Platelet-derived growth factor activates membrane-associated phosphatidylinositol 3-kinase and mediates its translocation from the cytosol. Detection of enzyme activity in detergent-solubilized cell extracts

Phosphorylation of Src by phosphoinositide 3-kinase regulates beta-adrenergic receptor-mediated EGFR transactivation

β2-Adrenergic receptors (β2AR) transactivate epidermal growth factor receptors (EGFR) through formation of a β2AR-EGFR complex that requires activation of Src to mediate signaling. Here, we show that both lipid and protein kinase activities of the bifunctional phosphoinositide 3-kinase (PI3K) enzyme are required for β2AR-stimulated EGFR transactivation. Mechanistically, the generation of phosphatidylinositol (3,4,5)-tris-phosphate (PIP3) by the lipid kinase function stabilizes β2AR-EGFR complexes while the protein kinase activity of PI3K regulates Src activation by direct phosphorylation. The protein kinase activity of PI3K phosphorylates serine residue 70 on Src to enhance its activity and induce EGFR transactivation following βAR stimulation. This newly identified function for PI3K, whereby Src is a substrate for the protein kinase activity of PI3K, is of importance since Src plays a key role in pathological and physiological signaling.

Galphaq binds two effectors separately in cells: evidence for predetermined signaling pathways

G-proteins transduce signals along diverse pathways, but the factors involved in pathway selection are largely unknown. Here, we have studied the ability of Galpha(q) to select between two effectors-mammalian inositide-specific phospholipase Cbeta (PLCbeta) and phosphoinositide-3-kinase (PI3K)-in human embryonic kidney 293 cells. These studies were carried out by measuring interactions between eCFP- and eYFP-tagged proteins using Forster resonance energy transfer in the basal state and during stimulation. Instead of association of Galpha(q) with effectors through diffusion and exchange, we found separate and stable pools of Galpha(q)-PLCbeta and Galpha(q)-PI3K complexes existing throughout the stimulation cycle. These separate complexes existed despite the ability of Galpha(q) to simultaneously bind both effectors as determined by in vitro measurements using purified proteins. Preformed G-protein/effector complexes will limit the number of pathways that a given signal will take, which may simplify predictive models.

Myeloproliferative disorder FOP-FGFR1 fusion kinase recruits phosphoinositide-3 kinase and phospholipase Cgamma at the centrosome

Background

The t(6;8) translocation found in rare and agressive myeloproliferative disorders results in a chimeric gene encoding the FOP-FGFR1 fusion protein. This protein comprises the N-terminal region of the centrosomal protein FOP and the tyrosine kinase of the FGFR1 receptor. FOP-FGFR1 is localized at the centrosome where it exerts a constitutive kinase activity.

Results

We show that FOP-FGFR1 interacts with the large centrosomal protein CAP350 and that CAP350 is necessary for FOP-FGFR1 localisation at centrosome. FOP-FGFR1 activates the phosphoinositide-3 kinase (PI3K) pathway. We show that p85 interacts with tyrosine 475 of FOP-FGFR1, which is located in a YXXM consensus binding sequence for an SH2 domain of p85. This interaction is in part responsible for PI3K activation. Ba/F3 cells that express FOP-FGFR1 mutated at tyrosine 475 have reduced proliferative ability. Treatment with PI3K pathway inhibitors induces death of FOP-FGFR1 expressing cells. FOP-FGFR1 also recruits phospholipase Cγ1 (PLCγ1) at the centrosome. We show that this enzyme is recruited by FOP-FGFR1 at the centrosome during interphase.

Conclusion

These results delineate a particular type of oncogenic mechanism by which an ectopic kinase recruits its substrates at the centrosome whence unappropriate signaling induces continuous cell growth and MPD.

Opposing effect of angiopoietin-1 on VEGF-mediated disruption of endothelial cell-cell interactions requires activation of PKC beta

Angiopoietin-1 (Ang1) and vascular endothelial growth factor (VEGF) cooperate in migration and survival of endothelial cells by activation of phosphatidylinositol-3 (PI-3) kinase and mitogen activating protein (MAP) kinase pathways. However, Ang1 opposes the effect of VEGF on vascular permeability. We found that Ang1 also blocks VEGF-mediated diffusion of fluoresin isothiocyanate (FITC)-labeled albumin across an endothelial cell monolayer. VEGF-mediated vascular permeability has been attributed, in part, to activation of phospholipase A(2) and subsequent formation of platelet activating factor. However, Ang1 had no effect on VEGF-induced activation of phospholipase A(2) or the release of arachidonic acid. VEGF-mediated permeability was associated with disruption of endothelial cell junctional complexes, dissociation of beta-catenin from VE-cadherin, and accumulation of beta-catenin in the cytosol. In contrast, Ang1 enhanced the interaction of beta-catenin with VE-cadherin and impaired VEGF-mediated dissociation of this complex. Ang1 also blocked VEGF-induced translocation of protein kinase C (PKC) and beta2 to the membrane, but had no effect on activation of PKC alpha. In addition, staurosporine and a PKC beta inhibitor, LY379196, blocked VEGF-mediated dissociation of beta-catenin from VE-cadherin, diffusion of albumin across the endothelial cell monolayer, and translocation of PKC beta isoforms. These data indicate that VEGF-mediated disruption of endothelial cell-cell interactions requires activation of PKC beta isoforms and that this pathway is blocked by Ang1.

Mechanisms by which cAMP increases bile acid secretion in rat liver and canalicular membrane vesicles

Bile acid secretion induced by cAMP and taurocholate is associated with recruitment of several ATP binding cassette (ABC) transporters to the canalicular membrane. Taurocholate-mediated bile acid secretion and recruitment of ABC transporters are phosphatidylinositol 3-kinase (PI3K) dependent and require an intact microtubular apparatus. We examined mechanisms involved in cAMP-mediated bile acid secretion. Bile acid secretion induced by perfusion of rat liver with dibutyryl cAMP was blocked by colchicine and wortmannin, a PI3K inhibitor. Canalicular membrane vesicles isolated from cAMP-treated rats manifested increased ATP-dependent transport of taurocholate and PI3K activity that were reduced by prior in vivo administration of colchicine or wortmannin. Addition of a PI3K lipid product, phosphoinositide 3,4-bisphosphate, but not its isomer, phosphoinositide 4,5-bisphosphate, restored ATP-dependent taurocholate in these vesicles. Addition of a decapeptide that activates PI3K to canalicular membrane vesicles increased ATP-dependent transport above baseline activity. In contrast to effects induced by taurocholate, cAMP-stimulated intracellular trafficking of the canalicular ABC transporters was unaffected by wortmannin, and recruitment of multidrug resistance protein 2, but not bile salt excretory protein (bsep), was partially decreased by colchicine. These studies indicate that trafficking of bsep and other canalicular ABC transporters to the canalicular membrane in response to cAMP is independent of PI3K activity. In addition, PI3K lipid products are required for activation of bsep in the canalicular membrane. These observations prompt revision of current concepts regarding the role of cAMP and PI3K in intracellular trafficking, regulation of canalicular bsep, and bile acid secretion.

