Fibroblasts transformed with v-src show enhanced formation of an inositol tetrakisphosphate

Tyrosine phosphorylation of HDAC3 by Src kinase mediates proliferation of HER2-positive breast cancer cells

The role of histone deacetylase 3 (HDAC3) is to repress the expression of various genes by eliminating acetyl group from histone. Thus, the regulation of HDAC3 activity is essential to maintain cellular homeostasis. In this study, we found that HDAC3 interacts with c-Src kinase. However, the interaction between HDAC3 and c-Src was previously reported, it has still been ambiguous whether c-Src phosphorylates HDAC3 and affects the function of HDAC3. First, we confirmed that HDAC3 directly binds to c-Src, and c-Src identified to interact with C-terminal domain (277-428 a.a.) of HDAC3. c-Src also phosphorylated three tyrosine sites of HDAC3 at tyrosine 325, 328, and 331. Importantly, wild-type c-Src increases HDAC3 activity, but not mutant c-Src^K298M (kinase inactive form). When these tyrosine residues are all substituted for alanine residues, the deacetylase activity of mutant HDAC3 was abolished. In addition, a proliferation of HER2-positive breast cancer cells expressing phosphorylation deficient mutant HDAC3 is decreased in comparison with control cells. Thus, our findings suggested that phosphorylation of HDAC3 by c-Src kinase regulates the HDAC3 activity and the proliferation of breast cancer cells.

Inositol 1,4,5-trisphosphate 3-kinases: functions and regulations

Inositol 1,4,5-trisphosphate 3-kinase (IP3 3-kinase/IP(3)K) plays an important role in signal transduction in animal cells by phosphorylating inositol 1,4,5-trisphosphate (IP3) to inositol 1,3,4,5-tetrakisphosphate (IP(4)). Both IP(3) and IP(4) are critical second messengers which regulate calcium (Ca(2+)) homeostasis. Mammalian IP3Ks are involved in many biological processes, including brain development, memory, learning and so on. It is widely reported that Ca(2+) is a canonical second messenger in higher plants. Therefore, plant IP3K should also play a crucial role in plant development. Recently, we reported the identification of plant IP3K gene (AtIpk2beta/AtIP3K) from Arabidopsis thaliana and its characterization. Here, we summarize the molecular cloning, biochemical properties and biological functions of IP3Ks from animal, yeast and plant. This review also discusses potential functions of IP3Ks in signaling crosstalk, inositol phosphate metabolism, gene transcriptional control and so on.

A role for rat inositol polyphosphate kinases rIPK2 and rIPK1 in inositol pentakisphosphate and inositol hexakisphosphate production in rat-1 cells

Over 30 inositol polyphosphates are known to exist in mammalian cells; however, the majority of them have uncharacterized functions. In this study we investigated the molecular basis of synthesis of highly phosphorylated inositol polyphosphates (such as inositol tetrakisphosphate, inositol pentakisphosphate (IP5), and inositol hexakisphosphate (IP6)) in rat cells. We report that heterologous expression of rat inositol polyphosphate kinases rIPK2, a dual specificity inositol trisphosphate/inositol tetrakisphosphate kinase, and rIPK1, an IP5 2-kinase, were sufficient to recapitulate IP6 synthesis from inositol 1,4,5-trisphosphate in mutant yeast cells. Overexpression of rIPK2 in Rat-1 cells increased inositol 1,3,4,5,6-pentakisphosphate (I(1,3,4,5,6)P5) levels about 2-3-fold compared with control. Likewise in Rat-1 cells, overexpression of rIPK1 was capable of completely converting I(1,3,4,5,6)P5 to IP6. Simultaneous overexpression of both rIPK2 and rIPK1 in Rat-1 cells increased both IP5 and IP6 levels. To reduce IPK2 activity in Rat-1 cells, we introduced vector-based short interference RNA against rIPK2. Cells harboring the short interference RNA had a 90% reduction of mRNA levels and a 75% decrease of I(1,3,4,5,6)P5. These data confirm the involvement of IPK2 and IPK1 in the conversion of inositol 1,4,5-trisphosphate to IP6 in rat cells. Furthermore these data suggest that rIPK2 and rIPK1 act as key determining steps in production of IP5 and IP6, respectively. The ability to modulate the intracellular inositol polyphosphate levels by altering IPK2 and IPK1 expression in rat cells will provide powerful tools to study the roles of I(1,3,4,5,6)P5 and IP6 in cell signaling.

Phosphoinositide 3-kinase facilitates antigen-stimulated Ca(2+) influx in RBL-2H3 mast cells via a phosphatidylinositol 3,4,5-trisphosphate-sensitive Ca(2+) entry mechanism

This study presents evidence that phosphoinositide 3-kinase (PI3K) plays a concerted role with phospholipase Cgamma in initiating antigen-mediated Ca(2+) signaling in mast cells via a phosphatidylinositol 3,4,5-trisphosphate (PI(3,4,5)P(3))-sensitive Ca(2+) entry pathway. Exogenous PI(3,4,5)P(3) at concentrations close to its physiological level induces instantaneous Ca(2+) influx into RBL-2H3 cells. This PI(3,4,5)P(3)-induced intracellular Ca(2+) increase is independent of phospholipase C activity or the depletion of internal stores. Moreover, inhibition of PI3K by LY294002 or by overexpression of the dominant negative inhibitor Deltap85 suppresses the Ca(2+) response to the cross-linking of the high affinity receptor for IgE (FcepsilonRI). Concomitant treatment of RBL-2H3 cells with LY294002 or Deltap85 and 2-aminoethyl diphenylborate, a cell-permeant antagonist of D-myo-inositol 1,4,5-trisphosphate receptors, abrogates antigen-induced Ca(2+) signals, whereas either treatment alone gives rise to partial inhibition. Conceivably, PI(3,4,5)P(3)-sensitive Ca(2+) entry and capacitative Ca(2+) entry represent major Ca(2+) influx pathways that sustain elevated [Ca(2+)]i to achieve optimal physiological responses. This study also refutes the second messenger role of D-myo-inositol 1,3,4,5-tetrakisphosphate in regulating FcepsilonRI-mediated Ca(2+) response. Considering the underlying mechanism, our data suggest that PI(3,4,5)P(3) directly stimulates a Ca(2+) transport system in plasma membranes. Together, these data provide a molecular basis to account for the role of PI3K in the regulation of FcepsilonRI-mediated degranulation in mast cells.

