Phospholipase C-gamma1: regulation of enzyme function and role in growth factor-dependent signal transduction

Downstream branches of receptor tyrosine kinase signaling act interdependently to shape the face

Background –

Previously we found that increasing fibroblast growth factor (FGF) signaling in the neural crest cells within the frontonasal process (FNP) of the chicken embryo caused dysmorphology that was correlated with reduced proliferation, disrupted cellular orientation, and lower MAPK activation but no change in PLCy and PI3K activation. This suggests RTK signaling may drive craniofacial morphogenesis through specific downstream effectors that affect cellular activities. In this study we inhibited three downstream branches of RTK signaling to determine their role in regulating cellular activities and how these changes affect morphogenesis of the FNP.

Results –

Small molecule inhibitors of MEK1/2, PI3K, and PLCy were delivered individually and in tandem to the right FNP of chicken embryos. All treatments caused asymmetric proximodistal truncation on the treated side and a mild expansion on the untreated side compared to DMSO control treated FNPs. Inhibiting each pathway caused similar decreased proliferation and disrupted cellular orientation, but did not affect apoptosis.

Conclusions –

Since RTK signaling is a ubiquitous and tightly regulated biochemical system we conclude that the downstream pathways are robust to developmental perturbation through redundant signaling systems.

The ErbB Signaling Network and Its Potential Role in Endometrial Cancer

Endometrial cancer (EC) is the second most common malignancy of the female reproductive system worldwide. The updated EC classification emphasizes the significant role of various signaling pathways such as PIK3CA-PIK3R1-PTEN and RTK/RAS/β-catenin in EC pathogenesis. Some of these pathways are part of the EGF system signaling network, which becomes hyperactivated by various mechanisms and participates in cancer pathogenesis. In EC, the expression of ErbB receptors is significantly different, compared with the premenopausal and postmenopausal endometrium, mainly because of the increased transcriptional activity of ErbB encoding genes in EC cells. Moreover, there are some differences in ErbB-2 receptor profile among EC subgroups that could be explained by the alterations in pathophysiology and clinical behavior of various EC histologic subtypes. The fact that ErbB-2 receptor expression is more common in aggressive EC histologic subtypes (papillary serous and clear cell) could indicate a future role of ErbB-targeted therapies in well-defined EC subgroups with overexpression of ErbB receptors.

Recent advances in understanding the molecular role of phosphoinositide-specific phospholipase C gamma 1 as an emerging onco-driver and novel therapeutic target in human carcinogenesis

Phosphoinositide metabolism is crucial intracellular signaling system that regulates a plethora of biological functions including mitogenesis, cell proliferation and division. Phospholipase C gamma 1 (PLCγ1) which belongs to phosphoinositide-specific phospholipase C (PLC) family, is activated by many extracellular stimuli including hormones, neurotransmitters, growth factors and modulates several cellular and physiological functions necessary for tumorigenesis such as cell survival, migration, invasion and angiogenesis by generating inositol 1,4,5-triphosphate (IP3) and diacylglycerol (DAG) via hydrolysis of phosphatidylinositol 4,5-biphosphate (PIP2). Cancer remains as a leading cause of global mortality and aberrant expression and regulation of PLCγ1 is linked to a plethora of deadly human cancers including carcinomas of the breast, lung, pancreas, stomach, prostate and ovary. Although PLCγ1 cross-talks with many onco-drivers and signaling circuits including PI3K, AKT, HIF1-α and RAF/MEK/ERK cascade, its precise role in carcinogenesis is not completely understood. This review comprehensively discussed the status quo of this ubiquitously expressed phospholipase as a tumor driver and highlighted its significance as a novel therapeutic target in cancer. Furthermore, we have highlighted the significance of somatic driver mutations in PLCG1 gene and molecular roles of PLCγ1 in several major human cancers, a knowledgebase that can be utilized to develop novel, isoform-specific small molecule inhibitors of PLCγ1.

Phospholipase Signaling in Breast Cancer

Breast cancer progression results from subversion of multiple intra- or intercellular signaling pathways in normal mammary tissues and their microenvironment, which have an impact on cell differentiation, proliferation, migration, and angiogenesis. Phospholipases (PLC, PLD and PLA) are essential mediators of intra- and intercellular signaling. They hydrolyze phospholipids, which are major components of cell membrane that can generate many bioactive lipid mediators, such as diacylglycerol, phosphatidic acid, lysophosphatidic acid, and arachidonic acid. Enzymatic processing of phospholipids by phospholipases converts these molecules into lipid mediators that regulate multiple cellular processes, which in turn can promote breast cancer progression. Thus, dysregulation of phospholipases contributes to a number of human diseases, including cancer. This review describes how phospholipases regulate multiple cancer-associated cellular processes, and the interplay among different phospholipases in breast cancer. A thorough understanding of the breast cancer-associated signaling networks of phospholipases is necessary to determine whether these enzymes are potential targets for innovative therapeutic strategies.

Vascular smooth muscle cell proliferation as a therapeutic target. Part 1: molecular targets and pathways

Cardiovascular diseases are a major cause of human death worldwide. Excessive proliferation of vascular smooth muscle cells contributes to the etiology of such diseases, including atherosclerosis, restenosis, and pulmonary hypertension. The control of vascular cell proliferation is complex and encompasses interactions of many regulatory molecules and signaling pathways. Herein, we recapitulated the importance of signaling cascades relevant for the regulation of vascular cell proliferation. Detailed understanding of the mechanism underlying this process is essential for the identification of new lead compounds (e.g., natural products) for vascular therapies.

Molecular characterization and expression analysis of olive flounder (Paralichthys olivaceus) phospholipase C gamma 1 and gamma 2

Phospholipase C gamma 1 and gamma 2 (PLCG1 and PLCG2) are influential in modulating Ca²⁺ and diacylglycerol, second messengers involved in tyrosine kinase-dependent signaling, including growth factor activation. Here, we used RACE (rapid amplification of cDNA ends) to clone cDNA encoding PLCG1 (PoPLCG1) and PLCG2 (PoPLCG2) in the olive flounder (Paralichthys olivaceus). The respective 1313 and 1249 amino acid sequences share high identity with human PLCG1 and PLCG2, and contain the following domains: pleckstrin homology (PH), EF-hand, catalytic X and Y, Src homology 2 (SH2), Src homology 3 (SH3), and C2. Phylogenic analysis and sequence comparison of PoPLCG1 and PoPLCG2 with other PLC isozymes showed a close relationship between the two PLCGs, supported by structural analysis. In addition, tissue expression analysis showed that PoPLCG1 was expressed predominantly in the brain, eye, and heart, whereas PoPLCG2 was expressed principally in gills, esophagus, spleen, and kidney. Following stimulation with LPS and Poly I:C, PoPLCG expression was compared with the expression of inflammatory cytokines IL-1β, IL-6, and TNF-α via reverse transcription-PCR and real-time quantitative PCR. Our results suggest that PoPLCG isozymes perform a critical immune function in olive flounder, being active in pathogen resistance and the inflammation process.