Selective PDZ protein-dependent stimulation of phosphatidylinositol 3-kinase by the adenovirus E4-ORF1 oncoprotein

While PDZ domain-containing proteins represent cellular targets for several different viral oncoproteins, including human papillomavirus E6, human T-cell leukemia virus type 1 Tax, and human adenovirus E4-ORF1, the functional consequences for such interactions have not been elucidated. Here we report that, at the plasma membrane of cells, the adenovirus E4-ORF1 oncoprotein selectively and potently stimulates phosphatidylinositol 3-kinase (PI3K), triggering a downstream cascade of events that includes activation of both protein kinase B and p70S6-kinase. This activity of E4-ORF1 could be abrogated by overexpression of its PDZ-protein targets or by disruption of its PDZ domain-binding motif, which was shown to mediate complex formation between E4-ORF1 and PDZ proteins at the plasma membrane of cells. Furthermore, E4-ORF1 mutants unable to activate the PI3K pathway failed to transform cells in culture or to promote tumors in animals, and drugs that block either PI3K or p70S6-kinase inhibited E4-ORF1-induced transformation of cells. From these results, we propose that the transforming and tumorigenic potentials of the adenovirus E4-ORF1 oncoprotein depend on its capacity to activate PI3K through a novel PDZ protein-dependent mechanism of action.

Mechanism by which cAMP activates PI3-kinase and increases bile acid secretion in WIF-B9 cells

Previous studies in rat bile canalicular membrane vesicles and WIF-B9 cells revealed that cAMP-induced trafficking of ATP-binding cassette (ABC) transporters to the canalicular membrane and their activation require phosphoinositide 3-kinase (PI3-K) products. In the present studies, canalicular secretion of fluorescein isothiocyanate-glycocholate in WIF-B9 cells was increased by cAMP and a decapeptide that enhances PI3-K activity; these effects were inhibited by wortmannin. To determine the mechanism(s) whereby cAMP activates PI3-K, we examined signal transduction pathways in WIF-B9 and COS-7 cells. cAMP activated PI3-K in both cell lines in a phosphotyrosine-independent manner. PI3-K activity increased in association with p110 beta in both cell lines. The effect of cAMP was KT-5720 sensitive, suggesting involvement of protein kinase A. Expression of a dominant-negative beta-adrenergic receptor kinase COOH terminus (beta-ARKct), which blocks G beta gamma signaling, decreased PI3-K activation in both cell lines. cAMP increased GTP-bound Ras in COS-7 but not WIF-B9 cells. Expression of dominant-negative Ras abolished cAMP-mediated PI3-K, which suggests that the effect is downstream of Ras and G beta gamma. These data indicate that cAMP activates PI3-K in a cell type-specific manner and provide insight regarding mechanisms of PI3-K activation required for bile acid secretion.

NERF2, a member of the Ets family of transcription factors, is increased in response to hypoxia and angiopoietin-1: a potential mechanism for Tie2 regulation during hypoxia

Vascular endothelial growth factor (VEGF) and angiopoietins regulate endothelial cell survival and migration and are essential for angiogenesis. Considerable progress has been made towards understanding hypoxia-mediated regulation of VEGF and its receptors. In contrast, little is known about the regulation of angiopoietins and their receptors in hypoxic cells. Using RT-PCR, RNAase protection assay, and Western blotting, we found that Tie1 and Tie2 mRNA and protein levels increased in response to hypoxia in human umbilical vein endothelial cells. Previously, we have shown that NERF2, a member of Ets family of transcription factors that is specifically expressed in endothelial cells, binds to the promoter region of Tie2 and transactivates Tie2 expression. In this study, we show that expression of NERF2 was increased under hypoxia and that this increase temporally correlated with the increase in Tie2 expression. Hypoxia-induced expression of NERF2 and Tie2 was blocked by angiopoietin-2, a competitive inhibitor of angiopoietin-1, and by recombinant soluble extracellular domain of Tie2 but not by VEGF-neutralizing antibodies. In addition, angiopoietin-1 directly induced expression of NERF2 in quiescent cells. These novel findings suggest that angiopoietin-1 regulates expression of NERF2 and its own receptor in hypoxic cells.

Hyaluronan oligosaccharides inhibit anchorage-independent growth of tumor cells by suppressing the phosphoinositide 3-kinase/Akt cell survival pathway

Hyaluronan oligosaccharides (molecular weight: approximately 2.5 x 10(3)) inhibit growth of several types of tumors in vivo. In vitro, the oligomers inhibit anchorage-independent growth of several tumor cell types. In accordance with this finding, the oligomers also induce apoptosis and stimulate caspase-3 activity under anchorage-independent conditions. Since inhibitors of phosphoinositide 3-kinase (PI 3-kinase) mimic the action of hyaluronan oligomers and since the PI 3-kinase/Akt (protein kinase B) cell survival pathway has previously been implicated in anchorage-independent growth of tumor cells, we examined the effect of oligomers on PI 3-kinase and its downstream activities in TA3/St murine mammary carcinoma and HCT 116 human colon carcinoma cells. We observed that 50-150 microg/ml hyaluronan oligomers inhibit PI 3-kinase activity and phosphorylation of Akt to approximately the same extent as optimal doses of wortmannin and LY294002, known inhibitors of PI 3-kinase. Similar inhibition of downstream events, i.e. phosphorylation of BAD and FKHR, was also observed. These effects were not observed on treatment with similar concentrations of chitin oligomers, chondroitin sulfate, or hyaluronan polymer. High molecular weight (approximately 2 x 10(6)) and low molecular weight (approximately 8 x 10(4)) preparations of hyaluronan polymer were equally ineffective. The effects of hyaluronan oligomers on these parameters were similar in magnitude to the effect of treatment with activity-blocking antibody against CD44. We interpret these results to indicate that the oligomers competitively block binding of endogenous hyaluronan polymer to CD44, consequently giving rise to attenuated signaling. Finally, we observed that hyaluronan oligomers, but not chitin oligomers, chondroitin sulfate, or hyaluronan polymer, stimulate expression of PTEN, a phosphatase that degrades the major signaling product of PI 3-kinase action, phosphoinositide 3,4,5-trisphosphate. We conclude that perturbation of hyaluronan-CD44 binding leads to suppression of the PI 3-kinase/Akt cell survival pathway and consequently to inhibition of anchorage-independent growth in culture and tumor growth in vivo.

Regulation of signaling in B cells through the phosphorylation of Syk on linker region tyrosines. A mechanism for negative signaling by the Lyn tyrosine kinase

The B cell antigen receptor (BCR) is coupled to the mobilization of Ca(2+) by the protein-tyrosine kinase, Syk. Syk, recruited to the clustered BCR, becomes phosphorylated on three tyrosines (Tyr-317, Tyr-342, and Tyr-346) located within the linker region that separates the C-terminal catalytic domain from the N-terminal tandem Src homology 2 domains. Phosphorylation within the linker region can be either activating or inhibitory to Ca(2+) mobilization depending on the sites that are modified. Syk that is not phosphorylated on linker region tyrosines couples the BCR to Ca(2+) mobilization through a phosphoinositide 3-kinase-dependent pathway. The phosphorylation of Tyr-342 and -346 enhances the phosphorylation and activation of phospholipase C-gamma and the early phase of Ca(2+) mobilization via a phosphoinositide 3-kinase-independent pathway. The phosphorylation of Tyr-317 strongly dampens the Ca(2+) signal. In cells that lack the Src family kinase, Lyn, the phosphorylation of the inhibitory Tyr-317 is suppressed leading to elevated production of inositol 1,4,5-trisphosphate and an amplified Ca(2+) signal. This provides a novel mechanism by which Lyn functions as an inhibitor of BCR-stimulated signaling. Thus, Syk and Lyn combine to determine the pathway through which the BCR is coupled to Ca(2+) mobilization as well as the magnitude and duration of the Ca(2+) flux.

