Platelet-derived growth factor induces rapid and sustained tyrosine phosphorylation of phospholipase C-gamma in quiescent BALB/c 3T3 cells

Phospholipase C

Phospholipase C (PLC) family members constitute a family of diverse enzymes. Thirteen different family members have been cloned. These family members have unique structures that mediate various functions. Although PLC family members all appear to signal through the bi-products of cleaving phospholipids, it is clear that each family member, and at times each isoform, contributes to unique cellular functions. This chapter provides a review of the current literature on PLC. In addition, references have been provided for more in-depth information regarding areas that are not discussed including tyrosine kinase activation of PLC. Understanding the roles of the individual PLC enzymes, and their distinct cellular functions, will lead to a better understanding of the physiological roles of these enzymes in the development of diseases and the maintenance of homeostasis.

Recent Advances in the Development of Anticancer Drugs that Act against Signalling Pathways

Cancer can be considered a disease of deranged intracellular signalling. The intracellular signalling pathways that mediate the effects of oncogenes on cell growth and transformation present attractive targets for the development of new classes of drugs for the prevention and treatment of cancer. This is a new approach to developing anticancer drugs and the potential, as well as some of the problems, inherent in the approach are discussed. Anticancer drugs that produce their effects by disrupting signalling pathways are already in clinical trial. Some properties of these drugs, as well as other inhibitors of signalling pathways under development as potential anticancer drugs, are reviewed.

Phosphoinositides: tiny lipids with giant impact on cell regulation

Phosphoinositides (PIs) make up only a small fraction of cellular phospholipids, yet they control almost all aspects of a cell's life and death. These lipids gained tremendous research interest as plasma membrane signaling molecules when discovered in the 1970s and 1980s. Research in the last 15 years has added a wide range of biological processes regulated by PIs, turning these lipids into one of the most universal signaling entities in eukaryotic cells. PIs control organelle biology by regulating vesicular trafficking, but they also modulate lipid distribution and metabolism via their close relationship with lipid transfer proteins. PIs regulate ion channels, pumps, and transporters and control both endocytic and exocytic processes. The nuclear phosphoinositides have grown from being an epiphenomenon to a research area of its own. As expected from such pleiotropic regulators, derangements of phosphoinositide metabolism are responsible for a number of human diseases ranging from rare genetic disorders to the most common ones such as cancer, obesity, and diabetes. Moreover, it is increasingly evident that a number of infectious agents hijack the PI regulatory systems of host cells for their intracellular movements, replication, and assembly. As a result, PI converting enzymes began to be noticed by pharmaceutical companies as potential therapeutic targets. This review is an attempt to give an overview of this enormous research field focusing on major developments in diverse areas of basic science linked to cellular physiology and disease.

The phospholipase C isozymes and their regulation

The physiological effects of many extracellular neurotransmitters, hormones, growth factors, and other stimuli are mediated by receptor-promoted activation of phospholipase C (PLC) and consequential activation of inositol lipid signaling pathways. These signaling responses include the classically described conversion of phosphatidylinositol(4,5)P(2) to the Ca(2+)-mobilizing second messenger inositol(1,4,5)P(3) and the protein kinase C-activating second messenger diacylglycerol as well as alterations in membrane association or activity of many proteins that harbor phosphoinositide binding domains. The 13 mammalian PLCs elaborate a minimal catalytic core typified by PLC-d to confer multiple modes of regulation of lipase activity. PLC-b isozymes are activated by Gaq- and Gbg-subunits of heterotrimeric G proteins, and activation of PLC-g isozymes occurs through phosphorylation promoted by receptor and non-receptor tyrosine kinases. PLC-e and certain members of the PLC-b and PLC-g subclasses of isozymes are activated by direct binding of small G proteins of the Ras, Rho, and Rac subfamilies of GTPases. Recent high resolution three dimensional structures together with biochemical studies have illustrated that the X/Y linker region of the catalytic core mediates autoinhibition of most if not all PLC isozymes. Activation occurs as a consequence of removal of this autoinhibition.

Phospholipase C

Phospholipase C (PLC) family members constitute a family of diverse enzymes. Thirteen different family members have been cloned. These family members have unique structures that mediate diverse functions. Although PLC family members all appear to signal through the bi-products of cleaving phospholipids, it is clear that each family member, and at times each isoform, contributes to unique cellular functions. This chapter provides a review of the current literature. In addition, references have been provided for more in depth information regarding areas that are discussed. Ultimately, understanding the roles of the individual PLC enzymes, and their distinct cellular functions, will lead to a better understanding of the development of diseases and the maintenance of homeostasis.

Proteomic analysis of post-translational modifications in conditioned Hermissenda

Post-translational modifications of proteins are a major determinant of biological function. Phosphorylation of proteins involved in signal transduction contributes to the induction and maintenance of several examples of cellular and synaptic plasticity. In this study we have identified phosphoproteins regulated by Pavlovian conditioning in lysates of Hermissenda nervous systems using two-dimensional electrophoresis (2DE) in conjunction with (32)P labeling, fluorescence based phosphoprotein in-gel staining, and mass spectrometry. Modification of protein phosphorylation regulated by conditioning was first assessed by densitometric analysis of (32)P labeled proteins resolved by 2DE from lysates of conditioned and pseudorandom control nervous systems. An independent assessment of phosphorylation regulated by conditioning was obtained from an examination of 2D gels stained with Pro-Q Diamond phosphoprotein dye. Mass spectrometric analysis of protein digests from phosphoprotein stained analytical gels or Coomassie Blue stained preparative gels provided for the identification of phosphoproteins that exhibited statistically significant increased phosphorylation in conditioned groups as compared to pseudorandom controls. A previously identified cytoskeletal related protein, Csp24 (24 kDa conditioned stimulus pathway phosphoprotein), involved in intermediate-term memory exhibited significantly increased phosphorylation detected 24 h post-conditioning. Our results show that proteins involved in diverse cellular functions such as transcriptional regulation, cell signaling, cytoskeletal regulation, metabolic activity, and protein degradation contribute to long-term post-translational modifications associated with Pavlovian conditioning.

PLC-gamma1 regulates fibronectin assembly and cell aggregation

Phospholipase C-gamma1 (PLC-gamma1) mediates cell adhesion and migration through an undefined mechanism. Here, we examine the role of PLC-gamma1 in cell-matrix adhesion in a hanging drop assay of cell aggregation. Plcg1 Null (-/-) mouse embryonic fibroblasts formed aggregates that were larger and significantly more resistant to dissociation than cells in which PLC-gamma1 is re-expressed (Null+ cells). Aggregate formation could be disrupted by inhibition of fibronectin interaction with integrins, indicating that fibronectin assembly may mediate aggregate formation. Fibronectin assembly was mediated by integrin alpha5beta1 in both cell lines, while assays measuring fibronectin assembly revealed increased assembly in the Null cells. Null and Null+ cells exhibited equivalent fibronectin mRNA levels and equivalent levels of fibronectin protein in pulse-labeling experiments. However, levels of secreted fibronectin in the conditioned medium were increased in Null cells. The data implicates a negative regulatory role for PLC-gamma1 in cell aggregation by controlling the secretion of fibronectin into the media and its assembly into fibrils.

New prospects for management and treatment of inoperable and recurrent skull base meningiomas

Skull base, including optic nerve, cavernous sinus, clival and foramen magnum tumors represent a major challenge for neurosurgeons and neuro-oncologists. Growth regulatory signaling pathways for these tumors are of increasing interest as potential targets for new chemotherapy. Those differentially activated in various grades of meningiomas are currently being identified as well. This article reviews some recent findings pathways that appear to regulate meningioma growth. Potential targets for novel therapies are also discussed.

