Human Histiocytic Lymphoma U937 Cells Research Articles

Photodynamic effect on cancer cells influenced by electromagnetic fields

The synergism of low-frequency electromagnetic field treatment and photodynamic effect on killing of human cancer cells is presented. The weak pulsating electromagnetic field (PEMF) generated by Helmholtz coils in the mT range influences the permeability of cell membranes for photosensitizers. Several types of sensitizers were excited by visible light during incorporation without and with two kinds of PEMF treatment. In the first part suitable photosensitizers were selected in the absorption range between 400 and 700 nm against human myeloid leukaemia K562 and human histiocytic lymphoma U937 cells by treatment of PEMF consisting of rectangular pulse groups. In the second part amplitude and frequency dependencies were measured using sinuous PEMF and white light with the result that after 12 min the PEMF treatment enhanced photodynamic effectivity by more than 40% over the control value. Taking into account the influence of many parameters, an additional optimization will be possible by photodynamic PEMF synergism for an increased drug delivery in general.

Reduction in drug-induced DNA double-strand breaks associated with beta1 integrin-mediated adhesion correlates with drug resistance in U937 cells.

We previously showed that adhesion of myeloma cells to fibronectin (FN) by means of beta1 integrins causes resistance to certain cytotoxic drugs. The study described here found that adhesion of U937 human histiocytic lymphoma cells to FN provides a survival advantage with respect to damage induced by the topoisomerase (topo) II inhibitors mitoxantrone, doxorubicin, and etoposide. Apoptosis induced by a topo II inhibitor is thought to be initiated by DNA damage. The neutral comet assay was used to determine whether initial drug-induced DNA damage correlated with cellular-adhesion-mediated drug resistance. Cellular adhesion by means of beta1 integrins resulted in a 40% to 60% reduction in mitoxantrone- and etoposide-induced DNA double-strand breaks. When the mechanisms regulating the initial drug-induced DNA damage were examined, a beta1 integrin-mediated reduction in drug-induced DNA double-strand breaks was found to correlate with reduced topo II activity and decreased salt-extractable nuclear topo IIbeta protein levels. Confocal studies showed changes in the nuclear localization of topo IIbeta; however, alterations in the nuclear-to-cytoplasmic ratio of topo IIbeta in FN-adhered cells were not significantly different. Furthermore, after a high level of salt extraction of nuclear proteins, higher levels of topo IIbeta-associated DNA binding were observed in FN-adhered cells than in cells in suspension. Together, these data suggest that topo IIbeta is more tightly bound to the nucleus of FN-adhered cells. Thus, FN adhesion by means of beta1 integrins appears to protect U937 cells from initial drug-induced DNA damage by reducing topo II activity secondarily to alterations in the nuclear distribution of topo IIbeta.

Photo-activation of hypericin with low doses of light promotes apparent photo-resistance in human histiocytic lymphoma U937 cells

We have observed that exposure of U937 cells, pre-incubated for 18 h with 0.2 μM hypericin, to 599 nm laser radiation with a fluence of 2.5 J/cm 2 renders them insensitive to higher light doses. In fact, pre-sensitized cells appear to be fully resistant to light doses that normally determine massive cellular apoptosis in experimental photo-dynamic therapy. The appearance of the photo-resistance, as measured by evaluating the changes in levels of expression of pro and anti apoptotic proteins, PARP fragmentation and cell viability is exclusively observed with exposure to light doses not exceeding 5–6 J/cm 2. Above this energy, necrosis replaces apoptosis upon photo-stimulation of U937 cells. Here, we describe the appearance of photo-resistance in hypericin-loaded U937 cells, but could not fully unravel the molecular mechanism underlying this process. However, the observed stimulation of the expression of the HSP-70 protein upon photo-induced stress may suggest its involvement in this process.

Chemopreventive agent resveratrol, a natural product derived from grapes, reversibly inhibits progression through S and G2 phases of the cell cycle in U937 cells

Resveratrol, a natural product derived from grapes, has been shown to prevent carcinogenesis in murine models. We report here that resveratrol induces antiproliferation and arrests the S phase in human histiocytic lymphoma U937 cells. Resveratrol induces arrest in the S phase at low concentrations (30–60 μM), but high concentrations do not induce S phase accumulation in U937 cells. Removal of resveratrol from the culture medium stimulates U937 cells to reenter the cell cycle synchronously, as judged by the expression patterns of cyclin E, A and by fluorescent activated cell sorting analysis. These data demonstrate that resveratrol causes S phase arrest and reversible cell cycle arrest. Thus, resveratrol provides an important new cell cycle blocker as well as a cancer chemopreventive agent.

