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84 (chronic myeloid leukemia), SKW-3 (human T-cell leukemia), HD-MY-Z (Hodgkinlymphoma), DOHH-2 (non-Hodgkin lymphoma), 5637 and EJ (human urinary bladdercarcinoma). The effects were assessed by the MTT-dye reduction assay in a wideconcentration range (1-500 µМ) with varying the exposure periods (24, 48, 72 or 96 h). Thereference angiostatic drug thalidomide (THAL) and the sesquiterpene lactonehydroxyartemisinine (ART-OH) (with established anticancer and antiangiogenic effects)were used as positive controls throughout the study (Table 1). The tested arginineanalogues demonstrated only marginal effects and failed to cause 50% inhibition of cellularviability even after 3 days exposure.Table 1. Antiproliferative effects of the tested compounds against a spectrum of humantumor cell lines, as assessed by the MTT-dye reduction assayIC 50 values (µM)Cell linesNNC NNA NsArg CAV THALART-OH72 h 96 h 72 h 96 h 72 h 96 h 72 h 96 h 72 h 72 hK-562 >500 >500 >500 >500 >500 >500 >500 >500 85 >100HL-60 >500 >500 >500 >500 >500 >500 >500 >500 39 >100HD-MY-Z >500 >500 >500 >500 >500 >500 >500 >500 70 >100DOHH-2 >500 >500 >500 >500 >500 >500 >500 >500 47 >1005637 >500 >500 >500 >500 >500 >500 >500 >500 29 >100EJ >500 >500 >500 >500 >500 >500 >500 >500 90 >100“Cell Death Detection” ELISAIn order to elucidate the mechanisms underlying the angiostatic effects of testedcompounds we analyzed their propensity to induce apoptosis in VEGF-stimulated HUVEC(Figure 2). To meet this objective we assessed the oligonucleosomal fragmentation of DNAin treated vs non-treated cells after 24 h exposure at concentrations higher than TGI.Compounds NNC, NNA and NCMe did not trigger apoptosis, which is in line with theMTT-data showing lack of cytotoxic effects. Some increase of the level of histoneassociated DNA-fragments was evoked only with CAV, which indicates that the inductionof apoptosis is at least partly implicated in the observed cytotoxic effects against VEGFstimulatedHUVEC.In conclusion we could say that new polyarginines were synthesized. The abnormalproliferative activity of vascular endothelium is a crucial event in tumor-inducedangiogenesis. The preliminary screening for antiproliferative and cytotoxic effects of testedbuilding amino acids against a spectrum of six human cell lines revealed that theestablished inhibitory activity, together with the low cytotoxicity, give us reason toconsider these compounds as prospective leads for development of antiangiogenic agents.AcknowledgmentsWe are grateful to the the Bulgarian Ministry of Education and Science and NFSR of Bulgaria(Contract MY-FS-13-07).References1. Tung, C., Weissleder, R. Adv. Drug Delivery Rev. 55, 281-294 (2003).2. Pajpanova, T., Stoev, S., Golovinsky, E., Krauss, G.-J., Miersch, J. Amino Acids 12, 191-204 (1997).3. Videnov, G., Aleksiev, B., Stoev, M., Pajpanova, T., Jung, G. Liebigs Ann. Chem. 941-945 (1993).4. Dzimbova, T., Pajpanova, T., Tabakova, S., Golovinsky, E. In: 5th Hellenic Forum on BioactivePeptides, TYPORAMA, Greece, Cordopatis P. (Ed.) 223-227 (2007).515