Detection of alpha-transducin in retinal rods but not cones.

The distribution in chicken retina of the alpha subunit of transducin, the guanine nucleotide--binding protein that couples light-dependent activation of rhodopsin with activation of guanosine 3',5'-monophosphate phosphodiesterase, was determined with the aid of a specific antiserum. alpha-Transducin was found in rod photoreceptor cells but was not detected in cones. These results show that rods and cones differ with respect to alpha-transducin content and suggest that the processes of phototransduction may differ correspondingly in rods and cones.

Binding of Gbetagamma subunits to cRaf1 downregulates G-protein-coupled receptor signalling.

Receptors of the seven transmembrane domain family are coupled to heterotrimeric G proteins [1]. Binding of ligand to these receptors induces dissociation of the heterotrimeric complex into free GTP-Galpha and Gbetagamma subunits, which then interact with their respective effector molecules to stimulate specific cellular responses. In some cases, these cellular responses involve mitogenic signalling [2]. The mitogen-activated protein (MAP) kinase cascade is initiated by the protein kinase cRaf1 and links growth factor receptor signalling to cell growth and differentiation [3]. The main activator of cRaf1 is the small GTP-binding protein Ras [4], and the binding of cRaf1 to GTP-Ras translocates cRaf1 to the plasma membrane, where it is activated [5]. It has been reported that cRaf1 associates directly with the beta subunit of heterotrimeric G proteins in vitro, and with the betagamma subunit complex in vivo [6], but the role of this association is not yet understood. Here, we show that cRaf1 associates with Gbeta1gamma2, and that this association in mammalian cells is significantly enhanced when active p21(Ras) is present or when cRaf1 is otherwise targeted to the membrane. Association with Gbeta1gamma2 has no effect on the kinase activity of cRaf1, but cRaf1 can affect Gbetagamma-mediated signalling events. Thus, membrane-localised cRaf1 inhibits G-protein-coupled receptor (GPCR)-stimulated activation of phospholipase Cbeta (PLCbeta) by sequestration of Gbetagamma subunits, an effect also observed with endogenous levels of cRaf1. Our data suggest that cRaf1 may be an important regulator of signalling by Gbetagamma, particularly in those GPCR systems that stimulate the MAP kinase cascade through the activation of p21(Ras).

TGFbeta1 represses proliferation of pancreatic carcinoma cells which correlates with Smad4-independent inhibition of ERK activation.

Transforming growth factor beta (TGFbeta) is a tumor suppressor acting as inhibitor of cell cycle progression of epithelial cells. We show that treatment of the pancreatic carcinoma cell lines PANC-1 and BxPC-3 with TGFbeta1 inhibits both growth factor-induced activation of the extracellular signal-regulated kinase 2 (ERK2) and translocation of the kinase to the nucleus. TGFbeta1 causes a concentration-dependent reduction of cell proliferation in both cell lines. By measuring ERK activation, we can show that TGFbeta1 is able to repress ERK activation induced by mitogenic stimuli such as EGF. This inhibitory effect of TGFbeta1 is not mediated by suppression of Ras or c-Raf-1 activation, but mediated by TGFbeta1-induced activation of a serine-threonine phosphatase, as demonstrated by inhibition of phosphatases by treatment with okadaic acid. Results obtained in the Smad4-deficient pancreatic carcinoma cell line BxPC-3, demonstrate that TGFbeta1-induced growth inhibition is mediated by a Smad4-independent prevention of ERK2 activation. In contrast to the effects of TGFbeta1 on epithelial cells, mesenchymal NIH3T3 fibroblasts exhibit elevated ERK2 activation and increased cell proliferation in response to TGFbeta1 treatment. Smad4-independent phosphatase-mediated inhibition of mitogen-activated ERK2 represents a novel effector pathway contributing to suppression of epithelial pancreatic carcinoma cell proliferation by TGFbeta1, in addition to the well-known Smad-induced tumor suppressor activity of TGFbeta. Oncogene (2000) 19, 4531 - 4541.