Contrasting role of phospholipase C-gamma1 in the expression of immediate early genes induced by epidermal or platelet-derived growth factors

While significant progress has been achieved in identifying the signal transduction elements that operate downstream of activated receptor tyrosine kinases, it remains unclear how different receptors utilize these signaling elements to achieve a common response. This study compares the capacity of epidermal growth factor (EGF) and platelet-derived growth factor (PDGF) to elicit the induction of immediate early gene (IEG) mRNAs in the presence or absence of phospholipase C-gamma1 (PLC-gamma1). The results show that while PDGF induction of nearly all IEG mRNAs is abrogated in plcg1 null cells, EGF induction of the same genes is variable in the null cells and exhibits three distinct responses. Five IEG mRNAs (Nup475, Cyr61, TF, Gly, TS7) are completely inducible by EGF in the presence or absence of PLC-gamma1, while three others (JE, KC, FIC) exhibit a stringent requirement for the presence of PLC-gamma1. The third type of response is exhibited by c-fos and COX-2. While these mRNAs are completely induced by EGF in the absence of PLC-gamma1, the time course of their accumulation is significantly delayed. No IEG was identified as completely inducible by EGF and PDGF in the absence of PLC-gamma1. Electrophoretic mobility shift assays (EMSA) demonstrate that PLC-gamma1 is necessary for nuclear extracts from PDGF-treated cells, but not EGF-treated cells, to interact with probes for AP-1 or NF-kappaB.

Mitogenic Signal Transduction Pathways in Meningiomas: Novel Targets for Meningioma Chemotherapy?

The roles of growth factor receptors and numerous downstream growth regulatory pathways are of increasing interest in neuro-oncology. These pathways have been extensively studied in gliomas but only recently analyzed in meningiomas. This article reviews current research on the growth factor receptor-Ras-Raf-1-MEK-1-MAPK, PI3K-Akt/PKB, PLC-gamma1-PKC, phospholipase A2-cyclooxygenase, and TGF-beta receptor-Smad pathways that appear to regulate meningioma growth and inhibit apoptosis. Sites along these receptor/kinase cascades that might be targeted by novel therapies are also discussed.

12-lipoxygenase is increased in glucose-stimulated mesangial cells and in experimental diabetic nephropathy

BACKGROUND

Arachidonic acid-derived 12-lipoxygenase (12-LO) products have potent growth and chemotactic properties. The present studies examined whether 12-LO and fibronectin are induced in cultured rat mesangial cells (MCs) exposed to high glucose and whether they are expressed in experimental diabetic nephropathy.

METHODS

To determine the effect of high glucose on MC 12-LO mRNA and protein expression, rat MCs were incubated with RPMI medium containing 100 (NG) or 450 mg/dL glucose (HG). For animal studies, rats were injected with diluent (control) or streptozotocin. The latter were left untreated (DM) or treated with insulin (DM + I). At sacrifice after four months, GAPDH, 12-LO, and fibronectin mRNA were measured by competitive reverse transcription-polymerase chain reaction (RT-PCR) in microdissected glomeruli (G). Renal sections were semiquantitatively scored (0 to 4+) for diabetic changes and for 12-LO and fibronectin by immunohistochemistry.

RESULTS

12-LO mRNA expression in MC exposed to HG (12.71 +/- 1.17 attm/microL) and DM G (1.78 +/- 0.65 x 10-3 attm/glomerulus) was significantly higher than those of MCs in NG media (6.71 +/- 0.78 attm/microL) and control G (0.34 +/- 0.12 x 10-3 attm/glomerulus, P < 0.005), respectively. Western blot revealed a 1.7- and a 2.8-fold increase in MC and G 12-LO protein expression, respectively (P < 0.05). The immunohistochemistry score for G 12-LO and diabetic nephropathy score was significantly greater in DM and DM + I than controls. MC and G GAPDH mRNA remained unchanged.

CONCLUSIONS

In MCs exposed to HG and in diabetic rat glomeruli, increments in 12-LO mRNA and protein are associated with changes modeling diabetic nephropathy. These findings suggest a role for the 12-LO pathway in the pathogenesis of diabetic nephropathy.

The role of phospholipase C-gamma1 in 1alpha,25-dihydroxyvitamin D(3) regulated keratinocyte differentiation

Phospholipase C-gamma1 (PLC-gamma1) is the most abundant member of the phospholipase C family expressed in human keratinocytes. PLC-gamma1 is induced by 1alpha,25-dihydroxyvitamin D(3) (1alpha,25(OH)(2)D(3)) in normal keratinocytes via a DR6-type vitamin D responsive element. This regulation is not observed in transformed keratinocytes. The role of PLC-gamma1 in mediating 1alpha,25(OH)(2)D(3) and calcium-regulated differentiation was then tested. Both specific PLC inhibitors and antisense constructs which selectively block PLC-gamma1 production prevented 1alpha,25(OH)(2)D(3) and calcium from inducing markers of differentiation such as involucrin and transglutaminase. These studies demonstrate that PLC-gamma1 induction by 1alpha,25(OH)(2)D(3) is critical to the ability of this hormone to regulate keratinocyte differentiation.

Activation of Stat3 preassembled with platelet-derived growth factor beta receptors requires Src kinase activity

Members of the STAT family of transcriptional regulators modulate the expression of a variety of gene products that promote cell proliferation, survival and transformation. Although initially identified as mediators of cytokine signaling, the STAT proteins are also activated by, and thus may contribute to the actions of, polypeptide growth factors. To define the mechanism by which these factors activate STATs, we examined the process of Stat3 activation in Balb/c-3T3 fibroblasts treated with platelet-derived growth factor (PDGF). As STATs are activated by tyrosine phosphorylation, and as PDGF receptors are ligand-activated tyrosine kinases, we considered the possibility that Stat3 interacts with and is phosphorylated by PDGF receptors. We find that Stat3 associates with PDGF beta receptors in both the presence and, surprisingly, the absence of PDGF. Moreover, Stat3 was phosphorylated on tyrosine in PDGF beta receptor immunoprecipitates of PDGF-treated but not untreated cells. Although required, receptor activation was insufficient for Stat3 activation. When added to cells in combination with a pharmacologic agent (PD180970) that specifically inhibits the activity of Src family tyrosine kinases, PDGF did not activate Stat3 as monitored by electrophoretic mobility shift assay. PD180970 did not affect MAPK activation by PDGF or the JAK-dependent activation of Stat3 by interleukin-6. The necessity of Src activity for Stat3 activation by PDGF was further evidenced by data showing the presence of Src in complexes containing both Stat3 and PDGF beta receptors in PDGF-treated cells. These results suggest a novel mechanism of STAT activation in which inactive Stat3 pre-assembles with inactive PDGF receptors, and in response to ligand binding and in a manner dependent on Src kinase activity, is rapidly phosphorylated and activated. Additional data demonstrate that Src kinase activity is also required for PDGF stimulation of DNA synthesis in density-arrested cells.

Activation of PLC and PI 3 kinase by PDGF receptor alpha is not sufficient for mitogenesis and migration in mesangial cells

BACKGROUND

Platelet-derived growth factor (PDGF) isoforms act through two distinct cell surface alpha and beta receptors. Glomerular mesangial cells express both receptors. PDGF BB and AB are potent mitogens for glomerular mesangial cells, and PDGF BB stimulates cell migration in a phosphatidylinositol 3 (PI 3) kinase-dependent manner. In this study, we investigated the effect of PDGF AA on cell migration, PI 3 kinase and phospholipase C (PLC) activation, and the role of these two enzymes in mediating biological responses in these cells in response to all three isoforms.

