Transport of ions in erythrocytes infected by plasmodia

Throughout its erythrocytic cycle the plasmodial parasite modifies the plasma membrane of its host cell. Some changes derive from parasite metabolism. Intraerythrocytic forms use glucose at more than 10-fold normal red cell rates. The H+ accompanying the lactate end-product is exported into the host cell cytoplasm by an electrogenic proton pump in the parasite membrane. This maintains a pH greater than 7.0 in the parasite cytoplasm, but lowers erythrocyte cytoplasmic pH from approximately 7.2 to 6.5. Ca2+ transport across parasite membranes is coupled to the proton pump, possibly a Ca2+/H+ antiporter. The Ca2+, Mg2+-ATPase and Na+,K+-ATPase activities of erythrocyte membranes from schizont-infected erythrocytes have been studied. Under optimal assay conditions (pH = 7.0; [ATP] = 1 mM; +/- calmodulin) membranes from infected cells showed a 30% reduction in Ca2+,Mg2+-ATPase activity but no difference from normal in Na+,K+-ATPase activity. The calmodulin levels of infected cells were depressed by about 30%. The [ATP] in the cytoplasm of infected erythrocytes was only 0.2 mM (as against 1.3 mM in normals) and at this ATP concentration the activities of both ATPases are only 30% of normal. Shifting the pH from 7.0 to 6.5 decreases Na+,K+-ATPase activity by an additional 50% but is without effect on the Ca2+,Mg2+-ATPase. The results provide a partial explanation for the increased Ca2+ permeability and altered Na+/K+ content of plasmodia-infected erythrocytes.

Epidermal growth factor receptor as a genetic therapy target for carcinoma cell radiosensitization

BACKGROUND

Exposure of human cancer cells to ionizing radiation activates the epidermal growth factor receptor (EGFR), which, in turn, mediates a cytoprotective response that reduces the cells' sensitivity to ionizing radiation. Overexpression of a dominant-negative EGFR mutant, EGFR-CD533, disrupts the cytoprotective response by preventing radiation-induced activation of the receptor and its downstream effectors. To investigate whether gene therapy with EGFR-CD533 has the potential to increase tumor cell radiosensitivity, we introduced an adenoviral vector containing EGFR-CD533 into xenograft tumors in nude mice and evaluated the tumor response to ionizing radiation.

METHODS

Xenograft tumors established from the human mammary carcinoma cell line MDA-MB-231 were transduced via infusion with the adenoviral vector Ad-EGFR-CD533 or a control vector containing the beta-galactosidase gene, Ad-LacZ. The transduced tumors were then exposed to radiation in the therapeutic dose range, and radiation-induced EGFR activation was assessed by examining the tyrosine phosphorylation of immunoprecipitated EGFR. Radiosensitization was determined in vitro by colony-formation assays. All statistical tests were two-sided.

RESULTS

The transduction efficiency of MDA-MB-231 tumors by Ad-LacZ was 44%. Expression of EGFR-CD533 in tumors reduced radiation-induced EGFR activation by 2.94-fold (95% confidence interval [CI] = 2.23 to 4.14). The radiosensitivity of Ad-EGFR-CD533-transduced tumors was statistically significantly higher (46%; P<.001) than that of Ad-LacZ-transduced tumors, yielding a dose-enhancement ratio of 1.85 (95% CI = 1.54 to 2.51).

CONCLUSIONS

Transduction of MDA-MB-231 xenograft tumors with Ad-EGFR-CD533 conferred a dominant-negative EGFR phenotype and induced tumor radiosensitization. Therefore, disruption of EGFR function through overexpression of EGFR-CD533 may hold promise as a gene therapeutic approach to enhance the sensitivity of tumor cells to ionizing radiation.

Ionizing radiation-induced, mitochondria-dependent generation of reactive oxygen/nitrogen

Transient generation of reactive oxygen or nitrogen (ROS/RNS), detected with dihydrodichlorofluoroscein by fluorescence microscopy, occurs within minutes of exposing cells to ionizing radiation. In the 1-10 Gy dose range, the amount of ROS/RNS produced/cell is constant, but the percentage of producing cells increases with dose (20 to 80%). Reversible depolarization of the mitochondrial membrane potential () and decrease in fluorescence of a mitochondria-entrapped dye, calcein, are observed coincidentally. Radiation-induced ROS/RNS, depolarization, and calcein fluorescence decrease are inhibited by the mitochondrial permeability transition inhibitor, cyclosporin A, but not the structural analogue, cyclosporin H. Radiation-stimulated ROS/RNS is also inhibited by overexpressing the Ca(2+)-binding protein, calbindin 28K, or treating cells with an intracellular Ca(2+) chelator. Radiation-induced ROS/RNS is observed in several cell types with the exception of rho(o) cells deficient in mitochondrial electron transport. rho(o) cells show neither radiation-induced ROS/RNS production nor depolarization. We propose that radiation damage in a few mitochondria is transmitted via a reversible, Ca(2+)-dependent mitochondrial permeability transition to adjacent mitochondria with resulting enhanced ROS/RNS generation. Measurements of radiation-induced mitogen-activated protein kinase activity indicate that this sensing/amplification mechanism is necessary for activation of some cytoplasmic signaling pathways by low doses of radiation.

The relative role of ErbB1-4 receptor tyrosine kinases in radiation signal transduction responses of human carcinoma cells

Activation of the epidermal growth receptor (ErbB1) occurs within minutes of a radiation exposure. Immediate downstream consequences of this activation are currently indistinguishable from those obtained with growth factors (GF), e.g. stimulation of the pro-proliferative mitogen-activated protein kinase (MAPK). To identify potential differences, the effects of GFs and radiation on other members of the ErbB family have been compared in mammary carcinoma cell lines differing in their ErbB expression profiles. Treatment of cells with EGF (ErbB1-specific) or heregulin (ErbB4-specific) resulted in a hierarchic transactivations of ErbB2 and ErbB3 dependent on GF binding specificity. In contrast, radiation indiscriminately activated all ErbB species with the activation profile reflecting that cell's ErbB expression profile. Downstream consequences of these ErbB interactions were examined with MAPK after specifically inhibiting ErbB1 (or 4) with tyrphostin AG1478 or ErbB2 with tyrphostin AG825. MAPK activation by GFs or radiation was completely inhibited by AG1478 indicating total dependance on ErbB1 (or 4) depending on which ErbB is expressed. Inhibiting ErbB2 caused an enhanced MAPK response simulating an amplified ErbB1 (or 4) response. Thus ErbB2 is a modulator of ErbB1 (or 4) function leading to different MAPK response profiles to GF or radiation exposure.

