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

Translocation of the Rac1 guanine nucleotide exchange factor Tiam1 induced by platelet-derived growth factor and lysophosphatidic acid

Several guanine nucleotide exchange factors for the Rho family of GTPases that induce activation by exchanging GDP for GTP have been identified. One of these is the tumor invasion gene product Tiam1, which acts on Rac1. In this study, we demonstrate that platelet-derived growth factor (PDGF) and lysophosphatidic acid induce the translocation of Tiam1 to the membrane fraction of NIH 3T3 fibroblasts in a time-dependent manner. Previously, we have shown that Tiam1 is phosphorylated by protein kinase C (PKC) and calcium/calmodulin kinase II (CaMK II) after stimulation with agonists. Here we show, by pretreatment of cells with kinase inhibitors, that CaMK II, but not PKC, is involved in the membrane translocation of Tiam1. Addition of the calcium ionophore ionomycin alone induced the translocation of Tiam1. However, the cell-permeable diacylglycerol oleoylacetylglycerol was without effect and did not enhance the effect of ionomycin. These data further indicated a role for CaMK II and not PKC. Inhibition of phosphoinositide 3-kinase by wortmannin had little effect on the translocation of Tiam1. The role of phosphorylation was further studied by comparing the phosphorylation pattern of Tiam1 in the membranes versus whole cell Tiam1. PDGF-induced phosphorylation of membrane-associated Tiam1 occurred more rapidly than that of the total Tiam1 pool, and CaMK II, but not PKC, played a significant role in this process. Furthermore, by using the p21-binding domain of PAK-3, we show that PDGF, but not lysophosphatidic acid, activates Rac1 in vivo and that this activation involves CaMK II and PKC, but not 3-phosphoinositides. Our results indicate that Tiam1 is translocated to and phosphorylated at membranes after agonist stimulation and that CaMK II, but not PKC, is involved in this process. Also, these kinases are involved in the activation of Rac in vivo.

Caveolin 1-mediated regulation of receptor tyrosine kinase-associated phosphatidylinositol 3-kinase activity by ceramide

Previous studies have indicated that proapoptotic stresses downregulate the phosphatidylinositol 3-kinase [PI(3)K]/Akt survival pathway via the activation of acid-sphingomyelinase (A-SMase) and ceramide production. Ceramide induces apoptosis and inhibits PI(3)K activity without altering expression, association, or phosphorylation of receptors, adapter proteins, or PI(3)K subunits. PI(3)K inhibition by ceramide is associated with recruitment of caveolin 1 to PI(3)K-associated receptor complexes within lipid raft microdomains. Overexpression of caveolin 1 alone is sufficient to alter PI(3)K activity and sensitizes fibroblasts to ceramide-induced cell death. Most importantly, antisense expression of caveolin 1 dramatically reduces ceramide-induced PI(3)K deregulation and results in a loss-of-function stress response similar to that in A-SMase-deficient cells. Stress-induced recruitment of caveolin 1 to receptor complexes was found to be dependent on A-SMase since cell lines deficient in A-SMase did not exhibit caveolin 1 association with PI(3)K receptor complexes. Thus, a genetic link between A-SMase activation and caveolin 1-induced inhibition of PI(3)K activity exists. These results led us to propose that stress-induced changes in raft microdomains lead to altered receptor tyrosine kinase signal transduction through the modulation of caveolin 1 by ceramide.

E5 oncoprotein mutants activate phosphoinositide 3-kinase independently of platelet-derived growth factor receptor activation

The E5 oncoprotein of bovine papillomavirus type 1 is a Golgi-resident, 44-amino acid polypeptide that can transform fibroblast cell lines by activating endogenous platelet-derived growth factor receptor beta (PDGF-R). However, the recent discovery of E5 mutants that exhibit strong transforming activity but minimal PDGF-R tyrosine phosphorylation indicates that E5 can potentially use additional signal transduction pathway(s) to transform cells. We now show that two classes of E5 mutants, despite poorly activating the PDGF-R, induce tyrosine phosphorylation and activation of phosphoinositide 3-kinase (PI 3-K) and that this activation is resistant to a selective inhibitor of PDGF-R kinase activity, tyrphostin AG1296. Consistent with this independence from PDGF-R signaling, the E5 mutants fail to induce significant cell proliferation in the absence of PDGF, unlike wild-type E5 or the sis oncoprotein. Despite differences in growth factor requirements, however, both wild-type E5 and mutant E5 cell lines form colonies in agarose. Interestingly, activation of PI 3-K occurs without concomitant activation of the ras-dependent mitogen-activated protein kinase pathway. The known ability of constitutively activated PI 3-K to induce anchorage-independent cell proliferation suggests a mechanism by which the mutant E5 proteins transform cells.

Interleukin-6 activates phosphatidylinositol-3 kinase, which inhibits apoptosis in human prostate cancer cell lines

BACKGROUND

A number of recent studies have identified interleukin (IL)-6 as an important regulator of prostate cancer growth. Here, we investigate the potential interaction of IL-6 with phosphatidylinositol (PI)-3 kinase, a key growth regulatory enzyme, in prostate cancer cell lines.

METHODS

Tyrosine phosphorylation of p85, the regulatory subunit of PI-3 kinase, in the human prostate cancer cell lines LNCaP and PC-3 was assessed by sequential immunoprecipitation with anti-p85 antibody and immunoblotting with anti-phosphotyrosine. The effects of wortmannin, an inhibitor of PI-3 kinase, and/or IL-6 on cell growth were assessed by MTT assays. DNA laddering experiments were performed to assay for programmed cell death.

RESULTS

Tyrosine phosphorylation of p85 is upregulated by IL-6 in both LNCaP and PC-3. IL-6 promotes coprecipitation of p85 with gp130, the signal-transducing component of the IL-6 receptor. Inhibition of PI-3 kinase with wortmannin induces programmed cell death in PC-3 cells. In contrast, wortmannin has no effect on LNCaP growth when used alone; however, combined with IL-6, wortmannin promotes apoptosis in these cells.

CONCLUSIONS

PI-3 kinase is involved in IL-6 signal transduction and delivers an antiapoptotic signal in human prostate cancer cell lines.

Modulation of cellular apoptotic potential: contributions to oncogenesis

The importance of apoptosis as a natural means to eliminate unwanted or damaged cells has been realized over the past decade. Many components required to exercise programmed cell death have been identified and shown to pre-exist in most, if not all, cells. Such ubiquity requires that apoptosis be tightly controlled and suggests the propensity of cells to trigger the cellular death machinery can be regulated. Recently, several signaling pathways have been demonstrated to impact the apoptotic potential of cells, most notably the phosphatidylinositol 3' kinase (PI3'K) pathway. The 3' phosphorylated lipid products generated by this enzyme promote activation of a protein-serine kinase, PKB/AKT, which is necessary and sufficient to confer cell PI3'K-dependent survival signals. The relevance of this pathway to human cancer was revealed by the recent finding that the product of the PTEN tumor suppressor gene acts to antagonize PI3'K. This review focuses on the regulation and mechanisms by which PKB activation protects cells and the oncologic consequences of dysregulation of the pathway.

Multiple forms of p55PIK, a regulatory subunit of phosphoinositide 3-kinase, are generated by alternative initiation of translation

A cDNA encoding p55PIK, one of the regulatory subunits of phosphoinositide (phosphatidylinositol) 3-kinase, was cloned from a cDNA library derived from the mouse mammary epithelial cell line C57MG. The cDNA coding for full-length p55PIK was transiently expressed in COS-7 cells. Western blot analysis of p55PIK expression using a specific antibody against p55PIK revealed that multiple protein products with different molecular masses were detected in COS-7 cell extracts. Experiments presented here demonstrate that multiple forms of p55PIK detected in COS-7 cells were produced by alternative initiation of translation. We also show that at least two in-frame start codons (AUG#2 and AUG#5) in p55PIK mRNA are used in COS-7 cells for the initiation of translation of p55PIK into proteins of 54 kDa and 50 kDa respectively. p55PIK mRNA was also alternatively translated into two proteins in PC cells, a mouse teratoma cell line, indicating that the alternative initiation of translation of p55PIK is not restricted to COS-7 cells. Results from immunoprecipitation and Western blot analysis showed that two forms (54 kDa and 50 kDa protein species) of p55PIK were detected in C57MG cells. Interestingly, when C57MG cells were treated with insulin, only p55PIK, but not p50PIK, bound to insulin receptor substrate-1 protein, providing evidence that different forms of p55PIKs may have specific distinct roles in signal transduction pathways.

