Remodeling of the actin cytoskeleton of target hepatocytes and NK cells during induction of apoptosis

Natural Killer cells are immune cells that recognize and eliminate altered and non-self cells from the circulation. To study the interaction between NK cells and target cells, we set up an experimental system consisting of rat Interleukin-2 activated Natural Killer cells (A-NK cells) and rat hepatocytes with a masked Major Histocompatibility Complex (MHC). The masking of the MHC induces recognition of the hepatocytes by the NK cells as non-self. We showed that in vitro apoptosis is rapidly induced in the hepatocytes [Blom et al., 1999] after co-incubation with A-NK cells. Now we describe the morphological changes that occur during and after interaction of A-NK cells with hepatocytes. Confocal laser scanning microscopy showed that the actin cytoskeleton of the NK cells was remodeled during attack of hepatocytes. Some NK cells were in close contact with the hepatocytes while others had formed actin-containing dendrites of varying length that made contact with the hepatocytes. However, dendrite formation is not obligatory for induction of apoptosis because cells that were unable to form these did induce FAS-dependent apoptosis in hepatocytes. Apparently both direct as well as distant contact resulted in apoptosis. Formation of the dendrites was calcium-dependent as EGTA largely prevented it. Importantly, chelation of the calcium also suppressed killing of the hepatocytes. Within 1 h after addition of the A-NK cells, morphological changes in hepatocytes that are characteristic of apoptosis, such as the formation of apoptotic bodies and fragmented nuclei, became apparent. Specifically, the actin cytoskeleton of the hepatocytes was remodeled resulting in the formation of the apoptotic bodies. Inhibition of caspase activity by z-Val-Ala-DL-Asp-fluoromethylketone (100 microM) partly protected against the rearrangement of the actin filaments in the hepatocytes.

Prevention of cycloheximide-induced apoptosis in hepatocytes by adenosine and by caspase inhibitors

The mechanism by which cycloheximide induces apoptosis in isolated rat hepatocytes was studied. Cycloheximide (1-300 microM) induced apoptosis within 3-4 hr in the hepatocytes. Specific apoptotic characteristics such as blebbing, phosphatidyl serine (PS) exposure, chromatin condensation, and nuclear fragmentation were induced. Cycloheximide (CHX) dose dependently activated the caspase-3-like proteases, but not the caspase-1-like proteases. Pretreatment of the hepatocytes with 100 microM of the caspase inhibitors z-Val-Ala-DL-Asp-fluoromethylketone or Ac-Asp-Glu-Val-Asp-aldehyde completely abrogated the caspase activation and the apoptosis. Addition of adenosine (100 microM) reduced phosphatidyl serine exposure and other morphological characteristics of apoptosis by 50%; however, it did not prevent the activation of the caspases, suggesting that adenosine inhibited downstream of caspase activation. The adenosine receptor antagonist 8-[4-[[[[(2-aminoethyl)amino]-carbonyl]methyl]oxy]phenyl]-1,3-dipropylxa nthine abolished the capacity of adenosine to prevent apoptosis, indicating that prevention was receptor-mediated. During apoptosis, the mitochondrial membrane potential in apoptotic cells (cells with PS exposition) was decreased to 50-60% of the control value; in the population viable cells, however, the mitochondrial membrane potential remained stable. Prevention of apoptosis by the caspase inhibitor z-Val-Ala-DL-Asp-fluoromethylketone or adenosine prevented the decrease in mitochondrial membrane potential. In conclusion, CHX rapidly induces apoptosis in isolated rat hepatocytes, which is inhibited by adenosine at a relatively late step.

Changes of G-actin localisation in the mitotic spindle region or nucleus during mitosis and after heat shock: a histochemical study of G-actin in various cell lines with fluorescent labelled vitamin D-binding protein