Mitogenic signaling by cyclic adenosine monophosphate in chromaffin cells involves phosphatidylinositol 3-kinase activation

Increase of intracellular cyclic adenosine monophosphate by the permeant cyclic adenosine monophosphate analog, 8-(4-chlorophenylthio)-adenosine 3':5'- cyclic monophosphate, is mitogenic for normal adult rat chromaffin cells. The mitogenic effect is blocked by the phosphatidylinositol 3-kinase inhibitor, LY294002, and is associated with accumulation of phosphorylated Akt and p70S6 kinase, suggesting that cyclic adenosine monophosphate activates Type l phosphatidylinositol 3-kinase. The mechanism of activation was examined in PC12 pheochromocytoma cells, which are neoplastic chromaffin cells that exhibit many of the biochemical characteristics of their normal counterparts. Incubation of PC12 cells with 8-(4-chlorophenylthio)-adenosine 3':5'- cyclic monophosphate led to a significant increase in total phosphatidylinositol 3-kinase activity that was sensitive to low concentrations of LY294002. The increase was maximal at 1 h and returned to basal levels within six hours. Immunoprecipitation studies showed no increase in phosphatidylinositol 3-kinase activity in anti-phosphotyrosine immune complexes from PC12 cells stimulated by 8-(4-chlorophenylthio)-adenosine 3':5'- cyclic monophosphate, in contrast to cells stimulated by nerve growth factor. Instead, activity was demonstrated in association with p110gamma and p85. These findings suggest that cyclic adenosine monophosphate causes activation of Types IA and IB phosphatidylinositol 3-kinase by a novel mechanism in chromaffin and pheochromocytoma cells. That activation may contribute to chromaffin cell proliferation and to the development and progression of pheochromocytomas. J. Cell. Biochem. Suppl. 36: 89-98, 2001.

Role of phosphoinositide 3-kinase in the aggressive tumor growth of HT1080 human fibrosarcoma cells

We have developed a model system of human fibrosarcoma cell lines that do or do not possess and express an oncogenic mutant allele of N-ras. HT1080 cells contain an endogenous mutant allele of N-ras, whereas the derivative MCH603 cell line contains only wild-type N-ras. In an earlier study (S. Gupta et al., Mol. Cell. Biol. 20:9294-9306, 2000), we had shown that HT1080 cells produce rapidly growing, aggressive tumors in athymic nude mice, whereas MCH603 cells produced more slowly growing tumors and was termed weakly tumorigenic. An extensive analysis of the Ras signaling pathways (Raf, Rac1, and RhoA) provided evidence for a potential novel pathway that was critical for the aggressive tumorigenic phenotype and could be activated by elevated levels of constitutively active MEK. In this study we examined the role of phosphoinositide 3-kinase (PI 3-kinase) in the regulation of the transformed and aggressive tumorigenic phenotypes expressed in HT1080 cells. Both HT1080 (mutant N-ras) and MCH603 (wild-type N-ras) have similar levels of constitutively active Akt, a downstream target of activated PI 3-kinase. We find that both cell lines constitutively express platelet-derived growth factor (PDGF) and PDGF receptors. Transfection with tumor suppressor PTEN cDNA into HT1080 and constitutively active PI 3-kinase-CAAX cDNA into MCH603 cells, respectively, resulted in several interesting and novel observations. Activation of the PI 3-kinase/Akt pathway, including NF-kappaB, is not required for the aggressive tumorigenic phenotype in HT1080 cells. Activation of NF-kappaB is complex: in MCH603 cells it is mediated by Akt, whereas in HT1080 cells activation also involves other pathway(s) that are activated by mutant Ras. A threshold level of activation of PI 3-kinase is required in MCH603 cells before stimulatory cross talk to the RhoA, Rac1, and Raf pathways occurs, without a corresponding activation of Ras. The increased levels of activation seen were similar to those observed in HT1080 cells, except for Raf and MEK, which were more active than HT1080 levels. This cross talk results in conversion to the aggressive tumorigenic phenotype. This latter observation is consistent with our previous observation that overstimulation of the activity of endogenous members of Ras signaling pathways, activated MEK in particular, is a prerequisite for aggressive tumorigenic growth.

Water-soluble HPMA copolymer-wortmannin conjugate retains phosphoinositide 3-kinase inhibitory activity in vitro and in vivo

Phosphoinositide kinases and ATM-related genes play a central role in many physiological processes. Activation of phosphoinositide 3-kinase (PI 3-kinase) is essential for signal transduction by many growth factors and oncogenes and may contribute to tumor progression. In the nanomolar range, Wortmannin (WM), a fungal metabolite, is a potent inhibitor of type I PI 3-kinase; it covalently modifies its catalytic subunit. Because WM is soluble only in organic solvents and unstable in water, there are difficulties in its use in vivo. To generate a water-soluble WM derivative, we used a conjugate of N-(2-hydroxypropyl)methacrylamide (HPMA) copolymer and 11-O-desacetylwortmannin (DAWM), which has a slightly lower inhibitory activity than WM. We covalently attached DAWM to HPMA copolymer containing oligopeptide (GFLG) side-chains. The final product had an estimated molecular mass of 20 kDa and contained 2 wt.% of DAWM. The HPMA copolymer (PHPMA)-DAWM conjugate inhibited type I PI 3-kinase activity in vitro and growth factor-stimulated activation of Akt in vivo; it possessed approximately 50% of the inhibitory activity of DMSO solubilized WM. The specificity and stability of the PHPMA-DAWM conjugate is currently under investigation. The new water-soluble form of WM may be useful in investigations of the role of PI 3-kinase in tumor progression and other cellular biological functions in vivo.

Platelet-derived growth factor requires epidermal growth factor receptor to activate p21-activated kinase family kinases

The platelet-derived growth factor (PDGF) receptor (PDGFR) transactivates the epidermal growth factor (EGF) receptor (ErbB1) to stimulate the cell migration of fibroblasts through an unknown mechanism (Li, J., Kim, Y. N. & Bertics, P. (2000) J. Biol. Chem. 275, 2951-2958). In this paper we provide evidence that the transactivation of the EGF receptor (EGFR) by PDGFR is essential for PDGF to activate p21-activated kinase (PAK) family kinases. Fetal calf serum (10%) transiently stimulates the PAK activity in NIH 3T3 fibroblasts. The activation of PAK was completely inhibited by either PDGFR-specific inhibitor (AG1295) or EGFR-specific inhibitor (AG1478), suggesting that serum requires either the PDGF- or EGF-dependent pathway or the combination of both to activate PAK. PDGF-induced activation of PAK is completely inhibited by either AG1295 or AG1478, indicating that PDGF requires both PDGFR and EGFR for PAK activation. In support of this notion, a mouse embryo fibroblast cell line derived from the EGFR -/- mouse (from Dr. Erwin Wagner) doesn't activate PAK in response to PDGF. Expression of human EGFR in this cell line restores the ability of the PDGF to induce PAK activation. Our results indicate that PDGF activates PAK through transactivation of ErbB1.