Radiosensitization of malignant glioma cells through overexpression of dominant-negative epidermal growth factor receptor

The epidermal growth factor receptor (EGFR) plays an important role in neoplastic growth control of malignant gliomas. We have demonstrated that radiation activates EGFR Tyr-phosphorylation (EGFR Tyr-P) and the proliferation of surviving human carcinoma cells, a likely mechanism of accelerated cellular repopulation, a major cytoprotective response after radiation. We now investigate the importance of radiation-induced activation of EGFR on the radiosensitivity of the human malignant glioma cells U-87 MG and U-373 MG. The function of EGFR was inhibited through a genetic approach of transducing cells with an Adenovirus (Ad) vector containing dominant-negative (DN) EGFR-CD533 (Ad-EGFR-CD533) at efficiencies of 85-90%. The resulting cells are referred to as U-87-EGFR-CD533 and U-373-EGFR-CD533. After irradiation at 2 Gy, both of the cell lines exhibited a mean 3-fold increase in EGFR Tyr-P. The expression of EGFR-CD533 completely inhibited the radiation-induced activation of EGFR. In clonogenic survival assays after a single radiation exposure, the radiation dose for a survival of 37% (D37) for U-87-EGFR-CD533 cells was 1.4- to 1.5-fold lower, relative to cells transduced with AdLacZ or untransduced U-87 MG cells. This effect was amplified with repeated radiation exposures (3 x 2 Gy) yielding a D37 ratio of 1.8-2.0. In clonogenic survival studies with U-373 MG cells, the radiosensitizing effect of EGFR-CD533 was similar. Furthermore, in vivo studies with U-87 MG xenografts confirmed the effect of EGFR-CD533 on tumor radiosensitization (dose enhancement ratio, 1.8). We conclude that inhibition of EGFR function via Ad-mediated gene transfer of EGFR-CD533 results in significant radiosensitization. As underlying mechanism, we suggest the disruption of a major cytoprotective response involving EGFR and its downstream effectors, such as mitogen-activated protein kinase. The experiments demonstrate for the first time that radiosensitization of malignant glioma cells through disruption of EGFR function may be achieved by genetic therapy approaches.

Signal transduction and cellular radiation responses

Exposure of cells to ionizing radiation results in complex cellular responses resulting in cell death and altered proliferation states. The underlying cytotoxic, cytoprotective and cellular stress responses to radiation are mediated by existing signaling pathways, activation of which may be amplified by intrinsic cellular radical production systems. These signaling responses include the activation of plasma membrane receptors, the stimulation of cytoplasmic protein kinases, transcriptional activation, and altered cell cycle regulation. From the data presented, there is increasing evidence for the functional links between cellular signal transduction responses and DNA damage recognition and repair, cell survival, or cell death through apoptosis or reproductive mechanisms.

Ionizing radiation stimulates existing signal transduction pathways involving the activation of epidermal growth factor receptor and ERBB-3, and changes of intracellular calcium in A431 human squamous carcinoma cells

Previous studies demonstrated that ionizing radiation activates the epidermal growth factor receptor (EGFR), as measured by Tyr autophosphorylation, and induces transient increases in cytosolic free [Ca2+], [Ca2+]f. The mechanistic linkage between these events has been investigated in A431 squamous carcinoma cells with the EGFR Tyr kinase inhibitor, AG1478. EGFR autophosphorylation induced by radiation at doses of 0.5-5 Gy or EGF concentrations of 1-10 ng/ml is inhibited by >75% at 100 nM AG1478. Activation of EGFR enhances IP3 production as a result of phospholipase C (PLC) activation. At the doses used, radiation stimulates Tyr phosphorylation of both, PLCgamma and erbB-3, and also mediates the association between erbB-3 and PLCgamma not previously described. The increased erbB-3 Tyr phosphorylation is to a significant extent due to transactivation by EGFR as >70% of radiation- and EGF-induced erbB-3 Tyr phosphorylation is inhibited by AG 1478. The radiation-induced changes in [Ca2+]f are dependent upon EGFR, erbB-3 and PLCgamma activation since radiation stimulated IP3 formation and Ca2+ oscillations are inhibited by AG1478, the PLCgamma inhibitor U73122 or neutralizing antibody against an extracellular epitope of erbB-3. These results demonstrate that radiation induces qualitatively and quantitatively similar responses to EGF in stimulation of the plasma membrane-associated receptor Tyr kinases and immediate downstream effectors, such as PLCgamma and Ca2+.

Downregulation of delta CaM kinase II in human tumor cells

Over two dozen alternative splice variants of CaMK-II, the type II Ca(2+)/CaM-dependent protein kinase, are encoded from four genes (alpha, beta, gamma and delta) in mammalian cells. Isozymes of alpha and beta CaMK-II are well characterized in brain; however, an understanding of the relative endogenous levels of CaMK-II isozymes in a wide variety of non-neuronal cells has not yet been described. In this study, we have demonstrated that CaMK-II consists primarily of the 54 kDa delta CaMK-II (delta(2) or delta(C)) isozyme in rodent fibroblasts. beta and gamma CaMK-II isozymes are minor and alpha CaMK-II was not expressed. The primary delta CaMK-II in human fibroblasts and the MCF10A mammary epithelial cell line was the 52 kDa delta(4) CaMK-II, an isozyme identical to delta(2) except for a missing 21-amino-acid C-terminal tail. delta CaMK-II levels were diminished in both human and rodent fibroblasts after SV40 transformation and in the mammary adenocarcinoma MCF7 cell line when compared to MCF10A cells. In fact, most tumor cells exhibited CaMK-II specific activities which were two- to tenfold lower than in untransformed fibroblasts. We conducted complementary CaMK-II studies on the NGF-induced differentiation of rat PC-12 cells. Although no new synthesis of CaMK-II occurs, neurite outgrowth in these cells is accompanied by a preferential activation of delta CaMK-II. Endogenous delta CaMK-II has a perinuclear distribution in fibroblasts and extends along neurites in PC-12 cells. These findings point to a role for delta CaMK-II isozymes in cellular differentiation.

Dominant negative EGFR-CD533 and inhibition of MAPK modify JNK1 activation and enhance radiation toxicity of human mammary carcinoma cells

Exposure of MDA-MB-231 human mammary carcinoma cells to an ionizing radiation dose of 2 Gy results in immediate activation and Tyr phosphorylation of the epidermal growth factor receptor (EGFR). Doxycycline induced expression of a dominant negative EGFR-CD533 mutant, lacking the COOH-terminal 533 amino acids, in MDA-TR15-EGFR-CD533 cells was used to characterize intracellular signaling responses following irradiation. Within 10 min, radiation exposure caused an immediate, transient activation of mitogen activated protein kinase (MAPK) which was completely blocked by expression of EGFR-CD533. The same radiation treatment also induced an immediate activation of the c-Jun-NH2-terminal kinase 1 (JNK1) pathway that was followed by an extended rise in kinase activity after 30 min. Expression of EGFR-CD533 did not block the immediate JNK1 response but completely inhibited the later activation. Treatment of MDA-TR15-EGFR-CD533 cells with the MEK1/2 inhibitor, PD98059, resulted in approximately 70% inhibition of radiation-induced MAPK activity, and potentiated the radiation-induced increase of immediate JNK1 activation twofold. Inhibition of Ras farnesylation with a concomitant inhibition of Ras function completely blocked radiation-induced MAPK and JNK1 activation. Modulation of EGFR and MAPK functions also altered overall cellular responses of growth and apoptosis. Induction of EGFR-CD533 or treatment with PD98059 caused a 3-5-fold increase in radiation toxicity in a novel repeated radiation exposure growth assay by interfering with cell proliferation and potentiating apoptosis. In summary, this data demonstrates that both MAPK and JNK1 activation in response to radiation occur through EGFR-dependent and -independent mechanisms, and are mediated by signaling through Ras. Furthermore, we have demonstrated that radiation-induced activation of EGFR results in downstream activation of MAPK which may affect the radiosensitivity of carcinoma cells.

