Biomedical subjects

T R Tritton

Publications and source records attributed to T R Tritton.

At least 19 recordsLinked to original sources

PubMed2001

Cell surface-directed interaction of anthracyclines leads to cytotoxicity and nuclear factor kappaB activation but not apoptosis signaling.

Anthracyclines are, above all, DNA intercalators, which induce genetic damage leading to cell death. However, increasing evidence firmly suggests that the underlying mechanism for anthracycline cytotoxicity is the induction of apoptosis through intracellular-mediated signaling pathways. Whether drug/DNA interaction is necessary for such apoptosis signaling is unknown. We investigated the cellular effects of the anthracyclines daunorubicin (DNR) and doxorubicin (DOX) using the myeloid leukemia cell line U937. By comparing free drug against agarose bead-immobilized drug iDNR and iDOX (which cannot accumulate within the cell), we observed that whereas both free and immobilized anthracyclines were cytotoxic, only the former induced apoptosis; the latter induced necrosis. Indeed, we did not observe ceramide generation, neutral sphingomyelinase activation, poly (ADP-ribose) polymerase cleavage, or other apoptotic events with iDNR or iDOX. However, both free and immobilized drug were similarly capable of triggering nuclear factor kappaB activation. These observations demonstrate that whereas activation of certain cellular signaling pathways can be achieved solely through membrane interaction, apoptosis signaling requires anthracycline internalization. These results also show that the initiation of cell survival pathways (illustrated by nuclear factor kappaB activation) is independent of intracellular drug/target interaction.

Antibiotics, Antineoplastic↗

PubMed2001

Integrated learning. Passing fad or foundation for the future?

On the surface, higher education has served the nation well. An increasing percentage of adults have attended college. Information is widely and freely available to most citizens; the economy is steady; and technological developments appear to offer ever-increasing improvements in the quality of life. Looking just beneath the surface, though, one realizes that knowledge is fragmented, access to knowledge is not universal, and there are no guarantees that past successes will sustain the future. Thus, it is worth examining how higher learning has contributed to the general prosperity and how education might evolve to address society's evident problems. This essay discusses the core approach of liberal arts in the context of the value--and obstacles--of integration across the traditional disciplines. Critical to the natural sciences is a firm grounding in the central importance of rigorous evidence, while the humanities keep us rooted in the importance of human values. Seeking linkages and connections between these realms is the lively challenge of this conference. The details of curricular design will be of less interest at this juncture than locating the critical issues and discussing how education might serve the goal of unifying knowledge and learning.

Education↗

PubMed1999

Asbestos-induced phosphorylation of epidermal growth factor receptor is linked to c-fos and apoptosis.

We examined the mechanisms of interaction of crocidolite asbestos fibers with the epidermal growth factor (EGF) receptor (EGFR) and the role of the EGFR-extracellular signal-regulated kinase (ERK) signaling pathway in early-response protooncogene (c-fos/c-jun) expression and apoptosis induced by asbestos in rat pleural mesothelial (RPM) cells. Asbestos fibers, but not the nonfibrous analog riebeckite, abolished binding of EGF to the EGFR. This was not due to a direct interaction of fibers with ligand, inasmuch as binding studies using fibers and EGF in the absence of membranes showed that EGF did not adsorb to the surface of asbestos fibers. Exposure of RPM cells to asbestos caused a greater than twofold increase in steady-state message and protein levels of EGFR (P < 0.05). The tyrphostin AG-1478, which inhibits the tyrosine kinase activity of the EGFR, but not the tyrphostin A-10, which does not affect EGFR activity, significantly ameliorated asbestos-induced increases in mRNA levels of c-fos but not of c-jun. Pretreatment of RPM cells with AG-1478 significantly reduced apoptosis in cells exposed to asbestos. Our findings suggest that asbestos-induced binding to EGFR initiates signaling pathways responsible for increased expression of the protooncogene c-fos and the development of apoptosis. The ability to block asbestos-induced elevations in c-fos mRNA levels and apoptosis by small-molecule inhibitors of EGFR phosphorylation may have therapeutic implications in asbestos-related diseases.

Animals↗

PubMed1999

Collateral methotrexate resistance in cisplatin-selected murine leukemia cells.