Catalytically inactive protein phosphatase 2A can bind to polyomavirus middle tumor antigen and support complex formation with pp60(c-src)

Interaction between the heterodimeric form of protein phosphatase 2A (PP2A) and polyomavirus middle T antigen (MT) is required for the subsequent assembly of a transformation-competent MT complex. To investigate the role of PP2A catalytic activity in MT complex formation, we undertook a mutational analysis of the PP2A 36-kDa catalytic C subunit. Several residues likely to be involved in the dephosphorylation mechanism were identified and mutated. The resultant catalytically inactive C subunit mutants were then analyzed for their ability to associate with a cellular (B subunit) or a viral (MT) B-type subunit. Strikingly, while all of the inactive mutants were severely impaired in their interaction with B subunit, most of these mutants formed complexes with polyomavirus MT. These findings indicate a potential role for these catalytically important residues in complex formation with cellular B subunit, but not in complex formation with MT. Transformation-competent MT is known to associate with, and modulate the activity of, several cellular proteins, including pp60(c-src) family kinases. To determine whether association of MT with an active PP2A A-C heterodimer is necessary for subsequent association with pp60(c-src), catalytically inactive C subunits were examined for their ability to form complexes containing pp60(c-src) in MT-expressing cells. Two catalytically inactive C subunit mutants that efficiently formed complexes with MT also formed complexes that included an active pp60(c-src) kinase, demonstrating that PP2A activity is not essential in cis in MT complexes for subsequent pp60(c-src) association.

Association between src-kinases and the polyoma virus oncogene middle T-antigen requires PP2A and a specific sequence motif

Polymoma virus encodes a potent oncogene, the middle T-antigen (MT), that induces cell transformation by copying the actions of tyrosine kinase associated growth factor receptors. A crucial component of MT transformation is its ability to bind and stimulate the activity of src-family kinases. However, the mechanism by which this is achieved remains unclear. Tyrosine phosphorylation of MT by src-kinases then provides binding sites for SH2 and PTB domain containing molecules in a paradigm of receptor action. We present evidence here that the MT/src complex contains equi-molar amounts of PP2A, and that phosphatase activity may be required for the interaction of MT with both PP2A and the src-family. PP2A, then, is a necessary component of the MT-src complex. We also show that two motifs in the 185 to 210 region of MT, each consisting of a basic area followed by a serine or threonine, are essential for interaction with src-kinases, but not PP2A. The spacing between the serine or threonine and the basic sequence also appears to be important. Substituting a cysteine residue in place of Thr203 in MT has no affect on the binding of pp60c-src, showing that these sites interact with src-kinases by a novel mechanism that does not require phosphorylation.

Separation and characterization of the activated pool of colony-stimulating factor 1 receptor forming distinct multimeric complexes with signalling molecules in macrophages

Colony-stimulating factor 1 (CSF-1) triggers the activation of intracellular proteins in macrophages through selective assembly of signalling complexes. The separation of multimeric complexes of the CSF-1 receptor (CSF-1R) by anion-exchange chromatography enabled the enrichment of low-stoichiometry complexes. A significant proportion of the receptor in CSF-1-stimulated cells that neither possessed detectable tyrosine kinase activity nor formed complexes was separated from the receptor pool displaying autokinase activity that formed chromatographically distinct multimeric complexes. A small pool of CSF-1R formed a multimeric complex with phosphatidylinositol-3 kinase (PI-3 kinase), SHP-1, Grb2, Shc, c-Src, Cbl, and a significant number of tyrosine-phosphorylated proteins in CSF-1-stimulated cells. The complex showed a considerable amount of CSF-1R complex-associated kinase activity. A detectable level of the complex was also present in untreated cells. PI-3 kinase in the multimeric complex displayed low lipid kinase activity despite the association with several proteins. The major pool of activated CSF-1R formed transient multimeric complexes with distinctly different tyrosine-phosphorylated proteins, which included STAT3 but also PI-3 kinase, Shc, SHP-1, and Grb2. A significant level of lipid kinase activity was detected in PI-3 kinase in the latter complexes. The different specific enzyme activities of PI-3 kinase in these complexes support the notion that the activity of PI-3 kinase is modulated by its association with CSF-1R and other associated cellular proteins. Specific structural proteins associated with the separate CSF-1R multimeric complexes upon CSF-1 stimulation and the presence of the distinct pools of the CSF-1R were dependent on the integrity of the microtubular network.

Role of phosphatidylinositol 3-kinase in angiotensin II regulation of norepinephrine neuromodulation in brain neurons of the spontaneously hypertensive rat

Chronic stimulation of norepinephrine (NE) neuromodulation by angiotensin II (Ang II) involves activation of the Ras-Raf-MAP kinase signal transduction pathway in Wistar Kyoto (WKY) rat brain neurons. This pathway is only partially responsible for this heightened action of Ang II in the spontaneously hypertensive rat (SHR) brain neurons. In this study, we demonstrate that the MAP kinase-independent signaling pathway in the SHR neuron involves activation of PI3-kinase and protein kinase B (PKB/Akt). Ang II stimulated PI3-kinase activity in both WKY and SHR brain neurons and was accompanied by its translocation from the cytoplasmic to the nuclear compartment. Although the magnitude of stimulation by Ang II was comparable, the stimulation was more persistent in the SHR neuron compared with the WKY rat neuron. Inhibition of PI3-kinase had no significant effect in the WKY rat neuron. However, it caused a 40-50% attenuation of the Ang II-induced increase in norepinephrine transporter (NET) and tyrosine hydroxylase (TH) mRNAs and [3H]-NE uptake in the SHR neuron. In contrast, inhibition of MAP kinase completely attenuated Ang II stimulation of NET and TH mRNA levels in the WKY rat neuron, whereas it caused only a 45% decrease in the SHR neuron. However, an additive attenuation was observed when both kinases of the SHR neurons were inhibited. Ang II also stimulated PKB/Akt activity in both WKY and SHR neurons. This stimulation was 30% higher and lasted longer in the SHR neuron compared with the WKY rat neuron. In conclusion, these observations demonstrate an exclusive involvement of PI3-kinase-PKB-dependent signaling pathway in a heightened NE neuromodulatory action of Ang II in the SHR neuron. Thus, this study offers an excellent potential for the development of new therapies for the treatment of centrally mediated hypertension.

Autophosphorylation of p110delta phosphoinositide 3-kinase: a new paradigm for the regulation of lipid kinases in vitro and in vivo

Phosphoinositide 3-kinases (PI3Ks) are lipid kinases which also possess an in vitro protein kinase activity towards themselves or their adaptor proteins. The physiological relevance of these phosphorylations is unclear at present. Here, the protein kinase activity of the tyrosine kinase-linked PI3K, p110delta, is characterized and its functional impact assessed. In vitro autophosphorylation of p110delta completely down-regulates its lipid kinase activity. The single site of autophosphorylation was mapped to Ser1039 at the C-terminus of p110delta. Antisera specific for phospho-Ser1039 revealed a very low level of phosphorylation of this residue in cell lines. However, p110delta that is recruited to activated receptors (such as CD28 in T cells) shows a time-dependent increase in Ser1039 phosphorylation and a concomitant decrease in associated lipid kinase activity. Treatment of cells with okadaic acid, an inhibitor of Ser/Thr phosphatases, also dramatically increases the level of Ser1039-phosphorylated p110delta. LY294002 and wortmannin blocked these in vivo increases in Ser1039 phosphorylation, consistent with the notion that PI3Ks, and possibly p110delta itself, are involved in the in vivo phosphorylation of p110delta. In summary, we show that PI3Ks are subject to regulatory phosphorylations in vivo similar to those identified under in vitro conditions, identifying a new level of control of these signalling molecules.