The inducible expression of dominant-negative epidermal growth factor receptor-CD533 results in radiosensitization of human mammary carcinoma cells

Ionizing radiation activates the epidermal growth factor receptor (EGFR) and downstream signaling involving the cytoprotective mitogen-activated protein kinase (MAPK) pathway. In our effort to investigate the role of EGFR in cellular responses to radiation, we generated mammary carcinoma cell clones, MCF-TR5-EGFR-CD533 and MDA-TR15-EGFR-CD533, that inducibly express EGFR-CD533, a truncated EGFR mutant lacking mitogenic and transformation activity. EGFR-CD533 expression inhibits radiation- and EGF-induced EGFR autophosphorylation and MAPK activation and, therefore, functions as a dominant-negative mutant without blocking the expression of EGFR or erbB-2, another member of the erbB receptor Tyr kinase family. Expression of EGFR-CD533 only minimally inhibited cell growth and did not alter radiosensitivity to single radiation exposures. However, repeated 2 Gy radiation exposures of cells, under conditions of EGFR-CD533 expression, essentially abolished their ability for subsequent cell growth. These results identify the inhibition of EGFR function through genetic manipulation as a potential therapeutic maneuver. The concept of such an intervention would be the radiosensitization of cells by counteracting a radiation-induced cytoprotective proliferation response.

Calcium-dependent stimulation of mitogen-activated protein kinase activity in A431 cells by low doses of ionizing radiation

Ionizing radiation at 2 Gy activates the epidermal growth factor receptor (EGFR) kinase activity in A431 squamous carcinoma cells and as a consequence transiently activates a downstream effector, mitogen-activated protein kinase (MAPK). A dose-response analysis shows fourfold activation 3-5 min after irradiation at 0.5 Gy with no additional activation after doses up to 4 Gy. Activation is independent of protein kinase C as defined by marginal effects of protein kinase C down-regulation and the protein kinase C inhibitor, chelerythrine. In contrast, an intracellular Ca2+ chelator (BAPTA/AM), a Ca2+ antagonist (TMB-8) and a phospholipase C inhibitor (U73223), which inhibits radiation-induced Ca2+ oscillations, all block MAPK stimulation. The upstream component, Raf-1, is also activated through a mechanism that is dependent on EGFR and Ca2+. Activation of Raf-1, monitored by tyrosine phosphorylation and co-immunoprecipitation with Ras, was inhibited by BAPTA/AM and TMB-8, indicating that the Ca2+-dependent step occurs at or before the interaction of Ras and Raf-1. Neither the Ras guanosine triphosphate exchange protein, SOS, nor Ca2+-activated tyrosine kinases linked to the MAPK pathway, focal adhesion kinase and PYK2, were stimulated by radiation. In contrast, EGF activated SOS as shown by the enhanced association of SOS with EGFR in co-immunoprecipitation experiments. These results suggest that activation of EGFR-dependent downstream signaling induced by radiation differs from that induced by the natural ligands of EGFR.

Radiation-induced proliferation of the human A431 squamous carcinoma cells is dependent on EGFR tyrosine phosphorylation

Accelerated cellular repopulation has been described as a response of tumors to fractionated irradiation in both normal tissue and tumor systems. To identify the mechanisms by which cells enhance their proliferative rate in response to clinically used doses of ionizing radiation (IR) we have studied human mammary and squamous carcinoma cells which are autocrine growth regulated by the epidermal growth factor receptor (EGFR) and its ligands, transforming growth factor-alpha and EGF. Both EGF and IR induced EGFR autophosphorylation, comparable levels of phospholipase C gamma activation as measured by inositol-1,4,5-triphosphate production, and as a consequence oscillations in cytosolic [Ca2+]. Activities of Raf-1 and mitogen-activated protein kinase (MAPK) were also stimulated by EGF and IR by Ca(2+)-dependent mechanisms. All these responses to EGF and IR were dependent upon activation of EGFR as judged by the use of the specific inhibitor of EGFR autophosphorylation, tyrphostin AG1478. Importantly, IR-induced proliferation of A431 cells was also inhibited by AG1478. This is the first report which demonstrates a link between IR-induced activation of proliferative signal transduction pathways and enhanced proliferation. We propose that accelerated repopulation of tumors whose growth is regulated by EGFR is initiated by an IR-induced EGFR activation mechanism that mimics the effects of growth factors.

Ionizing radiation induces a transient increase in cytosolic free [Ca2+] in human epithelial tumor cells

Free cytosolic [Ca2+] ([Ca2+]f) was measured at the single cell level using digitized video-intensified fluorescence microscopy and the fluorescent Ca2+ indicator, fura-2. Cells were irradiated at 1-4 Gy (dose rate of 6 Gy/min) with a 90Sr eye applicator mounted on the microscope. HeLa cells responding to ionizing radiation exhibited a maximal 3-4-fold transient rise in cytosolic and nuclear [Ca2+]f immediately upon irradiation, which persisted for at least 5 min before returning to baseline in most but not all cells. The radiation-induced rise in [Ca2+]f was blocked by 1 mM La3+, 100 nM nifedipine, or membrane potential depolarization, suggesting that HeLa cells possess a voltage-dependent Ca2+ channel that mediates the response to radiation. Experiments with Mn2+, a paramagnetic probe for extracellular Ca2+, showed that radiation stimulated an increase in Mn2+ influx, as witnessed by loss of fura-2 fluorescence. Thapsigargin and ryanodine, inhibitors of intracellular Ca2+ mobilization, also completely blocked the radiation effect, implying a linkage between the radiation-induced influx and mobilization of internal Ca2+ stores. Not all cells in an asynchronous culture responded identically to radiation. Upon synchronization with thymidine/aphidicolin, cells in the S- and G2/M-phase exhibited radiation-induced changes in [Ca2+]f, whereas G1-phase cells did not. In addition, the increased [Ca2+]f of irradiated G2/M phase cells did not fully return to pretreatment levels. Further studies utilizing MDA-MB-231 and MCF-7 human breast and HT-29 human colon cancer cell lines indicate that radiation can alter Ca2+ homeostasis in other epithelial cell types. In the case of MDA-MB-231 and HT-29 cells, oscillations in cytosolic [Ca2+]f levels were observed that persisted for up to 50 min. The kinetics and inhibitor sensitivities differed from HeLa cells, indicating a different type of mechanism for the radiation effects on cell [Ca2+]f. Survival studies with HeLa and MDA-MB-231 cells did not reveal a connection between the radiation effects on cellular Ca2+ homeostasis and cell survival.

A radiation-induced inhibitor of chromosome condensation and nuclear envelope breakdown in HeLa cells

An in vitro microscopic assay for mitosis-inducing activity in mitotic HeLa cells was developed and used to demonstrate that cells irradiated and arrested in G2 phase of the cell cycle contain an inhibitor of mitosis. This assay system has a number of advantages over other assays including the use of autologous components (HeLa nuclei and mitotic cell extracts) in contrast to the microinjection method with Xenopus oocytes and without the requirements for microinjection expertise and Xenopus oocytes. The radiation-inducible inhibitor was detected at the lowest radiation dose tested (2 Gy) with maximal activity achieved within 30 min after radiation. Inhibitor activity decayed with time after radiation (2 Gy) with no activity detected at 6 h even though the cells remained in G2 phase, suggesting that either synthesis or activation of additional components is necessary for recovery from G2 arrest. The inhibitor activity was not detected in irradiated cells treated with caffeine to induce premature recovery from G2 arrest.

