Evidence for two distinct phosphatidylinositol kinases in fibroblasts. Implications for cellular regulation

Production of novel polyphosphoinositides in vivo is linked to cell transformation by polyomavirus middle T antigen

Phosphatidylinositol 3-kinase associates with the polyomavirus middle T antigen (PyMTAg)-pp60c-src complex in polyomavirus-transformed cells. Here we show that anti-PyMTAg immunoprecipitates from PyMTAg-transformed NIH 3T3 cells have lipid kinase activities that phosphorylate phosphatidylinositol, phosphatidylinositol-4-bisphosphate, and phosphatidylinositol-4,5-bisphosphate at the D-3 position of the inositol ring to produce three new polyphosphoinositides: phosphatidylinositol-3-phosphate (PI-3-P), phosphatidylinositol-3,4-bisphosphate (PI-3,4-P2), and phosphatidylinositol trisphosphate (PIP3), respectively. PI-3-P was detected in intact parental and PyMTAg-transformed NIH 3T3 fibroblasts at both low and high cell densities. However, parental NIH 3T3 fibroblasts produced no detectable PI-3,4-P2 or PIP3 at high density. In contrast, growing, subconfluent cells and wild-type PyMTAg-transformed cells at high density had greatly enhanced incorporation of [3H]-inositol into these highly phosphorylated lipids. Cells transfected with a transformation-defective mutant of PyMTAg had undetectable levels of PI-3,4-P2 and PIP3 at high density. Thus, the synthesis of novel polyphosphoinositides by lipid kinase activity associated with PyMTAg correlates with cell growth and transformation.

Partial purification and characterization of phosphatidylinositol kinases from human platelets

Most of human platelet phosphatidylinositol (PI) kinase activity (approx. 80%) was associated with the membrane fraction and its majority was released by the extraction with Triton X-100 after KCl treatment. Two major activity peaks (mPIK-I and mPIK-III) were obtained by Mono Q column chromatography. They were distinct from each other with regard to Mr (76,000 and 80,000 as determined by gel-filtration chromatography), apparent Km values for ATP, effect of arachidonic acid and phosphatidylserine and detergent requirement. Triton X-100 inhibited the activity of mPIK-I but rather weakly enhanced the mPIK-III activity, and sodium cholate remarkably inhibited both mPIK-I and mPIK-III activities. Their products were identified to be phosphatidylinositol 4-phosphate. On the other hand, about 20% of PI kinase activity was recovered from the cytosolic fraction and two activity peaks (cPIK-I and cPIK-II) were resolved on Mono Q column chromatography. There were no significant differences in biochemical properties between cPIK-I and cPIK-II. Both of them had Mr approx. 550,000 as determined by gel-filtration chromatography and were activated by sodium cholate to a greater extent than by Triton X-100. The results suggest that the major PI kinases (mPIK-I and mPIK-III) are PI 4-kinase and mPIK-I is distinct from PI 4-kinases in other sources especially with regard to the effect of Triton X-100.

A point mutation at tyrosine-809 in the human colony-stimulating factor 1 receptor impairs mitogenesis without abrogating tyrosine kinase activity, association with phosphatidylinositol 3-kinase, or induction of c-fos and junB genes

Substitution of phenylalanine for tyrosine-809 in the human colony-stimulating factor 1 receptor (CSF-1R) inhibited its ability to transduce ligand-dependent mitogenic signals in mouse NIH 3T3 cells. When combined with an "activating" mutation at codon 301 that induces constitutive CSF-1R tyrosine kinase activity, the codon 809 mutation suppressed ligand-independent cell transformation. Comparative mapping of tryptic phosphopeptides from mutant and wild-type CSF-1R indicated that tyrosine-809 is a site of ligand-dependent receptor phosphorylation in vivo. The mutant receptor was active as a tyrosine kinase in vitro and in vivo, underwent CSF-1-dependent association with a phosphatidylinositol 3-kinase, and induced expression of the protooncogenes c-fos and junB, underscoring its ability to trigger some of the known cellular responses to CSF-1. The mutant receptor is likely to be impaired in its ability to interact with critical cellular effectors whose activity is required for mitogenesis.

A completely transformation-defective point mutant of polyomavirus middle T antigen which retains full associated phosphatidylinositol kinase activity

By using a random mutagenesis procedure combined with a recombinant retrovirus vector, mutants of polyomavirus middle T antigen (MTAg) were generated. Three new MTAg mutants with various degrees of transformation competence were more thoroughly characterized. All of the mutants produced a stable MTAg, as assessed by metabolic labeling or immunoblotting, and each mutant possessed wild-type levels of associated tyrosine kinase activity and associated phosphatidylinositol-3 (PI-3) kinase activity. One of these mutants, with a substitution of leucine for proline at amino acid 248 of MTAg (248m) was completely transformation defective, as measured in a focus-forming assay. Furthermore, the pattern of phosphorylation of 248m in vivo was identical to that of wild-type MTAg, and the kinetics of association of MTAg with an 85-kilodalton protein, the putative PI kinase, was not altered. Similarly, the pattern of PI derivatives obtained in an in vitro kinase assay was not altered by the substitution at amino acid 248. Since the single base pair mutation at amino acid 248 resulted in an MTAg that was completely transformation defective despite possessing wild-type levels of kinase activities, this suggests that neither tyrosine kinase nor PI-3 kinase activity nor the combination of both are sufficient for transformation by MTAg.