The presence and localisation of G-actin in various cell lines was studied using the highly G-actin specific, fluorescence-labelled vitamin D-binding protein. In various cell-types, pig kidney-derived cells (LLC-PK1), Chinese hamster ovary (CHO) cells, SV-40 transformed African green monkey kidney (COS) cells and human hepatoma (HepG2) cells, G-actin was only visible in the cytoplasm of interphase cells. However, in mitotic cells, depending on the mitotic phase, intense G-actin staining was found associated with the mitotic spindle (early mitosis) or overlapping the DNA-staining pattern (late mitosis). Also after heat shock (60-180 min at 43 degrees C), an intense nuclear staining of G-actin was observed. In LLC-PK1 cells, the increase of nuclear G-actin staining disappeared again after 24 h at 37 degrees C, but in COS, CHO and HepG2 cells, it was still present in the nucleus after 24 h at 37 degrees C, indicating that the process was not rapidly reversible in these cells; the increased nuclear G-actin was not associated with cell division. Comparison of the amount of G-actin present in the nucleus and in the cytosol before and after heat shock using Western blotting demonstrated that the total amount of G-actin in both nucleus and cytosol was unchanged after heat shock. This indicates that the increased G-actin staining is not a result of import of G-actin into the nucleus. These observations suggest a rearrangement of G-actin in the nucleus during both mitosis and heat shock, which may be due to changes in interaction of G-actin with chromosomes.

Induction of apoptosis and changes in nuclear G-actin are mediated by different pathways: the effect of inhibitors of protein and RNA synthesis in isolated rat hepatocytes

Stressor-induced changes in the cytoskeleton, of which actin is a major component, may lead to apoptosis. The role of drug-induced changes in nuclear G-actin and apoptosis was studied in freshly isolated hepatocytes. Several protein synthesis inhibitors, cycloheximide, puromycin, and emetine, induced 10 to 15% apoptosis in hepatocytes after 4 h, as was determined by changes in nuclear morphology and flow cytometric analysis of Annexin V-positive cells. Apoptosis induced by protein synthesis inhibition could be prevented by the caspase inhibitors Z-Val-Ala-DL-Asp fluormethylketone (zVAD-fmk) and Ac-Asp-Glu-Val-Asp-aldehyde (DEVD-cho). Several (chemical) stressors cause a rapid increase in nuclear G-actin staining in hepatocytes or cell lines (Meijerman et al., Biochem. Biophys. Res. Commun. 240, 697-700, 1997). The protein synthesis inhibitors also increased G-actin staining in nuclei after 2 h; this could not be inhibited by zVAD-fmk or DEVD-cho. Changes in the cytosolic F-actin pattern did not occur until nuclear G-actin staining had already increased. The mRNA synthesis inhibitor actinomycin D, also increased nuclear G-actin staining, but did not induce apoptosis within the studied time frame. The results suggest that the induction of apoptosis and the increased nuclear staining of G-actin by protein synthesis inhibition are differently controlled.

Nuclear accumulation of G-actin in isolated rat hepatocytes by adenine nucleotides

Extracellular ATP induces bleb formation in isolated rat hepatocytes. We examined the effect of extracellular ATP on the actin cytoskeleton of these hepatocytes. Exposure to 100 microM ATP caused pronounced nuclear accumulation of G-actin. ADP, AMP, adenosine, and dibutyryl-cAMP induced the same effect. Adenosine deaminase could inhibit both ATP- and adenosine-induced nuclear accumulation. The P2-receptor agonists, UTP and 2' & 3'-O-(4-benzoylbenzoyl)-adenosine 5'-triphosphate, did not induce this redistribution of G-actin. Phalloidin, which prevents depolymerisation of F-actin filaments to G-actin monomers, inhibited adenosine-induced nuclear accumulation of G-actin. These observations suggest that nuclear accumulation of G-actin is mediated by adenosine receptors.

Use of video microscopy to study the mechanisms of cytotoxicity in vitro in individual living cells

Recent technological innovations have made it possible to determine simultaneously multiple biochemical parameters in individual living cells. This new technique, commonly designated as video microscopy, makes use of fluorescent probes, specific for a certain cellular parameter. Cells are loaded with the probes and placed under a fluorescence microscope. After excitation at a specific wavelength the probes emit light of a longer wavelength. The intensity of the emitted light is proportional to the magnitude of the specific parameter. Emission is recorded with a video camera and the signal transferred to an image processor for extraction of quantitative data from the image, or to improve image quality. An overview of the technique itself and the necessary hardware, the major types of calculations and how to handle cells and probes is presented in this paper.

