tumor stem cell assay


Summary: A cytologic technique for measuring the functional capacity of tumor stem cells by assaying their activity. It is used primarily for the in vitro testing of antineoplastic agents.

Top Publications

  1. Rota L, Lazzarino D, Ziegler A, LeRoith D, Wood T. Determining mammosphere-forming potential: application of the limiting dilution analysis. J Mammary Gland Biol Neoplasia. 2012;17:119-23 pubmed publisher
    ..Here, we discuss the use and benefit of using a limiting dilution analysis to quantify sphere-forming frequency in primary mammary epithelial cells grown in nonadherent conditions. ..
  2. Bao B, Wang Z, Ali S, Ahmad A, Azmi A, Sarkar S, et al. Metformin inhibits cell proliferation, migration and invasion by attenuating CSC function mediated by deregulating miRNAs in pancreatic cancer cells. Cancer Prev Res (Phila). 2012;5:355-64 pubmed publisher
  3. Engesæter B, Sathermugathevan M, Hellenes T, Engebraten O, Holm R, Flørenes V, et al. Targeting inhibitor of apoptosis proteins in combination with dacarbazine or TRAIL in melanoma cells. Cancer Biol Ther. 2011;12:47-58 pubmed publisher
    ..Furthermore, the combined treatment reduced the ability of melanoma cells to engraft and form tumors in mice, actualizing the combination for future therapy of malignant melanoma. ..
  4. Wu L, Cai C, Wang X, Liu M, Li X, Tang H. MicroRNA-142-3p, a new regulator of RAC1, suppresses the migration and invasion of hepatocellular carcinoma cells. FEBS Lett. 2011;585:1322-30 pubmed publisher
    ..These results demonstrated that miR-142-3p directly and negatively regulates RAC1 in HCC cells, which highlights the importance of miRNAs in tumorigenesis. ..
  5. Zhang J, Stevens M, Laughton C, Madhusudan S, Bradshaw T. Acquired resistance to temozolomide in glioma cell lines: molecular mechanisms and potential translational applications. Oncology. 2010;78:103-14 pubmed publisher
    ..These cell lines provide valuable models for the development of strategies to combat temozolomide resistance. ..
  6. Mourtada Maarabouni M, Pickard M, Hedge V, Farzaneh F, Williams G. GAS5, a non-protein-coding RNA, controls apoptosis and is downregulated in breast cancer. Oncogene. 2009;28:195-208 pubmed publisher
  7. Zhang S, Huang F, Huang Y, Zhong W, Yu Z. Characterization of a cancer stem cell-like side population derived from human pancreatic adenocarcinoma cells. Tumori. 2010;96:985-92 pubmed
    ..To identify and partially characterize the side population cells derived from three human pancreatic adenocarcinoma cell lines...
  8. Kallifatidis G, Labsch S, Rausch V, Mattern J, Gladkich J, Moldenhauer G, et al. Sulforaphane increases drug-mediated cytotoxicity toward cancer stem-like cells of pancreas and prostate. Mol Ther. 2011;19:188-95 pubmed publisher
    ..No pronounced side effects were observed in normal cells or mice. Our data suggest that SF increases the effectiveness of various cytotoxic drugs against CSCs without inducing additional toxicity in mice. ..
  9. Hwang Verslues W, Kuo W, Chang P, Pan C, Wang H, Tsai S, et al. Multiple lineages of human breast cancer stem/progenitor cells identified by profiling with stem cell markers. PLoS ONE. 2009;4:e8377 pubmed publisher
    ..Taken together, these results suggest that similar to leukemia, several stem/progenitor cell-like subpopulations can exist in breast cancer. ..

More Information


  1. Vecchione A, Fassan M, Anesti V, Morrione A, Goldoni S, Baldassarre G, et al. MITOSTATIN, a putative tumor suppressor on chromosome 12q24.1, is downregulated in human bladder and breast cancer. Oncogene. 2009;28:257-69 pubmed publisher
    ..Our findings support the hypothesis that MITOSTATIN has many hallmarks of a classical tumor suppressor in solid tumors and may play an important role in cancer development and progression. ..
