Altered signal transduction in erbB-transformed cells. Implication of enhanced inositol phospholipid metabolism in erbB-induced transformation

Structural and functional aspects of platelet-derived growth factor and its receptors

Platelet-derived growth factor (PDGF) is a dimeric molecule that exists as homodimers or heterodimers of related polypeptide chains (A and B). Two types of PDGF receptor have been identified. The PDGF alpha-receptor binds all three isoforms with high affinity whereas the beta-receptor binds only PDGF-BB with high affinity, PDGF-AB with low affinity and does not appear to bind PDGF-AA. The alpha- and beta-receptors are structurally related, each having an intracellular protein tyrosine kinase domain. Ligand-induced functional activation of the receptors appears to involve receptor dimerization. Binding of PDGF to its receptor is followed by internalization and degradation of the ligand-receptor complex. Experiments with mutant receptors have shown that ligand-induced internalization is not absolutely dependent on the kinase activity of the beta-receptor. The v-sis oncogene of simian sarcoma virus (SSV) is a retroviral version of the PDGF B chain gene and SSV-transformation is mediated by an autocrine PDGF-like growth factor. Formal evidence that the expression of the PDGF beta-receptor is sufficient to confer susceptibility to SSV-transformation has been obtained using porcine endothelial cells expressing a recombinant human beta-receptor. PDGF is a chemotactic agent for several cell types. Recent experiments have shown that the PDGF beta-receptor mediates a chemotactic response and that this effect requires an intact protein tyrosine kinase activity.

Targeting of diacylglycerol degradation to M1 muscarinic receptors by beta-arrestins

Seven-transmembrane receptor (7TMR) signaling is transduced by second messengers such as diacylglycerol (DAG) generated in response to the heterotrimeric guanine nucleotide-binding protein Gq and is terminated by receptor desensitization and degradation of the second messengers. We show that beta-arrestins coordinate both processes for the Gq-coupled M1 muscarinic receptor. beta-Arrestins physically interact with diacylglycerol kinases (DGKs), enzymes that degrade DAG. Moreover, beta-arrestins are essential for conversion of DAG to phosphatidic acid after agonist stimulation, and this activity requires recruitment of the beta-arrestin-DGK complex to activated 7TMRs. The dual function of beta-arrestins, limiting production of diacylglycerol (by receptor desensitization) while enhancing its rate of degradation, is analogous to their ability to recruit adenosine 3',5'-monophosphate phosphodiesterases to Gs-coupled beta2-adrenergic receptors. Thus, beta-arrestins can serve similar regulatory functions for disparate classes of 7TMRs through structurally dissimilar enzymes that degrade chemically distinct second messengers.

A gene encoding phosphatidylinositol-4-phosphate 5-kinase is induced by water stress and abscisic acid in Arabidopsis thaliana

Phosphatidylinositol-4-phosphate 5-kinase (PIP5K) phosphorylates phosphatidylinositol-4-phosphate to produce phosphatidylinositol-4,5-bisphosphate as a precursor of two second messengers, inositol-1,4,5-triphosphate and diacylglycerol, and as a regulator of many cellular proteins involved in signal transduction and cytoskeletal organization. Despite PIP5K playing such an essential role in a number of physiological processes, much still remains to be made clear about its association with plants. Searching the Arabidopsis expression sequence tag database against already known yeast and mammalian PIP5K cDNAs, we identified two clones which partly encode the same Arabidopsis PIP5K and isolated a corresponding full-length cDNA encoding a protein that we designated AtPIP5K1. Recombinant AtPIP5K1 expressed in Escherichia coli possessed a PIP5K activity in vitro. Due to some structural and biochemical differences, AtPIP5K1 was not categorized as either a type I or type II PIP5K. The expression of the AtPIP5K1 mRNA was induced rapidly by treating Arabidopsis plants with drought, salt and abscisic acid, which suggests that AtPIP5K11 is involved in water-stress signal transduction. These data give evidence for a close link between phosphoinositide signaling cascades and water-stress responses in plants.

