Hormone-induced actin polymerization in rat hepatoma cells and human leucocytes

Vascular hypertrophy-associated hypertension of profilin1 transgenic mouse model leads to functional remodeling of peripheral arteries

Increased mechanical stress/hypertension in the vessel wall triggers the hypertrophic signaling pathway, resulting in structural remodeling of vasculature. Vascular hypertrophy of resistance vessels leads to reduced compliance and elevation of blood pressure. We showed before that increased expression of profilin1 protein in the medial layer of the aorta induces stress fiber formation, triggering the hypertrophic signaling resulting in vascular hypertrophy and, ultimately, hypertension in older mice. Our hypothesis is that profilin1 induced vascular hypertrophy in resistance vessels, which led to elevation of blood pressure, both of which contributed to the modulation of vascular function. Our results showed significant increases in the expression of alpha(1)- and beta(1)-integrins (280 + or - 6.3 and 325 + or - 7.4%, respectively) and the activation of the Rho/Rho-associated kinase (ROCK) II pathway (260 and 350%, respectively, P < 0.05) in profilin1 mesenteric arteries. The activation of Rho/ROCK led to the inhibition of endothelial nitric oxide synthase expression (39 + or - 5.4%; P < 0.05) and phosphorylation (35 + or - 4.5%; P < 0.05) but also an increase in myosin light chain 20 phosphorylation (372%, P < 0.05). There were also increases in hypertrophic signaling pathways in the mesenteric arteries of profilin1 mice such as phospho-extracellular signal-regulated kinase 1/2 and phospho-c-Jun NH(2)-terminal kinase (312.15 and 232.5%, respectively, P < 0.05). Functional analyses of mesenteric arteries toward the vasoactive drugs were assessed using wire-myograph and showed significant increases in the vascular responses of profilin1 mesenteric arteries toward phenylephrine, but significant decreases in response toward ROCK inhibitor Y-27632, ACh, sodium nitrite, and cytochalasin D. The changes in vascular responses in the mesenteric arteries of profilin1 mice are due to vascular hypertrophy and the elevation of blood pressure in the profilin1 transgenic mice.

Regulation of contractility by Hsp27 and Hic-5 in rat mesenteric small arteries

The regulation of small artery contractility by vasoconstrictors is important for vascular function, and actin cytoskeleton remodeling is required for contraction. p38 MAPK and tyrosine kinases are implicated in actin polymerization and contraction through heat shock protein 27 (Hsp27) and the cytoskeletal protein paxillin, respectively. We evaluated the roles of downstream targets of p38 MAPK and tyrosine kinases in cytoskeletal reorganization and contraction and whether the two signaling pathways regulate contraction independent of each other. We identified the expression of the paxillin homologue hydrogen peroxide-inducible clone-5 (Hic-5) and showed its activation by norepinephrine (NE) in a Src-dependent manner. Furthermore, we demonstrated a NE-induced interaction of proline-rich tyrosine kinase-2 (PYK2) but not Src or p125 focal adhesion kinase with Hic-5. This interaction was Src dependent, suggesting that Hic-5 was a substrate for PYK2 downstream from Src. The activation of Hic-5 induced its relocalization to the cytosol. The parallel activation of Hsp27 by NE was p38 MAPK dependent and led to its dissociation from actin filaments and translocation from membrane to cytosol and increased actin polymerization. Both Hsp27 and Hic-5 activation resulted in their association within the same time frame as NE-induced contraction, and the inhibition of either p38 MAPK or Src inhibited the interaction between Hsp27 and Hic-5 and the contractile response. Furthermore, combined p38 MAPK and Src inhibition had no greater effect on contraction than individual inhibition, suggesting that the two pathways act through a common mechanism. These data show that NE-induced activation and the association of Hsp27 and Hic-5 are required for the reorganization of the actin cytoskeleton and force development in small arteries.

