Insulin-induced formation of ruffling membranes of KB cells and its correlation with enhancement of amino acid transport

α2C-Adrenoceptors modulate L-DOPA uptake in opossum kidney cells and in the mouse kidney

Targeted deletion or selective pharmacological inhibition of α(2C)-adrenoceptors in mice results in increased brain tissue levels of dopamine and its precursor l-3,4-dihydroxyphenylalanine (l-DOPA), without significant changes in l-DOPA synthesis. l-DOPA uptake is considered the rate-limiting step in dopamine synthesis in the kidney. Since α(2C)-adrenoceptors may influence the transport of l-DOPA, we investigated the effect of α(2C)-adrenoceptor activation on l-DOPA uptake in a kidney cell line (opossum kidney cells). l-DOPA and dopamine kidney tissue levels in α(2C)-adrenoceptor knockout (α(2C)KO) mice and in mice treated with the selective α(2C)-adrenoceptor antagonist JP-1302 were also evaluated. The α(2)-adrenoceptor agonist medetomidine (0.1-1,000 nM) produced a concentration-dependent decrease in l-DOPA uptake in opossum kidney cells (IC(50): 2.5 ± 0.5 nM and maximal effect: 28 ± 5% of inhibition). This effect was abolished by a preincubation with JP-1302 (300 nM). Furthermore, the effect of medetomidine (100 nM) was abolished by a preincubation with U-0126 (10 μM), a MEK1/2 inhibitor. Kidney tissue levels of l-DOPA were significantly higher in α(2C)KO mice compared with wild-type mice (wild-type mice: 58 ± 2 pmol/g tissue and α(2C)KO mice: 81 ± 15 pmol/g tissue, P < 0.05) and in mice treated with JP-1302 (3 μmol/kg body wt) compared with control mice (control mice: 62 ± 2 pmol/g tissue and JP-1302-treated mice: 75 ± 1 pmol/g tissue, P < 0.05), both without significant changes in dopamine kidney tissue levels. However, mice treated with JP-1302 on a high-salt diet presented significantly higher dopamine levels in the kidney and urine compared with control animals on a high-salt diet. In conclusion, in a kidney cell line, α(2C)-adrenoceptor activation inhibits l-DOPA uptake, and in mice, deletion or blockade of α(2C)-adrenoceptors increases l-DOPA kidney tissue levels.

Insulin-induced endothelial cell cortical actin filament remodeling: a requirement for trans-endothelial insulin transport

Insulin's trans-endothelial transport (TET) is critical for its metabolic action on muscle and involves trafficking of insulin bound to its receptor (or at high insulin concentrations, the IGF-I receptor) via caveolae. However, whether caveolae-mediated insulin TET involves actin cytoskeleton organization is unknown. Here we address whether insulin regulates actin filament organization in bovine aortic endothelial cells (bAEC) and whether this affects insulin uptake and TET. We found that insulin induced extensive cortical actin filament remodeling within 5 min. This remodeling was inhibited not only by disruption of actin microfilament organization but also by inhibition of phosphatidylinositol 3-kinase (PI3K) or by disruption of lipid rafts using respective specific inhibitors. Knockdown of either caveolin-1 or Akt using specific small interfering RNA also eliminated the insulin-induced cortical actin filament remodeling. Blocking either actin microfilament organization or PI3K pathway signaling inhibited both insulin uptake and TET. Disruption of actin microfilament organization also reduced the caveolin-1, insulin receptor, and IGF-I receptor located at the plasma membrane. Exposing bAEC for 6 h to either TNFα or IL-6 blocked insulin-induced cortical actin remodeling. Extended exposure (24 h) also inhibited actin expression at both mRNA and protein levels. We conclude that insulin-induced cortical actin filament remodeling in bAEC is required for insulin's TET in a PI3K/Akt and plasma membrane lipid rafts/caveolae-dependent fashion, and proinflammatory cytokines TNFα and IL-6 block this process.

Signaling cascade of insulin-induced stimulation of L-dopa uptake in renal proximal tubule cells

The inward l-dihydroxyphenylalanine (L-dopa) transport supplies renal proximal tubule cells (PTCs) with the precursor for dopamine synthesis. We have previously described insulin-induced stimulation of L-dopa uptake into PTCs. In the present paper we examined insulin-related signaling pathways involved in the increase of l-dopa transport into isolated rat PTCs. Insulin (50-500 microU/ml) increased L-dopa uptake by PTCs, reaching the maximal increment (60% over the control) at 200 microU/ml. At this concentration, insulin also increased insulin receptor tyrosine phosphorylation. Both effects were abrogated by the tyrosine kinase inhibitor genistein (5 microM). In line, inhibition of the protein tyrosine phosphatase by pervanadate (0.2-100 microM) caused a concentration-dependent increase in both the uptake of L-dopa (up to 400%) and protein tyrosine phosphorylation. A synergistic effect between pervanadate and insulin on L-dopa uptake was observed only when threshold (0.2 microM), but not maximal (5 microM), concentrations of pervanadate were assayed. Insulin-induced stimulation of L-dopa uptake was also abolished by inhibition of phosphatidylinositol 3-kinase (PI3K; 100 nM wortmannin, and 25 microM LY-294002) and protein kinase C (PKC; 1 microM RO-318220). Insulin-induced activation of PKC-zeta was confirmed in vitro by its translocation from the cytosol to the membrane fraction, and in vivo by immunohistochemistry studies. Insulin caused a wortmannin-sensitive increase in Akt/protein kinase B (Akt/PKB) phosphorylation and a dose-dependent translocation of Akt/PKB to the membrane fraction. Our findings suggest that insulin activates PKC-zeta, and Akt/PKB downstream of PI3K, and that these pathways contribute to the insulin-induced increase of L-dopa uptake into PTCs.

Comparative study of protein tyrosine phosphatase-epsilon isoforms: membrane localization confers specificity in cellular signalling

To study the influence of subcellular localization as a determinant of signal transduction specificity, we assessed the effects of wild-type transmembrane and cytoplasmic protein tyrosine phosphatase (PTP) epsilon on tyrosine kinase signalling in baby hamster kidney (BHK) cells overexpressing the insulin receptor (BHK-IR). The efficiency by which differently localized PTPepsilon and PTPalpha variants attenuated insulin-induced cell rounding and detachment was determined in a functional clonal-selection assay and in stable cell lines. Compared with the corresponding receptor-type PTPs, the cytoplasmic PTPs (cytPTPs) were considerably less efficient in generating insulin-resistant clones, and exceptionally high compensatory expression levels were required to counteract phosphotyrosine-based signal transduction. Targeting of cytPTPepsilon to the plasma membrane via the Lck-tyrosine kinase dual acylation motif restored high rescue efficiency and abolished the need for high cytPTPepsilon levels. Consistent with these results, expression levels and subcellular localization of PTPepsilon were also found to determine the phosphorylation level of cellular proteins including focal adhesion kinase (FAK). Furthermore, PTPepsilon stabilized binding of phosphorylated FAK to Src, suggesting this complex as a possible mediator of the PTPepsilon inhibitory response to insulin-induced cell rounding and detachment in BHK-IR cells. Taken together, the present localization-function study indicates that transcriptional control of the subcellular localization of PTPepsilon may provide a molecular mechanism that determines PTPepsilon substrate selectivity and isoform-specific function.