Apoptosis-inducing activity of a driselase digest fraction of green tea residue.

We enzymatically digested green tea residue with Driselase, a crude preparation containing cellulase, pectinase and proteases, in order to examine the potential usefulness of the residue. A fraction of the digest soluble in 70% ethanol was found to induce the death of U937 human histiocytic lymphoma cells by apoptosis. Other enzyme preparations gave similar products with cell death-inducing activity of varing potency. The green tea residue may therefore be a useful source of potential agents with anti-cancer activity.

Enhancement of hyperthermia-induced apoptosis by a free radical initiator, 2,2′-azobis (2-amidinopropane) dihydrochloride, in human histiocytic lymphoma U937 cells

To elucidate the mechanism how a free radical initiator, 2,2′-azobis (2-amidinopropane) dihydrochloride (AAPH), induces cell death at hyperthermic temperatures, apoptosis in a human histiocytic lymphoma cell line, U937, was investigated. Free radical formation deriving from the thermal decomposition of AAPH was examined by spin trapping with 5,5-dimethyl-1-pyrroline-N-oxide (DMPO). An assay for DNA fragmentation, observation of nuclear morphological changes, and flow cytometry for phosphatidylserine (PS) externalization were used to detect apoptosis and revealed enhancement of 44.0°C hyperthermia-induced apoptosis by free radicals due to AAPH. However, free radicals alone derived from AAPH did not induce apoptosis. Hyperthermia induced the production of lipid peroxidation (LPO), an increase in intracellular Ca2+ concentration ([Ca2+]i) and enhanced expression of the type 1 inositol 1,4,5-trisphosphate receptor (IP3R1). The effects of hyperthermia on LPO and [Ca2+]i were enhanced markedly by the combination with AAPH. A significant decrease in Bcl-2 expression, increase in Bax expression, a loss of mitochondrial membrane potential (ΔΨm) and a marked increase in cytochrome c expression were found only in cells treated with hyperthermia and AAPH. Although an intracellular Ca2+ ion chelator, BAPTA-AM, completely inhibited DNA fragmentation, water-soluble vitamine E, Trolox, only partially suppressed DNA fragmentation and the increase in [Ca2+]i. In contrast, LPO was inhibited completely by Trolox, but no inhibition by BAPTA-AM was found. These results suggest that apoptosis induced by hyperthermia alone is due to the increase in [Ca2+]i arising from increased expression of IP3R1 and LPO. Additional increase in [Ca2+]i due to increased LPO and the activation of mitochondria-caspase dependent pathway play a major role in the enhancement of apoptosis by the combination with hyperthermia and AAPH.

Molecular and functional characterization of HSC54, a novel variant of human heat-shock cognate protein 70.

A novel variant of human heat-shock cognate protein 70 (HSC70) transcript, named heat-shock cognate protein 54 (HSC54), was identified and characterized. The transcript encodes the protein lacking 153 amino acid residues of HSC70 in a part of the protein-binding and variable domains, resulting in a calculated molecular mass of 53.5 kDa. HSC54 mRNA was detected in all human cells and tissues examined. The protein was also detected in peripheral mononuclear cells and U937 human histiocytic lymphoma cells. Heat treatment of U937 cells up-regulated the expression of HSC54. The chaperoning activity of HSC54 was examined by luciferase renaturation assay. HSC70 recovered the luciferase activity in the presence of reticulocyte lysate as a source of cochaperones. However, HSC54 did not facilitate the recovery of denatured luciferase; besides, HSC54 significantly inhibited the HSC70-mediated chaperoning activity. In pull-down experiments, HSC54 interacted with cochaperones, p60, HSP40, and p48, as HSC70 did. The resonant mirror detection analysis showed that p60 binds to HSC54 with a higher association rate constant than HSC70 with a similar affinity constant. These results suggest that HSC54 is constitutively expressed and also inducible by stress and may function as an endogenous inhibitory regulator of HSC70 by competing the cochaperones.

