Intracellular volume of osmotically regulated C-6 glioma cells

Breast cancer resistance protein (BCRP/ABCG2) localises to the nucleus in glioblastoma multiforme cells

The breast cancer resistance protein (BCRP), an ATP binding cassette (ABC) efflux transporter, plays a role in multiple drug resistance (MDR). Previous studies of the subcellular location of the ABC transporter P-glycoprotein indicated that this protein is expressed in nuclear membranes. This study examines the nuclear distribution of BCRP in seven human-derived glioblastoma (GBM) and astrocytoma cell lines. BCRP expression was observed in the nuclear extracts of 6/7 cell lines. Using the GBM LN229 cell line as a model, nuclear BCRP protein was detected by immunoblotting and confocal laser microscopy. Importantly, nuclear BCRP staining was found in a subpopulation of tumour cells in a human brain GBM biopsy. Mitoxantrone cytotoxicity in the LN229 cell line was determined with and without the BCRP inhibitor fumitremorgin C (FTC) and after downregulation of BCRP with small interfering RNA (siRNA). FTC inhibition of BCRP increased mitoxantrone cytotoxicity with a ~7-fold reduction in the IC₅₀ and this effect was further potentiated in the siRNA-treated cells. In conclusion, BCRP is expressed in the nuclear extracts of select GBM and astrocytoma cell lines and in a human GBM tumour biopsy. Its presence in the nucleus of cancer cells suggests new role for BCRP in MDR.

Oxalic acid alters intracellular calcium in endothelial cells

Patients with chronic renal failure (CRF) who undergo hemodialysis experience accelerated atherosclerosis and premature death. While the cause of uremic atherogenesis is unknown, we reported that uremic levels of oxalate, an excretory metabolite, severely inhibit proliferation and migration of human endothelial cells (EC) without affecting other cell types. Since the physical, cellular and molecular events of endothelial injury are clearly established as key factors in the development of plaque, and since inhibition of proliferation and migration would enhance endothelial injury, we have proposed that oxalate is an atherogenic toxin of uremia. In the current study, we used in situ cell counting and total DNA measurement to show that the inhibitory effect of oxalate on proliferation is exclusive to endothelial cells among human cell lines tested (endothelial cells, fibroblasts, aortic smooth muscle cells (SMC), glioblastoma and embryonic kidney cells). Using the fluorescent calcium indicators fura-2 and fluo-3, we correlated the inhibition of proliferation with a prolonged elevation in intracellular free calcium levels. We also demonstrated that all cells tested internalize 14C-oxalic acid. We conclude that plasma oxalate exerts its atherogenic effects by elevating intracellular calcium exclusively in endothelial cells and preventing re-endothelialization.

The cellular uptake of anandamide is coupled to its breakdown by fatty-acid amide hydrolase

Anandamide is an endogenous compound that acts as an agonist at cannabinoid receptors. It is inactivated via intracellular degradation after its uptake into cells by a carrier-mediated process that depends upon a concentration gradient. The fate of anandamide in those cells containing an amidase called fatty-acid amide hydrolase (FAAH) is hydrolysis to arachidonic acid and ethanolamine. The active site nucleophilic serine of FAAH is inactivated by a variety of inhibitors including methylarachidonylfluorophosphonate (MAFP) and palmitylsulfonyl fluoride. In the current report, the net uptake of anandamide in cultured neuroblastoma (N18) and glioma (C6) cells, which contain FAAH, was decreased by nearly 50% after 6 min of incubation in the presence of MAFP. Uptake in laryngeal carcinoma (Hep2) cells, which lack FAAH, is not inhibited by MAFP. Free anandamide was found in all MAFP-treated cells and in control Hep2 cells, whereas phospholipid was the main product in N18 and C6 control cells when analyzed by TLC. The intracellular concentration of anandamide in N18, C6, and Hep2 cells was up to 18-fold greater than the extracellular concentration of 100 nm, which strongly suggests that it is sequestered within the cell by binding to membranes or proteins. The accumulation of anandamide and/or its breakdown products was found to vary among the different cell types, and this correlated approximately with the amount of FAAH activity, suggesting that the breakdown of anandamide is in part a driving force for uptake. This was shown most clearly in Hep2 cells transfected with FAAH. The uptake in these cells was 2-fold greater than in vector-transfected or untransfected Hep2 cells. Therefore, it appears that FAAH inhibitors reduce anandamide uptake by cells by shifting the anandamide concentration gradient in a direction that favors equilibrium. Because inhibition of FAAH increases the levels of extracellular anandamide, it may be a useful target for the design of therapeutic agents.