ErbB Receptors and Cancer

The ErbB receptor family, also known as the EGF receptor family or type I receptor family, includes the epidermal growth factor (EGF) receptor (EGFR) or ErbB1/Her1, ErbB2/Her2, ErbB3/Her3, and ErbB4/Her4. Among all RTKs, EGFR was the first RTK identified and the first one linked to cancer. Thus, EGFR has also been the most intensively studied among all RTKs. ErbB receptors are activated after homodimerization or heterodimerization. The ErbB family is unique among the various groups of receptor tyrosine kinases (RTKs) in that ErbB3 has impaired kinase activity, while ErbB2 does not have a direct ligand. Therefore, heterodimerization is an important mechanism that allows the activation of all ErbB receptors in response to ligand stimulation. The activated ErbB receptors bind to many signaling proteins and stimulate the activation of many signaling pathways. The specificity and potency of intracellular signaling pathways are determined by positive and negative regulators, the specific composition of activating ligand(s), receptor dimer components, and the diverse range of proteins that associate with the tyrosine phosphorylated C-terminal domain of the ErbB receptors. ErbB receptors are overexpressed or mutated in many cancers, especially in breast cancer, ovarian cancer, and non-small cell lung cancer. The overexpression and overactivation of ErbB receptors are correlated with poor prognosis, drug resistance, cancer metastasis, and lower survival rate. ErbB receptors, especially EGFR and ErbB2 have been the primary choices as targets for developing cancer therapies.

Phosphoinositide-specific phospholipase C in health and disease

Phospholipases are widely occurring and can be found in several different organisms, including bacteria, yeast, plants, animals, and viruses. Phospholipase C (PLC) is a class of phospholipases that cleaves phospholipids on the diacylglycerol (DAG) side of the phosphodiester bond producing DAGs and phosphomonoesters. Among PLCs, phosphoinositide-specific PLC (PI-PLC) constitutes an important step in the inositide signaling pathways. The structures of PI-PLC isozymes show conserved domains as well as regulatory specific domains. This is important, as most PI-PLCs share a common mechanism, but each of them has a peculiar role and can have a specific cell distribution that is linked to a specific function. More importantly, the regulation of PLC isozymes is fundamental in health and disease, as there are several PLC-dependent molecular mechanisms that are associated with the activation or inhibition of important physiopathological processes. Moreover, PI-PLC alternative splicing variants can play important roles in complex signaling networks, not only in cancer but also in other diseases. That is why PI-PLC isozymes are now considered as important molecules that are essential for better understanding the molecular mechanisms underlying both physiology and pathogenesis, and are also potential molecular targets useful for the development of innovative therapeutic strategies.

Effect of acute acid-base disturbances on the phosphorylation of phospholipase C-γ1 and Erk1/2 in the renal proximal tubule

The renal proximal tubule (PT) plays a major role in whole-body pH homeostasis by secreting H(+) into the tubule lumen. Previous work demonstrated that PTs respond to basolateral changes in [CO2] and [HCO3-] by appropriately altering H(+) secretion-responses blocked by the ErbB inhibitor PD168393, or by eliminating signaling through AT1 angiotensin receptors. In the present study, we analyze phosphorylation of three downstream targets of both ErbBs and AT1: phospholipase C-γ1 (PLC-γ1), extracellular-regulated kinase 1 (Erk1), and Erk2. We expose rabbit PT suspensions for 5 and 20 min to our control (Ctrl) condition (5% CO2, 22 mmol/L HCO3-, pH 7.40) or one of several conditions that mimic acid-base disturbances. We found that each disturbance produces characteristic phosphorylation patterns in the three enzymes. For example, respiratory acidosis (elevated [CO2], normal [HCO3-]) at 20 min decreases PLC-γ1 phosphorylation at tyrosine-783 (relative to Ctrl). Metabolic acidosis (normal [CO2], decreased [HCO3-]) for 5 min increases Erk1 phosphorylation (p-Erk1) but not p-Erk2, whereas metabolic alkalosis (normal [CO2], elevated [HCO3-]) for 5 min decreases p-Erk1 and p-Erk2. In the presence of CO2/HCO3-, PD168393 blocks only two of eight induced decreases in phosphorylation. In two cases in which disturbances have no remarkable effects on phosphorylation, PD168393 unmasks decreases and in two others, increases. These drug effects provide insight into the roles of PD168393-sensitive kinases. Our results indicate that PLC-γ1.pY783, p-Erk1, and p-Erk2 in the PT change in characteristic ways in response to acute acid-base disturbances, and thus presumably contribute to the transduction of acid-base signals.

Increases in intracellular calcium via activation of potentially multiple phospholipase C isozymes in mouse olfactory neurons

Phospholipase C (PLC) and internal Ca(2+) stores are involved in a variety of cellular functions. However, our understanding of PLC in mammalian olfactory sensory neurons (OSNs) is generally limited to its controversial role in odor transduction. Here we employed single-cell Ca(2+) imaging and molecular approaches to investigate PLC-mediated Ca(2+) responses and its isozyme gene transcript expression. We found that the pan-PLC activator m-3M3FBS (25 μM) induces intracellular Ca(2+) increases in vast majority of isolated mouse OSNs tested. Both the response amplitude and percent responding cells depend on m-3M3FBS concentrations. In contrast, the inactive analog o-3M3FBS fails to induce Ca(2+) responses. The m-3M3FBS-induced Ca(2+) increase is blocked by the PLC inhibitor U73122, while its inactive analog U73433 has no effect. Removal of extracellular Ca(2+) does not change significantly the m-3M3FBS-induced Ca(2+) response amplitude. Additionally, in the absence of external Ca(2+), we found that a subset of OSNs respond to an odorant mixture with small Ca(2+) increases, which are significantly suppressed by U73122. Furthermore, using reverse transcription polymerase chain reaction and real-time quantitative polymerase chain reaction, we found that multiple PLC isozyme gene transcripts are expressed in olfactory turbinate tissue in various levels. Using RNA in situ hybridization analysis, we further show expression of β4, γ1, γ2 gene transcripts in OSNs. Taken together, our results establish that PLC isozymes are potent enzymes for mobilizing intracellular Ca(2+) in mouse OSNs and provide molecular insight for PLC isozymes-mediated complex cell signaling and regulation in the peripheral olfactory epithelium.

A review of platelet derived growth factor playing pivotal role in bone regeneration

This article is focused on the literature review and study of recent advances in the field of bone grafting, which involves platelet-derived growth factor (PDGF) as one of the facilitating factors in bone regeneration. This article includes a description of the mechanism of PDGF for use in surgeries where bone grafting is required, which promotes future application of PDGF for faster bone regeneration or inhibition of bone growth if required as in osteosarcoma. The important specific activities of PDGF include mitogenesis (increase in the cell populations of healing cells), angiogenesis (endothelial mitoses into functioning capillaries), and macrophage activation (debridement of the wound site and a second phase source of growth factors for continued repair and bone regeneration). Thus PDGF can be utilized in wound with bone defect to conceal the wound with repair of bony defect.

Ca2+-independent binding of anionic phospholipids by phospholipase C δ1 EF-hand domain

Recombinant EF-hand domain of phospholipase C δ1 has a moderate affinity for anionic phospholipids in the absence of Ca(2+) that is driven by interactions of cationic and hydrophobic residues in the first EF-hand sequence. This region of PLC δ1 is missing in the crystal structure. The relative orientation of recombinant EF with respect to the bilayer, established with NMR methods, shows that the N-terminal helix of EF-1 is close to the membrane interface. Specific mutations of EF-1 residues in full-length PLC δ1 reduce enzyme activity but not because of disturbing partitioning of the protein onto vesicles. The reduction in enzymatic activity coupled with vesicle binding studies are consistent with a role for this domain in aiding substrate binding in the active site once the protein is transiently anchored at its target membrane.