In vitro and in vivo inhibitory actions of morin on rat brain phosphatidylinositolphosphate kinase activity

Phosphatidylinositol-4,5-bisphosphate occupies a central role in signal transduction and in cellular transformation. Phosphatidylinositol-4,5-bisphosphate is produced by the enzymatic phosphorylation of phosphatidylinositol-4-phosphate by phosphatidylinositolphosphate kinase (EC 2.7.1.68). Inhibition of this enzyme might conceivably lowers the cellular pool of phosphatidylinositol-4,5-bisphosphate, thus constituting a feasible control point in regulating signal transduction and cellular transformation. Morin, a plant flavonoid, was demonstrated to exhibit in vitro inhibitory action on phosphatidylinositolphosphate kinase extracted from rat brain. This inhibition of enzymatic activity was found to be dose-dependent, with an IC50 value of approximately 10 microM morin. Lineweaver-Burk transformation of the inhibition data indicates that inhibition was competitive with respect to ATP. The Ki was calculated to be 5.15 x 10(-6) M. Inhibition was uncompetitive with respect to phosphatidylinositol-4-phosphate. The Ki was determined to be 0.94 x 10(-5) M. Administration of morin to rats led to a decrease in phosphatidylinositolphosphate kinase activity in brain extracts. This in vivo action of morin was found to be dose-dependent and time-dependent. These effects of morin on rat brain phosphatidylinositolphosphate kinase activity are discussed in relation to the other reported biological actions of this flavonoid.

Epidermal growth factor receptor stimulation of diacylglycerol kinase

The epidermal growth factor receptor (EGFR) activates formation of the phospholipid signal messenger phosphatidic acid (PA) on ligand binding. We explored the effects of chronic EGF stimulation on cellular PA in NIH3T3 cells expressing intact EGFR a mutant EGFR (EGFRvIII). The presence of EGFRvIII increased PA levels to twice those induced by chronic EGFR activation. Fatty acid methyl ester analysis revealed a marked increase in oleic acid containing PA. No apparent increase in phospholipase D (PLD) activity was detected, and diacylglycerol (DAG) kinase assays demonstrated a marked preference for dioleoyl DAG in the presence of activated EGFR or EGFRvIII. Levels of PA which were lower than would be predicted by DAG kinase activation are explained by increased phosphatidate phosphohydrolase activity. Specific inhibitors of EGFR kinase and DAG kinase suppressed DAG kinase activation and PA production by EGFRvIII. EGFR kinase activation by chronic exposure to ligand or by deletional mutation stimulates formation of a specific form of signalling PA.

Enhanced phosphoinositide metabolism in colorectal carcinoma cells derived from familial adenomatous polyposis patients

The production of the second messenger molecules diacylglycerol and inositol 1,4,5-trisphosphate is mediated by activated phosphatidylinositol-specific phospholipase C (PLC) enzymes. We report the enhancement of the phosphoinositide metabolism pathway in KMS-4 and KMS-8 cells, both of which are human colorectal carcinoma cell lines derived from familial adenomatous polyposis patients. In these cells, the cellular contents of diacylglycerol and inositol 1,4,5-trisphosphate were constitutively increased and the PLC activity in vitro was significantly high, as compared with those in normal colon cells or in other sporadic colorectal carcinoma cells. Northern and Western analyses showed the high expression levels of both PLC-gamma 1 and PLC-delta 1 in KMS-4 and KMS-8 cells. Moreover, we detected the enhancement of protein-tyrosine kinase activity and tyrosine phosphorylation of PLC-gamma 1 in these KMS cells. These results suggest the involvement of activated phosphoinositide signaling pathways in the colorectal tumorigenesis of familial adenomatous polyposis.

Comparative analysis of inositol phospholipid metabolism in viral and chemical transformation of mammalian cells

1. Inositol phospholipid metabolites were measured from virally and chemically transformed cells. 2. Increased levels of PIP and PIP2 were observed from both transformed cell lines as compared with controls. 3. Intracellular levels of IP3 were also increased approximately three folds in BPV-1 infected ID 13 cells and in 3-MC transformed NIH 3T3 cells. 4. The results suggest that phosphorylation of phosphatidylinositols and enhanced generation of IP3 second messenger molecules are the common signal transducing process leading to the cell transformation.