The role of a purinergic P2z receptor in calcium-dependent cell killing of isolated rat hepatocytes by extracellular adenosine triphosphate

Extracellular adenosine triphosphate (ATP o) (0.4 mmol/L), a P2-purinergic receptor agonist, induces cytolysis in several cell types including isolated rat hepatocytes. In this study, we investigated the P2-receptor involved in ATP o-induced, Ca2+-dependent cytotoxicity in hepatocytes. Pretreatment of hepatocytes with oxidized ATP, a P2z-receptor antagonist, or complexation of ATP(4-) (the agonist for the P2z-receptor) with an excess of Mg2+, prevented ATP o-induced cell death. Both protective treatments also prevented the development of a sustained high intracellular Ca2+ concentration as well as the subsequent accumulation of inorganic phosphate (Pi). The P2Z-receptor agonist 3'-O-'(4-benzoylbenzoyl)-ATP (BzATP) was twofold more potent than ATP in eliciting cytolysis, which was preceded by a sustained high intracellular Ca2+ concentration; pretreatment with oxidized ATP prevented both the increase in the intracellular Ca2+ concentration and cell death. Prevention of ATP o-induced cell death, as well as the increases in the intracellular Ca2+ concentration and inorganic phosphate (Pi) was also achieved by decreasing the pH o to 6.9. Together the findings indicate that Ca2+-dependent cell killing by extracellular ATP in hepatocytes is mediated by a P2Z-receptor. The cytolytic effects correlated specifically with a secondary "late" increase in the intracellular Ca2+ concentration.

Inhibition of succinate:ubiquinone reductase and decrease of ubiquinol in nephrotoxic cysteine S-conjugate-induced oxidative cell injury

The role of complex II in the cellular protection against oxidative stress was investigated in freshly isolated rat renal proximal tubular cells (PTC) with the use of the nephrotoxin S-(1,2-dichlorovinyl)-L-cysteine (DCVC). DCVC caused oxidative stress in PTC as determined by flow cytometry with dihydrorhodamine-123; this fluorescent probe is readily oxidized by primary hydroperoxides such as those formed during lipid peroxidation. The oxidative stress could be prevented by inhibition of the beta-lyase-mediated formation and covalent binding to cellular macromolecules of reactive DCVC metabolites, with amino oxyacetic acid (AOA), or by the antioxidant N,N'-diphenyl-p-phenylenediamine. Both AOA and DPPD also prevented cell death. The DCVC-induced oxidative stress was associated with a decrease in the succinate:ubiquinone reductase (SQR) activity of complex II, whereas NADH:ubiquinone reductase activity of complex I remained unaffected. AOA prevented the effect on SQR activity, whereas N,N'-diphenyl-p-phenylenediamine did not. Inhibition of SQR activity with thenoyl trifluoracetone (TTFA) potentiated the DCVC-induced oxidative cell injury, suggesting the involvement of SQR activity in an antioxidant pathway. To investigate this in greater detail, PTC were treated with an inhibitor of cytochrome-c-oxidase, KCN, in a buffer containing glycine, which prevents cell death by KCN. Glycine did not affect cell death by DCVC. KCN prevented the DCVC-induced oxidative stress and cell death. KCN cytoprotection could be prevented by inhibition of SQR activity with oxaloacetate or TTFA, whereas inhibition of either complex I or III with rotenone and antimycin, respectively, did not prevent it. The effect of DCVC on complex II was associated with a decrease in the cellular amount of reduced ubiquinone (QH2); the KCN-mediated cytoprotection was related to a 60% increase of cellular QH2. Rotenone almost completely inhibited ubiquinone reduction even in the presence of KCN, whereas oxaloacetate in combination with KCN resulted in QH2 levels comparable to control. This suggests that the SQR activity by complex II rather than the cellular content of reduced ubiquinone (QH2) is important as a part of the cellular antioxidant machinery in the cyto-protection against oxidative stress.

Role of mitochondrial Ca2+ in the oxidative stress-induced dissipation of the mitochondrial membrane potential. Studies in isolated proximal tubular cells using the nephrotoxin 1,2-dichlorovinyl-L-cysteine