Cdk7- and Cdc25A-Independent Dephosphorylation of Cdk2 during Phorbol Ester-Mediated Cell Cycle Arrest in U937 Cells

The molecular mechanism underlying protein kinase C (PKC)-mediated cell cycle arrest is poorly understood. We undertook to characterize phorbol ester-activated PKC-mediated cell cycle arrest. Treatment with phorbol ester inhibited cell growth of human histiocytic lymphoma U937 cells with 83% of the cells arrested in G1 phase. Reduced activity of cdk2 correlated with cdk2 dephosphorylation and accumulation of cdk2 inhibitor p21Waf in phorbol ester-treated cells. Dephosphorylation of cdk2 was not associated with cdk7 and cdc25A activity in phorbol ester-treated cells. Protein phosphatase inhibitor assays suggest that the dephosphorylation of cdk2 results in the activation of a specific protein tyrosine phosphatase. Thus, dephosphorylation of cdk2 as well as accumulation of cdk2 inhibitor is likely to contribute to the G1 phase arrest in phorbol ester-treated in U937 cells.

Membrane electroporation increases photodynamic effects

Abstract Electroporation of membranes is used widely for drug delivery. Photodynamic action consists of three main steps: (A) incorporation of the sensitizer through a membrane into cells; (B) photoxidation of cell constituents and (C) reoxidation of the reduced sensitizer by oxygen etc. The mechanisms of (B) and (C) have been studied widely in past decades. However, the mechanism of transport (A) of sensitizers to targets as the rate limiting step has not been studied to the same extent. Therefore we applied membrane and cell wall electroporation of human histiocytic lymphoma U937 and Saccharomyces cerevisiae cells in order to incorporate rapidly the reliable photodynamic agents thiopyronine, protoporphyrin, zinc phthalocyanine, copper phthalocyanine sulfonate, adriamycin and daunomysin, well-tried cytostatic agents. Depending on field strength and pulse width, 50–90% of cells become electroporated, then the dye diffuses rapidly into the cells, which reseal their membranes over a period of 6–10 min. Illumination for 10–15 min destroys all resealed cells faster than the same amount of unporated cells as in the case of the control (without pulse treatment) either by oxidation of cell components caused by excited dyes or singlet oxygen treatment. By this synergism of electroporation and photodynamic action at the same time, it is possible to kill all cells in a much shorter time than under usual conditions, e.g. in the control suspension. A combination of electrochemotherapeutic needle electrodes with a light conductor for a LASER connection will be effective for therapy.

Apoptosis-inducing activity of galloyl monosaccharides in human histiocytic lymphoma U937 cells.

Three galloyl monosaccharides contained in medicinal plants were examined for apoptosis-inducing activity in human histiocytic lymphoma U937 cells. Tetragalloyl glucose (TgG) induced apoptosis as found by chromatin condensation, DNA ladder formation, and inhibition by a caspase inhibitor. Digalloyl hamamelose had moderate activity, while monogalloyl glucose was only marginally active. These findings suggest that the number and disposition of their phenolic groups are important for apoptosis induction. TgG induced apoptosis in human colon and stomach cancer cell lines as well, indicating it is potentially useful as an anti-cancer agent.

Natural Splicing of Exon 2 of Human Interleukin-15 Receptor α-Chain mRNA Results in a Shortened Form with a Distinct Pattern of Expression

We report the existence of eight different interleukin-15 receptor alpha-chain (IL-15Ralpha) transcripts resulting from exon-splicing mechanisms within the IL-15Ralpha gene. Two main classes of transcripts can be distinguished that do or do not (Delta2 isoforms) contain the exon 2-coding sequence. Both classes were expressed in numerous cell lines and tissues (including peripheral blood lymphocytes) at comparable levels and could be transcribed in COS-7 cells, and the proteins were expressed at the cell surface. Both receptor forms displayed numerous glycosylation states, reflecting differential usage of a single N-glycosylation site as well as extensive O-glycosylations. Whereas IL-15Ralpha bound IL-15 with high affinity, Delta2IL-15Ralpha was unable to bind IL-15, thus revealing the indispensable role of the exon 2-encoded domain in cytokine binding. A large proportion of IL-15Ralpha was expressed at the nuclear membrane with some intranuclear localization, supporting a potential direct action of the IL-15.IL-15Ralpha complex at the nuclear level. In sharp contrast, Delta2IL-15Ralpha was found only in the non-nuclear membrane compartments, indicating that the exon 2-encoded domain (which is shown to contain a potential nuclear localization signal) plays an important role in receptor post-translational routing. Together, our data indicate that exon 2 splicing of human IL-15Ralpha is a natural process that might play regulatory roles at different levels.