  2. De Schutter H, Kimpe M, Isebaert S, Nuyts S. A systematic assessment of radiation dose enhancement by 5-Aza-2'-deoxycytidine and histone deacetylase inhibitors in head-and-neck squamous cell carcinoma. Int J Radiat Oncol Biol Phys. 2009;73:904-12 pubmed publisher
  3. Horst D, Scheel S, Liebmann S, Neumann J, Maatz S, Kirchner T, et al. The cancer stem cell marker CD133 has high prognostic impact but unknown functional relevance for the metastasis of human colon cancer. J Pathol. 2009;219:427-34 pubmed publisher
    ..Thus, we conclude that CD133 expression is a marker with high prognostic impact for colon cancer, while it seems to have no obvious functional role as a driving force of this malignancy. ..
  4. Wilson W, Stribbling S, Pruijn F, Syddall S, Patterson A, Liyanage H, et al. Nitro-chloromethylbenzindolines: hypoxia-activated prodrugs of potent adenine N3 DNA minor groove alkylators. Mol Cancer Ther. 2009;8:2903-13 pubmed publisher
    ..Nitro-CBIs are novel lead hypoxia-activated prodrugs that represent the first examples of hypoxia-selective generation of potent DNA minor groove alkylating agents. ..
  5. Heaney N, Pellicano F, Zhang B, Crawford L, Chu S, Kazmi S, et al. Bortezomib induces apoptosis in primitive chronic myeloid leukemia cells including LTC-IC and NOD/SCID repopulating cells. Blood. 2010;115:2241-50 pubmed publisher
    ..Because of known toxicities, including myelosuppression, the likely initial clinical application of bortezomib in CML would be in resistant and advanced disease. ..
  6. Al Assar O, Muschel R, Mantoni T, McKenna W, Brunner T. Radiation response of cancer stem-like cells from established human cell lines after sorting for surface markers. Int J Radiat Oncol Biol Phys. 2009;75:1216-25 pubmed publisher
    ..Likewise, CSLC characteristics were found in some but not all of the tested cell lines. The reported problems in testing for CSLC in cell lines may be overcome by additional techniques, beyond sorting for markers. ..
  7. Lin W, Zhang J, Zhang J, Liu X, Fei Z, Li X, et al. RNAi-mediated inhibition of MSP58 decreases tumour growth, migration and invasion in a human glioma cell line. J Cell Mol Med. 2009;13:4608-22 pubmed publisher
    ..In summary, our data highlight the importance of MSP58 in glioma progression and provided a biological basis for MSP58 as a novel candidate target for treatment of glioma. ..
  8. Ermler S, Scholze M, Kortenkamp A. The sensitivity of the MDA-kb2 cell in vitro assay in detecting anti-androgenic chemicals--identification of sources of variability and estimation of statistical power. Toxicol In Vitro. 2010;24:1845-53 pubmed publisher
    ..Using the modified assay protocol extensive concentration-response analyses were conducted with bisphenol A, BDE100 and vinclozolin. Our modified procedure improves considerably the reproducibility of the MDA-kb2 assay. ..
  9. Staab A, Fleischer M, Loeffler J, Said H, Katzer A, Plathow C, et al. Small interfering RNA targeting HIF-1? reduces hypoxia-dependent transcription and radiosensitizes hypoxic HT 1080 human fibrosarcoma cells in vitro. Strahlenther Onkol. 2011;187:252-9 pubmed publisher
  10. Ströfer M, Jelkmann W, Depping R. Curcumin decreases survival of Hep3B liver and MCF-7 breast cancer cells: the role of HIF. Strahlenther Onkol. 2011;187:393-400 pubmed publisher
    ..Effects of curcumin on cell growth and survival factor expression suggest its potential benefit in the treatment of cancer without a direct radiosensitizing influence of curcumin on these cells. ..
  11. Sroczynska P, Cruickshank V, Bukowski J, Miyagi S, Bagger F, Walfridsson J, et al. shRNA screening identifies JMJD1C as being required for leukemia maintenance. Blood. 2014;123:1870-82 pubmed publisher
    ..In both mouse and human leukemic cells, the growth defect upon JMJD1C depletion appears to be primarily due to increased apoptosis, which implicates JMJD1C as a potential therapeutic target in leukemia. ..
