation is not due to the presence of cytotoxic cells in the spleen of the tumor-allosensitized mice. Suppressor cells from neonatal mice do not inhibit the in vitro proliferation of tumor cells. SF injected iv into C57BL/6 mice decreases the mixed lymphocyte reactivity of the host spleen cells and decreases the ability of the host to reject skin allografts. We interpret these data to suggest that tumor-allosensitized spleen cells, and the SF they produce, not only affect lymphocyte function but also inhibit tumor cell proliferation. This dual effect of suppressor cells could be an important part of the immune surveillance against tumors.     

Modulation of EL-4 mouse lymphoma cell proliferation by macrophages and tumor related factors

It is well known that macrophages play an important role in the control of tumor growth. This control may be the result of a direct action of macrophages or mediated by several biologically active products or factors elaborated by these and other cell populations. Our studies on the proliferation of a murine T-cell lymphoma (EL-4) showed that the treatment of the ascitic fluid (from the peritoneum of EL-4 bearing mice) with carbonyl iron resulted in a depletion of phagocytes concomitant with a significant increase of [3H] thymidine uptake by EL-4 cells. Further, the growth of EL-4 cells cultured in semisolid agar was significantly inhibited by an underlayer of large quantities of macrophages both from normal and EL-4 bearing mice as well as when cultured in the presence of PGE2. The underlayer of tumor macrophages P388 D1 resulted in an increase of the EL-4 cell growth. Also, conditioned media obtained from in vitro liquid cultures of EL-4 cells and L 1210 cells (B-lymphoma) produced a remarkable inhibition of the in vitro cloning capacity and [3H] thymidine uptake by EL-4 cells. These data support the hypothesis that different factors from normal and hemopoietic tumor cells may control the tumor growth and point out that self-produced factors may modulate the proliferation of tumor cells.     

Antiproliferative activity of N(6)-isopentenyladenosine on HCT-15 colon carcinoma cell line

N(6)-Isopentenyladenosine (iPA), a member of the cytokinin family of plant hormones, exerts remarkable inhibition on tumor cell proliferation and apoptosis in several tumor cell lines. In this study, we report that iPA is able to inhibit the proliferation and promotes apoptosis in HCT-15 human colon cancer cells in a dose-dependent manner with a concentration of 2.5?μM, which causes 50% inhibition of cell viability. The cell cycle analysis by flow cytometry showed that iPA-induced growth arrest could be associated to apoptosis. Moreover, suppression of clonogenic activity occurs after exposure to iPA at a concentration of 2.5?μM for HCT-15.     

Inhibitory effect of Disulfiram/copper complex on non-small cell lung cancer cells

Non-small cell lung cancer (NSCLC) is the most common cause of cancer-related death in both men and women worldwide. Recently, Disulfiram has been reported to be able to inhibit glioblastoma, prostate, or breast cancer cell proliferation. In this study, the synergistic effect of Disulfiram and copper on NSCLC cell growth was investigated. Inhibition of cancer cell proliferation was detected by 1-(4,5-Dimethylthiazol-2-yl)-3,5-diphenylformazan (MTT) assay and cell cycle analysis. Liquid colony formation and tumor spheroid formation assays were used to evaluate their effect on cancer cell clonogenicity. Real-time PCR was performed to test the mRNA level of cancer stem cell related genes. We found that Disulfiram or copper alone did not potently inhibit NSCLC cell proliferation in vitro. However, the presence of copper significantly enhanced inhibitory effect of Disulfiram on NSCLC cell growth, indicating a synergistic effect between Disulfiram and copper. Cell cycle analysis showed that Disulfiram/copper complex caused NSCLC cell cycle arrest in G2/M phase. Furthermore, Disulfiram/copper significantly increased the sensitivity of cisplatin in NSCLC cells tested by MTT assay. Liquid colony formation assay revealed that copper dramatically increased the inhibitory effect of Disulfiram on NSCLC cell colony forming ability. Disulfiram combined with copper significantly attenuated NSCLC cell spheroid formation and recuded the mRNA expression of lung cancer stem cell related genes. Our data suggest that Disulfiram/copper complex alone or combined with other chemotherapy is a potential therapeutic strategy for NSCLC patients.     