Growth factor-dependent activation of the Ras-Raf-MEK-MAPK pathway in the human pancreatic carcinoma cell line PANC-1 carrying activated K-ras: implications for cell proliferation and cell migration.

Human ductal adenocarcinoma of the pancreas frequently carry activating point mutations in the K-ras protooncogene. We have analysed the activity of the Ras-Raf-MEK-MAPK cascade in the human pancreatic carcinoma cell line PANC-1 carrying an activating K-ras mutation. Serum-starved cells and cells grown in medium with serum did not show constitutively activated c-Raf, MEK-1, or p42 MAPK. Stimulation of cells with epidermal growth factor (EGF) or fetal calf serum (FCS) resulted in activation of N-Ras, but not K-Ras, as well as activation of c-Raf, MEK-1, and p42 MAPK. Preincubation of serum-starved cells with MEK-1 inhibitor PD98059 abolished EGF- and FCS-induced MAPK activation, identifying MEK as the upstream activator of MAPK. PANC-1 cells exhibited marked serum-dependence of anchorage-dependent and -independent cell growth as well as cell migration. EGF, alone or in combination with insulin and transferrin, did not induce cell proliferation of serum-starved PANC-1 cells, indicating that activation of MAPK alone was not sufficient to induce cell proliferation. FCS-induced DNA synthesis was inhibited by 40% by the MEK-1 inhibitor. On the other hand, treatment with either FCS or EGF alone resulted in marked, MEK-dependent increase of directed cell migration. Collectively, our results show that the activating K-ras mutation in PANC-1 cells does not result in constitutively increased Raf-MEK-MAPK signaling. Signal transduction via the Ras-Raf-MEK-MAPK cascade is maintained in these cells and is required for growth factor-induced cell proliferation and directed cell migration. Oncogene (2000).

A new functional Ras antagonist inhibits human pancreatic tumor growth in nude mice.

Constitutively active Ras proteins, their regulatory components, and overexpressed tyrosine kinase receptors that activate Ras, are frequently associated with cell transformation in human tumors. This suggests that functional Ras antagonists may have anti-tumor activity. Studies in rodent fibroblasts have shown that S-trans, transfarnesylthiosalicylic acid (FTS) acts as a rather specific nontoxic Ras antagonist, dislodging Ras from its membrane anchorage domains and accelerating its degradation. FTS is not a farnesyltransferase inhibitor, and does not affect Ras maturation. Here we demonstrate that FTS also acts as a functional Ras antagonist in human pancreatic cell lines that express activated K-Ras (Panc-1 and MiaPaCa-2). In Panc-1 cells, FTS at a concentration of 25-100 microM reduced the amount of Ras in a dose-dependent manner and interfered with serum-dependent and epidermal growth factor-stimulated ERK activation, thus inhibiting both anchorage-dependent and anchorage-independent growth of Panc-1 cells in vitro. FTS also inhibited tumor growth in Panc-1 xenografted nude mice, apparently without systemic toxicity. Daily FTS treatment (5 mg/kg intraperitoneally) in mice with tumors (mean volume 0.07 cm3) markedly decreased tumor growth (after treatment for 18 days, tumor volume had increased by only 23+/-30-fold in the FTS-treated group and by 127+/-66-fold in controls). These findings suggest that FTS represents a new class of functional Ras antagonists with potential therapeutic value.

Evidence for the presence of G-proteins, adenylyl cyclase and phospholipase C activities in lymphatic smooth muscle cell membranes.

