Biomedical subjects

M Falasca

Publications and source records attributed to M Falasca.

At least 19 recordsLinked to original sources

PubMed2003

Pyk2 regulates multiple signaling events crucial for macrophage morphology and migration.

The biological role of the protein tyrosine kinase, Pyk2, was explored by targeting the Pyk2 gene by homologous recombination. Pyk2-/- mice are viable and fertile, without overt impairment in development or behavior. However, the morphology and behavior of Pyk2-/- macrophages were impaired. Macrophages isolated from mutant mice failed to become polarized, to undergo membrane ruffling, and to migrate in response to chemokine stimulation. Moreover, the contractile activity in the lamellipodia of Pyk2-/- macrophages was impaired, as revealed by measuring the rearward movement toward the nucleus of fibronectin-coated beads on the lamellipodia in opposition to an immobilizing force generated by optical tweezers. Consistently, the infiltration of macrophages into a carageenan-induced inflammatory region was strongly inhibited in Pyk2-/- mice. In addition, chemokine stimulation of inositol (1, 4, 5) triphosphate production and Ca2+ release, as well as integrin-induced activation of Rho and phosphatidyl inositol 3 kinase, were compromised in Pyk2-/- macrophages. These experiments reveal a role for Pyk2 in cell signaling in macrophages essential for cell migration and function.

Actins↗

PubMed2001

Specificity in pleckstrin homology (PH) domain membrane targeting: a role for a phosphoinositide-protein co-operative mechanism.

Pleckstrin homology (PH) domains are protein modules found in proteins involved in many cellular processes. The majority of PH domain-containing proteins require membrane association for their function. It has been shown that most PH domains interact directly with the cell membrane by binding to phosphoinositides with a broad range of specificity and affinity. While a highly specific binding of the PH domain to a phosphoinositide can be necessary and sufficient for the correct recruitment of the host protein to the membrane, a weaker and less specific interaction may be necessary but not sufficient, thus probably requiring alternative, co-operative mechanisms.

Blood Proteins↗

PubMed2001

Modulation of oncogenic DBL activity by phosphoinositol phosphate binding to pleckstrin homology domain.

The Dbl family guanine nucleotide exchange factors (GEFs) contain a region of sequence similarity consisting of a catalytic Dbl homology (DH) domain in tandem with a pleckstrin homology (PH) domain. PH domains are involved in the regulated targeting of signaling molecules to plasma membranes by protein-protein and/or protein-lipid interactions. Here we show that Dbl PH domain binding to phosphatidylinositol 4,5-bisphosphate and phosphatidylinositol 3,4,5-triphosphate results in the inhibition of Dbl GEF activity on Rho family GTPase Cdc42. Phosphatidylinositol 4,5-bisphosphate binding to the PH domain significantly inhibits the Cdc42 interactive activity of the DH domain suggesting that the DH domain is subjected to the PH domain modulation under the influence of phosphoinositides (PIPs). We generated Dbl mutants unable to interact with PIPs. These mutants retained GEF activity on Cdc42 in the presence of PIPs and showed a markedly enhanced activating potential for both Cdc42 and RhoA in vivo while displaying decreased cellular transforming activity. Immunofluorescence analysis of NIH3T3 transfectants revealed that whereas the PH domain localizes to actin stress fibers and plasma membrane, the PH mutants are no longer detectable on the plasma membrane. These results suggest that modulation of PIPs in both the GEF catalytic activity and the targeting to plasma membrane determines the outcome of the biologic activity of Dbl.

3T3 Cells↗

PubMed2000

The role of the pleckstrin homology domain in membrane targeting and activation of phospholipase Cbeta(1).

Current studies involve an investigation of the role of the pleckstrin homology (PH) domain in membrane targeting and activation of phospholipase Cbeta(1) (PLCbeta(1)). Here we report studies on the membrane localization of the isolated PH domain from the amino terminus of PLCbeta(1) (PLCbeta(1)-PH) using fluorescence microscopy of a green fluorescent protein fusion protein. Whereas PLCbeta(1)-PH does not localize to the plasma membrane in serum-starved cells, it undergoes a rapid but transient migration to the plasma membrane upon stimulation of cells with serum or lysophosphatidic acid (LPA). Regulation of the plasma membrane localization of PLCbeta(1)-PH by phosphoinositides was also investigated. PLCbeta(1)-PH was found to bind phosphatidylinositol 3-phosphate most strongly, whereas other phosphoinositides were bound with lower affinity. The plasma membrane localization of PLCbeta(1)-PH induced by serum and LPA was blocked by wortmannin pretreatment and by LY294002. In parallel, activation of PLCbeta by LPA was inhibited by wortmannin, by LY294002, or by the overexpression of PLCbeta(1)-PH. Microinjection of betagamma subunits of G proteins in serum-starved cells induced the translocation of PLCbeta(1)-PH to the plasma membrane. These results demonstrate that a cooperative mechanism involving phosphatidylinositol 3-phosphate and the Gbetagamma subunit regulates the plasma membrane localization and activation of PLCbeta(1)-PH.