L-Dopa uptake and dopamine production in proximal tubular cells are regulated by beta(2)-adrenergic receptors

This study assessed the role of adrenergic receptors on the regulation of the uptake of L-dopa and the production of dopamine by renal tubular cells. Scatchard analysis showed two L-dopa uptake sites with different affinities (K(m) 0.316 vs 1.53 microM). L-Dopa uptake was decreased by the nonselective adrenergic agonists epinephrine or norepinephrine (40%), by the beta-selective agonist isoproterenol or the beta(2)-selective agonist terbutaline (60%), but not by alpha-selective agonists (all 1 microM). The effect of norepinephrine, isoproterenol, or terbutaline was unaffected by addition of the beta(1)-antagonist atenolol, abolished by ICI-118, 551, a beta(2)-antagonist (both 0.1 microM), and mimicked by the addition of dibutyryl-cAMP (1 microM). Preincubation with terbutaline decreased the number of high-affinity uptake sites (V(max) = 1.10 +/- 0.3 vs. 0.5 +/- 0.1 pmol. mg protein(-1). min(-1)) without changing their affinity. Norepinephrine or terbutaline decreased dopamine production by isolated cells, and this effect was abolished by ICI-118,551 (0.1 microM). In vivo administration of ICI-118,551 reduced the urinary excretion of L-dopa and increased the excretion of 3,4-dihydroxyphenylacetic acid without significant changes in plasma L-dopa concentrations. These results demonstrate that stimulation of beta(2)-adrenergic receptors decreases the number of high-affinity L-dopa uptake sites in isolated tubular cells resulting in a reduction of the uptake of L-dopa and the production of dopamine and provide evidence for the presence of this mechanism in the intact animal.

SH2-B is required for growth hormone-induced actin reorganization

The Src homology-2 (SH2) domain-containing protein SH2-Bbeta is a substrate of the growth hormone (GH) receptor-associated tyrosine kinase JAK2. Here we tested whether SH2-Bbeta is involved in GH regulation of the actin cytoskeleton. Based on cell fractionation and confocal microscopy, we find SH2-Bbeta present at the plasma membrane and in the cytosol. SH2-Bbeta colocalized with filamentous actin in GH and platelet-derived growth factor (PDGF)-induced membrane ruffles. To test if SH2-Bbeta is required for actin reorganization, we transiently overexpressed wild-type or mutant SH2-Bbeta in 3T3-F442A cells and assayed for GH- and PDGF-induced membrane ruffling and fluid phase pinocytosis. Overexpression of wild-type SH2-Bbeta enhanced ruffling and pinocytosis produced by submaximal GH but not submaximal PDGF. Point mutant SH2-Bbeta (R555E) and truncation mutant DeltaC555, both lacking a functional SH2 domain, inhibited membrane ruffling and pinocytosis induced by GH and PDGF. Mutant DeltaN504, which possesses a functional SH2 domain and enhances JAK2 kinase activity in overexpression systems, also inhibited GH-stimulated membrane ruffling. DeltaN504 failed to inhibit GH-induced nuclear localization of Stat5B, indicating JAK2 is active in these cells. Taken together, these results show that SH2-Bbeta is required for GH-induced actin reorganization by a mechanism discrete from the action of SH2-Bbeta as a stimulator of JAK2 kinase activity.

Insulin-induced actin filament remodeling colocalizes actin with phosphatidylinositol 3-kinase and GLUT4 in L6 myotubes

We examined the temporal reorganization of actin microfilaments by insulin and its participation in the localization of signaling molecules and glucose transporters in L6 myotubes expressing myc-tagged glucose transporter 4 (GLUT4myc). Scanning electron microscopy revealed a dynamic distortion of the dorsal cell surface (membrane ruffles) upon insulin treatment. In unstimulated cells, phalloidin-labeled actin filaments ran parallel to the longitudinal axis of the cell. Immunostaining of the p85 regulatory subunit of phosphatidylinositol 3-kinase was diffusely punctate, and GLUT4myc was perinuclear. After 3 minutes of insulin treatment, actin reorganized to form structures; these structures protruded from the dorsal surface of the myotubes by 10 minutes and condensed in the myoplasm into less prominent foci at 30 minutes. The p85 polypeptide colocalized with these structures at all time points. Actin remodeling and p85 relocalization to actin structures were prevented by cytochalasin D or latrunculin B. GLUT4myc recruitment into the actin-rich projections was also observed, but only after 10 minutes of insulin treatment. Irrespective of insulin stimulation, the majority of p85 and a portion (45%) of GLUT4 were recovered in the Triton X-100-insoluble material that was also enriched with actin. In contrast, vp165, a transmembrane aminopeptidase that morphologically colocalized with GLUT4 vesicles, was fully soluble in Triton X-100 extracts of both insulin-treated and control myotubes. Transient transfection of dominant inhibitory Rac1 (N17) into L6 myotubes prevented formation of dorsal actin structures and blocked insulin-induced GLUT4myc translocation to the cell surface. We propose that insulin-dependent formation of actin structures facilitates the association of PI3-K (p85) with GLUT4 vesicles and, potentially, the arrival of GLUT4 at the cell surface.

Actin filaments facilitate insulin activation of the src and collagen homologous/mitogen-activated protein kinase pathway leading to DNA synthesis and c-fos expression

The exact mechanism of the spatial organization of the insulin signaling pathway leading to nuclear events remains unknown. Here, we investigated the involvement of the actin cytoskeleton in propagation of insulin signaling events leading to DNA synthesis and expression of the immediate early genes c-fos and c-jun in L6 muscle cells. Insulin reorganized the cellular actin network and increased the rate of DNA synthesis and the levels of c-fos mRNA, but not those of c-jun mRNA, in undifferentiated L6 myoblasts. Similarly, insulin markedly elevated the levels of c-fos mRNA but not of c-jun mRNA in differentiated L6 myotubes. Disassembly of the actin filaments by cytochalasin D, latrunculin B, or botulinum C2 toxin significantly inhibited insulin-mediated DNA synthesis in myoblasts and abolished stimulation of c-fos expression by the hormone in myoblasts and myotubes. Actin disassembly abolished insulin-induced phosphorylation and activation of extracellulor signal-regulated kinases, activation of a 65-kda member of the p21-activated kinases, and phosphorylation of p38 mitogen-activated protein kinases but did not prevent activation of phosphatidylinositol 3-kinase and p70(S6k). Under these conditions, insulin-induced Ras activation was also abolished, and Grb2 association with the Src and collogen homologous (Shc) molecule was inhibited without inhibition of the tyrosine phosphorylation of Shc. We conclude that the actin filament network plays an essential role in insulin regulation of Shc-dependent signaling events governing gene expression by facilitating the interaction of Shc with Grb2.