Cell cycle-dependent biosynthesis of Plasmodium falciparum DNA polymerase-alpha

The DNA polymerase-alpha of Plasmodium falciparum was characterized according to aphidicolin sensitivity and immunological reactivity with monoclonal anti-sera against human DNA polymerase-alpha. Two major (105 and 72 kDa) and two minor (180 and 130 kDa) catalytic subunits of P. falciparum DNA polymerase-alpha were detected on activity gels. Activity gels did not indicate the presence of a DNA polymerase-beta in P. falciparum. Metabolically labeled polypeptides at 180, 105, 72, and 52 kDa were immunoprecipitated from Plasmodium nuclear extracts with the anti-KB cell DNA polymerase-alpha monoclonal antibody and, by size, correspond to the major subunits of mammalian DNA polymerase-alpha. The monoclonal antibody also neutralized Plasmodium DNA polymerase activity. Plasmodium DNA polymerase was synthesized predominantly at an early schizont stage at which time the parasite began to synthesize its DNA and multiply. No evidence for phosphorylation of the major catalytic subunit was obtained. Plasmodium growth, DNA synthesis, and DNA polymerase activity were inhibited significantly in parallel by aphidicolin. These results suggest that P. falciparum has a typical eukaryotic DNA polymerase-alpha and that regulation of its activity appears to be at the transcriptional level.

Comparison of adenylate cyclase and cAMP-dependent protein kinase in gametocytogenic and nongametocytogenic clones of Plasmodium falciparum

Adenylate cyclase and cAMP-dependent protein kinase activities in gametocytogenic (LE5) and nongametocytogenic (T9/96) clones of Plasmodium falciparum were compared to explore the role of cAMP in sexual differentiation of the parasite. Basal adenylate cyclase levels were equivalent in the 2 clones. However, cAMP-dependent histone II-A kinase activity was significantly higher in LE5 than in T9/96 over a range of cAMP concentrations. This difference was due to a decreased Vmax for the enzyme in the nongametocytogenic clone and not to an increased Ka for cAMP. Examination of parasite cAMP-binding proteins, likely to be kinase regulatory subunits, by both photoaffinity labeling with [32P]8-N3-cAMP and affinity chromatography of metabolically [35S]methionine-labeled cytosol of cAMP-agarose revealed a 53-kDa cAMP binding protein in both clones and a 49-kDa cAMP-binding protein in T9/96 that was absent in LE5. Our results suggest that T9/96 has lost the ability to undergo gametocytogenesis due to a substantial decrease in cAMP-dependent protein kinase activity rendering the parasite unable to respond to increased intracellular cAMP levels. Moreover, the reduction in cAMP-dependent protein kinase activity may be due to the presence of an alternative regulatory subunit of the kinase.

Effects of tamoxifen and protein kinase C inhibitors on hyperthermic cell killing

The effects of the anti-estrogen, tamoxifen, and the protein kinase C inhibitor, 1-(5-isoquinolinylsulfonyl)2-methylpiperazine (H7), on hyperthermic cytotoxicity were studied. Three cell lines were used, a human colon cancer cell line (HT-29), a human mammary carcinoma cell line (MCF-7), and Chinese hamster V79 lung fibroblasts. With all three cell lines, tamoxifen at concentrations greater than 7.5 microM during heating or with a 3-hr exposure prior to heating significantly sensitized cells to heat. When cells were preincubated with 10-20 microM tamoxifen for 1-2 hr at 37 degrees C prior to heat treatment, washed free of extracellular tamoxifen, heated to generate thermoresistance, and examined 18 hr later for thermoresistance, tamoxifen treated HT-29 and MCF-7 cells were significantly more heat sensitive than thermotolerant controls not previously exposed to tamoxifen. In contrast, the degree of induced thermoresistance of V79 cells was unchanged after tamoxifen treatment. H7, but not its structural analogue and low affinity protein kinase C inhibitor, HA1004, also sensitized cells to heat. Neither H7 nor HA1004 had any apparent effect on the degree of heat-induced thermoresistance in the three cell lines tested.

Plasmodium falciparum-infected erythrocytes contain an adenylate cyclase with properties which differ from those of the host enzyme

It has been postulated that differentiation of the human malaria parasite, Plasmodium falciparum, is controlled by cAMP levels. We have determined that P. falciparum synthesizes an adenylate cyclase with several properties distinct from those of the mammalian host cell enzyme. Adenylate cyclase activity was compared in P. falciparum-infected erythrocytes, isolated parasites free of host cell material, and uninfected erythrocyte membranes. The parasite enzyme was unaffected by GTP gamma S, AlF4-, and forskolin, while the erythrocyte enzyme was markedly stimulated by each of these compounds. The parasite adenylate cyclase also exhibited a striking preference for Mn2+ over Mg2+, which was not evident in the erythrocyte enzyme. Moreover, differing cation and pH sensitivities were observed for adenylate cyclase activity in the two cell types. When infected and uninfected erythrocytes were compared, the basal adenylate cyclase activity of infected cells was 7 and 49 times that measured in uninfected erythrocytes in the presence of Mg2+ and Mn2+, respectively. Furthermore, adenylate cyclase activity in infected cells exhibited properties typical of the parasite enzyme. This indicates that synthesis of the parasite enzyme rather than stimulation of the host enzyme accounts for the increased activity in infected cells.

Hyperthermia effects on cytosolic [Ca2+]: analysis at the single cell level by digitized imaging microscopy and cell survival

Digitized video-intensified fluorescence microscopy with the Ca2(+)-sensitive fluorescent dye fura-2 was used to measure cytosolic free Ca2+ [( Ca2+]f) in HT-29 human colon cancer cells. At 37 degrees C, the [Ca2+]f of individual cells ranged between 50 and 150 nM, with a mean of 120 nM. Raising the temperature to 41 degrees C for 1 h resulted in a slight reversible decrease (10-20%) in the mean [Ca2+]f. At 44 degrees C for 1 h, most (greater than 80%) cells exhibited a [Ca2+]f greater than 200 nM. This heat-induced rise in [Ca2+]f was not immediate but commenced after a lag time of 30 min. Postincubation at 37 degrees C for 2-6 h after heating, for 1 h at 44 degrees C resulted in a recovery of the basal [Ca2+]f in some but not all cells. A linear relationship was determined between percentage of cell killing and the number of cells with [Ca2+]f of greater than 200 nM after 37 degrees C post-heating incubation. Manipulation of extracellular [Ca2+] between 0.1 and 10 mM during heating did not modify the heat-induced changes in [Ca2+]f. No significant differences in survival at 37 degrees C or 44 degrees C were observed with cells incubated at 10, 1.0, and 0 [plus 1.0 mM ethylene glycol bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid] mM extracellular Ca2+. The Ca2+ channel blockers verapamil and nifedipine did not protect cells from heat treatment. These results suggest that irreversible heat-induced changes in intracellular Ca2+ homeostasis mechanisms may be a critical factor in heat cytotoxicity.