The adapter protein CrkII regulates neuronal Wiskott-Aldrich syndrome protein, actin polymerization, and tension development during contractile stimulation of smooth muscle

Actin polymerization has been shown to occur in tracheal smooth muscle tissues and cells in response to contractile stimulation, and there is evidence that the polymerization of actin is required for contraction. In tracheal smooth muscle, agonist-induced actin polymerization is mediated by activation of neuronal Wiskott-Aldrich syndrome protein (N-WASp) and the Arp (actin-related protein) 2/3 complex, and activation of the small GTPase Cdc42 regulates the activation of N-WASp. In the present study, the role of the adapter protein CrkII in the regulation of N-WASp and Cdc42 activation, actin polymerization, and tension development in smooth muscle tissues was evaluated. Stimulation of tracheal smooth muscle tissues with acetylcholine increased the association of CrkII with N-WASp. Plasmids encoding wild type CrkII or a CrkII mutant lacking the SH3 effector-binding ability, CrkII SH3N, were introduced into tracheal smooth muscle tissues, and the tissues were incubated for 2 days to allow for protein expression. Expression of the CrkII SH3N mutant in smooth muscle tissues inhibited the association of CrkII with N-WASp and the activation of Cdc42. The CrkII SH3N mutant also inhibited the increase in the association of N-WASp with Arp2, a major component of the Arp2/3 complex, in response to contractile stimulation, indicating inhibition of N-WASp activation. Expression of the CrkII SH3N mutant also inhibited tension generation and actin polymerization in response to contractile stimulation; however, it did not inhibit myosin light chain phosphorylation. These results suggest that CrkII plays a critical role in the regulation of N-WASp activation, perhaps by regulating the activation of Cdc42, and that it thereby regulates actin polymerization and active tension generation in tracheal smooth muscle. These studies suggest a novel signaling pathway for the regulation of N-WASp activation and active contraction in smooth muscle tissues.

Expression of non-phosphorylatable paxillin mutants in canine tracheal smooth muscle inhibits tension development

The adapter protein paxillin has been implicated in the regulation of cytoskeletal organization and cell motility. Paxillin undergoes tyrosine phosphorylation in response to the contractile stimulation of smooth muscle, and the depletion of paxillin by antisense inhibits smooth muscle contraction. In the present study, acetylcholine (ACh)-stimulation of tracheal smooth muscle tissues increased paxillin phosphorylation at tyr-31 and tyr-118 by three- to fourfold. The role of tyr-31 and tyr-118 phosphorylation of paxillin in smooth muscle was evaluated by introducing plasmids encoding wild type paxillin or paxillin mutants F31, F118 or F31/118 (phenylalanine substitution at tyrosine sites 31, 118) into tracheal smooth muscle strips by reversible permeabilization, and incubating the tissues for 2 days. The expression of recombinant proteins was confirmed by immunoblot and immunofluorescence analysis. Expression of the paxillin mutants F31, F118 or F31/118 inhibited the contractile response to ACh stimulation but did not inhibit the increase in myosin light chain phosphorylation. The expression of wild type paxillin had no significant affect on force or myosin light chain phosphorylation. ACh stimulation reduced G-actin/F-actin ratio in tissues expressing wild type paxillin; whereas the agonist-induced decrease in G-actin/F-actin was inhibited in strips expressing paxillin mutant F31/118. The paxillin mutant F31/118 showed a marked decrease in their interaction with the SH2/SH3 adaptor protein CrkII but not with vinculin or focal adhesion kinase. We conclude that paxillin phosphorylation at tyr-31 and tyr-118 regulates active tension development during contractile stimulation. Paxillin phosphorylation at these two sites may be important in regulating actin filament dynamics and organization during smooth muscle contraction.

