Evidence that plasma membrane fluidity of isolated hepatocytes is modified by exposure to microtubule-depolymerizing drugs

Membrane stabilizing effects of calcium and taxol during the cold storage of isolated rat hepatocytes

BACKGROUND

Calcium plays an important role in liver preservation and preservation induces depletion of cellular Ca. This may affect hepatocyte cytoskeleton integrity necessary for maintaining cell shape and organ viability. We tested the effects of a microtubular stabilizer (Taxol) in liver cell preservation.

METHODS

Isolated rat hepatocytes were preincubated with or without a microtubule stabilizing agent, 100 microM Taxol, at 37 degrees C for 20 min, then stored in the University of Wisconsin (UW) solution +/-1.5 mM CaC12 at 4 degrees C for up to 48 hr. After storage, the cells were rewarmed in Krebs-Henseleit buffer with air at 37 degrees C for 1 hr. Morphological changes in the plasma membrane (scanning electron microscopy) and cell viability (percentage of lactate dehydrogenase [LDH] release) before and after rewarming were studied.

RESULTS

Hepatocytes showed time-dependent increase in bleb formation (cytoskeleton disruption) during cold storage. Rewarming the cells caused even greater bleb formation and increased LDH release (cell death). Pretreatment of cells with Taxol and cold storage in the UW solution with 1.5 mM Ca suppressed both bleb formation and LDH release in 48-hr coldstored cells.

CONCLUSIONS

Cold storage of hepatocytes leads to reperfusion injury and cell death. This can be suppressed with Taxol and Ca. This suggests that hypothermia induces changes in cellular Ca and a disruption of the microtubules, leading to loss of cell viability. Improved liver preservation may require suppression of Ca-dependent disruption of the cytoskeleton system of liver cells.

Effect of vinblastine on cell membrane fluidity in vinblastine-sensitive and -resistant HeLa cells

The electron paramagnetic resonance method (EPR) was used to study the effects of vinblastine (VLB) on cell membrane fluidity in wild-type HeLa cells (HeLa K) and its subclone, which is resistant to several drugs (HeLa CA). HeLa CA cells, obtained by treatment of HeLa K with CDDP, were found to be more resistant to VLB than to CDDP. The experimentally observed EPR spectra were correlated with the calculated spectra obtained by computer simulation. The results indicate that the cell membrane of HeLa K and HeLa CA cells is heterogeneous and can be described with at least three types of coexisting domains with different fluidity characteristics. The two more fluid domains of HeLa CA cells were found to be more fluid than in HeLa K cells. The fluidity of the less fluid domain remained unchanged but its portion in the membrane was increased. VLB treatment did not affect the membrane fluidity of HeLa CA cells significantly. On the other hand, 1 h of treatment of HeLa K cells with 1 ng/ml VLB did not change the fluidity characteristics of membrane domains but decreased the portion of the less fluid domains. This was also reflected in an average increase of the entire membrane fluidity. The observed changes were detected at VLB concentrations which were far below the cytotoxic level.

Preliminary observations on the interference of antiblastic agents in membrane fluidity and leukocyte potential

A major problem in cancer treatment is the progressive desensitization of the cancer cells to chemotherapeutic drugs. Several hypotheses have been advanced to explain this property of neoplastic cells. In recent years, some calcium-channel blockers have successfully been used to restore drug-sensitivity in previously resistant tumors. The presence of a correlation between ion channels and membrane fluidity is well known. In the ambit of our studies on the activity of several chemotherapeutic drugs on tumors, we have studied the variations in membrane depolarization and fluidity in some leukemic cells as a result of polychemotherapeutic treatments. Our results demonstrate that the membrane fluidity and K(+)-induced depolarization of some types of leukemic cells in patients untreated and treated with some chemotherapeutic agents, are altered significantly as compared to those of normal leukocytes.

