Effect of cytochalasins on F-actin and morphology of Ehrlich ascites tumor cells

Diagnostic fragmentation filtering for the discovery of new chaetoglobosins and cytochalasins

RATIONALE

Microbial natural products are often biosynthesized as classes of structurally related compounds that have similar tandem mass spectrometry (MS/MS) fragmentation patterns. Mining MS/MS datasets for precursor ions that share diagnostic or common features enables entire chemical classes to be identified, including novel derivatives that have previously been unreported. Analytical data analysis tools that can facilitate a class-targeted approach to rapidly dereplicate known compounds and identify structural variants within complex matrices would be useful for the discovery of new natural products.

METHODS

A diagnostic fragmentation filtering (DFF) module was developed for MZmine to enable the efficient screening of MS/MS datasets for class-specific product ions(s) and/or neutral loss(es). This approach was applied to series of the structurally related chaetoglobosin and cytochalasin classes of compounds. These were identified from the culture filtrates of three fungal genera: Chaetomium globosum, a putative new species of Penicillium (called here P. cf. discolor: closely related to P. discolor), and Xylaria sp. Extracts were subjected to LC/MS/MS analysis under positive electrospray ionization and operating in a data-dependent acquisition mode, performed using a Thermo Q-Exactive mass spectrometer. All MS/MS datasets were processed using the DFF module and screened for diagnostic product ions at m/z 130.0648 and 185.0704 for chaetoglobosins, and m/z 120.0808 and 146.0598 for cytochalasins.

RESULTS

Extracts of C. globosum and P. cf. discolor strains revealed different mixtures of chaetoglobosins, whereas the Xylaria sp. produced only cytochalasins; none of the strains studied produced both classes of compounds. The dominant chaetoglobosins produced by both C. globosum and P. cf. discolor were chaetoglobosins A, C, and F. Tetrahydrochaetoglobosin A was identified from P. cf. discolor extracts and is reported here for the first time as a natural product. The major cytochalasins produced by the Xylaria sp. were cytochalasin D and epoxy cytochalasin D. A larger unknown "cytochalasin-like" molecule with the molecular formula C₃₈ H₄₇ NO₁₀ was detected from Xylaria sp. culture filtrate extracts and is a current target for isolation and structural characterization.

CONCLUSIONS

DFF is an effective LC/MS data analysis approach for rapidly identifying entire classes of compounds from complex mixtures. DFF has proved useful in the identification of new natural products and allowing for their partial characterization without the need for isolation.

The potential of compounds isolated from Xylaria spp. as antifungal agents against anthracnose

Anthracnose is a crop disease usually caused by fungi in the genus Colletotrichum or Gloeosporium. These are considered one of the main pathogens, causing significant economic losses, such as in peppers and guarana. The current forms of control include the use of resistant cultivars, sanitary pruning and fungicides. However, even with the use of some methods of controlling these cultures, the crops are not free of anthracnose. Additionally, excessive application of fungicides increases the resistance of pathogens to agrochemicals and cause harm to human health and the environment. In order to find natural antifungal agents against guarana anthracnose, endophytic fungi were isolated from Amazon guarana. The compounds piliformic acid and cytochalasin D were isolated by chromatographic techniques from two Xylaria spp., guided by assays with Colletotrichum gloeosporioides. The isolated compounds were identified by spectrometric techniques, as NMR and mass spectrometry. This is the first report that piliformic acid and cytochalasin D have antifungal activity against C. gloeosporioides with MIC 2.92 and 2.46 μmol mL⁻¹ respectively. Captan and difenoconazole were included as positive controls (MIC 16.63 and 0.02 μmol mL⁻¹, respectively). Thus, Xylaria species presented a biotechnological potential and production of different active compounds which might be promising against anthracnose disease.