3T3 Cells↗

PubMed2000

Novel functional PI 3-kinase antagonists inhibit cell growth and tumorigenicity in human cancer cell lines.

New efforts in cancer therapy are being focused at various levels of signaling pathways. With phosphoinositide 3-kinase (PI3-K) potentially being necessary for a range of cancer-related functions, we have investigated the influence of selected inositol tris- to hexakisphosphates on cell growth and tumorigenicity. We show that micromolar concentrations of inositol 1,3,4,5,6-pentakisphosphate and inositol 1,4,5,6-tetrakisphosphate [Ins(1,4,5,6)P(4)] inhibit IGF-1-induced [(3)H]-thymidine incorporation in human breast cancer (MCF-7) cells and the ability to grow in liquid medium and form colonies in agarose semisolid medium by small cell lung cancer (SCLC) cells, a human cancer cell line containing a constitutively active PI3-K. In an ovarian cancer cell line that also contains a constitutively active PI3-K (SKOV-3 cells), Ins(1,4,5,6)P(4) again inhibited liquid medium growth. Furthermore, when applied extracellularly, inositol 1,3,4,5-tetrakisphosphate was shown indeed to enter SCLC cells. These effects appeared specifically related to PH domains known to bind to phosphatidylinositol 3,4-bisphosphate [PtdIns(3,4)P(2)] and phosphatidylinositol 3,4,5-trisphosphate [PtdIns(3,4,5)P(3)], indicating involvement of the PI3-K downstream target protein kinase B (PKB/Akt). This was further supported by inhibition of PKB/Akt PH domain membrane targeting in COS-7 cells by Ins(1,4,5,6)P(4). Thus, we propose that specific inositol polyphosphates inhibit PI3-K by competing with PtdIns(3,4, 5)P(3)-binding PH domains and that this occurs mainly at the level of the downstream PI3-K target, PKB/Akt.

Animals↗

PubMed2000

Patterns within protein/polyphosphoinositide interactions provide specific targets for therapeutic intervention.

Signaling pathways involving the inositol polyphosphates and the polyphosphoinositides have become intricately linked with a number of disease states. More recently, this has principally involved the 3-phosphorylated products of phosphoinositide 3-kinase, an enzyme that itself shows oncogenic activity and has hence become of interest in the design of antitumorigenic drugs. The downstream effectors of phosphoinositide 3-kinase are involved in different aspects of cellular signaling and cytoskeleton and trafficking events that are linked to specific polyphosphoinositide binding properties of specific protein domains, which themselves have emerging roles in specific disease states. Our recent findings have demonstrated that there is a selectivity of the intracellular effects of extracellularly applied inositol polyphosphates in their abilities to inhibit a range of growth-related in vivo assay conditions, and that these can themselves be linked to the inhibition of the membrane localization of a green fluorescent protein (GFP) -tagged PH domain. We propose that GFP fusions of the polyphosphoinositides binding domains of specific proteins of interest can be used in high-throughput investigations of the therapeutic value of specific inositol polyphosphates analogs. Inhibition of in vivo membrane targeting of these domains from proteins involved in cell growth and tumorigenesis can thus be used in the search for new anticancer drugs.

Antineoplastic Agents↗

PubMed2000

A novel positive feedback loop mediated by the docking protein Gab1 and phosphatidylinositol 3-kinase in epidermal growth factor receptor signaling.