Novel function of phosphoinositide 3-kinase in T cell Ca2+ signaling. A phosphatidylinositol 3,4,5-trisphosphate-mediated Ca2+ entry mechanism

This study presents evidence that phosphoinositide (PI) 3-kinase is involved in T cell Ca(2+) signaling via a phosphatidylinositol 3,4, 5-trisphosphate PI(3,4,5)P(3)-sensitive Ca(2+) entry pathway. First, exogenous PI(3,4,5)P(3) at concentrations close to its physiological levels induces Ca(2+) influx in T cells, whereas PI(3,4)P(2), PI(4, 5)P(2), and PI(3)P have no effect on [Ca(2+)](i). This Ca(2+) entry mechanism is cell type-specific as B cells and a number of cell lines examined do not respond to PI(3,4,5)P(3) stimulation. Second, inhibition of PI 3-kinase by wortmannin and by overexpression of the dominant negative inhibitor Deltap85 suppresses anti-CD3-induced Ca(2+) response, which could be reversed by subsequent exposure to PI(3,4,5)P(3). Third, PI(3,4,5)P(3) is capable of stimulating Ca(2+) efflux from Ca(2+)-loaded plasma membrane vesicles prepared from Jurkat T cells, suggesting that PI(3,4,5)P(3) interacts with a Ca(2+) entry system directly or via a membrane-bound protein. Fourth, although D-myo-inositol 1,3,4,5-tetrakisphosphate (Ins(1,3,4, 5)P(4)) mimics PI(3,4,5)P(3) in many aspects of biochemical functions such as membrane binding and Ca(2+) transport, we raise evidence that Ins(1,3,4,5)P(4) does not play a role in anti-CD3- or PI(3,4,5)P(3)-mediated Ca(2+) entry. This PI(3,4,5)P(3)-stimulated Ca(2+) influx connotes physiological significance, considering the pivotal role of PI 3-kinase in the regulation of T cell function. Given that PI 3-kinase and phospholipase C-gamma form multifunctional complexes downstream of many receptor signaling pathways, we hypothesize that PI(3,4,5)P(3)-induced Ca(2+) entry acts concertedly with Ins(1,4,5)P(3)-induced Ca(2+) release in initiating T cell Ca(2+) signaling. By using a biotinylated analog of PI(3,4,5)P(3) as the affinity probe, we have detected several putative PI(3,4,5)P(3)-binding proteins in T cell plasma membranes.

Attenuation of LPA-mediated calcium signaling and inositol polyphosphate production in rat-1 fibroblasts transformed by the v-src oncogene

Alterations in cellular signaling underlie the transforming actions of many oncogenes. The vsrc oncogene tyrosine kinase, pp60vsrc, is known to alter multiple signal transduction pathways, including those involving phosphatidylinositol (PI) metabolism. In this study, we investigated the effects of vsrc-transformation on lysophosphatidic acid (LPA) receptor coupling to intracellular free calcium [Ca2+]i and PI turnover in rat-1 fibroblasts. In normal rat-1 cells, LPA rapidly elevated [Ca2+]i (EC50 = 10nM). In contrast, the ability of LPA to mobilize calcium was markedly attenuated in rat-1-vsrc cells. Further study revealed that the LPA-mediated generation of inositol (1,4,5)P3 and other inositol polyphosphates was also markedly attenuated in the vsrc-transformed cells. Although LPA caused a transient reduction in the level of PI(4,5)P2 in normal rat-1 cells, the agonist elevated the level of PI(4,5)P2 in the vsrc-transformed cells. These findings demonstrate that vsrc-transformation alters the coupling of LPA receptors to PI turnover and calcium signaling in rat-1 cells, and point to G protein-coupled receptor systems as targets for modulation by the vsrc kinase.

D-myo-inositol 1,4,5-trisphosphate 3-kinase A is activated by receptor activation through a calcium:calmodulin-dependent protein kinase II phosphorylation mechanism

D-myo-inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] 3-kinase, the enzyme responsible for production of D-myo-inositol 1,3,4,5-tetrakisphosphate, was activated 3- to 5-fold in homogenates of rat brain cortical slices after incubation with carbachol. The effect was reproduced in response to UTP in Chinese hamster ovary (CHO) cells overexpressing Ins(1,4,5)P3 3-kinase A, the major isoform present in rat and human neuronal cells. In ortho-32P-labelled cells, the phosphorylated 53 kDa enzyme could be identified after receptor activation by immunoprecipitation. The time course of phosphorylation was very similar to that observed for carbachol (or UTP)-induced enzyme activation. Enzyme phosphorylation was prevented in the presence of okadaic acid. Calmodulin (CaM) kinase II inhibitors (i.e. KN-93 and KN-62) prevented phosphorylation of Ins(1,4,5)P3 3-kinase. Identification of the phosphorylation site in transfected CHO cells indicated that the phosphorylated residue was Thr311. This residue of the human brain sequence lies in an active site peptide segment corresponding to a CaM kinase II-mediated phosphorylation consensus site, i.e. Arg-Ala-Val-Thr. The same residue in Ins(1,4,5)P3 3-kinase A was also phosphorylated in vitro by CaM kinase II. Phosphorylation resulted in 8- to 10-fold enzyme activation and a 25-fold increase in sensitivity to the Ca2+:CaM complex. In this study, direct evidence is provided for a novel regulation mechanism for Ins(1,4,5)P3 3-kinase (isoform A) in vitro and in intact cells.

Transformation of Rat-1 fibroblasts with the v-src oncogene induces inositol 1,4,5-trisphosphate 3-kinase expression

Transformation of Rat-1 fibroblasts with the v-src oncogene leads to a 6- to 8-fold enhancement of the activity of the Ins(1,4,5)P3 3-kinase in cytosolic extracts [Johnson, Wasilenko, Mattingly, Weber and Garrison (1989) Science 246, 121-124]. This study confirms these results using another v-src-transformed Rat-1 cell line (B31 cells) and investigates the molecular mechanism by which pp60v-src activates Ins(1,4,5)P3 3-kinase. The mRNA and protein levels for two rat isoforms of Ins(1,4,5)P3 3-kinase were determined in the v-src-transformed cell line. Both the mRNA and protein levels for isoform A were elevated in v-src-transformed Rat-1 cells while those for isoform B were not significantly affected. Moreover, stable expression of either form of Ins(1,4,5)P3 3-kinase in the B31 v-src-transformed Rat-1 cell line did not result in tyrosine phosphorylation of Ins(1,4,5)P3 3-kinase A or B. These results suggest that at least one mechanism by which the v-src oncogene increases the activity of the Ins(1,4,5)P3 3-kinase in the Rat-1 transformed fibroblast is by increasing the level of expression of Ins(1,4,5)P3 3-kinase A.

Transformation of Rat-1 fibroblasts with the v-src oncogene increases the tyrosine phosphorylation state and activity of the alpha subunit of Gq/G11

Two major intermediaries in signal transduction pathways are pp60v-sre family tyrosine kinases and heterotrimeric guanine nucleotide-binding proteins. In Rat-1 fibroblasts transformed by the v-src oncogene, endothelin-1 (ET-1)-induced inositol 1,4,5-trisphosphate accumulation is increased 6-fold, without any increases in the numbers of ET-1 receptors or in the response to another agonist, thrombin. This ET-1 hyperresponse can be inhibited by an antibody directed against the carboxyl terminus of the Gq/G11 alpha subunit, suggesting that the Gq/G11 protein couples ET-1 receptors to phospholipase C (PLC). While v-src transformation did not increase the expression of the Gq/G11 alpha subunit, immunoblotting with anti-phosphotyrosine antibodies and phosphoamino acid analysis demonstrated that the Gq/G11 alpha subunit becomes phosphorylated on tyrosine residues in v-src-transformed cells. Moreover, when the Gq/G11 protein was extracted from control and transformed cell lines and reconstituted with exogenous PLC, AIF*4-stimulated Gq/G11 activity was markedly increased in extracts from v-src-transformed cells. Our results demonstrate that the process of v-src transformation can increase the tyrosine phosphorylation state of the Gq/G11 alpha-subunit in intact cells and that the process causes an increase in the Gq/G11 alpha-subunit's ability to stimulate PLC following activation with AIF-4.