Dexamethasone induces rapid actin assembly in human endometrial cells without affecting its synthesis

Dexamethasone exerts a stimulatory effect of rapid-onset on the polymerization of actin. This has been documented in human endometrial adenocarcinoma Ishikawa cells, resulting in an acute, dose-dependent decrease in the G/total-actin ratio. In the present study we completely characterized this fast and apparently nongenomic effect of dexamethasone on actin assembly. We followed the morphological alterations of actin cytoskeleton and measured the time-dependent dynamics of actin polymerization both by ruling out any changes of total actin in the cells and by measuring its transcript. Rapid changes in actin polymerization were accurately measured using a highly sensitive and quantitative rhodamine-phalloidin fluorimetric assay. Ishikawa cells, exposed to 0.1 microM dexamethasone for various time periods up to 24 h, showed a highly significant, rapid, and transient increase in the polymerization of actin starting within 15 min of dexamethasone exposure and lasting 2 h. Treated cells showed a significant (1.79-fold) enhancement of the fluorescent signal compared to untreated cells at 15 min. This value decreased continuously in a time-dependent manner, reaching control levels after 120 min and remained so for the next 24 h. Confocal laser scanning microscopy studies confirmed these findings. Intensive coloration of microfilaments over several scanning sections suggested an enhanced degree of actin polymerization in cells preincubated for 15 min with 0.1 microM dexamethasone. Moreover, actin filaments were more resistant to cytochalasin B. Additionally, quantitative immunoblot analysis showed that the content of total cellular actin remained the same during this period, suggesting that the biosynthesis of actin was unaffected. Northern blot analysis showed that the concentration of the actin transcript was also unaffected. Our data suggest that glucocorticoids induce a fast and self-limited polymerization of actin in human endometrial cells without affecting its synthesis. These findings strengthen the hypothesis that glucocorticoids exert rapid, nongenomic cellular effects and that the actin-based cytoskeleton is an integral part of this pathway, playing an essential role in receiving and mediating steroid signals for the modulation of cellular responses.

Microinjection of ADP-ribosylated actin inhibits actin synthesis in hepatocyte-hepatoma hybrid cells

Treatment of hepatocyte-hepatoma hybrid cells with Clostridium botulinum C2 toxin led to a 167% increase in monomeric globular actin (G-actin) and to a 57% decrease in filamentous actin (F-actin) within 2 h. Simultaneously, the level of actin mRNA was specifically decreased to 49% and actin synthesis was significantly diminished. In contrast, treatment of hybrid cells with phalloidin led to a decrease in G-actin to 55% and to a reciprocal increase in actin mRNA to 244% and an increase in actin synthesis. These alterations of actin synthesis depending on the G-actin/F-actin ratio corresponded to the autoregulation of actin synthesis observed in primary cultures of rat hepatocytes. Microinjection of C2 toxin or of phalloidin into hepatocyte-hepatoma hybrid cells had the same effects on actin synthesis as incubation with either toxin in the culture medium. Microinjection of nonpolymerizable ADP-ribosylated G-actin into hepatocyte-hepatoma hybrid cells specifically decreased the incorporation of [35S]methionine into newly synthesized actin within 1 h. This decrease continued for at least 19 h. Microinjection of ADP-ribosylated actin led to rounding of cells and obvious disaggregation of actin filaments, which might be due to capping of actin filaments by the ADP-ribosylated actin. Because stabilization of actin filaments by phalloidin before microinjection of ADP-ribosylated actin also resulted in decreased actin synthesis, the concentration of monomeric G-actin seems to be responsible for the regulation of actin synthesis in hepatocyte-hepatoma hybrid cells, which can be regarded as immortalized hepatocytes.

Dexamethasone alters rapidly actin polymerization dynamics in human endometrial cells: evidence for nongenomic actions involving cAMP turnover