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.

Integrative analysis of cancer-related signaling pathways

Identification and classification of cancer types and subtypes is a major issue in current cancer research. Whole genome expression profiling of cancer tissues is often the basis for such subtype classifications of tumors and different signatures for individual cancer types have been described. However, the search for best performing discriminatory gene-expression signatures covering more than one cancer type remains a relevant topic in cancer research as such a signature would help understanding the common changes in signaling networks in these disease types. In this work, we explore the idea of a top down approach for sample stratification based on a module-based network of cancer relevant signaling pathways. For assembly of this network, we consider several of the most established cancer pathways. We evaluate our sample stratification approach using expression data of human breast and ovarian cancer signatures. We show that our approach performs equally well to previously reported methods besides providing the advantage to classify different cancer types. Furthermore, it allows to identify common changes in network module activity of those cancer samples.

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.

bFGF and PDGF-BB have a synergistic effect on the proliferation, migration and VEGF release of endothelial progenitor cells

We have investigated the synergistic effects of bFGF (basic fibroblast growth factor) and PDGF-BB (platelet-derived growth factor-BB) on the proliferation, migration and VEGF (vascular endothelial growth factor) release of EPCs (endothelial progenitor cells). The proliferation of EPCs was assayed by MTS [3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium]. EPCs migration was detected using the Transwell system. Real-time PCR was used to assess the transcription of PDGFRβ mRNA. PLC-γ (phospholipase C gamma) expression and VEGF release were analysed by Western blot and ELISA. bFGF and PDGF-BB could, respectively, or synergistically, promote the proliferation and migration of EPCs, and these effects of bFGF and PDGF-BB were implemented by enhancing PDGFRβ mRNA, PLC-γ and VEGF expression, while inhibitor of PDGF receptor kinase (AG1296) and the selective PLC inhibitor (U73122) could block these effects of bFGF and PDGF-BB. In the meantime, we proved that the amplification by bFGF and PDGF-BB-stimulated PDGFRβ mRNA, PLC-γ and VEGF expression was abrogated by anti-bFGF antibody, AG1296 and U73122. These results strongly suggest that the proliferation and migration of EPCs may depend on bFGF and/or PDGF-BB by PDGFRβ/PLC-γ signalling pathway, and bFGF and/or PDGF-BB stimulate VEGF release at a point downstream from PDGFRβ/PLC-γ in EPCs.

Roles of Phospholipase C Isozymes in Organogenesis and Embryonic Development

Phosphoinositide metabolism is an important intracellular signaling system that regulates a variety of cellular functions. Phospholipase C (PLC) is a key enzyme in this system. Recent studies on genetically manipulated mice have clarified the functions of PLC in vivo. This review focuses on the roles of PLC in organogenesis and embryonic development.

Receptor tyrosine kinases and respiratory motor plasticity

Protein kinases are a family of enzymes that transfer a phosphate group from adenosine tri-phosphate to an amino acid residue on a protein. The receptor tyrosine kinases (RTKs) are expressed on the outer cell membrane, bind extracellular protein ligands, and phosphorylate tyrosine residues on other proteins-essentially permitting communication between cells. Such activity regulates multiple aspects of cellular physiology including cell growth and differentiation, adhesion, motility, cell death, and morphological and synaptic plasticity. This review will focus on the role of RTKs in respiratory motor plasticity, with particular emphasis on long-term changes in respiratory motoneuron function. Reflecting the predominant literature, specific attention will be devoted to the role of tropomyosin-related kinase type B (TrkB) activation on phrenic motoneuron activity. However, many RTKs share similar patterns of expression and mechanisms of ligand-induced activation and downstream signaling. Thus, a perspective based on TrkB-induced phrenic motor plasticity may provide insight into the potential roles of other RTKs in the neural control of breathing. Finally, understanding how different RTKs affect respiratory motor output in the long-term may provide future avenues for pharmacological development with the goal of increasing respiratory motor output in disorders such as obstructive sleep apnea and after spinal cord injury. This is best illustrated in recent studies where we have used small, highly diffusible molecules to transactivate TrkB receptors near phrenic motoneurons to improve breathing after cervical spinal cord injury.

PLC-gamma1 and Rac1 coregulate EGF-induced cytoskeleton remodeling and cell migration

It is well established that epidermal growth factor (EGF) induces the cytoskeleton reorganization and cell migration through two major signaling cascades: phospholipase C-gamma1 (PLC-gamma1) and Rho GTPases. However, little is known about the cross talk between PLC-gamma1 and Rho GTPases. Here we showed that PLC-gamma1 forms a complex with Rac1 in response to EGF. This interaction is direct and mediated by PLC-gamma1 Src homology 3 (SH3) domain and Rac1 (106)PNTP(109) motif. This interaction is critical for EGF-induced Rac1 activation in vivo, and PLC-gamma1 SH3 domain is actually a potent and specific Rac1 guanine nucleotide exchange factor in vitro. We have also demonstrated that the interaction between PLC-gamma1 SH3 domain and Rac1 play a significant role in EGF-induced F-actin formation and cell migration. We conclude that PLC-gamma1 and Rac1 coregulate EGF-induced cell cytoskeleton remodeling and cell migration by a direct functional interaction.

Intramolecular regulation of phospholipase C-gamma1 by its C-terminal Src homology 2 domain

Phosphoinositide-specific phospholipase C-gamma1 (PLC-gamma1) is a key enzyme that governs cellular functions such as gene transcription, secretion, proliferation, motility, and development. Here, we show that PLC-gamma1 is regulated via a novel autoinhibitory mechanism involving its carboxy-terminal Src homology (SH2C) domain. Mutation of the SH2C domain tyrosine binding site led to constitutive PLC-gamma1 activation. The amino-terminal split pleckstrin homology (sPHN) domain was found to regulate the accessibility of the SH2C domain. PLC-gamma1 constructs with mutations in tyrosine 509 and phenylalanine 510 in the sPHN domain no longer required an intact amino-terminal Src homology (SH2N) domain or phosphorylation of tyrosine 775 or 783 for activation. These data are consistent with a model in which the SH2C domain is blocked by an intramolecular interaction(s) that is released upon cellular activation by occupancy of the SH2N domain.

Calcium signaling in the nucleus

Cytosolic Ca(2+) is a versatile secondary messenger that regulates a wide range of cellular activities. In the past decade, evidence has accumulated that free Ca(2+) within the nucleus also plays an important messenger function. Here we review the mechanisms and effects of Ca(2+) signals within the nucleus. In particular, evidence is reviewed that the nucleus contains the machinery necessary for production of inositol 1,4,5-trisphosphate and for inositol 1,4,5-trisphosphate receptor-mediated Ca(2+) release. The role of Ca(2+) signals within the nucleus is discussed including regulation of such critical cell functions as gene expression, activation of kinases, and permeability of nuclear pores.