Molecular study and regulation of D-myo-inositol 1,4,5-trisphosphate 3-kinase

D-myo-Inositol 1,4,5-trisphosphate (InsP3) is a critical second messenger involved in signal transduction, i.e., calcium homeostasis. InsP3-kinase directly regulates the levels of InsP3 and D-myo-inositol 1,3,4,5-tetrakisphosphate (InsP4). InsP3 3-kinase is a calmodulin (CaM)-dependent enzyme and is also a target for phosphorylation by protein kinase C (PKC). Molecular cloning of cDNA's encoding proteins presenting InsP3 3-kinase activity establish the existence of distinct isoenzymes (at least three: A, B and C). These isoforms are differentially expressed and regulated by calcium/CaM. Site-directed mutagenesis and chemical modification of InsP3 3-kinase A led to the identification of three charged residues involved in ATP/Mg2+ binding among the catalytic domain and a hydrophobic residue taking part of the CaM binding site.

pp60src-mediated phosphorylation of connexin 43, a gap junction protein

Several laboratories have demonstrated a decrease in gap junctional communication in cells transformed by the src oncogene of the Rous sarcoma virus. The decrease in gap junctional communication was associated with tyrosine phosphorylation of the gap junction protein, connexin 43 (Cx43). This study was initiated to determine if the phosphorylation of Cx43 is the result of a direct kinase-substrate interaction between the highly active tyrosine kinase, pp60v-src, and Cx43. Previous biochemical studies have been limited by the low levels of Cx43 protein in fibroblast cell lines. To obtain larger quantities of Cx43, we constructed a recombinant baculovirus expressing Cx43 in Spodoptera frugiperda (Sf-9) cells and subsequently purified the expressed Cx43 by immunoaffinity chromatography. We observed that this partially purified Cx43 was phosphorylated on tyrosine in vitro in the presence of kinase-active pp60src. Phosphotryptic peptide mapping indicated that the in vitro phosphorylated Cx43 contained phosphopeptides which comigrated with a subset of tryptic peptides prepared from Cx43 phosphorylated in vivo. Furthermore, coinfection of Sf-9 cells with recombinant baculoviruses encoding pp60v-src and Cx43 resulted in the accumulation of phosphotyrosine in Cx43. Taken together, the evidence presented in this paper demonstrates that kinase active pp60c-src is capable of phosphorylating Cx43 in a direct manner. Since the presence of phosphotyrosine on Cx43 is correlated with the down-regulation of gap-junctional communication, these results suggest that pp60v-src regulates gap junctional gating activity via tyrosine phosphorylation of Cx43.

Involvement of growth factor receptors in the mammalian UVC response

Irradiation of HeLa cells with short-wavelength ultraviolet light (UVC) induces the modification and activation of the preexisting transcription factors c-Fos-c-Jun (AP-1) and TCF/Elk-1, as well as the protein synthesis independent transcriptional activation of the c-fos and c-jun genes. This response to UVC is mediated via obligatory cytoplasmic signal transduction, involving Ras and Raf, Src, and MAP kinases. The UVC response is inhibited by prior down-modulation of growth factor receptor signaling upon growth factor prestimulation, by suramin (an inhibitor of receptor activation) or by expression of a dominant negative epidermal growth factor (EGF) receptor mutant. These data suggest the involvement of several growth factor receptors in the UVC response. Indeed, UVC induces the suramin-inhibitable immediate tyrosine phosphorylation of the EGF receptor.

Phosphoinositides and calcium signaling New aspects and diverse functions in cell regulation

Numerous circulating and locally produced hormones bind to specific cell-surface receptors and activate a variety of second-messenger pathways that evoke characteristic phenotypic responses in their target cells. One of the most ubiquitous signal transduction mechanisms is the phosphoinositide-calcium messenger system, which is activated by hormones, neurotransmitters, and growth factors. Stimulation of these receptors by their ligands causes a characteristic change in the metabolism of membrane phospholipids with production of diacylglycerol and a rapid increase in cytoplasmic Ca(2+) concentration, due to the release of stored intracellular Ca(2+) and stimulated Ca(2+) entry from the extracellular space. These intracettular signals act in concert to activate protein kinases that phosphorylate a variety of regulatory proteins. The link between phosphoinositide turnover and Ca(2+) mobilization is inositol 1,4,5-trisphosphate, the major Ca(2+)-mobilizing second messenger, which is produced from membrane phosphoinositides by activated phospholipase C enzymes. The mechanisms of ligand-regulated Ca(2+) influx and the additional regulatory role(s) of phosphoinositides and inositol phosphates are still being unfolded. This review and the following article summarize some recent developments and unsolved issues about this major signal transduction cascade that links calcium-mobilizing hormone receptors to the regulation of endocrine cell function.

Inositol polyphosphates are not increased by overexpression of Ins(1,4,5)P3 3-kinase but show cell-cycle dependent changes in growth factor-stimulated fibroblasts

NIH 3T3 fibroblasts were stably transfected with rat brain inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) 3-kinase to explore the relationship between increased production of Ins(1,3,4,5)P4 and the formation of InsP5 and InsP6. Mass measurements of InsP5 and InsP6 revealed no significant difference between kinase- and vector-transfected fibroblasts. However, such 3-kinase-transfected cells, when labeled with [3H]inositol for 48-72 h, showed lower levels of [3H]InsP5 and [3H]InsP6, as well as [3H]Ins(1,3,4,6)P4 and D/L[3H]Ins(1,4,5,6)P4, than their vector-transfected counterparts. Because Ins(1,4,5)P3 3-kinase-transfected cells grew less rapidly than vector-transfected controls, we determined whether the synthesis of InsP5 and InsP6 was related to a specific phase of the cell cycle. When NIH 3T3 cells prelabeled with [3H]inositol were synchronized by serum deprivation followed by stimulation with platelet-derived growth factor (PDGF), the amounts of labeled InsP5 and InsP6 began to increase only after 12 h of stimulation, when cells entered the S-phase as indicated by increased [3H]thymidine incorporation. The enhanced synthesis of these inositol polyphosphates was preceded by an early increase in Ins(1,4,5)P3 and its metabolites that was no longer evident by the fifth hour of PDGF action. There was also a prominent and biphasic increase in the level of D/L-Ins(1,4,5,6)P4 with an early peak at approximately 3 h and a second rise that paralleled the increases in InsP5 and InsP6. These results indicate that the formation of highly phosphorylated inositols is not tightly coupled to the receptor-mediated formation of Ins(1,4,5)P3 and its metabolites but is mainly determined by other factors that operate at specific points of the cell cycle.