Glucocorticoids, in addition to their well characterized effects on the genome, may affect cell function in a manner not involving genomic pathways. The mechanisms by which the latter is achieved are not yet clear. A possible means for this action may involve the actin cytoskeleton, since the dynamic equilibrium of actin polymerization changes rapidly following exposure to several stimuli, including hormones. The aim of the present work was to find out if glucocorticoids exert rapid, nongenomic effects on actin polymerization in Ishikawa human endometrial cells, which represent a well characterized in vitro cell model expressing functional glucocorticoid receptors. Short term exposure of the cells to the synthetic glucocorticoid dexamethasone resulted in an overall decrease of the G/total-actin ratio in a time- and dose-dependent manner. Specifically, in untreated Ishikawa cells the G/total-actin ratio was 0.48 +/- 0.01 (n = 26). It became 0.35 +/- 0.01 (n = 13, P < 0.01) following exposure to 10(-7) M dexamethasone for 15 min. This was induced by a significant decrease of the cellular G-actin level, without affecting the total actin content, indicating a rapid actin polymerization. This conclusion was fully confirmed by direct fluorimetry measurements, that showed a significant increase of the F-actin content by 44% (n = 6, P < 0.001) in cells treated with dexamethasone (10(-7)M, 15 min). The rapid dexamethasone-induced alterations of the state of actin polymerization were further supported by fluorescence microscopy. The latter studies showed that the microfilaments of cells pretreated with 10(-7)M dexamethasone for 15 min were more resistant to various concentrations of the antimicrofilament drug cytochalasin B, compared to untreated cells, implying microfilament stabilization. The action of dexamethasone on actin polymerization seems to be mediated via specific glucocorticoid binding sites, since the addition of the glucocorticoid antagonist RU486 completely abolished its effect. Moreover, it appears to act via non-transcriptional pathways, since actinomycin D did not block the dexamethasone-induced actin polymerization. In addition, cell treatment with 10(-7)M dexamethasone for 15 min fully reversed the forskolin-, but not the 8-bromo-cAMP-induced actin depolymerization. In line with these findings, the cAMP content of Ishikawa cells was decreased by 29.2% after a 15 min treatment with 10(-7)M dexamethasone (n = 4, P < 0.01). In conclusion, our results showed that dexamethasone induces rapid, time-, and dose-dependent changes in actin polymerization dynamics in Ishikawa cells. This action seems to be mediated via cAMP, involving probably nongenomic pathways. The above findings offer new perspectives for the understanding of the early cellular responses to glucocorticoids.

Characterization, cell-surface expression and ligand-binding properties of different truncated N-terminal extracellular domains of the ionotropic glutamate receptor subunit GluR1

To identify the location of the first transmembrane segment of the GluR1 glutamate receptor subunit artificial stop codons have been introduced into the N-terminal domain at amino acid positions 442, 510, and 563, namely just before and spanning the proposed first two transmembrane regions. The resultant truncated N-terminal fragments of GluR1, termed NT1, NT2, and NT3 respectively were expressed in Cos-7 cells and their cellular distribution and cell-surface expression analysed using an N-terminal antibody to GluR1. All of the fragments were fully glycosylated and were found to be associated with cell membranes but none was secreted. Differential extraction of the cell membranes indicated that both NT1 and NT2 behave as peripheral membrane proteins. In contrast NT3, like the full subunit, has integral membrane protein properties. Furthermore only NT3 is expressed at the cell surface as determined by immunofluorescence and cell-surface biotinylation. Protease protection assays indicated that only NT3 had a cytoplasmic tail. Binding studies using the selective ligand [(3)H]alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate ([(3)H]AMPA) demonstrated that NT3 does not bind ligand. Together these results indicate that the first transmembrane domain of the GluR1 subunit lies between residues 509 and 562, that the N-terminal domain alone cannot form a functional ligand-binding site and that this domain can be targeted to the cell surface provided that it has a transmembrane-spanning region.

Na+/Pi co-transport alters rapidly cytoskeletal protein polymerization dynamics in opossum kidney cells