A Swiss 3T3 variant cell line resistant to the effects of tumor promoters cannot be transformed by src

To study the relationship between oncogenesis by v-src and normal cellular signalling pathways, we determined the effects of v-src on 3T3-TNR9 cells, a Swiss 3T3 variant which does not respond mitogenically to tumor promoters such as 12-O-tetradecanoyl-phorbol-13-acetate (TPA). We found that src was unable to transform these variant cells, whether the oncogene was introduced by infection with a murine retrovirus vector or by transfection with plasmid DNA. 3T3-TNR9 cells were not inherently resistant to transformation, since infection with similar recombinant retroviruses containing either v-ras or v-abl did induce transformation. Further analysis of Swiss 3T3 and 3T3-TNR9 cell populations infected with the v-src-containing retrovirus revealed that although the amount of v-src DNA in each was approximately the same, the level of the v-src message and protein and the overall level of phosphotyrosine expressed in the infected variants was much less than in infected parental cells. Cotransfection experiments using separate v-src and neo plasmids revealed a decrease in the number of G418-resistant colonies when transfections of TNR9 cells occurred in the presence of the src-containing plasmid, suggesting a growth inhibitory effect of v-src on 3T3-TNR9 cells, as has also been found for TPA itself. Since v-src cannot transform this variant cell line, which does not respond mitogenically to the protein kinase C agonist TPA, we suggest that src makes use of the protein kinase C pathway as part of its signalling activities.

Purification and characterization of bovine brain type I phosphatidylinositol kinase

Investigation into the phosphatidylinositol kinase activities in bovine brain has revealed the presence of a type I PtdIns kinase activity. This classification is based upon potent inhibition by neutral detergent and the production of a phosphatidylinositol phosphate that can be distinguished from phosphatidyl-inositol-4-phosphate [PtdIns(4)P] by thin-layer chromatography. The enzyme has been substantially purified and the activity is associated with an 85-kDa polypeptide on SDS/polyacrylamide gel electrophoresis. Analysis of the product confirms the identification of the enzyme as a type I PtdIns kinase. The purified kinase has been characterized with respect to substrate dependence (Mg2+, ATP, PtdIns), substrate presentation (pure lipid versus mixed micelle) and specificity [PtdIns versus PtdIns(4)P and phosphatidylinositol 4,5-bisphosphate].

The novel inositol lipid phosphatidylinositol 3,4-bisphosphate is produced by human blood platelets upon thrombin stimulation

Radioactive PtdIns(3)P was detected in human platelets incubated with [32P]Pi, but remained unaffected by thrombin treatment. In contrast, [32P]PtdIns(3,4)P2 was absent from resting platelets, but was produced by thrombin-activated platelets in a dose- and time-dependent manner. [32P]PtdInsP3 was never found under these conditions. These changes are similar to those elicited in other cells by platelet-derived growth factor or the oncogene product pp60c-src.

Phosphatidylinositol metabolism in cells transformed by polyomavirus middle T antigen

Associated with the middle T antigen of polyomavirus is a novel phosphatidylinositol (PtdIns) kinase activity which phosphorylates PtdIns at the D-3 position of the inositol ring. We have undertaken an analysis of myo-[3H]inositol-containing compounds in a panel of NIH 3T3 cell lines stably transfected with transforming and nontransforming middle T antigen mutants. All cell lines from which PtdIns 3-kinase activity coprecipitated with middle T antigen exhibited modestly elevated levels of PtdIns(3)P and compounds with predicted PtdIns(3,4)P2 and PtdIns(3,4,5)P3 structures. Complex formation between middle T antigen and PtdIns 3-kinase correlated not with an increase in total inositol phosphate levels but rather with elevated levels of InsP2 and InsP4. A specific increase in the level of an InsP2 species which comigrated in high-pressure liquid chromatography analysis with Ins(3,4)P2 was observed. These results suggest that association of the polyomavirus middle T antigen with PtdIns 3-kinase activates a distinct inositol metabolic pathway.

A Swiss 3T3 variant cell line resistant to the effects of tumor promoters cannot be transformed by src

To study the relationship between oncogenesis by v-src and normal cellular signalling pathways, we determined the effects of v-src on 3T3-TNR9 cells, a Swiss 3T3 variant which does not respond mitogenically to tumor promoters such as 12-O-tetradecanoyl-phorbol-13-acetate (TPA). We found that src was unable to transform these variant cells, whether the oncogene was introduced by infection with a murine retrovirus vector or by transfection with plasmid DNA. 3T3-TNR9 cells were not inherently resistant to transformation, since infection with similar recombinant retroviruses containing either v-ras or v-abl did induce transformation. Further analysis of Swiss 3T3 and 3T3-TNR9 cell populations infected with the v-src-containing retrovirus revealed that although the amount of v-src DNA in each was approximately the same, the level of the v-src message and protein and the overall level of phosphotyrosine expressed in the infected variants was much less than in infected parental cells. Cotransfection experiments using separate v-src and neo plasmids revealed a decrease in the number of G418-resistant colonies when transfections of TNR9 cells occurred in the presence of the src-containing plasmid, suggesting a growth inhibitory effect of v-src on 3T3-TNR9 cells, as has also been found for TPA itself. Since v-src cannot transform this variant cell line, which does not respond mitogenically to the protein kinase C agonist TPA, we suggest that src makes use of the protein kinase C pathway as part of its signalling activities.