Affinity Modulation of Very Late Antigen-5 Through Phosphatidylinositol 3-Kinase in Mast Cells

Adhesiveness of integrins is up-regulated rapidly by a number of molecules, including growth factors, cytokines, chemokines, and other cell surface receptors, through a mechanism termed inside-out signaling. The inside-out signaling pathways are thought to alter integrin affinity for ligand, or cell surface distribution of integrin by diffusion/clustering. However, it remains to be clarified whether any physiologically relevant agonists induce a rapid change in the affinity of β1 integrins and how ligand-binding affinity is modulated upon stimulation. In this study, we reported that affinity of β1 integrin very late Ag-5 (VLA-5) for fibronectin was rapidly increased in bone marrow-derived mast cells by Ag cross-linking of FcεRI. Ligand-binding affinity of VLA-5 was also augmented by receptor tyrosine kinases when the phospholipase Cγ-1/protein kinase C pathway was inhibited. Wortmannin suppressed induction of the high affinity state VLA-5 in either case. Conversely, introduction of a constitutively active p110 subunit of phosphatidylinositol 3-kinase (PI 3-kinase) increased the binding affinity for fibronectin. Failure of a constitutively active Akt to stimulate adhesion suggested that the affinity modulation mechanisms mediated by PI 3-kinase are distinct from the mechanisms to control growth and apoptosis by PI 3-kinase. Taken together, our findings demonstrated that the increase of affinity of VLA-5 was induced by physiologically relevant stimuli and PI 3-kinase was a critical affinity modulator of VLA-5.

Inhibition of adipose differentiation by phosphatidylinositol 3-kinase inhibitors

Phosphatidylinositol (PI) 3-kinase plays an important role in various cellular signaling mechanisms in several cell systems. The role of PI 3-kinase in adipose differentiation was investigated. For this purpose, we examined the effect of specific inhibitors of PI 3-kinase on the differentiation of two adipogenic cell lines, 1246 and 3T3-L1. The results show that two structurally different inhibitors of PI 3-kinase, i.e., LY294002 and wortmannin, blocked adipose differentiation in a time and dose-dependent fashion. The results from time- course studies indicated that PI 3-kinase activity is most important in the early phase (day 4 to day 6) of the differentiation program. The effect of PI 3-kinase inhibitor on the expression of the peroxisome proliferator-activated receptor (PPAR) gamma, a master regulator in adipogenesis induced during the differentiation process, was also examined. LY294002 significantly inhibited the induction of PPARgamma mRNA expression. During the initiation phase of adipogenesis (day 4 to day 6), the expression of PPARgamma was induced and LY294002 blocked the increase of expression of PPARgamma mRNA. The inhibition of expression of PPARgamma may provide a molecular mechanism for the action of PI 3-kinase inhibitors on adipose differentiation.

Lysophosphatidylcholine generates superoxide anions through activation of phosphatidylinositol 3-kinase in human neutrophils

Lysophosphatidylcholine (LPC) accumulates in inflammatory tissues, where neutrophils are recruited to generate superoxide anions (O2.-). Here, we show that LPC stimulates O2.- generation in human neutrophils and that the activity is inhibited with phosphatidylinositol 3-kinase (PI3 kinase) inhibitors, but not with protein kinase C (PKC) inhibitors. Furthermore, we demonstrate that LPC activates PI3 kinase in neutrophils. Thus, LPC might contribute to host defense by generating O2.- in neutrophils through PI3 kinase activation, but not through PKC activation.

Structure and function of phosphoinositide 3-kinases

Phosphoinositide kinases (PI3Ks) play an important role in mitogenic signaling and cell survival, cytoskeletal remodeling, metabolic control and vesicular trafficking. Here we summarize the structure-function relationships delineating the activation process of class I PI3Ks involving various domains of adapter subunits, Ras, and interacting proteins. The resulting product, PtdIns(3,4,5)P3, targets Akt/protein kinase B (PKB), Bruton's tyrosine kinase (Btk), phosphoinositide-dependent kinases (PDK), integrin-linked kinase (ILK), atypical protein kinases C (PKC), phospholipase Cgamma and more. Surface receptor-activated PI3Ks function in mammals, insects, nematodes and slime mold, but not yeast. While many members of the class II family have been identified and characterized biochemically, it is presently unknown how these C2-domain containing PI3Ks are activated, and which PI substrate they phosphorylate in vivo. PtdIns 3-P is produced by Vps34p/class III PI3Ks and operates via the PtdIns 3-P-binding proteins early endosomal antigen (EEA1), yeast Vac1p, Vps27p, Pip1p in lysosomal protein targeting. Besides the production of D3 phosphorylated lipids, PI3Ks have an intrinsic protein kinase activity. For trimeric GTP-binding protein-activated PI3Kgamma, protein kinase activity seems to be sufficient to trigger mitogen-activated protein kinase (MAPK). Recent disruption of PI3K genes in slime mold, Caenorhabditis elegans, Drosophila melanogaster and mice further underlines the importance of PI3K signaling systems and elucidates the role of PI3K signaling in multicellular organisms.

Oligodendroglial survival factors, PDGF-AA and CNTF, activate similar JAK/STAT signaling pathways

Platelet-derived growth factor (PDGF) and ciliary neurotrophic factor (CNTF) have pleiotropic actions on many cell types. In the presence of these factors, oligodendroglia respond by enhanced survival when deprived of trophic factors or in the presence of the cytotoxic cytokine, tumor necrosis factor-alpha (TNF-alpha). To determine whether these two oligodendroglial survival factors converge in their signaling cascades, we examined their JAK/STAT pathways in enriched oligodendrocyte (OL) progenitors and in the progenitor OL cell line, central glia-4 (CG-4). Cytokine pathways such as JAK/STAT have been characterized extensively in hematopoietic cells; however, it is increasingly evident that the same cytokines that play a role in hematopoiesis also play a role during development and injury of the central nervous system. This is the first study that clearly defines the presence and activation of JAK/STAT proteins in OL progenitors and compares the signal transduction pathway of two well-known oligodendroglial survival factors. In this study, we report that PDGF- and CNTF-induced OL progenitors responded with a rapid tyrosine phosphorylation of JAK1, JAK2, STAT1alpha/beta, and STAT3. We feel that these identified JAK/STAT signaling molecules play a large role in the cellular response to these factors. Because both PDGF and CNTF enhance OL progenitor survival, these JAK/STATs may play a role in regulating this important cellular process.

Analysis of platelet-derived growth factor-induced phospholipase D activation in mouse embryo fibroblasts lacking phospholipase C-gamma1

Platelet-derived growth factor (PDGF) activates phospholipase D (PLD) in mouse embryo fibroblasts (MEFs). In order to investigate a role for phospholipase C-gamma1 (PLC-gamma1), we used targeted disruption of the Plcg1 gene in the mouse to develop Plcg1(+/+) and Plcg1(-/-) cell lines. Plcg1(+/+) MEFs treated with PDGF showed a time- and dose-dependent increase in the production of total inositol phosphates that was substantially reduced in Plcg1(-/-) cells. Plcg1(+/+) cells also showed a PDGF-induced increase in PLD activity that had a similar dose dependence to the PLC response but was down-regulated after 15 min. Phospholipase D activity, however, was markedly reduced in Plcg1(-/-) cells. The PDGF-induced inositol phosphate formation and the PLD activity that remained in the Plcg1(-/-) cells could be attributed to the presence of phospholipase C-gamma2 (PLC-gamma2) in the Plcg1(-/-) cells. The PLC-gamma2 expressed in the Plcg1(-/-) cells was phosphorylated on tyrosine in response to PDGF treatment, and a small but significant fraction of the Plcg1(-/-) cells showed Ca2+ mobilization in response to PDGF, suggesting that the PLC-gamma2 expressed in the Plcg1(-/-) cells was activated in response to PDGF. The inhibition of PDGF-induced phospholipid hydrolysis in Plcg1(-/-) cells was not due to differences in the level of PDGF receptor or in the ability of PDGF to cause autophosphorylation of the receptor. Upon treatment of the Plcg1(-/-) cells with oleoylacetylglycerol and the Ca2+ ionophore ionomycin to mimic the effect of PLC-gamma1, PLD activity was restored. The targeted disruption of Plcg1 did not result in universal changes in the cell signaling pathways of Plcg1(-/-) cells, because the phosphorylation of mitogen-activated protein kinase was similar in Plcg1(+/+) and Plcg1(-/-) cells. Because increased plasma membrane ruffles occurred in both Plcg1(+/+) and Plcg1(-/-) cells following PDGF treatment, it is possible neither PLC nor PLD are necessary for this growth factor response. In summary, these data indicate that PLC-gamma is required for growth factor-induced activation of PLD in MEFs.