EGF induces recycling membrane to form ruffles

KB cells are know to respond to epidermal growth factor (EGF) by producing prodigious ruffles in the plasma membrane within minutes. The signal transduction pathway underlying this effect in fibroblasts is mediated through Rac, a member of the Ras-like family of GTPases. As ruffles are rich in components of the cytoskeleton--particularly in actin and ezrin--it has been suggested that ruffles arise when activated Rac modulates the actin cytoskeleton to push out a membrane protrusion. We set out to see whether the surface of new ruffles arises from neighbouring membrane, or whether it comes from an intracellular pool of endocytosed membrane. If it arose by exocytosis from endosomes, it would be expected to be enriched in those recycling receptors that are concentrated in coated pits in the endocytic side of the cycle. On the other hand, if it arose passively from the adjacent plasma membrane, a uniform distribution of these receptors would be expected. Here, we show that as soon as ruffles appear on KB cells in response to EGF, their membrane surfaces are enriched in both transferrin and low density lipoprotein (LDL) receptors. Both these proteins are known to be selectively concentrated into endosomal membranes by clathrin-mediated endocytosis. Our results reveal that the surfaces of ruffles arise by exocytosis of internal membrane from the endocytic cycle and, therefore, that a primary action of Rac is to redirect the exocytosis of recycling membrane into just those specific sites where ruffles form.

Profilin and gelsolin stimulate phosphatidylinositol 3-kinase activity

Actin-binding proteins such as profilin and gelsolin bind to phosphatidylinositol (PI) 4,5-bisphosphate (PI 4,5-P2) and regulate the concentration of monomeric actin. We report here that profilin and gelsolin stimulate PI 3-kinase-mediated phosphorylation of PI 4,5-P2 (lipid kinase activity) in a concentration-dependent manner. This effect is specific to profilin and gelsolin because other cytoskeletal proteins such as tau or actin do not affect PI 3-kinase activity. In addition to lipid kinase activity, PI 3-kinase also has protein kinase activity: it phosphorylates proteins (p85 subunit of PI 3-kinase). However, the protein kinase activity of PI 3-kinase was not affected in the presence of profilin. Kinetic analysis, as a function of varying concentrations of ATP and PI 4,5-P2, showed that profilin affects the Vmax of PI 3-kinase without affecting k(m). Profilin may also affect PI 3-kinase activity by its direct association to the enzyme because dot-blot analysis using antibody to glutathione S-transferase (GST) suggested that GST-85 kDa, a fusion protein of PI 3-kinase, binds to profilin. However, PI 3-kinase did not affect the actin-sequestering ability of profilin (determined by pyrene-labeled actin), which indicates that actin and p85 do not share a common binding site on profilin. These studies suggest that profilin and gelsolin may control the generation of 3-OH phosphorylated phosphoinositides, which in turn may regulate the actin polymerization.

Identification of IQGAP as a putative target for the small GTPases, Cdc42 and Rac1

Cdc42 and Rac1 have been implicated in the regulation of various cell functions such as cell morphology, polarity, and cell proliferation. We have partially purified a Cdc42- and Rac1-associated protein with molecular mass of about 170 kDa (p170) from bovine brain cytosol. This protein interacted with guanosine 5'-(3-O-thio)triphosphate (GTPgammaS).glutathione S-transferase (GST)-Cdc42 and GTPgammaS++.GST-Rac1 but not with the GDP.GST-Cdc42, GDP.GST-Rac1, or GTPgammaS.GST-RhoA). We identified p170 as an IQGAP, which is originally identified as a putative Ras GTPase-activating protein. Recombinant IQGAP specifically interacted with GTPgammaS.Cdc42 and GTPgammaS.Rac1. The C-terminal fragment of IQGAP was responsible for their interactions. IQGAP was specifically immunoprecipitated with dominant-active Cdc42(Val12) or Rac1(Val12) from the COS7 cells expressing Cdc42(Val12) or Rac1(Val12), respectively. Immunofluorescence analysis revealed that IQGAP was accumulated at insulin- or Rac1-induced membrane ruffling areas. This accumulation of IQGAP was blocked by the microinjection of the dominant-negative Rac1(Asn17) or Cdc42(Asn17). Moreover, IQGAP was accumulated at the cell-cell junction in MDCK cells, where alpha-catenin and ZO-1 were localized. These results suggest that IQGAP is a novel target molecule for Cdc42 and Rac1.

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.

bFGF and aFGF induce membrane ruffling in breast cancer cells but not in normal breast epithelial cells: FGFR-4 involvement

Acidic and basic fibroblast growth factors (aFGF and bFGF) are growth factors which may have a physiological role in the normal breast and in breast cancer. A study of the effects of aFGF and bFGF on a variety of breast cell lines and epithelial cells purified from normal breast organoids showed that whereas normal breast cells did not exhibit membrane ruffling in response to either of these growth factors, some breast cancer cell lines did. This difference was not due to lack of receptor since all the cell lines tested were mitogenically stimulated by bFGF. Dominant negative mutations of FGF receptor 3 (FGFR-3) and the small GTP-binding protein p21rac inhibited membrane ruffling, showing that receptor dimerization and phosphorylation and p21rac activation are prerequisites for membrane ruffling in response to aFGF and bFGF. Transient transfection of individual FGFRs into cos-7 cells showed that FGFR-1, FGFR-2 and FGFR-3 could not mediate a membrane ruffling response whereas FGFR-4 could. These studies elucidate one signalling mechanism of FGF and point to differences in the response of normal and cancer breast epithelial cells which may be important in cell motility.

Lipid dynamics and peripheral interactions of proteins with membrane surfaces

A large body of evidence strongly indicates biomembranes to be organized into compositionally and functionally specialized domains, supramolecular assemblies, existing on different time and length scales. For these domains and intimate coupling between their chemical composition, physical state, organization, and functions has been postulated. One important constituent of biomembranes are peripheral proteins whose activity can be controlled by non-covalent binding to lipids. Importantly, the physical chemistry of the lipid interface allows for a rapid and reversible control of peripheral interactions. In this review examples are provided on how membrane lipid (i) composition (i.e., specific lipid structures), (ii) organization, and (iii) physical state can each regulate peripheral binding of proteins to the lipid surface. In addition, a novel and efficient mechanism for the control of the lipid surface association of peripheral proteins by [Ca2+], lipid composition, and phase state is proposed. The phase state is, in turn, also dependent on factors such as temperature, lateral packing, presence of ions, metabolites and drugs. Confining reactions to interfaces allows for facile and cooperative large scale integration and control of metabolic pathways due to mechanisms which are not possible in bulk systems.

rac p21 is involved in insulin-induced membrane ruffling and rho p21 is involved in hepatocyte growth factor- and 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced membrane ruffling in KB cells