The Ste20 kinases SPAK and OSR1 travel between cells through exosomes

Proteomics studies have identified Ste20-related proline/alanine-rich kinase (SPAK) and oxidative stress response 1 (OSR1) in exosomes isolated from body fluids such as blood, saliva, and urine. Because proteomics studies likely overestimate the number of exosome proteins, we sought to confirm and extend this observation using traditional biochemical and cell biology methods. We utilized HEK293 cells in culture to verify the packaging of these Ste20 kinases in exosomes. Using a series of centrifugation and filtration steps of conditioned culture medium isolated from HEK293 cells, we isolated nanovesicles in the range of 40-100 nm. We show that these small vesicles express the tetraspanin protein CD63 and lack endoplasmic reticulum and Golgi markers, consistent with these being exosomes. We show by Western blot and immunogold analyses that these exosomes express SPAK, OSR1, and Na-K-Cl cotransporter 1 (NKCC1). We show that exosomes are not only secreted by cells, but also accumulated by adjacent cells. Indeed, exposing cultured cells to exosomes produced by other cells expressing a fluorescently labeled kinase resulted in the kinase finding its way into the cytoplasm of these cells, consistent with the idea of exosomes serving as cell-to-cell communication vessels. Similarly, coculturing cells expressing different fluorescently tagged proteins resulted in the exchange of proteins between cells. In addition, we show that both SPAK and OSR1 kinases entering cells through exosomes are preferentially expressed at the plasma membrane and that the kinases in exosomes are functional and maintain NKCC1 in a phosphorylated state.

Role of cytoskeleton network in anisosmotic volume changes of intact and permeabilized A549 cells

Recently we found that cytoplasm of permeabilized mammalian cells behaves as a hydrogel displaying intrinsic osmosensitivity. This study examined the role of microfilaments and microtubules in the regulation of hydrogel osmosensitivity, volume-sensitive ion transporters, and their contribution to volume modulation of intact cells. We found that intact and digitonin-permeabilized A549 cells displayed similar rate of shrinkage triggered by hyperosmotic medium. It was significantly slowed-down in both cell preparations after disruption of actin microfilaments by cytochalasin B, suggesting that rapid water release by intact cytoplasmic hydrogel contributes to hyperosmotic shrinkage. In hyposmotic swelling experiments, disruption of microtubules by vinblastine attenuated the maximal amplitude of swelling in intact cells and completely abolished it in permeabilized cells. The swelling of intact cells also triggered ~10-fold elevation of furosemide-resistant (86)Rb+ (K+) permeability and the regulatory volume decrease (RVD), both of which were abolished by Ba2+. Interestingly, RVD and K+ permeability remained unaffected in cytocholasin/vinblastine treated cells demonstrating that cytoskeleton disruption has no direct impact on Ba2+-sensitive K+-channels involved in RVD. Our results show, for the first time, that the cytoskeleton network contributes directly to passive cell volume adjustments in anisosmotic media via the modulation of the water retained by the cytoplasmic hydrogel.

Ion channels involved in cell volume regulation: effects on migration, proliferation, and programmed cell death in non adherent EAT cells and adherent ELA cells

This mini review outlines studies of cell volume regulation in two closely related mammalian cell lines: nonadherent Ehrlich ascites tumour cells (EATC) and adherent Ehrlich Lettre ascites (ELA) cells. Focus is on the regulatory volume decrease (RVD) that occurs after cell swelling, the volume regulatory ion channels involved, and the mechanisms (cellular signalling pathways) that regulate these channels. Finally, I shall also briefly review current investigations in these two cell lines that focuses on how changes in cell volume can regulate cell functions such as cell migration, proliferation, and programmed cell death.

Physiology of cell volume regulation in vertebrates

The ability to control cell volume is pivotal for cell function. Cell volume perturbation elicits a wide array of signaling events, leading to protective (e.g., cytoskeletal rearrangement) and adaptive (e.g., altered expression of osmolyte transporters and heat shock proteins) measures and, in most cases, activation of volume regulatory osmolyte transport. After acute swelling, cell volume is regulated by the process of regulatory volume decrease (RVD), which involves the activation of KCl cotransport and of channels mediating K(+), Cl(-), and taurine efflux. Conversely, after acute shrinkage, cell volume is regulated by the process of regulatory volume increase (RVI), which is mediated primarily by Na(+)/H(+) exchange, Na(+)-K(+)-2Cl(-) cotransport, and Na(+) channels. Here, we review in detail the current knowledge regarding the molecular identity of these transport pathways and their regulation by, e.g., membrane deformation, ionic strength, Ca(2+), protein kinases and phosphatases, cytoskeletal elements, GTP binding proteins, lipid mediators, and reactive oxygen species, upon changes in cell volume. We also discuss the nature of the upstream elements in volume sensing in vertebrate organisms. Importantly, cell volume impacts on a wide array of physiological processes, including transepithelial transport; cell migration, proliferation, and death; and changes in cell volume function as specific signals regulating these processes. A discussion of this issue concludes the review.