METHODS

3H-thymidine incorporation and modified Boyden chamber assay were used to determine DNA synthesis and directed migration, respectively, in response to all three PDGF isoforms. Differential activation of alpha and beta receptors was studied by immunecomplex tyrosine kinase assay of corresponding receptor immunoprecipitates. PLC gamma 1 activity was determined by measuring total inositol phosphates in response to different PDGF isoforms. PI 3 kinase activity was determined in antiphosphotyrosine or PDGF receptor immunoprecipitates.

RESULTS

Both PDGF BB and AB resulted in stimulation of DNA synthesis and directed migration of mesangial cells. AA was neither chemotactic nor mitogenic. However, all three isoforms increased tyrosine phosphorylation of a 180 kD protein in antiphosphotyrosine immunoprecipitates, suggesting activation of respective receptors. Direct immunecomplex tyrosine kinase assay of alpha and beta receptors demonstrated significant activation of both of these receptors when cells are treated with PDGF BB or AB. PDGF AA increased tyrosine kinase activity of the alpha receptor but not the beta receptor. All three isoforms significantly stimulated the production of inositol phosphates with order of potency being BB > AB > AA. PDGF AA also dose dependently stimulated PI 3 kinase activity measured in antiphosphotyrosine immunoprecipitates of treated cells. A comparison of PI 3 kinase activity in antiphosphotyrosine immunoprecipitates from mesangial cells stimulated with three different PDGF isoforms showed significant activation of this enzyme with a decreasing order of activity: BB > AB > AA.

CONCLUSION

Taken together, these data demonstrate that all three isoforms of PDGF significantly stimulate PLC gamma 1 and PI 3 kinase, two enzymes necessary for both DNA synthesis and directed migration. However, activation of alpha receptor by PDGF AA with a subsequent increase in PLC and PI 3 kinase activities is not sufficient to induce these biological responses in mesangial cells. These data indicate that the extent of activation of signal transduction pathways may be a major determinant of the biological activity of different PDGF isoforms in mesangial cells.

Growth factor-induced promoter activation of murine phospholipase C delta4 gene

Phospholipase C delta4 (PLCdelta4) is one of the delta-type PLC isozymes, the expression of which is induced in nuclei by treatment with serum and also in some cancer cells. We isolated and analyzed a promoter region of the murine PLCdelta4 gene. DNA sequence analysis showed that this region is GC-rich and has no TATA box, and the region from -143 to -127 was found, by luciferase activity and gel mobility-shift assay, to be essential for transcription of PLCdelta4. We also found that the promoter activity of PLCdelta4 was stimulated by treatment with growth factors such as bradykinin, lysophosphatidic acid, and Ca2+ ionophore in addition to serum. In parallel, we detected PLCdelta4 mRNA induction and an increase in complex formation of the promoter region and nuclear protein from HeLa cells on stimulation with these growth factors. Finally, we found that trapping the growth factor-induced cytoplasmic Ca2+-inhibited activation of the promoter activity and protein induction in nuclei. These results show that PLCdelta4 may have an important role in nuclei in response to growth factors, and its expression may be partially regulated by an increase in cytoplasmic Ca2+.

Pancreatic stellate cells are activated by proinflammatory cytokines: implications for pancreatic fibrogenesis

BACKGROUND

The pathogenesis of pancreatic fibrosis is unknown. In the liver, stellate cells play a major role in fibrogenesis by synthesising increased amounts of collagen and other extracellular matrix (ECM) proteins when activated by profibrogenic mediators such as cytokines and oxidant stress.

AIMS

To determine whether cultured rat pancreatic stellate cells produce collagen and other ECM proteins, and exhibit signs of activation when exposed to the cytokines platelet derived growth factor (PDGF) or transforming growth factor beta (TGF-beta).

METHODS

Cultured pancreatic stellate cells were immunostained for the ECM proteins procollagen III, collagen I, laminin, and fibronectin using specific polyclonal antibodies. For cytokine studies, triplicate wells of cells were incubated with increasing concentrations of PDGF or TGF-beta.

RESULTS

Cultured pancreatic stellate cells stained strongly positive for all ECM proteins tested. Incubation of cells with 1, 5, and 10 ng/ml PDGF led to a significant dose related increase in cell counts as well as in the incorporation of 3H-thymidine into DNA. Stellate cells exposed to 0.25, 0.5, and 1 ng/ml TGF-beta showed a dose dependent increase in alpha smooth muscle actin expression and increased collagen synthesis. In addition, TGF-beta increased the expression of PDGF receptors on stellate cells.

CONCLUSIONS

Pancreatic stellate cells produce collagen and other extracellular matrix proteins, and respond to the cytokines PDGF and TGF-beta by increased proliferation and increased collagen synthesis. These results suggest an important role for stellate cells in pancreatic fibrogenesis.

Phosphorylation of phospholipase Cgamma1 on tyrosine residue 783 by platelet-derived growth factor regulates reorganization of the cytoskeleton

It is known that platelet-derived growth factor (PDGF) induces the phosphorylation of phospholipase C (PLC) gamma1 and that phosphorylation on tyrosine (Tyr) 783 of PLCgamma1 is essential for phosphatidylinositol 4,5-bisphosphate hydrolyzing activity in vivo, while phosphorylation does not affect the catalytic activity in vitro. To study the roles of Tyr-783 phosphorylation in vivo, we developed a polyclonal antibody that recognizes PLCgamma1 containing phosphotyrosine 783 (alpha-PLCgamma1 PY). Tyr-783-phosphorylated PLCgamma1 was not detected in the absence of PDGF, appeared after stimulation, increased for 30 min, and then decreased to near the prestimulation level. Immunostaining of cells showed that PDGF-produced Tyr-783-phosphorylated PLCgamma1 localized predominantly at membrane ruffles and stress fibers where it colocalized with actin filaments within 30 min. Ninety minutes after PDGF stimulation, the actin filaments were disassembled to short fragments, and the levels of Tyr-783-phosphorylated PLCgamma1 were remarkably decreased in membrane ruffles and cytoskeleton. Furthermore, the depolymerization of actin filaments and membrane ruffling caused by PDGF stimulation were blocked by microinjecting alpha-PLCgamma1 PY, as occurred following the microinjection of the PLCgamma1-2SH2 domain, which is expected to associate with phosphorylated PDGF receptors and to block PLCgamma1 binding. It is worth noting that the microinjection of tyrosine-phosphorylated peptide (consisting of 13 amino acids containing Tyr-783) induced the disassembly of actin filaments and membrane ruffling as observed in PDGF-stimulated cells, while nonphosphorylated peptide did not cause any effect. These data suggest that the phosphorylation of PLCgamma1 on tyrosine 783 by PDGF plays an important role in cytoskeletal reorganization in addition to mitogenesis.

Signal transduction via platelet-derived growth factor receptors

Platelet-derived growth factor (PDGF) exerts its stimulatory effects on cell growth and motility by binding to two related protein tyrosine kinase receptors. Ligand binding induces receptor dimerization and autophosphorylation, allowing binding and activation of cytoplasmic SH2-domain containing signal transduction molecules. Thereby, a number of different signaling pathways are initiated leading to cell growth, actin reorganization migration and differentiation. Recent observations suggest that extensive cross-talk occurs between different signaling pathways, and that stimulatory signals are modulated by inhibitory signals arising in parallel.