Differential localization and pH dependency of phosphoinositide 1,4,5-IP3, 1,3,4,5-IP4 and IP6 receptors in rat and human brains

It is well established that the inositol lipids mediate signal transduction in several cellular populations. Many neurotransmitters, hormones and growth factors act at plasma membrane receptors to induce the hydrolysis of phosphatidylinositols and hence the generation of various inositol phosphates (IP). The best known member of this family is 1,4,5-IP3, which is associated with the release of Ca2+ from intracellular pools. It has also been proposed that two others inositides, 1,3,4,5-IP4 and IP6, may be involved in Ca2+ homeostasis. In order to study the possible relevance of these various inositides in neuronal tissues, we have localized the respective receptors in rat and human brain under both acidic and basic pH conditions. In the hippocampal formation, [3H]1,3,4,5-IP4 binding sites are concentrated in the hilus and the molecular layer while a clearly different pattern of distribution is seen for [3H]1,4,5-IP3, its highest concentration of labelling being concentrated in the oriens and radiatum laminae. This contrasting profile of distribution is also observed in other brain areas such as the caudate-putamen, the septo-hippocampal area, and the molecular and granular layers of the cerebellum. Moreover, while highest amounts of specific [3H]1,4,5-IP3 binding are obtained at pH 8.5, the opposite is found for [3H]1,3,4,5-IP4, with high binding levels seen under acidic conditions. [3H]IP6 binding sites are broadly distributed with specific labelling concentrated in areas enriched with neuronal perikarya such as the granular cell layer of the dentate gyrus, the pyramidal cell layers of the hippocampus and the granular cell layer of the cerebellum.(ABSTRACT TRUNCATED AT 250 WORDS)

Dynamics of connexin43 phosphorylation in pp60v-src-transformed cells

Connexin43 phosphorylation was analysed in non-transformed and pp60v-src-transformed Rat-1 fibroblasts. Connexin43 appeared to be the primary connexin expressed in these cells. Although gap-junctional communication was disrupted in pp60v-src-transformed cells, they contained more connexin43 protein and RNA than their non-transformed counterpart. Connexin43 was phosphorylated within minutes of its synthesis in both cell types and appeared to be degraded while in the phosphorylated state. Phosphopeptide and phosphoamino acid analyses suggested that connexin43 in both cell types contained at least five fragments with serine phosphorylation. The major difference in connexin43 phosphorylation between the pp60v-src-transformed and non-transformed cells was that, whereas approx. 70% of the phosphorylated connexin43 in the former contained phosphotyrosine, this phosphoamino acid was not detected in connexin43 isolated from the latter cells. These data support the hypothesis that phosphorylation of connexin43 on tyrosine is critical for the blockade of gap-junctional communication which occurs concomitantly with transformation by the pp60v-src oncogene.

v-Src activates a unique phospholipase D activity that can be distinguished from the phospholipase D activity activated by phorbol esters

Phospholipase D (PLD) activity, as measured by the transphosphatidylation of cellular phospholipids, is elevated in BALB/c 3T3 cells transformed by v-Src. Phorbol esters that activate protein kinase C (PKC) also increase PLC activity in BALB/c 3T3 cells. v-Src-induced PLD activity could be distinguished from phorbol ester-induced PLD activity by differential radiolabelling of phospholipids, which are the substrates of PLD. Both v-Src- and phorbol ester-induced PLD activity could be detected when phospholipids were prelabelled with either radiolabelled myristate or palmitate; however, only phorbol ester-induced PLD activity could be detected when either arachidonate or 1-O-alkyl-sn-glyceryl-3-phosphorylcholine (alkyl-lysoPC) was used to prelabel the phospholipids. The increased PLD activity in v-Src-transformed cells was not detected when the cells were prelabelled with either arachidonic acid or alkyl-lysoPC, which contains an ether linkage at sn-1 of the glycerol backbone. As both arachidonic acid and alkyl-lysoPC are incorporated into phosphatidylcholine (PC), the substrate for v-Src-induced PLD activity, these data suggest that the PLD activated by v-Src can distinguish PCs lacking arachidonic acid and ether linkages. Consistent with v-Src activating a PLD activity that is distinct from that activated by phorbol esters that activate PKC directly, neither depleting cells of PKC nor treatment with the protein kinase inhibitor, staurosporine, had any effect on v-Src-induced PLD activity, whereas both PKC depletion and staurosporine inhibited phorbol ester-induced PLD activity. Taken together, these data suggest that v-Src activates a PKC-independent PLD activity that is specific for a subpopulation of PC and distinct from the PLD activity induced by PKC activity induced by phorbol esters. The diacylglycerol produced from PC by the action of the v-Src-induced PLD may therefore be responsible for the activation of PKC by v-Src.

Effects of high extracellular calcium and strontium on inositol polyphosphates in bovine parathyroid cells

The addition of Ca2+ or a variety of divalent cations increases intracellular Ca2+ in parathyroid cells and suppresses secretion. Since 1,4,5-inositol trisphosphate (IP3) and 1,3,4,5-inositol tetrakisphosphate (IP4) mediate Ca2+ mobilization in other systems, we examined high Ca(2+)- and Sr(2+)-induced accumulation of IP3 and IP4 isomers by anion-exchange HPLC and measured 1,4,5-IP3 mass in parathyroid cells. Raising extracellular [Ca2+] from 0.5 to 3.0 mM increased 3H-1,4,5-IP3 within 5 s, which was confirmed by mass measurements. 3H-1,3,4-IP3 rose gradually by 10 s and increased for 60 s after the addition of Ca2+. Although we detected no change in 3H-1,3,4,5-IP4, the increase in 3H-1,3,4-IP3 suggests that 3H-1,3,4,5-IP4 was being formed. The addition of 4 mM SrCl2 produced similar changes in 1,4,5-IP3, which were confirmed by mass assay. 3H-1,3,4,5-IP4 did not change. However, Sr2+ induced a gradual increase in 3H-1,3,4-IP3, which remained above control levels for 5 minutes. Isotopic labeling studies in this system may underestimate changes in 1,4,5-IP3 mass, but both mass and radioisotopic analyses indicate that high extracellular Ca2+ and Sr2+ stimulate substantial increases in 1,4,5-IP3 without significant accumulation of 1,3,4,5-IP4. These studies suggest a role for 1,4,5-IP3 in intracellular Ca2+ mobilization by divalent cations in parathyroid cells.

Sub-second oscillations of inositol 1,4,5-trisphosphate and inositol 1,3,4,5-tetrakisphosphate during platelet activation by ADP and thrombin: lack of correlation with calcium kinetics

The hypothesis that ADP and thrombin liberate Ins(1,4,5)P3 in blood platelets, with kinetics consistent for releasing Ca2+ within 2s, was tested by quenched-flow techniques. Both agonists stimulated transient and equal synthesis of Ins(1,4,5)P3 and Ins(1,3,4,5)P4 near 200 ms and later short-lived peaks, which were not correlated with the slower steady increase in intracellular [Ca2+] between 0.5 to 2 s detected by Indo-1. Shear forces alone caused transient liberation of these inositol phosphates within 0.5 s and up to 4 s, yet failed to increase intracellular [Ca2+].

Comparison of the levels of inositol metabolites in transformed haemopoietic cells and their normal counterparts

We have compared the levels of inositol metabolites in three pairs of normal and transformed cells which have been matched with respect to their cell lineage, differentiation and proliferation status: (i) normal human myeloid blast cells and the human promyelocytic leukaemic cell line, HL60; (ii) human umbilical-cord T-helper cells and C8166 cells, a HTLV-1-transformed T-helper cell line; and (iii) an interleukin 3-dependent long-term culture of murine pro-B-cells (BAF3) and BAF3 cells transformed by transfection with the bcr-abl oncogene. Complex patterns of inositol metabolites were present in each of the cell populations. Although there were a number of differences in the levels of certain inositol metabolites between individual cell populations in the paired groups, we did not observe any consistent difference in the levels of inositol metabolites between the proliferating normal and transformed cells. In particular, our data do not support the reported correlation between elevated glycerophosphoinositol (GroPIns) levels and transformation of cells by membrane and cytoplasmic oncogenes which has been reported by other workers. All the cells contained high concentrations of Ins(1,3,4,5,6)P5 (between 12 and 55 microM) and InsP6 (between 37 and 105 microM). The HTLV1-transformed T-helper cells had particularly high levels of total inositol phosphates (predominantly GroPIns, an unidentified inositol bisphosphate and InsP6). The observations are discussed with reference to cell transformation and to the differentiation status of the paired populations.