Insulin-stimulated cell growth in insulin receptor substrate-1-deficient ZR-75-1 cells is mediated by a phosphatidylinositol-3-kinase-independent pathway

In many human breast cancers and cultured cell lines, insulin receptor expression is elevated, and insulin, via its own insulin receptor, can stimulate cell growth. It has recently been demonstrated that the enzyme phosphatidylinositol-3-kinase (PI3-K) mediates various aspects of insulin receptor signaling including cell growth. In order to understand the mechanisms for insulin-stimulated cell growth in human breast cancer, we measured insulin-stimulable PI3-K activity in a non-transformed breast epithelial cell line, MCF-10A, and in two malignantly transformed cell lines, ZR-75-1 and MDA-MB157. All three cell lines express comparable amounts of insulin receptors whose tyrosine autophosphorylation is increased by insulin, and in these cell lines insulin stimulates growth. In MDA-MB157 and MCF-10A cells, insulin stimulated PI3-K activity three- to fourfold. In ZR-75-1 cells, however, insulin did not stimulate PI3-K activity. In ZR-75-1 cells PI3-K protein was present, and its activity was stimulated by epidermal growth factor, suggesting that there might be a defect in insulin receptor signaling upstream of PI3-K and downstream of the insulin receptor. Next, we studied insulin receptor substrate-1 (IRS-1), a major endogenous substrate for the insulin receptor which, when tyrosine is phosphorylated by the insulin receptor, interacts with and activates PI3-K. In ZR-75-1 cells, there were reduced levels of protein for IRS-1. In these cells, both Shc tyrosine phosphorylation and mitogen-activated protein kinase (MAP-K) activity were increased by the insulin receptor (indicating that the p21ras pathway may account for insulin-stimulated cell growth in ZR-75-1 cells). The PI3-K inhibitor LY294002 (50 microM) reduced insulin-stimulated growth in MCF-10A and MDA-MB157 cell lines, whereas it did not modify insulin effect on ZR-75-1 cell growth. The MAP-K/Erk (MEK) inhibitor PD98059 (50 microM) consistently reduced insulin-dependent growth in all three cell lines. Taken together, these data suggest that in breast cancer cells insulin may stimulate cell growth via PI3-K-dependent or-independent pathways.

Phosphoinositide 3-kinase: the key switch mechanism in insulin signalling

Insulin plays a key role in regulating a wide range of cellular processes. However, until recently little was known about the signalling pathways that are involved in linking the insulin receptor with downstream responses. It is now apparent that the activation of class 1a phosphoinositide 3-kinase (PI 3-kinase) is necessary and in some cases sufficient to elicit many of insulin's effects on glucose and lipid metabolism. The lipid products of PI 3-kinase act as both membrane anchors and allosteric regulators, serving to localize and activate downstream enzymes and their protein substrates. One of the major ways these lipid products of PI 3-kinase act in insulin signalling is by binding to pleckstrin homology (PH) domains of phosphoinositide-dependent protein kinase (PDK) and protein kinase B (PKB) and in the process regulating the phosphorylation of PKB by PDK. Using mechanisms such as this, PI 3-kinase is able to act as a molecular switch to regulate the activity of serine/threonine-specific kinase cascades important in mediating insulin's effects on endpoint responses.

Suppression of diacylglycerol levels by antibodies reactive with the c-erbB-2 (HER-2/neu) gene product p185c-erbB-2 in breast and ovarian cancer cell lines

Seven of 10 murine monoclonal antibodies reactive with the extracellular domain of p185c-erbB-2 inhibited the anchorage independent growth of the SKBr3 breast cancer cell line that overexpressed p185c-erbB-2. Significant inhibition (56-72%) of diacylglycerol (DAG) levels (P < 0.0001) was observed with the 10 antibodies that inhibited SKBr3 growth (RC1, NB3, RC6, PB3, 741F8, DB5, ID5), whereas the 3 antibodies (TA1, 520C9, 454C11) that failed to inhibit SKBr3 growth also failed to affect DAG levels. Thus, DAG levels correlated with antibody-mediated growth regulation for each of the 10 monoclonal reagents. Antibody-induced inhibition of anchorage-independent growth of SKBr3 could be reversed by incubation with phorbol myristate acetate. The ID5 antibody inhibited growth of the SKBr3, SKOv3, and OVCA 432 tumor cell lines, but not of OVCA 420, OVCA 429, and OVCA 433. DAG levels were significantly decreased after ID5 treatment of the SKBr3 and SKOv3 cell lines, but not the OVCA 420, OVCA 429, and OVCA 433 lines. In the 432 line, there was a decrease which did not reach significance. Consequently, changes in DAG levels correlated with growth regulation in 5 of 6 breast and ovarian carcinoma cell lines tested with a trend toward correlation in the sixth. Decreases in DAG may be one mediator of the growth regulatory signals produced by anti-p185c-erbB-2 antibodies.

Overexpression of EGFR and c-erbB2 causes enhanced cell migration in human breast cancer cells and NIH3T3 fibroblasts

Overexpression of EGFR and c-erbB2 frequently occurs in human breast cancers, correlating with poor prognosis. Here we show that overexpression of EGFR and c-erbB2 in cell lines increases cell migration, an important step in metastasis formation. The effect of EGFR on migration is dependent on the addition of EGF to the cells. In contrast, c-erbB2 seems to act independently of its ligand in these assays. Overexpression of this receptor is sufficient to induce cell migration. In addition, we investigated the involvement of a number of signal transduction pathways known to be activated by the EGFR. We found that inactivation of MAPKK results in a decreased migration, while inactivation of PI3K increases migration.

Modification of phosphatidylinositol 3-kinase SH2 domain binding properties by Abl- or Lck-mediated tyrosine phosphorylation at Tyr-688

In cells expressing the oncogenic Bcr-Abl tyrosine kinase, the regulatory p85 subunit of phosphatidylinositol 3-kinase is phosphorylated on tyrosine residues. We report that this phosphorylation event is readily catalyzed by the Abl and Lck protein-tyrosine kinases in vitro, by Bcr-Abl or a catalytically activated Lck-Y505F in co-transfected COS cells, and by endogenous kinases in transfected Jurkat T cells upon triggering of their T cell antigen receptor. Using these systems, we have mapped a major phosphorylation site to Tyr-688 in the C-terminal SH2 domain of p85. Tyrosine phosphorylation of p85 in vitro or in vivo was not associated with detectable change in the enzymatic activity of the phosphatidylinositol 3-kinase heterodimer, but correlated with a strong reduction in the binding of some, but not all, phosphoproteins to the SH2 domains of p85. This provides an additional candidate to the list of SH2 domains regulated by tyrosine phosphorylation and may explain why association of phosphatidylinositol 3-kinase with some cellular ligands is transient or of lower stoichiometry than anticipated.

The p85 and p110 Subunits of Phosphatidylinositol 3-Kinase-α Are Substrates, In Vitro, for a Constitutively Associated Protein Tyrosine Kinase in Platelets

Phosphatidylinositol 3-kinase (PI3K) is a heterodimer lipid kinase consisting of an 85-kD subunit bound to a 110-kD catalytic subunit that also possesses intrinsic, Mn2+-dependent protein serine kinase activity capable of phosphorylating the 85-kD subunit. Here, we examine the Mn2+-dependent protein kinase activity of PI3Kα immunoprecipitated from normal resting or thrombin-stimulated platelets, and characterize p85/p110 phosphorylation, in vitro. Phosphoamino acid analysis of phosphorylated PI3Kα showed p85 and p110 were phosphorylated on serine, but in contrast to previous results, were also phosphorylated on threonine and tyrosine. Wortmannin and LY294002 inhibited p85 phosphorylation; however, p110 phosphorylation was also inhibited suggesting p110 autophosphorylation on serine/threonine. The protein tyrosine kinase inhibitor, erbstatin analog, partially inhibited p85 and p110 phosphorylation but did not appear to affect PI3K lipid kinase activity. The in vitro phosphorylation of p85α or p110α derived from thrombin-stimulated platelets was no different than that of resting platelets, but we confirm that in thrombin receptor-stimulated platelets enhanced levels of p85α and PI3K lipid kinase activity were recovered in antiphosphotyrosine antibody immunoprecipitates. These results suggest PI3Kα can autophosphorylate on serine and threonine, and both p85α and p110α are substrates for a constitutively-associated protein tyrosine kinase in platelets.