CAIR-1/BAG-3 modulates cell adhesion and migration by downregulating activity of focal adhesion proteins

CAIR-1/BAG-3 is a stress and survival protein that has been shown to bind SH3 domain-containing proteins through its proline-rich (PXXP) domain. Because stress and survival pathways are active during invasion and metastasis, we hypothesized that CAIR-1 is a regulator of signaling pathways that modulate cell adhesion and migration. MDA-435 human breast carcinoma cells were stably transfected with full-length CAIR-1 (FL) or a proline-rich domain deleted mutant (dPXXP). FL cells migrated poorly through collagen IV-coated filters to serum (14% of control, p=0.0004), whereas migration of dPXXP cells was more robust (228%, p=0.00001). Adhesion to collagen IV-coated surfaces was reduced in FL cells and augmented in dPXXP cells (FL 64%, p=0.03; dPXXP 138%, p=0.01). Rhodamine-phalloidin staining highlighted more stress fibers and thicker filopodial protrusions in dPXXP cells. Fewer focal adhesions were also seen in FL cells. A reduction in tyrosine phosphorylation of focal adhesion kinase (FAK) and paxillin occurred in FL cells under these conditions. In contrast, increased FAK and paxillin phosphorylation was documented in dPXXP cells. Differential FAK phosphorylation occurred at the major autophosphorylation site Y(397) and Src phosphorylation site Y(861). Concordant with these findings, there was decreased interaction between FAK and its downstream partners p(130)Cas and Crk observed in FL cells but not in dPXXP cells. These results collectively indicate that CAIR-1 may negatively regulate adhesion, focal adhesion assembly, signaling, and migration via its PXXP domain.

Akt binds to and phosphorylates phospholipase C-gamma1 in response to epidermal growth factor

Both phospholipase (PL) C-gamma1 and Akt (protein kinase B; PKB) are signaling proteins that play significant roles in the intracellular signaling mechanism used by receptor tyrosine kinases, including epidermal growth factor (EGF) receptor (EGFR). EGFR activates PLC-gamma1 directly and activates Akt indirectly through phosphatidylinositol 3-kinase (PI3K). Many studies have shown that the PLC-gamma1 pathway and PI3K-Akt pathway interact with each other. However, it is not known whether PLC-gamma1 binds to Akt directly. In this communication, we identified a novel interaction between PLC-gamma1 and Akt. We demonstrated that the interaction is mediated by the binding of PLC-gamma1 Src homology (SH) 3 domain to Akt proline-rich motifs. We also provide a novel model to depict how the interaction between PLC-gamma1 SH3 domain and Akt proline-rich motifs is dependent on EGF stimulation. In this model, phosphorylation of PLC-gamma1 Y783 by EGF causes the conformational change of PLC-gamma1 to allow the interaction of its SH3 domain with Akt proline-rich motifs. Furthermore, we showed that the interaction between PLC-gamma1 and Akt resulted in the phosphorylation of PLC-gamma1 S1248 by Akt. Finally, we showed that the interaction between PLC-gamma1 and Akt enhanced EGF-stimulated cell motility.

Re-distribution of phospholipase C gamma 2 in macrophage precursors is mediated by the actin cytoskeleton under the control of the Src kinases

Macrophage colony-stimulating factor (M-CSF) is a growth factor that is known to trigger several signalling pathways through receptor tyrosine kinase activation. We investigated the specific requirements for the activation of phospholipase C gamma 2 (PLC-gamma2) during the differentiation of mouse bone marrow-derived macrophage precursors. M-CSF stimulation induced rapid PLC-gamma2 translocation and phosphorylation from the cytosolic compartment to the cell periphery. Both events were dependent on cytoskeleton integrity and Src kinase activity, but only PLC-gamma2 phosphorylation did not require PI3-kinase activity. Biochemical experiments as well as confocal microscopy analyses indicate that the translocation of PLC-gamma2 is mediated by the direct association of this protein with the actin cytoskeleton. Using GST-fusion proteins containing various deletions of the PLC-gamma2 Src homology region, it was found that PLC-gamma2 binds to F-actin via its SH2 domains, a feature that has equally been found in a co-sedimentation assay. This association, which is increased during actin reorganisation and disrupted by cytoskeleton inhibitors, seems to be a primary means to recruit this enzyme to the cell periphery. These results indicate that, upon M-CSF stimulation, PLC-gamma2 cellular localisation and phosphorylation are strongly dependent on cytoskeleton architecture of the macrophage precursor as well as the PI3-kinase and the Src kinases.

The phosphorylation of phospholipase C-gamma1, Raf-1, MEK, and ERK1/2 induced by a conserved retroviral peptide

A synthetic 17-amino acid peptide (CKS-17) homologous to a highly conserved region of human and animal retroviral transmembrane proteins has been found to exhibit suppressive properties for numerous immune functions. It has been shown that CKS-17 causes an imbalance of human types 1 and 2 cytokines and inhibition of the immune responses of lymphocytes, monocytes, and macrophages. CKS-17 induced increased intracellular levels of cAMP, which plays an important role in regulation of cytokine biosynthesis. In this study, using a Jurkat T-cell line and Western blot analysis, CKS-17 induced phosphorylation of PLC-gamma1, Raf-1, MEK and ERK1/2. Using a PLC selective inhibitor U73122 or PLC-gamma1-deficient Jurkat cell line, phosphorylation induced by CKS-17 of ERK1/2, PLC-gamma1, or Raf-1, respectively, were undetectable or significantly reduced. Reintroduction of PLC-gamma1 into the PLC-gamma1-deficient Jurkat cells restored the phosphorylation of ERK1/2 and PLC-gamma1 induced by CKS-17. Further, pretreatment of Jurkat cells with PKC inhibitors blocks the phosphorylation of Raf-1, MEK, and ERK1/2 induced by CKS-17. These results indicate that CKS-17 induces the PLC-gamma1-PKC-Raf-1-MEK-ERK1/2 signaling pathway.

Mesangial cell-reduced Ca2+signaling in high glucose is due to inactivation of phospholipase C-β3by protein kinase C

In high glucose, glomerular mesangial cells (MCs) demonstrate impaired Ca2+signaling in response to seven-transmembrane receptor stimulation. To identify the mechanism, we first postulated decreased release from intracellular stores. Intracellular Ca2+was measured in fluo-3-loaded primary cultured rat MCs using confocal fluorescence microscopy. In high glucose (HG) 30 mM for 48 h, the 25 nM ionomycin-stimulated intracellular Ca2+response was reduced to 82% of that observed in normal glucose (NG). In NG 5.6 mM, Ca2+responses to endothelin (ET)-1 and platelet-derived growth factor (PDGF) were unchanged in cells cultured in 50 nM Ca2+vs. 1.8 mM Ca2+. Depletion of intracellular Ca2+stores with thapsigargin eliminated ET-1-stimulated Ca2+responses. Incubation in 30 mM glucose (HG) for 48 h or stimulation with phorbol myristate acetate (PMA) for 10 min eliminated the Ca2+response to ET-1 but had no effect on the PDGF response. Downregulation of protein kinase C (PKC) with 24-h PMA or inhibition with Gö6976 in HG normalized the Ca2+response to ET-1. Because ET-1 and PDGF stimulate Ca2+signaling through different phospholipase C pathways, we hypothesized that, in HG, PKC selectively phosphorylates and inhibits PLC-β3. Using confocal immunofluorescence imaging, in NG, a 1.6- to 1.7-fold increase in PLC-β3Ser1105phosphorylation was observed following PMA or ET-1 stimulation for 10 min. In HG, immunofluorescent imaging and immunoblotting showed increased PLC-β3phosphorylation, without change in total PLC-β3, which was reversed with 24-h PMA or Gö6976. We conclude that reduced Ca2+signaling in HG cannot be explained by reduced Ca2+stores but is due to conventional PKC-dependent phosphorylation and inactivation of PLC-β3.