Mass spectrometric analysis of arachidonyl-containing phospholipids in human U937 cells.

The human histiocytic lymphoma U937 cell line contains a rich source of the 85 kDa cytosolic phospholipase A2 (cPLA2). DMSO-differentiated U937 cells were used as a model to investigate the free arachidonic acid release, the arachidonate distribution and the phospholipid source of arachidonate upon Ca2+ ionophore stimulation. A combination of several chromatographic and mass spectrometric techniques was employed in this study. The amount of free arachidonic acid (AA) released upon stimulation, the arachidonate content in total lipids and in each of the phospholipid classes were determined by gas chromatography/mass spectrometry (GC/MS). Glycerophosphoethanolamine (GPE) was found to be the major pool of arachidonate in differentiated human U937 cells (55%) and glycerophosphocholine (GPC) and glycerophosphoinositol (GPI) contributed 22 and 8%, respectively. Upon Ca2+ ionophore stimulation, GPE class lost the largest amount of arachidonate, followed by GPC class. GPI class, however, gained a substantial amount of arachidonate. Most of the arachidonate depleted from GPE and GPC was recovered as free AA, some of which was rapidly esterified into GPI species. GC/MS with electron capture negative chemical ionization provided excellent sensitivity for the measurement of arachidonic acid which was derivatized to its pentafluorobenzyl ester. Intact phospholipid molecular species including the arachidonyl-containing phospholipid species were identified using capillary high-performance liquid chromatography/continuous-flow liquid secondary ion mass spectrometry (CF-LSIMS). No specificity was found for releasing free AA among the arachidonyl-containing GPE and GPC species upon Ca2+ ionophore stimulation. CF-LSIMS provided a sensitive and effective means of detecting intact phospholipid species.

Lipopolysaccharide Inhibits TNF-Induced Apoptosis: Role of Nuclear Factor-κB Activation and Reactive Oxygen Intermediates

AbstractLPS, a component of the cell wall in Gram-negative bacteria, induces inflammation and septic shock syndrome by stimulating various inflammatory cytokines including TNF. How LPS affects the TNF-mediated cellular responses, however, is not understood. In this study, the effect of LPS on TNF-mediated apoptosis in human histiocytic lymphoma U-937 cells was investigated. We found that treatment of cells with LPS completely abolished TNF-mediated cytotoxicity and activation of caspase-3. LPS-chelating antibiotic, polymyxin B, suppressed the antiapoptotic activity, indicating the specificity of the effect. Within minutes, LPS through CD14 induced the activation of NF-κB, degradation of IκBα (inhibitory subunit of NF-κB) and IκBβ, and nuclear translocation of p65. An antioxidant, pyrrolidine dithiocarbamate, which blocked LPS-induced NF-κB activation, also abolished the antiapoptotic effects of LPS at the same time. Besides TNF, the apoptosis induced by taxol and okadaic acid was also sensitive to LPS-induced NF-κB activation, whereas that induced by H2O2, doxorubicin, daunomycin, vincristine, and vinblastine was NF-κB insensitive. Tumor cells that constitutively expressed NF-κB also showed resistance to the apoptotic effects of TNF, taxol, and okadaic acid, but sensitivity to all other agents, indicating the critical role of NF-κB in blocking apoptosis induced by certain agents. Overall, these results indicate that LPS induces resistance to the apoptotic effects of TNF and other agents, and that NF-κB activation, whether induced or constitutive, inhibits this apoptosis.

Interleukin-4 Down-regulates Both Forms of Tumor Necrosis Factor Receptor and Receptor-mediated Apoptosis, NF-κB, AP-1, and c-Jun N-Terminal Kinase: COMPARISON WITH INTERLEUKIN-13