In plasma membranes derived from bovine mesenteric lymphatic smooth muscle cells, guanine nucleotide and forskolin stimulated adenylyl cyclase (AC) activity in a concentration-dependent manner, indicative of the presence of the stimulatory G-protein Gs linked to AC. There was no significant enzyme inhibition by low concentrations of guanine nucleotide and no effect on basal or guanine nucleotide-stimulated activity following pertussis toxin treatment of cells, suggesting the absence of Gi linked to inhibition of AC. Furthermore, there was no effect of adrenaline, isoprenaline or clonidine on basal or forskolin-stimulated activities, nor was there any specific binding of the beta-adrenoceptor ligand [125I]cyanopindolol to membranes, suggesting that catecholamine receptors do not modulate AC activity in these membranes. Pertussis toxin-mediated ADP ribosylation of membrane proteins and Western immunoblotting analysis revealed the presence of G-protein subunits G alpha i2, G alpha q, and G beta 1. In experiments designed to identify a possible effector enzyme for these G-proteins, membranes were screened with a range of antibodies raised against phospholipase C (PLC) beta, gamma and delta isozymes. Though no evidence was obtained by Western blotting for any of these proteins, PLC activity was concentration-dependently stimulated by Ca2+, but not by AIF4-, GTP[S], or purified G beta gamma subunits. Finally, no specific binding to membranes of the alpha 1-adrenoceptor ligand [3H]prazosin or the alpha 2-adrenoceptor ligand [3H]yohimbine was obtained. In conclusion, this study provides evidence for a Gs-dependent stimulation of AC, and for the presence of Gi2 and Gq/11, which do not appear to regulate a PLC activity also identified in lymphatic smooth muscle cell membranes. Furthermore, neither AC nor PLC appear to be associated with catecholamine receptors.

Functional reconstitution of a receptor-activated signal transduction pathway in Xenopus laevis oocytes using the cloned human C5a receptor.

We have used the polymerase chain reaction to isolate and clone the cDNA encoding the human C5a receptor, and have injected the cDNA-derived receptor cRNA into Xenopus laevis oocytes for functional characterization of the receptor protein. Receptor activity was determined either electrophysiologically by measuring the agonist-dependent opening of [Ca2+]i-dependent Cl- channels, or by analysing the agonist-dependent efflux of 45Ca2+ from the oocytes. Using both methodologies, injection of pure C5a receptor cRNA failed to confer C5a sensitivity on the oocytes. In contrast, marked responses to C5a were observed when the receptor cRNA was supplemented with poly(A)+ RNA isolated from undifferentiated HL-60 cells, which is devoid of C5a receptor mRNA. Binding studies using radioiodinated C5a revealed that the C5a receptor polypeptide was in fact synthesized and targeted to the oocyte plasma membrane in oocytes injected with receptor cRNA alone, and that the level of receptor expression was not influenced by coinjection of poly(A)+ RNA from undifferentiated HL-60 cells. These results strongly suggest that the human C5a receptor requires a specific cofactor(s) lacking in Xenopus oocytes but present in undifferentiated HL-60 cells, to generate intracellular signals in oocytes. Identification and characterization of this factor will provide important information about the molecular mechanisms by which G-protein-coupled receptors activate phospholipase C.

Desensitization of adenylate cyclase and increase of Gi alpha in cardiac hypertrophy due to acquired hypertension.

The present study investigated whether reduced adenylate cyclase activity and an increase in inhibitory guanine nucleotide binding proteins (Gi alpha), which have been observed in the failing human heart, already occur in myocardial hypertrophy before the stage of heart failure. In membranes of hypertrophic hearts from rats with different forms of experimentally induced hypertension without heart failure (one-kidney, one clip rats, deoxycorticosterone-treated rats, and rats with reduced renal mass), basal as well as isoprenaline-, 5'-guanylylimidodiphosphate-, and forskolin-stimulated adenylate cyclase activity was reduced. The activity of the catalyst was depressed in deoxycorticosterone but unchanged in one-kidney, one clip and reduced renal mass compared with controls. The number of beta-adrenergic receptors was similar in all groups. Radioimmunological quantification of Gi alpha proteins revealed an increase by 73% in one-kidney, one clip, 67% in reduced renal mass, but only 20% in deoxycorticosterone compared with sham-operated, age-matched control rats. The increase of Gi alpha was accompanied by smaller changes of pertussis toxin-induced [32P]ADP-ribosylation of a 40-kd membrane protein. It is concluded that Gi alpha contributes to the reduced adenylate cyclase activity in cardiac hypertrophy in one-kidney, one clip and reduced renal mass and to a smaller extent in deoxycorticosterone. It is suggested that an enhanced expression of Gi alpha could occur not only in severe heart failure but also in cardiac hypertrophy and could, therefore, contribute to myocardial depression and progression of disease in heart failure. In addition, Gi alpha might represent an important regulatory mechanism for cardiac adenylate cyclase activity and thus, might play an important role in various cardiac diseases.