Resistance to anticancer drugs is a major cause of failure of many therapeutic protocols. A variety of mechanisms have been proposed to explain this phenomenon. The exact mechanism depends upon the drug of interest as well as the tumor type treated. While studying a cell line selected for its resistance to cisplatin we noted that the cells expressed a > 25,000-fold collateral resistance to methotrexate. Given the magnitude of this resistance we elected to investigate this intriguing collateral resistance. From a series of investigations we have identified an alteration in a membrane protein of the resistant cell as compared to the sensitive cells that could be the primary mechanism of resistance. Our studies reviewed here indicate decreased tyrosine phosphorylation of a protein (molecular mass = 66) in the resistant cells, which results in little or no transfer of methotrexate from the medium into the cell. Since this is a relatively novel function for tyrosine phosphorylation, this information may provide insight into possible pharmacological approaches to modify therapeutic regimens by analyzing the status of this protein in tumor samples for a better survival of the cancer patients.

Animals↗

PubMed1998

Drug resistance results in alterations in expression of immune recognition molecules and failure to express Fas (CD95).

It is demonstrated that methotrexate/cisplatin-sensitive L1210 cells express low levels of major histocompatibility complex (MHC) class II relative to the high levels expressed on methotrexate (MTX)/cisplatin-resistant L1210/DDP cells. L1210 cells express cell-surface Fas, while the L1210/DDP cells express no cell-surface Fas. Expression of costimulatory molecules B7-1/B7-2 and Fas is increased on L1210 cells, but not L1210/DDP, in the presence of methotrexate or trimetrexate (TMTX). Therefore, a component of the mechanism of action of some anti-cancer agents may be to facilitate immune recognition and T cell-directed, Fas-induced cell death. Loss of cell-surface Fas expression and failure of Fas (CD95)-dependent apoptotic death has been observed when cells develop drug resistance. The defect in apoptosis can be overcome by anti-cancer agents or experimental manipulation that induce Fas expression on the drug-resistant cells.

Animals↗

PubMed1997

Inhibition of epidermal growth factor receptor-associated tyrosine kinase blocks glioblastoma invasion of the brain.

OBJECTIVE: Glioblastoma multiforme is a malignant primary brain tumor associated with short patient survival despite aggressive treatment, in part because of its propensity to aggressively infiltrate into brain tissue. Glioblastoma multiforme is also unique because it is the only nonepithelial human tumor for which excessive activation of epidermal growth factor receptor (EGFR) has been consistently linked to tumor growth and patient survival, and EGFR activation promotes glioblastoma multiforme infiltration in vitro. METHODS: Cocultures of human glioblastoma spheroids (derived from three separate patients) and fetal rat brain aggregates were examined for infiltration using confocal microscopy, in the presence of 0 to 100 mumol/L genistein, a tyrosine kinase (TK) inhibitor, and 3 mumol/L tyrphostin A25, a specific EGFR-TK inhibitor. RESULTS: Infiltration (not attachment) was completely inhibited by genistein at 10 mumol/L, the IC20 for monolayer growth inhibition in two cell lines. Tyrphostin A25 at 3 mumol/L (the IC20 for monolayers) reduced invasion in a third cell line from 38.8 +/- 6.1% invasion-hour per hour (n = 5) to 2.9 +/- 1.2% invasion-hour per hour (n = 6) (P = 0.0002, two-tailed t test, 93% inhibition), and from 0.54 +/- 0.065% per hour (slope) to 0.028 +/- 0.018% per hour (P = 0.00001, 95% inhibition). Maximal percent invasion was reduced from 100 +/- 0 to 7.4 +/- 5.6% of the fetal rat brain aggregate. No change was detected in EGFR-associated tyrosine phosphorylation at those doses in monolayers by 32P immunolabeling, consistent with the known effects of low concentrations of TK inhibitors. An increase in expression of wild-type and truncated EGFR was demonstrated by Western blotting. Invasion was equally well inhibited by a monoclonal antibody to the high-affinity ligand binding domain of EGFR and not by antibody to an inactive domain. CONCLUSION: Our observations support the role of EGFR activation as a determinant by which glioblastoma invades normal brain tissue, and we show that invasion can be effectively inhibited at much lower concentrations of TK inhibitors than are necessary for growth suppression.

Animals↗

PubMed1996

Asbestos causes stimulation of the extracellular signal-regulated kinase 1 mitogen-activated protein kinase cascade after phosphorylation of the epidermal growth factor receptor.