Oncogenicity of AKR mink cell focus-inducing murine leukemia virus correlates with induction of chronic phosphatidylinositol signal transduction

Naturally occurring recombinant murine leukemia viruses (MuLVs), termed mink cell focus-inducing (MCF) viruses, are the proximal leukemogens in spontaneous thymic lymphomas of AKR mice. The mechanism by which these viruses transform lymphocytes is not clear. Previous studies have implicated either integrational activation of proto-oncogenes, chronic autocrine immune stimulation, and/or autocrine stimulation of growth factor receptors (e.g., interleukin 2 receptors) via binding of the viral env glycoprotein (gp70) to these receptors. Any one of these events could also involve activation of second messenger signaling pathways in the cell. We examined whether infection with oncogenic AKR-247 MCF MuLV induced transmembrane signaling cascades in thymocytes of AKR mice. Cyclic AMP levels were not changed, but there was enhanced turnover of phosphatidylinositol phosphates, with concomitant increases in diacyglycerol and inositol 1,4,5-triphosphate. Thus, phospholipase C activity was increased. Protein kinase C activity was also elevated in comparison to that in uninfected thymocytes. The above events occurred in parallel with MCF expression in the thymus and were chronically maintained thereafter. No changes in phospholipid turnover occurred in an organ which did not replicate the MCF virus (spleen) or in thymocytes of AKR mice infected with a thymotropic, nononcogenic MCF virus (AKV-1-C36). Therefore, only the oncogenic MCF virus induced phosphatidylinositol signal transduction. Flow cytometric comparison of cell surface gp70 revealed that AKR-247 MCF virus-infected thymocytes expressed more MCF virus gp70 than did thymocytes from AKV-1-C36 MCF virus-infected mice, suggesting that certain threshold quantities of MCF virus env glycoproteins may be involved in this signaling. This type of signal transduction is not induced by stimulation of the interleukin 2 receptor but is involved in certain oncogene systems (e.g., ras and fms). Its chronic induction by oncogenic MCF MuLV may thus initiate thymocyte transformation.

v-erbB oncogene expression accounts for most variations in protein synthesis after avian erythroblastosis virus infection of chicken embryo fibroblasts: a two-dimensional electrophoresis study

The effect of the v-erbA and/or v-erbB oncogenes on cellular gene expression was investigated after separation by two-dimensional polyacrylamide gel electrophoresis of [35S]methionine-labelled proteins from chicken embryo fibroblasts (CEF), infected by either the avian erythroblastosis virus (AEV) carrying both oncogenes, or by viruses carrying only one of them. We observed significant changes in the synthesis of 34 proteins in AEV-transformed CEF as compared with control cells. The synthesis of 24 of them was increased while the synthesis of the other 10 proteins was decreased. The expression of v-erbB alone is necessary and sufficient to induce changes in the synthesis of 27 proteins while the 7 remaining modifications are observed only in cells expressing v-erbB together with v-erbA. Moreover, the deregulation of protein synthesis by v-erbB-expressing viruses was correlated with the morphological transformation state of cells.

The regulatory role of EF-hand motifs of pig 80K diacylglycerol kinase as assessed using truncation and deletion mutants

To elucidate the regulatory function of EF-hand motifs of pig 80K diacylglycerol (DG) kinase, we constructed and expressed several truncation and deletion mutants of the enzyme in E. coli or COS-7 cells. The bacterially expressed EF-hand region could bind Ca2+ and was suggested to undergo conformational change like calmodulin. A mutant enzyme lacking EF-hands lost Ca(2+)-binding activity, but could be fully activated by phosphatidylserine (PS) or deoxycholate in the absence of Ca2+. The full activation of the wild-type enzyme by PS, on the other hand, was totally dependent on Ca2+. Further, the wild-type enzyme expressed in COS-7 cells was exclusively soluble, whereas the EF-hand-deleted mutant was considerably associated with the membranes. The results suggest that under Ca(2+)-free condition, the EF-hand masks the PS-binding site of the DG kinase, and that the Ca(2+)-binding results in the exposure of the PS-binding site through the conformational change of the EF-hand region.