Full activation of the platelet-derived growth factor beta-receptor kinase involves multiple events

Activation of receptor tyrosine kinases is thought to involve ligand-induced dimerization, which promotes receptor transphosphorylation and thereby increases the receptor's phosphotransferase activity. We used two platelet-derived growth factor beta-receptor (beta-PDGFR) mutants to identify events that are required for full engagement (autophosphorylation and activation of the kinase activity) of the beta-PDGFR kinase. The F79/81 receptor (Tyr to Phe substitution at 579 and 581 in the juxtamembrane domain of the receptor) was capable of only very modest ligand-dependent autophosphorylation and also failed to associate with numerous SH2 domain-containing proteins. Furthermore, stimulation with platelet-derived growth factor (PDGF) did not increase the kinase activity of the F79/81 mutant toward exogenous substrates. However, the F79/81 receptor had basal kinase activity and could be artificially stimulated by incubation with ATP. Because the low kinase activity of the F857 mutant (Tyr to Phe substitution at 857 in the putative activation loop) could not be increased by incubation with ATP, failure to phosphorylate Tyr-857 may be the reason why the F79/81 mutant has low kinase activity. Surprisingly, the F857 mutant underwent efficient PDGF-dependent autophosphorylation. Thus the PDGF-dependent increase in the kinase activity of the receptor is not required for autophosphorylation. We conclude that full activation of the beta-PDGFR kinase requires at least two, apparently distinct events.

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.

Differential regulation of vitamin D responsive elements in normal and transformed keratinocytes

Squamous cell carcinomas (SCC) derived from human epidermis fail to differentiate normally under the influence of 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] despite the presence of the vitamin D receptor. Previous studies from our laboratory showed that phospholipase C-gamma1 (PLC-gamma1) was upregulated transcriptionally by 1,25(OH)2D3 in normal human keratinocytes, and a vitamin D responsive element (VDRE) in its promoter region has been identified. To examine the inducibility of human PLC-gamma1 transcription by 1,25(OH)2D3 and/or retinoic acid in SCC cell lines, we transiently transfected SCC4 and SCC12B2 cells with human PLC-gamma1 promoter-luciferase constructs containing the VDRE and tested the response of these constructs to 1,25(OH)2D3 and/or all-trans retinoic acid. The induction of the human PLC-gamma1 VDRE by 1,25(OH)2D3 was synergistic with all-trans retinoic acid in normal human keratinocytes, but none of the constructs was induced by 1,25(OH)2D3 and/or all-trans retinoic acid in SCC4 and SCC12B2 cells. In contrast, the construct containing the VDRE of the human 24-hydroxylase gene was induced several fold by 1,25(OH)2D3 in normal human keratinocytes and by both 1,25(OH)2D3 and all-trans retinoic acid in SCC4 and SCC12B2 cells. DNA mobility shift assays showed that both the vitamin D receptor and the retinoic acid receptor in SCC4 and SCC12B2 cells bound the human PLC-gamma1 VDRE similarly to that seen in normal keratinocytes. The data indicate that the VDRE in the human PLC-gamma1 gene is not functional in SCC4 and SCC12B2 cells, unlike normal human keratinocytes, even though vitamin D receptors bind normally to it. Failure of transcriptional control of the PLC-gamma1 gene by 1,25(OH)2D3 suggests the lack of a cofactor(s) linking the VDRE to the transcriptional machinery.

The Rho GTPase and a putative RhoGEF mediate a signaling pathway for the cell shape changes in Drosophila gastrulation

The Rho GTPases mediate actin rearrangements that are likely to be required for the numerous cell shape changes in a developing embryo. In a genetic screen for Rho signaling pathway components in Drosophila, we identified a gene, DRhoGEF2, that encodes a predicted Rho-specific guanine nucleotide exchange factor. Embryos lacking DRhoGEF2 fail to gastrulate due to a defect in cell shape changes required for tissue invagination, and expression of a dominant-negative Rho GTPase in early embryos results in similar defects. Evidence is also presented that DRhoGEF2 mediates these specific cell shape changes in response to the extracellular ligand, Fog. Together, these results establish a Rho-mediated signaling pathway that is essential for the major morphogenetic events in Drosophila gastrulation.

Dissociation of tyrosine phosphorylation and activation of phosphoinositide phospholipase C induced by the protein kinase C inhibitor Ro-31-8220 in Swiss 3T3 cells treated with platelet-derived growth factor

Platelet-derived growth factor (PDGF) stimulates the hydrolysis of phosphatidylinositol 4,5-bisphosphate (Ptd InsP2) via phospholipase C-gamma1 (PLC-gamma1) in Swiss 3T3 cells. Treatment of cells with the protein kinase C (PKC) inhibitor Ro-31-8220 greatly decreased PDGF-induced tyrosine phosphorylation of PLC-gamma1, but paradoxically enhanced the production of inositol phosphates (InsPs). The inhibitor also caused an increase of PDGF receptor tyrosine phosphorylation at later times. The changes in phosphorylation of the receptor were correlated with alterations in PLC-gamma1 translocation to the particulate fraction. Thus, although activation of PLC-gamma1 was associated with phosphorylation of the receptor and translocation of the enzyme to the particulate fraction, it was dissociated from its tyrosine phosphorylation. A non-receptor-associated, cytosolic tyrosine kinase also was found to phosphorylate PLC-gamma1 in a PDGF-dependent manner, but was not inhibited by Ro-31-8220 in vitro. PKC depletion by phorbol ester treatment decreased the tyrosine phosphorylation of PLC-gamma1 induced by PDGF and slowed the translocation of PLC-gamma1, but Ro-31-8220 produced further effects. The effect of Ro-31-8220 to enhance the production of InsPs could not be attributed to inhibition of PKC since InsPs production with PDGF was decreased in PKC-depleted cells and a stimulatory effect of the inhibitor was still evident. Interestingly, Ro-31-8220 decreased the radioactivity in phosphatidylinositol and increased that in phosphatidylinositol 4-phosphate and PtdInsP2 in cells labeled with myo[3H]inositol. The increased synthesis of PtdInsP2 could contribute to the increased production of InsPs induced by Ro-31-8220. In summary, these results support the conclusion that the activation of PLC-gamma1 in response to PDGF requires autophosphorylation of the receptor and membrane association of PLC-gamma1, but not phosphorylation of the enzyme. Furthermore, the effects of Ro-31-8220 on the tyrosine phosphorylation and activity of PLC-gamma1, and on PtdInsP2 synthesis cannot be attributed to inhibition of PKC.

