The effect of GTPase activating protein upon ras is inhibited by mitogenically responsive lipids

Hydrolysis of phosphatidylcholine by phospholipase D is a common response to mitogens which stimulate inositol lipid hydrolysis in Swiss 3T3 fibroblasts

The stimulated hydrolysis of inositol lipids and phosphatidylcholine (PtdCho) by bombesin, [Arg8]vasopressin ([Arg8]Vp) and prostaglandin F2 alpha (PGF2 alpha) was analysed in Swiss 3T3 cells pre-labelled to isotopic equilibrium with either [methyl-3H]choline, myo-[2-3H]inositol or [9,10 (n)-3H]palmitic acid. All three agonists activated the phospholipase D-catalysed hydrolysis of PtdCho as determined by the release of [3H]choline (Cho) and the formation of [3H]phosphatidylbutanol (PtdBut). The release of [3H]choline by each agonist exhibited similar sensitivity to prolonged pre-exposure to the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA). The release of [3H]choline exhibited the same dose dependency as the production of total inositol phosphates for each mitogen suggesting that the two responses might be mediated through identical receptors. Acute pre-treatment with TPA allowed the dissociation of inositol lipid hydrolysis from PtdCho breakdown, since it inhibited inositol phosphate accumulation but stimulated choline generation. The loss of mitogen stimulated choline release in cells pre-treated with the phorbol ester for 48 h was not due to loss of stimulated inositol phosphate production which was reproducibly enhanced in these 'down-regulated' cells.

The role of phospholipases and phospholipid-derived signals in cell activation

The complexity of receptor-regulated breakdown and modification of phospholipids continues to grow. New developments extend our concepts of signalling enzymes and possible messengers.

Signal transduction by interferon-alpha through arachidonic acid metabolism

Molecular mechanisms that mediate signal transduction by growth inhibitory cytokines are poorly understood. Type I (alpha and beta) interferons (IFNs) are potent growth inhibitory cytokines whose biological activities depend on induced changes in gene expression. IFN-alpha induced the transient activation of phospholipase A2 in 3T3 fibroblasts and rapid hydrolysis of [3H]arachidonic acid (AA) from prelabeled phospholipid pools. The phospholipase inhibitor, bromophenacyl bromide (BPB), specifically blocked IFN-induced binding of nuclear factors to a conserved, IFN-regulated enhancer element, the interferon-stimulated response element (ISRE). BPB also caused a dose-dependent inhibition of IFN-alpha-induced ISRE-dependent transcription in transient transfection assays. Specific inhibition of AA oxygenation by eicosatetraynoic acid prevented IFN-alpha induction of factor binding to the ISRE. Treatment of intact cells with inhibitors of fatty acid cyclooxygenase or lipoxygenase enzymes resulted in amplification of IFN-alpha-induced ISRE binding and gene expression. Thus, IFN-alpha receptor-coupled AA hydrolysis may function in activation of latent transcription factors by IFN-alpha and provides a system for studying the role of AA metabolism in transduction of growth inhibitory signals.

Serum stimulation of NIH 3T3 cells induces the production of lipids able to inhibit GTPase-activating protein activity

Quiescent NIH 3T3 cells were stimulated with serum prior to the extraction of total cellular lipids. These lipids were fractionated on thin-layer chromatography plates, and individual fractions were tested for the ability to inhibit GTPase-activating protein (GAP) activity. Two separate GAP inhibitory lipids were produced. One behaved similarly to arachidonic acid during silica gel chromatography, whereas the other was related to a phosphoinositide. Further study of the arachidonic acid-related material indicated that it was produced between 1 and 5 min after serum addition but was never observed in high-density, contact-inhibited cultures. The identity of these lipids is under investigation. The possibility raised by these results, that a metabolite of arachidonic acid is involved in mitogenic signaling, was supported by the finding that several lipoxygenase products of arachidonic acid efficiently inhibited GAP activity. These results provide further support for the hypothesis that lipids, GAP, and ras activity function together in the control of cellular proliferation.