Sources of diradylglycerols generated during cell growth and phorbol ester stimulation in Madin-Darby canine kidney cells

The molecular species of diacylglycerol and alkylacylglycerol of Madin-Darby canine Kidney (MDCK) cells were analyzed to determine the sources of diradylglycerols generated during cell growth and phorbol ester stimulation. MDCK cells in log phase growth contained higher levels of diacylglycerol and alkylacylglycerol than confluent cells. Both subclasses of diradylglycerol showed higher levels of saturated and monoenoic species during log phase. Glycerol incorporation into diradylglycerols was increased during growth, consistent with an increase in their synthesis de novo. Stimulation with 12-O-tetradecanoylphorbol-13-acetate (TPA), a potent activator of protein kinase C, caused an increase in the level of diacylglycerol but not alkylacylglycerol. Log phase MDCK cells showed a greater response to TPA treatment than confluent cells. The molecular species of diacylglycerol generated during stimulation with either TPA or dioctanoylglycerol closely resembled the species of phosphatidylcholine. These results indicate that TPA and synthetic diacylglycerol stimulate endogenous diacylglycerol production through the hydrolysis of phosphatidylcholine. In contrast, the higher content of diacylglycerol and alkylacylglycerol in replicating MDCK cells is the result of an increase in their synthesis de novo.

Regulation of the metabolism of 1,2-diacylglycerols and inositol phosphates that respond to receptor activation

This review assimilates information on the regulation of the metabolism of those inositol phosphates and diacylglycerols that respond to receptor activation. Particular emphasis is placed on the regulation of specific enzymes, the occurrence of isoenzymes, and metabolic compartmentalization; the overall aim is to demonstrate the significance of these activities in relation to the physiological impact of the various cell signalling processes.

Diacylglycerol kinase: a key modulator of signal transduction?

Diacylglycerol kinase (DGK) plays a central role in the metabolism of diacylglycerol released as a second messenger in agonist-stimulated cells. The major purified form of the enzyme (80 kDa DGK) is highly abundant in lymphocyte cytosol and may become membrane-associated via phosphorylation by protein kinase C. In addition, there are several kinase subspecies immunologically distinct from the 80 kDa enzyme, which differ markedly in their responses to several compounds such as sphingosine and R59022. Thus, further work on each enzyme species is needed to define the function of DGK in stimulated cells.

Phosphoinositide metabolism and oncogenes

The hydrolysis of phosphatidylinositol 4,5-bisphosphate plays important roles in growth factor- or oncogene-induced cell proliferation. However, it is still unknown whether the hydrolysis of phosphatidylinositol 4,5-bisphosphate is an essential step for cell growth. To solve this problem, we developed a monoclonal antibody against the lipid. Injection of the antibody into ras-transformed cells caused reversible and dose-dependent decrease in DNA synthesis and reverted the cell morphology to that of the untransformed cells. The antibody also inhibited the proliferation of erbB- and src-transformed cells but not the proliferation of untransformed or myc-transformed cells.

Synthesis of diacylglycerol de novo is responsible for permanent activation and down-regulation of protein kinase C in transformed cells

We measured the synthesis of diacylglycerol de novo in normal NIH/3T3 fibroblasts and in cells transformed by ras, src, sis and abl oncogenes. Analysis of the incorporation of glucose-derived 14C into diacylglycerol indicated that neosynthesis of diacylglycerol was constitutively active in the transformed cell lines. Elevated levels of diacylglycerol and persistent activation/down-regulation of protein kinase C reduced the binding of phorbol dibutyrate to transformed cells. This phenomenon could be reversed by blocking the glycolytic pathway, thus indicating that neosynthesized diacylglycerol was responsible for persistent activation and down-regulation of protein kinase C. In transformed cells, protein kinase C activity could not be stimulated by the addition of diolein; however, inhibition of glycolysis restored the ability of transformed cells to respond to diolein. Taken together these data indicate that constitutive synthesis of diacylglycerol de novo is responsible for activation and down-regulation of protein kinase C in transformed cells, and it may play a role in altered mitogenic signalling.