Asbestos fibers are human carcinogens with undefined mechanisms of action. In studies here, we examined signal transduction events induced by asbestos in target cells of mesothelioma and potential cell surface origins for these cascades. Asbestos fibers, but not their nonfibrous analogues, induced protracted phosphorylation of the mitogen-activated protein (MAP) kinases and extracellular signal-regulated kinases (ERK) 1 and 2, and increased kinase activity of ERK2. ERK1 and ERK2 phosphorylation and activity were initiated by addition of exogenous epidermal growth factor (EGF) and transforming growth factor-alpha, but not by isoforms of platelet-derived growth factor or insulin-like growth factor-1 in mesothelial cells. MAP kinase activation by asbestos was attenuated by suramin, which inhibits growth factor receptor interactions, or tyrphostin AG 1478, a specific inhibitor of EGF receptor tyrosine kinase activity (IC50 = 3 nM). Moreover, asbestos caused autophosphorylation of the EGF receptor, an event triggering the ERK cascade. These studies are the first to establish that a MAP kinase signal transduction pathway is initiated after phosphorylation of a peptide growth factor receptor following exposure to asbestos fibers.

Animals↗

PubMed1996

Expression of c-fos precedes MDR3 in vincristine and adriamycin selected multidrug resistant murine erythroleukemia cells.

Expression of murine P-glycoprotein (P-gp), encoded by mdrl or mdri3, confers a multidrug resistance phenotype. Higher expression of c-fos and c-jun has also been demonstrated in multidrug resistant human and murine cells. We detected increased expression of c-fos early in the derivation of two series of murine erythroleukemia sublines selected for resistance to vincristine or adriamycin which eventually overexpress mdr3. We speculate that early expression of c-fos prepares cells for overexpression of other genes, such as mdr3, that contribute to the multidrug resistance phenotype.

ATP Binding Cassette Transporter, Subfamily B↗

PubMed1996

Correlation of altered tyrosine phosphorylation with methotrexate resistance in a cisplatin-resistant subline of L1210 cells.

Collateral resistance to cisplatin and methotrexate has been reported in several cell lines. A murine leukemia cell line (L1210/DDP) selected for cisplatin resistance also has been shown to be highly resistant to methotrexate. Of the mechanisms proposed for methotrexate resistance, only changes in methotrexate transport into the cells were found in an earlier report. Methotrexate enters mammalian cells via an active transport system. In the present study, we demonstrated that the transport into the cell may be impaired in the resistant cells due to altered tyrosine phosphorylation of a membrane protein with a molecular mass of 66 kDa. This alteration was manifested by altered tyrosine phosphorylation of the 66 kDa protein and may be an underlying modification that renders the cells resistant to methotrexate. These results suggest involvement of tyrosine phosphorylation in folate transport and methotrexate resistant in L1210/DDP cells.

Affinity Labels↗

PubMed1996

Tamoxifen induces TGF-beta 1 activity and apoptosis of human MCF-7 breast cancer cells in vitro.

We report here that the antiestrogen tamoxifen (TAM) induces cell death in human breast cancer cell line MCF-7. We assessed the type of cell death induced by TAM in this breast cancer cell line on the basis of morphological and biochemical characteristics. Dying cells showed morphological characteristics of apoptosis, such as chromatin condensation and nuclear disintegration. DNA isolated from these cells revealed a pattern of distinctive DNA bands on agarose gel. The DNA fragmentation in MCF-7 cells induced by TAM could also be detected by terminal deoxynucleotidyl transferase-mediated dUTP-biotin end labeling. Northern blot hybridization revealed a substantial increase in the amounts of TRPM-2 and TGF-beta 1 mRNAs in MCF-7 cells after treatment with TAM. In contrast, the mRNA level of the estrogen-induced pS2 gene was strongly suppressed. The biological activity of TGF-beta was increased at least fourfold in the media from MCF-7 cells treated with TAM. The results presented in this study suggest that TAM induces apoptosis of MCF-7 cells and it may be mediated by the secretion of active TGF-beta.

Apoptosis↗

PubMed1996

Investigations on the mechanisms of methotrexate resistance in a cisplatin-resistant L1210 murine leukemia cell subline.

We report a murine leukemia cell variant (L1210/DDP), selected for cisplatin (DDP) resistance, to be cross-resistant to methotrexate (MTX). Cross-resistance of L1210 cells to DDP and MTX has been observed by others, and has also been recorded in P388 murine leukemia and SSC-25 human squamous carcinoma cells. We demonstrated that MTX resistance is not due to dihydrofolate reductase (DHFR) gene amplification, increased DHFR enzyme activity or decreased MTX binding to the target enzyme. Of the mechanisms commonly proposed for MTX resistance, only differences in transport were observed when comparing sensitive (L1210/0) and resistant (L1210/DDP) cells. Our results suggest that MTX resistance in L1210/DDP cells is due to altered methotrexate uptake.