Cell motility assays on tissue culture dishes via non-invasive confinement and release of cells

In vitro cell migration assays are useful for screening bioactive agents that regulate angiogenesis, tumor metastasis, would healing, and immune responses by effecting changes in the rate of cell migration. Here we have developed a noninvasive in vitro migration assay that operates through release of confluent groups of cells initially confined within patterns of cell-resistant polyelectrolyte. Cell-resistant patterns of polyelectrolyte, separating groups of confluent cells, are rendered cell adhesive by adsorption of a second, cell adhesive polyelectrolyte of opposite charge; thereby, resulting in migration of cells into the separating regions. By dynamically controlling cell-surface interactions through self-assembly of cell-adhesive and cell resistant polyelectrolytes, this method eliminates the need to mechanically wound cells, as is done in current cell migration assays. The utility of this technique in identifying molecules and mechanisms that regulate cell migration is demonstrated by its application as an assay for the effects of platelet derived growth factors, cytoskeleton disrupting agents, and Merlin overexpression, on the migration of NIH 3T3 fibroblasts.

Molecular cloning and characterization of a novel phospholipase C, PLC-eta

PLC (phospholipase C) plays an important role in intracellular signal transduction by hydrolysing phosphatidylinositol 4,5-bisphosphate, a membrane phospholipid. To date, 12 members of the mammalian PLC isoforms have been identified and classified into five isotypes beta, gamma, delta, epsilon and zeta, which are regulated by distinct mechanisms. In the present study, we describe the identification of a novel PLC isoform in the brains of human and mouse, named PLC-eta, which contains the conserved pleckstrin homology domain, X and Y domains for catalytic activity and the C2 domain. The first identified gene encoded 1002 (human) or 1003 (mouse) amino acids with an estimated molecular mass of 115 kDa. The purified recombinant PLC-eta exhibited Ca2+-dependent catalytic activity on phosphatidylinositol 4,5-bisphosphate. Furthermore, molecular biological analysis revealed that the PLC-eta gene was transcribed to several splicing variants. Although some transcripts were detected in most of the tissues we examined, the transcript encoding 115 kDa was restricted to the brain and lung. In addition, the expression of the 115 kDa protein was defined in only nerve tissues such as the brain and spinal cord. In situ hybridization analysis with brain revealed that PLC-eta was abundantly expressed in various regions including cerebral cortex, hippocampus, zona incerta and cerebellar Purkinje cell layer, which are neuronal cell-enriched regions. These results suggest that PLC-eta may perform fundamental roles in the brain.

A New Tyrosine Phosphorylation Site in PLCγ1: The Role of Tyrosine 775 in Immune Receptor Signaling

Phospholipase Cgamma (PLCgamma) is a ubiquitous gatekeeper of calcium mobilization and diacylglycerol-mediated events induced by the activation of Ag and growth factor receptors. The activity of PLCgamma is regulated through its controlled membrane translocation and tyrosine (Y) phosphorylation. Four activation-induced tyrosine phosphorylation sites have been previously described (Y472, Y771, Y783, and Y1254), but their specific roles in Ag receptor-induced PLCgamma1 activation are not fully elucidated. Unexpectedly, we found that the phosphorylation of a PLCgamma1 construct with all four sites mutated to phenylalanine was comparable with that observed with wild-type PLCgamma1, suggesting the existence of an unidentified site(s). Sequence alignment with known phosphorylation sites in PLCgamma2 indicated homology of PLCgamma1 tyrosine residue 775 (Y775) with PLCgamma2 Y753, a characterized phosphorylation site. Tyrosine 775 was characterized as a phosphorylation site using phospho-specific anti-Y775 antiserum, and by mutational analysis. Phosphorylation of Y775 did not depend on the other tyrosines, and point mutation of PLCgamma1 Y775, or the previously described Y783, substantially reduced AgR-induced calcium, NF-AT, and AP-1 activation. Mutation of Y472, Y771, and Y1254 had no effect on overall PLCgamma1 phosphorylation or activation. Although the concomitant mutation of Y775 and Y783 abolished downstream PLCgamma1 signaling, these two tyrosines were sufficient to reconstitute the wild-type response in the absence of functional Y472, Y771, and Y1254. These data establish Y775 as a critical phosphorylation site for PLCgamma1 activation and confirm the functional importance of Y783.

Single and repeated stress-induced modulation of phospholipase C catalytic activity and expression: role in LH behavior

PI-PLC, a critical enzyme of the phosphoinositide (PI) signaling pathway, mediates many physiological functions in the brain, including cellular plasticity. Stress-induced learned helplessness (LH) in animals serves as a model of behavioral depression. Recently, we observed that repeated stress prolongs the duration of LH behavior in rats, enabling us to compare neurobiologic abnormalities in acute and chronic depression. Here we examine whether LH behavior is associated with alterations in phospholipase C (PLC), and whether repetition of inescapable shock has similar or dissimilar effects on PLC to those of the single-stress paradigm. Rats were exposed to inescapable shock either once on day 1, or twice, on days 1 and 7. Rats were tested for escape latency on days 2 and 4 after day 1 inescapable shock or on days 2, 8, and 14 after day 1 and 7 inescapable shock. PI-PLC activity and mRNA and protein expression of three different PLC isozymes were determined in the frontal cortex and hippocampus. Higher escape latencies were observed in LH rats tested on day 2 after single inescapable shock and on day 14 after repeated inescapable shock. Single inescapable shock reduced PI-PLC activity in the frontal cortex and hippocampus of LH rats. On the other hand, repeated inescapable shock not only reduced PI-PLC activity in these brain areas of LH rats but also selectively decreased the expression of PLC beta1 and PLC gamma1 isozymes. Our results suggest different responsiveness at the level of PI-PLC after single vs repeated stress, and that reductions in PLC may be critical in the pathophysiology of depression and other stress-related disorders.

A NHERF binding site links the betaPDGFR to the cytoskeleton and regulates cell spreading and migration

The Na(+)/H(+) exchanger regulatory factor, NHERF, is a multifunctional adapter protein involved in a wide range of physiological activities. NHERF associates with merlin and the ezrin/radixin/moesin (MERM) family of membrane-actin cytoskeletal linker proteins through its C-terminus and is capable of interacting via its PDZ1 domain to the betaPDGF receptor (betaPDGFR). Thus, NHERF, potentially links the betaPDGFR to the actin cytoskeleton through its interaction with MERM proteins. In the present study, we have examined whether abolishing the interaction of betaPDGFR with NHERF results in actin cytoskeletal rearrangements. We have stably expressed a wild-type betaPDGFR, a mutant betaPDGFR (L1106A) that is incapable of interacting with NHERF, as well as a kinase defective mutant receptor (K634R), in PDGFR-deficient mouse embryonic fibroblasts. Our observations indicate that cells expressing betaPDGFR (L1106A) were impaired in their ability to spread and migrate on fibronectin compared with wild-type and K634R cells. L1106A mutant cells also revealed an increased number of focal adhesions, a condensed F-actin ring at the cell periphery and a decrease in total focal adhesion kinase (FAK) tyrosine phosphorylation. Further, we show that NHERF and MERM proteins could act as intermediary bridging proteins between betaPDGFR and FAK. Thus, the interaction of betaPDGFR with NHERF may provide an essential link between the cell membrane and the cortical actin cytoskeleton independent of receptor activity.