The relationship between mitochondrial Ca2+, oxidative stress, and a dissipation of the mitochondrial membrane potential (delta psi) was investigated in proximal tubular kidney cells. Freshly isolated proximal tubular cells from rat kidney were exposed to the nephrotoxin 1,2-dichlorovinyl-L-cysteine (DCVC). DCVC stimulated the formation of hydroperoxides as determined by flow cytometry using the hydroperoxide-sensitive compound dichlorofluorescein. This was prevented by the antioxidant diphenylphenylenediamine (DPPD) and the iron chelator desferrioxamine. Studies in individual cells with video-intensified fluorescence microscopy showed that a DCVC-induced increase in the intracellular free calcium concentration ([Ca2+]i) was accompanied by an increase in the mitochondrial free calcium concentration ([Ca2+]m). The latter increase was selectively prevented by an inhibitor of the mitochondrial calcium uniporter, ruthenium red (RR). Chelation of cellular Ca2+ with EGTA acetoxymethyl ester (EGTA/AM) completely prevented the formation of hydroperoxides, whereas inhibition of the uptake of Ca2+ by the mitochondria with RR reduced it. This indicates that the increase in [Ca2+]m is important for the induction of oxidative stress by DCVC. DPPD and desferrioxamine did not protect against a DCVC-induced increase in [Ca2+]i and [Ca2+]m, indicating that oxidative stress is the consequence rather than the cause of the cellular calcium perturbations. DCVC decreased delta psi and caused cell death; both effects were clearly delayed by EGTA/AM and RR, although they could not prevent a decrease in delta psi. The latter decrease was completely prevented by inhibition of the beta-lyase-mediated metabolism of DCVC with aminooxyacetic acid. Like EGTA/AM, inhibition of oxidative stress with DPPD and desferrioxamine delayed the decrease in delta psi. This strongly suggests that the decrease in delta psi caused by metabolites of DCVC directly is potentiated by Ca(2+)-dependent DCVC-induced hydroperoxide formation. The importance of both hydroperoxide formation and mitochondrial damage in DCVC-induced cell killing is discussed.

Mitochondrial K+ as modulator of Ca(2+)-dependent cytotoxicity in hepatocytes. Novel application of the K(+)-sensitive dye PBFI (K(+)-binding benzofuran isophthalate) to assess free mitochondrial K+ concentrations

In isolated rat hepatocytes a sustained high increase in intracellular free Ca2+ ([Ca2+]i), induced by extracellular ATP, is associated with mitochondrial dysfunction and cell death. The Ca(2+)-induced effects are Pi-dependent and less severe when the intracellular K+ content is low. In this study, the involvement of mitochondrial K+ processing in Ca(2+)-induced loss of mitochondrial membrane potential (MMP) and viability was investigated. The recently introduced K(+)-sensitive dye PBFI (K(+)-binding benzofuran isophthalate) has been used in combination with video-microscopy to assess intramitochondrial free K+ concentration ([K+]mito) in rat liver mitochondria in situ. After rapid permeabilization of the plasma membrane to remove cytosolic PBFI, the remaining PBFI was localized in mitochondria, and a 'resting' [K+]mito of approx. 15 mM could be measured. Increased [K+]mito levels were measured after induction of a prolonged increase in [Ca2+]i by ATP. Much lower [K+]mito, more comparable with control levels, were observed when intracellular K+ was depleted by omission of extracellular K+. In permeabilized cells the Ca(2+)-induced, Pi-dependent, dissipation of the MMP was markedly delayed in the absence of K+. These observations suggest involvement of [K+]mito as modulating agent in Ca(2+)-induced cytotoxicity in hepatocytes.

The relationship between intracellular Ca2+ and the mitochondrial membrane potential in isolated proximal tubular cells from rat kidney exposed to the nephrotoxin 1,2-dichlorovinyl-cysteine

The effects of 1,2-dichlorovinyl-cysteine (DCVC) on the intracellular free calcium concentration ([Ca2+]i) and the mitochondrial membrane potential (delta phi) were investigated in freshly isolated rat kidney proximal tubular cells (PTC). Prior to cell death, DCVC induced a rise in [Ca2+]i and a decrease in the delta phi. Omission of extracellular calcium still resulted in a DCVC-induced increase of [Ca2+]i, indicating that calcium was released from intracellular stores. The beta-lyase inhibitor amino-oxyacetic acid completely protected against mitochondrial damage and cell death, indicating that the DCVC effects are dependent on beta-lyase metabolism. Incubation of the PTC with DCVC together with the intracellular-calcium complexing agents EDTA/acetoxy-methyl (AM), EGTA/AM or Quin-2/AM delayed (but did not prevent) the decrease of the delta phi and cell death, which indicates a relationship between [Ca2+]i and the decrease of delta phi. In individual cells four different responses induced by DCVC were observed; an increase of [Ca2+]i without an effect on delta phi, a decrease of delta phi and an increase of [Ca2+]i occurring simultaneously; an increase of [Ca2+]i preceded by a decrease of delta phi and a decrease of delta phi without any increase of [Ca2+]i. This indicates that DCVC-induced effects on [Ca2+]i and delta phi can appear independently. The data show that mitochondrial damage is potentiated by an elevation of [Ca2+]i, thereby creating a situation which rapidly leads to cell death.