Animals↗

PubMed1993

Protection from adriamycin cytotoxicity in L1210 cells by brefeldin A.

We present studies which suggest that the cytotoxic action of Adriamycin (ADR) may involve intracellular pathways of vesicular transport. The movement of proteins or lipids from the endoplasmic reticulum to the plasma membrane via the Golgi organelle and associated compartments exhibits several temperature-sensitive steps between 15 degrees C and 20 degrees C. In this same temperature range, ADR loses its cytotoxic capacity. Using the inhibitor brefeldin A (BFA), we have investigated the possible role of intravesicular trafficking in the loss of ADR activity and the induction of protection from cytotoxicity at low temperature in L1210 cells. We show here that cells pretreated at 37 degrees C for 2 h with BFA could be protected from a subsequent exposure to ADR. The concentration causing 50% inhibition, determined by cloning in soft agar, was increased approximately 3.5 fold. L1210 cells could also be protected from the topoisomerase II inhibitors etoposide and amsacrine, but to a lesser extent; the concentration causing 50% inhibition for the latter inhibitors were increased 2-fold. Spectrofluorometric analysis of intracellular ADR accumulation revealed that there was no significant difference in the level of ADR in cells with or without BFA pretreatment. In addition, examination of ADR-induced cleavable complex formation by alkaline elution showed no significant difference in the level of DNA strand breaks in cells which had been pretreated with BFA even though there was a large difference in survival. Further examination of the persistence of DNA damage after a period of up to 6 h of repair revealed that cells which were pretreated with BFA removed DNA strand breaks at rates equivalent to those of cells which had received ADR directly. These results suggest that the protective effect induced by brefeldin A does not involve uptake, DNA damage, or repair but instead implicates protein or lipid interactions which may be independent of DNA damage and which may influence critical events that take place after the topoisomerase II-DNA complex has been formed.

Amsacrine↗

PubMed1993

Activity of two novel anthracene-9,10-diones against human leukemia cells containing intercalator-sensitive or -resistant forms of topoisomerase II.

We have examined the activities of two novel aza-anthracene-9,10-diones (aza), 1-aza and 2-aza, in HL-60 human leukemia cell lines containing type II topoisomerases with different sensitivities to inhibition by other intercalating agents. The sensitive line, HL-60, was sensitive to 2-aza but not to 1-aza, whereas the resistant HL-60/AMSA was sensitive to neither agent. Measurements of 1- and 2-aza-induced, topoisomerase II-mediated DNA cross-linking in the cells revealed patterns of resistance and sensitivity that paralleled the results in the cytotoxicity assays. However, measurements of drug-induced topoisomerase II-mediated DNA cross-linking using purified HL-60 and HL-60/AMSA topoisomerase II indicated that both agents could stabilize a covalent complex between DNA and the HL-60 enzyme. HL-60/AMSA topoisomerase II resisted stabilization by either agent. This suggests that the resistance of HL-60 cells to 1-aza is not due to the inability of this drug to inhibit topoisomerase II but rather to another, undefined mechanism.

Anthraquinones↗

PubMed1993

Anomalous expression of P-glycoprotein in highly drug-resistant human KB cells.

KB-A1 and KB-A10 are 2 multi-drug-resistant cell lines which are 100- and 1,000-fold resistant to Adriamycin, respectively. We have examined the expression of P-glycoprotein at the molecular and cellular levels in these human carcinoma cells. Both MDR cell lines, when compared to the parental KB-3-1, show characteristic increases in mdr 1 gene copy number, an increase in mdr 1 mRNA expression, a corresponding increase in transcription rate and a consequent over-expression of P-glycoprotein. However, the more highly resistant KB-A10 cells have a lower gene copy number, express less mdr 1 mRNA and contain less P-glycoprotein than the A1 cell line. To determine whether higher levels of cellular resistance were attributable to enhanced efficacy of P-glycoprotein or to other cellular regulatory mechanisms, we examined other major cellular properties known to be associated with the mdr phenotype. Both the KB-A1 and KB-A10 lines exhibit similar increases in protein kinase C activity as compared to the drug-sensitive parent. In addition, neither glutathione-S-transferase nor topoisomerase II activities account for enhanced resistance of the KB-A10 cells. The above observations are contrary to the premise that the level of drug resistance is necessarily proportional to expression of P-glycoprotein or to other common factors thought to participate in drug insensitivity; consequently, new mechanisms of resistance must be in operation in these cells.