Human scavenger receptor class B type II (SR-BII) and cellular cholesterol efflux

Although studies in recombinant cells indicate that scavenger receptor class B, type I (SR-BI) can promote cholesterol efflux, investigations in transgenic mice overexpressing or deficient in SR-BI endorse its physiological function as selectively sequestering cholesteryl esters from high-density lipoproteins (HDLs). Less clear is the role of SR-BII, a splice variant of the SR-B gene that differs only in the C-terminal cytoplasmic domain. Here, we identify several putative signalling motifs in the C-terminus of human SR-BII, which are absent from SR-BI, and hypothesize that these motifs interact with signalling molecules to mobilize stored cholesteryl esters and/or promote the efflux of intracellular free cholesterol. 'Pull-down' assays using a panel of tagged SH3 (Src homology 3) domains showed that cytoplasmic SR-BII, but not cytoplasmic SR-BI, bound the SH3 domain of phospholipase C-gamma1; this interaction was not, however, detected under more physiological conditions. Specific anti-peptide antisera identified SR-BII in human monocyte/macrophage THP-1 cells and, in recombinant cells, revealed receptor localization to caveolae, a plasma membrane microdomain that concentrates signal-transducer molecules and acts as a conduit for cholesterol flux between cells and lipoproteins. Consistent with its caveolar localization, expression of human SR-BII in recombinant Chinese hamster ovary cells (CHO-SR-BII) was associated with increased HDL-mediated cholesterol efflux. Nevertheless, when CHO-SR-BII cells were pre-loaded with cholesteryl [(3)H]oleate and incubated with HDL, cholesteryl ester stores were not reduced compared with control cells. We conclude that although human SR-BII is expressed by macrophages, contains cytoplasmic signalling motifs and localizes to caveolae, its ability to stimulate cholesterol efflux does not reflect enhanced hydrolysis of stored cholesteryl esters.

Phospholipase Cgamma in distinct regions of the ventral tegmental area differentially modulates mood-related behaviors

Neurotrophic factor signaling pathways modulate cellular and behavioral responses to drugs of abuse. In addition, chronic exposure to morphine increases expression of phospholipase Cgamma1 (PLCgamma1) (a protein involved in neurotrophic signaling) in the ventral tegmental area (VTA), a neural substrate for many drugs of abuse. Using viral-mediated gene transfer to locally alter the activity of PLCgamma1, we show that overexpression of PLCgamma1 in rostral portions of the VTA (R-VTA) results in increased morphine place preference, whereas PLCgamma1 overexpression in the caudal VTA (C-VTA) results in avoidance of morphine-paired compartments. In addition, overexpression of PLCgamma1 in R-VTA causes increased preference for sucrose and increased anxiety-like behavior but does not affect responses to stress or nociceptive stimuli. In contrast, overexpression of PLCgamma1 in C-VTA decreases preference for sucrose and increases sensitivity to stress and nociceptive stimuli, although there was a tendency for increased anxiety-like behavior as seen for the R-VTA. These results show that levels of PLCgamma1 in the VTA regulate responsiveness to drugs of abuse, natural rewards, and aversive stimuli and point to the possibility that distinct topographical regions within the VTA mediate generally positive versus negative responses to emotional stimuli. Moreover, these data also support a role for drug-induced elevations in PLCgamma1 expression in the VTA in mediating long-term adaptations to drugs of abuse and aversive stimuli.

Phosphorylated inositol compounds in beta -cell stimulus-response coupling

Pancreatic beta-cell function is essential for the regulation of glucose homeostasis in humans, and its impairment leads to the development of type 2 diabetes. Inputs from glucose and cell surface receptors act together to initiate the beta-cell stimulus-response coupling that ultimately leads to the release of insulin. Phosphorylated inositol compounds have recently emerged as key players at all levels of the stimulus-secretion coupling process. In this current review, we seek to highlight recent advances in beta-cell phosphoinositide research by dividing our examination into two sections. The first involves the events that lead to insulin secretion. This includes both new roles for inositol polyphosphates, particularly inositol hexakisphosphate, and both conventional and 3-phosphorylated inositol lipids. In the second section, we deal with the more novel concept of the autocrine role of insulin. Here, released insulin initiates signal transduction cascades, principally through the activity of phosphatidylinositol 3-kinase. This new round of signal transduction has been established to activate key beta-cell genes, particularly the insulin gene itself. More controversially, this insulin feedback has also been suggested to either terminate or enhance insulin secretion events.

Activation of phospholipase Cgamma2 by tyrosine phosphorylation

Phospholipase Cgamma2 (PLCgamma2) has been implicated in collagen-induced signal transduction in platelets and antigen-dependent signaling in B-lymphocytes. It has been suggested that tyrosine kinases activate PLCgamma2. We expressed the full-length cDNA for human PLCgamma2 in bacteria and purified the recombinant enzyme. The recombinant enzyme was Ca(2+)-dependent with optimal activity in the range of 1 to 10 microM Ca(2+). In vitro phosphorylation experiments with recombinant PLCgamma2 and recombinant Lck, Fyn, and Lyn tyrosine kinases showed that phosphorylation of PLCgamma2 led to activation of the recombinant enzyme. Using site-directed mutagenesis, we investigated the role of specific tyrosine residues in activation of PLCgamma2. A mutant form of PLCgamma2, in which all three tyrosines at positions 743, 753, and 759 in the SH2-SH3 linker region were replaced by phenylalanines, exhibited decreased Lck-induced phosphorylation and completely abolished the Lck-dependent activation of PLCgamma2. Individual mutations of these tyrosine residues demonstrated that tyrosines 753 and 759, but not 743, were responsible for Lck-induced activation of PLCgamma2. To confirm these results, we procured a phosphospecific antibody to a peptide containing phosphorylated tyrosines corresponding to residues 753 and 759. This antibody recognized phosphorylated wild-type PLCgamma2 on Western blots but did not interact with unphosphorylated PLCgamma2 or with PLCgamma2 containing mutated tyrosine residues at 753 and 759. Using this antibody, we showed in intact platelets that collagen, a PLCgamma2-dependent agonist, induces phosphorylation of PLCgamma2 at Y753 and Y759. These studies demonstrate the importance of these two tyrosine residues in regulating the activity of PLCgamma2.