  12. Nör C, Sassi F, de Farias C, Schwartsmann G, Abujamra A, Lenz G, et al. The histone deacetylase inhibitor sodium butyrate promotes cell death and differentiation and reduces neurosphere formation in human medulloblastoma cells. Mol Neurobiol. 2013;48:533-43 pubmed publisher
    ..In addition, our study suggests that HDAC inhibition might promote the neuronal differentiation of MB cells and provides the first evidence that an HDAC inhibitor might suppress the expansion or survival of MB cancer stem cells. ..
  13. Yin X, Xiang T, Li L, Su X, Shu X, Luo X, et al. DACT1, an antagonist to Wnt/?-catenin signaling, suppresses tumor cell growth and is frequently silenced in breast cancer. Breast Cancer Res. 2013;15:R23 pubmed publisher
    ..Our study demonstrates that DACT1 could function as a tumor suppressor but was frequently downregulated in breast cancer. ..
  14. Han E, Im C, Park S, Moon E, Hong S. Combined treatment with peroxisome proliferator-activated receptor (PPAR) gamma ligands and gamma radiation induces apoptosis by PPAR?-independent up-regulation of reactive oxygen species-induced deoxyribonucleic acid damage signals in non-small cell. Int J Radiat Oncol Biol Phys. 2013;85:e239-48 pubmed publisher
    ..Taken together, these results indicated that the combined treatment of PPAR? ligands and ?-radiation synergistically induced DNA damage and apoptosis, which was regulated by ROS. ..
  15. Cui X, Xu D, Lv C, Qu F, He J, Chen M, et al. Suppression of MED19 expression by shRNA induces inhibition of cell proliferation and tumorigenesis in human prostate cancer cells. BMB Rep. 2011;44:547-52 pubmed
    ..Knockdown of MED19 caused S-phase arrest and induced apoptosis via modulation of Bid and Caspase 7. It was suggested that MED19 serves as a novel proliferation regulator that promotes growth of prostate cancer cells. ..
  16. Ma T, Zhang Y, Zhang C, Luo J, Kong L. Downregulation of TIGAR sensitizes the antitumor effect of physapubenolide through increasing intracellular ROS levels to trigger apoptosis and autophagosome formation in human breast carcinoma cells. Biochem Pharmacol. 2017;143:90-106 pubmed publisher
    ..Taken together, these results indicate that downregulation of TIGAR increased PB-induced apoptosis and autophagosomes associated cell death through promoting ROS generation in MDA-MB-231 and MCF-7 cells. ..
  17. Ding L, Huang X, Zheng F, Xie J, She L, Feng Y, et al. FZD2 inhibits the cell growth and migration of salivary adenoid cystic carcinomas. Oncol Rep. 2016;35:1006-12 pubmed publisher
    ..Knockdown of FZD2 downregulated the expression of PAI-1. Our results suggest that FZD2 may be a tumor suppressor gene in SACCs that inhibits cell growth and migration. ..
  18. Zeng H, Yuan Z, Zhu H, Li L, Shi H, Wang Z, et al. Expression of hPNAS-4 radiosensitizes Lewis lung cancer. Int J Radiat Oncol Biol Phys. 2012;84:e533-40 pubmed publisher
    ..Therefore, it may be a potential radiosensitive treatment of radiation therapy for lung cancer. ..
  19. Gerashchenko O, Zhuravel E, Skachkova O, Khranovska N, Filonenko V, Pogrebnoy P, et al. Biologic activities of recombinant human-beta-defensin-4 toward cultured human cancer cells. Exp Oncol. 2013;35:76-82 pubmed
    ..Recombinant human beta-defensin-4 is biologically active peptide capable to cause oppositely directed effects toward biologic features of cancer cells in vitro dependent on its concentration. ..
  20. Ondrusova L, Reda J, Záková P, Tuhácková Z. Inhibition of mTORC1 by SU6656, the selective Src kinase inhibitor, is not accompanied by activation of Akt/PKB signalling in melanoma cells. Folia Biol (Praha). 2013;59:162-7 pubmed
  21. Hong X, Chedid K, Kalkanis S. Glioblastoma cell line-derived spheres in serum?containing medium versus serum-free medium: a comparison of cancer stem cell properties. Int J Oncol. 2012;41:1693-700 pubmed publisher
    ..U251n cells are distinguished from other tumor cells due to their potential for multilineage differentiation. ..