Bimodal lipid substrate dependence of phosphatidylinositol kinase

Phosphatidylinositol (PI) kinase activity was solubilized from rat liver microsomes and partially purified by chromatography on hydroxyapatite and Reactive Green 19-Superose. Examination of the ATP dependence using a mixed micellar assay gave a Km of 120 microM. The dependence of reaction rate on PI was more complicated. PI kinase bound a large amount of Triton X-100, and as expected for a micelle-associated enzyme utilizing a micelle-associated lipid substrate, the reaction rate was dependent on the micellar mole fraction, PI/(PI + Triton X-100), with a Km of 0.02 (unitless). Activity showed an additional dependence on bulk PI concentration at high micelle dilution. These results demonstrated two kinetically distinguishable steps leading to formation of a productive PI/enzyme(/ATP) complex. The rate of the first step, which probably represents exchange of PI from the bulk micellar pool into enzyme-containing micelles, depends on bulk PI concentration. The rate of the second step, association of PI with enzyme within a single micelle, depends on the micellar mole fraction of PI. Depression of the apparent Vmax at low ionic strength suggested that electrostatic repulsion between negatively charged PI/Triton X-100 mixed micelles inhibits PI exchange, consistent with a model in which intermicellar PI exchange depends on micellar collisions.

PDGF-induced activation of phospholipase C is not required for induction of DNA synthesis

Platelet-derived growth factor (PDGF) induction of DNA synthesis is believed to involve activation of phospholipase C (PLC) and subsequent accumulation of inositol 1,4,5-triphosphate [I(1,4,5)P3], increase in intracellular Ca2+, activation of protein kinase C (PKC), and receptor down regulation. Generation of these events is triggered by the tyrosine protein kinase (TPK) activity of the PDGF receptor. The TPK inhibitor genistein blocked PDGF induction of these events, including DNA synthesis, with the exception of receptor down regulation. PDGF-induced phosphotyrosine phosphorylations, including receptor autophosphorylation, were inhibited by genistein. Removal of genistein and PDGF resulted in DNA synthesis without the occurrence of PLC activation. These findings indicate that these early events, with the exception of receptor down regulation, are not necessary for PDGF-induced DNA synthesis.

Association of type I phosphatidylinositol kinase activity with mutationally activated forms of human pp60c-src

Chicken embryo fibroblast cells overexpressing activated mutant forms of human pp60c-src, but not those overexpressing normal human pp60c-src, exhibited high levels of type I phosphatidylinositol (PI) kinase activity associated with pp60c-src. Levels of PI kinase activity were positively correlated with src tyrosine protein kinase activity and not with absolute levels of pp60c-src. Our results suggest that a linkage exists between certain forms of pp60c-src and the PI signal transduction pathway.

Association of type I phosphatidylinositol kinase activity with mutationally activated forms of human pp60c-src

Chicken embryo fibroblast cells overexpressing activated mutant forms of human pp60c-src, but not those overexpressing normal human pp60c-src, exhibited high levels of type I phosphatidylinositol (PI) kinase activity associated with pp60c-src. Levels of PI kinase activity were positively correlated with src tyrosine protein kinase activity and not with absolute levels of pp60c-src. Our results suggest that a linkage exists between certain forms of pp60c-src and the PI signal transduction pathway.

Platelet-derived growth factor (PDGF)-dependent association of phospholipase C-gamma with the PDGF receptor signaling complex

We investigated the interaction of phospholipase C-gamma (PLC-gamma) with wild-type and mutant forms of the platelet-derived growth factor (PDGF) beta-receptor both in vivo and in vitro. After PDGF treatment of CHO cell lines expressing wild-type or either of two mutant (delta Ki and Y825F) PDGF receptors, PLC-gamma became tyrosine phosphorylated and associated with the receptor proteins. The receptor association and tyrosine phosphorylation of PLC-gamma correlated with the ability of these receptors to mediate ligand-induced phosphatidylinositol turnover. However, both the delta Ki and Y825F mutant receptors were deficient in transmitting mitogenic signals, suggesting that the PDGF-induced tyrosine phosphorylation and receptor association of PLC-gamma are not sufficient to account for the growth-stimulatory activity of PDGF. Wild-type and delta Ki mutant PDGF receptor proteins expressed with recombinant baculovirus vectors also associated in vitro with mammalian PLC-gamma. However, baculovirus-expressed c-fms, v-fms, c-src, and Raf-1 proteins failed to associate with PLC-gamma under similar conditions. Phosphatase treatment of the baculovirus-expressed PDGF receptor greatly decreased its association with PLC-gamma. This requirement for receptor phosphorylation was also observed in vivo, where PLC-gamma could not associate with a mutant PDGF receptor (K602A) defective in autophosphorylation. PLC-gamma also coimmunoprecipitated with two other putative receptor substrates, the serine-threonine kinase Raf-1 and the 85-kilodalton phosphatidylinositol-3' kinase, presumably through its association with the ligand-activated receptor. Furthermore, baculovirus-expressed Raf-1 phosphorylated purified PLC-gamma in vitro at sites which showed increased serine phosphorylation in vivo in response to PDGF. These results suggest that PDGF directly influences PLC activity by inducing the association of PLC-gamma with a receptor signaling complex, resulting in increased tyrosine and serine phosphorylation of PLC-gamma.

