Antiphosphotyrosine recovery of phospholipase C activity after EGF treatment of A-431 cells

Activation Mechanisms and Diverse Functions of Mammalian Phospholipase C

Phospholipase C (PLC) plays pivotal roles in regulating various cellular functions by metabolizing phosphatidylinositol 4,5-bisphosphate in the plasma membrane. This process generates two second messengers, inositol 1,4,5-trisphosphate and diacylglycerol, which respectively regulate the intracellular Ca²⁺ levels and protein kinase C activation. In mammals, six classes of typical PLC have been identified and classified based on their structure and activation mechanisms. They all share X and Y domains, which are responsible for enzymatic activity, as well as subtype-specific domains. Furthermore, in addition to typical PLC, atypical PLC with unique structures solely harboring an X domain has been recently discovered. Collectively, seven classes and 16 isozymes of mammalian PLC are known to date. Dysregulation of PLC activity has been implicated in several pathophysiological conditions, including cancer, cardiovascular diseases, and neurological disorders. Therefore, identification of new drug targets that can selectively modulate PLC activity is important. The present review focuses on the structures, activation mechanisms, and physiological functions of mammalian PLC.

Phospholipase C: underrated players in microbial infections

During bacterial infections, one or more virulence factors are required to support the survival, growth, and colonization of the pathogen within the host, leading to the symptomatic characteristic of the disease. The outcome of bacterial infections is determined by several factors from both host as well as pathogen origin. Proteins and enzymes involved in cellular signaling are important players in determining the outcome of host-pathogen interactions. phospholipase C (PLCs) participate in cellular signaling and regulation by virtue of their ability to hydrolyze membrane phospholipids into di-acyl-glycerol (DAG) and inositol triphosphate (IP3), which further causes the activation of other signaling pathways involved in various processes, including immune response. A total of 13 PLC isoforms are known so far, differing in their structure, regulation, and tissue-specific distribution. Different PLC isoforms have been implicated in various diseases, including cancer and infectious diseases; however, their roles in infectious diseases are not clearly understood. Many studies have suggested the prominent roles of both host and pathogen-derived PLCs during infections. PLCs have also been shown to contribute towards disease pathogenesis and the onset of disease symptoms. In this review, we have discussed the contribution of PLCs as a determinant of the outcome of host-pathogen interaction and pathogenesis during bacterial infections of human importance.

Comprehensive review of targeted therapy for colorectal cancer

Colorectal cancer (CRC) is among the most lethal and prevalent malignancies in the world and was responsible for nearly 881,000 cancer-related deaths in 2018. Surgery and chemotherapy have long been the first choices for cancer patients. However, the prognosis of CRC has never been satisfying, especially for patients with metastatic lesions. Targeted therapy is a new optional approach that has successfully prolonged overall survival for CRC patients. Following successes with the anti-EGFR (epidermal growth factor receptor) agent cetuximab and the anti-angiogenesis agent bevacizumab, new agents blocking different critical pathways as well as immune checkpoints are emerging at an unprecedented rate. Guidelines worldwide are currently updating the recommended targeted drugs on the basis of the increasing number of high-quality clinical trials. This review provides an overview of existing CRC-targeted agents and their underlying mechanisms, as well as a discussion of their limitations and future trends.

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.

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.

Engineered N-cadherin and L1 biomimetic substrates concertedly promote neuronal differentiation, neurite extension and neuroprotection of human neural stem cells