Shc proteins are phosphorylated and regulated by the v-Src and v-Fps protein-tyrosine kinases

The mammalian shc gene encodes two overlapping proteins of 46 and 52 kDa, each with a C-terminal Src homology 2 (SH2) domain and an N-terminal glycine/proline-rich sequence, that induce malignant transformation when overexpressed in mouse fibroblasts. p46shc, p52shc, and an additional 66-kDa shc gene product become highly tyrosine phosphorylated in Rat-2 cells transformed by the v-src or v-fps oncogene. Experiments using temperature-sensitive v-src and v-fps mutants indicate that Shc tyrosine phosphorylation is rapidly induced upon activation of the v-Src or v-Fps tyrosine kinases. These results suggest that Shc proteins may be directly phosphorylated by the v-Src and v-Fps oncoproteins in vivo. In cells transformed by v-src or v-fps, or in normal cells stimulated with epidermal growth factor, Shc proteins complex with a poorly phosphorylated 23-kDa polypeptide (p23). Activated tyrosine kinases therefore regulate the association of Shc proteins with p23 and may thereby control the stimulation of an Shc-mediated signal transduction pathway. The efficient phosphorylation of Shc proteins and the apparent induction of their p23-binding activity in v-src- and v-fps-transformed cells are consistent with the proposition that the SH2-containing Shc polypeptides are biologically relevant substrates of the oncogenic v-Src and v-Fps tyrosine kinases.

The inositol phosphates in WRK1 rat mammary tumour cells

1. A detailed structural survey has been made of the inositol phosphates of unstimulated and vasopressin-stimulated WRK-1 rat mammary tumour cells. Inositol phosphate peaks were separated by h.p.l.c., and structural assignments were made for more than 20 compounds by combinations of: (a) co-chromatography with labelled standards; (b) site-specific enzymic dephosphorylation; (c) complete and partial periodate oxidation, followed by h.p.l.c. of polyols and their stereospecific oxidation by dehydrogenases; and (d) ammoniacal hydrolysis. 2. The 'inositol monophosphates' fraction from unstimulated cells included an uncharacterized peak, probably containing some glycerophosphoinositol, and Ins(1:2-cyclic)P. Stimulation provoked accumulation of both Ins1P and Ins3P, of Ins2P, and of Ins5P and/or the enantiomers Ins4P and Ins6P. The proportions of Ins1P and Ins3P were determined by partial periodate oxidation and enantiomeric identification of the resulting glucitols. 3. Three inositol bisphosphate peaks were detected in unstimulated cells: Ins(1,4)P2 [this was distinguished chemically from its enantiomer Ins(3,6)P2], Ins(3,4)P2 and/or Ins(1,6)P2, and Ins(4,5)P2 and/or Ins(5,6)P2. On stimulation, Ins(1,4)P2 and Ins(3,4)P2 [and/or Ins(1,6)P2] levels increased, and Ins(1:2-cyclic,4)P2 and Ins(1,3)P2 were also formed. 4. Three inositol trisphosphate peaks were obtained from unstimulated cells: all increased during stimulation. These were Ins(1,3,4)P3 [with some Ins(1:2-cyclic,4,5)P3], Ins(1,4,5)P3 and Ins(3,4,5)P3 [and/or Ins(1,5,6)P3]. During stimulation, another compound, probably Ins(1,4,6)P3, appeared in the 'Ins(1,4,5)P3 peak'. The 'Ins(3,4,5)P3 peak' contained a second trisphosphate, probably Ins(2,4,5)P3. 5. Three inositol tetrakisphosphates, namely Ins(1,3,4,6)P4, Ins(1,3,4,5)P4, were present in unstimulated cells, and all accumulated during stimulation. 6. Ins(1,3,4,5,6)P5, which is the most abundant inositol polyphosphate in these cells, a less abundant inositol pentakisphosphate and inositol hexakisphosphate were all unresponsive to stimulation.

Comparison of neuronal inositol 1,4,5-trisphosphate 3-kinase and receptor mRNA distributions in the adult rat brain using in situ hybridization histochemistry

As a result of its interaction with a specific receptor, inositol 1,4,5-trisphosphate mobilizes intracellular calcium. The metabolism of inositol 1,4,5-trisphosphate is rather complex: inositol 1,4,5-trisphosphate 3-kinase produces inositol 1,3,4,5-tetrakisphosphate, a putative second messenger. In order to elucidate inositol 1,3,4,5-tetrakisphosphate function, a comparative in situ hybridization study of the distributions of inositol 1,4,5-trisphosphate 3-kinase and receptor mRNAs was performed in the adult rat brain using oligonucleotides derived from their cDNA sequences. The neuronal distributions of the mRNA for the receptor were larger than for the kinase. Highest levels of both mRNAs were found in the cerebellar Purkinje cells, where they were enriched in their neuronal perikarya and to a lesser extent in their dendrites. In addition to the cerebellum, mRNAs were mainly detected in the hippocampal pyramidal cells of the CA1 sector of the Ammon's horn and in the granule cells of the dentate gyrus, and also in a majority of the neurons in the cortical layers II-III and V, especially in the frontal cortex and cingulate cortex; caudate-putamen, accumbens, olfactory tubercle and Calleja islets; claustrum; anterior olfactory nucleus; taenia tecta; piriform cortex; dorsolateral septum; bed nucleus stria terminalis; amygdala; hippocampal CA2-4 sectors and subiculum. The inositol 1,4,5-trisphosphate receptor mRNA but not kinase mRNA was found in a majority of the neurons in the thalamus, especially in the parafascicular nucleus; hypothalamus, especially the medial hypothalamus; substantia nigra pars compacta and ventral tegmental area; superior colliculus; lateral interpeduncular nucleus and central gray. Taking into account the limitation in sensitivity of the technique, both mRNAs were not detected in glial cells and in the olfactory bulb; basal nucleus of Meynert, diagonal band nuclei; medial septal nucleus; substantia innominata; globus pallidus; entopeduncular nucleus; substantia nigra pars reticulata; ventral pallidum; subthalamic nucleus; spinal cord and dorsal root ganglia. In conclusion, cerebellum and hippocampus appear to contain almost similar levels of kinase mRNA. This is in contrast to receptor mRNA levels which were at much higher levels in the cerebellum when compared with the hippocampus. For this reason, we have chosen hippocampal CA1 pyramidal cells and dentate gyrus granule cells for studying inositol 1,4,5-trisphosphate 3-kinase function.

Activation of 6-phosphofructo-2-kinase by pp60v-src is an indirect effect

6-Phosphofructo-2-kinase (PFK-2) catalyses the synthesis of fructose 2,6-bisphosphate (Fru-2,6-P2), a potent stimulator of glycolysis. In chick-embryo fibroblasts, PFK-2 activity and Fru-2,6-P2 concentration increase upon transformation by Rous sarcoma virus. We show here that the increase in PFK-2 activity required more than 2 h after shifting fibroblasts infected with a thermosensitive mutant of Rous sarcoma virus from the restrictive to the permissive temperature. Pretreatment of the cells with actinomycin D prevented this increase in PFK-2 activity, suggesting a requirement for RNA synthesis. However, the increase in PFK-2 activity did not correspond to an increase in immunoprecipitable PFK-2. Moreover, the thermostability of PFK-2 and the affinity of this enzyme for its substrate fructose 6-phosphate were increased upon transformation by Rous sarcoma virus. Staurosporine, an inhibitor of protein kinase C, prevented the increase in PFK-2 activity brought about by the shift to the permissive temperature. This, together with a comparison of the effects of phorbol esters on PFK-2 activity, suggests that pp60v-src stimulates, via protein kinase C, the transcription of a gene whose products is a distinct PFK-2 isoenzyme or a protein that activates PFK-2.