The mitogenic activities of phosphatidate are acyl-chain-length dependent and calcium independent in C3H/10T1/2 cells

Phosphatidates (PA or phosphatidic acid) were shown to have mitogenic properties, including the stimulation of DNA synthesis and calcium mobilization in C3H/10T1/2 cells. Their continuous presence for a minimum of 7 h induced DNA synthesis with kinetics similar to that observed when 10% fetal bovine serum was used as a mitogen. PAs with long chain saturated fatty acid moieties were more mitogenic, in a dose-dependent fashion, than PAs with short saturated or unsaturated fatty acid moieties. When compared with lysostearoyl-PA (LSPA), distearoyl-PA (DSPA) was as potent with respect to the induction of DNA synthesis. Lysooleoyl-PA (LOPA) was slightly more potent than dioleoyl-PA (DOPA), but much weaker than DSPA and LSPA. Preincubation with dilauroyl-PA (DLPA) reduces the mitogenic effect of DSPA by 85%. The pattern of mitogenic inhibition suggests that a chain-length-independent, yet PA-specific, mechanism is involved. Both DSPA and DLPA are equally taken up by the cells after 30 min. LOPA, but not LSPA, produced a large calcium transient (1.3 microM), which we found to be derived from intracellular sources. DSPA, the most mitogenic PA tested, produced a weaker transient (0.6 microM). Interestingly, LSPA did not produce any detectable calcium transient. These results suggest that the chain-length-specific step in the signaling mechanism of PA occurs after the initial chain-length-independent partitioning and/or binding to the membrane and that the induction of DNA synthesis is not related to the observed calcium transients.

The ras superfamily of small GTP-binding proteins

Considerable advances have recently been made in understanding the structure and function of the proteins encoded by the ras proto-oncogenes. In addition, a large number of ras-related small GTP-binding proteins with very diverse activities have now been identified. This review explores developments in this rapidly expanding field.

Serum stimulation of NIH 3T3 cells induces the production of lipids able to inhibit GTPase-activating protein activity

Quiescent NIH 3T3 cells were stimulated with serum prior to the extraction of total cellular lipids. These lipids were fractionated on thin-layer chromatography plates, and individual fractions were tested for the ability to inhibit GTPase-activating protein (GAP) activity. Two separate GAP inhibitory lipids were produced. One behaved similarly to arachidonic acid during silica gel chromatography, whereas the other was related to a phosphoinositide. Further study of the arachidonic acid-related material indicated that it was produced between 1 and 5 min after serum addition but was never observed in high-density, contact-inhibited cultures. The identity of these lipids is under investigation. The possibility raised by these results, that a metabolite of arachidonic acid is involved in mitogenic signaling, was supported by the finding that several lipoxygenase products of arachidonic acid efficiently inhibited GAP activity. These results provide further support for the hypothesis that lipids, GAP, and ras activity function together in the control of cellular proliferation.

A cytoplasmic protein inhibits the GTPase activity of H-Ras in a phospholipid-dependent manner

A cytoplasmic protein has been identified that inhibits the guanosine triphosphatase (GTPase) activity of bacterially synthesized, cellular H-Ras protein. This GTPase inhibiting protein is able to counteract the activity of GTPase activating protein (GAP), which has been postulated to function as a negative regulator of Ras activity. The potential biological importance of the GTPase inhibiting protein is further supported by its interaction with lipids. Phospholipids produced in cells as a consequence of mitogenic stimulation increase the activity of the GTPase inhibiting protein, as well as inhibit the activity of GAP. The interaction of such lipids with each of these two regulatory proteins would, therefore, tend to increase the biological activity of Ras and stimulate cell proliferation.