Platelet-derived growth factor stimulates sodium-dependent Pi transport in osteoblastic cells via phospholipase Cgamma and phosphatidylinositol 3' -kinase

Inorganic phosphate (Pi) is a major regulator of cell metabolism. The Pi transport activity in the plasma membrane is a main determinant of the intracellular level of this ion. In bone-forming cells, Pi transport is important for the calcification of the bone matrix. In this study, the effect of platelet-derived growth factor (PDGF) on Pi transport activity and the signaling mechanism involved in this cellular response were analyzed. The results indicate that PDGF is a potent and selective stimulator of sodium-dependent Pi transport in the mouse calvaria-derived MC3T3-E1 osteoblast-like cells. The change in Pi transport induced by PDGF-BB was dependent on translational processes and affected the Vmax of the Pi transport system. These observations suggested that enhanced Pi transport activity in response to PDGF resulted from insertion of newly synthesized Pi transporters in the plasma membrane. The role of activation of mitogen activated protein (MAP) kinase, phospholipase C (PLC)gamma or phosphatidylinositol 3-kinase (PI-3-kinase), in mediating this effect of PDGF, was investigated. A selective inhibitor of the PDGF receptor tyrosine kinase activity (CGP 53716) completely blocked PDGF-induced protein tyrosine phosphorylation of several proteins including the PDGF receptor, PLCgamma, MAP kinase, and association of the p85 subunit of PI-3'-kinase. Associated with this effect, the increase in Pi transport induced by PDGF was completely blunted by 5 microM CGP 53716. Inhibition of MAP kinase activity by cAMP agonists did not influence Pi transport stimulation induced by PDGF. However, inhibitors of protein kinase C completely blocked this response. A selective inhibitor of PI-3-kinase, LY294002, also significantly reduced this effect of PDGF. In summary, these results indicate that PDGF is a potent and selective stimulator of Pi transport in osteoblastic cells. The mechanism responsible for this effect is not mediated by MAP kinase but involves tyrosine phosphorylation-dependent activation of PLCgamma and PI-3-kinase.

Cloning and functional analysis of the hematopoietic cell-specific phospholipase C(gamma)2 promoter

Phospholipase C(gamma)2 (PLCgamma2) is a phospholipid-converting enzyme which, upon receptor stimulation, is activated within membrane-bound signalling complexes. In contrast to the highly ubiquitous PLCgamma1, PLCgamma2 is expressed predominantly in B-lymphocytes. Associated with antigen-coupling receptors it is activated by tyrosine phosphorylation after the triggering of B-cell surface immunoglobulin. We have cloned and sequenced the human PLCgamma2 promoter. Primer extension analysis reveals the existence of a major transcriptional start site. The TATA-less promoter contains G+C-rich stretches with a cluster of contiguous SP1 consensus sites, an NF1, and an AP2 site between bp -220 to -70. A construct containing the region from -189 to +78 confers full promoter activity, as shown by fusion to a luciferase reporter gene construct. The distal part of the promoter between bp -662 to -293 containing an SRE, EBF and CACCC box contributed negatively to promoter activity in the B-cell line Raji but not in three adherent cell lines. In Raji cells, PLCgamma2 mRNA is expressed at low levels with a half life greater than 4 h. After treatment with serum, TPA, retinoic acid, or with 5-azacytidine increased levels of PLCgamma2 mRNA were induced in B-cells.

Inhibition of the phosphorylation of a myristoylated alanine-rich C kinase substrate by methyl methanesulfonate in cultured NIH 3T3 cells

The effect of methyl methanesulfonate (MMS) on the phosphorylation of an acidic 80-kDa myristoylated alanine-rich C kinase substrate (MARCKS) protein was investigated in NIH 3T3 fibroblasts. An alkylating agent, MMS inhibited protein kinase C activity and the phosphorylation of MARCKS. MMS treatment also lowered the cellular amounts of second messengers of inositol-1,4,5-trisphosphate and diacylglycerol. Data suggest that MMS decreased the phosphorylation of phospholipase C, a protein whose activity is influenced by its phosphorylation state. We present here the first report that MMS intervenes in a signal cascade by inhibiting the phosphorylation of phospholipase C, which in turn leads to the inactivation of protein kinase C and the subsequent inhibition of MARCKS phosphorylation.

Requirement for phosphatidylinositol 3'-kinase activity in platelet-derived growth factor-stimulated tyrosine phosphorylation of p125 focal adhesion kinase and paxillin

The role of phosphatidylinositol 3'-kinase (PI 3'-kinase) activity in platelet-derived growth factor (PDGF)-stimulated tyrosine phosphorylation of focal adhesion kinase (p125FAK) and paxillin has been examined. The tyrosine phosphorylation of p125FAK and paxillin in response to PDGF was markedly inhibited by wortmannin in a dose-dependent manner. PDGF-stimulated PI 3'-kinase activity, membrane ruffle formation, and tyrosine phosphorylation of p125FAK and paxillin were all inhibited by the same low concentrations of wortmannin (>90% inhibition at 40nM). In contrast, tyrosine phosphorylation of p125FAK and paxillin in response to bombesin, endothelin, and phorbol 12,13-dibutyrate was not inhibited by wortmannin in these cells. Furthermore, LY294002, an inhibitor of PI 3'-kinase structurally unrelated to wortmannin, also inhibited PDGF-stimulated p125FAK tyrosine phosphorylation. PDGF was shown to stimulate the tyrosine phosphorylation of p125FAK in porcine aortic endothelial (PAE) cells transfected with the wild type PDGF-beta receptors, but not in PAE cells transfected with PDGF-beta receptors in which the PI 3'-kinase binding sites (Tyr-740/751) were replaced by phenylalanine. PDGF-stimulated, PI 3'-kinase-dependent tyrosine phosphorylation of p125FAK was not inhibited by rapamycin, and thus it was dissociated from the activation of p70 S6 kinase, previously identified as a molecular downstream target of PI 3'-kinase. Thus, we have identified a PI 3'-kinase-dependent signal transduction pathway in the action of PDGF, which leads to the phosphorylation of p125FAK and paxillin.

EGF or PDGF receptors activate atypical PKClambda through phosphatidylinositol 3-kinase

Overexpression of a TPA-insensitive PKC member, an atypical protein kinase C (aPKClambda), results in an enhancement of the transcriptional activation of TPA response element (TRE) in cells stimulated with epidermal growth factor (EGF) or platelet-derived growth factor (PDGF). EGF or PDGF also caused a transient increase in the in vivo phosphorylation level and a change in the intracellular localization of aPKClambda from the nucleus to the cytosol, indicating the activation of aPKClambda in response to this growth factor stimulation. These immediate signal-dependent changes in aKPClambda were observed for a PDGF receptor add-back mutant (Y40/51) that possesses only two of the five major autophosphorylation sites and binds PI3-kinase, and were inhibited by wortmannin, an inhibitor of PI3-kinase. Furthermore, an N-terminal fragment of the catalytic subunit of PI3-kinase, p110alpha, inhibited aPKClambda-dependent activation of TRE in Y40/51 cells stimulated with PDGF. Overexpression of p110alpha resulted in an enhancement of TRE expression in response to PDGF and the regulatory domain of aPKClambda inhibited this TRE activation in Y40/51 cells. These results provide the first in vivo evidence supporting the presence of a novel signalling pathway from receptor tyrosine kinases to aPKClambda through PI3-kinase.

A small GTP-binding protein, Rho, associates with the platelet-derived growth factor type-beta receptor upon ligand binding

Ligand binding to the platelet-derived growth factor (PDGF) receptor initiates a complex and diverging cascade of signaling pathways. GTP-binding proteins with intrinsic GTPase activity (G-proteins) frequently link cell surface receptors to intracellular signaling pathways, but no close associations of the PDGF receptor and any small G-proteins, nor any such associations activated by ligand binding to the receptor have been previously reported. We demonstrate that a small GTP-binding protein binds specifically to the murine and human PDGF type-beta receptor. In response to PDGF-BB stimulation, there is an increase in the amount of labeled small G-protein associated with the PDGF type-beta receptor. The GTP-binding protein did not undergo ligand-induced association with a mutant receptor protein that was unable to bind ATP. Proteolytic cleavage analysis, together with two-dimensional separation techniques, identified the small G-protein specifically associating with the PDGF type-beta receptor after ligand binding as a member of the Rho family. This was confirmed by demonstration that the small G-protein coimmunoprecipitated by the anti-PDGF receptor antibody was a substrate for the ADP-ribosyltransferase C3 exoenzyme. Thus, the PDGF type-beta receptor may form a complex with one or more small G-proteins upon binding PDGF-BB, and the Rho small G-protein is likely to be an important component of the proteins making up the multimeric signaling complex of the PDGF type-beta receptor.