Insulin and hepatocyte growth factor (HGF) induced morphologically different membrane rufflings in KB cells. Insulin-induced membrane ruffling was inhibited by microinjection of rho GDI, an inhibitory GDP/GTP exchange regulator for both rho p21 and rac p21 small GTP-binding proteins, but not inhibited by microinjection of botulinum exoenzyme C3, known to selectively ADP-ribosylate rho p21 and to impair its function. This rho GDI action was prevented by comicroinjection with guanosine 5'-(3-O-thio)triphosphate (GTP gamma S)-bound rac1 p21. In contrast, HGF-induced membrane ruffling was inhibited by microinjection of rho GDI or C3. This rho GDI action was prevented by comicroinjection with GTP gamma S-bound rhoA p21, and this C3 action was prevented by comicroinjection with GTP gamma S-bound rhoAIle-41 p21, which is resistant to C3. Microinjection of either GTP gamma S-bound rac1 p21 or rhoA p21 alone induced membrane ruffling in the absence of the growth factors. The rac1 p21-induced membrane ruffling was morphologically similar to the insulin-induced kind, whereas rhoA p21-induced ruffling was apparently different from both the insulin- and HGF-induced kinds. Membrane ruffling was also induced by 12-O-tetradecanoylphorbol-13-acetate (TPA), a protein kinase C-activating phorbol ester, but not by Ca2+ ionophore or microinjection of a dominant active Ki-ras p21 mutant (Ki-rasVal-12 p21). The phorbol ester-induced membrane ruffling was morphologically similar to the rhoA p21-induced kind and inhibited by microinjection of rho GDI or C3. These results indicate that rac p21 and rho GDI are involved in insulin-induced membrane ruffling and that rho p21 and rho GDI are involved in HGF- and phorbol ester-induced membrane rufflings.

The serum-induced phosphorylation of mammalian hsp27 correlates with changes in its intracellular localization and levels of oligomerization

The oligomeric small heat-shock protein hsp27, also denoted hsp28, is constitutively expressed in several mammalian cells and displays a phosphorylation status that is related to cellular growth and differentiation. This protein is related to alpha-crystallin and has strong sequence similarity with an in vitro inhibitor of actin polymerization. Here, we have analyzed hsp27 phosphorylation, cellular localization and structural organization following serum stimulation of serum-starved HeLa cells. hsp27 is dephosphorylated in starved cells and quantitatively recovered in the form of small structures (< 200 kDa) present in the soluble phase of the cytoplasm. Immediately after the addition of serum to starved cells, a rapid phosphorylation and complex changes in the intracellular distribution and structural organization of hsp27 are observed. Phosphorylation essentially occurs at the level of small hsp27 structures (< 200 kDa) and is concomitant with the increased molecular mass (up to 700 kDa) of a fraction of this protein. Serum treatment also induced the detergent-sensitive association of another fraction of hsp27, still in the form of small and dephosphorylated structures, with cellular particulate fractions. Contrasting with these observations, hsp70 had the tendency to concentrate into nucleoli during serum starvation.

Inhibition of glycogen synthesis by epidermal growth factor in hepatocytes. The role of cell density and pertussis toxin-sensitive GTP-binding proteins

Epidermal growth factor (EGF) counteracts the stimulation of glycogen synthesis by insulin in hepatocytes, but it is not known whether this is due to inhibition of glycogen synthesis or to inhibition of the insulin-signalling mechanism. This study investigates the mechanisms by which EGF affects the basal rate and the insulin stimulation of glycogen synthesis. The basal rate of glycogen synthesis is higher at low than at high cell density. EGF inhibits the basal rate of glycogen synthesis at low cell density but not in confluent cultures and abolishes the difference due to density. However, EGF inhibits the stimulation of glycogen synthesis by insulin irrespective of cell density. Increasing glycogen synthesis by increasing the [glucose] does not abolish the difference in rates of glycogen synthesis due to cell density, neither does it induce responsiveness to EGF at high cell density, establishing that responsiveness to EGF is a function of cell density and not of the basal rate and that inhibition of the insulin stimulation also cannot be accounted for by the higher rate of glycogen synthesis. Cytochalasin D and phalloidin, which alter cell morphology through interactions with the microfilament cytoskeleton, mimic the cell-density-dependent inhibition of glycogen synthesis by EGF. The inhibition of glycogen synthesis by EGF and cytochalasin D is additive and cytochalasin D potentiates the inhibition of glycogen synthesis by EGF, suggesting involvement of a cytoskeletal mechanism. Exogenous phospholipase C inhibits glycogen synthesis at both low and high cell density and the inhibition at low cell density is not additive with that caused by either EGF or cytochalasin D, suggesting that these agonists inhibit glycogen synthesis through changes in Ca2+ and/or diacylglycerol. The inhibition of glycogen synthesis by EGF in the absence of insulin stimulation is blocked by neomycin, which inhibits Ca2+ release from intracellular stores but not by antagonists of protein kinase C. It was also inhibited by pertussis toxin (50%), suggesting that it may involve GTP-binding-protein-mediated release of Ca2+ from intracellular stores. The inhibition of the stimulation of glycogen synthesis by insulin was not affected by neomycin and was only marginally inhibited by pertussis toxin or guanosine 5'-O-[3-thio]triphosphate (GTP[S]). We infer from these findings that the inhibition by EGF of the basal rate of glycogen synthesis and of the insulin stimulation are mediated by different mechanisms. The latter is pertussis toxin insensitive and independent of cell density, whereas the former is expressed only at low cell density, it is potentiated by cytochalasin D and inhibited by pertussis toxin.

rac p21 is involved in insulin-induced membrane ruffling and rho p21 is involved in hepatocyte growth factor- and 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced membrane ruffling in KB cells

Insulin and hepatocyte growth factor (HGF) induced morphologically different membrane rufflings in KB cells. Insulin-induced membrane ruffling was inhibited by microinjection of rho GDI, an inhibitory GDP/GTP exchange regulator for both rho p21 and rac p21 small GTP-binding proteins, but not inhibited by microinjection of botulinum exoenzyme C3, known to selectively ADP-ribosylate rho p21 and to impair its function. This rho GDI action was prevented by comicroinjection with guanosine 5'-(3-O-thio)triphosphate (GTP gamma S)-bound rac1 p21. In contrast, HGF-induced membrane ruffling was inhibited by microinjection of rho GDI or C3. This rho GDI action was prevented by comicroinjection with GTP gamma S-bound rhoA p21, and this C3 action was prevented by comicroinjection with GTP gamma S-bound rhoAIle-41 p21, which is resistant to C3. Microinjection of either GTP gamma S-bound rac1 p21 or rhoA p21 alone induced membrane ruffling in the absence of the growth factors. The rac1 p21-induced membrane ruffling was morphologically similar to the insulin-induced kind, whereas rhoA p21-induced ruffling was apparently different from both the insulin- and HGF-induced kinds. Membrane ruffling was also induced by 12-O-tetradecanoylphorbol-13-acetate (TPA), a protein kinase C-activating phorbol ester, but not by Ca2+ ionophore or microinjection of a dominant active Ki-ras p21 mutant (Ki-rasVal-12 p21). The phorbol ester-induced membrane ruffling was morphologically similar to the rhoA p21-induced kind and inhibited by microinjection of rho GDI or C3. These results indicate that rac p21 and rho GDI are involved in insulin-induced membrane ruffling and that rho p21 and rho GDI are involved in HGF- and phorbol ester-induced membrane rufflings.