  22. Flanagan S, Cooper K, Mannava S, Nikiforov M, Shewach D. Short hairpin RNA suppression of thymidylate synthase produces DNA mismatches and results in excellent radiosensitization. Int J Radiat Oncol Biol Phys. 2012;84:e613-20 pubmed publisher
    ..Importantly, shRNA suppression of TS avoids FP-mediated TS elevation and its negative prognostic role. These studies support the further exploration of TS suppression as a novel radiosensitizing strategy. ..
  23. Schilling D, Bayer C, Emmerich K, Molls M, Vaupel P, Huber R, et al. Basal HIF-1? expression levels are not predictive for radiosensitivity of human cancer cell lines. Strahlenther Onkol. 2012;188:353-8 pubmed publisher
    ..Our data suggest that basal HIF-1? levels in human tumor cell lines do not predict their radiosensitivity under normoxia. ..
  24. Techawathanawanna S, Nimmannit A, Akewanlop C. Clinical characteristics and disease outcome of UICC stages I-III colorectal cancer patients at Siriraj Hospital. J Med Assoc Thai. 2012;95 Suppl 2:S189-98 pubmed
    ..Multi-modality treatments with surgery, adjuvant chemotherapy and radiation resulted in comparable survival as in Western countries. Independent risk factors for worse survival in this cohort were stage III disease and male gender. ..
  25. Karst A, Levanon K, Drapkin R. Modeling high-grade serous ovarian carcinogenesis from the fallopian tube. Proc Natl Acad Sci U S A. 2011;108:7547-52 pubmed publisher
    ..With this model, we will now be able to assess the transformative effects of specific genetic alterations on FTSECs in order to characterize their respective roles in HGSOC development...
  26. Yang F, Luo L, Zhang L, Wang D, Yang S, Ding L, et al. MiR-346 promotes the biological function of breast cancer cells by targeting SRCIN1 and reduces chemosensitivity to docetaxel. Gene. 2017;600:21-28 pubmed publisher
    ..Taken together, miR-346 may function as an oncogenic miRNA and mediate chemosensitivity to docetaxel through targeting SRCIN1 in breast cancer, targeted modulation of miR-346 expression may became a potential strategy for the treatment. ..
  27. Kang K, Zhu C, Wong Y, Gao Q, Ty A, Wong M. Gefitinib radiosensitizes stem-like glioma cells: inhibition of epidermal growth factor receptor-Akt-DNA-PK signaling, accompanied by inhibition of DNA double-strand break repair. Int J Radiat Oncol Biol Phys. 2012;83:e43-52 pubmed publisher
    ..Gefitinib differentially enhances radiosensitivity of stem-like gliomaspheres by reducing EGFR-Akt activation and DNA-PKcs expression, accompanied by enhanced irradiation-induced DNA DSBs and inhibition of DSB repair. ..
  28. Song Y, Xue L, Du S, Sun M, Hu J, Hao L, et al. Caveolin-1 knockdown is associated with the metastasis and proliferation of human lung cancer cell line NCI-H460. Biomed Pharmacother. 2012;66:439-47 pubmed publisher
    ..The expression of ?-catenin increased and the level of E-cadherin decreased. In summary, our findings provide experimental evidence that CAV-1 may function as a proproliferative and antimetastatic gene in NCI-H460 cell line. ..
  29. Jin X, Kim S, Jeon H, Beck S, Sohn Y, Yin J, et al. Interferon regulatory factor 7 regulates glioma stem cells via interleukin-6 and Notch signalling. Brain. 2012;135:1055-69 pubmed publisher
  30. Wang X, Liu Q, Hou B, Zhang W, Yan M, Jia H, et al. Concomitant targeting of multiple key transcription factors effectively disrupts cancer stem cells enriched in side population of human pancreatic cancer cells. PLoS ONE. 2013;8:e73942 pubmed publisher
    ..Specifically, the present study establishes the combination of Sox2/Oct4/c-Myc targeting as a potential anti-pancreatic cancer agent worthy of further studies in preclinical settings. ..
  31. Zhou W, Feng X, Ren C, Jiang X, Liu W, Huang W, et al. Over-expression of BCAT1, a c-Myc target gene, induces cell proliferation, migration and invasion in nasopharyngeal carcinoma. Mol Cancer. 2013;12:53 pubmed publisher
    ..The results suggest that BCAT1 may be a novel molecular target for the diagnosis and treatment of NPC. ..