Compromising the constitutive p16INK4a expression sensitizes human neuroblastoma cells to Hsp90 inhibition and promotes premature senescence

The Hsp90 chaperone has become the attractive pharmacological target to inhibit tumor cell proliferation. However, tumor cells can evolve with mechanisms to overcome Hsp90 inhibition. Using human neuroblastoma, we have investigated one such limitation. Here, we demonstrate that neuroblastoma cells overcome the interference of tumor suppressor p16^INK4a in cell proliferation, which is due to its latent interaction with CDK4 and CDK6. Cells also displayed impedance to the pharmacological inhibition of cancer chaperone Hsp90 inhibition with respect to induced cytotoxicity. However, the p16^INK4a knockdown has triggered the activation of cyclin-CDK6 axis and enhanced the cell proliferation. These cells are eventually sensitized to Hsp90 inhibition by activating the DNA damage response mediated through p53-p21^WAF-1 axis and G1 cell cycle exit. While both CDK4 and CDK6 have exhibited low affinity to p16^INK4a, CDK6 has exhibited high affinity to Hsp90. Destabilizing the CDK6 interaction with Hsp90 has prolonged G2/M cell cycle arrest fostering to premature cellular senescence. The senescence driven cells exhibited compromised metastatic potential both in vitro as well as in mice xenografts. Our study unravels that cancer cells can be adapted to the constitutive expression of tumor suppressors to overcome therapeutic interventions. Our findings display potential implication of Hsp90 inhibitors to overcome such adaptations.     

Design,synthesis, and evaluation of quinazoline T cell proliferation inhibitors

We report here on a class of quinazoline molecules that inhibit T cell proliferation. The most potent compound N-p-tolyl-2-(3,4,5-trimethoxyphenyl)quinazolin-4-amine (S101) and its close analogs were found to inhibit the proliferation of T cells from human peripheral blood mononuclear cells (PBMC) and Jurkat cells, with IC₅₀ in the sub-micromolar range. The inhibitor induced G2 cell cycle arrest but did not inhibit IL-2 secretion. The anti-proliferative effect correlated with inhibition of the tyrosine phosphorylation of SLP-76, a molecular element in the signaling pathway of the T cell receptor (TCR). The inhibitor restrained proliferation of lymphocytes with much higher potency than non-hematopoietic cells. This new class of specific T cell proliferation inhibitors may serve as lead molecules for the development of agents aimed at diseases in which T cell signaling plays a role and agents to induce tolerance to grafted tissues or organs.     

Indomethacin Inhibits Endothelial Cell Proliferation by Suppressing Cell Cycle Proteins and PRB Phosphorylation: A Key to Its Antiangiogenic Action?

Nonsteroidal anti-inflammatory drugs (NSAIDs) inhibit angiogenesis in vivo and in vitro, but the mechanism of this action is unclear. Angiogenesis—formation of new capillary vessels—requires endothelial proliferation, migration, and tube formation. It is stimulated by basic fibroblast growth factor (bFGF) and vascular endothelial growth factor (VEGF). The cell cycle is regulated positively by cyclins and negatively by cyclin-dependent kinase inhibitors (CKI) and the retinoblastoma protein (pRb). Since the effects of NSAIDs on cell cycle-regulatory proteins in endothelial cells remain unknown, we examined the effect of indomethacin on bFGF-stimulated endothelial cell proliferation and on cell cycle regulatory proteins in rat primary aortic endothelial cells (RAEC). Indomethacin significantly inhibited basal and bFGF-stimulated endothelial cell proliferation. This inhibition correlated significantly with reduced cyclin D1 and increased p21 protein expression. Furthermore, indomethacin reduced pRb phosphorylation. These findings suggest that indomethacin arrests endothelial cell proliferation essential for angiogenesis by modulating cell cycle protein levels.     