Cell volume regulation: physiology and pathophysiology

Cell volume perturbation initiates a wide array of intracellular signalling cascades, leading to protective and adaptive events and, in most cases, activation of volume-regulatory osmolyte transport, water loss, and hence restoration of cell volume and cellular function. Cell volume is challenged not only under physiological conditions, e.g. following accumulation of nutrients, during epithelial absorption/secretion processes, following hormonal/autocrine stimulation, and during induction of apoptosis, but also under pathophysiological conditions, e.g. hypoxia, ischaemia and hyponatremia/hypernatremia. On the other hand, it has recently become clear that an increase or reduction in cell volume can also serve as a specific signal in the regulation of physiological processes such as transepithelial transport, cell migration, proliferation and death. Although the mechanisms by which cell volume perturbations are sensed are still far from clear, significant progress has been made with respect to the nature of the sensors, transducers and effectors that convert a change in cell volume into a physiological response. In the present review, we summarize recent major developments in the field, and emphasize the relationship between cell volume regulation and organism physiology/pathophysiology.

Microfilament dynamics during HaCaT cell volume regulation

Cell volume is an important parameter in many physiological processes, and is closely regulated in many cell types. In those cells, swelling induced by hypotonic media is followed by an ion-driven regulatory volume decrease. In many cell types, this regulatory volume decrease requires an intact actin cytoskeleton. Therefore, we investigated the changes in the structure and polymerization state of the actin cytoskeleton in HaCaT keratinocytes during cell swelling and regulatory volume decrease. Disruption of the actin cytoskeleton by 2microM cytochalasin D inhibits regulatory volume decrease in HaCaT cells. Cells swollen in the presence of low concentrations of cytochalasin D (0.8microM, 305-250mosM) keep the elevated volume even after cytochalasin D removal. A further decrease of tonicity (250-200mosM) is again counteracted by regulatory volume decrease reaching the volume, which has been established at 250mosM. In contrast, no visible changes occurred in actin cytoskeleton morphology of EGFP-actin-transfected HaCaT cells during swelling or regulatory volume decrease. However, biochemical analysis showed an increase in total F-actin levels 90s after the onset of hypotonicity. The ratio of Triton-soluble to -insoluble actin also increased after hypotonic shock, suggesting that the measured increase in F-actin is primarily due to de novo polymerization and formation of short actin filaments, i.e., actin oligomers. These results show that a rapid reorganization of the actin cytoskeleton takes place after hypotonic treatment. This reorganization can influence signaling in response to hypotonicity either indirectly by means of sequestering or releasing actin-associated proteins, or directly by the interaction of short actin filaments with plasma membrane ion channels, and may be involved in determining a new volume set point.

KCNQ channels are involved in the regulatory volume decrease response in primary neonatal rat cardiomyocytes

Cardiomyocytes may experience significant cell swelling during ischemia and reperfusion. Such changes in cardiomyocyte volume have been shown to affect the electrical properties of the heart, possibly leading to cardiac arrhythmia. In the present study the regulatory volume decrease (RVD) response of neonatal rat cardiomyocytes was studied in intact single cells attached to coverslips, i.e. with an intact cytoskeleton. The potential contribution of KCNQ (Kv7) channels to the RVD response and the possible involvement of the F-actin cytoskeleton were investigated. The rate of RVD was significantly inhibited in the presence of the KCNQ channel blocker XE-991 (10 and 100 microM). Electrophysiological experiments confirmed the presence of an XE-991 sensitive current and Western blotting analysis revealed that KCNQ1 channel protein was present in the neonatal rat cardiomyocytes. Hypoosmotic cell swelling changes the structure of the F-actin cytoskeleton, leading to a more rounded cell shape, less pronounced F-actin stress fibers and patches of actin. In the presence of cytochalasin D (1 microM), a potent inhibitor of actin polymerization, the RVD response was strongly reduced, confirming a possible role for an intact F-actin cytoskeleton in linking cell swelling to activation of ion transport in neonatal rat cardiomyocytes.