Dynamin inhibits phosphatidylinositol 3-kinase in hematopoietic cells

Phosphatidylinositol 3-kinase (PI 3-kinase) plays a role in late stages of endocytosis as well as in cellular proliferation and transformation. The SH3 domain of its regulatory p85 subunit stimulates the GTPase activity of dynamin in vitro. Dynamin is a GTPase enzyme required for endocytosis of activated growth factor receptors. An interaction between these proteins has not been demonstrated in vivo. Here, we report that dynamin associates with PI 3-kinase in hematopoietic cells. We detected both p85 and PI 3-kinase activity in dynamin immune complexes from IL-3-dependent BaF3 cells. However, this association was significantly reduced in BaF3 cells transformed with the BCR/abl oncogene. After transformation only a 4-fold increase in PI 3-kinase activity was detected in dynamin immune complexes, whereas grb2 associated activity was elevated 20-fold. Furthermore, dynamin inhibited the activity of both purified recombinant and immunoprecipitated PI 3-kinase. In BaF3 cells expressing a temperature-sensitive mutant of BCR/abl, a significant decrease in p85 and dynamin association was observed 4 h after the induction of BCR/abl activity. In contrast, in IL-3-stimulated parental BaF3 cells, this association was increased. Our results demonstrate an in vivo association of PI 3-kinase with dynamin and this interaction regulates the activity of PI 3-kinase.

Cell swelling-induced translocation of rat liver Na(+)/taurocholate cotransport polypeptide is mediated via the phosphoinositide 3-kinase signaling pathway

Cell swelling stimulates phosphoinositide 3-kinase (PI3K) and mitogen-activated protein kinase (MAPK) in hepatocytes, and the PI3K signaling pathway is involved in cAMP-mediated translocation of sinusoidal Na(+)/taurocholate (TC) cotransporter (Ntcp) to the plasma membrane. We determined whether cell swelling also stimulates TC uptake and Ntcp translocation via the PI3K and/or MAPK signaling pathway. All studies were conducted in isolated rat hepatocytes. Hepatocyte swelling induced by hypotonic media resulted in: 1) time- and medium osmolarity-dependent increases in TC uptake, 2) an increase in the V(max) of Na(+)/TC cotransport, and 3) wortmannin-sensitive increases in TC uptake and plasma membrane Ntcp mass. Hepatocyte swelling also induced wortmannin-sensitive activation of PI3K, protein kinase B, and p70(S6K). Rapamycin, an inhibitor of p70(S6K), inhibited cell swelling-induced activation of p70(S6K) but failed to inhibit cell swelling-induced stimulation of TC uptake. Because PD98095, an inhibitor of MAPK, did not inhibit cell swelling-induced increases in TC uptake, it is unlikely that the effect of cell swelling on TC uptake is mediated via the MAPK signaling pathway. Taken together, these results indicate that 1) cell swelling stimulates TC uptake by translocating Ntcp to the plasma membrane, 2) this effect is mediated via the PI3K, but not MAPK, signaling pathway, and 3) protein kinase B, but not p70(S6K), is a likely downstream effector of PI3K.

Heregulin-beta is especially potent in activating phosphatidylinositol 3-kinase in nontransformed human mammary epithelial cells

The neu differentiation factors/heregulins (HRGs) comprise a family of polypeptide growth factors that activate p185(erbB-2) through direct binding to either erbB-3 or erbB-4 receptor tyrosine kinases. We have previously shown that HRG-beta is mitogenic for various human mammary epithelial cell lines that coexpress c-erbB-2 and c-erbB-3. Phosphatidylinositol 3-kinase (PI3K) is activated by p185(erbB-2) /erbB-3 heterodimers in cells stimulated by HRG, and PI3K is constitutively activated by p185(erbB-2) /erbB-3 in breast carcinoma cells that overexpress c-erbB-2. To better understand the relative abilities of HRGs, epidermal growth factor (EGF), or insulin to activate PI3K under normal physiological conditions, we compared the levels of recruitment of the 85-kDa regulatory subunit of PI3K when activated by the type I (erbB) or type II [insulin-like growth factor (IGF)] receptor tyrosine kinases in two different nontransformed human mammary epithelial cell lines. The nontransformed H16N-2 cells isolated from normal tissue express EGFR, p185(erbB-2), and erbB-3, and are highly responsive to the mitogenic effects of HRG-beta as well as to the combination of EGF and insulin in serum-free culture. We measured the stoichiometry of p85 recruited by tyrosine-phosphorylated proteins induced in H16N-2 cells by either the alpha or the beta isoform of HRG. HRG-beta was greater than 10-fold more potent in inducing p85 recruitment than was the less biologically active HRG-alpha isoform. HRG-beta was also a more potent inducer of p85 recruited by tyrosine-phosphorylated proteins than was either EGF, insulin, or EGF and insulin combined. Furthermore, erbB-3 principally mediated the direct recruitment of p85 in cells stimulated by HRG or EGF, indicating that, in addition to the high-level activation of PI3K by p185(erbB-2) / erbB-3, EGFR/erbB-3 heterodimer interaction is essential for the weak but significant level of PI3K activated by EGF in cells that express normal EGFR levels. Studies using the PI3K inhibitor wortmannin also indicated that PI3K activation was required for the proliferation of H16N-2 cells induced by either HRG-beta or EGF and insulin in serum-free culture. Finally, HRG-beta was also an especially potent inducer of PI3K in the nontransformed MCF-10A cells, which were derived spontaneously from normal reduction mammoplasty tissue. These data show, for the first time, a side-by-side quantitative comparison of the relative degree of PI3K activated by different growth factors in nontransformed growth factor-dependent cells under precisely defined conditions in culture.

Inactivation of wild-type BCR/ABL tyrosine kinase in hematopoietic cells by mild hyperthermia

Temperature-sensitive mutants of BCR/ABL tyrosine kinase have been extensively used to study the mechanisms of cell transformation and signal transduction. However, little is known about the effect of temperature on the activity of wild-type BCR/ABL gene product. In this study, we demonstrate that in vivo tyrosine kinase activity of p210, p190 BCR/ABL and v-abl are temperature-sensitive when expressed in hematopoietic cells and decline when temperature is raised 2 degrees C above normal range. In vitro tyrosine kinase activities of purified recombinant Abl and immunoprecipitated p210 BCR/ABL were also sensitive to increased temperature. Tyrosine phosphorylation of cellular proteins was markedly reduced in BCR/ABL transformed cells after 16 h at 39 degrees C, whereas the expression of BCR/ABL was unchanged. Temperature-induced downregulation of BCR/ABL kinase activity was reversible when cells were shifted back to 37 degrees C. The downregulation of Abl tyrosine kinase activity was not influenced by mutation or deletion of SH2 or SH3 domains or mutation of the GRB2 binding site. No increase in functional activity or expression of protein-tyrosine phosphatases, PTP-1B, SH-PTP1 or SH-PTP2 was detected in cells grown at 39 degrees C. Temperature-induced downregulation in tyrosine kinase activity correlated with decline in phosphotyrosine-associated PI 3-kinase whereas there was no change in growth factor independence of transformed hematopoietic cells. In conclusion, Abl tyrosine kinase has intrinsic sensitivity to temperature and BCR/ABL expressed in hematopoietic cells is downregulated by increasing temperature 2 degrees C. These observations provide a unique opportunity to identify cellular factor(s) which regulate BCR/ABL kinase in vivo and suggests possible novel treatment of CML by a mild hyperthermia.