Cardiac adenylyl cyclase, beta-adrenergic receptors, and G proteins in salt-sensitive hypertension.

The present study investigated whether high salt intake (8%) in Dahl salt-sensitive and salt-resistant rats with and without hypertension produces a heterologous desensitization of cardiac adenylyl cyclase as observed in various types of hypertension and human heart failure. In membranes from Dahl salt-sensitive rats on a high-salt diet (8%) basal, isoproterenol-, 5'-guanylylimidodiphosphate-, and forskolin-stimulated adenylyl cyclase was reduced compared with the low-salt (0.4%) group and Dahl salt-resistant rats on either 0.4% or 8% sodium chloride. The activity of the catalyst was depressed, and the expression of the immunodetectable inhibitory G proteins Gi alpha was increased in Dahl salt-sensitive rats on 8% sodium chloride, whereas the density of beta-adrenergic receptors and the activity of the stimulatory G protein Gs alpha reconstituted into Gs alpha-deficient S49 cyc- mouse lymphoma cell membranes were unchanged in any condition studied. We conclude that high salt intake in salt-sensitive hypertensive Dahl rats produces hypertension, cardiac hypertrophy, and heterologous desensitization of cardiac adenylyl cyclase. The latter alteration is due to an increase of Gi alpha proteins and a depressed catalyst activity of adenylyl cyclase. The results demonstrate that heterologous adenylyl cyclase desensitization can precede the development of contractile dysfunction in later stages and can occur independently of changes in beta-adrenergic receptors.

Farnesylcysteine analogues inhibit chemotactic peptide receptor-mediated G-protein activation in human HL-60 granulocyte membranes.

Analogues of S-prenylated cysteine like N-acetyl-S-trans,trans-farnesyl-L-cysteine (AFC) have previously been shown to inhibit the carboxyl methylation of proteins carrying a C-terminal S-prenylated cysteine residue and to block the endotoxin-activated serum-elicited chemotactic response of mouse macrophages. Here, we show that AFC inhibits both basal and formyl peptide receptor-stimulated binding of guanosine 5'-O-(3-thiotriphosphate) (GTP[S]) to and hydrolysis of GTP by membranes of myeloid differentiated HL-60 granulocytes. Receptor-stimulated GTP[S] binding and GTP hydrolysis are more sensitive to AFC inhibition than basal G-protein functions. Inhibition of formyl peptide receptor-mediated G-protein activation is also observed for S-trans,trans-farnesyl-3-thiopropionic acid, but not for N-acetyl-S-trans-geranyl-L-cysteine, N-acetyl-L-cysteine, or the methyl ester of AFC, suggesting that the farnesyl moiety and the carboxyl group, but not the peptide bond of AFC are required for inhibition. The observations that exogeneous S-adenosyl-L-methionine is apparently not required for and S-adenosyl-L-homocysteine does not attenuate the inhibitory action of AFC raise the distinct possibility that AFC inhibits receptor-mediated G-protein interaction by a mechanism other than inhibition of protein carboxyl methylation.

The tryptic and chymotryptic fragments of the beta-subunit of guanine nucleotide binding proteins in brain are identical to those of retinal transducin.

The 35-kDa beta-subunit of transducin purified from rod outer segment membranes is cleaved into 2 major fragments by trypsin, and 7 major fragments by chymotrypsin. Identical fragments are visualized by immunoblotting with transducin-beta specific antisera after proteolysis of rod outer segment membranes, purified brain guanine nucleotide binding proteins, and brain membranes. The results indicate that the beta-subunits of transducin and of brain guanine nucleotide binding proteins are not only similar structurally, but are also similarly oriented in membranes with respect to accessibility to proteolytic enzymes.