The activity of tumor necrosis factor (TNF), a proinflammatory cytokine, is regulated by a number of other cytokines, including interleukin (IL)-4. How IL-4 regulates various activities of TNF is not fully understood. In the present report, we investigated the effect of IL-4 on the cell surface TNF receptors in human histiocytic lymphoma U-937 cells. Pretreatment of cells with IL-4 down-regulated TNF receptors in a dose- and time-dependent manner; an almost 90% decrease occurred with 10 ng/ml IL-4 treatment for 24 h. Scatchard analysis revealed that the decrease was due to receptor number and not affinity. IL-13, which shares a common receptor subunit and various biological activities with IL-4, had no effect on TNF receptors. IL-4's effect on TNF receptors was not cell type-specific, since decreases also occurred on various epithelial and T cells. Both the p60 and p80 forms of the TNF receptor were down-regulated to the same extent. Western blot showed that IL-4 induced shedding of the TNF receptors. The decrease of TNF receptors by IL-4 was accompanied by down-regulation of TNF-induced activities, including cytotoxicity, caspase-3 activation, NF-kappaB and AP-1 activation, and c-Jun N-terminal kinase induction. Wortmannin reversed the IL-4-induced TNF receptor down-regulation and all other measured cellular responses, indicating a critical role of phosphatidylinositol 3-kinase. Rapamycin also blocked the effect of IL-4-induced regulation, thus suggesting the role of p70 S6 kinase. Overall, our results suggest that TNF receptor down-regulation by IL-4 plays a critical role in the antagonistic effects of IL-4 on TNF-induced cellular responses and that this mechanism differs from that of IL-13.

Bactericidal activities of the pyrrole derivative BM212 against multidrug-resistant and intramacrophagic Mycobacterium tuberculosis strains.

The pyrrole derivative BM212 [1, 5-diaryl-2-methyl-3-(4-methylpiperazin-1-yl)methyl-pyrrole] was shown to possess strong inhibitory activity against both Mycobacterium tuberculosis and some nontuberculosis mycobacteria. BM212 was inhibitory to drug-resistant mycobacteria and also exerted bactericidal activity against intracellular bacilli residing in the U937 human histiocytic lymphoma cell line.

Tetracycline derivatives induce apoptosis selectively in cultured monocytes and macrophages but not in mesenchymal cells.

Evidence for a non-antibiotic activity displayed by certain tetracycline derivatives is presented. This activity is a selective cytotoxicity toward cells of the monocytic lineage (the human histiocytic lymphoma U937 cell line and the mouse macrophage line RAW264) but not toward various cells of a mesenchymal lineage (including primary ovine articular chondrocytes and meniscal cells, murine calvarial osteoblasts and MG-63 osteosarcoma cells, and primary human neonatal foreskin fibroblasts). Cells were incubated with various chemically modified tetracycline derivatives (CMTs) or doxycycline for 24 hrs at a range of concentrations between zero and 50 micrograms/mL in both serum-containing and serum-free culture conditions. Assessment of cell viability by means of the MTT assay demonstrated a potent dose-dependent cytotoxic effect induced by compound CMT-3 and a less potent effect induced by doxycycline, but no apparent cytotoxic effect in the presence of either CMT-2 or CMT-5. Cytospin preparations analyzed by the labeling of DNA fragments indicated the same trends and suggested that cell death was via an apoptotic mechanism. The cytotoxic potency of these tetracyclines toward cells of the monocytic lineage could be diminished but not abolished by either the presence of 10% fetal calf serum within the culture medium, or pre-treatment with phorbol esters to promote a more macrophage-like phenotype. These data provide evidence that, in addition to well-characterized antibiotic and MMP-inhibitory characteristics, tetracyclines may function by a novel mechanism to induce selective apoptosis.

Acquisition of cellular resistance to 9-nitro-camptothecin correlates with suppression of transcription factor NF-kappa B activation and potentiation of cytotoxicity by tumor necrosis factor in human histiocytic lymphoma U-937 cells.

Resistance of tumor cells to chemotherapeutic agents is a major problem in cancer therapy. Continuous exposure of human histiocytic lymphoma U-937 cells to 9-nitro-camptothecin (9NC), an inhibitor of the nuclear DNA topoisomerase I, induces resistance to this drug. Because of the involvement of the nuclear factor NF-kappa B in the expression of several growth regulatory genes, we examined the activation of this transcription factor in 9NC-resistant U-937 cells. We found that resistance to increasing concentrations of 9NC correlated with resistance to tumor necrosis factor (TNF)-dependent activation of NF-kappa B. However, the constitutive synthesis of NF-kappa B proteins remained unaffected. Cellular resistance was not unique to TNF, as other activators of NF-kappa B, including interleukin-1, phorbol ester and hydrogen peroxide, also had no effect. There was no difference between 9NC-sensitive and -resistant cells in the activation of NF-kappa B by okadaic acid. Other transcription factors, including AP-1 and Oct-1, were not affected in the resistant cells. When examined for the inhibitory subunit of NF-kappa B (I kappa B alpha), resistant cells showed a faster rate of resynthesis than the control. Interestingly, although 9NC resistance correlated with resistance to TNF-dependent NF-kappa B activation, TNF-dependent cytotoxicity in these cells was enhanced by several hundred fold despite a significant decrease in the number of TNF receptors. In conclusion, our results suggest that NF-kappa B activation may play a role in tumor cell killing by 9NC but not by TNF.