  32. Takebayashi K, Mekata E, Sonoda H, Shimizu T, Endo Y, Tani T. Clinical potential of the anticancer drug sensitivity test for patients with synchronous stage IV colorectal cancer. Cancer Chemother Pharmacol. 2013;72:217-22 pubmed publisher
    ..4 days in Group A and 518.5 days in Group B (p = 0.0061). The CD-DST can define chemoresistant and chemosensitive tumors. The use of CD-DST might be one of the tools to supplement informed consent prior to initiation of therapy. ..
  33. Hagelgans A, Nacke B, Zamaraeva M, Siegert G, Menschikowski M. Silibinin down-regulates expression of secreted phospholipase A2 enzymes in cancer cells. Anticancer Res. 2014;34:1723-9 pubmed
  34. Pandelidou M, Dimas K, Georgopoulos A, Hatziantoniou S, Demetzos C. Preparation and characterization of lyophilised egg PC liposomes incorporating curcumin and evaluation of its activity against colorectal cancer cell lines. J Nanosci Nanotechnol. 2011;11:1259-66 pubmed
    ..In vivo studies based on the recent findings are in progress in our laboratory. ..
  35. Quelen C, Lippert E, Struski S, Demur C, Soler G, Prade N, et al. Identification of a transforming MYB-GATA1 fusion gene in acute basophilic leukemia: a new entity in male infants. Blood. 2011;117:5719-22 pubmed publisher
    ..Taken together, these results establish, for the first time, a link between a recurrent chromosomal translocation and the development of this particular subtype of infant leukemia. ..
  36. Brauer P, Zheng Y, Evans M, Dominguez Brauer C, Peehl D, Tyner A. The alternative splice variant of protein tyrosine kinase 6 negatively regulates growth and enhances PTK6-mediated inhibition of ?-catenin. PLoS ONE. 2011;6:e14789 pubmed publisher
    ..Fully understanding functions of ALT-PTK6 and its impact on PTK6 signaling will be critical for development of therapeutic strategies that target PTK6 in cancer. ..
  37. Mozet C, Stoehr M, Dimitrova K, Dietz A, Wichmann G. Hedgehog targeting by cyclopamine suppresses head and neck squamous cell carcinoma and enhances chemotherapeutic effects. Anticancer Res. 2013;33:2415-24 pubmed
    ..Our findings define SMO--a Hh component- as a potential target in HNSCC and suggest the utility of Hh targeting in future multimodal treatment regimens for HNSCC. ..
  38. Forti F, Armelin H. Arginine vasopressin controls p27(Kip1) protein expression by PKC activation and irreversibly inhibits the proliferation of K-Ras-dependent mouse Y1 adrenocortical malignant cells. Biochim Biophys Acta. 2011;1813:1438-45 pubmed publisher
    ..These data show that the survival destabilisation of K-Ras-dependent Y1 malignant cells by AVP requires large quantities of the p27(Kip1) protein as well as phosphorylation of the p27(Kip1) protein at both Ser10 and Thr187. ..
  39. Miyatake Y, Oliveira A, Jarboui M, Ota S, Tomaru U, Teshima T, et al. Protective roles of epithelial cells in the survival of adult T-cell leukemia/lymphoma cells. Am J Pathol. 2013;182:1832-42 pubmed publisher
    ..Combined evidence suggests that interactions with normal epithelial cells augment the resistance of ATL cells to TSA-induced apoptosis and facilitate immune evasion by ATL cells. ..
  40. Ahmad A, Ali S, Ahmed A, Ali A, Raz A, Sakr W, et al. 3, 3'-Diindolylmethane enhances the effectiveness of herceptin against HER-2/neu-expressing breast cancer cells. PLoS ONE. 2013;8:e54657 pubmed publisher
  41. Polireddy K, Chavan H, Abdulkarim B, Krishnamurthy P. Functional significance of the ATP-binding cassette transporter B6 in hepatocellular carcinoma. Mol Oncol. 2011;5:410-25 pubmed publisher
    ..Collectively, these findings, obtained by loss of function and gain of function analysis, suggest that ABCB6 plays a role in cell growth and proliferation by targeting the cell cycle...