We studied with biochemical and immunofluorescent techniques the interactions between the actin microfilament and tubulin microtubule cytoskeleton and Na+/P1 co-transport in opossum kidney cells, a line with proximal tubular characteristics. On brief (5 min) incubation of the cells with a low (0.1 mM) concentration of Pi, a rapid F-actin depolymerization takes place, which fails to occur in cells incubated under similar conditions with 1 mM Pi. The disassembly of actin microfilaments could be quantitatively expressed as a 33% increase in the ration of monomeric G-actin to polymerized F-actin (G/F-actin ration from 0.80 +/- 0.03 to 1.06 +/- 0.06, n = 28, P<0.01), owing to a significant decrease in the latter. Under these conditions microfilaments were also markedly destabilized, as shown by their diminished resistance to graded cytochalasin B concentrations. In addition, incubation of opossum kidney cells with low Pi concentrations (0.1 mM) resulted within 5 min in a substantial depolymerization of microtubules, shown by immunofluorescence microscopy and measured as a 70.9 +/- 6.9% (n = 11, P<0.01) decrement by immunoblot analysis. These changes, which occur only when extracellular Pi concentrations are kept low, seem to be related to a significant increase within 5 min in the rate of cellular Pi uptake by 25.5% under these conditions. The shifts in the dynamic equilibria between monomeric and polymerized actin and tubulin in response to cellular Pi uptake were transient, being fully reversible within 30 min. Moreover, the effect of Pi seemed to be specific because inhibition of its uptake by phosphonoformic acid blunted microtubular disassembly markedly. In contrast, measurement of Pi uptake in the presence of agents known to stabilize cytoskeletal structures showed a substantial decrease with phallacidin, which stabilized microfilaments, whereas the microtubule stabilizer taxol had no apparent effect. These results indicate that acute alterations in the polymerization dynamics and stability of both microfilaments and microtubules are involved in the modulation of Na+/Pi co-transport and suggest important cytoskeletal participation in proximal tubular transport functions.

Hormones and the cytoskeleton of animals and plants

It is often overlooked that a cell can exert its specific functions only after it has acquired a specific morphology: function follows form. The cytoskeleton plays an important role in establishing this form, and a variety of hormones can influence it. The cytoskeletal framework has also been shown to function in a variety of cellular processes, such as cell motility (important for behavior), migration (important for the interrelationship between the endocrine and immune systems, e.g., chemotaxis), intracellular transport of particles, mitosis and meiosis, maintenance of cellular morphology, spatial distribution of cell organelles (e.g., nucleus and Golgi system), cellular responses to membrane events (e.g., endocytosis and exocytosis), intracellular communication including conductance of electrical signals, localization of mRNA, protein synthesis, and--more specifically in plants--ordered cell wall deposition, cytoplasmic streaming, and spindle function followed by phragmoplast function. All classes of hormones seem to make use of the cytoskeleton, either during their synthesis, transport, secretion, degradation, or when influencing their target cells. In this review special attention is paid to cytoskeleton-mediated effects of selected hormones related to growth, transepithelial transport, steroidogenesis, thyroid and parathyroid functioning, motility, oocyte maturation, and cell elongation in plants.

Autoregulation of actin synthesis by physiological alterations of the G-actin level in hepatocytes

Hypotonic treatment of cultured rat hepatocytes significantly decreased the monomeric G-actin level by 18% after 120 min while the level of filamentous F-actin remained essentially unchanged. Simultaneously the level of cellular actin mRNA was increased by 53%. Incubation of hepatocytes for 120 min with the F-actin stabilizing toxin phalloidin from Amanita phalloides led to a decrease of G-actin by 70% and an increase of F-actin by 55%. Although the toxin dependent decrease of G-actin was much more pronounced than the decrease after hypotonic treatment, the increase of actin mRNA was similar under both conditions. Simultaneous treatment with hypotonic medium did not result in a further decrease of the G-actin level. On the other hand, the G-actin elevating C2 toxin from Clostridium botulinum completely blocked the effects of osmotic stress on G-actin and actin-mRNA content. The results demonstrate that already an essentially physiological decrease of G-actin without alterations of F-actin results in a substantial enhancement of the actin mRNA level, indicating the physiological significance of this autoregulation.