Receptor-mediated ADP-ribosylation of a phospholipase C-stimulating G protein.

In membranes of myeloid differentiated HL 60 cells, the chemotactic peptide FMLP stimulates phospholipase C via a pertussis toxin-sensitive G protein. FMLP markedly stimulates the cholera toxin-dependent ADP-ribosylation of a 40 kDa protein in these membranes. This effect of FMLP is inhibited by GTP and GTP[S], and is almost completely abolished in membranes of pertussis toxin-pretreated HL 60 cells. Treatment of HL 60 membranes with cholera toxin and NAD markedly inhibits FMLP-stimulated high affinity GTPase. These results suggest that a 40 kDa G protein sensitive to both pertussis and cholera toxin functionally interacts with the formyl peptide receptor of HL 60 cells and, thus, very likely is the G protein that stimulates phospholipase C in this system.

Angiotensin II stimulates angiotensinogen synthesis in hepatocytes by a pertussis toxin-sensitive mechanism.

The role of intracellular messengers in the stimulatory effect of angiotensin II on angiotensinogen synthesis and secretion in hepatocytes was examined. Angiotensinogen secretion was not influenced by modulators of intracellular calcium (calmidazolium, A 23187, Bay K 8644, methoxamine). In contrast, agents decreasing intracellular cAMP (angiotensin II, guanfacine) stimulated, and those increasing cAMP (isoproterenol, glucagon, forskolin) depressed angiotensinogen secretion. An inverse relationship was also observed between cAMP and angiotensinogen mRNA. Pretreatment of hepatocytes with pertussis toxin abolished the stimulation by angiotensin II. It is concluded that angiotensin II-induced stimulation of angiotensinogen synthesis is initiated by inhibition of adenylate cyclase.

Antibodies against a retinal guanine nucleotide-binding protein cross-react with a single plasma membrane protein in non-retinal tissues.

Antisera (AS/1-AS/6) to purified bovine retinal transducin, a guanine nucleotide-binding protein, were produced in 6 rabbits. Immunoblots showed that the antisera varied in their reactivity with the subunits of transducin; AS/1 reacted strongly with all 3 subunits, while the others reacted with only the beta and/or gamma subunits. Only AS/1 specifically immunoprecipitated the alpha subunit radiolabeled with non-covalently bound guanine nucleotides. Immunostaining of plasma membrane proteins from non-retinal tissues with AS-1 revealed a single protein (approx. 35 kDa), most likely representing the beta subunit of the guanine nucleotide-binding proteins (Gs and Gi) associated with adenylate cyclase. Cerebral cortex showed the highest content of this protein. Antisera against transducin provide a highly specific and sensitive probe for quantitation of the beta subunit of Gs and Gi.

Changes in the guanine nucleotide-binding proteins, Gi and Go, during differentiation of 3T3-L1 cells.

Differentiation of 3T3-L1 cells from fibroblasts to adipocytes is accompanied by increased adenylate cyclase response to lipolytic agents. We used pertussis toxin and specific antibodies to measure the inhibitory guanine nucleotide-binding protein, Gi, and the novel G-protein, Go, in membranes from 3T3-L1 cells. Pertussis toxin-dependent labeling of a 39-40 kDa protein showed an initial 30% rise, followed by an 80% fall during differentiation. Immunoblots showed that 3T3-L1 cells contain Go, as well as Gi, and that changes in the former parallel the changes in pertussis toxin labeling. Changes in Gi and GO may contribute to altered adenylate cyclase response during 3T3-L1 cell differentiation.

The GTP-binding regulatory proteins of neuroblastoma x glioma, NG108-15, and glioma, C6, cells. Immunochemical evidence of a pertussis toxin substrate that is neither Ni nor No.