Nitric oxide-induced anti-mitogenic effects in high and low responder rat strains.

Nitric oxide (NO) has multiple biologic functions: in the brain it acts as a neuronal messenger; elsewhere, it causes smooth muscle relaxation, inhibition of platelet aggregation, inhibition of leukocyte adhesion, inhibition of tumor growth, and microbiostasis. Our studies show that production of NO is responsible for the unusual unresponsiveness of BN rat spleen cells to mitogens. NG-monomethyl-L-arginine (NGMMA), a potent competitive inhibitor for NO synthase, reverses this defect. Lysed RBC or NGMMA were shown to enhance mitogen-induced spleen cell proliferation only one- to twofold in Lewis rats (that have normal mitogen responsiveness) but act to stimulate BN rat T cells by 10- to 100-fold. NGMMA-enhanced proliferation was significantly diminished by prior depletion of macrophages. Surprisingly, NO did not inhibit IL-2 production in 48-h cultures of BN rat spleen cells, and exogenous IL-2 was ineffective in releasing NO-mediated suppression. These studies indicate that NO produced by macrophages can completely and reversibly inhibit T cell proliferation. The BN rat appears to be unique in its production of very high levels of NO, making it an especially useful animal model for studying the biologic control and functional consequences of NO generation.     

A study on the immune receptors for polysaccharides from the roots of Astragalus membranaceus, a Chinese medicinal herb

The immunopotentiating effect of the roots of Astragalus membranaceus, a medicinal herb, has been associated with its polysaccharide fractions (Astragalus polysaccharides, APS). We herein demonstrate that APS activates mouse B cells and macrophages, but not T cells, in terms of proliferation or cytokine production. Fluorescence-labeled APS (fl-APS) was able to selectively stain murine B cells, macrophages and a also human tumor cell line, THP-1, as determined in flow cytometric analysis and confocal laser scanning microscopy. The specific binding of APS to B cells and macrophages was competitively inhibited by bacterial lipopolysaccharides. Rabbit-anti-mouse immunoglobulin (Ig) antibody was able to inhibit APS-induced proliferation of, and APS binding to, mouse B cells. Additionally, APS effectively stimulated the proliferation of splenic B cells from C3H/HeJ mice that have a mutated TLR4 molecule incapable of signal transduction. These results indicate that APS activates B cells via membrane Ig in a TLR4-independent manner. Interestingly, macrophages from C3H/HeJ mice were unable to respond to APS stimulation, suggesting a positive involvement of the TLR4 molecule in APS-mediated macrophage activation. Monoclonal Ab against mouse TLR4 partially inhibited APS binding with macrophages, implying direct interaction between APS and TLR4 on cell surface. These results may have important implications for our understanding on the molecular mechanisms of immunopotentiating polysaccharides from medicinal herbs.     

Britannin stabilizes T cell activity and inhibits proliferation and angiogenesis by targeting PD-L1 via abrogation of the crosstalk between Myc and HIF-1α in cancer