Insulin does not disrupt actin microfilaments, microtubules, and in vitro aortic endothelial wound repair

In the face of small denuding injuries, the endothelium undergoes a process of rapid repair involving actin microfilaments, microtubules, and centrosomes to reestablish an intact monolayer. Failure to maintain an intact endothelial monolayer is an important factor in the pathogenesis of the atherosclerotic plaque. It was hypothesized that increased susceptibility to atherosclerosis in diabetes mellitus may be, in part, due to delayed reendothelialization following endothelial injury. To test this, the effects of high insulin concentrations on the reendothelialization of small wounds were examined using an in vitro porcine aortic endothelial cell wound model. Elevated concentrations of insulin did not disrupt the confluent endothelial monolayer or alter endothelial cell shape. Insulin also did not induce detectable alterations in the distribution of microtubules and microfilaments in the confluent monolayer. High insulin did not reduce the extent of reendothelialization of a linear wound made in the confluent monolayer. Centrosomal reorientation was similar to that of control wounded cultures as was the reorganization of the microfilaments and microtubules. The data suggest that the atherogenic effects of hyperinsulinemia are not due to disruption of endothelial repair.

Microcompartmentation, metabolic channelling and carbohydrate metabolism

The inter-organelle cytoplasm of eukaryotic cells was once considered to be a homogeneous solution in which many of the enzymes of intermediary metabolism are soluble; however, advances in cell biology have revealed an intricate picture at the microscopic level of cytoplasm structure. Consequently, a great deal of constraint is required when extrapolating to the intact cell from enzyme studies in vitro, a point made frequently in the literature of the last decade or so. The idea of spatial organization is now accepted and covers a wide variety of local microenvironments and possibly localized metabolic channelling. The latter, although accepted as a phenomenon, is controversial in terms of its physiological significance. This review covers evidences showing that both glycolytic and glycogenolytic enzymes are microcompartmentalized. The potential significance of this compartmentation appears to involve metabolic chanelling, a process by which rearrangement of enzymes on a dynamic cytomatrix leads to "channels" in which metabolic substrates are passed from one enzyme to the next. The combined effects of such enzyme proximity and their activation as a result of the altered kinetic properties conferred upon the enzymes by their cytoskeletal associations favours maximal rate of reaction. These and other aspects of microcompartmentation and metabolic channelling are discussed.

Hepatocyte swelling leads to rapid decrease of the G-/total actin ratio and increases actin mRNA levels

Exposure of isolated rat hepatocytes to hypotonic (190 mosmol/l) incubation media lowered the cellular G-actin level without affecting the total actin content: here the G-/total actin ratio decreased by 15.5 +/- 1.4% (n = 7). Similar effects were observed following isotonic cell swelling by either addition of glutamine (10 mM) or insulin (100 nM), resulting in a decrease of the G-/total actin ratios by 13.5 +/- 2.1% (n = 5) and 14.1 +/- 1.1% (n = 11), respectively. The effects of hypotonic exposure, glutamine and insulin on the G-/total actin ratio largely occurred within 1 min and persisted for at least 2 h in presence of the respective effectors. After a 120 min exposure to hypotonic media, glutamine or insulin the actin mRNA levels were increased 2.4-, 2.0- and 3.6-fold, respectively. Hypertonic exposure lowered the G-/total actin ratio by only 4.9 +/- 2.5% (n = 4) and increased actin mRNA levels only 1.2-fold. There was a close relationship between glutamine- and hypotonicity-induced cell swelling and the decrease of G-/total actin ratios. The data suggest that cell swelling exerts rapid and marked effects on the state of actin polymerization and increases actin mRNA levels. Thus, cytoskeletal alterations in response to cell swelling may be involved in the regulation of hepatic metabolism by cell volume.

The effects of insulin, cycloheximide and phalloidin on the content of actin and p35 in extracts prepared from the nuclear fraction of Krebs II ascites cells