The p85 and p110 Subunits of Phosphatidylinositol 3-Kinase-α Are Substrates, In Vitro, for a Constitutively Associated Protein Tyrosine Kinase in Platelets

Phosphatidylinositol 3-kinase (PI3K) is a heterodimer lipid kinase consisting of an 85-kD subunit bound to a 110-kD catalytic subunit that also possesses intrinsic, Mn2+-dependent protein serine kinase activity capable of phosphorylating the 85-kD subunit. Here, we examine the Mn2+-dependent protein kinase activity of PI3Kα immunoprecipitated from normal resting or thrombin-stimulated platelets, and characterize p85/p110 phosphorylation, in vitro. Phosphoamino acid analysis of phosphorylated PI3Kα showed p85 and p110 were phosphorylated on serine, but in contrast to previous results, were also phosphorylated on threonine and tyrosine. Wortmannin and LY294002 inhibited p85 phosphorylation; however, p110 phosphorylation was also inhibited suggesting p110 autophosphorylation on serine/threonine. The protein tyrosine kinase inhibitor, erbstatin analog, partially inhibited p85 and p110 phosphorylation but did not appear to affect PI3K lipid kinase activity. The in vitro phosphorylation of p85α or p110α derived from thrombin-stimulated platelets was no different than that of resting platelets, but we confirm that in thrombin receptor-stimulated platelets enhanced levels of p85α and PI3K lipid kinase activity were recovered in antiphosphotyrosine antibody immunoprecipitates. These results suggest PI3Kα can autophosphorylate on serine and threonine, and both p85α and p110α are substrates for a constitutively-associated protein tyrosine kinase in platelets.

Cellular functions regulated by Src family kinases

Src family protein tyrosine kinases are activated following engagement of many different classes of cellular receptors and participate in signaling pathways that control a diverse spectrum of receptor-induced biological activities. While several of these kinases have evolved to play distinct roles in specific receptor pathways, there is considerable redundancy in the functions of these kinases, both with respect to the receptor pathways that activate these kinases and the downstream effectors that mediate their biological activities. This chapter reviews the evidence implicating Src family kinases in specific receptor pathways and describes the mechanisms leading to their activation, the targets that interact with these kinases, and the biological events that they regulate.

Density dependent elevation of phosphatidylinositol-3 kinase level in rat 3Y1 cells

We have explored the levels of phosphatidylinositol-3 kinase protein during culture of rat 3Y1 cells. Confluent cell cultures exhibited a higher level of phosphatidylinositol-3 kinase compared with that of growing cells. After replating of the cells on fresh dishes, the level of phosphatidylinositol-3 kinase returned to that of growing cells within 24h. This density-dependent regulation of the phosphatidylinositol-3 kinase level was not lined to cell growth, because growth arrest by serum starvation did not cause elevation of the phosphatidylinositol-3 kinase level. Northern blotting analysis revealed that this regulation was based on the transcriptional level. After cell growth was arrested by contact inhibition, elevation of the level of phosphatidylinositol-3,4,5-trisphosphate was detected suggesting that phosphatidylinositol-3 kinase was activated in these cells. These effects were not seen in src-transformed 3Y1 cells, suggesting that this regulation was lost in transformed cells.

Granulocyte-macrophage colony-stimulating factor and IL-5 activate mitogen-activated protein kinase through Jak2 kinase and phosphatidylinositol 3-kinase in human eosinophils

Mitogen-activated protein (MAP) kinases are activated by the sequential activation of Ras, Raf, and MEK (MAP kinase kinase) and regulate a wide variety of cell functions. To determine the kinase cascade for granulocyte-macrophage colony-stimulating factor (GM-CSF)- and IL-5-induced MAP kinase activation in eosinophils, we studied the effect of inhibitors of Jak2 kinase, tyrosine kinases, phosphatidylinositol 3-kinase, and protein kinase C on GM-CSF- and IL-5-induced MAP kinase activation in human eosinophils. GM-CSF and IL-5 activated 40, 42, and 44 kilodalton MAP kinase isoforms in eosinophils. This was indicated by the electrophoretic mobility shift of the three isoforms of MAP kinase in immunoblotting with anti-MAP kinase antibody and also by in-gel MAP kinase assay. MAP kinase activation was time- and dose-dependent, becoming maximal 3 to 15 minutes after stimulation. A Jak2 kinase inhibitor AG-490, a tyrosine kinase inhibitor genistein, and a phosphatidylinositol 3-kinase inhibitor wortmannin inhibited GM-CSF- and IL-5-induced MAP kinase activation in eosinophils, whereas a protein kinase C inhibitor staurosporine had a weak inhibitory effect. Furthermore, AG-490 and genistein prevented GM-CSF-induced tyrosine phosphorylation of Jak2 kinase in eosinophils. Taken together, these results indicate that GM-CSF and IL-5 activate MAP kinases through the signaling pathway of Jak2 kinase-tyrosine phosphorylated beta chain-phosphatidylinositol 3-kinase-Ras in eosinophils.

Influence of tyrosine phosphorylation on protein interaction with FcgammaRIIa

The cytoplasmic tail of Fc(gamma)RIIa present on human neutrophils shares with other antigen receptors a common amino acid sequence called ITAM (Immunoreceptor Tyrosine-based Activation Motif). After receptor ligation, the tyrosine residues within this motif become phosphorylated. We prepared a recombinant fusion protein of the cytoplasmic tail of Fc(gamma)RIIa (containing the ITAM) with glutathione-S-Transferase (GST-CT) to characterize the phosphorylation of Fc(gamma)RIIa and its ability to interact with other proteins involved in signal transduction. The GST-CT became phosphorylated in the presence of Lyn, Hck and Syk (immunoprecipitated from human neutrophils), but not in the presence of Fgr. Of the active kinases, only Lyn (mainly present in the membrane fraction) was found to associate with the GST-CT in the absence of ATP. This association was also observed in immunoprecipitates of Fc(gamma)RIIa from resting neutrophils, suggesting that Lyn might be the kinase responsible for the initial Fc(gamma)RIIa phosphorylation. Moreover, we observed specific association of Syk and the p85 subunit of PI 3-kinase after incubation of the GST-CT with neutrophil cytosol. This interaction was dependent on tyrosine phosphorylation of the GST-CT. Substitution of 269Tyr by Phe almost completely abolished tyrosine phosphorylation of the fusion protein. Substitution of either 253Tyr or 269Tyr eliminated Syk binding, but only 253Tyr appeared to be essential for p85 binding. We hypothesize that, upon activation, the membrane-associated Lyn is responsible for the initial tyrosine phosphorylation of Fc(gamma)RIIa, thus creating a docking site for Syk and PI 3-kinase.

pp60c-src binding to polyomavirus middle T-antigen (MT) requires residues 185 to 210 of the MT sequence

Interaction with the src family of tyrosine kinases is crucial to the transforming action of polyomavirus middle T-antigen (MT). Association with MT activates the tyrosine kinase activity of pp60(c-src) and, through subsequent MT phosphorylation, creates binding sites for signalling molecules whose stimulation culminates in cell transformation. Despite this importance, and many studies, little is known of the mechanisms by which pp60(c-src) binds to MT. We report here isolation of the first MT mutants that disrupt pp60(c-src) binding without affecting the interaction between MT and protein phosphatase 2A (PP2A). Through deletion analysis we established that interaction with pp60(c-src) requires the sequences between amino acids 185 and 210 of MT, but these residues have no effect on PP2A binding. Cells expressing these mutants showed few altered properties, indicating that the PP2A-MT interaction alone has little influence on cell phenotype. Subcellular location of these mutant MT molecules was indistinguishable by immunofluorescence analysis from that of wild-type MT but was altered markedly on loss of PP2A binding. This suggests a possible role for PP2A in specifying subcellular distribution.