We investigated the design of neurotrophic biomaterial constructs for human neural stem cells, guided by neural developmental cues of N-cadherin and L1 adhesion molecules. Polymer substrates fabricated either as two-dimensional (2-D) films or three-dimensional (3-D) microfibrous scaffolds were functionalized with fusion chimeras of N-cadherin-Fc alone and in combination with L1-Fc, and the effects on differentiation, neurite extension and survival of H9 human-embryonic-stem-cell-derived neural stem cells (H9-NSCs) were quantified. Combinations of N-cadherin and L1-Fc co-operatively enhanced neuronal differentiation profiles, indicating the critical nature of the two complementary developmental cues. Notably, substrates presenting low levels of N-cadherin-Fc concentrations, combined with proportionately higher L1-Fc concentration, most enhanced neurite outgrowth and the degree of MAP2+ and neurofilament-M+ H9-NSCs. Low N-cadherin-Fc alone promoted improved cell survival following oxidative stress, compared to higher concentrations of N-cadherin-Fc alone or combinations with L1-Fc. Pharmacological and antibody blockage studies revealed that substrates presenting low levels of N-cadherin are functionally competent so long as they elicit a threshold signal mediated by homophilic N-cadherin and fibroblast growth factor signaling. Overall, these studies highlight the ability of optimal combinations of N-cadherin and L1 to recapitulate a "neurotrophic" microenvironment that enhances human neural stem cell differentiation and neurite outgrowth. Additionally, 3-D fibrous scaffolds presenting low N-cadherin-Fc further enhanced the survival of H9-NSCs compared to equivalent 2-D films. This indicates that similar biofunctionalization approaches based on N-cadherin and L1 can be translated to 3-D "transplantable" scaffolds with enhanced neurotrophic behaviors. Thus, the insights from this study have fundamental and translational impacts for neural-stem-cell-based regenerative medicine.

Mammalian phospholipase C

Phospholipase C (PLC) converts phosphatidylinositol 4,5-bisphosphate (PIP(2)) to inositol 1,4,5-trisphosphate (IP(3)) and diacylglycerol (DAG). DAG and IP(3) each control diverse cellular processes and are also substrates for synthesis of other important signaling molecules. PLC is thus central to many important interlocking regulatory networks. Mammals express six families of PLCs, each with both unique and overlapping controls over expression and subcellular distribution. Each PLC also responds acutely to its own spectrum of activators that includes heterotrimeric G protein subunits, protein tyrosine kinases, small G proteins, Ca(2+), and phospholipids. Mammalian PLCs are autoinhibited by a region in the catalytic TIM barrel domain that is the target of much of their acute regulation. In combination, the PLCs act as a signaling nexus that integrates numerous signaling inputs, critically governs PIP(2) levels, and regulates production of important second messengers to determine cell behavior over the millisecond to hour timescale.

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.

Requirement for PLCγ2 in IL-3 and GM-CSF-stimulated MEK/ERK phosphorylation in murine and human hematopoietic stem/progenitor cells

Even though the involvement of intracellular Ca(2+) Ca(i)(2+) in hematopoiesis has been previously demonstrated, the relationship between Ca(i)(2+) signaling and cytokine-induced intracellular pathways remains poorly understood. Herein, the molecular mechanisms integrating Ca(2+) signaling with the extracellular signal-regulated kinase 1/2 (ERK1/2) pathway in primary murine and human hematopoietic stem/progenitor cells stimulated by IL-3 and GM-CSF were studied. Our results demonstrated that IL-3 and GM-CSF stimulation induced increased inositol 1,4,5-trisphosphate (IP(3) ) levels and Ca(i)(2+) release in murine and human hematopoietic stem/progenitor cells. In addition, Ca(i)(2+) signaling inhibitors, such as inositol 1,4,5-trisphosphate receptor antagonist (2-APB), PKC inhibitor (GF109203), and CaMKII inhibitor (KN-62), blocked phosphorylation of MEK activated by IL-3 and GM-CSF, suggesting the participation of Ca(2+) -dependent kinases in MEK activation. In addition, we identify phospholipase Cγ2 (PLCγ2) as a PLCγ responsible for the induction of Ca(2+) release by IL-3 and GM-CSF in hematopoietic stem/progenitor cells. Furthermore, the PLCγ inhibitor U73122 significantly reduced the numbers of granulocyte-macrophage colony-forming units after cytokine stimulation. Similar results were obtained in both murine and human hematopoietic stem/progenitor cells. Taken together, these data indicate a role for PLCγ2 and Ca(2+) signaling through the modulation of MEK in both murine and human hematopoietic stem/progenitor cells.