Plasma membrane fluidity of keratinocytes of normal and psoriatic skin: a study using fluorescence anisotropy of trimethylammoniumdiphenylhexatriene (TMA-DPH)

The aim of this study was to investigate plasma membrane fluidity in human keratinocytes using fluorescence anisotropy of 1,6-diphenyl-1,3,5-hexatriene (DPH) and its cationic derivative 1-[4-(trimethylamino)-phenyl]-6-phenyl-1,3,5-hexatriene (TMA-DPH). Keratinocytes from normal or psoriatic skin were isolated using trypsin-EDTA or dispase. In keratinocytes isolated from normal skin, TMA-DPH anisotropy values were higher than those observed using DPH; the difference must be related to the different localization of the two probes. In fact, DPH in whole cells localizes in plasma as well as intracellular membranes, yielding an average value of fluidity, while the cationic derivative TMA-DPH resides in the plasma membrane of the whole cells for a sufficient time for anisotropy measurements. Moreover, it has to be considered that plasma membrane is more ordered than intracellular membranes. The kinetics of incorporation of TMA-DPH was similar in keratinocytes isolated using trypsin-EDTA and those isolated using; dispase, however, the fluorescence anisotropy values were lower in keratinocytes isolated with dispase (0.260 +/- 0.01 vs 0.270 +/- 0.01, p = 0.029). This difference is probably related to modifications of lipid-protein interactions after trypsin treatment. Since no damage to plasma membrane after incubation with dispase seems to have been reported, we decided to use this separation procedure to study plasma membrane fluidity in psoriasis, a human pathological condition characterized by excessive cell proliferation and incomplete differentiation. Lower anisotropy values (0.260 +/- 0.01 vs 0.270 +/- 0.01, p = 0.001), indicating an increase in fluidity, were observed in keratinocytes isolated from skin of psoriatic patients than in epidermal cells isolated from normal human skin. We suggest that the measurement of fluorescence anisotropy in living cells is a convenient and useful tool to study membrane fluidity in human keratinocytes isolated from normal and diseased skin. Its application represents a technical advance because plasma membrane fluidity can be measured using very limited amounts of tissue, as obtained from biopsies.

A comparison of the fluorescence properties of TMA-DPH as a probe for plasma membrane and for endocytic membrane

In earlier studies, the fluorescence probe 1-(4-(trimethylamino)phenyl)-6-phenylhexa-1,3,5-triene (TMA-DPH) was shown to interact with living cells by instantaneous incorporation into the plasma membrane, according to a water (probe not fluorescent)/membrane (probe highly fluorescent) partition equilibrium. This made it interesting both as a fluorescence anisotropy probe for plasma membrane fluidity determinations and as a quantitative tracer for endocytosis and intracellular membrane traffic. In order to ascertain the limiting concentrations for its use in these applications, we performed a systematic study of its fluorescence properties (intensity, lifetime, anisotropy) in the plasma membrane and in endocytic membranes of intact L929 mouse fibroblasts. Some of the experiments were repeated on mouse-bone-marrow-derived macrophages and on phospholipidic LUV to confirm the results. Rather unexpectedly, it was observed that: (i) the incorporation of TMA-DPH into the membranes, monitored by UV absorption measurements, remained proportional to the probe concentration over the wide range explored (5 x 10(-7) M-2.5 x 10(-5) M); (ii) however, concerning fluorescence, quenching effects occurred in the membranes above certain critical concentrations. These effects were shown to result from Förster-type resonance auto-transfer; (iii) strikingly, the critical concentrations were considerably higher in early-endocytic-vesicle membranes than in the bulk plasma membrane. It was established that membrane fluidity was involved and this was confirmed by the parallel study on phospholipidic vesicles. Potential applications of these properties as a novel approach for evaluating membrane fluidity are suggested.