G1 arrest of U937 cells by onconase is associated with suppression of cyclin D3 expression, induction of p16INK4A, p21WAF1/CIP1 and p27KIP and decreased pRb phosphorylation.

Onconase is a 12 kDa protein homologous to pancreatic RNase A isolated from amphibian oocytes which shows cytostatic and cytotoxic activity in vitro, inhibits growth of tumors in mice and is in phase III clinical trials. The present study was aimed to reveal mechanisms by which onconase perturbs the cell cycle progression. Human histiocytic lymphoma U937 cells were treated with onconase and expression of cyclins D3 and E, as well as of the cyclin-dependent kinase inhibitors (CKIs) p16INK4A, p21WAF1/CIP1 and p27KIP1 (all detected immunocytochemically) was measured by multiparameter flow cytometry, in relation to the cell cycle position. Also monitored was the status of phosphorylation of retinoblastoma protein (pRb) by a novel method utilizing mAb which specifically detects underphosphorylated pRb in individual cells. Cell incubation with 170 nM onconase for 24 h and longer led to their arrest in G1 which was accompanied by a decrease in expression of cyclin D3, no change in cyclin E, and enhanced expression of all three CKIs. pRb was underphosphorylated in the onconase arrested G1 cells but was phosphorylated in the cells that were still progressing through S and G2/M in the presence of onconase. The cytostatic effect of onconase thus appears to be mediated by downregulation of cyclin D3 combined with upregulation of p27KIP1, p16INK4A and p21WAF1/CIP1, the events which may prevent phosphorylation of pRb during G0/1 and result in cell arrest at the restriction point controlled by Cdk4/6 and D type cyclins.

HIV-Tat Protein Activates c-Jun N-Terminal Kinase and Activator Protein-1

Human immunodeficiency virus-1 tat (HIV-tat) protein, like other proinflammatory cytokines (such as TNF), activates a wide variety of cellular responses, some of which play a critical role in progression of HIV infection. Whether HIV-tat, like TNF, also activates c-Jun N-terminal kinase (JNK) and the transcription factor activator protein (AP)-1 is not known. We show that treatment of human histiocytic lymphoma U937 cells with the HIV-tat protein causes activation of JNK and AP-1 in a time- and dose-dependent manner. Transfection of a T cell line, H9 cells with the HIV-tat gene also resulted in an activation of JNK that was not further increased by treatment of cells with exogenous HIV-tat protein. Neutralizing Ab against HIV-tat inhibited the HIV-tat-mediated JNK activation. The activation of JNK by HIV-tat appears to be mediated through generation of free radical species, since pretreatment of cells with N-acetylcysteine (NAC) abolished the effect. Overall our results demonstrate that HIV-tat activates JNK and AP-1, which may contribute to the pathogenesis of AIDS.

Cytotoxic anionic tribromo platinum(II) complexes containing benzothiazole and benzoxazole donors: synthesis, characterization, and structure-activity correlation

A series of platinum(II) complexes of the type [NEt 4][PtLBr 3] where L is a 2-styrylbenzothiazole or 2-styrlbenzole was prepared. The complexes were characterized by their melting points, elemental analyses, UV-Vis and far-infrared spectra, and 1H and 195Pt NMR spectroscopy. The NMR data are consistent with coordination of the benzoheterazole ligands to the Pt ion through the nitrogen atom. The ligands and the complexes were assayed for cytotoxicity against U937 human histiocytic lymphoma cells. It was observed that the ligands displayed very poor cytotoxicity, while the Pt(II) complexes demonstrated marked cytotoxicities and a structure-activity correlation was established. Pt(II) complexes containing S as the heteroatom in the benzoheterazole ring were generally more active than their O-containing counterpart. The position and type of substituents in the styryl moeity were also important for biological activity. Our data indicate that adjacent methoxy functionalities were present in the most potent drugs tested. Further development of active Pt(II) complexes with improved solubilities is warranted.