Plasmodium falciparum: ATP/ADP transport across the parasitophorous vacuolar and plasma membranes

Previous studies have shown that ATP is required for the growth of the intracellular parasite, Plasmodium, outside its host cell, the erythrocyte, and that bongkrekic acid, an inhibitor of mitochondrial ATP/ADP transporter, inhibits intraerythrocytic Plasmodium maturation. We have characterized ATP/ADP transport of Plasmodium falciparum, isolated by either immune lysis or N2-cavitation. [3H]ATP uptake was due to ATP/ADP exchange since ADP efflux was dependent on exogenous ATP in an approximate 1:1 stoichiometry and both ATP influx and ADP efflux were equally inhibited by atractyloside (Ki = 100 nM). ATP uptake was not inhibited by the nucleoside transport inhibitor, nitrobenzylthioinosine. Conversely, adenosine and hypoxanthine transport were insensitive to atractyloside. ATP influx was characterized by a Km = 0.14 mM and Vmax = 1.2 nmol ATP/min/10(6) cells. Substrate specificity studies for nucleotide-induced ADP efflux indicated a preference for an adenosine ring and triphosphate, but transport did not require a hydrolyzable phosphate bond. Protein synthesis was measured with free parasites starved of glucose. Addition of 1.0 mM ATP resulted in a 40% recovery of total protein synthetic capacity in a process inhibited by 500 nM atractyloside, suggesting that uptake of erythrocyte-derived ATP by P. falciparum may be essential for maintaining maximal rates of protein synthesis during specific stages of intra-erythrocytic parasite maturation.

Effects of hyperthermia on the membrane potential and Na+ transport of V79 fibroblasts

The effects of hyperthermia (41-43 degrees C) on the membrane potential (calculated from the transmembrane distribution of [3H]tetraphenylphosphonium) and Na+ transport of Chinese hamster V79 fibroblasts were studied. At 41 degrees C, hyperthermia induced a membrane hyperpolarization of log phase cells (5 to 26 mV) that was reversible upon returning to 37 degrees C. The hyperpolarization was inhibited 50% by 1 mM ouabain or 0.25 mM amiloride, an inhibitor of Na+:H+ exchange. Shifting temperature to 41 degrees C increased ouabain-sensitive Rb+ uptake indicating activation of the electrogenic Na+ pump. At 43 degrees C for 60 min, the membrane potential of log phase cells depolarized (20-35 mV). Parallel studies demonstrated enhanced Na+ uptake at 41 degrees C only in the presence of ouabain. At 43 degrees C, Na+ uptake was increased relative to controls with or without ouabain present. At both 41 and 43 degrees C, 0.25 mM amiloride inhibited heat-stimulated Na+ uptake. Na+ efflux was enhanced at 41 degrees C in a process inhibited by ouabain. Thus, one consequence of heat treatment at 41 degrees C is activation of Na+:H+ exchange with the resultant increase in cytosolic [Na+] activating the electrogenic Na+ pump. At temperatures greater than or equal to 43 degrees C, the Na+ pump is inhibited.

Cyclic AMP- and Ca2(+)-dependent protein kinases in Plasmodium falciparum

The cyclic AMP- and Ca2(+)-dependent protein kinase activities of Plasmodium falciparum were partially characterized after purification of parasites from host erythrocytes by N2 cavitation and Percoll gradient centrifugation. Proteins of molecular weights 80, 54, 51, and 31.5 kDa were phosphorylated in a cAMP-dependent manner in cytosolic extracts of isolated P. falciparum. Cytosolic extracts also contained cAMP-dependent histone II-A kinase activity with an average Vmax of 131.1 pmol/32P/min/mg protein and a Km for cAMP of 85nM. Upon photoaffinity labeling with [32P]-8-N3-cAMP, a 53-kDa protein was specifically labeled in parasite cytosol. A metabolically labeled protein of the same molecular weight was identified by cAMP-agarose affinity chromatography. The 53-kDa protein cochromatographed with cAMP-dependent histone II-A kinase activity on DEAE-cellulose, suggesting that it is the regulatory subunit of the kinase. Ca2(+)-dependent phosphorylation of proteins of molecular weights 195, 158, 51, 47.5, and 15 kDa was demonstrated in a membrane fraction from parasites free of the erythrocyte membrane. This activity was not stimulated by either calmodulin or phospholipid plus diacylglycerol and was absent from the membranes of uninfected erythrocytes. Of several exogenous substrates tested, none were found to be a substrate for this Ca2(+)-dependent kinase. Both cAMP- and Ca2(+)-dependent kinases phosphorylated serine and threonine residues.

Cytotoxic hyperthermia and Ca2+ homeostasis: the effect of heat on Ca2+ uptake by nonmitochondrial intracellular Ca2+ stores

The effect of cytotoxic hyperthermia on Ca2+ transport by intracellular, nonmitochondrial Ca2+ stores of the human colon cancer cell line, HT-29, was studied using cells permeabilized with saponin. Saponin treatment permitted equilibration of the cytosol with a defined extracellular medium consisting of an intracellular-like ionic composition, ATP and an ATP-regenerating system, and Ca2+/EGTA buffers to adjust the free [Ca2+]. Under the conditions employed, ATP-dependent Ca2+ uptake in saponin-permeabilized cells was demonstrated to be exclusively due to nonmitochondrial Ca2+ stores, e.g., endoplasmic reticulum or calciosomes. Heat treatment for 120 min at 44.5 degrees C sufficient to kill 80% of the cells inhibited ATP-dependent Ca2+ uptake by 50% in terms of rate and total Ca2+ accumulated. With cells made thermotolerant by either arsenite or heat treatment 24 h prior to challenge heating, ATP-dependent Ca2+ uptake was resistant to a second equivalent heat dose. Efflux of Ca2+ from saponin-permeabilized cells when measured at 37 degrees C was unaffected by a prior heat treatment (44.5 degrees C for 120 min).

The role of lipids in Plasmodium falciparum invasion of erythrocytes: a coordinated biochemical and microscopic analysis

The role of lipids in Plasmodium falciparum invasion of erythrocytes was investigated by biochemical and fluorescent microscopic analysis. Metabolic incorporation of radioactive oleate or palmitate and fractionation of radiolabeled phospholipids by thin-layer chromatography revealed no difference in the major phospholipid classes of schizonts and early ring forms after merozoite invasion. Fluorescent anthroyloxy derivatives of oleate and palmitate were also metabolically incorporated into parasite phospholipids. By microscopic analysis, the fluorescent phospholipids were seen localized in the plasma membrane and, within the merozoite, concentrated near the apical end. During invasion fluorescent phospholipid appeared to be injected from the apical end of the merozoite into the host membrane, both within and outside the parasite-host membrane junctions. After invasion fluorescent lipid was only found in the parasite plasma membrane and/or parasitophorous vacuole membrane. Parallel experiments with a fluorescent cholesterol derivative, incorporated into parasite membranes by exchange, revealed neither heterogeneous distribution of label within the parasite nor evidence for cholesterol transfer from merozoite to host cell membrane. Results suggest that during invasion no major covalent alteration of parasite lipids, such as lysophospholipid formation, occurs. However, invasion and formation of the parasitophorous vacuolar membrane apparently involves insertion of parasite phospholipids into the host membrane.

Synthesis of a somatostatin-like peptide by Plasmodium falciparum

The intracellular parasite, Plasmodium falciparum, was found to synthesize a peptide similar to mammalian somatostatin. High performance liquid chromatography of acetic acid extracts of Plasmodium-infected erythrocytes revealed a metabolically labeled peptide that co-eluted with rat somatostatin and that was reactive with antibody against rat somatostatin. Bioassay of partially purified Plasmodium peptide demonstrated somatostatin activity. Acetic acid extracts from non-synchronized infected cultures were shown by radioimmunoassay to contain the equivalent of 150 molecules of somatostatin per parasite. Somatostatin was not detectable in erythrocytes of non-infected cultures.