  42. Ramezani Rad P, Geng H, Hurtz C, Chan L, Chen Z, Jumaa H, et al. SOX4 enables oncogenic survival signals in acute lymphoblastic leukemia. Blood. 2013;121:148-55 pubmed publisher
    ..Collectively, these studies identify SOX4 as a central mediator of oncogenic PI3K/AKT and MAPK signaling in ALL. ..
  43. Hu Y, Li K, Asaduzzaman M, Cuella R, Shi H, Raguz S, et al. MiR-106b~25 cluster regulates multidrug resistance in an ABC transporter-independent manner via downregulation of EP300. Oncol Rep. 2016;35:1170-8 pubmed publisher
    ..Thus, miR-106b~25 cluster controls transporter-independent MDR by apoptosis evasion via downregulation of EP300. ..
  44. Diamond J, Eckhardt S, Tan A, Newton T, Selby H, Brunkow K, et al. Predictive biomarkers of sensitivity to the aurora and angiogenic kinase inhibitor ENMD-2076 in preclinical breast cancer models. Clin Cancer Res. 2013;19:291-303 pubmed publisher
  45. Wu Z, Cui F, Yu F, Peng X, Jiang T, Chen D, et al. Up-regulation of CHAF1A, a poor prognostic factor, facilitates cell proliferation of colon cancer. Biochem Biophys Res Commun. 2014;449:208-15 pubmed publisher
    ..Hence, CHAF1A upregulation functions as a poor prognostic indicator of colon cancer, potentially contributing to its progression by mediating cancer cell proliferation. ..
  46. Kwon H, Song K, Han C, Zhang J, Lu L, Chen W, et al. Epigenetic Silencing of miRNA-34a in Human Cholangiocarcinoma via EZH2 and DNA Methylation: Impact on Regulation of Notch Pathway. Am J Pathol. 2017;187:2288-2299 pubmed publisher
    ..Consequently, these signaling cascades may represent potential therapeutic targets for effective treatment of human CCA. ..
  47. Master A, Malamas A, Solanki R, Clausen D, Eiseman J, Sen Gupta A. A cell-targeted photodynamic nanomedicine strategy for head and neck cancers. Mol Pharm. 2013;10:1988-97 pubmed publisher
    ..Altogether our results show significant promise toward a cell-targeted photodynamic nanomedicine for effective treatment of H&N carcinomas. ..
  48. Yang X, Kessler E, Su L, Thorburn A, Frankel A, Li Y, et al. Diphtheria toxin-epidermal growth factor fusion protein DAB389EGF for the treatment of bladder cancer. Clin Cancer Res. 2013;19:148-57 pubmed publisher
    ..The intravesical delivery of targeted toxin fusion proteins is a novel treatment approach for non-muscle-invasive urinary bladder cancer. With appropriate targeting, the treatments are effective and well-tolerated in vivo. ..
  49. Xu D, He X, Chang Y, Xu C, Jiang X, Sun S, et al. Inhibition of miR-96 expression reduces cell proliferation and clonogenicity of HepG2 hepatoma cells. Oncol Rep. 2013;29:653-61 pubmed publisher
    ..Thus, the use of a synthetic inhibitor of miR-96 may be a promising approach for the treatment of HCC. ..
  50. Tang Q, Xie X, Wang J, Chen Q, Han A, Zou C, et al. Glycogen synthase kinase-3?, NF-?B signaling, and tumorigenesis of human osteosarcoma. J Natl Cancer Inst. 2012;104:749-63 pubmed publisher
    ..2 months). GSK-3? activity may promote osteosarcoma tumor growth, and therapeutic targeting of the GSK-3? and/or NF-?B pathways may be an effective way to enhance the therapeutic activity of anticancer drugs against osteosarcoma. ..
  51. Anchoori R, Khan S, Sueblinvong T, Felthauser A, Iizuka Y, Gavioli R, et al. Stressing the ubiquitin-proteasome system without 20S proteolytic inhibition selectively kills cervical cancer cells. PLoS ONE. 2011;6:e23888 pubmed publisher
    ..Importantly, RAMB1 synergistically triggers cell death of cervical cancer cells when combined with the lysosome inhibitor Chloroquine. ..
  52. Kraljacic B, Arguello M, Amri A, Cormack G, Borden K. Inhibition of eIF4E with ribavirin cooperates with common chemotherapies in primary acute myeloid leukemia specimens. Leukemia. 2011;25:1197-200 pubmed publisher