Platelet-derived growth factor (PDGF)-dependent association of phospholipase C-gamma with the PDGF receptor signaling complex

We investigated the interaction of phospholipase C-gamma (PLC-gamma) with wild-type and mutant forms of the platelet-derived growth factor (PDGF) beta-receptor both in vivo and in vitro. After PDGF treatment of CHO cell lines expressing wild-type or either of two mutant (delta Ki and Y825F) PDGF receptors, PLC-gamma became tyrosine phosphorylated and associated with the receptor proteins. The receptor association and tyrosine phosphorylation of PLC-gamma correlated with the ability of these receptors to mediate ligand-induced phosphatidylinositol turnover. However, both the delta Ki and Y825F mutant receptors were deficient in transmitting mitogenic signals, suggesting that the PDGF-induced tyrosine phosphorylation and receptor association of PLC-gamma are not sufficient to account for the growth-stimulatory activity of PDGF. Wild-type and delta Ki mutant PDGF receptor proteins expressed with recombinant baculovirus vectors also associated in vitro with mammalian PLC-gamma. However, baculovirus-expressed c-fms, v-fms, c-src, and Raf-1 proteins failed to associate with PLC-gamma under similar conditions. Phosphatase treatment of the baculovirus-expressed PDGF receptor greatly decreased its association with PLC-gamma. This requirement for receptor phosphorylation was also observed in vivo, where PLC-gamma could not associate with a mutant PDGF receptor (K602A) defective in autophosphorylation. PLC-gamma also coimmunoprecipitated with two other putative receptor substrates, the serine-threonine kinase Raf-1 and the 85-kilodalton phosphatidylinositol-3' kinase, presumably through its association with the ligand-activated receptor. Furthermore, baculovirus-expressed Raf-1 phosphorylated purified PLC-gamma in vitro at sites which showed increased serine phosphorylation in vivo in response to PDGF. These results suggest that PDGF directly influences PLC activity by inducing the association of PLC-gamma with a receptor signaling complex, resulting in increased tyrosine and serine phosphorylation of PLC-gamma.

Activated type I phosphatidylinositol kinase is associated with the epidermal growth factor (EGF) receptor following EGF stimulation

We have shown that a type I phosphatidylinositol (PI) kinase activity is associated with the epidermal growth factor (EGF) receptor in a mouse fibroblast cell line expressing human EGF receptors (NRHER5) and that this activity increases dramatically upon treatment of cells with physiologically relevant concentrations of EGF. EGF stimulated a time-dependent increase in EGF receptor-associated PI kinase activity measured in EGF receptor immunoprecipitates. Activation was detected 15 min after the addition of EGF, and it peaked between 1 and 2 hr. Activation of PI kinase was detected with EGF concentrations as low as 10 pM and maximal stimulation occurred at approximately 1 nM. Analysis of deacylated PI phosphate products, and inhibition of the PI kinase activity by nonionic detergent, indicated that the PI kinase described here was type I or PI 3' kinase. These results demonstrate the regulation of a type I PI kinase by EGF and suggest a potential role in the EGF receptor signal transduction pathway.

PDGF beta-receptor stimulates tyrosine phosphorylation of GAP and association of GAP with a signaling complex

Platelet-derived growth factor (PDGF) stimulated the tyrosine phosphorylation of the GTPase activating protein (GAP) in 3T3 cells and in CHO cells expressing wild-type PDGF receptors, but not in several CHO cell lines expressing mutant receptors defective in transmitting mitogenic signals. Following PDGF treatment of cells, GAP physically associated with the PDGF receptor and with Raf-1, phospholipase c-gamma, and PI-3 kinase, suggesting that PDGF induced the formation of complexes of signaling molecules. The association of GAP with the PDGF receptor and the phosphorylation of GAP with the PDGF receptor and the phosphorylation of GAP were reconstituted in vitro using purified protein and in insect cells expressing murine PDGF receptor and human GAP. However, in cells transformed by activated c-Ha-ras, which are defective in certain responses to PDGF, GAP failed to associate with the PDGF receptor or increase its phosphotyrosine content in response to PDGF. The association of GAP with ligand-activated PDGF receptors may directly link PDGF and ras signaling pathways.

Purification and kinetic properties of a membrane-bound phosphatidylinositol kinase of the bovine adrenal medulla

Phosphatidylinositol (PI) kinase (EC 2.7.1.67), an integral membrane protein of chromaffin granule ghosts of the bovine adrenal medulla, was found to phosphorylate PI in the 4-position of the inositol ring. The PI kinase was purified about 200-fold from a membrane fraction containing chromaffin granules and microsomes by extraction with Triton X-114, followed by phase partition (clouding) and heparin Sepharose chromatography. The PI kinase preparation (specific activity of 5.1 nmol PIP/mg protein per min) was free from other enzymatic activities that metabolize polyphosphoinositides. Km values of 55 microM and 40 microM for ATP and PI, respectively, were estimated for the purified enzyme. Concentrations of Triton X-100 above the critical micellar concentration (0.01%, w/v) were necessary to support significant enzyme activity, which was optimal at about 0.1% (w/v). Its dependence of pH was similar to that of the membrane-bound enzyme, with a broad optimum around pH 7. Mes in the millimolar concentration range was found to strongly inhibit the activity of the purified PI kinase (I50 at about 4 mM). The enzyme was almost totally inhibited by low micromolar concentrations of free calcium, and stimulated by hydrophilic cations, e.g., Mg2+ and poly(L-lysine), with the same potencies as for the membrane-bound enzyme. The amphiphilic cation trifluoperazine, however, stimulated the activity of purified PI kinase less effectively than the membrane-bound enzyme (Husebye, E.S. and Flatmark, T. (1988) Biochem. Pharmacol. 37, 449-456), whereas the inhibitory effect of near millimolar concentrations of trifluoperazine was the same for the two forms of the enzyme. It is concluded that the membrane-bound PI kinase of this tissue is of type II according to the classification of Cantley and co-workers (Whitman et al. (1987) Biochem. J. 247, 165-174).