Protein tyrosine kinase involvement in learning-produced changes in Hermissenda type B photoreceptors

Learning-correlated changes in the excitability and photoresponses of Hermissenda's ocular type B photoreceptors are mediated by reductions in two distinct K(+) currents, I(A) and I(K-Ca). The suppression of these K(+) currents has been linked to conditioning-produced activation of protein kinase C (PKC). The question of whether PKC accounts completely for the changes in excitability and K(+) currents or whether other kinase(s) are involved has received little attention. In the present experiments, we asked whether protein tyrosine kinases (PTKs) might also contribute to conditioning-produced alterations in B cells. We found that the PTK inhibitors genistein and lavendustin A greatly reduced cumulative depolarization of type B cells, a short-term correlate of associative learning. This disruption occurred even when PKC activation had been either occluded by preexposure of type B cells to a phorbol ester or otherwise prevented by the pseudosubstrate inhibitor peptide PKC[19-31]. PTK inhibitors also increased the amplitude of the transient (I(A)) and delayed (I(Delayed)) components of voltage-dependent K(+) current that have previously been shown to be selectively reduced by conditioning and to contribute to cumulative depolarization. Genistein partially prevented the reduction of I(A) and I(Delayed) due to in vitro conditioning and blocked the changes in their voltage dependencies. Ionophoresis of pervanadate ion, a potent inhibitor of protein tyrosine phosphatases, depolarized type B photoreceptors and occluded conditioning-produced cumulative depolarization. Pervanadate also suppressed I(A) and I(Delayed), reduced their voltage dependence, and altered inactivation kinetics for I(A), mimicking conditioning. Western blot analysis using a phosphotyrosine antibody indicated that conditioning increased the phosphotyrosine content of many proteins within the Hermissenda CNS. Collectively, our results suggest that in addition to PKC, one or more PTKs play an important role in conditioning-produced changes in type B cell excitability. PTKs and PKCs converge to effect reductions in B cell K(+) currents during conditioning, apparently through distinct biophysical mechanisms.

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.

Changes in intracellular Ca2+ levels induced by cytokines and P2 agonists differentially modulate proliferation or commitment with macrophage differentiation in murine hematopoietic cells

The role of intracellular Ca2+ (Ca2+i) on hematopoiesis was investigated in long term bone marrow cultures using cytokines and agonists of P2 receptors. Cytokines interleukin 3 and granulocyte/macrophage colony stimulator factor promoted a modest increase in Ca2+i concentration ([Ca2+]i) with activation of phospholipase Cgamma, MEK1/2, and Ca2+/calmodulin kinase II. Involvement of protein kinase C was restricted to stimulation with interleukin 3. In addition, these cytokines promoted proliferation (20 times) and an increase in the Gr-1(-)Mac-1+ population with participation of gap junctions (GJ). Nevertheless ATP, ADP, and UTP promoted a large increase in [Ca2+]i, moderate proliferation (6 times), a reduction in the primitive Gr-1(-)Mac-1(-)c-Kit+ population, and differentiation into macrophages without participation of GJ. It is likely that Ca2+i participates as a regulator of hematopoietic signaling: moderate increases in [Ca2+]i would be related to cytokine-dependent proliferation with participation of GJ, whereas high increases in [Ca2+]i would be related to macrophage differentiation without maintenance of the primitive population.