Binding of the radioligand [35S]adenosine 5'-O-(2-thiodiphosphate) and intracellular calcium response in rat liver parenchymal cells

The use of the radioligand [35S]adenosine 5'-O-(2-thiodiphosphate) (ADP beta 35S) for the determination of P2y-purinoceptors on turkey erythrocyte membranes has recently been described. In the present study, we were able to demonstrate specific binding of this radioligand in intact rat liver parenchymal cells. Within 10 min a thermodynamic equilibrium was obtained which lasted for 25 min with a subsequent decline. Displacement studies with several nucleotides were performed yielding Ki values of 1.5 +/- 0.47 microM for UTP, 1.8 +/- 0.35 microM for adenosine 5'-O-(2-thiodiphosphate) (ADP beta S), 31 +/- 6.2 microM for ATP and 35 +/- 6.1 microM for GTP. In addition, we showed that ADP beta 35S is highly resistant to degradation by ecto-nucleotidases, with only 14.5 +/- 1.4% of total ADP beta 35S present being degraded after 1 hr, and that the binding of ADP beta 35S to its binding sites was modulated by EDTA. The Ki value of ATP shifted to 8.1 +/- 1.2 microM upon the addition of 1 mM EDTA to the incubation medium. In these rat liver parenchymal cells all nucleotides promoted calcium entry in a dose-dependent manner with EC50 values of 3.5 +/- 0.22 microM for UTP, 20.7 +/- 3.1 microM for ATP, 38.3 +/- 6.4 microM for ADP beta S and 73.6 +/- 13.7 microM for GTP, with GTP being a partial agonist. Based on the data derived from the present study we discuss the possible correlation between binding and functional experiments and conclude that the described receptor resembles most closely the P2u-purinoceptor and/or "nucleotide receptor", in that UTP is at least as active as ATP.

Calcium-induced cytotoxicity in hepatocytes after exposure to extracellular ATP is dependent on inorganic phosphate. Effects on mitochondrial calcium

In isolated mitochondria extensive uptake of Ca2+ in the presence of an "inducing agent," e.g. inorganic phosphate (Pi), causes permeabilization of the mitochondrial inner membrane and a collapse of the mitochondrial membrane potential. In this study we tested whether the effect of phosphate occurs in intact hepatocytes. Rat hepatocytes were incubated with ATP to induce a sustained increase in intracellular Ca2+ ([Ca2+]i), dissipation of the mitochondrial membrane potential, and cell death (Zoeteweij, J. P., van de Water, B., de Bont, H. J. G. M., Mulder, G. J., and Nagelkerke, J. F. (1992) Biochem. J. 288, 207-213). Omission of Pi from the incubation medium delayed the loss of viability. The nonhydrolyzable ATP analog adenosine 5'-O-(thiotriphosphate) (ATP gamma S) had similar effects on [Ca2+]i and viability, but now omission of extracellular Pi completely protected against cytotoxicity. Exposure to ATP or ATP gamma S induced a large cellular uptake of Pi. With the use of video-microscopy a significant increase in mitochondrial free calcium was observed before the onset of cell death. Accumulation of mitochondrial calcium was reduced when extracellular Pi was omitted. These results suggest that, after induction of high [Ca2+]i by ATP in hepatocytes, 1) mitochondria accumulate calcium which is associated with cell toxicity and 2) intracellular Pi increases which stimulates mitochondrial calcium uptake. These observations support a calcium-dependent mitochondrial dysfunction, induced by phosphate, as a valid model for ATP-induced cytotoxicity in hepatocytes.