Transforming signals generated by the polyoma virus tumor antigens

The secretion of a threshold amount of SAGF can elicit one of the major features of the transformed phenotype, anchorage-independent growth. It appears that, like other cells, NIH 3T3 cells make and secrete SAGF, but not enough to enable them to proliferate in soft agar. The stream of signals from polyoma virus MTAg:pp60c-src:PI-kinase complexes attached to the inner surface of the cell membrane is not enough to raise SAGF secretion to the threshold level for agar growth, but these signals can powerfully enhance the responsiveness to added SAGF. Only when NIH 3T3 cells express all three polyoma T antigens do they secrete enough SAGF and become responsive enough to the factor to form colonies in soft agar. This is in contrast to F111 cells which produce adequate amounts of, and respond to, SAGF after MTAg expression alone. The reasons for this difference remain to be investigated. Obviously, factors expressed during the course of the spontaneous establishment are different for these two lines and they affect the cells' response to MTAg. Clearly, knowing how polyoma virus transforms cells will require a deeper understanding of the deregulation of membrane signaling enzymes by the viral MTAg and the identification and characterization of the autocrine SAGF's that mediate at least part of the neoplastic response to the viral signals.

The colony stimulating factor-1 receptor associates with and activates phosphatidylinositol-3 kinase

Colony stimulating factor-1 (CSF-1) is a lineage-specific growth factor required for proliferation and survival of mononuclear phagocytes and their precursors. The CSF-1 receptor belongs to a family of ligand-activated protein-tyrosine kinases. Activation of the platelet-derived growth factor receptor, but not the CSF-1 receptor, leads to an increase in phospholipase C activity and a subsequent elevation in intracellular calcium. Recent studies have shown that a novel phosphoinositol (PtdIns) kinase, termed PtdIns-3 kinase, is stimulated by the platelet-derived growth factor receptor and certain oncogenes in the protein-tyrosine kinase family. PtdIns-3 kinase phosphorylates the D-3 hydroxyl position of the inositol ring of PtdIns, and its products do not participate in the generation of the second messenger inositol 1,4,5-trisphosphate (Ins(1,4,5)P3). Here we report that addition of CSF-1 is followed by activation of PtdIns-3 kinase in a macrophage cell line (P388 D1), which contains CSF-1 receptors, and in BALB/c fibroblasts made to express the human CSF-1 receptor. Furthermore, we show that activation of the CSF-1 receptor results in the accumulation in intact cells of polyphosphoinositides phosphorylated at the D-3 position of the inositol ring. Thus activation of the CSF-1 receptor stimulates PtdIns-3 kinase activity, indicating a novel pathway for CSF-1 receptor-mediated signal transduction.

Phosphorylation of middle T by pp60c-src: a switch for binding of phosphatidylinositol 3-kinase and optimal tumorigenesis

Substitution of phenylalanine for tyrosine 315 of the polyoma virus middle T (mT) protein lowers the incidence and limits the spectrum of tumors induced following inoculation of the virus into newborn mice. This substitution removes the major site of phosphorylation by pp60c-src without altering the ability of mT to associate with or to activate pp60c-src. The mutant mT fails to show binding of a phosphatidylinositol 3-kinase (Ptdlns 3-kinase) activity that is normally present in wild-type mT complexes. Furthermore, an anti-peptide antiserum that specifically recognizes mT lacking phosphate at tyrosine 315 precipitates binary (mT-pp60c-src) but not ternary (mT-pp60c-src-Ptdlns 3-kinase) complexes from wild-type infected cell extracts. Reprecipitation with either anti-pp60c-src or anti-mT serum brings down ternary complexes containing mT phosphorylated on tyrosine 315. Phosphorylation of mT by pp60c-src in vivo is therefore a critical event for binding of Ptdlns 3-kinase and for expression of the full tumorigenic potential of the virus.

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

The tyrosine kinase pp60v-src, encoded by the v-src oncogene, seems to regulate phosphatidylinositol metabolism. The effect of pp60v-src on control points in inositol phosphate production was examined by measuring the amounts of inositol polyphosphates in Rat-1 cells expressing wild-type or mutant forms of the protein. Expression of v-src-resulted in a five- to sevenfold elevation in the steady-state amount of an isomer of inositol tetrakisphosphate, whereas the concentrations of inositol trisphosphates or other inositol tetrakisphosphates were not affected. The activity of a key enzyme in the formation of inositol tetrakisphosphates, inositol (1,4,5)-trisphosphate 3-kinase, was increased six- to eightfold in cytosolic extracts prepared from the v-src-transformed cells, suggesting that this enzyme may be one target for the pp60v-src kinase and that it may participate in the synthesis of novel, higher order inositol phosphates.

Phosphatidylinositol kinase activity associates with viral p60src protein

Immunoprecipitates of p60v-src proteins from chicken embryo fibroblasts infected with Rous sarcoma virus were assayed for phosphatidylinositol (PI) kinase activity in the absence of detergents. The product of the PI kinase reaction, phosphatidylinositol monophosphate (PIP), migrated slightly slower than did the authentic phosphatidylinositol-4-monophosphate marker in thin-layer chromatography and was indistinguishable from phosphatidylinositol-3-monophosphate produced by PI kinase type I. Furthermore, the deacylated product comigrated with glycerophosphoinositol-3-phosphate in high-performance liquid chromatography. Both sucrose gradient fractionation and the heat stability of PI kinase activity from cells infected with temperature-sensitive mutants suggest that the PI kinase activity is not intrinsic to p60v-src but is a property of another molecule complexed with p60v-src. All transforming variants of p60src were associated with PI kinase activity, whereas this enzyme activity was hardly detectable in immunoprecipitates from cells infected with nontransforming viruses encoding p60c-src or an enzymatically inactive variant. However, PI kinase activity was found in p60src immunoprecipitates from cells infected with nonmyristylated, nontransforming mutants as well as temperature-sensitive mutants at the nonpermissive temperature, which indicated that simple association of PI kinase activity with p60src is not sufficient for cell transformation.