Mechanisms of transformation by polyoma virus middle T antigen

This review addresses a fundamental question of polyoma virus biology: What is the molecular mechanism by which the polyoma virus middle T antigen (MTAg) transforms cells in culture? Since MTAg has no known intrinsic biochemical activity, it is believed to act by modulating the properties of the host cell's proteins (see review by Courtneidge [26]). Experiments to date have largely focused on the interaction of MTAg with the cellular tyrosine kinase, pp60c-src. However, recent data from a number of laboratories have demonstrated the importance of other MTAg-associating cellular proteins in MTAg-mediated transformation, including pp62c-yes and a phosphatidylinositol kinase. In this review, we will summarize what is presently known about the proteins interacting with MTAg. The extent to which the currently known details of the biochemistry of MTAg and its associated proteins can explain the transforming properties of the various mutant alleles of MTAg will be assessed.

Functions of sphingolipids and sphingolipid breakdown products in cellular regulation

The discovery that breakdown products of cellular sphingolipids are biologically active has generated interest in the role of these molecules in cell physiology and pathology. Sphingolipid breakdown products, sphingosine and lysosphingolipids, inhibit protein kinase C, a pivotal enzyme in cell regulation and signal transduction. Sphingolipids and lysosphingolipids affect significant cellular responses and exhibit antitumor promoter activities in various mammalian cells. These molecules may function as endogenous modulators of cell function and possibly as second messengers.

Antiphosphotyrosine recovery of phospholipase C activity after EGF treatment of A-431 cells

A tenfold increase in phospholipase C activity specific for phosphatidylinositol 4,5-bisphosphate (PIP2) was immunopurified from extracts of A-431 epidermoid carcinoma cells stimulated with epidermal growth factor. This finding suggests a biochemical link between growth factor-stimulated tyrosine kinase activity and PIP2 hydrolysis.

Disappearance of diacylglycerol kinase translocation in ras-transformed cells

Oncogenic transformation has been considered to be in part a consequence of the elevated levels of 1,2-diacylglycerol(DG), resulting in the permanent activation of protein kinase C. DG content in transformed cells with v-H-ras, c-K-ras and N-ras oncogene increased 1.5-fold compared to that in non-transformed NIH/3T3 cells. DG kinase activity of membrane fractions, which plays an important role in DG attenuation, was significantly lower in all ras-transformed cells. On the contrary, DG kinase activity in cytosol fractions in ras-transformed cells was found to be increased. DG kinase translocated very markedly from cytosol to membranes in non-transformed NIH/3T3 cells by the treatment of phospholipase C. On the other hand, translocation of DG kinase in ras-transformed cells was slight, though the formation of DG by the treatment of phospholipase C was almost same between ras-transformed and NIH/3T3 cells. These results strongly support the idea that the increased DG content in ras-transformed cells is, at least partly due to the defect of DG kinase translocation, which may lead to the sustained activation of protein kinase C.

Effect of transforming growth factor-alpha on inositol phospholipid metabolism in human epidermoid carcinoma cells

Transforming growth factor-alpha (TGF-alpha) stimulates (in a dose-dependent manner) the incorporation of [32P]Pi into phosphatidylinositol (PI), phosphatidylinositol 4-phosphate (PIP), phosphatidylinositol 4,5-bisphosphate (PIP2), and phosphatidic acid (PA) in the human epidermoid carcinoma cell line (A431). The effect of TGF-alpha on the incorporation was found to be similar to that of EGF. On the other hand, a striking difference in the activation of diacylglycerol (DG) kinase activity was seen between TGF-alpha and EGF. At least 100 times more TGF-alpha was required to achieve maximal stimulation of DG kinase activity relative to EGF. These results suggest that the activation of DG kinase by TGF-alpha may involve a mechanism independent from or subsequent to activation of the EGF receptor.