Complex subcellular distribution of sodium-dependent amino acid transport systems in kidney cortex and LLC-PK1/Cl4 cells

To characterize the amino acid transport system in basolateral membranes and to test for possible intracellular loci of amino acid transport activity, we surveyed the distribution of L-alanine transport activity in rabbit proximal tubular cells and LLC-PK1/Cl4 cells. A three-dimensional separation procedure based on differential sedimentation, density gradient centrifugation, and counter-current distribution resolved 21 physically and biochemically distinct membrane populations from rabbit cortex. Inhibition of L-alanine transport by phenylalanine and N-(methylamino)isobutyric acid was used to delineate parallel amino acid transport pathways. Population n was identified as brush border membranes by virtue of its 16-fold maltase enrichment; 94% of its Na(+)-dependent alanine transport activity was mediated by systems previously shown to be characteristic of brush border membranes. Two populations, c' and c", which accounted for 25% of the total Na,K-ATPase activity, were identified as basalateral membranes on the basis of Na,K-ATPase cumulative enrichment factors of 15 and 21; 82% of the total alanine transport in these populations was mediated by a Na(+)-independent system similar to the classical system L. Na,K-ATPase, Na(+)-independent and Na(+)-dependent alanine transport activities were associated with intracellular membrane populations as well as with the plasma membranes. The major intracellular locus of Na,K-ATPase activity, population i accounted for roughly 31% of the Na,K-ATPase, maximally enriched ninefold; it contained 29% of the total system L transport activity. Population l, which was identified as endoplasmic reticulum because it was the major locus of membrane-bound NADPH cytochrome c reductase activity, contained 44% of the total system A transport. Three distinct Golgi-derived populations, m', m", and o, accounted for 39% of the total system A transport. A survey of the amino acid transport systems in LLC-PK1/Cl4 cells showed that the majority of system A-mediated amino acid transport was present in membranes of intracellular and possibly apical origin. The presence of large intracellular pools of amino acid transport activities might reflect newly synthesized transport proteins, ongoing membrane recycling or, perhaps, intracellular reserves available for rapid recruitment to the plasma membrane.

Membrane surface ruffling in cells that over-express Alzheimer amyloid beta/A4 C-terminal peptide

Deposition of beta/A4 amyloid in brain is a defining characteristic of Alzheimer disease (AD); however, the extent to which amyloid deposits may interfere with normal cellular processes is incompletely understood. We examined this issue by means of PC12 cells. After transfection with DNA coding for 97 amino acids of the beta/A4 C-terminal region of the amyloid precursor protein, beta/A4 antigen was visible at the cell membrane. We report that normal unstimulated PC12 cells exhibit ruffling activity at the cell surface when plated on a plastic substrate. Relative to control cells, however, those that over-expressed the beta/A4 C-terminal peptide had significantly higher levels of ruffling activity, suggesting a structural and/or functional membrane modification. Similar cellular alterations, if present, in Alzheimer brain cells, may indicate disturbances in membrane-associated functions, including intercellular communication.

Ruffles induced by Salmonella and other stimuli direct macropinocytosis of bacteria

Ruffles are specialized plasma membrane ultrastructures of mammalian cells though to be integral to growth, development and locomotion. Induced by growth factors, mitogens or oncogene expression, ruffles are sites of filamentous actin rearrangement and are temporally associated with enhanced pinocytosis. But the function of ruffles, their mechanism of induction and their role in pinocytosis are not understood. We have observed formation of structures resembling ruffles associated with the site of entry of invasive Salmonella typhimurium. Here we report that ruffles elicited by invasive Salmonella directly mediate internalization of non-invasive bacteria in a macropinocytotic fashion, a phenomenon we term 'passive entry'. Furthermore, ruffles induced in the absence of Salmonella also facilitate passive entry. We present evidence that ruffles, common to many signalling events, comprise the macropinocytotic machinery mediating pinocytosis and are subverted by Salmonella so as to enter mammalian cells.

Production of a unique antibody specific for membrane ruffles and its use to characterize the behavior of two distinct types of ruffles

I have produced a new monoclonal antibody, YF-169, against membrane ruffle specific 55-kD protein. YF-169 stained membrane ruffles of chick embryo fibroblasts so definitely that it enabled clear and reliable analyses of membrane ruffles. Fibroblasts organized two distinct types of membrane ruffles. One type of the ruffles were transiently formed in serum-starved cells (Type I) when stimulated by serum or platelet-derived growth factor. After spontaneous degradation of Type I ruffles, the other type of ruffles containing many microspikes were gradually organized at leading edges (Type II). The formation of Type I ruffles was not affected by either nocodazole, a microtubule-disrupting drug, or taxol, a microtubule-stabilizing reagent. However, Type II ruffles were entirely destroyed not only by nocodazole but also by taxol, suggesting that regulated organization of microtubule network is important to maintain Type II ruffles. H8, a protein kinase inhibitor prevented the spontaneous degradation of Type I ruffles and also reduced the destructive effect of nocodazole on Type II ruffles without affecting microtubule-disrupting activity. Protein kinases may be involved in the degradation processes of both types of ruffles. W7, a calmodulin antagonist, strongly inhibited Type I ruffle formation and completely destroyed Type II ruffles. W7 was also found to induce a remarkable change of 55-kD protein localization. After degradation of Type II ruffles, most of 55-kD protein was incorporated into newly formed unusual thick fibers. These results suggest that regulated organization of microtubule network is not necessary to form Type I ruffles but is important to maintain Type II ruffles, while calmodulin function is essential for both types of membrane ruffles.

Ruffling and locomotion: role in cell resistance to growth factor-induced proliferation

It has long been known that the growth rate of cells in vitro can be retarded by providing substrates of restricted area. Such experiments were performed with adhesive islets, made by depositing metals onto agarose layers through templates of various sizes. Since normal cells are unable to adhere to agarose, they become confined to the metallic surface. Using such haptotactic islets, we have studied the role of membrane ruffling and cell locomotion in the resistance of AG1523 human fibroblasts to growth factor-induced mitogenesis. Cells plated on small substrates, i.e., 2,150 microns 2 in area, initially showed vigorous ruffling, which was suppressed by 8 h after plating but had resumed again by 12 h. In contrast, cells on larger-size islets showed a rapid decline and stabilization of ruffling activity. When the growth rate was measured for single cells cultured on haptotactic islets, it was found to increase linearly from areas of 4,280 microns 2 up to 425,000 microns 2. Since the area needed to saturate the growth response was approximately 50-fold larger than the area occupied by a single cell, the growth inhibition was attributed in part to an interference with locomotion. The implication that locomotion provided positive input into growth control mechanisms was subjected to a direct test by evaluating the effect of nine polypeptide growth factors on the motility of serum-starved cells. All except TGF-beta 1 stimulated movement. Finally, the mitogenic effect of growth factors was measured by [3H]thymidine incorporation and found to be proportional to motile activities, as quantitatively assayed. We conclude that locomotion suppression is a factor in AG1523 cell resistance to growth factor-induced mitogenesis.