Lipid kinase and protein kinase activities of G-protein-coupled phosphoinositide 3-kinase gamma: structure-activity analysis and interactions with wortmannin

Signalling via seven transmembrane helix receptors can lead to a massive increase in cellular PtdIns(3,4,5)P3, which is critical for the induction of various cell responses and is likely to be produced by a trimeric G-protein-sensitive phosphoinositide 3-kinase (PI3Kgamma). We show here that PI3Kgamma is a bifunctional lipid kinase and protein kinase, and that both activities are inhibited by wortmannin at concentrations equal to those affecting the p85/p110alpha heterodimeric PI3K (IC50 approx. 2 nM). The binding of wortmannin to PI3Kgamma, as detected by anti-wortmannin antisera, closely followed the inhibition of the kinase activities. Truncation of more than the 98 N-terminal amino acid residues from PI3Kgamma produced proteins that were inactive in wortmannin binding and kinase assays. This suggests that regions apart from the core catalytic domain are important in catalysis and inhibitor interaction. The covalent reaction of wortmannin with PI3Kgamma was prevented by preincubation with phosphoinositides, ATP and its analogues adenine and 5'-(4-fluorosulphonylbenzoyl)adenine. Proteolytic analysis of wortmannin-prelabelled PI3Kgamma revealed candidate wortmannin-binding peptides around Lys-799. Replacement of Lys-799 by Arg through site-directed mutagenesis aborted the covalent reaction with wortmannin and the lipid kinase and protein kinase activities completely. The above illustrates that Lys-799 is crucial to the phosphate transfer reaction and wortmannin reactivity. Parallel inhibition of the PI3Kgamma-associated protein kinase and lipid kinase by wortmannin and by the Lys-799-->Arg mutation reveals that both activities are inherent in the PI3Kgamma polypeptide.

CD40-triggered protein tyrosine phosphorylation on Vav and on phosphatidylinositol 3-kinase correlates with survival of the Ramos-Burkitt lymphoma B cell line

Signals transduced through CD40 rescue cells of the Ramos-Burkitt lymphoma (Ramos-BL) B cell line from surface immunoglobulin M (sIgM)-triggered growth arrest and apoptosis. This study investigates whether protein tyrosine kinase (PTK) activity and tyrosine phosphorylation on p95(vav) and on the p85 regulatory subunit of phosphatidylinositol 3-kinase (PI3 kinase) play a role in the regulation of Ramos-BL B cell survival. The PTK inhibitor herbimycin A (HA) triggers significant growth arrest prior to apoptosis from the G1-phase of the cell cycle, indicating that tyrosine phosphorylation of key proteins is critical for Ramos-BL cell cycle progression and survival. Indeed, signals transduced through CD40 fail to rescue Ramos-BL B cells from HA-triggered growth arrest and apoptosis. Since Vav and PI3 kinase are intimately involved in the regulation of cellular growth, their tyrosine phosphorylation status was determined in unstimulated and anti-IgM- and anti-CD40-treated Ramos-BL B cells: Vav and p85 are devoid of tyrosine-phosphorylated epitopes in control cells whereas p85, but not Vav, is significantly phosphorylated following ligation of sIgM and anti-CD40 triggers tyrosine phosphorylation on both proteins. Thus, tyrosine-phosphorylated Vav may be a critical effector of CD40-mediated survival. As tyrosine-phosphorylated PI3 kinase is common to both sIgM-triggered death and CD40-triggered survival pathways, its lipid kinase activity was correlated with tyrosine phosphorylation on p85: Ramos-BL B cells exhibit high basal levels of PI3 kinase activity, determined by immunoprecipitation with anti-p85 and 32P incorporation into phosphatidylinositol, which is not significantly affected by stimulation with anti-IgM but which is elevated by 36 +/- 2.9% following ligation of CD40. Thus, tyrosine phosphorylation on p85 correlates with the CD40-triggered increase in PI3 kinase activity but not with basal levels nor with sIgM-triggered levels of enzymatic activity: these data suggest the presence of different PI3 kinase isoforms or the existence of multiple regulatory pathways for the same PI3 kinase isotype in Ramos-BL B cells.

Wortmannin blocks goldfish retinal phosphatidylinositol 3-kinase and neurite outgrowth

The goldfish retina has been used extensively for the study of nerve regeneration. A role for phosphatidylinositol 3-kinase (PI3K) in neurite outgrowth from goldfish retinal explants has been examined by means of wortmannin (WT), a selective inhibitor of the enzyme. The presence of PI3K in retinal extracts was determined by means of immunoprecipitation as well as by an in vitro assay system for catalytic activity. The relative amount of the p85 subunit of PI3K detected by western blot in the retina following optic nerve crush was unchanged. WT inhibited goldfish brain PI3K activity at concentrations as low as 10(-9) M, approximating that reported for inhibition of mammalian PI3K's. Daily addition of 10(-8) M WT to retinal explants, activated by prior crush of the optic nerve, significantly inhibited neurite outgrowth during a 7 day in vitro culture period, while a single addition of WT to freshly explanted retina had no effect on neurite outgrowth. These results suggest that a PI3K-mediated process may be critical for nerve regrowth.

Tyrosine phosphorylation-dependent activation of phosphatidylinositide 3-kinase occurs upstream of Ca2+-signalling induced by Fcgamma receptor cross-linking in human neutrophils

The effect of wortmannin on IgG-receptor (FcgammaR)-mediated stimulation of human neutrophils was investigated. The Ca2+ influx induced by clustering of both Fcgamma receptors was inhibited by wortmannin, as was the release of Ca2+ from intracellular stores. Wortmannin also inhibited, with the same efficacy, the accumulation of Ins(1,4,5)P3 observed after FcgammaR stimulation, but did not affect the increase in Ins(1,4,5)P3 induced by the chemotactic peptide, formyl-methionine-leucine-phenylalanine. Because wortmannin is, in the concentrations used here, an inhibitor of PtdIns 3-kinase, these results suggested a role for PtdIns 3-kinase upstream of Ca2+ signalling, induced by FcgammaR cross-linking. Support for this notion was obtained by investigating the effect of another inhibitor of PtdIns 3-kinase, LY 294002, and by studying the kinetics of PtdIns 3-kinase activation. We found translocation of PtdIns 3-kinase to the plasma membrane and increased PtdIns 3-kinase activity in the membrane as soon as 5 s after FcgammaR cross-linking, even before the onset of the Ca2+ response. Moreover, the translocation of PtdIns 3-kinase to the plasma membrane was inhibited by co-cross-linking of either FcgammaRIIa and FcgammaRIIIb with the tyrosine phosphatase, CD45, indicating a requirement for protein tyrosine phosphorylation in the recruitment of PtdIns 3-kinase to the plasma membrane. Taken together, our results suggest a role for PtdIns 3-kinase in early signal transduction events after FcgammaR cross-linking in human neutrophils.

Binding of HIV-1 to its receptor induces tyrosine phosphorylation of several CD4-associated proteins, including the phosphatidylinositol 3-kinase

Cell surface CD4 molecules are known to be important in several physiological responses of T lymphocytes. The use of human immunodeficiency virus (HIV) particles or purified gp120 molecules as CD4 cross-linking agents has been shown to result in a cascade of intracellular biochemical events. In addition, we and other have provided evidence suggesting that virus-mediated CD4 multimerization can lead to modulation of HIV-1 long terminal repeat-dependent activity and virus production. We were thus interested in measuring the effect of HIV-1 particles on intracellular tyrosine-phosphorylation levels, mostly of CD4-associated proteins. Using the T cell line CEM-T4, we observed that HIV-1 induces an increase in tyrosine phosphorylation of four major proteins physically complexed to the CD4 molecule. Immunoblot analysis permitted the identification of two of these proteins, p56lck and phosphatidylinositol 3-kinase (PI 3-kinase) p85 alpha. No concomitant variation in the level of these two CD4-associated proteins was observed after HIV-1-induced CD4 cross-linking. To our knowledge, this is the first report linking HIV-1-mediated CD4 multimerization to an increase in tyrosine phosphorylation of the PI 3-kinase complex. The four CD4-associated molecules described in this report are most likely implicated in virus-induced CD4-linked signaling events.