The role of pump number and intracellular sodium and potassium in determining Na,K pump activity in human erythrocytes

The factors that determine the activity of the Na,K pump in vivo were investigated by measuring Na,K pump activity under in vivo conditions in human red cells and relating it to the intracellular content of sodium ([Na]i) and potassium ([K]i) and the number of pump units per cell (pump number). Na,K pump activity was measured as ouabain-sensitive K+ influx, pump number was determined from the maximal binding of 3H-ouabain to intact cells, and [Na]i and [K]i were measured by atomic absorption spectrophotometry in washed, packed cells. In the 81 samples studied, pump activity per cell was significantly correlated with pump number (r = .64, P less than 0.001), but was negatively correlated with [Na]i (r = -.28, P less than 0.02) and was not correlated with [K]i. An inverse relationship was found between pump number and [Na]i. When pump activity was expressed as activity per pump unit, rather than per cell, a significant relationship was seen between pump activity and [Na]i (r = .50, P less than 0.001), and a negative correlation existed between the activity per pump unit and [K]i (r = -.29, P less than 0.01). The effect of intracellular Na+ at physiologic levels on pump activity was not strong, with the activity per pump unit increasing only 25% with a doubling of [Na]i. These results indicate that pump number is the major determinant of pump activity in human red cells in vivo, while [Na]i and [K]i are of secondary importance.(ABSTRACT TRUNCATED AT 250 WORDS)

Furosemide-sensitive Na+ and K+ transport and human erythrocyte volume

The relationship between cation transport and cell volume in human erythrocytes was investigated by measuring ouabain-sensitive K+ influx, ouabain-resistant, furosemide-sensitive K+ influx, and ouabain + furosemide-resistant K+ influx, and maximal ouabain binding in microcytic, normocytic and macrocytic red cells. A significant correlation was found between the mean corpuscular volume and furosemide-sensitive K+ influx normalized either to cell number (r = 0.636, P less than 0.001) or to cell volume (r = 0.488, P less than 0.001). No relationship was seen between mean corpuscular volume and ouabain-sensitive K+ influx, and the number of ouabain-binding sites per cell was only weakly correlated with mean corpuscular volume (r = 0.337, P less than 0.05). A slight, negative relationship existed between mean corpuscular volume and ouabain + furosemide-resistant K+ influx expressed per volume of cells (r = -0.359, P less than 0.01), and an apparent relationship between furosemide-sensitive K+ influx and mean corpuscular hemoglobin concentration (r = 0.446, P less than 0.01) disappeared when microcytic samples were excluded from analysis. Furosemide-sensitive transport, including Na+ influx and K+ and Na+ efflux, was completely absent in microcytic cells from one patient with alpha-thalassemia minor. In addition, these cells exhibited a furosemide-resistant, Cl(-)-dependent K+ influx. Exposure of normal erythrocytes to hypotonic conditions (196 mosM) increased furosemide-sensitive K+ influx by a mean of 45% (P less than 0.05), while exposure to hypertonic conditions (386 mosM) had no significant effect. The results indicate that furosemide-sensitive transport and cell volume are interrelated in human erythrocytes. However, the inability to fully recreate this relationship with in vitro manipulation of cell volume suggest that this relationship is established prior to red cell maturation.

Proliferative rates of cloned malignant mammary epithelial cells as a measure of clonal heterogeneity in human breast carcinomas

Malignant mammary epithelial cells (MMECs) were isolated from 8 human breast carcinomas and 1 adenoma as single cells or organoids and established in vitro. Depending on the cellularity of the tumor, between 9 X 10(4) and 4 X 10(6) cells were released per gram of tumor tissue. With the use of conditioned media and growth-promoting agents, a high proportion of cells (ranging from 0.5 to 11.4%) could be established in culture. A high degree of tumor cell heterogeneity in breast carcinomas was suggested by the observation that significantly different proliferative rates were found for 50 mammary epithelial cells cloned from 2 different tumors during the first subpassage in vitro prior to significant expansion of the cell colonies. The computed doubling times of these clones varied between 16 hours and more than 48 hours. The mammary epithelial nature of the cells was confirmed by their surface reactivity with monoclonal antibodies specific for MMECs. Studies on clones from 2 tumors revealed a positive correlation between proliferative rates of MMECs, their lactate production, and specific proteins synthesized as analyzed by two-dimensional macromolecular protein maps.

Membrane potential of erythrocytic stages of Plasmodium chabaudi free of the host cell membrane

Free parasites were isolated from Plasmodium chabaudi-infected rat erythrocytes by N2-cavitation and purified on Percoll gradients. The membrane potential of the free parasites determined from the transmembrane distribution of the lipophilic cation, tetraphenylphosphonium, was -93 +/- 10 mV for late stage parasites and -90 +/- 3 mV for ring forms. Studies with intact infected erythrocytes demonstrated that the membrane potential of ring forms was much smaller compared to late trophozoites and schizonts and thus the present findings with free parasites suggest that host cell cytoplasmic factors may determine the magnitude of the parasite membrane potential. Both extracellular pH and [Na+] were found to modify the membrane potential of free parasites. Electrogenic protonophores, the H+-ATPase inhibitor dicyclohexylcarbodiimide and orthovanadate collapsed the potential of free parasites. Ouabain (or its membrane permeant derivative, strophanthidin), and oligomycin were without effect. These inhibitor studies suggest that an electrogenic H+-ATPase similar to that found in yeast generates in part the membrane potential of malaria parasites. Using weak acid distribution or a pH sensitive fluorescent dye, it was demonstrated that free parasites maintain an alkaline intracellular pH at extracellular pH greater than 6.5. The pH gradient was partially collapsed by orthovanadate or dicyclohexylcarbodiimide and by substitution of Na+ for K+ in the suspending buffer. The H+-ATPase and K+:H+ exchange may therefore both contribute to regulation of intracellular pH in Plasmodium.

Extracellular development of Plasmodium knowlesi erythrocytic stages in an artificial intracellular medium

The development of erythrocytic stages of Plasmodium knowlesi separated from their host cells has been determined in terms of the capacity of the isolated organisms to carry out the synthesis and secretion of proteins. P. knowlesi trophozoites and schizonts were released from host cells by nitrogen decompression and cultivated in a medium consisting of 20 mM Na+; 120 mM K+; 1 mM Mg2+; no Ca2+; 100 mM Cl-; 20 mM HCO3-; 5 mM Hepes [pH 6.73], glucose, vitamins, amino acids and 10% fetal calf serum. The yield was about 97% intact parasites, judging by their ability to maintain a membrane potential, and these parasites had more than 80% the capacity of infected cells for nuclear replication and macromolecule biosynthesis. Pulse and pulse-chase labeling studies with [35S]methionine show that parasite-synthesized proteins with Mr 160 000, 140 000, 100 000 and 58 000 are exported from the parasite in soluble form. Proteins with Mr 140 000, 100 000, 58 000-60 000, 40 000 were recovered in a particulate fraction isolated from the parasite culture fluid. An Mr 62 000 protein synthesized in large amounts by isolated parasites during the last 2h of the developmental cycle, could not be detected in infected erythrocytes, and a minor early Mr 74 000 protein becomes prominent in free parasites but not infected cells toward the end of the developmental cycle. Parasite-synthesized proteins with Mr 230 000, 160 000, 140 000, 62 000, 58 000 and 45 000 were labeled by incubation with radioactive N-acetylglucosamine during short term incubation in vitro. About 80% of label incorporation occurred via N-glycosylation supported by dolichol derived from the blood, and about 20% via glycolytic intermediates.