Increased phosphatidylinositol kinase activity in psoriatic epidermis

Phosphatidylinositol (PI) kinase is activated by growth factors, such as epidermal growth factor (EGF), and is thought to be involved in cellular proliferation. Psoriasis is a hyperproliferative epidermal disease in which EGF receptor expression is altered and phospholipase C activity is increased. Considering the potential importance of growth factor stimulated phosphoinositide metabolism in the genesis of abnormal growth, we measured PI kinase activity in epidermal keratome biopsies from normal skin and the lesional and nonlesional skin of psoriatic patients. The PI kinase activity in 10 psoriatic involved plaques was increased 6.7-fold (Vmax = 67.1 +/- 23.9 pmol formed/min/mg protein +/- SE) when compared with 11 normal epidermal biopsies (Vmax = 10.0 +/- 1.3 pmol/min/mg protein, p less than 0.025). Similar results were noted when enzyme activity was standardized using DNA content. The apparent Km of PI kinase for ATP in involved psoriatic biopsies (0.45 +/- 0.14 mM) was also significantly (p less than 0.025) increased compared with normals (0.11 +/- 0.02 mM). The PI kinase activity in 11 biopsies of nonlesional psoriatic epidermis was not statistically different from normal epidermis. Both psoriatic and normal PI kinases required Mg++ and were inhibited by Ca++. The polyamine, spermine, a known activator of PI kinase in other tissues, stimulated normal but not psoriatic epidermal PI kinase. Both normal and psoriatic PI kinase activities had an apparent mol wt of 85,000. Increased synthesis of phosphoinositides by PI kinase in psoriatic tissue may provide more substrate for phospholipase C; a key enzyme in growth factor-mediated signal transduction.

Separation and identification of two phosphatidylinositol 4-kinase activities in bovine uterus

Growth factor-activated second messenger pathways are mediated in part via breakdown products of phosphoinositides. We have separated two phosphatidylinositol (PtdIns) 4-Kinases from bovine uteri which appear to be regulated independently. The predominant type II enzyme previously was purified to apparent homogeneity; the type I enzyme has been purified approximately 1000 fold (specific activity, approximately 30 nmoles/mg/min). The type I and type II enzymes differ sharply in apparent Km for ATP and response to divalent cations. In contrast to type II enzyme, type I PtdIns kinase was resistant to inhibition by adenosine, inhibited by increasing concentrations of Triton X-100, and less stable to storage than type II enzyme at pH values below 6.5 and above 8.5. Type I PtdIns 4-kinase has an apparent molecular mass of approximately 200 kD and type II enzyme of approximately 80 kD. Using both enzymatic and chemical criteria, both enzymes specifically phosphorylated the fourth hydroxyl group of PtdIns. The results thus establish the presence of two distinct and separate enzymes catalyzing PtdIns 4-kinase activity with different physical, kinetic, and regulatory properties, suggesting an important site for the regulation of second messenger signals transducing the responsiveness of cells to growth factors.

PDGF-dependent tyrosine phosphorylation stimulates production of novel polyphosphoinositides in intact cells

A phosphatidylinositol (PI) kinase activity associated with certain protein tyrosine kinases important in cell proliferation phosphorylates the 3' hydroxyl position of PI to produce phosphatidylinositol-3-phosphate (PI-3-P). Here we report that, in addition to PI-3' kinase activity, anti-phosphotyrosine (alpha-P-tyr) immunoprecipitates from platelet-derived growth factor (PDGF)-stimulated smooth muscle cells (SMC) contain lipid kinase activities that utilize the substrates phosphatidylinositol-4-phosphate (PI-4-P) and phosphatidylinositol-4,5-bisphosphate (PI-4,5-P2). These activities are absent in alpha-P-tyr immunoprecipitates from quiescent SMC. The product of PI-4-P phosphorylation appears to be phosphatidylinositol-3,4-bisphosphate (PI-3,4-P2), a lipid not previously reported. The product of PI-4,5-P2 phosphorylation is phosphatidylinositol-trisphosphate (PIP3). PI-3-P was detected in quiescent SMC and increased only slightly in response to PDGF. PIP3 and the putative PI-3,4-P2 appeared only after the addition of mitogen. Both the temporal production of these novel phospholipids after PDGF stimulation and the observation of the enzymatic activities that produce them in alpha-P-tyr immunoprecipitates suggest that these phospholipids are excellent candidates for mediators of the PDGF mitogenic response.

Phosphatidylinositol kinase activity associates with viral p60src protein

Immunoprecipitates of p60v-src proteins from chicken embryo fibroblasts infected with Rous sarcoma virus were assayed for phosphatidylinositol (PI) kinase activity in the absence of detergents. The product of the PI kinase reaction, phosphatidylinositol monophosphate (PIP), migrated slightly slower than did the authentic phosphatidylinositol-4-monophosphate marker in thin-layer chromatography and was indistinguishable from phosphatidylinositol-3-monophosphate produced by PI kinase type I. Furthermore, the deacylated product comigrated with glycerophosphoinositol-3-phosphate in high-performance liquid chromatography. Both sucrose gradient fractionation and the heat stability of PI kinase activity from cells infected with temperature-sensitive mutants suggest that the PI kinase activity is not intrinsic to p60v-src but is a property of another molecule complexed with p60v-src. All transforming variants of p60src were associated with PI kinase activity, whereas this enzyme activity was hardly detectable in immunoprecipitates from cells infected with nontransforming viruses encoding p60c-src or an enzymatically inactive variant. However, PI kinase activity was found in p60src immunoprecipitates from cells infected with nonmyristylated, nontransforming mutants as well as temperature-sensitive mutants at the nonpermissive temperature, which indicated that simple association of PI kinase activity with p60src is not sufficient for cell transformation.