The Gab1 protein is tyrosine phosphorylated in response to various growth factors and serves as a docking protein that recruits a number of downstream signaling proteins, including phosphatidylinositol 3-kinase (PI-3 kinase). To determine the role of Gab1 in signaling via the epidermal growth factor (EGF) receptor (EGFR) we tested the ability of Gab1 to associate with and modulate signaling by this receptor. We show that Gab1 associates with the EGFR in vivo and in vitro via pTyr sites 1068 and 1086 in the carboxy-terminal tail of the receptor and that overexpression of Gab1 potentiates EGF-induced activation of the mitogen-activated protein kinase and Jun kinase signaling pathways. A mutant of Gab1 unable to bind the p85 subunit of PI-3 kinase is defective in potentiating EGFR signaling, confirming a role for PI-3 kinase as a downstream effector of Gab1. Inhibition of PI-3 kinase by a dominant-interfering mutant of p85 or by Wortmannin treatment similarly impairs Gab1-induced enhancement of signaling via the EGFR. The PH domain of Gab1 was shown to bind specifically to phosphatidylinositol 3,4,5-triphosphate [PtdIns(3,4,5)P3], a product of PI-3 kinase, and is required for activation of Gab1-mediated enhancement of EGFR signaling. Moreover, the PH domain mediates Gab1 translocation to the plasma membrane in response to EGF and is required for efficient tyrosine phosphorylation of Gab1 upon EGF stimulation. In addition, overexpression of Gab1 PH domain blocks Gab1 potentiation of EGFR signaling. Finally, expression of the gene for the lipid phosphatase PTEN, which dephosphorylates PtdIns(3,4, 5)P3, inhibits EGF signaling and translocation of Gab1 to the plasma membrane. These results reveal a novel positive feedback loop, modulated by PTEN, in which PI-3 kinase functions as both an upstream regulator and a downstream effector of Gab1 in signaling via the EGFR.

Adaptor Proteins, Signal Transducing↗

PubMed2000

Different subcellular localization and phosphoinositides binding of insulin receptor substrate protein pleckstrin homology domains.

Insulin evokes diverse biological effects through receptor-mediated tyrosine phosphorylation of the insulin receptor substrate (IRS) proteins. Here, we show that, in vitro, the IRS-1, -2 and -3 pleckstrin homology (PH) domains bind with different specificities to the 3-phosphorylated phosphoinositides. In fact, the IRS-1 PH domain binds preferentially to phosphatidylinositol 3,4,5-trisphosphate (PtdIns-3,4,5-P3), the IRS-2 PH domain to phosphatidylinositol 3,4-bisphosphate (PtdIns-3,4-P2), and the IRS-3 PH domain to phosphatidylinositol 3-phosphate. When expressed in NIH-IR fibroblasts and L6 myocytes, the IRS-1 and -2 PH domains tagged with green fluorescent protein (GFP) are localized exclusively in the cytoplasm. Stimulation with insulin causes a translocation of the GFP-IRS-1 and -2 PH domains to the plasma membrane within 3-5 min. This translocation is blocked by the phosphatidylinositol 3-kinase (PI 3-K) inhibitors, wortmannin and LY294002, suggesting that this event is PI 3-K dependent. Interestingly, platelet-derived growth factor (PDGF) did not induce translocation of the IRS-1 and -2 PH domains to the plasma membrane, indicating the existence of specificity for insulin. In contrast, the GFP-IRS-3 PH domain is constitutively localized to the plasma membrane. These results reveal a differential regulation of the IRS PH domains and a novel positive feedback loop in which PI 3-K functions as both an upstream regulator and a downstream effector of IRS-1 and -2 signaling.

Animals↗

PubMed1998

Specificity and promiscuity in phosphoinositide binding by pleckstrin homology domains.