Role of PI 3-kinase in angiopoietin-1-mediated migration and attachment-dependent survival of endothelial cells

Angiopoietin-1 is a unique growth factor which induces Tie2 receptor autophosphorylation and interaction with signal transduction molecules, GRB2 and p85 subunit of PI 3-kinase, but no detectable mitogenic response. Here we show that PI 3-kinase-dependent activation of Akt and attachment to extracellular matrix are required for angiopoietin-1-mediated endothelial cell survival. Apoptosis of growth factor-deprived cells grown in monolayer was decreased by angiopoietin-1 and correlated with Akt activation. In contrast, angiopoietin-1, bFGF or VEGF failed to protect cells in suspension culture. Ceramide, an intermediate of several apoptotic pathways, interferes with growth factor-mediated Akt activation. Ceramide induced endothelial cell death and abolished angiopoietin-1-mediated activation of Akt and the effect on cell survival. In addition, we found that PI 3-kinase activity is necessary for migration of endothelial cells in response to Angiopoietin-1. A transient activation of MAPK/ERKs was also detected within 10 min after stimulation with angiopoietin-1. In contrast to VEGF-mediated biological effects, inhibition of MAPK/ERKs by PD98059 in endothelial cells did not affect angiopoietin-1 mediated survival or migration. These findings indicate significant differences in intracellular signaling between VEGF and angiopoietin-1 and that PI 3-kinase lipid products are key mediators of the biological effects of angiopoietin-1.

Phosphoinositide 3-kinase lipid products regulate ATP-dependent transport by sister of P-glycoprotein and multidrug resistance associated protein 2 in bile canalicular membrane vesicles

Bile acid transport and secretion in hepatocytes require phosphatidylinositol (PI) 3-kinase-dependent recruitment of ATP-dependent transporters to the bile canalicular membrane and are accompanied by increased canalicular PI 3-kinase activity. We report here that the lipid products of PI 3-kinase also regulate ATP-dependent transport of taurocholate and dinitrophenyl-glutathione directly in canalicular membranes. ATP-dependent transport of taurocholate and dinitrophenyl-glutathione in isolated canalicular vesicles from rat liver was reduced 50-70% by PI 3-kinase inhibitors, wortmannin, and LY294002, at concentrations that are specific for Type I PI 3-kinase. Inhibition was reversed by addition of lipid products of PI 3-kinase (PI 3,4-bisphosphate and, to a lesser extent, PI 3-phosphate and PI 3,4,5-trisphosphate) but not by PI 4, 5-bisphosphate. A membrane-permeant synthetic 10-mer peptide that binds polyphosphoinositides and leads to activation of PI 3-kinase in macrophages doubled PI 3-kinase activity in canalicular membrane vesicles and enhanced taurocholate and dinitrophenyl-glutathione transport in canalicular membrane vesicles above maximal ATP-dependent transport. The effect of the peptide was blocked by wortmannin and LY294002. PI 3-kinase activity was also necessary for function of the transporters in vivo. ATP-dependent transport of taurocholate and PI 3-kinase activity were reduced in canalicular membrane vesicles isolated from rat liver that had been perfused with taurocholate and wortmannin. PI 3,4-bisphosphate enhanced ATP-dependent transport of taurocholate in these vesicles above control levels. Our results indicate that PI 3-kinase lipid products are necessary in vivo and in vitro for maximal ATP-dependent transport of bile acid and nonbile acid organic anions across the canalicular membrane. Our results demonstrate regulation of membrane ATP binding cassette transporters by PI 3-kinase lipid products.

Phosphatidylinositol 3-kinase is a target for protein tyrosine nitration

A major mechanism of injury associated with the production of nitric oxide (NO*) in vivo is due to its diffusion-limited reaction with superoxide to form peroxynitrite, which in turn may cause nitration of protein tyrosine residues. To assess the physiological role of tyrosine nitration, it is crucial to identify the proteins that become nitrated. Therefore, we treated lysates from RAW 264.7 cells with 1 mM peroxynitrite and immunoprecipitated tyrosine nitrated proteins. This treatment resulted in the nitration of several proteins, with molecular weights ranging from 60-250 kD. One of these proteins was immunologically identified as the p85 regulatory subunit of the phosphatidylinositol 3-kinase, a key enzyme involved in the signal transduction cascade initiated by many agonists including growth factors. Treatment of RAW 264.7 macrophages with the NO* donor spermine NONOate also induced a nitration of the p85 subunit, demonstrating that this covalent modification also occurs in intact cells. Immunoprecipitation of the p110 catalytic subunit of the phosphatidylinositol 3-kinase co-immunoprecipitated p85 in control lysates. However, p85 could not be detected in the same immunoprecipitates when the lysates had been preincubated with 1 mM peroxynitrite, indicating that the nitration of the p85 subunit may abrogate its interaction with the p110 subunit.

The role of phosphoinositide 3-kinase in taurocholate-induced trafficking of ATP-dependent canalicular transporters in rat liver

Recent studies indicate that wortmannin, a potent inhibitor of phosphatidylinositol (PI) 3-kinase, interferes with bile acid secretion in rat liver; taurocholate induces recruitment of ATP-dependent transporters to the bile canalicular membrane, and PI 3-kinase products are important in intracellular trafficking. We investigated the role of PI 3-kinase in bile acid secretion by studying the in vivo effect of taurocholate, colchicine, and wortmannin on bile acid secretion, kinase activity, and protein levels in canalicular membrane vesicle (CMV) and sinusoidal membrane vesicle (SMV) fractions from rat liver. Treatment of rats or perfusion of isolated liver with taurocholate significantly increased PI 3-kinase activity in both membrane fractions. Taurocholate increased protein content of ATP-dependent transporters, which were detected only in CMVs, whereas increased levels of p85 and a cell adhesion molecule, cCAM 105, were observed in both fractions. Colchicine prevented taurocholate-induced changes in all proteins studied, as well as the increase in PI 3-kinase activity in CMVs, but it resulted in further accumulation of PI 3-kinase activity, p85, and cCAM 105 in SMVs. These results indicate that taurocholate-mediated changes involve a microtubular system. Wortmannin blocked taurocholate-induced bile acid secretion. The effect was more profound when wortmannin was administered prior to treatment with taurocholate. When wortmannin was given after taurocholate, the protein levels of each ATP-dependent transporter were maintained in CMVs, whereas the levels of p85 and cCAM decreased in both membrane fractions. Perfusion of liver with wortmannin before taurocholate administration blocked accumulation of all proteins studied in CMVs and SMVs. These results indicate that PI 3-kinase is required for intracellular trafficking of itself, as well as of ATP-dependent canalicular transporters.