Shrinkage insensitivity of NKCC1 in myosin II-depleted cytoplasts from Ehrlich ascites tumor cells

Protein phosphorylation/dephosphorylation and cytoskeletal reorganization regulate the Na(+)-K(+)-2Cl(-) cotransporter (NKCC1) during osmotic shrinkage; however, the mechanisms involved are unclear. We show that in cytoplasts, plasma membrane vesicles detached from Ehrlich ascites tumor cells (EATC) by cytochalasin treatment, NKCC1 activity evaluated as bumetanide-sensitive (86)Rb influx was increased compared with the basal level in intact cells yet could not be further increased by osmotic shrinkage. Accordingly, cytoplasts exhibited no regulatory volume increase after shrinkage. In cytoplasts, cortical F-actin organization was disrupted, and myosin II, which in shrunken EATC translocates to the cortical region, was absent. Moreover, NKCC1 activity was essentially insensitive to the myosin light chain kinase (MLCK) inhibitor ML-7, a potent blocker of shrinkage-induced NKCC1 activity in intact EATC. Cytoplast NKCC1 activity was potentiated by the Ser/Thr protein phosphatase inhibitor calyculin A, partially inhibited by the protein kinase A inhibitor H89, and blocked by the broad protein kinase inhibitor staurosporine. Cytoplasts exhibited increased protein levels of NKCC1, Ste20-related proline- and alanine-rich kinase (SPAK), and oxidative stress response kinase 1, yet they lacked the shrinkage-induced plasma membrane translocation of SPAK observed in intact cells. The basal phosphorylation of p38 mitogen-activated protein kinase (p38 MAPK) was increased in cytoplasts compared with intact cells, yet in contrast to the substantial activation in shrunken intact cells, p38 MAPK could not be further activated by shrinkage of the cytoplasts. Together these findings indicate that shrinkage activation of NKCC1 in EATC is dependent on the cortical F-actin network, myosin II, and MLCK.

Cholesterol modulates the volume-regulated anion current in Ehrlich-Lettre ascites cells via effects on Rho and F-actin

The mechanisms controlling the volume-regulated anion current (VRAC) are incompletely elucidated. Here, we investigate the modulation of VRAC by cellular cholesterol and the potential involvement of F-actin, Rho, Rho kinase, and phosphatidylinositol-(4,5)-bisphosphate [PtdIns(4,5)P2] in this process. In Ehrlich-Lettre ascites (ELA) cells, a current with biophysical and pharmacological properties characteristic of VRAC was activated by hypotonic swelling. A 44% increase in cellular cholesterol content had no detectable effects on F-actin organization or VRAC activity. A 47% reduction in cellular cholesterol content increased cortical and stress fiber-associated F-actin content in swollen cells. Cholesterol depletion increased VRAC activation rate and maximal current after a modest (15%), but not after a severe (36%) reduction in extracellular osmolarity. The cholesterol depletion-induced increase in maximal VRAC current was prevented by F-actin disruption using latrunculin B (LB), while the current activation rate was unaffected by LB, but dependent on Rho kinase. Rho activity was decreased by ∼20% in modestly, and ∼50% in severely swollen cells. In modestly swollen cells, this reduction was prevented by cholesterol depletion, which also increased isotonic Rho activity. Thrombin, which stimulates Rho and causes actin polymerization, potentiated VRAC in modestly swollen cells. VRAC activity was unaffected by inclusion of a water-soluble PtdIns(4,5)P2analogue or a PtdIns(4,5)P2-blocking antibody in the pipette, or neomycin treatment to sequester PtdIns(4,5)P2. It is suggested that in ELA cells, F-actin and Rho-Rho kinase modulate VRAC magnitude and activation rate, respectively, and that cholesterol depletion potentiates VRAC at least in part by preventing the hypotonicity-induced decrease in Rho activity and eliciting actin polymerization.

Lymphocyte calcium signaling from membrane to nucleus

Ca(2+) signals control a variety of lymphocyte responses, ranging from short-term cytoskeletal modifications to long-term changes in gene expression. The identification of molecules and channels that modulate Ca(2+) entry into T and B lymphocytes has both provided details of the molecular events leading to immune responses and raised controversy. Here we review studies of the pathways that allow Ca(2+) entry, the function of Ca(2+) in the regulation of cell polarity and motility and the principles by which Ca(2+)-dependent transcription regulates lymphocyte function.

Using jasplakinolide to turn on pathways that enable the isolation of new chaetoglobosins from Phomospis asparagi

The isolation and structure elucidation of three new secondary metabolites, chaetoglobosin-510 (1), -540 (2), and -542 (3), are described. These compounds were produced by cultures of the marine-derived fungus Phomopsis asparagi, challenged with the known F-actin inhibitor jasplakinolide. Chaetoglobosin-542 (3) displayed antimicrofilament activity and was cytotoxic toward murine colon and leukemia cancer cell lines.