Pleckstrin homology (PH) domains are small protein modules involved in recruitment of signaling molecules to cellular membranes, in some cases by binding specific phosphoinositides. We describe use of a convenient "dot-blot" approach to screen 10 different PH domains for those that recognize particular phosphoinositides. Each PH domain bound phosphoinositides in the assay, but only two (from phospholipase C-delta1 and Grp1) showed clear specificity for a single species. Using soluble inositol phosphates, we show that the Grp1 PH domain (originally cloned on the basis of its phosphatidylinositol 3,4,5-trisphosphate (PtdIns(3,4,5)P3) binding) binds specifically to D-myo-inositol 1,3,4,5-tetrakisphosphate (Ins(1,3,4,5)P4) (the PtdIns(3,4,5)P3 headgroup) with KD = 27.3 nM, but binds D-myo-inositol 1,3,4-trisphosphate (Ins(1,3,4)P3) or D-myo-inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) over 80-fold more weakly. We show that this specificity allows localization of the Grp1 PH domain to the plasma membrane of mammalian cells only when phosphatidylinositol 3-kinase (PI 3-K) is activated. The presence of three adjacent equatorial phosphate groups was critical for inositol phosphate binding by the Grp1 PH domain. By contrast, another PH domain capable of PI 3-K-dependent membrane recruitment (encoded by EST684797) does not distinguish Ins(1,3,4)P3 from Ins(1,3,4,5)P3 (binding both with very high affinity), despite selecting strongly against Ins(1,4,5)P3. The remaining PH domains tested appear significantly less specific for particular phosphoinositides. Together with data presented in the literature, our results suggest that many PH domains bind similarly to multiple phosphoinositides (and in some cases phosphatidylserine), and are likely to be regulated in vivo by the most abundant species to which they bind. Thus, using the same simple approach to study several PH domains simultaneously, our studies suggest that highly specific phosphoinositide binding is a characteristic of relatively few cases.

Animals↗

PubMed1998

Release of the mitogen lysophosphatidylinositol from H-Ras-transformed fibroblasts; a possible mechanism of autocrine control of cell proliferation.

Lysophosphatidylinositol (LysoPtdIns) is formed by a constitutively-active phosphoinositide-specific phospholipase A2 in Ras-transformed cells and can stimulate cell proliferation. To evaluate whether LysoPtdIns could function as an autocrine modulator of cell growth, we examined whether LysoPtdIns can be released in the medium of Ras-transformed FRT-Fibro fibroblasts and thyroid cells. Here, we report that LysoPtdIns accumulates in the extracellular space of these lines and reaches levels up to tenfold higher than in the case of normal cells. Moreover, the ionophore A23187 increased the levels of the lysolipid in the extracellular medium. Extracellular LysoPtdIns was rapidly hydrolyzed to inositol 1:2-cyclic phosphate. LysoPtdIns induced thymidine incorporation in FRT-Fibro Ha-Ras fibroblasts, whereas inositol cyclic 1:2-cyclic phosphate did not affect cell growth per se, nor did it interfere with the LysoPtdIns mitogenic activity. We hypothesize that in Ras-transformed fibroblasts the formation and release of LysoPtdIns may function as an autocrine mechanism that participates in the Ras-dependent stimulation of cell growth.

Animals↗

PubMed1998

Activation of phospholipase C gamma by PI 3-kinase-induced PH domain-mediated membrane targeting.

Signaling via growth factor receptors frequently results in the concomitant activation of phospholipase C gamma (PLC gamma) and phosphatidylinositol (PI) 3-kinase. While it is well established that tyrosine phosphorylation of PLC gamma is necessary for its activation, we show here that PLC gamma is regulated additionally by the lipid products of PI 3-kinase. We demonstrate that the pleckstrin homology (PH) domain of PLC gamma binds to phosphatidylinositol 3,4,5-trisphosphate [PdtIns(3,4,5)P3], and is targeted to the membrane in response to growth factor stimulation, while a mutated version of this PH domain that does not bind PdtIns(3,4,5)P3 is not membrane targeted. Consistent with these observations, activation of PI 3-kinase causes PLC gamma PH domain-mediated membrane targeting and PLC gamma activation. By contrast, either the inhibition of PI 3-kinase by overexpression of a dominant-negative mutant or the prevention of PLC gamma membrane targeting by overexpression of the PLC gamma PH domain prevents growth factor-induced PLC gamma activation. These experiments reveal a novel mechanism for cross-talk and mutual regulation of activity between two enzymes that participate in the control of phosphoinositide metabolism.

Amino Acid Sequence↗

PubMed1998

Activation of the c-fos serum response element by phosphatidyl inositol 3-kinase and rho pathways in HeLa cells.