Cyclic adenosine monophosphate-mediated protection against bile acid-induced apoptosis in cultured rat hepatocytes

Cyclic adenosine monophosphate (cAMP) has been shown to modulate apoptosis. To evaluate the role of cAMP in bile acid-induced hepatocyte apoptosis, we studied the effect of agents that increase cAMP on the induction of apoptosis by glycochenodeoxycholate (GCDC) in cultured rat hepatocytes. GCDC induced apoptosis in 26.5%+/-1.1% of hepatocytes within 2 hours. Twenty-minute pretreatment of hepatocytes with 100 micromol/L 8-(4-chlorothiophenyl) cAMP (CP-cAMP) resulted in a reduction in the amount of apoptosis to 35.2%+/-3.8% of that seen in hepatocytes treated with GCDC alone. Other agents that increase intracellular cAMP, including dibutyryl cAMP (100 micromol/L), glucagon (200 nmol/L), and a combination of forskolin (20 micromol/L) and 3-isobutyl-1-methylxanthine (20 micromol/L), also inhibited GCDC-induced apoptosis to a similar extent. Pretreatment with the protein kinase A (PKA) inhibitor, KT5720, prevented the protective effect of CP-cAMP and inhibited CP-cAMP-induced activation of PKA activity. Inhibitors of phosphatidylinositol 3-kinase (PI3K), wortmannin (50 nmol/L), or Ly 294002 (20 micromol/L) also prevented the cytoprotective effect of cAMP. PI3K assays confirmed that wortmannin (50 nmol/L) inhibited PI3K activity, while CP-cAMP had no effect on the activity of this lipid kinase. GCDC increased mitogen-activated protein kinase (MAPK) activity, but had no effect on stress-activated protein kinase (SAPK) activity in hepatocytes. cAMP decreased basal and GCDC-induced MAPK activity and increased SAPK activity. The MAPK kinase inhibitor, PD 98059, inhibited both GCDC-mediated MAPK activation and GCDC-induced apoptosis.

IN CONCLUSION

1) agents that increase intracellular cAMP protect against hepatocyte apoptosis induced by hydrophobic bile acids; 2) activation of MAPK by GCDC may be involved in bile acid-induced apoptosis; and 3) cAMP-mediated cytoprotection against bile acid-induced apoptosis appears to involve PKA, MAPK, and PI3K.

Intracellular localization of phosphatidylinositide 3-kinase and insulin receptor substrate-1 in adipocytes: potential involvement of a membrane skeleton

Phosphatidylinositide (PI) 3-kinase binds to tyrosyl-phosphorylated insulin receptor substrate-1 (IRS-1) in insulin-treated adipocytes, and this step plays a central role in the regulated movement of the glucose transporter, GLUT4, from intracellular vesicles to the cell surface. PDGF, which also activates PI 3-kinase in adipocytes, has no significant effect on GLUT4 trafficking in these cells. We propose that this specificity may be mediated by differential localization of PI 3-kinase in response to insulin versus PDGF activation. Using subcellular fractionation in 3T3-L1 adipocytes, we show that insulin- and PDGF-stimulated PI 3-kinase activities are located in an intracellular high speed pellet (HSP) and in the plasma membrane (PM), respectively. The HSP is also enriched in IRS-1, insulin-stimulated tyrosyl-phosphorylated IRS-1 and intracellular GLUT4-containing vesicles. Using sucrose density gradient sedimentation, we have been able to segregate the HSP into two separate subfractions: one enriched in IRS-1, tyrosyl-phosphorylated IRS-1, PI 3-kinase as well as cytoskeletal elements, and another enriched in membranes, including intracellular GLUT4 vesicles. Treatment of the HSP with nonionic detergent, liberates all membrane constituents, whereas IRS-1 and PI 3-kinase remain insoluble. Conversely, at high ionic strength, membranes remain intact, whereas IRS-1 and PI 3-kinase become freely soluble. We further show that this IRS-1-PI 3-kinase complex exists in CHO cells overexpressing IRS-1 and, in these cells, the cytosolic pool of IRS-1 and PI 3-kinase is released subsequent to permeabilization with Streptolysin-O, whereas the particulate fraction of these proteins is retained. These data suggest that IRS-1, PI 3-kinase, as well as other signaling intermediates, may form preassembled complexes that may be associated with the actin cytoskeleton. This complex must be in close apposition to the cell surface, enabling access to the insulin receptor and presumably other signaling molecules that somehow confer the absolute specificity of insulin signaling in these cells.

Tyrosine phosphorylation of the CD3-epsilon subunit of the T cell antigen receptor mediates enhanced association with phosphatidylinositol 3-kinase in Jurkat T cells

T cell receptor signaling results both in T cell proliferation and apoptosis. A key enzyme at the intersection of these downstream pathways is phosphatidylinositol 3'-kinase (PI 3-kinase). In a previous report, we showed that the p85alpha subunit of the PI 3-kinase preferentially associated with the CD3-zeta membrane-proximal immunoreceptor tyrosine-based activation motif of the zeta chain (zetaA-ITAM) (Exley, M., Varticovski, L., Peter, M., Sancho, J., and Terhorst, C. (1994) J. Biol. Chem. 269, 15140-15146). Here, we demonstrate that tyrosine phosphorylation of CD3-epsilon can recruit the PI 3-kinase enzyme in a T cell activation-dependent manner. In vivo studies with Jurkat cells stably transfected with a CD8-CD3-epsilon chimera (termed CD8-epsilon) shows that ligation of endogenous CD3-epsilon or CD8-epsilon by specific antibodies induces tyrosine phosphorylation of CD3-epsilon or CD8-epsilon, respectively. Increased tyrosine phosphorylation correlates with increased binding of p85alpha PI 3-kinase and recruitment of PI 3-kinase enzymatic activity to CD3-epsilon or CD8-epsilon proteins. Mutagenesis studies in COS-7 cells, transiently transfected with CD8-epsilon, p85alpha, and Fyn cDNAs in various combinations, show that both Tyr170 and Tyr181 within the CD3-epsilon-ITAM are required for efficient binding of p85alpha PI 3-kinase. Thus, replacement of Tyr170 by Phe (Y170F), or Tyr181 by Phe (Y181F) significantly reduces binding of p85alpha PI 3-kinase, whereas it does not affect binding of Fyn. Further in vitro experiments suggest that a direct binding of the tandem SH2 domains of p85alpha PI 3-kinase to the two phosphorylated tyrosines in a single CD3-epsilon-ITAM may occur. The data also support a model in which a single CD3 subunit can recruit distinct effector molecules by means of TCR-mediated differential ITAM phosphorylation.

Isolation and characterization of the droPIK57 gene encoding a new regulatory subunit of phosphatidylinositol 3-kinase from Drosophila melanogaster

Mammalian phosphatidylinositol 3-kinase (PI 3-kinase) plays an important role in the regulation of various cellular, receptor tyrosine kinase-mediated processes, such as mitogenesis and transformation. PI 3-kinase is composed of a 110-kDa catalytic subunit and a regulatory subunit of 85 kDa or 55 kDa. We have cloned a gene for a regulatory subunit from Drosophila melanogaster, named droPIK57, from head-specific cDNA libraries. The droPIK57 gene encodes a protein containing two SH2 domains with significant sequence homology to those in p85 and p55. Like the p55 subunits, DroPIK57 is missing the SH3 domain and the bcr homology region of the p85 subunit. The short N-terminus as well as the C-terminus of the DroPIK57 protein show no identity to the known PI 3-kinase subunits, suggesting that it is a new member in the family of regulatory subunits. In-situ hybridization and Northern blot analysis indicate a widespread function of this gene during embryogenesis and in the CNS.