Morphological changes in Krebs II ascites tumour cells induced by insulin are associated with differences in protein composition and altered amounts of free, cytoskeletal-bound and membrane-bound polysomes

A three-step sequential detergent/salt extraction procedure was used in order to isolate three distinct subcellular fractions containing free (FP), cytoskeletal-bound (CBP) and membrane-bound polysomes (MBP), respectively, from Krebs II ascites cells (Vedeler et al., Mol Cell Biochem 100: 183-193, 1991). The purpose was to study changes in the distribution of polysomes in these three fractions during long-term incubation with insulin under either stationary conditions or in roller suspension culture. Insulin caused a redistribution of polysomes between FP, CBP and MBP fractions. The hormone appeared to promote an entry of ribosomes into polysomes both in CBP and MBP populations. When cells were grown in stationary culture in the presence of insulin and thus promoted to attach to the substratum and undergo morphological changes, a diversion of ribosomes from CBP into MBP was observed. The level of protein synthesis was apparently very high in this latter fraction since more than 70% of ribosomes were in polysomes. Morphological changes observed following insulin treatment were accompanied by a shift of certain proteins among subcellular fractions (for example actin and p35). The fibronectin content was about 20% higher in attached compared to non-attached cells. The results suggest that morphological changes induced by stimulation with insulin are associated with an increased activity of MBP, presumably reflecting a requirement for an increased synthesis of membrane proteins.

The role of cell swelling in the stimulation of glycogen synthesis by insulin

In hepatocyte cultures, insulin stimulates cellular accumulation of K+, partly (approximately 20%) by net replacement of cell Na+, but largely (approximately 80%) by increasing the cell K++Na+ content, with a consequent increase in cell volume. An increase in cation content occurred within 5 min of exposure to insulin and was not secondary to metabolic changes. Insulin also increased the cation content, by increasing the Na+ content, in a K(+)-free medium or when K+ uptake was inhibited with 1 mM-ouabain. However, insulin did not increase the cation content in a Na(+)-free medium. The stimulation of glycogen synthesis by insulin, like the increase in cation content, was blocked in a Na(+)-free medium, but not when K+ uptake was inhibited. Hypo-osmotic swelling restored the stimulation of glycogen synthesis in a Na(+)-free medium, indicating that the lack of effect of insulin in the iso-osmotic Na(+)-free medium was not due to a direct requirement for Na+ for glycogen synthesis, but to a secondary mechanism, dependent on Na+ entry, that can be mimicked by hypo-osmotic swelling. Quinine increased cell volume further and caused a further increase in glycogen synthesis. The hypothesis that cellular uptake of K+ may be part of the mechanism by which insulin controls metabolism was discounted, because inhibition of K+ uptake does not block the metabolic effects of insulin [Czech (1977) Annu. Rev. Biochem. 46, 359-384]. The present results support the hypothesis that an increase in cell cation content, and thereby cell volume, rather than K+ uptake, is part of the mechanism by which insulin stimulates glycogen synthesis in hepatocytes.

Differential sensitivity of insulin- and adaptive-regulation-induced system A activation to microtubular function in skeletal muscle

1. Insulin and adaptive regulation are known to stimulate system A amino acid transport activity in skeletal muscle. The present study was designed to investigate whether activation of system A in muscle is a consequence of processes which rely on microtubule or microfilament function. To that end, extensor digitorum longus (EDL) muscles were incubated in the presence of colchicine and cytochalasin D, well-known inhibitors of microtubule and microfilament activity respectively. 2. Basal alpha-(methyl)aminoisobutyric acid (MeAIB) uptake decreased after incubation with 5 microM-colchicine in a time-dependent manner. In keeping with this, adaptive regulation of MeAIB uptake caused by prolonged incubation in the absence of amino acids was substantially decreased in the presence of colchicine. 3. Under these conditions, stimulation of MeAIB uptake by insulin was unaltered in muscle in the presence of colchicine. This contrasted with the insulin-induced stimulation of MeAIB uptake by isolated rat hepatocytes, which was markedly decreased by colchicine. 4. Cytochalasin D, an agent that disrupts microfilaments, did not inhibit basal or insulin-stimulated MeAIB uptake by the incubated muscle. 5. Neither colchicine nor cytochalasin D modified the stimulatory effect of insulin on 3-O-methylglucose uptake by EDL muscle. 6. We conclude that up-regulation of system A by synthesis of new carriers depends on the integrity of microtubular function both in skeletal muscle and in hepatocytes. Microtubules might play a role in the movement of system A-containing vesicles from the Golgi network to the plasma membrane.

Glioma-derived PDGF-related protein presents as 17 kd intracellularly and assembled form induces actin reorganization

We have electrophoretically obtained platelet-derived growth factor (PDGF)-related protein from human glioma (glioma derived PDGF-related protein: GD-PDGF) and produced rabbit antiserum against the monomer of GD-PDGF. By methods of immunoaffinity chromatography and Western blotting, we analyzed GD-PDGF in cultured human glioma cells and conditioned medium. The intracellular GD-PDGF was only detected at 17 kd molecular weight by the purified rabbit antibody. When the intracellular 17 kd monomer was purified by the IgG-coupled immunoaffinity chromatography, the eluted protein was not detected at 17 kd but at 52 kd. The 52 kd GD-PDGF was spontaneously and immediately converted to 56 kd, which was partly degraded to 32 and 35 kd within 24 hours. On the other hand, in the conditioned media of glioma cell lines GD-PDGF presents mainly as 56 kd. The assembled forms of GD-PDGF exhibited a powerful activity to induce membrane ruffle formation and reorganization of actin filaments in cultured glial cells and glioma cells. These results indicated that GD-PDGF is intracellularly stored as 17 kd monomer and exists extracellularly as assembled forms, which may act as an autocrine and paracrine effect on the surrounding cells.

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.