Amounts of the guanine nucleotide binding regulatory proteins which are also pertussis toxin substrates (such as Ni and No) were measured in rat glioma, C6BU-1, cells and in neuroblastoma X glioma, NG108-15, hybrid cells. Measurements were performed both by quantitating pertussis toxin catalyzed ADP-ribosylation and by quantitative immunoblotting with affinity purified antibodies specific for Ni or No. The amounts of pertussis toxin substrate in C6 and NG108-15 cells are 7.5 and 0.6 pmol/mg membrane protein, respectively. These levels are minimum values and higher estimates of the total amounts of N proteins in the two cells are obtained by quantitative immunoblot analysis of the beta-subunit common to all N proteins. Immunoblots with specific antibodies show that NG108-15 cells contain 3.8 pmol/mg of No and detectable but small (less than 0.1 pmol/mg) amounts of Ni. In contrast, C6 cell membranes contain no detectable No and only 0.14 pmol/mg Ni. Thus, C6 cells contain large amounts of a pertussis toxin substrate which is neither Ni nor No.

Mutational analysis of a putative polyphosphoinositide binding site in phospholipase C-beta 2.

The phosphatidylinositol 4,5-bisphosphate (PtdIns-P2)-regulated actin-binding protein gelsolin and most phosphoinositide-specific phospholipases C (PLCs) comprise a basic amino acid motif ((K/R)xxxKxK(K/R); x denotes any amino acid) which was previously suggested to represent a PtdInsP2-binding site commonly present in these proteins. We have challenged this hypothesis for PLC beta 2 by replacing one or several residues of this motif (KILIKNKK; residues 457-464) and examining the functional consequences of these alterations. The results show that the integrity of the basic motif is important for PtdInsP2 hydrolysis by PLC beta 2. Replacement of lysines 463 or 461 by arginine led to reduction or complete loss, respectively, of enzyme activity. The results provide further support to the concept that the function of the basic motif within the various PLCs is to bind the enzyme substrate PtdInsP2.

Interaction of small G proteins with photoexcited rhodopsin.

Bovine rod outer segment (ROS) membranes contain in addition to the heterotrimeric G protein transducin, several small GTP-binding proteins (23-27 kDa). Furthermore, these membranes contain two substrate proteins (about 22 and 24 kDa) for botulinum C3 ADP-ribosyltransferase known to ADP-ribosylate small G proteins in any mammalian cell type studied so far. Most interestingly, [32P]ADP-ribosylation of ROS membrane small G proteins by C3 is regulated by light and guanine nucleotides in a manner similar to pertussis toxin-catalyzed [32P]ADP-ribosylation of the alpha-subunit of transducin. These findings suggest that not only the heterotrimeric G protein transducin but also the C3 substrate small G proteins present in ROS membranes interact with photoexcited rhodopsin and thus contribute to its signalling action.

A peptide corresponding to a potential polyphosphoinositide binding site of phospholipase C-beta 2 enhances its catalytic activity.

A peptide corresponding to a basic consensus amino acid motif present in both actin-binding proteins and phosphoinositide-specific phospholipases C was synthesized and its effect on the activity of a recombinant phospholipase C-beta 2 (PLC beta 2) expressed in baculovirus-infected insect cells was studied. The peptide markedly and specifically stimulated the activity of the enzyme. This stimulatory effect required a particular primary and/or secondary structure of the peptide and occurred without lowering the affinity of the enzyme for Ca2+. The function of the PLC beta 2 segment corresponding to the peptide might be to bind and offer the substrate to the catalytic domain of this enzyme in a more favorable configuration or, alternatively, to interact with a hypothetical inhibitory constraint.

Activation of phosphatidylinositol lipid-specific phospholipase C-beta 3 by G-protein beta gamma subunits.

A novel member of the inositol lipid-specific phospholipase C family, PtdIns-PLC beta 3, is shown to be activated by beta gamma subunits of the heterotrimeric GTP-binding protein, transducin. The activation is a direct effect since it is observed with the purified proteins. Furthermore, the activation is blocked by the GDP-liganded alpha subunit of transducin, confirming that the effect is due to free beta gamma subunits. The implications with respect to receptor-PtdIns-PLC coupling are discussed.