Epidermal growth factor-mediated activation of the ETS domain transcription factor Elk-1 requires nuclear calcium

Cytosolic and nuclear Ca(2+) have been shown to differentially regulate transcription. However, the impact of spatially distinct Ca(2+) signals on mitogen-activated protein kinase-mediated gene expression remains unknown. Here we investigated the role of nuclear and cytosolic Ca(2+) signals in epidermal growth factor (EGF)-induced transactivation of the ternary complex factor Elk-1 using a GAL4-Elk-1 construct. EGF increased Ca(2+) in both the nucleus and cytosol of HepG2 or 293 cells. Pretreatment with the intracellular Ca(2+) chelator bis(2-aminophenyl)ethyleneglycol-N,N,N',N'-tetraacetic acid significantly reduced EGF-induced transactivation of Elk-1, indicating that EGF-stimulated Elk-1 transcriptional activity is dependent on intracellular Ca(2+). To determine the relative contribution of nuclear and cytosolic Ca(2+) signals during EGF-mediated Elk-1 transactivation, Ca(2+) signals in either compartment were selectively impaired by targeted expression of the Ca(2+)-binding protein parvalbumin to either the nucleus or cytosol. Suppression of nuclear but not cytosolic Ca(2+) signals inhibited EGF-induced transactivation of Elk-1. However, suppression of nuclear Ca(2+) signals did not affect the ability of ERK either to become phosphorylated or to undergo translocation to the nucleus in response to EGF. Elk-1 phosphorylation and nuclear localization following EGF stimulation were also unaffected by suppressing nuclear Ca(2+) signals. These results suggest that nuclear Ca(2+) is required for EGF-mediated transcriptional activation of Elk-1 and that phosphorylation of Elk-1 alone is not sufficient to induce its transcriptional activation in response to EGF. Thus, subcellular targeting of parvalbumin reveals a distinct role for nuclear Ca(2+) signals in mitogen-activated protein kinase-mediated gene transcription.

Epidermal growth factor stimulates serine/threonine phosphorylation of the focal adhesion protein paxillin in a MEK-dependent manner in normal rat kidney cells

Epidermal growth factor (EGF)-stimulated proliferation of renal epithelial cells plays an important role in the recovery of kidney tubule epithelia following exposure to insult. Numerous studies have demonstrated that tyrosine phosphorylation of the focal adhesion protein paxillin mediates in part the effects of growth factors on cell growth, migration, and organization of the actin-based cytoskeleton. The experiments in this report were designed to determine the effect of EGF on paxillin phosphorylation in normal rat kidney (NRK) epithelial cells. Interestingly, treatment of NRK cells with EGF stimulated paxillin serine/threonine phosphorylation, which caused a reduction in the mobility of paxillin on sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). The EGF-stimulated mobility shift of paxillin was independent of an intact cytoskeleton, phosphatidylinositol 3-kinase (PI 3-kinase) activation, protein kinase C (PKC) activation, and cellular adhesion. However, inhibitors of the mitogen-activated protein kinase/extracellular signal-regulated kinase kinase abrogated the EGF-stimulated change in paxillin mobility. In addition, the EGF-stimulated change in paxillin serine/threonine phosphorylation was not accompanied by a profound reorganization of the actin cytoskeleton. These results identify paxillin as a component EGF signaling in renal epithelial cells and implicate members of the MAP kinase pathway as critical regulators of paxillin serine/threonine phosphorylation.

PLC-gamma1 enzyme activity is required for insulin-induced DNA synthesis

Previously, we had shown that inhibition of PLC activity impaired the ability of insulin to activate ERK in 3T3-L1 adipocytes. In this study, we confirmed that the insulin receptor and PLC-gamma1 are physically associated in hIRcB fibroblasts, insulin stimulates PLC-gamma1 enzyme activity, and inhibition of PLC activity impairs activation of ERK. We subsequently investigated whether PLC-gamma1 is required for insulin-stimulated mitogenesis. First, inhibition of PLC activity using U73122 impairs the ability of insulin to stimulate DNA synthesis. Second, disruption of the interaction of the insulin receptor with PLC-gamma1 by microinjection of SH2 domains derived from PLC-gamma1 or Grb2 but not Shc similarly blocks insulin-induced DNA synthesis. Third, microinjection of neutralizing antibodies to PLC-gamma1 blocks DNA synthesis, but nonneutralizing antibodies do not. The blockade in all three cases is rescued by synthetic diacylglycerols but not by inositol-1,4,5-trisphosphate, indicating a requirement for PLC enzyme activity. These experimental data point to a requirement for PLC-gamma1 in insulin-stimulated mitogenesis in hIRcB cells.

Cancer cell motility--on the road from c-erbB-2 receptor steered signaling to actin reorganization

Cell migration depends mainly on actin polymerization and intracellular organization, which are influenced by a vast variety of actin binding proteins (ABPs). Regulation of ABP activity is mediated by second messengers such as phosphoinositides and calcium. Signaling via these second messengers is initiated and regulated by membrane receptors, e.g., receptor tyrosine kinases (RTKs), and by adhesion molecule interactions (e.g., integrins and selectins) and focal adhesion kinases. A major role in steering second-messenger signaling and thus in actin cytoskeleton reorganization and motility of cancer cells is played by the RTK c-erbB-2. This occurs through a number of signaling pathways which involve mainly enzymes, e.g., phospholipase Cgamma1 and GTPases, which modify signaling molecules. Furthermore large multiprotein complexes including actin-related protein 2/3, Wiskott-Aldrich syndrome protein, profilin, and capping protein among others play an important role in regulating actin reorganization. The complex picture of the mode of actin reorganization, which is involved in tumor cell migration, is slowly emerging from the mists of cellular signaling pathways, but this is still by no means a clear view.

Four tyrosine residues in phospholipase C-gamma 2, identified as Btk-dependent phosphorylation sites, are required for B cell antigen receptor-coupled calcium signaling

Activation of phospholipase C-gamma2 (PLCgamma2) is the critical step in B cell antigen receptor (BCR)-coupled calcium signaling. Although genetic dissection experiments on B cells have demonstrated that Bruton's tyrosine kinase (Btk) and Syk are required for activating PLCgamma2, the exact activation mechanism of PLCgamma2 by these kinases has not been established. We identify the tyrosine residues 753, 759, 1197, and 1217 in rat PLCgamma2 as Btk-dependent phosphorylation sites by using an in vitro kinase assay. To evaluate the role of these tyrosine residues in phosphorylation-dependent activation of PLCgamma2, PLCgamma2-deficient DT40 cells were reconstituted with a series of mutant PLCgamma2s in which the phenylalanine was substituted for tyrosine. Substitution of all four tyrosine residues almost completely eliminated the BCR-induced PLCgamma2 phosphorylation, indicating that these residues include the major phosphorylation sites upon BCR engagement. Cells expressing PLCgamma2 with a single substitution exhibited some extent of reduction in calcium mobilization, whereas those expressing quadruple mutant PLCgamma2 showed greatly reduced calcium response. These findings indicate that the phosphorylations of the tyrosine residues 753, 759, 1197, and 1217, which have been identified as Btk-dependent phosphorylation sites in vitro, coordinately contribute to BCR-induced activation of PLCgamma2.

Transient upregulation of the glial glutamate transporter GLAST in response to fibroblast growth factor, insulin-like growth factor and epidermal growth factor in cultured astrocytes

Although expression of the glial glutamate transporter GLAST is tightly regulated during development and under pathophysiological conditions, little is known about endogenous modulators of GLAST expression. Because growth factors are generally believed to regulate glial functions, we addressed their possible contribution to GLAST regulation in cultured rat astrocytes. Of the six growth factors tested (basic fibroblast growth factor (bFGF), insulin-like growth factor-1 (IGF-1), epidermal growth factor (EGF), insulin, platelet-derived growth factor, and hepatocyte growth factor), bFGF, IGF-1 and EGF enhanced [3H]glutamate transport activity in a concentration-dependent manner. These effects were accompanied by an increase in the Vmax value for transport activity and in GLAST protein and mRNA levels, which suggests that GLAST expression is transcriptionally regulated by the growth factors. Interestingly, the effects reached a peak after 36 hours of exposure to growth factors, and rapidly returned to baseline by 48 hours. A combination of IGF-1 with either bFGF or EGF showed an additive effect on the glutamate uptake activity, but a combination of bFGF and EGF did not. Pharmacological blockade of protein kinase C inhibited the effects of IGF-1 and EGF, but not bFGF. By contrast, genistein, an inhibitor of tyrosine kinases, blocked the effects of bFGF and EGF without affecting the effect of IGF-1. These results suggest that the growth factors activate different signaling pathways for GLAST upregulation. The present study may indicate a novel regulatory system of glial glutamate transporters.