Tyrosine phosphorylation of G protein alpha subunits by pp60c-src

A number of lines of evidence suggest that cross-talk exists between the cellular signal transduction pathways involving tyrosine phosphorylation catalyzed by members of the pp60c-src kinase family and those mediated by guanine nucleotide regulatory proteins (G proteins). In this study, we explore the possibility that direct interactions between pp60c-src and G proteins may occur with functional consequences. Preparations of pp60c-src isolated by immunoprecipitation phosphorylate on tyrosine residues the purified G-protein alpha subunits (G alpha) of several heterotrimeric G proteins. Phosphorylation is highly dependent on G-protein conformation, and G alpha(GDP) uncomplexed by beta gamma subunits appears to be the preferred substrate. In functional studies, phosphorylation of stimulatory G alpha (G alpha s) modestly increases the rate of binding of guanosine 5'-[gamma-[35S]thio]triphosphate to Gs as well as the receptor-stimulated steady-state rate of GTP hydrolysis by Gs. Heterotrimeric G proteins may represent a previously unappreciated class of potential substrates for pp60c-src.

Estrogen-dependent, tamoxifen-resistant tumorigenic growth of MCF-7 cells transfected with HER2/neu

Since the poor prognosis associated with HER2 amplified breast cancers might be explained by a mechanistic association between p185HER2 overexpression and therapeutic resistance, we assessed the chemo-endocrine sensitivity of estrogen receptor (ER) containing MCF-7 breast cancer cells transfected with full-length HER2 cDNA. Of the 36 isolated MCF/HER2 subclones, 7 were found to overexpress p185HER2 surface receptor at levels 3 to 45-fold greater than parental or control transfected cells (MCF/neo). The overexpressing transfectants possessed increased inositol-1,4,5-triphosphate-3'-kinase activity comparable to enzyme activity in the endogenously HER2 amplified breast cancer cell lines SK-Br-3 and BT-474. The anti-p185HER2 monoclonal antibody and receptor-specific partial agonist, muMAb4D5 (4D5), known to inhibit growth of SK-Br-3 and BT-474 cells, produced no significant growth inhibitory effect on any of the transfectants including the 45-fold overexpressing MCF/HER2-18 cells which were studied in greater detail. MCF/HER2-18 cells contained at least partially functioning exogenous receptor since 4D5 (3 micrograms/ml) specifically stimulated phosphorylation of p185HER2 and its co-precipitating ptyr56 substrate within 5 min, and this was followed at 1 h by a transient induction of c-myc but not c-fos mRNA. ER content and the in vitro sensitivity of MCF/HER2-18 cells to 5-fluorouracil and adriamycin were identical to those of control transfectants and parental cells. However, these highly overexpressing transfectants had acquired low level (2 to 4-fold) resistance to cisplatin and were no longer sensitive to the antiestrogen tamoxifen (TAM). To compare the hormone-dependent tumorigenicity of the HER2 transfectants, MCF/HER2-18 and control cells (MCF, MCF/neo-3) were implanted into ovariectomized athymic nude mice. No tumors were produced in the absence of estradiol (E2) administration. In E2 supplemented mice, MCF/HER2-18 tumors grew most rapidly. When E2 treatment was stopped and daily TAM injections were initiated, MCF-7 and MCF/neo-3 tumor growth ceased immediately, while MCF/HER2-18 tumors continued to show an accelerated growth rate lasting weeks. This pattern of hormone-dependent, TAM-resistant growth exhibited by the MCF/HER2-18 tumors in nude mice supports the possibility that p185HER2 overexpression in human breast cancers may be linked to therapeutic resistance.

Ultrastructural localization of inositol 1,4,5-trisphosphate 3-kinase in rat cerebellar cortex

Subcellular localization of inositol 1,4,5-trisphosphate 3-kinase in the rat cerebellar cortex was studied immunohistochemically using a monoclonal antibody. Electron microscopy revealed intense immunoreactivity in the dendritic spines of Purkinje cells forming synapses with the parallel fibers, climbing fibers and recurrent collaterals of Purkinje cell axons. The labelling was associated with the hypolemmal cisternae, surrounding matrix and plasmalemma including the postsynaptic densities. Weaker immunoreactivity was present in the dendritic spines of basket cells and in certain segments of Purkinje cell recurrent collaterals. The postsynaptic regions of the dendritic trunks of Purkinje and basket cells were negative. These results indicate that inositol 1,4,5-trisphosphate 3-kinase is distributed amongst the spines of various synaptic relations with different electrophysiological properties, and that axon terminals of certain cell types are another functional site for the enzyme.

Regulation of natural antibody binding and susceptibility to natural killer cells through Zn(++)-inducible ras oncogene expression

Changes in the natural resistance phenotype were examined for the 2H1, 10T 1/2 cells expressing the activated human H-ras oncogene under the transcriptional regulation of the zinc-inducible mouse metallothionein-I promoter. Culture of the cells in 50 microM ZnSO4 induced an increase in ras protein p21 levels which were maximal within 1 day. Natural-antibody (NAb) binding was significantly increased following 2 days of cell culture in ZnSO4 and continued to increase up to 4 days. The increased NAb binding returned to uninduced levels within 2 days following the removal of added zinc ions from the culture medium. The cells also exhibited a significant increase in natural killer (NK) cell sensitivity following 2 days in ZnSO4. This was maintained as long as the zinc was in the medium, but returned to uninduced levels within 1 day following its removal. The results show that NAb binding and susceptibility to NK cells increased following ras oncogene expression in 10T 1/2 cells and that both parameters were regulated by p21 expression. Repeated i.v. administration of whole-serum NAb prior to tumor inoculation reduced the number of early tumors following s.c. injection of Zn(++)-induced 2Hl cells into Zn(++)-treated C3H/HeN mice, consistent with an in vivo role for NAb in the defense against ras-transformed cells. In contrast, small but statistically significant reductions in NAb binding were observed following v-H-ras transformation of NIH 3T3 cells or v-src transformation of 10T 1/2. The data argue for an NAb- and NK-cell-susceptible phase of ras-induced tumor development which is a prerequisite for these mediators to contribute to a first line of defense against incipient neoplasia, and suggest that characteristics of the recipient cell and the transforming oncogene are important in determining the natural resistance phenotype.

Okadaic acid inhibits angiotensin II stimulation of Ins(1,4,5)P3 and calcium signalling in rat hepatocytes

OKA2 and CL-A significantly inhibit the ability of angiotensin II, ATP and vasopressin to raise [Ca2+]i in rat hepatocytes, with a partial inhibition of the initial spike, and a complete inhibition of the following plateau. In contrast, the [Ca2+]i response to thapsigargin, which releases intracellular calcium stores through a mechanism independent of inositol phosphates, is much less affected. The ability of angiotensin II to stimulate Ins(1,4,5)P3 production is also reduced by OKA, with kinetics consistent with the inhibited [Ca2+]i response. Since OKA and CL-A are potent and selective inhibitors of phosphoprotein phosphatases, these results provide further evidence that agonist-stimulated Ins(1,4,5)P3 signalling can be inhibited by protein phosphorylation.