Effect of harvesting methods, growth conditions and growth phase on diacylglycerol levels in cultured human adherent cells

The cellular mass of sn-1,2-diacylglycerols, which are intracellular second messengers which activate protein kinase C, were quantitatively determined with an enzymatic assay. The method employed to harvest cultured human skin fibroblasts or human epidermal A431 cells prior to extraction of lipid into chloroform/methanol affected diacylglycerol (DAG) levels. Scraping or trypsinization significantly increased DAG levels. A method was devised to allow reliable and reproducible DAG measurements from adherent cells. The addition of methanol prior to scraping was shown to stop cellular metabolism and to permit accurate quantitation. Importantly, this solvent was compatible with cultures grown on plastic. Using this method, growth conditions which could affect DAG levels were investigated. Changes in the osmolality of the culture medium did not affect the DAG levels of A431 cells; exposure of A431 cells to acidic pH or elevated temperature lowered DAG levels. In contrast to fibroblasts, the total DAG levels of A431 cells continued to increase during serum deprivation. The highest DAG levels, normalized to phospholipids, were observed during the exponential growth phase. This ratio dropped when the cultures reached confluency. These experiments also demonstrated that A431 cells possess higher DAG levels than do normal fibroblasts. The function of DAG in cellular regulation is discussed.

Modified responsiveness of v-Ha-ras-transfected rat fibroblasts to growth factors and a tumor promoter

The correlation of the phenotypic changes of v-Ha-ras transfected cells with the expression of p21ras and the modified responses to growth factors and a tumor promoter were examined. Transfection of the v-Ha-ras gene together with the neomycin-resistance gene into 208F rat fibroblasts yielded transformed clones characterized by morphological changes, anchorage-independent growth, and tumorigenicity in nude mice. The degrees of these biological alterations were parallel with the expression of mRNA and protein of the ras gene. In ras-transformed cells, anchorage-independent growth was stimulated by epidermal growth factor (EGF), insulin, bombesin, and fibroblast growth factor, whereas in the parental 208F cells, anchorage-independent growth was observed only in the presence of EGF, and there were many fewer EGF-induced colonies than those in the ras-transformed clones. A tumor promoter, 12-O-tetradecanoylphorbol-13-acetate (TPA) also augmented anchorage-independent growth of ras-transformed cells and induced morphological changes in monolayer cultures without altering the expression of the ras gene or phosphorylation of the p21ras protein. Retinoic acid inhibited the TPA-induced anchorage-independent growth. These results showed a good correlation of the expression of p21ras with the phenotypic changes and the increased sensitivity of the p21ras-expressing cells to the stimulation of growth factors and tumor promoter.

Common elements in growth factor stimulation and oncogenic transformation: 85 kd phosphoprotein and phosphatidylinositol kinase activity

The phosphorylation of proteins on tyrosine in vivo and in vitro was examined in 3T3 cells stimulated by platelet-derived growth factor (PDGF) and transformed by polyoma middle T antigen (MTAg) by using an antibody directed against phosphotyrosine (P-tyr). Two common events were observed upon PDGF stimulation or MTAg transformation of cells: the appearance in the immunoprecipitates of an 85 kd phosphoprotein, and increased phosphatidylinositol (PI) kinase activity. In PDGF-stimulated cells, the 85 kd phosphoprotein and PI kinase activity appeared rapidly, within 1 min of growth factor addition. The PI kinase activity and 85 kd phosphorylation were also increased in anti-P-tyr immunoprecipitates from cells transformed by v-fms and v-sis, but not by SV40 T antigen. The presence of the tyrosine-phosphorylated 85 kd protein correlated with PI kinase activity during several purification steps. These results suggest that the 85 kd phosphoprotein, a putative PI kinase, is a substrate for both the PDGF receptor and MTAg/pp60c-src tyrosine kinase activities.

Phospholipase C activation and diacylglycerol kinase inactivation lead to an increase in diacylglycerol content in spontaneously hypertensive rat

Activities of three kinases, phosphatidylinositol (PI), phosphatidylinositol 4-phosphate (PIP), and diacylglycerol (DG) kinases, and phospholipase C were measured in erythrocyte ghosts from spontaneously hypertensive rats (SHR) and normotensive Wistar Kyoto rats (WKY). PI kinase activity was significantly higher in SHR than WKY but there was no significant difference in PIP kinase activity between SHR and WKY. The activity of phospholipase C, which hydrolyzes PIP2, was also increased in SHR. However, DG kinase activity was, on the contrary, decreased in SHR. These results suggest that there is a tendency to accumulate DG in SHR. Indeed, DG content in erythrocytes of SHR increased 1.7-fold compared to that of WKY. Such DG accumulation may cause the sustained activation of protein kinase C in SHR, since DG is a physiological activator for protein kinase C.