The cellular functions of small GTP-binding proteins

A substantial number of novel guanine nucleotide binding regulatory proteins have been identified over the last few years but the function of many of them is largely unknown. This article will discuss a particular family of these proteins, structurally related to the Ras oncoprotein. Approximately 30 Ras-related small guanosine triphosphate (GTP)-binding proteins are known, and from yeast to man they appear to be involved in controlling a diverse set of essential cellular functions including growth, differentiation, cytoskeletal organization, and intracellular vesicle transport and secretion.

ras p21 and GAP inhibit coupling of muscarinic receptors to atrial K+ channels

The signal-transducing G protein Gk couples muscarinic receptors to K+ (K+[ACh]) channels in atrial cells. Recombinant human ras p21 GAP (GTPase activating protein) at subnanomolar concentrations inhibited GTP-dependent channel opening in isolated atrial cell membranes. This inhibition depended on interaction of GAP with ras p21 in the isolated membranes. In addition, recombinant ras p21 proteins blocked the currents; this effect could be blocked by prior incubation of membranes with specific anti-GAP antibodies. We therefore propose that ras p21 GTP complexed with GAP (ras p21-GAP) blocks K+[ACh] currents. The channel block could be overcome by GTP gamma S activation of endogenous Gk; this indicates that ras p21-GAP does not interfere with interaction of Gk with the K+[ACh] channel directly, but prevents coupling of the muscarinic receptor to endogenous Gk.

Oncogenes and anti-oncogenes

The study of oncogenes offers insights into many steps in signal transduction. Rapid progress is possible because of the combination of biochemistry and genetics--unique in vertebrate cell biology--the availability of specific clones and antibodies, sequence information, dominant mutants, and biochemical assays of function. The wealth of detail on oncogenes and proto-oncogenes continues to increase dramatically. Hopefully, in the next year or two some of the gaps will be filled in and all the steps along at least one pathway from the cell membrane to the nucleus will be understood.

Multiple GTP-binding proteins from cholinergic synaptic vesicles

Cholinergic synaptic vesicles purified from the electric organ of the marine ray, Discopyge ommata, contain 2 different size classes of GTP-binding proteins: one or more with an apparent molecular weight (MW) between 37 and 41 kDa, and 3 major and at least 2 minor proteins with MW between 20 and 29 kDa. These GTP-binding proteins were detectable using the alpha 32P-GTP overlay technique and covalent modification with bacterial toxins. The higher MW GTP-binding proteins are ADP-ribosylated by pertussis toxin and 2 of the lower MW GTP-binding proteins are sensitive to botulinum toxin.

Inhibition by phospholipids of the interaction between R-ras, rho, and their GTPase-activating proteins

Certain lipids were found to inhibit the interaction between rho and R-ras proteins and their respective GTPase-activating proteins (GAP). Inhibitory lipids were similar for each protein but differed significantly from those previously found to inhibit the interaction between ras protein and GAP activity. These data raise the possibility that ras and related proteins are controlled biologically by interactions between lipids and GAP molecules.

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.

Rapid actions of 1 alpha,25-dihydroxyvitamin D3 on Ca2+ and phospholipids in isolated rat liver nuclei

The effects of 1 alpha,25-dihydroxyvitamin D3 (1 alpha,25-(OH)2D3) on Ca2+ levels and phospholipid metabolism were studied in isolated nuclei prepared from rat liver. Nuclear Ca2+ concentration was estimated with the fluorescent indicator Fura 2. In agreement with previous reports, ATP (1 mM) produced a rapid increase in nuclear Ca2+ from 188 +/- 25 to 593 +/- 121 nM. Exposure to 1 alpha,25-(OH)2D3 (20 nM) also produced a rapid increase in nuclear Ca2+ to 402 +/- 71 nM. The 1 beta epimer of 1 alpha,25-(OH)2D3 had no effect. Nuclear phosphatidylinositol was labeled by incubation with [gamma-32P]ATP for 3 h. 1 alpha,25-(OH)2D3 produced a two-fold increase in [32P]lysophosphatidylinositol (LPI) within 5 min from 44 +/- 11 to 87 +/- 19 cpm/2.5 x 10(7) nuclei. 1 beta,25-(OH)2D3 had no effect on [32P]LPI production. Exposure of nuclei to exogenous LPI (15 microM) produced an instantaneous increase in nuclear Ca2+ to 372 +/- 81 nM, comparable to ATP and 1 alpha,25-(OH)2D3. The rapid effects of 1 alpha,25-(OH)2D3 on phospholipid metabolism and Ca2+ in isolated nuclei suggest that the steroid may exert effects distinct from the well-characterized receptor-mediated changes in gene expression.