Metabolic and structural evidence for the existence of a third species of polyphosphoinositide in cells: D-phosphatidyl-myo-inositol 3-phosphate

When human 1321 N1 astrocytoma cells were labelled to steady state with [3H]inositol and briefly with [32P]orthophosphate, a compound which contained both radiotracers and which co-migrated with phosphatidylinositol-myo-inositol 4-phosphate during t.l.c. could be extracted in acidic chloroform/methanol. Treatment with methylamine under conditions which lead to deacylation of conventional glycerophospholipids yielded a water-soluble moiety which was labelled with both radioisotopes and was eluted from an anion-exchange h.p.l.c. column with a retention time similar to, but distinct from, that of glycerophosphoinositol 4-phosphate. Experiments using sodium periodate and selective phosphatase enzymes to degrade this compound systematically generated a series of products which suggested the structure of the parent phospholipid was phosphatidyl-myo-inositol 3-phosphate (PtdIns3P). PtdIns3P is metabolically closely related to the pool(s) of inositol phospholipid(s) that serves as substrate(s) for an agonist-sensitive phosphoinositidase C, as the levels of PtdIns3P fell significantly when 1321 N1 cells were stimulated with carbachol. The relative rate of turnover of the inositol moiety of PtdIns3P is similar to that of both of the major polyphosphoinositides and significantly higher than that of total cellular phosphatidyl-myo-inositol. This suggests that all three polyphosphoinositides are synthesized from a common, rapidly metabolized, pool of phosphatidyl-myo-inositol.

Role of phosphatidylinositol kinase in PDGF receptor signal transduction

The molecules with which the platelet-derived growth factor (PDGF) receptor interacts to elicit the biochemical reactions responsible for cell proliferation have not been identified. Antisera directed against specific PDGF receptor peptides coprecipitated a phosphatidylinositol (PI) kinase and the PDGF receptor. Immunoprecipitates from PDGF-stimulated cells contained 10 to 50 times as much PI kinase as those from unstimulated cells. Mutation of the PDGF receptor by deletion of its kinase insert region resulted in a receptor markedly less effective than the wild type in eliciting cell proliferation and defective in PDGF-stimulated PI kinase, but still capable of PDGF-induced receptor autophosphorylation and phosphoinositide hydrolysis. These data show that the PDGF receptor is physically associated with a PDGF-sensitive PI kinase that is distinct from tyrosine kinase and is not required for PDGF-induced PI hydrolysis. The finding that the mutant PDGF receptor missing the kinase insert domain elicited known early biochemical responses to PDGF, but did not associate with or regulate PI kinase, suggests a novel role for the receptor-associated PI kinase in the transmission of mitogenic signals.

Phosphorylation of diacylglycerol kinase in vitro by protein kinase C

We investigated the effects of enzyme phosphorylation in vitro on the properties of diacylglycerol kinase. Diacylglycerol kinase and protein kinase C, both present as Mr-80,000 proteins, were highly purified from pig thymus cytosol. Protein kinase C phosphorylated diacylglycerol kinase (up to 1 mol of 32P/mol of enzyme) much more actively than did cyclic AMP-dependent protein kinase. Phosphorylated and non-phosphorylated diacylglycerol kinase showed a similar pI, approx. 6.8. Diacylglycerol kinase phosphorylated by either protein kinase C or cyclic AMP-dependent protein kinase was almost exclusively associated with phosphatidylserine membranes. In contrast, soluble kinase consisted of the non-phosphorylated form. The catalytic properties of the lipid kinase were not much affected by phosphorylation, although phosphorylation-linked binding with phosphatidylserine vesicles resulted in stabilization of the enzyme activity.

Purification and kinetic properties of a soluble phosphatidylinositol-4-phosphate kinase of the bovine adrenal medulla with emphasis on its inhibition by calcium ions

A phosphatidylinositol-4-phosphate kinase (PIP kinase, EC 2.7.1.68) was purified about 1000-fold from the cytosolic fraction of bovine adrenal medulla by ammonium sulfate precipitation, anion exchange, phosphocellulose and gel permeation chromatography. The PIP kinase preparation was free from other polyphosphoinositide metabolizing activities, and some of its catalytic properties were studied using both membrane-bound and liposomal/micellar phosphatidylinositol 4-phosphate (PIP) as the substrate. Using a native substrate (chromaffin granule ghosts) enriched in PIP, the Km value for MgATP was about 130 microM at 5 mM Mg2+, and optimum activity was obtained at pH 7.2. ADP inhibited the PIP kinase activity in a mixed non-competitive manner. The enzyme was stimulated by Mg2+ and spermidine, whereas free calcium effectively inhibited the PIP kinase activity (I50 at about 0.1 microM Ca2+). This inhibition was independent of calmodulin and CaATP (80 microM) was not a substrate for the PIP kinase. The presence of near intracellular concentrations of potassium decreased the sensitivity to calcium ions (I50 of about 50 microM), but the inhibitory effect was still evident in the physiologically interesting concentration range. The concentration for half-maximal inhibition was three orders of magnitude higher (I50 of about 200 microM) using a liposomal/micellar suspension of purified PIP as the substrate, which demonstrates the importance of using a native substrate in studying the regulatory properties of this enzyme. It is concluded that the high sensitivity of the PIP kinase activity to calcium ions is likely to be physiologically significant, as recently proposed for the phosphatidylinositol (PI) kinase (Husebye, E.S. and Flatmark, T. (1988) Biochim. Biophys. Acta 968, 261-265), and may represent a negative feedback control of the cytosolic calcium concentration. The cationic amphiphile trifluoperazine was a potent inhibitor of PIP kinase activity (I50 at about 15 microM), and may represent a useful tool to study PI kinase selectively in membranes or cells containing both kinases.