Mutations of Ros differentially effecting signal transduction pathways leading to cell growth versus transformation

The signaling functions of the oncogenic protein-tyrosine kinase v-Ros were studied by systematically mutating the tyrosine residues in its cytoplasmic domain. The carboxyl mutation of Tyr-564 produces the most pronounced inhibitory effect on v-Ros autophosphorylation and interaction with phospholipase Cgamma. A cluster of 3 tyrosine residues, Tyr-414, Tyr-418, and Tyr-419, within the PTK domain of v-Ros plays an important role in modulating its kinase activity. The mutant F419 and the mutant DI, deleting 6-amino acids near the catalytic loop, retain wild type protein tyrosine kinase and mitogenic activities, but have dramatically reduced oncogenicity. Both mutant proteins are able to phosphorylate or activate components in the Ras/microtubule-associated protein kinase signaling pathway. However, F419 mutant protein is unable to phosphorylate insulin receptor substrate 1 (IRS-1) or promote association of IRS-1 with phosphatidylinositol 3-kinase. This tyrosine residue in the context of the NDYY motif may define a novel recognition site for IRS-1. Both F419 and DI mutants display impaired ability to induce tyrosine phosphorylation of a series of cytoskeletal and cell-cell interacting proteins. Thus the F419 and DI mutations define v-Ros sequences important for cytoskeleton signaling, the impairment of which correlates with the reduced cell transforming ability.

Role of Rho family proteins in phospholipase D activation by growth factors

Treatment of fibroblasts with growth factors results in activation of phospholipase D (PLD). In order to determine the role of the Rho family of small GTPases in growth factor-mediated PLD activation, we used cells transfected with wild type and mutant Rac1. In response to epidermal growth factor (EGF), PLD activity was greatly increased in Rat1 fibroblasts expressing wild type Rac1 (wtRac1), and completely abrogated in cells expressing dominant negative N17Rac1, consistent with Rac1 mediating the action of this growth factor. In contrast, in cells treated with platelet-derived growth factor (PDGF) or phorbol ester, the wtRac1 cells showed little or no enhancement of PLD activity, and the response was not affected in the N17Rac1 cells, implying that Rac1 played a minimal role in the activation of PLD by PDGF or protein kinase C. Both growth factors produced an attenuated PLD response in cells expressing constitutively active V12Rac1, but these cells showed other changes, including altered morphology, increased basal PLD, and decreased growth factor receptor autophosphorylation. The effects of EGF and PDGF on phosphoinositide phospholipase C activity were not enhanced in cells expressing wtRac1 or inhibited in those expressing N17Rac1. In cells expressing constitutively active V12Rac1, basal phosphoinositide phospholipase C was elevated, but there were no significant effects of EGF or PDGF. We used C3 transferase of Clostridium botulinum, which ADP-ribosylates and inactivates RhoA, to investigate the involvement of RhoA in the activation of PLD by PDGF. Cells expressing wtRac1 and N17Rac1 showed a decreased PLD in response to PDGF when treated with C3 transferase, indicating a role for RhoA. In summary, these data indicate a major role for Rac1 in the activation of PLD by EGF, but not PDGF or protein kinase C.

Differential effects of constitutively active phosphatidylinositol 3-kinase on glucose transport, glycogen synthase activity, and DNA synthesis in 3T3-L1 adipocytes

Phosphatidylinositol 3-kinase (PI3K) activation is necessary for many insulin-induced metabolic and mitogenic responses. However, it is unclear whether PI3K activation is sufficient for any of these effects. To address this question we increased PI3K activity in differentiated 3T3-L1 adipocytes by adenovirus-mediated expression of both the inter-SH2 region of the regulatory p85 subunit of PI3K (iSH2) and the catalytic p110 alpha subunit (p110). Coexpression resulted in PI3K activity that exceeded insulin-stimulated activity by two- to fivefold in cytosol, total membranes, and the low density microsome (LDM) fraction, the site of greatest insulin stimulation. While insulin increased glucose transport 15-fold, coexpression of iSH2-p110 increased transport (5.2-) +/- 0.7-fold with a parallel increase in GLUT4 translocation to the plasma membrane. Constitutive activation of PI3K had no effect on maximally insulin-stimulated glucose transport. Neither basal nor insulin-stimulated activity of glycogen synthase or mitogen-activated protein kinase was altered by iSH2-p110 coexpression. DNA synthesis was increased twofold by insulin in control 3T3-L1 adipocytes transduced with beta-galactosidase-encoding recombinant adenovirus, while iSH2-p110 coexpression increased DNA synthesis fivefold. These data indicate that (i) increased PI3K activity is sufficient to activate some but not all metabolic responses to insulin, (ii) activation of PI3K to levels exceeding the effect of insulin in adipocyte LDM results in only a partial stimulation of glucose transport, and (iii) increased PI3K activity in the absence of growth factor or oncoprotein stimulation is a potent stimulus of DNA synthesis.

Functional interaction between the SH2 domain of Fyn and tyrosine 324 of hamster polyomavirus middle-T antigen

Middle-T antigen of mouse polyomavirus (MomT) associates with the cellular tyrosine kinases c-Src, c-Yes, and Fyn, while middle-T antigen of hamster polyomavirus (HamT) exclusively binds Fyn. This interaction is essential for polyomavirus-mediated transformation of cells in culture and tumor formation in animals. Here we show that the kinase domain of Fyn is sufficient for association with MomT but not for binding of HamT. We further demonstrate that a Fyn mutant lacking the SH2 domain is able to bind MomT but fails to associate with HamT, indicating that the SH2 domain of Fyn is essential for stable association with HamT. HamT, but not MomT, contains a tyrosine residue, Tyr-324, in the sequence context YEEI. Mutation of Tyr-324 to phenylalanine led to a drastic reduction of associated Fyn and abolished the oncogenicity of HamT. This suggests that Tyr-324 is the major phosphotyrosine residue mediating the binding of HamT to the SH2 domain of Fyn. These findings show that mouse and hamster polyomaviruses use different strategies to target Src-related tyrosine kinases.

The E5 gene product of rhesus papillomavirus is an activator of endogenous Ras and phosphatidylinositol-3'-kinase in NIH 3T3 cells

We examined the effect of two rhesus papillomavirus 1 (RhPV) oncogenes on cytokine-induced signal transduction pathways leading to the possible activation of Ras protein (p21ras) and phosphatidylinositol kinase. p21ras in both the activated (GTP-bound) and inactivated (GDP-bound) states were quantitated. NIH 3T3 cell lines expressing the RhPV 1 E5 gene or epidermal growth factor receptor cDNA had about a sixfold higher ratio of p21ras-bound GTP to p21ras-bound GDP as compared with parental NIH 3T3 cells or a cell line expressing the RhPV 1 E7 gene under normal culture conditions, yet expressed similar levels of p21ras. Quiescent cells had dramatically reduced levels of activated p21ras, except those containing RhPV 1 E7. Levels were restored by stimulation with epidermal growth factor or platelet-derived growth factor. Both epidermal growth factor and platelet-derived growth factor receptor of RhPV 1 E5- and E7-containing cells responded to cytokine stimulation. Endogenous phosphatidylinositol-3'-kinase was up-regulated in NIH 3T3 cells transformed with the E5 genes of RhPV 1 and bovine papillomavirus 1. These results suggest that E5 genes of papillomaviruses play a major role in the regulation of transduction pathways.