Characterization of myosin-II binding to Golgi stacks in vitro

In addition to important roles near the actin-rich cell cortex, ample evidence indicates that multiple myosins are also involved in membrane movements in the endomembrane system. Nonmuscle myosin-II has been shown to have roles in anterograde and retrograde trafficking at the Golgi. Myosin-II is present on Golgi stacks isolated from intestinal epithelial cells and has been localized to the Golgi in several polarized and unpolarized cell lines. An understanding of roles of myosin-II in Golgi physiology will be facilitated by understanding the molecular arrangement of myosin-II at the Golgi. Salt-washing removes endogenous myosin-II from isolated Golgi and purified brush border myosin-II can bind in vitro. Brush border myosin-II binds to a tightly bound Golgi peripheral membrane protein with a K(1/2) of 75 nM and binding is saturated at 0.7 pmol myosin/microg Golgi. Binding studies using papain cleavage fragments of brush border myosin-II show that the 120-kDa rod domain, but not the head domain, of myosin heavy chain can bind directly to Golgi stacks. The 120-kDa domain does not bind to Golgi membranes when phosphorylated in vitro with casein kinase-II. These results suggest that phosphorylation in the rod domain may regulate the binding and/or release of myosin-II from the Golgi. These data support a model in which myosin-II is tethered to the Golgi membrane by its tail and actin filaments by its head. Thus, translocation along actin filaments may extend Golgi membrane tubules and/or vesicles away from the Golgi complex.

Isoproterenol induces actin depolymerization in human airway smooth muscle cells via activation of an Src kinase and GS

In a previous study, we showed that isoproterenol induced actin depolymerization in human airway smooth muscle cells by both protein kinase A (PKA)-dependent and -independent signaling pathways. We now investigate the signaling pathway of PKA-independent actin depolymerization induced by isoproterenol in these cells. Cells were briefly exposed to isoproterenol or PGE1 in the presence and absence of specific inhibitors of Src-family tyrosine kinases, phosphatidylinositol-3-kinase (PI3 kinase), or MAP kinase, and actin depolymerization was measured by concomitant staining of filamentous actin with FITC-phalloidin and globular actin with Texas red DNase I. Isoproterenol, cholera toxin, and PGE1 induced actin depolymerization, indicated by a decrease in the intensity of filamentous/globular fluorescent staining. Pretreatment with the Src kinase inhibitors 4-amino-5-(4-chlorophenyl)-7-( t-butyl)pyrazolo[3,4-d]pyriimidine (PP2) or geldanamycin or the PKA inhibitor Rp-cAMPS only partly inhibited isoproterenol- or PGE1-induced actin depolymerization. In contrast, PP2 and geldanamycin did not inhibit forskolin-induced actin depolymerization, and AG-213 (an EGF receptor tyrosine kinase inhibitor) did not inhibit isoproterenol- or PGE1-induced actin depolymerization. PI3 kinase or MAP kinase inhibition did not inhibit isoproterenol-induced actin depolymerization. Moreover, isoproterenol but not forskolin induced tyrosine phosphorylation of an Src family member at position 416. These results further confirm that both PKA-dependent and PKA-independent pathways mediate actin depolymerization in human airway smooth muscle cells and that the PKA-independent pathway by which isoproterenol induces actin depolymerization in human airway smooth muscle cells involves Src protein tyrosine kinases and the Gs protein.

Curcumin inhibits cell motility and alters microfilament organization and function in prostate cancer cells

Curcumin is a dietary phytochemical associated with anti-tumorigenic effects, but the mechanisms by which it inhibits cancer cell growth and metastasis are not completely understood. For example, little information is available regarding the effects of curcumin on cytoskeletal organization and function. In this study, time-lapse video and immunofluorescence labeling methods were used to demonstrate that curcumin significantly alters microfilament organization and cell motility in PC-3 and LNCaP human prostate cancer cells in vitro. Curcumin rapidly arrests cell movements and subsequently alters cell shape in the highly motile PC-3 cell line, but has a less noticeable effect on the relatively immobile LNCaP cell line. Stress fibers are augmented, and the overall quantity of f-actin appears to increase in both types of cells following curcumin treatment. Cytochalasin B (CB) disrupts microfilament organization in both cell lines, and causes vigorous membrane blebbing in PC-3 cells, but not LNCaP cells. Pre-treatment of cells with curcumin suppresses changes in microfilament organization caused by CB, and blocks PC-3 membrane blebbing. At least some of the effects of curcumin appear to be mediated by protein kinase C (PKC), as treatment with the PKC inhibitor bisindolylmaleimide inhibits the ability of curcumin to block CB-induced membrane blebbing. These findings demonstrate that curcumin exerts significant effects on the actin cytoskeleton in prostate cancer cells, including altering microfilament organization and function. This is a novel observation that may represent an important mechanism by which curcumin functions as a chemopreventative agent, and as an inhibitor of angiogenesis and metastasis.