Regulation of phospholipase C gamma isoforms in haematopoietic cells: why one, not the other?

Phospholipase C gamma (PLCgamma) isoforms are critical for the generation of calcium signals in haematopoietic systems in response to the stimulation of immune receptors. PLCgamma is unique amongst phospholipases in that it is tightly regulated by the action of a number of tyrosine kinases. It is itself directly phosphorylated on a number of tyrosines and contains several domains through which it can interact with other signalling proteins and lipid products such as phosphatidylinositol 3,4,5-trisphosphate. Through this network of interactions, PLCgamma is activated and recruited to its substrate, phosphatidylinositol 4,5-bisphosphate, at the membrane. Both isoforms of PLCgamma, PLCgamma1 and PLCgamma2, are present in haematopoietic cells. The signalling cascade involved in the regulation of these two isoforms varies between cells, though the systems are similar for both PLCgamma1 and PLCgamma2. We will compare these cascades for both PLCgamma1 and PLCgamma2 and discuss possible reasons as to why one form of PLCgamma and not the other is required for signalling in specific haematopoietic cells, including T lymphocytes, B lymphocytes, platelets, and mast cells.

Specific involvement of G(alphai2) with epidermal growth factor receptor signaling in rat hepatocytes, and the inhibitory effect of chronic ethanol

We have previously shown that chronic alcohol consumption inhibits liver regeneration by impairing epidermal growth factor receptor (EGFR)-operated phospholipase C-(gamma1) (PLC-(gamma1)) activation and the resultant rise in intracellular [Ca(2+)](i). In hepatocytes, activation of PLC-(gamma1) by EGFR requires involvement of a pertussis toxin-sensitive inhibitory guanine nucleotide-binding regulatory protein (G(alphai)) as an intermediate. In the present study, we first identified the G(alphai) protein isoform associated with the activated EGFR, and then examined whether the toxic effect of alcohol on EGFR signaling and liver cell proliferation was exerted on this association. In cultured hepatocytes from control rats, EGF rapidly induced association between EGFR and G(alphai2) but not other G(alphai) isoforms. In hepatocytes from rats fed alcohol for 16 weeks, EGF failed to stimulate this association of G(alphai2) with the EGFR. The impairment of EGFR-G(alphai2) complex formation caused by alcohol was associated with a decreased level of G(alphai2) in the plasma membrane fraction (approximately 50% control). Pertussis toxin, an inhibitor of G(alphai) function, produced an analogous disruption of the association between G(alphai2) and the EGFR, as well as inhibiting EGF-induced DNA synthesis. It is concluded that, in hepatocytes, G(alphai2) is specific among G(alphai) isoforms in coupling activation of the EGFR to other signaling pathways that control cell proliferation. Impaired coupling of G(alphai2) of EGFR could contribute to the mechanism by which chronic alcohol exposure inhibits liver regeneration.

Differential stimulation of c-Kit mutants by membrane-bound and soluble Steel Factor correlates with leukemic potential

The authors investigated the roles of PI3-kinase and PLC-γ in stimulation by Steel Factor (SLF) through c-Kit. c-Kit mutants YF719, YF728, and a YF719/YF728 double mutant were expressed in 32D myelomonocytic cells. KitYF719 fails to recruit PI3-kinase after stimulation with SLF, whereas KitYF728 fails to stimulate PLC-γ phosphorylation or mobilize Ca++. Both single mutants responded mitogenically to soluble SLF (sSLF) in a manner indistinguishable from wild type (WT), although sSLF failed to stimulate or promote the survival of cells expressing the double mutant. In contrast, although cells expressing WT or YF719 were mitogenically stimulated by membrane-bound SLF (mSLF), stimulation of cells expressing KitYF728 was impaired. Similarly, cells expressing WT or YF719 receptors were stimulated by plate-bound anti-Kit antibodies, whereas cells expressing the YF728 receptor were not stimulated. Neomycin sulfate, a PLC antagonist, inhibited cells expressing YF719 receptors stimulated by sSLF. Neomycin also inhibited cells expressing the WT receptor that were stimulated by mSLF or immobilized anti-Kit antibodies but did not inhibit stimulation of cells expressing WT or YF719 receptors by sSLF. 32D cells expressing KitWT, KitYF719, or KitYF728 were injected into mice and the presence of cells was evaluated by colony assays 6 to 7 weeks later. Although both KitWT and KitYF719 expressing cells could be recovered from the spleen and bone marrow, recovery of KitYF728 cells from these organs was severely reduced. These results indicate that Kit tyrosine 728 is of particular importance for mitogenic stimulation by mSLF or immobilized ligand and is required for full maintenance of cells in vivo, likely through activation of PLC-γ.

Pleiotropic contributions of phospholipase C-gamma1 (PLC-gamma1) to T-cell antigen receptor-mediated signaling: reconstitution studies of a PLC-gamma1-deficient Jurkat T-cell line

Phospholipase C-gamma1 (PLC-gamma1) plays a crucial role in the coupling of T-cell antigen receptor (TCR) ligation to interleukin-2 (IL-2) gene expression in activated T lymphocytes. In this study, we have isolated and characterized two novel, PLC-gamma1-deficient sublines derived from the Jurkat T-leukemic cell line. The P98 subline displays a >90% reduction in PLC-gamma1 expression, while the J.gamma1 subline contains no detectable PLC-gamma1 protein. The lack of PLC-gamma1 expression in J.gamma1 cells caused profound defects in TCR-dependent Ca(2+) mobilization and NFAT activation. In contrast, both of these responses occurred at normal levels in PLC-gamma1-deficient P98 cells. Unexpectedly, the P98 cells displayed significant and selective defects in the activation of both the composite CD28 response element (RE/AP) and the full-length IL-2 promoter following costimulation with anti-TCR antibodies and phorbol ester. These transcriptional defects were reversed by transfection of P98 cells with a wild-type PLC-gamma1 expression vector but not by expression of mutated PLC-gamma1 constructs that lacked a functional, carboxyl-terminal SH2 [SH2(C)] domain or the major Tyr(783) phosphorylation site. On the other hand, the amino-terminal SH2 [SH2(N)] domain was not essential for reconstitution of RE/AP- or IL-2 promoter-dependent transcription but was required for the association of PLC-gamma1 with LAT, as well as the tyrosine phosphorylation of PLC-gamma1 itself, in activated P98 cells. These studies demonstrate that the PLC-gamma1 SH2(N) and SH2(C) domains play functionally distinct roles during TCR-mediated signaling and identify a non-Ca(2+)-related signaling function linked to the SH2(C) domain, which couples TCR plus phorbol ester-CD28 costimulation to the activation of the IL-2 promoter in T lymphocytes.