Many growth factors rapidly induce transcription of the c-fos proto-oncogene. We have investigated the pathways for induction of the c-fos promoter by serum and epidermal growth factor (EGF) in HeLa cells. Induction of the serum response element (SRE) of the c-fos promoter could be split into two parts, one involving the serum response factor-associated ternary complex factor (TCF) factors and the second mediated by core SRE sequences. Serum induction was mediated primarily by the core SRE, whereas EGF used both the TCF and core SRE pathways. Using activated and inhibitory signaling proteins, we found that phosphatidyl inositol 3-kinase (PI3K) and rho family members could mediate activation by serum. Activation by PI3K was mediated by core SRE sequences and was dependent upon rac and rho, suggesting a PI3K-to-rac-to-rho pathway for core SRE activation. The PI3K target Akt was also capable of activating the SRE but functioned through the TCF pathway, suggesting that Akt does not mediate the primary PI3K pathway to the SRE and that Akt is capable of activating TCF family members. Serum and EGF induction of the core SRE was partially inhibited by rho and PI3K inhibitors. The use of these inhibitors demonstrates the complexity of signaling pathways to the SRE and suggests that serum activates rho by PI3K-dependent and -independent pathways.

3T3 Cells↗

PubMed1997

Fast receptor-induced formation of glycerophosphoinositol-4-phosphate, a putative novel intracellular messenger in the Ras pathway.

Glycerophosphoinositols are phosphoinositide metabolites whose levels are constitutively elevated in Ras-transformed cells. Here, we show that one of these compounds, glycerophosphoinositol-4-phosphate (GroPIns-4-P) responds acutely to the stimulation of the epidermal growth factor receptor, with a fast, massive and transient increase. The mechanism leading to GroPIns-4-P formation involves the activation of phosphoinositide-3 kinase and the small GTP-binding protein Rac, since GroPIns-4-P was neither formed in cells expressing the dominant negative form of Rac nor in cells treated with the phosphoinositide-3 kinase inhibitor wortmannin. GroPIns-4-P has been previously shown to inhibit adenylyl cyclase. Accordingly, epidermal growth factor also decreased the basal, cholera toxin-stimulated, and forskolin-stimulated cyclic AMP levels with kinetics similar to those of GroPIns-4-P formation, suggesting that GroPIns-4-P mediates this inhibitory effect. The hormone-induced formation of GroPIns-4-P was detected in several cell lines of various origin, suggesting that GroPIns-4-P is a novel intracellular messenger of the Ras pathway, possibly able to convey information from tyrosine kinase receptors to the cyclic AMP cascade.

3T3 Cells↗

PubMed1997

Phosphatidylinositol 3-kinase mediates epidermal growth factor-induced activation of the c-Jun N-terminal kinase signaling pathway.

The signaling events which mediate activation of c-Jun N-terminal kinase (JNK) are not yet well characterized. To broaden our understanding of upstream mediators which link extracellular signals to the JNK pathway, we investigated the role of phosphatidylinositol (PI) 3-kinase in epidermal growth factor (EGF)-mediated JNK activation. In this report we demonstrate that a dominant negative form of PI 3-kinase as well as the inhibitor wortmannin blocks EGF-induced JNK activation dramatically. However, wortmannin does not have an effect on JNK activation induced by UV irradiation or osmotic shock. In addition, a membrane-targeted, constitutively active PI 3-kinase (p110beta) was shown to produce in vivo products and to activate JNK, while a kinase-mutated form of this protein showed no activation. On the basis of these experiments, we propose that PI 3-kinase activity plays a role in EGF-induced JNK activation in these cells.

Androstadienes↗

PubMed1996

Changes in the levels of glycerophosphoinositols during differentiation of hepatic and neuronal cells.

Glycerophosphoinositols are metabolites formed by a phosholipase A2 and a lysolipase specifically acting on membrane phosphoinositol lipids. High levels of these compounds characterize epithelial cells and fibroblasts transformed by ras and other cellular oncogenes. Here we have analyzed the glycerophosphoinositol levels in cells that are considered models of cell differentiation. Using rat hepatocytes at different stages of liver development we have shown that the glycerophosphoinositol basal levels of fetal cells were up to fourfold higher than in adult hepatocytes. No changes in glycerophosphoinositol were observed in regenerating rat liver, a model of differentiated cells proliferating in a synchronous manner, where only glycerophosphoinositol 4-phosphate increased by 80%. Similarly to fetal hepatocytes, a modest but significant increase (30%) in the levels of glycerophosphoinositols was observed in undifferentiated NG-108-15 cells as compared to the same cells induced to differentiate by cAMP. In a different neuronal cell line, PC12 cells, increased glycerophosphoinositol levels characterized the differentiated cells. Based on these observations we suggest that high glycerophosphoinositol levels characterize cellular phenomena associated with the activation of ras/mitogen-activated protein kinase pathways.