Involvement of actin in the electrotonic conductivity of mixed synapses in Mauthner neurons in the goldfish

The effects of phalloidin, a preperation which highly specifically and selectively polymerizes actin and which binds to actin, on the electrotonic conductivity and structure of mixed synapses were studied in goldfish Mauthner neurons (MN). These experiments showed that paired subthreshold electrical stimulation of the afferent input in the presence of phalloidin led to increases in the amplitude of MN responses to the second stimulus by an average of 80%. In controls, this amplitude increased by only 10% and only when suprathreshold stimuli were used, while subthreshold stimuli were ineffective. We regard these results as demonstrating increases in the conductivity of mixed synapses, this being induced by polymerization of actin. At the ultrastructural level, application of phalloidin to MN and their mixed synapses induced increases in the sizes and numbers of actin-containing desmosome-like contacts, and in the numbers of fibrillar bridges in the clefts of these contacts. Use of colloidal gold as a label for phalloidin demonstrated that bridges were made of actin. The interdependent morphofunctional changes seen in mixed synapses provide grounds for suggesting a role for actin in the conduction of the electrotonic signal through mixed synapses. The structural substrate for this process may be provided by bridges in the clefts of desmosome-like contacts.

Cell swelling activates cloned Ca(2+)-activated K(+) channels: a role for the F-actin cytoskeleton

Cloned Ca(2+)-activated K(+) channels of intermediate (hIK) or small (rSK3) conductance were expressed in HEK 293 cells, and channel activity was monitored using whole-cell patch clamp. hIK and rSK3 currents already activated by intracellular calcium were further increased by 95% and 125%, respectively, upon exposure of the cells to a 33% decrease in extracellular osmolarity. hIK and rSK3 currents were inhibited by 46% and 32%, respectively, by a 50% increase in extracellular osmolarity. Cell swelling and channel activation were not associated with detectable increases in [Ca(2+)](i), evidenced by population and single-cell measurements. In addition, inhibitors of IK and SK channels significantly reduced the rate of regulatory volume decrease (RVD) in cells expressing these channels. Cell swelling induced a decrease, and cell shrinkage an increase, in net cellular F-actin content. The swelling-induced activation of hIK channels was strongly inhibited by cytochalasin D (CD), in concentrations that caused depolymerization of F-actin filaments, indicating a role for the F-actin cytoskeleton in modulation of hIK by changes in cell volume. In conclusion, hIK and rSK3 channels are activated by cell swelling and inhibited by shrinkage. A role for the F-actin cytoskeleton in the swelling-induced activation of hIK channels is suggested.

The actin cytoskeleton and cytotoxic T lymphocytes: evidence for multiple roles that could affect granule exocytosis-dependent target cell killing

One important mechanism cytotoxic T lymphocytes (CTLs) use to kill virus-infected, transplanted or tumour targets is exocytosis of granules that contain cytotoxic agents such as perforin and granzymes. Granule exocytosis-dependent target cell killing is a complex process, involving initial T-cell receptor (TCR)-dependent signalling that includes Ca2+ influx and activation of protein kinase C, shape changes that serve to bind the CTL to the target and, finally, exocytosis of lytic granules at the site of contact with the target cell. Although there is reason to propose that multiple steps in the lytic process could involve the actin cytoskeleton of CTLs, few studies have examined this issue, and those that have do not allow the specific step(s) involved to be determined. We have used the potent membrane-permeant actin cytoskeleton-modifying drugs jasplakinolide and latrunculin A to investigate the actin dependence of defined processes that are expected to be important for granule exocytosis-dependent killing. Our results, obtained using TALL-104 human leukaemic CTLs as a model system, are consistent with the idea that a functional actin cytoskeleton is required for TCR/CD3-dependent signalling, for activation of store-dependent Ca2+ influx and for CTL shape changes. When cells were stimulated with solid-phase anti-CD3 antibodies, treatment with either jasplakinolide or latrunculin A abolished granule exocytosis. However, when cells were stimulated in a manner that bypasses TCR/CD3-dependent signalling, granule exocytosis was not significantly altered, suggesting that the actin cytoskeleton does not function as a barrier to exocytosis.