Extracellular pH, transmembrane distribution and cytotoxicity of chlorambucil

The effects of extracellular pH (6.2 to 7.3) on uptake and cytotoxicity of the weak acid anti-tumor drug chlorambucil were investigated. Decreasing extracellular pH from 7.3 to 6.5 had a negligible effect on the intracellular pH of Chinese hamster V79 fibroblasts, thus resulting in the formation of a transmembrane pH gradient (intracellular alkaline). Addition of high concentrations of acetate or bicarbonate partially collapsed the pH gradient. Chlorambucil (pKa = 5.8) behaved as a weak acid with enhanced accumulation and cytotoxicity at extracellular pH less than 7.0. As predicted for a weak acid, partial collapse of the transmembrane pH gradient decreased both uptake and cell killing. Since the interstitial pH of micrometastases and solid tumors of many cancers is low relative to normal tissues, these results have potential implications for both in vitro drug testing and in vivo therapy.

The isolation and characterization of Ca++-accumulating subcellular membrane fractions from cerebral arteries

A study was undertaken using differential centrifugation methods to isolate from rabbit cerebral arteries the subcellular microsomal protein fractions capable of actively sequestering Ca++. One isolated protein fraction displayed a relatively large adenosine triphosphate (ATP)-dependent Ca++-accumulating capacity that was completely inhibited by NaN3, and was therefore designated the "mitochondrial fraction." Electron microscopy confirmed that this fraction consisted of numerous mitochondrial elements. Another isolated membrane fraction possessed a Ca++-accumulating capacity dependent on ATP and oxalate and only partially sensitive to NaN3. In the presence of mersalyl acid or the Ca++ ionophore, A23187, Ca++ uptake by this fraction was inhibited 98.0% and 87.4%, respectively. Electron microscopy revealed that this fraction consisted of numerous membrane vesicles, and measurements of Na+-K+-ATPase (adenosine triphosphatase) activity indicated minimal plasma membrane contamination. It was concluded that this microsomal fraction consisted primarily of sarcoplasmic reticulum. At physiological free [Ca++] levels, Ca++ uptake by this fraction was inhibited by norepinephrine through a process sensitive to tolazoline but not propranolol. The effects on Ca++ uptake of added cyclic adenosine monophosphate (cAMP) alone or with rabbit or bovine protein kinase were inconclusive. The organic Ca++ channel blockers, nifedipine and verapamil, significantly inhibited Ca++ uptake by sarcoplasmic reticulum.

Calcium transport of Plasmodium chabaudi-infected erythrocytes

The calcium content and transport processes of Plasmodium chabaudi-infected rat erythrocytes were analyzed by atomic absorption spectroscopy and 45Ca2+ flux measurements. Infected erythrocytes, after fractionation on metrizamide gradients according to stage of parasite development, exhibited progressively increasing levels of Ca2+ with schizont and gametocytes containing 10- to 20-fold greater calcium levels than normal cells (0.54 +/- 0.25 nmol/10(8) cells). 45Ca2+ flux experiments showed both increased influx and decreased efflux in infected erythrocytes. Tris/NH4Cl lysis of normal erythrocytes preloaded with 45Ca2+ with the Ca2+ ionophore A23187 released less than 90% of cell calcium after incubation in ethyleneglycol bis(aminoethylether) N,N'-tetraacetic acid containing buffer, whereas lysis of the infected erythrocyte membrane resulted in release of 10-20% cell Ca2+, with the remaining portion associated with the isolated parasite fraction. This information together with the effects of various metabolic inhibitors indicates the presence of a parasite Ca2+ compartment in P. chabaudi-infected erythrocytes. Dicyclohexylcarbodiimide (DCCD) an inhibitor of proton ATPases of chloroplasts, bacteria, yeast, and mitochondria, and the proton ionophore, carbonyl cyanide m-chlorophenylhydrazone (CCCP), inhibited Ca2+ influx and stimulated efflux from infected cells. These results combined with evidence for a DCCD- and CCCP-sensitive membrane potential in P. chabaudi-infected cells (Mikkelsen et al., accompanying manuscript) suggest that Ca2+ transport of intraerythrocytic parasites is coupled to a proton-motive force across the Plasmodia plasma membrane.

Membrane potential of Plasmodium-infected erythrocytes

The membrane potential (Em) of normal and Plasmodium chabaudi-infected rat erythrocytes was determined from the transmembrane distributions of the lipophilic anion, thiocyanate (SCN), and cation, triphenylmethylphosphonium (TPMP). The SCN- and TPMP-measured Em of normal erythrocytes are -6.5 +/- 3 mV and -10 +/- 4 mV, respectively. The TPMP-measured Em of infected cells depended on parasite developmental stage; "late" stages (schizonts and gametocytes) were characterized by a Em = -35 mV "early stages (ring and copurifying noninfected) by a low Em (-16 mV). The SCN-determined Em of infected cells was -7 mV regardless of parasite stage. Studies with different metabolic inhibitors including antimycin A, a proton ionophore (carbonylcyanide m-chlorophenylhydrazone [CCCP] ), and a H+ -ATPase inhibitor (N,N'-dicyclohexylcarbodiimide, [DCCD] ) indicate that SCN monitors the Em across the erythrocyte membrane of infected and normal cells whereas TPMP accumulation reflects the Em across the plasma membranes of both erythrocyte and parasite. These inhibitor studies also implicated proton fluxes in Em-generation of parasitized cells. Experiments with weak acids and bases to measure intracellular pH further support this proposal. Methylamine distribution and direct pH measurement after saponin lysis of erythrocyte membranes demonstrated an acidic pH for the erythrocyte matrix of infected cells. The transmembrane distributions of weak acids (acetate and 5,5-dimethyloxazolidine-2,4-dione) indicated a DCCD-sensitive alkaline compartment. The combined results suggest that the intraerythrocyte parasite Em and delta pH are in part the consequence of an electrogenic proton pump localized to the parasite plasma membrane.

Plasmodial modifications of erythrocyte surfaces

The maturation of malarial parasites in red blood cells produces major alterations in the composition and properties of the host cell surfaces. Existing surface-exposed proteins are modified and new antigenic glycoproteins are synthesized by the parasite and inserted into the membrane. Some of the neoproteins are associated with surface excrescences on the host cells and in some cases these foster adhesion of those cells to capillary endothelium. The erythrocyte endoskeleton is degraded and in association the infected cells become deformed and lose pliability. An increase in intracellular Ca2+ may contribute to the changes in host cells surfaces.