The nuclear fraction isolated from Krebs II ascites cells following cell disruption by nitrogen cavitation was separated into four fractions by salt/detergent extraction: NP-40 soluble fraction, 130 mM KCl extract, DOC/Triton x 100 soluble fraction and salt/detergent treated nuclei. The protein composition of the individual fractions was studied by SDS-PAGE and the relative amounts of actin and a 35 kDa protein (p35) were measured from gel scans. There was a time-dependent shift of actin from the 130 mM KCl extract to the NP-40 soluble fraction upon storage of the nuclear fraction on ice, indicating a progressive depolymerization of microfilaments. Compared with actin there was a slower release of p35 into the NP-40 soluble fraction. The results suggest that p35 is not integrated in the microfilament network. Phalloidin, which stabilizes the microfilaments, enriched the amount of both proteins in the 130 mM KCl extracts, together with a series of other proteins in the range 50-205 kDa. The presence of phalloidin also resulted in a large increase in the actin content in both the DOC/Triton x 100 extract and the fraction containing salt/detergent treated nuclei. Incubation of cells with insulin and/or cycloheximide enriched the amount of actin in the 130 mM KCl fraction. The results show that short term incubation of cells with phalloidin, insulin or cycloheximide increases the actin content of the nuclear fraction and also affects the presence of several other proteins.

Microenvironmental factors and the binding of glycolytic enzymes to contractile filaments

1. In reviewing the microenvironmental factors involved in the binding of the glycolytic enzymes to contractile filaments, consideration has been given to the significance of molecular crowding in maintaining these interactions under cellular conditions, and the influence of hormones, metabolites, pH and enzyme modifications on these phenomena. 2. Overall, these data serve to emphasize the biological reality of these associations, and their micro-organizational adaptations during physiological activities.

Insulin induces changes in the subcellular distribution of actin and 5'-nucleotidase

An increase in the amount of actin associated with the plasma membrane was visualized by immunocytochemistry 5 min after the addition of insulin to Krebs II ascites tumour cells maintained in serum-free medium. At 1 h of incubation the rim of fluorescence at the plasma membrane as measured by image analysis, was about 30% more intense than in control cells indicating that the initial accumulation of actin at the plasma membrane was not of a transient nature. Since an increase in the total cellular actin content in ascites cells did not occur until after a lag period of about 15 min then the increased amount of actin at the plasma membrane seen at 5 min was attributed to a stimulation of the polymerization of actin. An increase in the association of actin at the plasma membrane was also observed in 3T3 fibroblasts in areas of membrane ruffling, while in some cells there was also increased actin accumulation in the perinuclear area. The putative plasma membrane-microfilament linking protein 5'-nucleotidase was shown to be present in association with actin in the cytoskeletal fraction. Incubation of cells with insulin resulted in a shift of the enzyme toward the bottom of gradients indicating association with actin filaments of a greater length. The results demonstrate that insulin causes a stimulation of actin polymerization and that the hormone can be therefore assigned a role in the regulation of the cytoskeleton.

Reorganization of microfilaments in macrophages after LPS stimulation

Lipopolysaccharide (LPS), a potent activating substance of macrophages, induced the reorganization of microfilaments in macrophages obtained from C3H/HeN mice. At 1 min after LPS addition, a slight disassembly of actin was observed. At 2 to 4 min, there was a gradual assembly; then, at 5 and 6 min, a subsequent rapid disassembly occurred. We employed two methods to observe this process. One was the RITC-phalloidin staining of actin filaments and the other was the extraction of monomeric actin and unstable actin filaments with Triton X-100 solution. The results obtained by the two methods were basically in agreement. Nevertheless, there was a discrepancy between the results from the two methods, concerning the ratio of assembly and disassembly. The RITC-phalloidin staining was more sensitive in detecting actin assembly and less sensitive in detecting the disassembly than the extraction with Triton X-100 solution was. This difference suggests that some of the unstable filaments, which were extracted with Triton X-100 solution and fixed with formalin, were formed during the LPS-induced reorganization process. This reversible actin assembly could not be observed in the LPS-nonresponder, C3H/HeJ mouse macrophages. We concluded that the observed process could be attributed to LPS-signal triggering pathways subsequent to LPS binding and that a necessary component to initiate effective LPS-signaling, which is probably deficient in C3H/HeJ mice, is involved in this reorganization process of LPS-stimulated macrophages.