Decreased actin solubility observed during ATP-depletion is mimicked by severing agents but not depolymerizing agents in isolated and cultured proximal tubular cells

The microvilli of the apical membrane of proximal tubule (PT) cells are supported by the underlying actin cytoskeleton. Ischaemic or anoxic ATP-depletion leads to the disruption of the actin cytoskeleton, resulting in microvillar retraction and loss of membrane polarity. Using isolated PT cells, we have previously demonstrated that actin filaments (F-actin) are likely severed during ATP-depletion. A sequential extraction protocol revealed a decrease in actin solubility, resulting in the sequestration of a distinct F-actin pool with the insoluble cellular complex in ATP-depleted PT cells. We demonstrate here that decreased actin solubility is not only a reliable end-marker of ATP-depletion induced injury in freshly isolated PT cells, but also serves as a biochemical marker in the cultured proximal tubular cell line LLC-PK1. In the present studies, we also investigated specific actin-binding drugs to determine if they mimic the effects observed during energy depletion. Jasplakinolide (JP), a compound which binds F-actin and prevents depolymerization, did not effect actin solubility during ATP-depletion. Furthermore, swinholide A (SA), an F-actin severing agent, resulted in decreased actin solubility, mimicking the effects of ATP-depletion. Interestingly, latrunculin A (LA), an agent which depolymerizes F-actin, did not reduce actin solubility, but rather resulted in an increase in digitonin-soluble actin. Taken collectively, our results support previous work and suggest that disruption of the actin cytoskeleton during ATP-depletion is mediated by F-actin severing/fragmentation and not depolymerization. The differential effects of F-actin disrupting agents and the consistencies observed in both models of ischaemic injury will provide a basis for a more detailed understanding of the pathological events of PT-cell dysfunction.

Possible interrelationship between changes in F-actin and myosin II, protein phosphorylation, and cell volume regulation in Ehrlich ascites tumor cells

Osmotic shrinkage of Ehrlich ascites tumor cells (EATC) elicited translocation of myosin II from the cytosol to the cortical region, and swelling elicits concentration of myosin II in the Golgi region. Rho kinase and p38 both appeared to be involved in shrinkage-induced myosin II reorganization. In contrast, the previously reported shrinkage-induced actin polymerization [Pedersen et al. (1999) Exp. Cell Res. 252, 63-74] was independent of Rho kinase, p38, myosin light chain kinase (MLCK), and protein kinase C (PKC), which thus do not exert their effects on the shrinkage-activated transporters via effects on F-actin. The subsequent F-actin depolymerization, however, appeared MLCK- and PKC-dependent, and the initial swelling-induced F-actin depolymerization was MLCK-dependent; both effects were apparently secondary to kinase-mediated effects on cell volume changes. NHE1 in EATC is activated both by osmotic shrinkage and by the serine/threonine phosphatase inhibitor Calyculin A (CL-A). Both stimuli caused Rho kinase-dependent myosin II relocation to the cortical cytoplasm, but in contrast to the shrinkage-induced F-actin polymerization, CL-A treatment elicited a slight F-actin depolymerization. Moreover, Rho kinase inhibition did not significantly affect NHE1 activation, neither by shrinkage nor by CL-A. Implications for the possible interrelationship between changes in F-actin and myosin II, protein phosphorylation, and cell volume regulation are discussed.

Actin depolymerization via the beta-adrenoceptor in airway smooth muscle cells: a novel PKA-independent pathway

Actin is a major functional and structural cytoskeletal protein that mediates such diverse processes as motility, cytokinesis, contraction, and control of cell shape and polarity. While many extracellular signals are known to mediate actin filament polymerization, considerably less is known about signals that mediate depolymerization of the actin cytoskeleton. Human airway smooth muscle cells were briefly exposed to isoproterenol, forskolin, or the cAMP-dependent protein kinase A (PKA) agonist stimulatory diastereoisomer of adenosine 3',5'-cyclic monophosphate (Sp-cAMPS). Actin polymerization was measured by concomitant staining of filamentous actin with FITC-phalloidin and globular actin with Texas red DNase I. Isoproterenol, forskolin, or Sp-cAMPS induced actin depolymerization, indicated by a decrease in the intensity of filamentous/globular fluorescent staining. The PKA inhibitor Rp diastereomer of adenosine 3',5'-cyclic monophosphothioate (Rp-cAMPS) completely inhibited forskolin-stimulated depolymerization, whereas it only partially inhibited isoproterenol-induced depolymerization. The protein tyrosine kinase inhibitors genistein or tyrphostin A23 also partially inhibited isoproterenol-induced actin depolymerization. In contrast, the combination of Rp-cAMPS and either tyrosine kinase inhibitor had an additive effect at inhibiting isoproterenol-induced actin depolymerization. These results suggest that both PKA-dependent and -independent pathways mediate actin depolymerization in human airway smooth muscle cells.