A neuroprotective compound, aurin tricarboxylic acid, stimulates the tyrosine phosphorylation cascade in PC12 cells

Aurin tricarboxylic acid (ATA), a general nuclease inhibitor, was reported to prevent PC12 cells from cell death caused by serum starvation (1). In our study, ATA also protected PC12 cells, but not NIH3T3 cells, from serum-starved cell death. When we investigated the mechanism of action of ATA on these cells, ATA was found to increase tyrosine phosphorylation in PC12 cells, but not in NIH3T3 cells. Further investigation on tyrosine-phosphorylated proteins revealed that ATA, similar to nerve growth factor and epidermal growth factor, induced tyrosine phosphorylation of mitogen-activated protein kinases. Since the tyrosine phosphorylation of mitogen-activated protein kinases is thought to play an important role inn growth factor-dependent signal pathways, this finding suggests that the action of ATA on PC12 cells is mediated by tyrosine phosphorylation cascade, similar to growth factor signaling. In addition, we found that Shc proteins, phosphatidylinositol 3-kinase, and phospholipase C-gamma were also phosphorylated in ATA-treated PC12 cells. These key proteins in signal transduction pathways are known to associate with ligand-activated growth factor receptors and are phosphorylated on tyrosine. Thus, the phosphorylation of these three proteins by ATA stimulation supports the speculation that ATA activates a certain receptor tyrosine kinase.

Electroporation of pp60c-src antibodies inhibits the angiotensin II activation of phospholipase C-gamma 1 in rat aortic smooth muscle cells

Our previous study has shown that angiotensin II induces the rapid tyrosine phosphorylation and activation of phospholipase C-gamma 1 in cultured rat aortic smooth muscle (RASM) cells (Marrero, M.B., Paxton, W.G., Duff, J. L., Berk, B. C., and Bernstein, K. E. (1994) J. Biol. Chem, 269, 10935-10939). This signaling pathway is initiated by ligand binding to the AT1 receptor, a cell surface G protein-coupled receptor. Antibodies to pp60c-src were introduced into RASM cells by electroporation. Angiotensin II-stimulated tyrosine phosphorylation of phospholipase C-gamma 1 was eliminated by the anti-pp60c-src antibodies but not by anti-mouse IgG or bovine serum albumin. Angiotensin II also induced the rapid tyrosine phosphorylation of pp120, a known pp60c-src kinase substrate, and this phosphorylation was also specifically inhibited by anti-pp60c-src antibodies. Electroporation of RASM cells with anti-pp60c-src antibodies had no effect on platelet-derived growth factor-stimulated tyrosine phosphorylation of PLC-gamma 1. Anti-pp60c-src also reduced the angiotensin II-stimulated inositol 1,4,5-trisphosphate production by 78%, while it had no effect on the platelet-derived growth factor-stimulated inositol 1,4,5-trisphosphate production. These data provide the first evidence for a direct involvement of pp60c-src kinase in angiotensin II-mediated PLC-gamma 1 phosphorylation and activation. Furthermore, it also describes a pathway in which a seven-transmembrane receptor can stimulate an intracellular tyrosine kinase.

Inhibitors of phospholipid intracellular signaling as antiproliferative agents

The improved understanding of oncogenesis and the involvement of oncogenes and tumor suppressor genes, has led to a rational approach of specific target-directed anti-cancer drug development. Cancer genes have been found to be important not only in the control of cell proliferation but also in the mediation of processes such as drug resistance, metastasis, neo-vascularization (angiogenesis), and apoptosis. These are all important targets in their own right and the development of drugs against specific "upstream" targets in oncogenic or growth factor signal transduction cascades it may be possible to inhibit multiple "downstream" targets. Ultimately, to test the hypothesis that signaling pathways offer good targets for anticancer drug development will take several years of careful clinical study and we cannot say at this time whether the approach will work. There are a small number of compounds in the early stages of clinical development as anticancer agents that may act by inhibiting growth factor signaling pathways. In all cases the activity of the compounds on intracellular signaling pathways was discovered after their identification as antiproliferative agents. There are also compounds in preclinical development that have been specifically developed as inhibitors of growth factor signaling, although their selectivity for tumor cells compared to normal tissue remains to be investigated fully in appropriate animal tumor models. It is possible that a single antisignaling drug by itself may not have the power to completely inhibit tumor growth and a combination of drugs may be needed. It may also take a combination of drugs to prevent the emergence of resistance. Clearly there are several challenges to developing this new class of anticancer drugs, and there will undoubtedly be others that must be faced.

Trapidil inhibits human mesangial cell proliferation: effect on PDGF beta-receptor binding and expression

Mesangial cell (MC) proliferation, a histopathologic feature common to many human glomerular diseases, is regulated by several growth factors through their binding to specific cell surface receptors. Platelet-derived growth factor (PDGF) is a peptide exerting a potent mitogenic activity on MC. Recently, an increased expression of both PDGF protein and its receptor has been localized in the mesangial areas of several experimental as well as human proliferative glomerulonephritides (GN). Thus, it may be postulated that the inhibition of PDGF action could prevent MC proliferation during mesangial proliferative GN. Trapidil, an antiplatelet drug, has been shown to inhibit the growth of several cell types both in vitro and in vivo. The present study was aimed at evaluating the effect of Trapidil on human MC in vitro. The addition of 100 to 400 micrograms/ml Trapidil significantly reduced cell proliferation induced by different growth factors (FCS, PDGF-BB, bFGF, EGF), the highest inhibitory effect being on PDGF-BB stimulated MC growth. The effect of the drug was dose-dependent and seemingly specific: aspirin was devoid of any anti-proliferative action, while dypiridamole proved to be toxic. Receptor binding experiments showed that Trapidil competitively inhibited 125I-PDGF-BB binding to its cell surface receptors, without inducing receptor internalization, at least after short-term (2 hr) incubation. In contrast, long-term (48 hr) exposure to 400 micrograms/ml Trapidil caused a sharp increase of PDGF-BB binding. Similar effects on cell proliferation and 125I-PDGF-BB binding were observed when NIH-3T3 fibroblasts were exposed to the test substance.(ABSTRACT TRUNCATED AT 250 WORDS)

A rapid bioassay for platelet-derived growth factor beta-receptor tyrosine kinase function

We have extended a melanophore-based bioassay for G-protein coupled receptors to include the functional expression of the murine platelet-derived growth factor (PDGF) beta-receptor. The homodimeric ligand PDGF-BB induced activation of the transiently expressed receptor in melanophore cells. This led to dose dependent pigment dispersion whereas it did not induce pigment dispersion in wild type cells. The effective concentration of PDGF-BB giving half-maximal pigment dispersion (EC50) was 1nM after 30 minutes exposure. PDGF-AA had no ability to induce pigment dispersion in melanophore cells transiently expressing the beta-PDGF receptor. PDGF-BB-induced pigment dispersion could be blocked by the bis-indolylmaleimide Ro 31-8220 which is an inhibitor of protein kinase C isoenzymes. Functional expression of the PDGF beta-receptor extends the use of the pigment translocation assay to include transmembrane signaling receptor tyrosine kinases. It opens the opportunity for the discovery of potent agonists and antagonists through massive drug screening and investigations of functional ligand-receptor interactions for single transmembrane domain receptors.