Animals↗

PubMed1995

Signalling pathways involved in the mitogenic action of lysophosphatidylinositol.

Lysophosphatidylinositol has been previously shown to stimulate cell proliferation in differentiated and in K-ras transformed thyroid cells. Increased levels of lysophosphatidylinositol, but not lysophosphatidylcholine or lysophosphatidylethanolamine, are present in thyroid as well as in other ras-transformed cell lines. We have now investigated the mechanism of action of this lysolipid by analysing its effects in a differentiated thyroid cell line. Lysophosphatidylinositol did not increase the levels of cAMP, the main stimulator of cell proliferation in the thyroid, whereas it stimulated phosphoinositide breakdown, mobilization of cytosolic Ca2+ and arachidonic acid release, suggesting that it activates both phospholipases C and A2. None of the effects of lysophosphatidylinositol were prevented by pretreatment of cells with pertussis toxin. Instead, the tyrosine kinase inhibitors, tyrphostins AG18 and AG561, completely blocked its mitogenic action. The effects of lysophosphatidylinositol were distinguishable from those of the well known mitogen lysophosphatidic acid, which affected differently the signalling pathways analysed and was not mitogenic in ras-transformed cells. These results suggest that the mitogenic activity of lysophosphatidylinositol is associated with the activation of phospholipase C and phospholipase A2 and is relatively specific for ras-transformed cells.

Animals↗

PubMed1995

Physiological concentrations of thyrotropin increase cytosolic calcium levels in primary cultures of human thyroid cells.

The activity of TSH, the main regulator of growth and differentiation in the thyroid, has been mainly related to the activation of the adenylyl cyclase cascade. TSH also activates phospholipase-C and -A2; these effects, however, have been reported to require concentrations of the hormone up to 1000-fold higher than those effective on adenylyl cyclase, suggesting that the main physiological mechanism involved in the action of TSH is the activation of this enzyme. Using primary cultures of human thyroids, we here show that physiological concentrations of TSH (0.01-10 mU/L) are also able to increase intracellular Ca2+ levels. Cells were loaded with the fluorescent Ca2+ probe fura-2 and analyzed by single cell Ca2+ recording. The basal Ca2+ level was 105 +/- 30 nmol/L, and physiological concentrations of TSH increased it by 2- to 7-fold. The Ca2+ increase was transient and lasted up to 10 min. It is also shown that the TSH-dependent Ca2+ increase involves both the activation of phospholipase-C and the entry of extracellular Ca2+. TSH (100-10000 mU/L) increased cAMP levels by up to 20-fold in parallel experiments performed on the same cell preparations. These data demonstrate that physiological concentrations of TSH are able to increase cytosolic Ca2+ levels, indicating that this second messenger might directly mediate the action of this hormone in the thyroid.

Adenosine Triphosphate↗

PubMed1994

Elevated levels and mitogenic activity of lysophosphatidylinositol in k-ras-transformed epithelial cells.

In cell lines stably (KiKi) or reversibly (Ts) transformed by the k-ras oncogene originated from a differentiated rat thyroid line (FRTL5 cells), k-ras-induced transformation has been associated with an increased phospholipase A2 activity. Here we provide evidence that this enzymic activity is phosphoinositide specific and leads to the formation of lysophosphatidylinositol. The levels of this lysolipid increased by 2-3-fold in ras-transformed cells (KiKi cells and Ts cells at the permissive temperature of 33 degrees C) as compared to differentiated cells (FRTL5) or to Ts cells maintained at 39 degrees C, i.e. at the temperature where ras-p21, the product of the ras oncogene, is inactive. Since another lysoderivative, lysophosphatidic acid, has been shown to be a mitogen, we have tested whether lysophosphatidylinositol could have a similar activity on thyroid cells. Lysophosphatidylinositol (10-100 microM) induced a 5-10-fold increase in [3H]thymidine incorporation in both FRTL5 and KiKi cells, whereas lysophosphatidic acid was active only in differentiated cells. Lysophosphatidylinositol (approximately 25 microM) and lysophosphatidic acid (50-100 microM) acted synergistically with insulin in increasing [3H]thymidine incorporation. Moreover, lysophosphatidylinositol at concentrations three-fold higher than those found to be mitogenic, inhibited the activity of the GTPase-activating protein. We conclude that lysophosphatidylinositol is a mitogen that might play a role in the modulation of k-ras transformed cell proliferation.

Animals↗