Inhibition of geranylgeranylation blocks agonist-induced actin reorganization in human airway smooth muscle cells

To determine whether RhoA isoprenylation (geranylgeranylation) is required for agonist-induced actin cytoskeleton reorganization (measured by an increase in the filamentous F- to monomeric G-actin ratio), human airway smooth muscle cells were treated for 72 h with inhibitors of geranylgeranyltransferase I. Geranylgeranyltransferase inhibitor (GGTI)-2147 or -286 pretreatment completely blocked the increase in the F- to G-actin fluorescence ratio when cells were stimulated with lysophosphatidic acid (LPA), endothelin, or carbachol. In contrast, LPA or endothelin induced actin cytoskeletal reorganization in cells treated with farnesyltransferase inhibitor (FTI)-277 to inactivate Ras. Forskolin-induced adenylyl cyclase activity was inhibited by carbachol in GGTI-2147-pretreated cells, demonstrating that the effect of geranylgeranyltransferase I inhibition on stress fiber formation was not due to uncoupling of signaling between the heterotrimeric G(i) protein (the Ggamma subunit is isoprenylated) and distal effectors. These results demonstrate that selective GGTIs can inhibit agonist-induced actin reorganization.

The cytoskeleton and cell volume regulation

Although the precise mechanisms have yet to be elucidated, early events in osmotic signal transduction may involve the clustering of cell surface receptors, initiating downstream signaling events such as assembly of focal adhesion complexes, and activation of, e.g. Rho family GTPases, phospholipases, lipid kinases, and tyrosine- and serine/threonine protein kinases. In the present paper, we briefly review recent evidence regarding the possible relation between such signaling events, the F-actin cytoskeleton, and volume-regulatory membrane transporters, focusing primarily on our own work in Ehrlich ascites tumer cells (EATC). In EATC, cell shrinkage is associated with an increase, and cell swelling with a decrease in F-actin content, respectively. The role of the F-actin cytoskeleton in cell volume regulation in various cell types has largely been investigated using cytochalasins to disrupt F-actin and highly varying effects have been reported. Findings in EATC show that the effect of cytochalasin treatment cannot always be assumed to be F-actin depolymerization, and that, moreover, there is no well-defined correlation between effects of cytochalasins on F-actin content and their effects on F-actin organization and cell morphology. At a concentration verified to depolymerize F-actin, cytochalasin B (CB), but not cytochalasin D (CD), inhibited the regulatory volume decrease (RVD) and regulatory volume increase (RVI) processes in EATC. This suggests that the effect of CB is related to an effect other than F-actin depolymerization, possibly its F-actin severing activity.

Oxytocin and lysophosphatidic acid induce stress fiber formation in human myometrial cells via a pathway involving Rho-kinase

The actin cytoskeleton is important for stress fiber formation and contributes to the initiation and maintenance of smooth muscle contraction. To determine if oxytocin and lysophosphatidic acid (LPA) induce stress fiber formation, cultured human myometrial cells were exposed to oxytocin (10(-5) M) or LPA (10(-6) M), and filamentous (F) and globular (G) actin pools were stained with fluorescein isothiocyanate-phalloidin and Texas red DNase I, respectively. The F- to G-actin fluorescent-staining ratio was measured by fluorescence microscopy. Oxytocin and LPA increased stress fiber formation, as indicated by an increase in the F- to G-actin fluorescent-staining ratio. The Rho-kinase inhibitor Y-27632 markedly attenuated this increase. Oxytocin-induced stress fiber formation was completely inhibited in the presence of the oxytocin antagonist compound VI. Tyrosine kinase inhibition with tyrphostin A23 partially blocked the increase induced by oxytocin but had no effect on LPA-induced stress fiber formation. Stress fiber formation was not blocked by pertussis toxin, mitogen-activated protein kinase, or protein kinase C inhibitors. Our results show that human myometrial cells respond to oxytocin and LPA with the formation of stress fibers that may be involved in the maintenance of uterine contractions. Rho-kinase appears to be a key signaling factor in this pathway.