Inhibition by phospholipids of the interaction between R-ras, rho, and their GTPase-activating proteins

Certain lipids were found to inhibit the interaction between rho and R-ras proteins and their respective GTPase-activating proteins (GAP). Inhibitory lipids were similar for each protein but differed significantly from those previously found to inhibit the interaction between ras protein and GAP activity. These data raise the possibility that ras and related proteins are controlled biologically by interactions between lipids and GAP molecules.

Lysophosphatidate-induced cell proliferation: identification and dissection of signaling pathways mediated by G proteins

Lysophosphatidate (LPA), the simplest natural phospholipid, is highly mitogenic for quiescent fibroblasts. LPA-induced cell proliferation is not dependent on other mitogens and is blocked by pertussis toxin. LPA initiates at least three separate signaling cascades: activation of a pertussis toxin-insensitive G protein mediating phosphoinositide hydrolysis with subsequent Ca2+ mobilization and stimulation of protein kinase C; release of arachidonic acid in a GTP-dependent manner, but independent of prior phosphoinositide hydrolysis; and activation of a pertussis toxin-sensitive Gi protein mediating inhibition of adenylate cyclase. The peptide bradykinin mimics LPA in inducing the first two responses but fails to activate Gi and to stimulate DNA synthesis. Our data suggest that the mitogenic action of LPA occurs through Gi or a related pertussis toxin substrate and that the phosphoinositide hydrolysis-protein kinase C pathway is neither required nor sufficient, by itself, for mitogenesis. The results further suggest that LPA or LPA-like phospholipids may have a novel role in G protein-mediated signal transduction.

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.

Phospholipids containing polyunsaturated fatty acyl groups are mitogenic for normal mouse mammary epithelial cells in serum-free primary cell culture

Epithelial cells obtained by collagenase digestion of mammary glands from virgin BALB/c mice were cultured in collagen gels in serum-free basal medium containing insulin (10 micrograms/ml), to which lipids or growth factors were added. Synthetic phospholipids were added as liposomes. Dilinoleoyl phosphatidic acid or phosphatidylserine or epidermal growth factor stimulated multifold growth. The optimum mitogenic effect of the phospholipids was dependent upon the presence of a polyunsaturated fatty acid esterified to the sn-2 position of the glycerol moiety. Dilinoleoyl phosphatidylcholine also stimulated growth but was generally less stimulatory than phosphatidylserine or phosphatidic acid, and phosphatidylethanolamine did not stimulate growth. Studies using phospholipids radiolabeled in either the sn-2 fatty acyl group or the glycerol backbone showed that the relative effect of phospholipids on growth did not correlate directly with the extent of their incorporation into cellular lipid, indicating that phospholipid turnover was the more important determinant for mitogenesis. Analysis of phosphatidic acid-stimulated growth suggested that both cAMP-dependent and cAMP-independent pathways were involved. Thus, mitogenic phospholipids stimulate proliferation by activating (directly or indirectly) multiple growth-regulatory pathways in mammary epithelial cells.

Expression of membrane receptors and (proto)oncogenes during the ontogenic development and neoplastic transformation of the intestinal mucosa

The functional relationship between membrane receptors involved in signal transduction and (proto) oncogene expression has been explored during the ontogenic development and differentiation of the intestinal mucosa in man and rat. The present review develops detailed picture of the current understanding of some mechanisms underlying growth and function of normal, immortalized and cancerous intestinal epithelial cells.

Protein kinases in neutrophils: a review

In chemotactic factor-stimulated neutrophils, rapid increases of intracellular levels of cyclic AMP, calcium, and diacylglycerol have been observed and may be linked to protein kinase activation. The study of the physiological role and regulation of protein kinases in the neutrophil and the identification of their substrates has provided valuable information on the molecular mechanism of neutrophil activation. The focus of this review is on those aspects of protein kinases that are relevant to neutrophil activation and on the substrate proteins for these protein kinases. The possible role of protein phosphorylation in neutrophil function is also discussed.