Angiotensin II activates phosphatidylinositol 3-kinase in vascular smooth muscle cells

Phosphatidylinositol 3-kinase (PI3K) is an important component of the signal transduction systems activated by tyrosine kinase receptors. It has not been established, however, whether PI3K is also an essential mediator for G protein-coupled receptors. The potential involvement of PI3K in G protein-linked angiotensin II (Ang II)-dependent signaling was assessed in a primary cell culture system of porcine coronary artery smooth muscle cells (SMCs). Treatment of quiescent SMCs with Ang II (10(-5) to 10(-8) mol/L) resulted in a dose-dependent activation of PI3K when assayed in vivo and in vitro. The Ang II receptor antagonists losartan and PD123319 were used to establish that Ang II stimulates PI3K through the Ang II type-1 (AT1) receptor. Immunofluorescent microscopy revealed that Ang II (10(-6) mol/L) stimulated the translocation of p85, the regulatory subunit of PI3K, from the perinuclear region to distinct foci throughout the cell within 15 minutes. Western blot analysis of p85 subcellular distribution demonstrated that p85 concentrations were also increased within 15 minutes in the membrane fraction and concomitantly decreased in the cytoskeletal and nuclear fractions. These changes in PI3K location and activity were paralleled by increased tyrosine phosphorylation of p85. A potential correlation between angiotensin-mediated PI3K activation and SMC growth was found using LY294002, a specific inhibitor of PI3K, which blocked the increase in DNA and RNA synthesis as well as cellular hyperplasia generated by Ang II (10(-6) mol/L) stimulation of quiescent SMCs. These data indicate that PI3K may operate as a mediator of vascular SMC growth after stimulation with Ang II.

Fluoroaluminate induces pertussis toxin-sensitive protein phosphorylation: differences in MC3T3-E1 osteoblastic and NIH3T3 fibroblastic cells

Fluoride is an acknowledged bone-forming agent that may act through stimulation of osteoblast proliferation. Fluoride's action on osteoblasts and bone is potentiated by aluminum, which can form a complex with fluoride (fluoroaluminate) and activate heterotrimeric G proteins. Here we examined signaling pathways activated by fluoroaluminate in MC3T3-E1 osteoblastic and in NIH3T3 fibroblastic cells. In MC3T3-E1 cells, fluoroaluminate induced a decrease in cAMP levels and an increase in MAP and p70 S6 kinase phosphorylations. These responses were partially or completely prevented by pertussis toxin, an inhibitor of G alpha i proteins. In NIH3T3 cells, fluoroaluminate induced weaker tyrosine and MAP kinase phosphorylations. Fluoroaluminate, but not PDGF, induced a long-lasting tyrosine phosphorylation of a 130 kDa protein only in MC3T3-E1 cells. The expression of G alpha i2, but not of G alpha s and G alpha q/11 proteins was about 10-fold higher in MC3T3-E1 cells. Thus, different signaling in osteoblastic and fibroblastic cells may be due to differential expression of G alpha i proteins and tyrosine kinase substrates and could underlie fluoride's pharmacological action in bone.

Divergence in signal transduction pathways of platelet-derived growth factor (PDGF) and epidermal growth factor (EGF) receptors. Involvement of sphingosine 1-phosphate in PDGF but not EGF signaling

Platelet-derived growth factor (PDGF) and serum, but not epidermal growth factor (EGF), stimulated sphingosine kinase activity in Swiss 3T3 fibroblasts and increased intracellular concentrations of sphingosine 1-phosphate (SPP), a sphingolipid second messenger (Olivera, A., and Spiegel, S. (1993) Nature 365, 557-560). We report herein that DL-threo-dihydrosphingosine (DHS), a competitive inhibitor of sphingosine kinase that prevents PDGF-induced SPP formation, specifically inhibited the activation of two cyclin-dependent kinases (p34(cdc2) kinase and Cdk2 kinase) induced by PDGF, but not by EGF. SPP reversed the inhibitory effects of DHS on PDGF-stimulated cyclin-dependent kinases and DNA synthesis, demonstrating that the DHS effects were mediated via inhibition of sphingosine kinase. DHS also markedly reduced PDGF-stimulated but not EGF-stimulated mitogen-activated protein kinase activity and DNA binding activity of activator protein-1. Examination of the early signaling events of PDGF action revealed that DHS did not affect PDGF-induced autophosphorylation of the growth factor receptor or phosphorylation of the SH2/SH3 adaptor protein Shc and its association with Grb2. This sphingosine kinase inhibitor did not abrogate activation of phosphatidylinositol 3-kinase by PDGF. In agreement, treatment with SPP had no effect on these responses but did, however, potently stimulate phosphorylation of Crk, another SH2/SH3 adaptor protein. Moreover, DHS inhibited PDGF-stimulated, but not EGF-stimulated, Crk phosphorylation. Thus, regulation of sphingosine kinase activity defines divergence in signal transduction pathways of PDGF and EGF receptors leading to mitogen-activated protein kinase activation.

Binding to the platelet-derived growth factor receptor transiently activates the p85alpha-p110alpha phosphoinositide 3-kinase complex in vivo

Ligand stimulation of the platelet-derived growth factor (PDGF) receptor results in its association with phosphoinositide 3-kinase activity and a corresponding synthesis of 3'-phosphorylated lipids. Early studies that examined this interaction in vivo employed anti-phosphotyrosine antiserum or antiserum against the PDGF receptor. The recent identification of multiple isoforms of both the regulatory and the catalytic subunit of the enzyme have led us to utilize antisera against p85alpha and p110alpha to characterize the association of this particular phosphoinositide 3-kinase complex with the PDGF receptor following ligand stimulation of murine fibroblasts. Both the p85alpha and p110alpha subunits rapidly associated with the ligand-activated receptor resulting in a transient, 2-fold increase in the total pool of p110alpha lipid kinase activity. This association was stable for 15 min after initial stimulation. Subsequently, both subunits began to dissociate from the receptor with similar kinetics. By 60 min this process was complete, demonstrating that p85alpha and p110alpha both associate with the receptor and dissociate from the receptor as a dimeric complex. At this time, marked PDGF receptor down-regulation was observed. Immunoprecipitation from metabolically labeled cells revealed that p85alpha is constitutively phosphorylated on serine residues in quiescent cultures. Upon PDGF stimulation, this phosphorylation upon serine residues was maintained in addition to tyrosine phosphorylation of this subunit. No phosphorylation of the p110alpha subunit was detected in either quiescent or PDGF-stimulated cells. Quantitation of Western blot analysis demonstrated that only 5% of the total pool of p85alpha associated with the PDGF receptor upon ligand stimulation. The 2-fold increase in the lipid kinase activity measured in immunoprecipitates using either anti-p85alpha or anti-p110alpha antiserum therefore reflects a far greater increase in the specific activity of the enzyme upon its association with the PDGF receptor.

Coordinated effects of electromagnetic field exposure on erythropoietin-induced activities of phosphatidylinositol-phospholipase C and phosphatidylinositol 3-kinase

Initial studies with the erythropoietin-sensitive human hematopoietic cell line, TF1, demonstrated both multifarious effects of pulsed electromagnetic field (EMF) exposure on lipid signal transduction and antiproliferative effects of EMF. Stimulation of TF1 cells with erythropoietin resulted in increased phosphatidylinositol 3-kinase activity within 2 min. Addition of wortmannin, an inhibitor of phosphatidylinositol 3-kinase, produced a decrease in cell proliferation as measured by accumulation of cells in the G0/G1 phase of the cell cycle and suppression of erythropoietin-induced DNA synthesis. Similar effects on cell proliferation were seen under EMF treatment. Phosphatidylinositol 3-kinase activity in erythropoietin-stimulated TF1 cells, measured in whole-cell extracts, increased 34% within 2 min and remained above basal levels for at least 20 min. EMF decreased erythropoietin-stimulated phosphatidylinositol 3-kinase activity to lower than basal levels. Additionally, translocation of the 85-kDa regulatory subunit (p85) of phosphatidylinositol 3-kinase to the membrane was prevented by EMF. Phosphatidylinositol-specific phospholipase C was activated, as reflected by increases in diacylglycerol and inositol trisphosphate at 15-60 s after EMF treatment. These results provide the first evidence of subtle coordinated changes by EMF associated with loss of phosphatidylinositol 3-kinase activity, inhibition of the translocation of p85 to the membrane, and activation of phosphatidylinositol-phospholipase C.