D-3 phosphoinositide metabolism in cells treated with platelet-derived growth factor

Despite extensive analysis of phosphoinositide 3-hydroxykinases (PI 3-kinases) at the molecular level, comparatively little is known about the mechanisms by which products of these enzymes exert their expected second-messenger functions. This study examines the metabolism of D-3 phosphoinositides in mouse Ph-N2 fibroblasts lacking the platelet-derived growth factor (PDGF) alpha-receptor. Treatment of these cultures with BB PDGF, but not AA PDGF, resulted in transient activation of PI 3-kinase activity measured in vitro. Treatment of myo-[3H]inositol-labelled Ph-N2 cells with BB PDGF resulted in the rapid induction of PtdIns(3,4)P2 and PtdIns(3,4,5)P3 and, to a smaller extent, PtdIns3P. The appearance of PtdIns(3,4,5)P3 preceded that of PtdIns(3,4)P2 and PtdIns3P after the addition of PDGF, suggesting that PtdIns(4,5)P2 is the preferred substrate of the agoniststimulated PI 3-kinase in intact cells. Treatment of both resting and PDGF-stimulated cells with the fungal metabolite wortmannin resulted in pronounced, selective effects on the levels of all D-3 phosphoinositides. Kinetic studies with this PI 3-kinase inhibitor revealed the presence of at least two independent routes for the biosynthesis of D-3 phosphoinositides in PDGF-treated cells.

Platelet-derived growth factor-induced formation of tensin and phosphoinositide 3-kinase complexes

Tensin is an SH2 domain-containing cytoskeletal protein that binds to and caps actin filaments. Investigation of signal transduction mechanisms associated with tensin revealed the presence of phosphoinositide 3-kinase (PI 3-kinase) activity in tensin immunoprecipitates from platelet-derived growth factor-treated cells. Association of PI 3-kinase activity with tensin was transitory, and the amount of activity was approximately 1% of the total PI 3-kinase activity found in anti-phosphotyrosine (anti-pY) immunoprecipitates. In vitro, PI 3-kinase activity associated with the SH2 domain of tensin in a platelet-derived growth factor-dependent manner. The optimal phosphopeptide binding specificity of the SH2 domain of tensin was determined to be phospho-Y (E or D), N, (I, V, or F). Synthetic phosphopeptides containing the sequence YENI could specifically block the association of PI 3-kinase activity with tensin in a dose-dependent manner. These results suggest that PI 3-kinase interacts with the cytoskeleton via the SH2 domain of tensin and may play an important role in platelet-derived growth factor-induced cytoskeletal reorganization that is concomitant with cell migration and proliferation.

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.

Association of phosphatidylinositol 3 kinase to protein kinase C zeta during interleukin-2 stimulation

Interleukin-2 induces a serine-phosphorylated phosphatidylinositol 3 kinase activity in the mouse T cell line TS1 alpha beta. Moreover, protein kinase C (PKC) zeta directly or indirectly associates with the phosphatidylinositol 3 kinase and the association appears to be necessary for the serine-phosphorylated phosphatidylinositol 3 kinase activity, since release of zeta PKC by competition of binding with peptides spanning the p110 sequence from amino acids 907 to 925 abolishes the serine-phosphorylated phosphatidylinositol 3 kinase activity. This kinase activity is also blocked when zeta PKC expression is inhibited by antisense oligonucleotide. Inhibition of phosphatidylinositol 3 kinase activity by wortmannin does not abolish zeta PKC association.

Oxidative stress induces tyrosine phosphorylation of PDGF alpha-and beta-receptors and pp60c-src in mesangial cells

Reactive oxygen species are autocrine and paracrine modulators of cell behavior. Hydrogen peroxide, a cellular oxidant, has been shown to stimulate mesangial cell proliferation. In the present study we analyzed the H2O2-induced early signaling events. Immunofluorescence analysis revealed a H2O2 induced dose-dependent increase in tyrosine phosphorylation. Short treatment (2 or 5 min) with 5 mM H2O2 induced a mitogenic response and a significant (P < 0.01) increase in the number of cells compared to non-treated controls. Proteins extracted from H2O2 (0.1 to 10 mM) treated cells were separated on SDS-PAGE and subjected to immunoblot analysis with anti-phosphotyrosine. A dose-dependent induction of tyrosine phosphorylation of 180 kDa, 120 kDa and 60 kDa proteins was observed within 1 to 10 minutes. By sequentially using immunoprecipitation and immunoblotting the 180 kDa tyrosine phosphorylated band was shown to represent both PDGF alpha- and beta-receptors. The tyrosine phosphorylated 60 kDa protein was identified as the cytoplasmic protein tyrosine kinase pp60c-src. The c-src phosphorylation was associated with an inhibition of c-src kinase activity, suggesting phosphorylation of tyrosine 527 in the c-src regulatory domain. Pretreatment with catalase completely abrogated the H2O2-induced PDGF receptor and c-src tyrosine phosphorylation. These data support the notion that the activation of a signaling pathway involving the PDGF receptors and c-src contributes to the mitogenic effects of reactive oxygen species.

Cpk is a novel class of Drosophila PtdIns 3-kinase containing a C2 domain

We report the identification of a novel class of phosphatidylinositol (PtdIns) 3-kinases whose members contain C-terminal C2 domains. We have isolated Drosophila and murine genes (termed cpk and cpk-m respectively) by polymerase chain reaction amplification of cDNA libraries with degenerate primers corresponding to conserved regions of PtdIns kinases. The amino acid sequences of Cpk and Cpk-m are most similar to that of p110, a family of PtdIns 3-kinases that mediates the responses of cells to mitogenic stimuli. The Cpk and Cpk-m sequences are similar to a large, central region of p110, but differ from p110 at their N and C termini. The N termini of the Cpk proteins do not contain any recognizable protein motif, while the C termini contain "C2 domains," a feature unique among PtdIns kinases. Cpk has an intrinsic PtdIns kinase activity and can phosphorylate PtdIns and PtdIns-4-P, but not PtdIns(4,5)P2, at the D3 position of the inositol ring. Cpk is the first PtdIns 3-kinase identified with this particular substrate specificity. We have identified two potential Cpk-binding proteins, p90 and p190, and have determined that both Cpk and p190 may be tyrosine phosphorylated. This finding suggests that Cpk function may be regulated by tyrosine kinases.

Brush border phosphatidylinositol 3-kinase mediates epidermal growth factor stimulation of intestinal NaCl absorption and Na+/H+ exchange

In terminally differentiated ileal villus Na+-absorptive cells, epidermal growth factor (EGF) stimulates NaCl absorption and its component brush border Na+/H+ exchanger, acting via basolateral membrane receptors, and as we confirm here, a brush border tyrosine kinase. In the present study we show that brush border phosphatidylinositol 3-kinase (PI 3-kinase) is involved in EGF stimulation of NaCl absorption and brush border Na+/H+ exchange. In rabbit ileum studied with the Ussing chamber-voltage clamp technique, EGF stimulation of active NaCl absorption is inhibited by the selective PI 3-kinase inhibitor wortmannin. PI 3-kinase, a largely cytosolic enzyme, translocates specifically to the brush border of ileal absorptive cells following EGF treatment. This translocation occurs as early as 1 min after EGF treatment and remains increased at the brush border for at least 15 min. EGF also causes a rapid (1 min) and large (4-5-fold) increase in brush border PI 3-kinase activity. Involvement of PI 3-kinase activity in intestinal Na+ absorption is established further by studies done in the human colon cancer cell line, Caco-2, stably transfected with the intestinal brush border isoform of the Na+/H+ exchanger, NHE3 (Caco-2/NHE3 cells). Brush border Na+/H+ exchange activity was measured using the pH-sensitive fluorescent dye 2'7'-bis(carboxyethyl)5-(6)-carboxyfluorescein. EGF added to the basolateral surface but not apical surface of Caco-2/NHE3 cells increased brush border Na+/H+ exchange activity. The EGF-induced increase in brush border Na+/H+ exchange activity was completely abolished in cells pretreated with wortmannin. EGF treatment caused increased tyrosine phosphorylation of PI 3-kinase in both ileal brush border membranes and Caco-2/NHE3 cells, suggesting that a tyrosine kinase upstream of the PI 3-kinase is involved in the EGF effects on Na+ absorption. In conclusion, the present study provides evidence in two separate intestinal models, the ileum and a human colon cancer cell line, that PI 3-kinase is an intermediate in EGF stimulation of intestinal Na+ absorption.