Effects of vinblastine on cell membrane fluidity and the growth of SA-1 tumor in mice

Cell membranes can be targets of some anti-cancer drugs. Therefore, the purpose of this study was to determine whether vinblastine (VLB) can also affect the tumor cell membrane. On the in vivo SA-1 tumor model, alteration of cell membrane fluidity (measured by electron paramagnetic resonance, EPR), cytotoxicity and morphological changes of the SA-1 tumor cells after VLB treatment were studied. The cytotoxic effect of VLB was biphasic, with an initial fast increase in cytotoxicity followed by a plateau. The surviving cells had increased membrane fluidity and were morphologically changed. The dose-response curve of VLB on membrane fluidity was also biphasic with an initial fast increase in membrane fluidity followed by a plateau. Since dose-response curves of VLB cytotoxicity and its effect on membrane fluidity were similar, there was a high correlation between both effects. The effect of VLB on membrane fluidity was the most pronounced at 24 h and 48 h after treatment. The results of this study indicate that VLB affects cell membrane by increasing the membrane fluidity of SA-1 tumor cells in vivo in a dose-and time-dependent manner. Therefore, this finding may be beneficially implemented also in priming cells for other cytotoxic drugs and for appropriate timing of drug sequence in combined schedules.

Microtubule-dependent transport of bile salts through hepatocytes: cholic vs. taurocholic acid

Studies with taurine-conjugated bile salts have demonstrated two pathways for hepatocellular delivery of bile salts to bile: a cytosolic, microtubule-independent pathway and a membrane-based, microtubule-dependent pathway. However, a significant portion of circulating bile salts may be unconjugated. To determine whether free bile salts utilize similar pathways, we examined the effect of colchicine on the biliary excretion of intravenously administered cholic acid and taurocholate in intact rats. Basal rats were pretreated with low-dose colchicine or its inactive isomer, lumicolchicine, 1 hr before placement of intravenous and biliary cannulas and 2.75 hr before intravenous injection of [14C]cholic acid and [3H]taurocholate. Superfused rats were prepared as above but with intravenous infusion of taurocholate at 200 nmol/min.100 gm beginning 0.75 hr before [14C]cholic acid/[3H]taurocholate injection. Depleted/reinfused rats were subjected to biliary diversion for 20 hr before colchicine or lumicolchicine pretreatment, infusion of taurocholate and [14C]cholic acid/[3H]taurocholate injection. In each group, biliary excretion of [14C]taurocholate and [3H]taurocholate was inhibited equally by colchicine; for peak excretion rates the respective inhibition values were 33% and 35% in basal rats, 63% and 65% in superfused rats, and 74% and 76% in depleted/reinfused rats. Biliary excretion of [14C]taurocholate occurred consistently later than excretion of [3H]taurocholate, and maximal rates of excretion were reduced. In contrast, plasma uptake rates of [14C]cholic acid and [3H]taurocholate were essentially the same in depleted/reinfused rats. Deconvolution analysis of [14C]taurocholate vs. [3H]taurocholate biliary excretion curves revealed no significant differences among experimental groups. We conclude that conversion of [14C]cholic acid to [14C]taurocholate slightly retards its biliary excretion and diminishes its peak excretion rate compared with exogenous [3H]taurocholate.(ABSTRACT TRUNCATED AT 250 WORDS)

Reversal of carbon tetrachloride induced changes in microviscosity and lipid composition of liver plasma membrane by colchicine in rats

Colchicine is beneficial in the treatment of cirrhotic patients, it prevents changes in plasma membrane bound enzymes induced by CCl4 intoxication. In this study, lipid composition and microviscosity were measured in liver plasma membranes isolated from rats given CCl4. Microviscosity values increased in rats given CCl4 for six weeks but fell considerably in those given CCl4 for 10 weeks. Both these changes were absent when colchicine was given with CCl4. The cholesterol/phospholipid molar ratios and lipid peroxide values increased but plasma membrane phospholipids, the length of fatty acyl chains, and the unsaturation index fell significantly after CCl4 intoxication. Colchicine treatment also prevented these changes. Changes in the lipid composition of liver plasma membranes were significantly correlated with lipid peroxidation. Colchicine prevents changes in the physicochemical properties of liver plasma membranes induced by longterm CCl4 treatment, probably by blocking peroxidation of unsaturated fatty acids.