Polyomavirus transforms rat F111 and mouse NIH 3T3 cells by different mechanisms

Polyomavirus middle tumor antigen (mT) was expressed in a line of mouse NIH 3T3 cells under control of the dexamethasone-regulatable mouse mammary tumor virus promotor. Contrary to rat F111 cells which were rendered anchorage independent by mT expression alone (L. Raptis, H. Lamfrom, and T.L. Benjamin, Mol. Cell. Biol. 5:2476-2487, 1985), mT-producing NIH 3T3 cells were unable to grow in agar even after full mT induction. The mT:pp60c-src-associated phosphatidylinositol kinase was activated in these cells to a degree similar to that in fully transformed cells expressing the small and large T antigens, in addition to mT. We therefore propose that the stimulation of this phosphatidylinositol kinase, although apparently necessary, is not sufficient for transformation of NIH 3T3 cells by polyomavirus.

Mechanisms of transformation by polyoma virus middle T antigen

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

Stimulation of phosphatidylinositol 4,5-bisphosphate phospholipase C activity by phosphatidic acid

Phosphatidic acid was a potent activator of the phosphatidylinositol 4,5-bisphosphate (PtdIns-P2) phospholipase C activity associated with human platelet membranes. Lysophosphatidic acid was half as active as phosphatidic acid, and shortening the fatty acid chain reduced the effectiveness of the corresponding phosphatidic acid. Compounds lacking either the phosphate group (diacylglycerol or phorbol ester) or the fatty acid (glycerol phosphate) were not activators. When the negative charge was contributed by a carboxyl group (fatty acid or phosphatidylserine), stimulation of phospholipase C was weak but detectable. Structural analogs of phosphatidic acid (lipopolysaccharide, lipid A, and 2,3-diacylglucosamine 1-phosphate) were less effective but also enhanced PtdIns-P2 hydrolysis. Phosphatidic acid potentiated the activation of phospholipase C by alpha-thrombin, chelators, and guanine nucleotides. Phosphatidylinositol 4-phosphate and PtdIns-P2 were also effective activators of PtdIns-P2 degradation. Other phospholipids were without effect. The production of inositol 1,4,5-trisphosphate and diacylglycerol via the activation of phospholipase C provides a rationale for the cellular responses evoked by phosphatidic acid and the ability of this phospholipid to potentiate and initiate hormonal responses.

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

Bacterially synthesized c-Ha-ras protein (Ras) was incubated with guanosine triphosphatase (GTPase) activating (GA) protein in the presence of various phospholipids. The stimulation of Ras GTPase activity by GA protein was inhibited in some cases. Among the lipids most active in blocking GA protein activity were lipids that show altered metabolism during mitogenic stimulation. These included phosphatidic acid (containing arachidonic acid), phosphatidylinositol phosphates, and arachidonic acid. Other lipids, including phosphatidic acid with long, saturated side chains, diacylglycerols, and many other common phospholipids, were unable to alter GA protein activity. The interaction of lipids with GA protein might be important in the regulation of Ras activity during mitogenic stimulation.

Cyclic GMP-dependent protein kinase stimulates the plasmalemmal Ca2+ pump of smooth muscle via phosphorylation of phosphatidylinositol

The effect of phosphorylation by cyclic GMP-dependent protein kinase (G-kinase) on the activity of the plasmalemmal Ca2+-transport ATPase was studied on isolated plasma membranes and on the ATPase purified from pig erythrocytes and from the smooth muscle of pig stomach and pig aorta. Incubation with G-kinase resulted, in both smooth-muscle preparations, but not in the erythrocyte ATPase, in a higher Ca2+ affinity and in an increase in the maximal rate of Ca2+ uptake. Cyclic AMP-dependent protein kinase (A-kinase) did not exert such an effect. The stimulation of the (Ca2+ + Mg2+)-dependent ATPase activity of the purified Ca2+ pump reconstituted in liposomes depended on the phospholipid used for reconstitution. The stimulation of the (Ca2+ + Mg2+)-ATPase activity by G-kinase was only observed in the presence of phosphatidylinositol (PI). G-kinase, but not A-kinase, stimulated the phosphorylation of PI to phosphatidylinositol phosphate (PIP) in a preparation of (Ca2+ + Mg2+)-ATPase obtained by calmodulin affinity chromatography from smooth muscle, but not in a similar preparation from erythrocytes. Adenosine inhibited both the phosphorylation of PI and the stimulation of the (Ca2+ + Mg2+)-ATPase by G-kinase. In the absence of G-kinase the (Ca2+ + Mg2+)-ATPase was stimulated by the addition of PIP, but not by PI. In contrast with previous results of Furukawa & Nakamura [(1987) J. Biochem (Tokyo) 101, 287-290], no convincing evidence for a phosphorylation of the (Ca2+ + Mg2+)-ATPase was found. Evidence is presented showing that the apparent phosphorylation occurs in a contaminant protein, possibly myosin light-chain kinase. It is proposed that G-kinase stimulates the plasmalemmal Ca2+ pump of smooth-muscle cells indirectly via the phosphorylation of an associated PI kinase.