Binding and growth-inhibitory effect of heparin and oligo-heparin (2kDa) in Balb/c 3T3 cells: lack of effect on PDGF- or serum-induced inositol lipid turnover

1. The ability of heparins (bovine heparin sm 1026, Av. mol. wt. 36.9 kDa and bovine heparin EP 756, Av. mol. wt. 12.9 kDa) and heparin fractions of different molecular weights (low molecular weight heparin, LMW 2123/OP, Av. mol. wt. 4.5 kDa and oligo-heparin, Av. mol. wt. 2 kDa) to inhibit the proliferation and signalling of Balb/c 3T3 fibroblasts was investigated. 2. Heparin and heparin fractions of 4.5 and 2 kDa significantly inhibited DNA synthesis as monitored by [2H]-thymidine incorporation. 3. 3H-labelled heparin fractions of 4.5 and 2 kDa were prepared by gel-chromatography fractionation on Sephadex G-75 of an 3H-labelled commercial heparin after treatment with heparinase. 4. The binding of unfractionated and oligo-heparin of 2 kDa to Balb/c 3T3 fibroblasts was studied; we determined the specificity of heparin and oligo-heparin binding to the cells by means of displacement of bound 3H-labelled compound in response to increasing concentrations of unlabelled compounds. Scatchard analysis of binding data obtained using [3H]-heparin as ligand revealed the presence of a single class of high affinity binding sites (Kd = 28 nM) for heparin. Scatchard analysis of binding data obtained using [3H]-oligo-heparin as ligand revealed the presence of a single class of low affinity binding sites (Kd = 3.2 microM) for oligo-heparin. 5. In addition heparin displaced [3H]-oligo-heparin at a concentration of approximately 100 fold of the Kd determined in displacement studies. Furthermore, oligo-heparin significantly displaced [3H]-heparin at a concentration of approximately 10 fold of the Kd determined by displacement studies. 6. Both heparin and oligo-heparin exert their inhibitory effects on Balb/c 3T3 DNA synthesis stimulated by PDGF or serum. However these molecules did not affect the inositol lipid turnover triggered by PDGF at a concentration which did not produce maximal response. The increase of inositol phosphate metabolism produced by 20% serum was also unaffected by heparin. This concentration of serum elicited a response comparable to that induced by a submaximal concentration of PDGF.

Phosphoinositide hydrolysis in Ki-ras-transformed fibroblasts stimulated by platelet-derived growth factor and bradykinin

Signal transduction in response to platelet-derived growth factor (PDGF)-BB and bradykinin (BK) have been examined by measuring inositol polyphosphate formation in NIH3T3 fibroblast and v-Ki-ras-transformed NIH3T3 fibroblast (DT). The PDGF-induced inositol polyphosphate formation in NIH3T3 was greater than that in DT cells, in which autophosphorylation of PDGF receptor and tyrosine phosphorylation of phospholipase C (PLC)-gamma 1 were suppressed when examined by immunoblotting with anti-phosphotyrosine antibody. On the other hand, BK-stimulation produced a much higher level of inositol polyphosphate in DT cells which have a greater number of BK receptors. These results indicate that in Ki-ras transformed cells the decrease (caused by PDGF) and the increase (caused by BK) in phosphoinositide hydrolysis are due to the defective autophosphorylation of PDGF receptors leading to a reduction in PLC-gamma 1 tyrosine phosphorylation and the overexpression of BK receptors, respectively.

Differences in the regulation of phosphatidylinositol-specific phospholipase C in normal and neoplastic keratinocytes

The induction of epidermal differentiation by Ca2+ in vitro is associated with enhanced activity of phosphatidylinositol-specific phospholipase C (PLC). Neoplastic keratinocyte cell lines expressing a mutant c-Ha-ras gene and normal keratinocytes transformed to the neoplastic phenotype by transduction with the v-Ha-ras gene (v-Ha-ras keratinocytes) have elevated constitutive activity of PLC that increases further in response to Ca2+, but the cells do not differentiate normally. PLC-gamma 1 (145 kDa) is the major isoform detected by immunoblotting of extracts from control, v-Ha-ras, and neoplastic keratinocyte cell lines cultured in 0.05 mM Ca2+ medium. The amount of PLC-gamma 1 protein was higher in neoplastic cell lines than in normal and v-Ha-ras keratinocytes that had similar PLC-gamma 1 protein levels. Thus, higher PLC-gamma 1 protein levels cannot account for the elevated constitutive activity PLC in v-Ha-ras keratinocytes. After induction of differentiation by Ca2+, the amount of PLC-gamma 1 protein increased in all cell types, and PLC-delta 1 (85 kDa), barely detectable in 0.05 mM Ca2+, increased. PLC-beta 1 was not detected at any Ca2+ concentration. PLC-gamma 1 and PLC-delta 1 mRNA did not increase after elevation of extracellular Ca2+, suggesting that posttranscriptional mechanisms can regulate PLC-gamma 1 and PLC-delta 1 protein levels in normal and neoplastic keratinocytes. Activation of protein kinase C by treatment with 12-O-tetradecanoylphorbol-13-acetate (TPA) inhibited the stimulation of inositol phosphate (InsP) formation by Ca2+ but did not alter basal InsP levels in normal keratinocytes. In contrast, TPA treatment reduced both Ca(2+)-stimulated and basal InsP formation in neoplastic cells lines and v-Ha-ras keratinocytes. In both normal and v-Ha-ras keratinocytes labeled with [32P]orthophosphate, antibodies against PLC-gamma 1 immunoprecipitated a complex of 32P-labeled proteins. The relative labeling of the PLC-gamma 1 band was greater in normal than in v-Ha-ras keratinocytes. Furthermore, treatment with TPA specifically increased the relative phosphorylation of PLC-gamma 1 in v-Ha-ras keratinocytes but not in normal keratinocytes. These results suggest that the negative regulation of constitutive activity of PLC by protein kinase C differs in normal and neoplastic keratinocytes and that this could be the mechanism of increased PLC activity produced by an oncogenic ras gene in keratinocytes.

PDGF-mediated activation of phosphatidylinositol 3 kinase in human mesangial cells

Platelet-derived growth factor (PDGF) stimulates mitogenesis and exerts other biologic activities in glomerular mesangial cells. The precise mechanism of PDGF-induced mitogenesis in these cells is not clear. The activation of a signal transducing enzyme, phosphatidylinositol 3 kinase (PI 3 kinase) is associated with mitogenesis. Activation of PI 3 kinase results from stimulation of tyrosine kinase and G-protein-coupled classes of receptors. The synthesis of D3 phosphorylated inositides, the products of this enzymatic reaction, in non-nucleated cells such as blood platelets is dependent upon protein kinase C activation and G-proteins. We studied the activation of PI 3 kinase in response to PDGF in human glomerular mesangial cells. Using a PI 3 kinase 85 kD subunit specific antibody, we detected mesangial cell PI 3 kinase protein as 110 and 85 kD heterodimer. PDGF stimulated PI 3 kinase activity in antiphosphotyrosine immunoprecipitates in a dose-dependent manner showing maximum activation at 12 ng/ml. The antiphosphotyrosine associated PI 3 kinase activity showed biphasic kinetics with a fast peak within two minutes followed by a second peak at 10 minutes. Antiphosphotyrosine and PI 3 kinase immunoprecipitation studies indicated the association of the 85 kD PI 3 kinase subunit with PDGFR. Direct immunoprecipitation with PDGFR beta antibody showed the association of PI 3 kinase activity with the PDGF-receptor. The isoquinoline sulfonyl piperazine compound H7 at concentrations that inhibit PDGF-stimulated PKC activity had no effect on PDGF-stimulated PI 3 kinase activity in antiphospotyrosine immunoprecipitates. These data indicate that PI3 kinase activation is insensitive to PKC. Treatment of mesangial cells with pertussis toxin at concentrations that partially inhibited PDGF-induced DNA synthesis in human mesangial cells did not inhibit PDGF-induced PI 3 kinase activation. These data indicate that PDGF activates PI 3 kinase in mesangial cells and that pertussis toxin-sensitive G-proteins are not involved in PI 3 kinase activation. The data further dissociate activation of PI 3 kinase from mitogenesis in human mesangial cells.