Actin cytoskeletal network in aging and cancer

The cytoskeleton is being recognized as an important modulator of metabolic functions of the cell. The actin cytoskeletal network, in particular, is involved in events regulating cell proliferation and differentiation. The state of actin in a variety of cell types is regulated by signals arising from the cell surface through a wide spectrum of interactions. In this review, we explore the role of actin cytoskeletal network in a series of events which are known to influence cell proliferation and differentiation. These include interaction of actin network with extracellular matrix proteins, cell surface membranes, second messengers, cytoplasmic enzymes and the nucleus. Because of the involvement of the actin network in such diverse interactions, we propose that alterations in the actin cytoskeletal function may be an important aspect of generalized decrease in cellular functions associated with aging. Preliminary data indicate that alterations in the cytoskeletal network do occur in cells obtained from older individuals. Alterations in actin state are also reported during malignant transformation of cells in culture, and in naturally occurring tumors. Taken together, the existing data seem to suggest that changes in the actin cytoskeletal network may be a part of the aging process as well as malignant transformation. Therefore, the study of the actin cytoskeletal network and its regulation has the potential to yield important information regarding cellular senescence and neoplastic transformation.

CTP:cholinephosphate cytidylyltransferase in human and rat lung: association in vitro with cytoskeletal actin

CTP:cholinephosphate cytidylyltransferase activities were compared in saline homogenates of immature fetal (15-16 weeks gestation) and adult human lung. There were no differences in subcellular enzyme distribution, in Vmax activity, or in the phosphatidylglycerol-mediated stimulation of soluble enzyme activity. These results provide no support for a developmental translocation of cytidylyltransferase from a cytosolic to a microsomal location in human lung, such as that proposed to accompany the maturation of pulmonary surfactant phosphatidylcholine biosynthesis in rat. Soluble cytidylyltransferase activity from human but not rat lung was increased after manipulation in vitro. Resolution of human H form (greater than 10(3) kDa) and L form (200 kDa) enzyme by gel filtration led to an activity increase of 200%. Incubation at 37 degrees C for 2 h increased soluble enzyme recovery, although prior centrifugal removal of generated actin-rich aggregates was necessary in adult lung fractions. In contrast, 85% of soluble rat lung cytidylyltransferase was actin aggregate-associated after incubation. The apparent heteroassociation of rat and human lung enzyme with actin in the presence of poly(ethylene glycol) at 4 degrees C strongly suggested close in vitro and potential in vivo linkage. A partial co-purification of adult human lung cytidylyltransferase with actin was also consistent with this idea. We propose that some reported cytidylyltransferase translocation phenomena may be mediated by cytoskeletal interactions in vitro.

Polymerization of additional actin is not required for capping of surface antigens in B-lymphocytes

CH12 is a murine B-cell lymphoma whose surface immunoglobulin (sIg) and concanavalin A (Con A) receptors patch and cap readily. Actin may be involved in CH12 patching and capping, since fodrin and F-actin collect under the cap, and cytochalasin D inhibits sIg capping. We have examined the state of the actin cytoskeleton during patching and capping. A wide range of concentrations of rabbit anti-mouse antibody (RAM) and Con A were used to patch or cap CH12 cells. G-actin was quantitated by DNase I inhibition, F-actin was quantitated by fluorescence-activated cell sorter analysis of fluorescent phalloidin staining, and actin nucleation sites were measured by pyrene actin polymerization. None of these methods detected any significant changes in actin when compared to control cells or untreated cells, leading us to conclude that increased actin polymerization is not necessary for capping to occur. The significance of these data to the membrane flow and cytoskeletal models of capping is discussed.

Reappraisal of AIDS--is the oxidation induced by the risk factors the primary cause?

The emergence of AIDS as a recognizable disease, its epidemiology, the clinical and laboratory data and the way in which they have been interpreted to deduce the currently acceptable hypothesis of its aetiology and mechanism of transmission are critically examined. There is no compelling reason for preferring the viral hypothesis of AIDS to one based on the activity of oxidizing agents. In fact, the latter is to be preferred, since unlike the viral hypothesis it leads to possible methods of prevention and treatment using currently available therapeutic substances.