Thermosensitivity of the membrane potential of normal and simian virus 40-transformed hamster lymphocytes

The effects of temperature in the fever range (37-42 degrees) on the membrane potentials of normal and simian virus 40-transformed hamster lymphocytes were analyzed. The transmembrane distributions of radiolabeled triphenylmethylphosphonium and thiocyanate were measured, and they provide upper and lower limits for the normal cell membrane potential at 37 degrees of -48 +/- 6 (S.D.) and -31 +/- 5 mV and for the tumor cells, -36 +/- 4 and -19 +/- 2 mV. The mitochondrial contribution to the triphenylmethylphosphonium-measured membrane potential, 5 to 10 mV for both splenocytes and simian virus 40-transformed lymphocytes, was estimated by utilizing antimycin A and carbonylcyanide-m-chlorophenylhydrazone to inhibit generation of a mitochondrial membrane potential. Incubation for 1 to 2 hr at 38-42 degrees resulted in a 6- to 15-mV depolarization of normal cells and a 2- to 6-mV hyperpolarization of tumor cells. Both depolarization and hyperpolarization were fully reversible by subsequent incubation at 37 degrees and insensitive to antimycin A and carbonyl-cyanide-m-chlorophenylhydrazone. The membrane potential of normal splenocytes when measured with triphenylmethylphosphonium at 37 degrees was depolarized by 35% with 1 mM ouabain and thermally induced depolarization was blocked. The membrane potential of tumor cells at 37 degrees was insensitive to ouabain; however, the hyperpolarization at 40 degrees was inhibited. The membrane potential of normal lymphocytes stimulated with phytohemagglutinin was depolarized relative to that of nonstimulated control cells and assumed the thermal response characteristics of tumor cells.

Isolation and partial characterization of an acetylcholine receptor-enriched membrane fraction from skeletal muscle

A procedure for purification of the bungarotoxin-binding fraction of sarcolemma from rabbit skeletal muscle is described. Muscle is homogenized in 0.25M sucrose without high salt extraction and membrane fractions separated initially by differential centrifugation procedures. An ultracentrifugation pellet enriched in cell surface and sarcoplasmic reticulum markers is further fractionated on a dextran gradient (density = 1.0 to 1.09). Two fractions are identified as sarcolemma according to high specific activities for lactoperoxidase-iodination, Na+, K+-ATPase and alpha-bungarotoxin-binding. No Ca++, Mg++-ATPase activity is found in these fractions. A third fraction, the dextran gradient pellet, is enriched in Ca++, Mg++-ATPase activity and lactoperoxidase iodinatable material and characterized by low bungarotoxin binding. This fraction represents a mixture of sarcoplasmic reticulum and transverse tubules with some sarcolemma contamination.

Neutrophil plasma membranes. I. High-yield purification of human neutrophil plasma membrane vesicles by nitrogen cavitation and differential centrifugation

Neutrophil chemotaxis, phagocytosis, and oxygen-dependent microbicidal activity are initiated by interactions of stimuli with the plasma membrane. However, difficulties in neutrophil plasma membrane isolation have precluded studies on the precise structure or function of this cellular component. In this paper, a method is described for the isolation of representative human neutrophil plasma membrane vesicles, using nitrogen cavitation for cell disruption and a combination of differential centrifugation and equilibrium ultracentrifugation in Dextran gradients for membrane fractionation. Multiple biochemical markers and galactose oxidase-tritiated sodium borohydride surface labeling were employed to follow the yield, purity, and distribution of plasma membranes, nuclei, lysosomes, endoplasmic reticulum, mitochondria, and cytosol. According to these markers, neutrophil plasma membranes were exposed to minimal lysosomal hydrolytic enzymes and could be isolated free of other subcellular organelles. In contrast, disruption of neutrophils by mechanical homogenization resulted in > 20% lysosomal rupture and significant plasma membrane proteolysis. Electron microscopy demonstrated that plasma membranes isolated after nitrogen cavitation appeared to be sealed vesicles with striking homogeneity.

Concanavalin A induces an intraluminal alkalinization of thymocyte membrane vesicles

Weak acid distribution methods demonstrate that mitogenic levels of concanavalin A induce an intravesicular alkalinization of isolated thymocyte membrane vesicles. Experiments with chemical reagents that crosslink the high affinity concanavalin A receptor and extensive correlation with known cellular events suggest that a "membrane Bohr effect" may participate in the initiation of mitogenesis.

Calcium binding to plasma membranes from normal and SV40 transformed hamster lymphocytes

The calcium binding properties of isolated plasma membranes from normal and SV40 transformed hamster lymphocytes were compared over the Ca2+ concentration range of 10(-5)M to 5 x 10(-3)M and at physiological ionic strength. At all Ca2+ concentrations, normal membranes bound more Ca2+ than tumor membranes; at blood Ca2+ levels (1-2 mM) plasma membranes of normal cells bind twice as much as membranes from tumor cells. Normal plasma membranes demonstrated positive cooperative Ca2+ binding whereas tumor membranes displayed non-interacting Ca2+ binding sites. Ca2+ binding to both membranes was insensitive to Mg2+ (0.1 to 2.5 mM). A pH shift from 7 to 6 resulted in a 70% decrease of normal membrane-bound Ca2+ compared to a 40% decrease observed with tumor membranes. Extracellular surface Ca2+ binding to intact cells was also studied after a 72-hour equilibration of cells with 45 Ca2+ and with ethyleneglycol-bis-(beta-amino-ethyl ether) N,N'-tetraacetate chelation as marker for surface Ca2+. Tumor cell surface Ca2+ binding was only 10% of that observed with quiescent lymphocytes. Normal lymphocytes stimulated to divide with phytohemagglutinin also showed a decreased level of surface Ca2+ (50%). However, plasma membranes isolated from non-dividing and phytohemagglutinin-stimulated lymphocytes exhibited equivalent Ca2+ binding.

Effect of transmembrane ion gradients on Raman spectra of sealed, hemoglobin-free erythrocyte membrane vesicles

Sealed hemoglobin-free erythrocyte vesicles have been isolated. Imposition of transmembrane cation gradients increases the intensity of Raman scattering in the CH3-stretching region as observed with unsealed ghosts at temperatures greater than 38 degrees C and pH less than 7.0 [Verma, S. P. & Wallach, D. F. H. (1976) Proc. Natl. Acad. Sci. USA 73, 3358--3561]. Modifications in the amide I and amide III frequencies consistent with increased helicity of membrane proteins are observed upon imposition of a cation gradient. Spectrin-free vesicles also demonstrate cation gradient-sensitive intensity changes in the CH3-stretching region. However, no evidence for cation gradient-related protein conformation changes is found with these vesicles. The transmembrane potential of these vesicles has been altered by variations in anion composition and the electrogenic activity of Na+,K+-ATPase. The membrane potential was monitored by cyanine dye fluorescence. Imposition of a membrane potential (negative inside) also increased the intensity of Raman scattering in the CH3-stretching region. These results suggest that a transmembrane potential (negative inside) and/or cation gradient can energize membranes by compression of the apolar region and transfer of protein methyl residues into polar regions.

Temperature sensitivity of the erythrocyte membrane potential as determined by cyanine dye fluorescence

We have used the cyanine dye fluorescence technique to measure the membrane potential of human erythrocytes as a function of temperature. With erythrocytes starved of glucose, there is an abrupt decrease in membrane potential centered at 38 degrees which is reversible up to 41 degrees, and irreversible at higher temperatures. With erythrocytes supplemented with glucose, the thermally induced transition is centered at 41 degrees and is reversible up to the highest temperature measured, 45 degrees. These results extend previous spectroscopic studies with erythrocyte membranes which demonstrated a thermally induced transition in protein tertiary or quaternary structure that is irreversible above 42 degrees.