Membrane targeting of the nucleotide exchange factor Sos is sufficient for activating the Ras signaling pathway

Activation of growth factor receptors results in tyrosine autophosphorylation and recruitment of SH2 domain-containing effectors, including Grb2. Grb2 recruitment mediates activation of the Ras nucleotide exchanger Sos by an unknown mechanism. To examine the role of membrane recruitment, we prepared Sos derivatives containing either myristoylation or farnesylation signals. This resulted in plasma membrane targeting of Sos and stimulation of the Ras signaling pathway, including ERK and AP-1 activities leading to oncogenic transformation. Sos derivatives with nonfunctional myristoylation or farnesylation sequences were inactive. Farnesylation of Sos also activated Ras signaling in yeast. In both mammalian cells and yeast, membrane-targeted Sos derivatives lacking the C-terminal region were considerably more active. Therefore, targeting of Sos to the plasma membrane in the vicinity of Ras appears to be the primary mechanism leading to activation of the Ras pathway. A secondary mechanism could involve relief of the inhibitory effect of the Sos C-terminal region.

Tyrosine 508 of the 85-kilodalton subunit of phosphatidylinositol 3-kinase is phosphorylated by the platelet-derived growth factor receptor

The mechanisms by which growth factors and oncogenic agents activate phosphatidylinositol 3-kinase (PI3 kinase) are unknown. Previously, we reported that the 85-kDa regulatory subunit of PI3 kinase is tyrosine-phosphorylated both in vitro by the platelet-derived growth factor beta-receptor (PDGFR) tyrosine kinase and in fibroblasts in response to PDGF. As a first step in determining the role of tyrosine phosphorylation in PDGF signaling through PI3 kinase, we investigated which tyrosines on p85 are phosphorylated by the PDGFR. Recombinant p85 was phosphorylated with recombinant PDGF receptors, and tryptic phosphopeptides were purified by HPLC and analyzed by Edman degradation. By this approach and by mutational analysis, Y508 was identified as the major in vitro phosphorylation site. Tryptic phosphopeptide mapping demonstrated Y508 to also be phosphorylated in vivo in COS cells. Comparison of these data with a previous report [Hayashi, H., Nishioka, Y., Kamohara, S., Kanai, F., Ishii, K., Fukui, Y., Shibasaki, F., Takenawa, T., Kido, H., Katsunuma, N., & Ebina, Y. (1993) J. Biol. Chem. 268, 7107-7117] suggests that p85 is phosphorylated differently by the PDGF and insulin receptor tyrosine kinases. Therefore, p85 may be regulated differently by PDGF and insulin. Mapping of phosphorylation sites on p85 may lead to new insights into the regulation of signal transduction through PI3 kinase.

Naturally occurring ether-linked phosphatidylcholine activates phosphatidylinositol 3-kinase and stimulates cell growth

Phosphatidylcholine (PC) from marine invertebrates is enriched in ether-linked forms. PCs from ray fish, Dasyatis sp., and bivalve, Macoma birmanica, used in the present study, contain 65% and 75% (w/w of total PC) of ether-linked forms, respectively. Ether-linked PCs also occur in mammalian membranes. Agonist-mediated hydrolysis of PC generates second messengers which participate in cellular responses. In this study, we tested whether PCs from marine invertebrates directly affect mammalian cell growth and activity of phosphatidylinositol (Pl-3-kinase). Pl-3-kinase participates in mitogenesis initiated by a variety of growth factors. Pl-3-kinase converts polyphosphoinositides to 3' phosphorylated isomers and these products accumulate in response to mitogenic stimuli. Whether cell membrane lipids regulate Pl-3-kinase activity is not known. The marine animal-derived PCs and dioleoyl DAG (dioleoylglycerol) stimulated growth of murine pre-B lymphocytes, whereas chicken PC (egg lecithin) inhibited growth of these cells. Egg lecithin is also a potent inhibitor of Pl-3-kinase activity in vitro. We studied the effect of PCs and DAG on Pl-3-kinase activity. Unlike egg lecithin, marine animal PCs enhanced Pl-3-kinase activity. We investigated the effect of lipids on Pl-3-kinase substrate utilization. PCs enriched in ether-linked species increased utilization of substrates by Pl-3-kinase. PCs purified from marine organisms also contain a substantially higher percentage of the cis-unsaturated fatty acids, especially of the -omega 3 series (25% and 30% of total fatty acids for Dasyatis sp. and Macoma birmanica, respectively), as compared to vertebrate sources. In spite of differences in fatty acid composition, marine PCs and dioleoyl DAG showed similar effects on cell growth and Pl-3-kinase activity. These findings indicate that ether-linked phospholipids activate Pl-3-kinase and may participate in mitogenic responses.

Platelet-derived growth factor induces phosphatidylinositol 3-kinase release from the middle T-pp60c-src complex and association with the platelet-derived growth factor receptor

Both platelet-derived growth factor (PDGF) and epidermal growth factor (EGF) induce mitogenesis in normal rat kidney (NRK) fibroblasts transformed by the polyoma virus middle T (pmt) oncogene. In unstimulated pmt-NRK cells phosphatidylinositol 3-kinase forms a complex with the middle T protein and pp60c-src. PDGF treatment causes a release of phosphatidylinositol 3-kinase activity from the complex and a simultaneous increase in activity associated with the PDGF receptor. In contrast after treatment with EGF the majority of phosphatidylinositol 3-kinase activity remains associated with the middle T-pp60c-src complex. Proliferation of NRK fibroblasts transformed by the v-src oncogene is already maximal, and no further stimulation is observed with either PDGF or EGF. Neither growth factor induces dissociation of the complex between phosphatidylinositol 3-kinase and pp60v-src. These observations suggest that the complex between phosphatidylinositol 3-kinase, the middle T protein and pp60c-src is dissociable, and that phosphatidylinositol 3-kinase plays different roles in mitogenic signal transduction by the PDGF and EGF receptors.

Novel inositol containing phospholipids and phosphates: their synthesis and possible new roles in cellular signalling

Details of the widely employed PtdIns(4,5)P2 hydrolysis receptor-stimulated signalling pathway continue to be elucidated rapidly. However, it has recently become apparent that numerous other inositol lipids and phosphates are widespread and are likely to have important cellular functions. In this review, we focus particularly on three rapidly progressing areas: the synthesis and possible functions of 3-phosphorylated inositol lipids, particularly phosphatidylinositol 3,4,5-trisphosphate; the roles of inositol 1,4,5-trisphosphate and inositol 1,3,4,5-tetrakisphosphate in coordinating intracellular Ca2+ mobilization and Ca2+ influx in stimulated cells; and the metabolism and possible functions of other inositol polyphosphates and of inositol polyphosphate pyrophosphates.