Regulation of [gamma-3H]aminobutyric acid transport by Ca2+ in isolated synaptic plasma membrane vesicles

We studied the effect of Ca2+ on the transport of the gamma-aminobutyric acid (GABA) by synaptic plasma membrane (SPM) vesicles isolated from sheep brain cortex and observed that intravesicular Ca2+ inhibits the [3H]GABA accumulation in a concentration-dependent manner. This inhibitory effect of Ca2+ exhibited two distinct components: one in the micromolar range of Ca2+ concentration, and the other in the millimolar range. Previous EGTA washing of the membranes, or incorporation of trifluoperazine into the vesicular space reduced the inhibitory action of Ca2+, particularly at low Ca2+ (1-5 microM). Okadaic acid (1 microM) also relieved the Ca2+ inhibition at low, but not at high Ca2+ concentrations (1 mM), whereas the calpain inhibitor I did not alter the effect of the low Ca2+, but it partially reduced (approximately 28%) the effect of Ca2+ in the millimolar range. The results indicate that the GABA transporter is regulated by low Ca2+ concentration (microM) and probably its effect is mediated by the (Ca2+ x calmodulin)-stimulated phosphatase 2B (calcineurin). In contrast, the GABA uptake inhibition observed at high Ca2+ concentrations (1 mM) is less specific, and probably it is partially related to the proteolytic activity of membrane bound calpain II.

Membrane potential manipulation in synaptic plasma membrane vesicles for studying neurotransmitter uptake and release

Synaptic plasma membrane (SPM) vesicles represent a membrane fraction very useful in studying non-vesicular neurotransmitter release. The procedure described here to isolate SPM vesicles from a crude synaptosomal fraction of sheep brain cortex is quick, simple (ultracentrifugation in a discontinuous density gradient of dextran T110), and combines a high yield (130 micrograms/g brain) with a satisfactory grade of purification. The preparation of SPM vesicles consists of vesicles (approximately 0.54 +/- 0.8 micron diameter) delimited by a single membrane with the native orientation. We are able to ascertain these characteristics on the basis of morphology studies (electron microscopy observations), enzyme activities (Na+/K(+)-ATPase, Ca2+/Mg(2+)-ATPase, acetylcholinesterase and glucose-6-phosphatase), biochemical composition (lipid and protein analysis) and the tetrodotoxin sensitivity of the veratridine-induced gamma-aminobutyric acid (GABA) release. Isolating the SPM vesicles by the proposed procedure permits manipulating the ionic gradients across the membrane by changing the ion concentrations on either side or by utilizing specific ionophores. The vesicles retain their various activities, including their capacity for neurotransmitter uptake and release assays for at least 3 months, when preserved at -70 degrees C. Furthermore, the vesicles permit depicting the electrochemical gradients across the membranes into chemical and electrical components. We describe the use of the tetraphenylphosphonium cation (TPP+) to dissipate the membrane potential (delta psi) of the vesicles, while preserving ionic gradients. The characteristics of the lipid-soluble cation TPP+ allows a massive inflow of this cation into vesicular compartments and a consequent depolarization.

Dual role of K+ and Na+ on the transport of [3H]-gamma-aminobutyric acid by synaptic plasma membrane vesicles

The influence of the monovalent cations (Na+ and K+) and of the electrical gradient on the high-affinity [3H]-gamma-aminobutyric acid ([3H]GABA) transport was investigated in synaptic plasma membrane (SPM) vesicles isolated from sheep brain cortex. This process specifically requires internal K+, since when it is replaced by Li+, the delta psi remains of the same order of magnitude, but no uptake of [3H]GABA occurs. The influence of the external Na+ concentration on the rate of [3H]GABA uptake suggests that this mechanism exhibits two components, whose characteristics are determined by the delta psi. Depolarization reduces the Jmax of [3H]GABA influx and enhances the binding of Na+ associated to [3H]GABA transport. Nevertheless, depolarization does not affect the K0.5 of binding sites for Na+ and the stoichiometry of translocation. These results suggest that intravesicular K+ and external Na+ have a dual role on the mechanism of [3H]GABA uptake: K+ acts directly on the carrier and determines the membrane polarization; Na+ is cotransported with GABA and, according to the polarization state of the membrane, it modulates the operation of the carrier in its inward GABA translocation.

Alterations of membrane integrity and cellular constituents by arachidonic acid in neuroblastoma and glioma cells

Effects of arachidonic acid on cellular metabolism, cation content, lipid peroxidation, sodium pump activities and release of labeled arachidonic acid were studied in C-6 glioma cells and N18TG2 neuroblastoma cells. Arachidonic acid caused a significant increase in intracellular sodium levels concomitant with a decrease in intracellular potassium in both cell lines. Both (Na+ + K+)-ATPase and p-nitrophenyl phosphatase of glioma cells were inhibited by arachidonic acid whereas only the p-nitrophenyl phosphatase of neuroblastoma cell was inactivated. Low concentrations of arachidonic acid stimulated lactic acid release whereas high concentrations had an opposite effect. In addition, the lipid peroxide content of glioma cells was increased abruptly by 50 microM arachidonic acid whereas only a slight increase of malondialdehyde was observed in neuroblastoma cells. When the cultured cells of both cell lines were incubated with exogenous labeled arachidonic acid, 78-95% of the label was incorporated into membrane phospholipids. Only a very small fraction of prostaglandin E2 and prostaglandin F2 alpha was synthesized. Exogenous arachidonic acid and free radicals generated with xanthine-xanthine oxidase caused a significant release of endogenous labeled arachidonic acid from cellular membrane phospholipids. These data further support our hypothesis that the arachidonic acid and its oxygen radical metabolites induce pathological alterations in membrane permeability and cellular volume.