ATP Binding Cassette Transporter, Subfamily B, Mem↗

PubMed1992

Adriamycin: protection from cell death by removal of extracellular drug.

Adriamycin is a cytotoxic drug which has enjoyed considerable success in the treatment of cancer. This agent has a bewildering variety of biological effects both within and on the surface of cells exposed to drug, and it has proved difficult to unambiguously assign a single mechanism of action. In this report we are able to separate intracellular and extracellular actions by taking advantage of the complete lack of Adriamycin-induced cytotoxicity at low temperature. For example, cells exposed to 100 microM Adriamycin at 0 degree C are not killed by the drug, even though this concentration is orders of magnitude higher than the concentration needed to cause 100% cell death at 37 degrees C. If cells exposed to 100 microM Adriamycin at 0 degree C are shifted to fresh drug-free medium at 37 degrees C, there is a time-dependent decrease in survival. However, if the drug-free medium contains calf thymus DNA (1.5 mg/ml) to act as a reservoir for Adriamycin binding of effluxed drug, there is no ensuing cytotoxicity. Thus, the results show that no matter how much drug is present inside the cell, there must also be extracellular drug available for membrane interaction in order to initiate nuclear DNA damage and the cytotoxic cascade.

Animals↗

PubMed1992

Expression of c-fos in human and murine multidrug-resistant cells.

In both mouse sarcoma 180 and human KB cells selected for the multiple drug resistance (MDR) phenotype, there is an elevation in the steady state mRNA level of c-fos. There is no detectable gene amplification for c-fos, nor is there any significant change in the rate of mRNA transcription or degradation, suggesting that other factors are responsible for the increased expression level in resistance. Cells selected for resistance to methotrexate, a drug not in the MDR group, do not have an increase in c-fos mRNA expression. When drug-sensitive cells are exposed for 30 min to an ED50 concentration of vinblastine, Adriamycin, colchicine, or VP-16, but not to methotrexate or cisplatin, there is a 3-6-fold induction in the level of c-fos message. Because the former drugs are members of the MDR class and the latter are not, the results are consistent with the hypothesis that induction of c-fos by low levels of cytotoxic drugs may be an early event in the acquisition of the MDR phenotype. If this were the case, then c-fos would be expected to act in concert with c-jun to control transcription by binding to a specific DNA regulatory site. Consistent with this explanation is the existence of an AP-1 sequence in the promotor region for the P-glycoprotein gene (mdr1), as well as the fact that c-jun is also overexpressed in MDR cells.

ATP Binding Cassette Transporter, Subfamily B, Mem↗

PubMed1991

Cell surface actions of adriamycin.

Adriamycin has a vast range of reported actions on the structural and functional properties of cells. This review summarizes the literature on the ability of the drug to modulate the cell surface membrane and attempts to address the question of how such actions could be linked to cytotoxicity. In addition, we consider the use of polymer immobilization of adriamycin to separate intracellular from plasma membrane effects of the drug, and show how this approach has been helpful in interpreting the pharmacology of adriamycin. Finally, a range of biophysical and spectroscopic approaches to defining the molecular details of adriamycin-bilayer interactions is surveyed, and the results used to discuss a model for how this antineoplastic agent binds to membranes.

Cell Membrane↗

PubMed1990

Transverse location of anthracyclines in lipid bilayers. Paramagnetic quenching studies.

Quenching of anthracycline fluorescence by a series of spin-labeled fatty acids was used to probe the transverse location of the drug in phosphatidylcholine bilayers in the form of small unilamellar vesicles. Stern-Volmer plots of the quenching data indicate that the fluorophore moiety of the anthracycline is intercalated into the hydrocarbon region of the bilayer, with deeper penetration observed in fluid-phase than in solid-phase vesicles. 31P-NMR parameters (T1 and nuclear Overhauser enhancement (NOE] are unaffected by the presence of drug, consistent with a binding site removed from the interfacial region. Comparison of intensity (F0/F) plots with lifetime (tau 0/tau) data shows that the predominant mechanism of anthracycline quenching by membrane-bound nitroxides is static. Since the membrane-bound drug is also accessible to quenching by I-, the binding site in the membrane must create a channel which is accessible to solvent. Two other fluorescent probes, 12-(9-anthroyloxy)stearate (12-AS) and diphenylhexatriene (DPH), were employed to confirm the results obtained with the anthracyclines, giving quenching data representative of their location in the bilayer.

Antibiotics, Antineoplastic↗