Signalling pathways as targets for anticancer drug development

Intracellular signalling pathways mediating the effects of oncogenes on cell growth and transformation offer novel targets for the development of anticancer drugs. With this approach, it may be sufficient to target a component of the signalling pathway activated by the oncogene rather than the oncogene product itself. In this review, the abilities of some antiproliferative drugs to inhibit signalling targets are considered. There are some anticancer drugs already in clinical trial that may act by inhibiting signalling targets, as well as drugs in preclinical development. Some problems that may be encountered in developing this new class of anticancer drugs are discussed.

Dissociation of PDGF receptor tyrosine kinase activity from PDGF-mediated inhibition of gap junctional communication

Gap junction-mediated intercellular communication (GJC) may play an important role in cell proliferation and transformation since GJC is inhibited by growth factors, oncogenes, tumor promoters, and carcinogens. We have studied inhibition of GJC by platelet-derived growth factor-BB (PDGF) in the mouse fibroblast cell line C3H/10T1/2 and have sought to determine whether PDGF-induced inhibition of GJC is mediated by the PDGF receptor tyrosine kinase (RTK). PDGF-mediated inhibition of GJC was rapid and transient, with maximal inhibition occurring 40 min after PDGF addition and GJC returning to control levels after 70 min. The effect of PDGF on GJC was concentration-dependent, with maximal inhibition of 90% or greater occurring at 10 ng/ml PDGF. Stimulation of RTK activity, as determined by antiphosphotyrosine immunoblot analysis of PDGF receptor and the receptor substrates phospholipase C-gamma I (PLC-gamma I) and guanosine triphosphatase activating protein (GAP), was also concentration-dependent. Inhibition of GJC required a greater concentration of PDGF than did stimulation of RTK activity. The tyrosine kinase inhibitor genistein blocked PDGF-induced RTK activity, as measured by PDGF receptor, PLC-gamma I, and GAP tyrosine phosphorylation, in a concentration-dependent manner but did not affect PDGF-mediated inhibition of GJC. Genistein alone had no effect on GJC or PDGF receptor expression. PDGF treatment in the presence or absence of genistein resulted in phosphorylation of the connexin 43 protein on nontyrosine residues. These results suggest that inhibition of GJC by ligand-activated PDGF receptor is dissociable from the RTK activity responsible for PDGF, PLC-gamma I, and GAP phosphorylation.

A qualitative and quantitative protein database approach identifies individual and groups of functionally related proteins that are differentially regulated in simian virus 40 (SV40) transformed human keratinocytes: an overview of the functional changes associated with the transformed phenotype

A qualitative and quantitative two-dimensional (2-D) gel database approach has been used to identify individual and groups of proteins that are differentially regulated in simian virus 40 (SV40) transformed human keratinocytes (K14). Five hundred and sixty [35S]methionine-labeled proteins (462 isoelectric focusing, IEF; 98 nonequilibrium pH gradient electrophoresis, NEPHGE), out of the 3038 recorded in the master keratinocyte database, were excised from dry, silver-stained gels of normal proliferating primary keratinocytes and K14 cells and the radioactivity was determined by liquid scintillation counting. Two hundred and thirty five proteins were found to be either up- (177) or down-regulated (58) in the transformed cells by 50% or more, and of these, 115 corresponded to known proteins in the keratinocyte database (J.E. Celis et al., Electrophoresis 1993, 14, 1091-1198). The lowest abundance acidic protein quantitated was present in about 60,000 molecules per cell, assuming a value of 10(8) molecules per cell for total actin. The results identified individual, and groups of functionally related proteins that are differentially regulated in K14 keratinocytes and that play a role in a variety of cellular activities that include general metabolism, the cytoskeleton, DNA replication and cell proliferation, transcription and translation, protein folding, assembly, repair and turnover, membrane traffic, signal transduction, and differentiation. In addition, the results revealed several transformation sensitive proteins of unknown identity in the database as well as known proteins of yet undefined functions. Within the latter group, members of the S100 protein family--whose genes are clustered on human chromosome 1q21--were among the highest down-regulated proteins in K14 keratinocytes. Visual inspection of films exposed for different periods of time revealed only one new protein in the transformed K14 keratinocytes and this corresponded to keratin 18, a cytokeratin expressed mainly by simple epithelia. Besides providing with the first global overview of the functional changes associated with the transformed phenotype of human keratinocytes, the data strengthened previous evidence indicating that transformation results in the abnormal expression of normal genes rather than in the expression of new ones.

Inhibitors of phosphatidylinositol signalling as antiproliferative agents

Intracellular signalling pathways mediating the effects of oncogenes on cell growth and transformation offer novel targets for the development of anticancer drugs. With this approach it may be sufficient to target a component of the signalling pathway activated by the oncogene rather than the oncogene product itself. Phosphatidylinositol (PtdIns) is a key component of two growth factor signalling pathways. It acts as a substrate for PtdIns specific phospholipase C (PtdInsPLC) and for PtdIns-3-kinase. In this review the antiproliferative properties of some inhibitors of PtdInsPLC and PtdIns-3-kinase are considered. There are some compounds already in clinical trial as anticancer drugs that may act by inhibiting PtdIns signalling, as well as several compounds in preclinical development. Some problems that may be encountered in developing this new class of anticancer drugs are discussed.

The epidermal growth factor receptor is associated with phospholipase C-gamma 1 in meningiomas

In this study we evaluated phospholipase C-gamma 1 (PLC-gamma 1) expression, activity, and association with the epidermal growth factor (EGF) receptor in a series of human meningiomas as well as cultured meningioma cells. Phospholipase C-gamma 1 was detectable by immunoblot and immunohistochemistry in 13 of 13 meningioma specimens. Epidermal growth factor receptors were detected by immunoblot in six of nine meningiomas (67%) and by immunohistochemistry in 13 of 19 meningiomas (68%) but not in normal leptomeningeal cells. In two of three meningiomas EGF receptors and/or a 170-kd phosphotyrosine band precipitated with a PLC-gamma 1 antiserum. Both PLC-gamma 1 and EGF receptors also exhibited the same pattern of immunostaining on meningioma tissue sections. Phospholipase C-gamma 1 catalytic activity, measured in a PIP2 hydrolysis assay, was higher in nine EGF receptor-positive meningiomas than in six EGF receptor-negative meningiomas (P = .05; t test). Finally, treatment of cultured meningioma cells with transforming growth factor-alpha induced a 78% increase in PLC-gamma 1 catalytic activity. Thus, these data are consistent with the possibility that the EGF receptor tyrosine kinase regulates PLC-gamma 1 activity in native meningioma tissue.