Phosphoinositides and cell growth

The findings described above illustrate how the src kinase can influence several new pathways of inositol phosphate metabolism, both at the membrane level with the production of novel D-3 phosphoinositides and the activation of PI-3 kinase, and at the cytosolic level by altering the expression of certain inositol polyphosphates, in particular Ins(1,4,5,6)P4. At present, it is difficult to speculate on the role these phenomena play in cellular transformation by src, since the functions of D-3 phosphoinositides and most inositol polyphosphates are unclear. There is evidence, however, that these new pathways of phosphoinositide metabolism occur in response to other types of cellular stimulations besides src transformation. Novel D-3 phosphoinositides are expressed in a variety of nonneoplastic cells, including human platelets treated with thrombin, smooth muscle cells and stimulated neutrophils. In addition, unusual InsP4 isomers such as D/L-Ins(1,4,5,6)P4 are found in chicken erythrocytes, murine macrophages, AR4-2J rat pancreatoma cells and adrenal glomerulosa cells, to name only a few. Recently, associations have been reported between PI-3 kinases and cytoskeletal elements in thrombin- stimulated platelets, and between activated ras proteins in rat liver epithelial cells. The latter discovery is particularly intriguing since GTP-binding proteins such as ras are known to influence cell shape and serve as downstream effector proteins in the signal transduction pathways of numerous growth factor receptors. Thus, one function of novel phosphoinositides and their metabolites may lie at the level of cytoskeletal and cell shape regulation. Clearly, additional roles for phosphoinositides exist in cells besides their traditional use as precursors for the generation of Ins(1,4,5)P3 and diacylglycerol.(ABSTRACT TRUNCATED AT 250 WORDS)

v-Src increases diacylglycerol levels via a type D phospholipase-mediated hydrolysis of phosphatidylcholine

Activating the protein-tyrosine kinase of v-Src in BALB/c 3T3 cells results in rapid increases in the intracellular second messenger, diacylglycerol (DAG). v-Src-induced increases in radiolabeled DAG were most readily detected when phospholipids were prelabeled with myristic acid, which is incorporated predominantly into phosphatidylcholine. Consistent with this observation, v-Src increased the level of intracellular choline. No increase in DAG was observed when cells were prelabeled with arachidonic acid, which is incorporated predominantly into phosphatidylinositol. Inhibiting phosphatidic acid (PA) phosphatase, which hydrolyzes PA to DAG, blocked v-Src-induced DAG production and enhanced PA production, implicating a type D phospholipase. Consistent with the involvement of a type D phospholipase, v-Src increased transphosphatidylation activity, which is characteristic of type D phospholipases. Thus, v-Src-induced increases in DAG most likely result from the activation of a type D phospholipase/PA phosphatase-mediated signaling pathway.

v-Src increases diacylglycerol levels via a type D phospholipase-mediated hydrolysis of phosphatidylcholine

Activating the protein-tyrosine kinase of v-Src in BALB/c 3T3 cells results in rapid increases in the intracellular second messenger, diacylglycerol (DAG). v-Src-induced increases in radiolabeled DAG were most readily detected when phospholipids were prelabeled with myristic acid, which is incorporated predominantly into phosphatidylcholine. Consistent with this observation, v-Src increased the level of intracellular choline. No increase in DAG was observed when cells were prelabeled with arachidonic acid, which is incorporated predominantly into phosphatidylinositol. Inhibiting phosphatidic acid (PA) phosphatase, which hydrolyzes PA to DAG, blocked v-Src-induced DAG production and enhanced PA production, implicating a type D phospholipase. Consistent with the involvement of a type D phospholipase, v-Src increased transphosphatidylation activity, which is characteristic of type D phospholipases. Thus, v-Src-induced increases in DAG most likely result from the activation of a type D phospholipase/PA phosphatase-mediated signaling pathway.

Altered levels of phosphoinositide metabolites and activation of guanine-nucleotide dependent phospholipase C in rat hepatic tumors

The metabolism of phosphatidylinositol was studied in normal quiescent hepatocytes, hepatocellular carcinomas induced by single dose of diethylnitrosamine, followed by 2-acetylaminofluorene and partial hepatectomy (Solt-Farber model), and in an established hepatoma cell line, JB1. The JB1 hepatoma cell line and hepatocellular carcinomas demonstrated a 4- to 5-fold higher rate of turnover of [3H]-inositol and [3H]-glycerol than the control hepatocytes. Significantly, elevated levels of second messengers inositol 1,4,5-trisphosphate and sn-1,2-diacylglycerol were noted in hepatic tumor cells within 4 hr of labeling with precursor molecules, whereas no detectable level of 3H-labeled inositol trisphosphate was noted in quiescent hepatocytes, even after incubation with 10 mM LiCl for 30 min. Approximately 2.5-fold higher specific activities of a guanine nucleotide and Ca+2 dependent phosphatidylinositol 4,5-bisphosphate specific phospholipase C were detected in the hepatocellular carcinoma cells. The cellular location of the phospholipase C activity was also different, being membrane bound in hepatocytes and equally distributed between cytosolic and membrane factions in the hepatomas. These data are consistent with the hypothesis that the enhanced production of diacylglycerol and inositol 1,4,5-trisphosphate in hepatocellular carcinomas may be due to the activation of a guanine nucleotide dependent phosphatidylinositol 4,5-bisphosphate specific phospholipase C. These data are the first to compare phosphoinositide turnover in normal liver and hepatic tumor cells and suggest that the sustained levels of second messengers is closely associated with the transformation and enhanced growth rate in hepatic tumor cells.

Phosphorylation and activation of epidermal growth factor receptors in cells transformed by the src oncogene

Because functionally significant substrates for the tyrosyl protein kinase activity of pp60v-src are likely to include membrane-associated proteins involved in normal growth control, we have tested the hypothesis that pp60v-src could phosphorylate and alter the signaling activity of transmembrane growth factor receptors. We have found that the epidermal growth factor (EGF) receptor becomes constitutively phosphorylated on tyrosine in cells transformed by the src oncogene and in addition displays elevated levels of phosphoserine and phosphothreonine. High-performance liquid chromatography phosphopeptide mapping revealed two predominant sites of tyrosine phosphorylation, both of which differed from the major sites of receptor autophosphorylation; thus, the src-induced phosphorylation is unlikely to occur via an autocrine mechanism. To determine whether pp60v-src altered the signaling activity of the EGF receptor, we analyzed the tyrosine phosphorylation of phospholipase C-gamma, since phosphorylation of this enzyme occurs in response to activation of the EGF receptor but not in response to pp60v-src alone. We found that in cells coexpressing pp60v-src and the EGF receptor, phospholipase C-gamma was constitutively phosphorylated, a result we interpret as indicating that the signaling activity of the EGF receptor was altered in the src-transformed cells. These findings suggest that pp60v-src-induced alterations in phosphorylation and function of growth regulatory receptors could play an important role in generating the phenotypic changes associated with malignant transformation.