On the principles of functional ordering in biological membranes

Integrating the available data on lipid-protein interactions and ordering in lipid mixtures allows to emanate a refined model for the dynamic organization of biomembranes. An important difference to the fluid mosaic model is that a high degree of spatiotemporal order should prevail also in liquid crystalline, "fluid" membranes and membrane domains. The interactions responsible for ordering the membrane lipids and proteins are hydrophobicity, coulombic forces, van der Waals dispersion, hydrogen bonding, hydration forces and steric elastic strain. Specific lipid-lipid and lipid-protein interactions result in a precisely controlled yet highly dynamic architecture of the membrane components, as well as in its selective modulation by the cell and its environment. Different modes of organization of the compositionally and functionally differentiated domains would correspond to different functional states of the membrane. Major regulators of membrane architecture are proposed to be membrane potential controlled by ion channels, intracellular Ca2+, pH, changes in lipid composition due to the action of phospholipase, cell-cell coupling, as well as coupling of the membrane with the cytoskeleton and the extracellular matrix. Membrane architecture is additionally modulated due to the membrane association of ions, lipo- and amphiphilic hormones, metabolites, drugs, lipid-binding peptide hormones and amphitropic proteins. Intermolecular associations in the membrane and in the membrane-cytoskeleton interface are further selectively controlled by specific phosphorylation and dephosphorylation cascades involving both proteins and lipids, and regulated by the extracellular matrix and the binding of growth factors and hormones to their specific receptor tyrosine kinases. A class of proteins coined architectins is proposed, as a notable example the pp60src kinase. The functional role of architectins would be in causing specific changes in the cytoskeleton-membrane interface, leading to specific configurational changes both in the membrane and cytoskeleton architecture and corresponding to (a) distinct metabolic/differentiation states of the cell, and (b) the formation and maintenance of proper three dimensional membrane structures such as neurites and pseudopods.

Cholera toxin inhibits signal transduction by several mitogens and the in vitro growth of human small-cell lung cancer

Cholera toxin (CT) inhibited the in vitro growth of three of four human small-cell lung carcinoma (SCLC) cell lines with a 50% inhibitory concentration of 27-242 ng/ml. Loss of surface membrane ruffling and the capacity of [Tyr4]-bombesin, vasopressin, and fetal calf serum to stimulate increases in intracellular free calcium clearly preceded effects on cellular metabolic activity and cell growth. 125I-[Tyr4]-bombesin binding was unaffected by CT treatment but [Tyr4]-bombesin stimulated phospholipase C activity was decreased in membranes from CT-treated SCLC cells. CT stimulated a rapid but transient increase in intracellular cyclic AMP ([cAMP]i) in SCLC. The effects of CT on susceptible SCLC were not reproduced by elevations of [cAMP]i induced by forskolin or cyclic AMP analogues. GM1 ganglioside, the cellular binding site for CT, was highly expressed in the CT-sensitive but not the CT-resistant SCLC cell lines. In contrast, expression of guanine nucleotide binding protein substrates for ADP-ribosylation by CT was similar. These data demonstrate the existence of a CT-sensitive growth inhibitory pathway in SCLC-bearing GM1 ganglioside. Addition of CT results in decreased responsiveness to several mitogenic stimuli. These results suggest novel therapeutic approaches to human SCLC.

Epidermal growth factor induces the accumulation of calpactin II on the cell surface during membrane ruffling

Confluent and proliferatively quiescent T51B rat liver epithelial cells provide a cellular model for the study of epidermal growth factor (EGF) effects in non-neoplastic cells. Immunoreactive calpactin II, a well-known substrate for EGF-receptor kinase, was found predominantly in the cytosol, although a second immunoreactive pool was found in a Triton X-100-extractable membrane fraction. Stimulation with EGF resulted in a rapid and transient (2-5 min) formation of ruffles at the cell surface and at the cell-cell contacts. Both calpactin II and filamentous actin were found co-localized at the membrane ruffles. Immunoprecipitations of membrane-bound calpactin II from 32P-labeled cells indicate a transient EGF-dependent phosphorylation of calpactin II correlating with membrane ruffling. These results suggest a temporal (2-5 min) function for calpactin II at the plasma membrane during the EGF-induced mitogenesis of T51B cells.

Possible involvement of normal p21 H-ras in the insulin/insulinlike growth factor 1 signal transduction pathway

Expression of a mutant H-ras gene confers a transformed phenotype to rat-1 fibroblasts which is basically independent of exogenous growth factors (GFs). Rat-1 cells induced to express high levels of the normal H-ras gene were also found to display a transformed phenotype. In contrast to cells expressing mutant H-ras, these cells were dependent on GFs. We used this difference in GF dependence to analyze a possible involvement of exogenous GFs in H-ras function. Compared with untransformed rat-1 cells, cells overexpressing normal H-ras displayed an elevated response toward insulinlike growth factor 1 (IGF-1), insulin, and bombesin and an increased sensitivity toward phosphatidic acids. It was found that 8-bromo-cyclic AMP inhibited the responses to all GFs in rat-1 cells but had no effect on mutant-H-ras-transformed cells. In cells overexpressing normal H-ras, 8-bromo-cyclic AMP inhibited the responses to all GFs except those to insulin and IGF-1. This implies that overexpression of normal H-ras in the presence of insulin/IGF-1 is functionally similar to the expression of mutant H-ras, since mutant H-ras can circumvent this block by itself. These and other results strongly suggest a functional linkage between insulin/IGF-1 and normal p21 H-ras.

ras p21 oncoprotein is autoregulated and acts as a potential mediator of insulin action or the H-ras1 promoter

Rat fibroblast cells carrying an exogenous normal or mutant T24 human H-ras1 gene were transfected with plasmids carrying the normal or mutant T24 H-ras1 gene promoter linked to the reporter chloramphenicol acetyl transferase (CAT) gene and the cells were treated with insulin. We found that the H-ras1 gene was positively autoregulated and that insulin potentiated the response of the T24 ras p21 to the H-ras1 gene promoter. We have also examined the effect of insulin directly on the H-ras1 promoter by treating stable transfectants obtained after transfection of rat fibroblasts with plasmids carrying the normal or mutant T24 H-ras1 gene promoter linked to the reporter CAT gene and the selectable marker aminoglycoside phosphotransferase (aph) gene. We found that insulin appeared to have no direct effect on the H-ras1 promoter in this case, suggesting that the effect is mediated through the ras p21 oncogene product. We suggest that the mutant T24 H-ras p21 protein mediates the action of insulin.

Possible involvement of normal p21 H-ras in the insulin/insulinlike growth factor 1 signal transduction pathway

Expression of a mutant H-ras gene confers a transformed phenotype to rat-1 fibroblasts which is basically independent of exogenous growth factors (GFs). Rat-1 cells induced to express high levels of the normal H-ras gene were also found to display a transformed phenotype. In contrast to cells expressing mutant H-ras, these cells were dependent on GFs. We used this difference in GF dependence to analyze a possible involvement of exogenous GFs in H-ras function. Compared with untransformed rat-1 cells, cells overexpressing normal H-ras displayed an elevated response toward insulinlike growth factor 1 (IGF-1), insulin, and bombesin and an increased sensitivity toward phosphatidic acids. It was found that 8-bromo-cyclic AMP inhibited the responses to all GFs in rat-1 cells but had no effect on mutant-H-ras-transformed cells. In cells overexpressing normal H-ras, 8-bromo-cyclic AMP inhibited the responses to all GFs except those to insulin and IGF-1. This implies that overexpression of normal H-ras in the presence of insulin/IGF-1 is functionally similar to the expression of mutant H-ras, since mutant H-ras can circumvent this block by itself. These and other results strongly suggest a functional linkage between insulin/IGF-1 and normal p21 H-ras.