Fasting increases the susceptibility of rat hepatocytes to the cytotoxic effects of N-hydroxy-acetylaminofluorene. Effects on mitochondrial respiration and membrane potential

Isolated rat hepatocytes were incubated with the carcinogen N-hydroxy-2-acetylaminofluorene (N-OH-AAF). Cells from fasted rats were much more susceptible to the cytotoxic effects of 1 mM N-OH-AAF than cells from fed rats: after approximately 90 min exposure the former were all dead but the latter still viable. Even after 240 min 25% of the "fed" cells were still viable. The loss of viability was preceded by a decrease in mitochondrial membrane potential (MMP) and inhibition of respiration; the mitochondrial respiration as measured in permeabilized cells appeared uncoupled. Addition of 15 mM fructose prevented cell death and the loss of MMP in cells both from fed and fasted rats to a large extent; however, uncoupling was not prevented. After incubation of hepatocytes from fasted rats with 1 mM [3H]N-OH-AAF for 120 min, 12 nmol [3H]N-OH-AAF became bound per mg cell protein. Addition of fructose decreased this to 7 nmol. In cells from fed animals 4 nmol [3H]N-OH-AAF became bound after 120 min, in this case fructose had no effect. Part of the protective effect of fructose might be explained by a decrease in intracellular ATP, which prevents the formation of reactive intermediates of N-OH-AAF resulting in a decrease of covalent binding, in addition, fructose protects via a yet to be determined mechanism.

Involvement of intracellular Ca2+ and K+ in dissipation of the mitochondrial membrane potential and cell death induced by extracellular ATP in hepatocytes

Isolated rat hepatocytes were incubated with extracellular ATP to induce a prolonged increase in intracellular Ca2+ ([Ca2+]i) and a loss of viability within 2 h. By using video-intensified fluorescence microscopy, the effects of exposure to extracellular ATP on [Ca2+]i, mitochondrial membrane potential (MMP) and cell viability were determined simultaneously in individual living hepatocytes. The increase in [Ca2+]i on exposure to ATP was followed by a decreasing MMP; there were big differences between individual cells. Complete loss of the MMP occurred before cell death was observed. Omission of K+ from the incubation medium decreased the cytotoxicity of ATP; under these conditions, intracellular K+ was decreased by more than 80%. Treatment with nigericin also depleted intracellular K+ and decreased ATP-induced toxicity. Protection against loss of viability by means of a decrease in intracellular [K+] was reflected by maintenance of the MMP. These observations suggest that ATP-induced cell death may be caused by a mechanism that has been described for isolated mitochondria: after an increase in Ca2+ levels, a K+ influx into mitochondria is induced, which finally disrupts the MMP and leads to cell death.

Role of microtubuli in secretion of very-low-density lipoprotein in isolated rat hepatocytes: early effects of thiol reagents

The secretion of very-low-density lipoprotein from hepatocytes proceeds through the microtubules. In this study, the role of glutathione in the maintenance of intact microtubules and the secretion of very-low-density lipoprotein has been investigated. When rat hepatocytes were incubated with reagents that deplete glutathione (e.g., diethylmaleate, alpha-bromoisovalerylurea or allyl alcohol) or reacted directly with protein thiols (disulfiram), the secretion of very-low-density lipoprotein by the cells was inhibited and the microtubules were severely damaged as visualized by immunofluorescence staining. Both events occurred within 30 min; long before, an effect on the energy status of the cells became evident. The observed inhibition of the secretion therefore seems due to an effect of the toxicants on the microtubules. For alpha-bromoisovalerylurea, diethylmaleic acid and allyl alcohol, it may be related to glutathione depletion; preincubation of the hepatocytes with N-acetyl-L-cysteine reduced the decrease of glutathione by alpha-bromoisovalerylurea and allyl alcohol (but not of diethylmaleic acid) and almost completely prevented the inhibition of very-low-density lipoprotein secretion and microtubule damage. Depletion of glutathione may result in modification of a small group of essential free protein thiols. Disulfiram did not deplete glutathione, and N-acetyl-L-cysteine could not prevent the effects of disulfiram on microtubules. The binding to protein thiols of radiolabeled disulfiram, which binds to microtubules in vitro, was determined. At 0.2 mmol/L disulfiram, only 3% of total cellular protein thiols were conjugated, but secretion of very-low-density lipoprotein was already inhibited by 25%, and microtubules were severely affected. We propose that modification of a small fraction of cellular protein thiols results in the loss of microtubular ultrastructure and thereby leads to inhibition of very-low-density lipoprotein secretion.