Second messengers derived from inositol lipids

Many hormones, growth factors, and neurotransmitters stimulate their target cells by promoting the hydrolysis of plasma-membrane phosphoinositides to form the two second messengers, diacylglycerol and inositol 1,4,5-trisphosphate [Ins(1,4,5)P3]. In such cells, ligand-receptor interaction stimulates specific phospholipases that are activated by guanyl nucleotide regulatory G proteins or tyrosine phosphorylation. In many cells, the initial rise in cytoplasmic calcium due to Ins(1,4,5)P3-induced mobilization of calcium from agonist-sensitive stores is followed by a sustained phase of cytoplasmic calcium elevation that maintains the target-cell response, and is dependent on influx of extracellular calcium. Numerous inositol phosphates are formed during metabolism of the calcium-mobilizing messenger, inositol 1,4,5-trisphosphate [Ins(1,4,5)P3], to lower and higher phosphorylated derivatives. The cloning of several phospholipase-C isozymes, as well as the Ins(1,4,5)P3-5 kinase and the Ins(1,4,5)P3 receptor, have clarified several aspects of the diversity and complexity of the phosphoinositide-calcium signaling system. In addition to their well-established roles in hormonal activation of cellular responses such as secretion and contraction, phospholipids and their hydrolysis products have been increasingly implicated in the actions of growth factors and oncogenes on cellular growth and proliferation.

Inositol 1,4,5-trisphosphate 3-kinase distribution in the rat brain. High levels in the hippocampal CA1 pyramidal and cerebellar Purkinje cells suggest its involvement in some memory processes

The distribution of inositol 1,4,5-trisphosphate (InsP3) 3-kinase was studied in the adult rat brain, using polyclonal antibodies raised against the purified 50,000-Da rat brain enzyme by immunohistochemistry and Western blot, in addition to enzymatic assay. Immunohistochemically, the enzyme was detected in neurons, where it was localized in the dendrites and at the periphery of the cell bodies. Using selective toxin lesions, the highest enzyme levels were found in the dendrites of hippocampal CA1 pyramidal cells and in neurons in the dorsal portion of the lateral septum, regions both involved in long-term potentiation; and in the dendrites of Purkinje cell subpopulations in the cerebellum, a region involved in long-term depression. High levels were found in neurons in the cortex; in the anterior olfactory nucleus; in the striatum (caudate, putamen, olfactory tubercle, Calleja islets and accumbens); in the central nucleus of the amygdala; in the hippocampal dentate gyrus and in the subiculum. The enzyme was not detected in other brain regions. By Western blot, a 50,000-Da immunoreactive band was present in the cortex, caudate-putamen and cerebellum. This band was most highly stained in the hippocampus. InsP3 3-kinase activity, stimulated by calcium/calmodulin, corresponded to 6172-2638 pmol of InsP4 produced/min/mg protein in the hippocampus followed by frontal and parietotemporal cortex and cerebellum. This activity was below 400 in the brainstem and spinal cord.(ABSTRACT TRUNCATED AT 250 WORDS)

Phosphorylation and activation of epidermal growth factor receptors in cells transformed by the src oncogene

Because functionally significant substrates for the tyrosyl protein kinase activity of pp60v-src are likely to include membrane-associated proteins involved in normal growth control, we have tested the hypothesis that pp60v-src could phosphorylate and alter the signaling activity of transmembrane growth factor receptors. We have found that the epidermal growth factor (EGF) receptor becomes constitutively phosphorylated on tyrosine in cells transformed by the src oncogene and in addition displays elevated levels of phosphoserine and phosphothreonine. High-performance liquid chromatography phosphopeptide mapping revealed two predominant sites of tyrosine phosphorylation, both of which differed from the major sites of receptor autophosphorylation; thus, the src-induced phosphorylation is unlikely to occur via an autocrine mechanism. To determine whether pp60v-src altered the signaling activity of the EGF receptor, we analyzed the tyrosine phosphorylation of phospholipase C-gamma, since phosphorylation of this enzyme occurs in response to activation of the EGF receptor but not in response to pp60v-src alone. We found that in cells coexpressing pp60v-src and the EGF receptor, phospholipase C-gamma was constitutively phosphorylated, a result we interpret as indicating that the signaling activity of the EGF receptor was altered in the src-transformed cells. These findings suggest that pp60v-src-induced alterations in phosphorylation and function of growth regulatory receptors could play an important role in generating the phenotypic changes associated with malignant transformation.

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.

Cloning and expression in Escherichia coli of a rat brain cDNA encoding a Ca2+/calmodulin-sensitive inositol 1,4,5-trisphosphate 3-kinase

Inositol 1,4,5-trisphosphate (InsP3) 3-kinase catalyses the phosphorylation of InsP3 to inositol 1,3,4,5-tetrakisphosphate (InsP4). InsP3 3-kinase activity was stimulated by Ca2+ in the presence of calmodulin (CaM) and the protein was associated with two silver-stained bands which migrated with an apparent Mr of approx. 50,000 on SDS/polyacrylamide gels. Upon limited proteolysis with trypsin, the native InsP3 3-kinase was converted into polypeptides of Mr 44,000 and 36,000. Both tryptic fragments displayed InsP3 3-kinase activity that was Ca2+/CaM-sensitive. A cDNA clone, C5, that encodes the C-terminal part of the InsP3 3-kinase, was isolated by immunoscreening of a rat brain cDNA library. The 5' end of this clone was used in turn to probe the same library, yielding a clone (CP16) containing the entire coding sequence of InsP3 3-kinase. The encoding protein of 459 amino acids (calculated Mr 50,868) has several putative phosphorylation sites for cyclic AMP-dependent protein kinase, protein kinase C and CaM-dependent protein kinase II. When clone C5 was expressed in Escherichia coli, the truncated fusion protein showed Ca2+/CaM-sensitive InsP3 3-kinase activity. Our data demonstrate that the N-terminal part of the protein is not essential for either enzymic or CaM-regulatory properties.

Effect of guanine nucleotides on polyphosphoinositide synthesis in rat liver plasma membranes

The effect of guanosine 5'-[gamma-thio]triphosphate (GTP[S]) on PtdIns and PtdIns(4)P kinase activities was measured in rat liver plasma membranes. The addition of [32P]ATP resulted in the rapid incorporation of 32P into PtdIns(4)P and PtdIns(4,5)P2, with maximal levels reached within 30 s. GTP[S] (25-500 microM) increased the rate and magnitude of [32P]PtdIns(4)P and [32P]PtdIns(4,5)P2 formation by 50 and 120% respectively. Similar stimulatory effects were induced by guanosine 5'-[beta gamma-imido]triphosphate, GTP, GDP and guanosine 5'-[beta-thio]diphosphate. The stimulation of PtdIns phosphorylation by GTP[S] occurred in the presence of 2 mM-EGTA, a condition which fully inhibited phosphoinositide-specific phospholipase C. GTP[S] did not stimulate phosphomonoesterase activity, and its action was not due to the binding of magnesium. However, the overall ATP-hydrolysing activity of the membrane preparation was inhibited by GTP[S] and the other guanine nucleotides. There was a direct correlation between the extent of this inhibition and the stimulation of polyphosphoinositide formation. The results indicate that stimulation of polyphosphoinositide formation by guanine nucleotides in rat liver plasma membranes can be accounted for by an inhibition of ATP hydrolysis. These data are inconsistent with a specific GTP-binding protein (G-protein)-mediated stimulation of PtdIns or PtdIns(4)P kinase.