Regulation by intracellular Ca2+ and cyclic AMP of the growth factor-induced ruffling membrane formation and stimulation of fluid-phase endocytosis and exocytosis

Insulin, insulin-like growth factor-I (IGF-I), and epidermal growth factor (EGF) induce formation of ruffling membranes [T. Kadowaki et al. (1986) J. Biol. Chem. 261, 16,141-16,147] and stimulate the fluid-phase endocytosis and exocytosis [Y. Miyata et al. (1988) Exp. Cell Res. 178, 73-83] in human epidermoid carcinoma KB cells. An increase in intracellular Ca2+ concentration by treatment with A23187, a calcium ionophore, or an increase in intracellular cAMP level by treatment with dibutyryl cAMP or forskolin almost completely inhibited the insulin-, IGF-I-, or EGF-induced formation of ruffling membranes. Increases in Ca2+ or cAMP concentration also inhibited almost completely the stimulation of fluid-phase endocytosis and exocytosis elicited by these growth factors. These results suggest that the growth factor-induced ruffling membrane formation and the stimulation of fluid-phase endocytosis and exocytosis have a common regulatory mechanism involving intracellular concentrations of Ca2+ and cAMP. 125I-EGF binding assays and immunoprecipitation experiments with anti-phosphotyrosine antibody revealed that treatment of KB cells with A23187, dibutyryl cAMP, or forskolin did not inhibit the EGF binding to the cells nor subsequent tyrosine autophosphorylation of its receptors. These results indicate that Ca2+- and/or cAMP-sensitive intracellular reactions exist downstream from the receptor kinase activation in the process of these early cellular responses.

Fourier analysis of cell motility: correlation of motility with metastatic potential

We report the development of a computerized, mathematical system for quantitating the various types of cell motility. This Fourier analysis method simultaneously quantifies for individual cells (i) temporal changes in cell shape represented by cell ruffling, undulation, and pseudopodal extension, (ii) cell translation, and (iii) average cell size and shape. This spatial-temporal Fourier analysis was tested on a series of well-characterized animal tumor cell lines of rat prostatic cancer to study in a quantitative manner the correlation of cell motility with increasing in vivo metastatic potential. Fourier motility coefficients measuring pseudopodal extension correlated best with metastatic potential in the cell lines studied. This study demonstrated that Fourier analysis provides quantitative measurement of cell motility that may be applied to the study of biological processes. This analysis should aid in the study of the motility of individual cells in various areas of cellular and tumor biology.

Rapid stimulation of fluid-phase endocytosis and exocytosis by insulin, insulin-like growth factor-I, and epidermal growth factor in KB cells

Effects of growth factors on fluid-phase endocytosis and exocytosis in human epidermoid carcinoma KB cells were examined by measuring horseradish peroxidase (HRP) as a marker. Insulin, insulin-like growth factor-I (IGF-I), and epidermal growth factor (EGF) promoted HRP accumulation. They also stimulated the efflux of the preloaded HRP from the cells. From these results it follows that these growth factors stimulate the influx as well as the efflux of HRP, because the accumulation rate is the sum of the influx rate and the efflux rate. The stimulation of both HRP accumulation and HRP efflux was rapidly induced within 2-4 min of the addition of growth factors and persisted for at least 60 min. The concentrations eliciting half-maximal stimulatory effects of insulin, IGF-I, and EGF were about 5 X 10(-7), 1 X 10(-9), and 5 X 10(-10) M, respectively. aIR-3 (anti-type I IGF receptor antibody) completely blocked the stimulation of HRP accumulation by IGF-I but very slightly inhibited the stimulation by insulin. The 528 IgG (anti-EGF receptor antibody) inhibited the stimulation of HRP accumulation by EGF. These results indicated that each of these growth factors stimulates the HRP accumulation mediated by the corresponding (homologous) growth factor receptors. The rapid stimulation of fluid-phase influx and efflux may constitute one of the common early cellular responses to growth factors.

Induction of circular membrane ruffling on human fibroblasts by platelet-derived growth factor

One of the earliest effects of platelet-derived growth factor (PDGF) on human fibroblasts in culture is an induction of membrane ruffling. The morphology of the ruffles induced by PDGF is unique in that they form circular arrangements on the dorsal side of the cells. Here we report that the induction of circular ruffle arrangements is an effect specific for PDGF, dose-dependent and inhibitable by anti-PDGF antibodies. We have attempted to utilize this effect to design a rapid and sensitive bioassay for PDGF. The "membrane ruffling assay" is compared with other methods to measure PDGF and its specificity with regard to the different dimeric forms of PDGF is discussed. Introduction of Ca2+ into the cells via the Ca2+ ionophore A23187 or the addition of the tumor-promor 12-O-tetradecanoylphorbol-13-acetate (TPA), which is a stimulator of protein kinase C, does not induce circular ruffle formations on human fibroblasts, neither does the addition of the combination of these two agents. However, addition of TPA almost completely inhibits PDGF-induced circular ruffle formations. Further, we find a shift in the time-course of the PDGF-induced circular ruffle formations by sodium orthovanadate, an inhibitor of protein tyrosine phosphatases. This may indicate the involvement of protein phosphorylation in the regulation of PDGF-induced membrane ruffling.

Alteration in growth, cell morphology, and cytoskeletal structures of KB cells induced by epidermal growth factor and transforming growth factor-beta

Long-term biological effects of epidermal growth factor (EGF), insulin, insulin-like growth factor-I (IGF-I), and transforming growth factor-beta (TGF-beta) were examined with human epidermoid carcinoma KB cells. EGF inhibited the growth of KB cells in both serum-containing and serum-free synthetic media by reducing the growth rate and by lowering the saturation density. The cells cultured with EGF showed relatively high motility and grew dispersely as single cells, whereas the cells cultured in the absence of EGF grew in clusters. Although TGF-beta itself did not inhibit the growth of KB cells, it augmented the growth inhibition by EGF. TGF-beta also affected the cell morphology. In the presence of TGF-beta, the cells became flattened and actin stress fibers were well developed compared to those cultured in its absence. The effects of EGF on growth, cell motility, and cell morphology were reversible. Tyrosine phosphorylation of EGF receptors was continuously observed for at least 50 h in the presence of EGF. TGF-beta did not increase the phosphorylation induced by EGF. These results suggested that signals continuously transmitted through EGF receptors caused the changes in cell growth and morphology and that TGF-beta did not act on the cells by modulating binding of EGF to its receptors or activation of the receptor kinase. In contrast to EGF and TGF-beta, neither insulin nor IGF-I affected cell morphology or growth, although KB cells express their receptors and the receptor kinases were also continuously activated during exposure of the cells to insulin or IGF-I.