BackgroundProgrammed cell death-ligand 1 (PD-L1) is overexpressed in tumor cells, which causes tumor cells to escape T cell killing, and promotes tumor cell survival, cell proliferation, migration, invasion, and angiogenesis. Britannin is a natural product with anticancer pharmacological effects.PurposeIn this work, we studied the anticancer potential of britannin and explored whether britannin mediated its effect by inhibiting the expression of PD-L1 in tumor cells.MethodsIn vitro, the mechanisms underlying the inhibition of PD-L1 expression by britannin were investigated by MTT assay, homology modeling and molecular docking, RT-PCR, western blotting, co-immunoprecipitation, and immunofluorescence. The changes in tumor killing activity, cell proliferation, cell cycle, migration, invasion, and angiogenesis were analyzed by T cell killing assays, EdU labeling, colony formation, flow cytometry, wound healing, matrigel transwell invasion, and tube formation, respectively. In vivo, the antitumor activity of britannin was evaluated in the HCT116 cell xenograft model.ResultsBritannin reduced the expression of PD-L1 in tumor cells by inhibiting the synthesis of the PD-L1 protein but did not affect the degradation of the PD-L1 protein. Britannin also inhibited HIF-1α expression through the mTOR/P70S6K/4EBP1 pathway and Myc activation through the Ras/RAF/MEK/ERK pathway. Mechanistically, britannin inhibited the expression of PD-L1 by blocking the interaction between HIF-1α and Myc. In addition, britannin could enhance the activity of cytotoxic T lymphocytes and inhibit tumor cell proliferation and angiogenesis by inhibiting PD-L1. Finally, in vivo observations were confirmed by demonstrating the antitumor activity of britannin in a murine xenograft model.ConclusionBritannin inhibits the expression of PD-L1 by blocking the interaction between HIF-1α and Myc. Moreover, britannin stabilizes T cell activity and inhibits proliferation and angiogenesis by inhibiting PD-L1 in cancer. The current work highlights the anti-tumor effect of britannin, providing insights into the development of cancer therapeutics via PD-L1 inhibition.     

Tumor cytostasis mediated by a monoclonal IgM antibody promoting adhesion between macrophages and tumor cells. Evidence for a lectin-like behavior.

We have shown previously that an IgM mAb (A10) recognizing Ehrlich tumor (ET) cell surface carbohydrates, inhibits in vivo ET growth by a macrophage-dependent mechanism. The inhibition mechanism involving both IgM and macrophages was unclear because receptors for IgM on macrophages are controversial and another monoclonal IgM (E1), also recognizing ET cell surface carbohydrates, was completely unable to show any protective effect. Here we show that A10, but not E1, was able to promote adhesion between macrophages and ET cells by a receptor for IgM-independent mechanism. Immunofluorescence studies showed that A10, but not E1, did react with macrophages if these cells were preincubated with a source of Ag spontaneously released from ET cells. This Ag release appeared to be required for A10-mediated adhesion, because adhesion was not obtained when ET cells fixed with paraformaldehyde were used. Cytostasis studies performed with macrophages stimulated with L-929 conditioned medium and ET cells showed that A10, but not E1 nor unrelated IgM, was able to inhibit ET cell proliferation in vitro by a mechanism involving cell contact between both cell populations. Therefore, IgM inhibition of ET growth, both in vivo and in vitro, could be explained by a lectin-like mechanism, where IgM, recognizing Ag of tumor origin, bridges macrophages to tumor cells.     

ERK1/2 inhibition enhances apoptosis induced by JAK2 silencing in human gastric cancer SGC7901 cells

Recent studies suggest JAK2 signaling may be a therapeutic target for treatment of gastric cancer (GC). However, the exact roles of JAK2 in gastric carcinogenesis are not very clear. Here, we have targeted JAK2 to be silenced by shRNA and investigated the biological functions and related mechanisms of JAK2 in GC cell SGC7901. In this study, JAK2 is commonly highly expressed in GC tissues as compared to their adjacent normal tissues (n = 75, p < 0.01). Specific down-regulation of JAK2 suppressed cell proliferation and colony-forming units, induced G2/M arrest in SGC7901 cells, but had no significant effect on cell apoptosis in vitro or tumor growth inhibition in vivo. Interestingly, JAK2 silencing-induced activation of ERK1/2, and inactivation of ERK1/2 using the specific ERK inhibitor PD98059 markedly enhanced JAK2 shRNA-induced cell proliferation inhibition, cell cycle arrest and apoptosis. Ultimately, combination of PD98059 and JAK2 shRNA significantly inhibited tumor growth in nude mice. Our results implicate JAK2 silencing-induced cell proliferation inhibition, cell cycle arrest, and ERK1/2 inhibition could enhance apoptosis induced by JAK2 silencing in SGC7901 cells.