G proteins in signal transduction: the regulation of phospholipase C

The hydrolysis of phosphatidylinositol 4,5-bisphosphate by specific phospholipase C (PLC) enzymes produces two second messengers, inositol 1,4,5-trisphosphate and diacylglycerol. Heterotrimeric guanine nucleotide-binding regulatory proteins (G proteins) of the Gq subfamily activate the PLC beta 1 isoform of PLC. We have purified three isozymes of PLC beta: PLC beta 1 and PLC beta 3 from rat brain and PLC beta 2 from HL-60 cells. Whereas the beta 1 and beta 2 isozymes appear restricted to a few cell types, beta 3 is broadly distributed. Gq alpha (the alpha subunit of the Gq subfamily) can activate all three isoforms but PLC beta 2 is much less sensitive. Thus all three enzymes are potential effectors for pertussis toxin-insensitive regulation by hormones. The three beta isozymes can also be activated by purified beta gamma subunits. The PLC beta 3 isoform gives the greatest activation with beta gamma; PLC beta 1 is least responsive. The results indicate that all the known isoforms of mammalian PLC beta can be regulated at unique sites by both Gq alpha and beta gamma subunits. The effect of beta gamma subunits may provide a pathway for the regulation of PLC beta isozymes by pertussis toxin-sensitive G proteins or may indicate that the alpha subunit of Gq and its associated beta gamma both participate in regulation of the same phospholipase molecule.

Signal-response coupling mediated by the transduced colony-stimulating factor-1 receptor and its oncogenic fms variants in naive cells

Colony-stimulating factor-1 (CSF-1 or M-CSF) supports the proliferation and survival of mononuclear phagocytes by binding to a receptor (CSF-1R) encoded by the c-fms proto-oncogene. Whereas the CSF-1R kinase is normally regulated by ligand, receptors bearing 'activating mutations' act constitutively as enzymes and can transform fibroblasts and haemopoietic cells of different lineages. Introduction of human CSF-1R enables mouse NIH-3T3 cells to form colonies in agar in response to human CSF-1 and to proliferate in serum-free medium supplemented with CSF-1, albumin, transferrin and insulin. Similarly, expression of human CSF-1R in interleukin 3-dependent mouse FDC-P1 myeloid cells enables them to grow in CSF-1. High levels of CSF-1R expression in FDC-P1 cells can induce factor-independent growth which is abrogated by a 'neutralizing' monoclonal antibody to the receptor. Therefore, critical mutations in the c-fms gene or overexpression of CSF-1R in immature myeloid precursors might each contribute to leukaemia.

Epidermal growth factor/transforming growth factor alpha receptors and psoriasis

The abnormal growth and differentiation in psoriasis is reflected in the abnormal regulation of Epidermal Growth Factor/Transforming Growth Factor Alpha (EGF/TGFalpha) receptor metabolism. In psoriasis and other hyperproliferative skin conditions these receptors are persistently expressed throughout the interfollicular epidermis as long as the growth stimulatory signal persists. One of the first biochemical signs of effective therapy of psoriasis is the return of the EGF/TGFalpha receptor pattern toward the primarily basilar distribution seen in normal human adult skin. Whether the abnormal expression of TGFalpha in the involved skin induces the persistent expression of EGF receptors is not known nor is the signal that causes the increased production of TGFalpha. Studies to determine what factors regulate EGF receptor expression and TGFalpha induction may yield important new insights into the pathogenesis and therapy of psoriasis.

Distinct roles of phosphoinositide-3 kinase and phospholipase Cgamma2 in B-cell receptor-mediated signal transduction

During B-cell receptor (BCR) signaling, phosphoinositide-3 kinase (PI3K) is thought to function upstream of phospholipase Cgamma2 (PLCgamma2). PLCgamma2 deficiency specifically impedes transitional type 2 (T2) to follicular (FO) mature B-cell transition. Here, we demonstrate that PI3K deficiency specifically impaired T2-to-FO mature B-cell transition and marginal zone B-cell development. Furthermore, we investigated the functional relationship between PI3K and PLCgamma2 using PI3K-/-, PLCgamma2-/-, and PI3K-/- PLCgamma2-/- B cells. Interestingly, PLCgamma2 deficiency had no effect on BCR-mediated PI3K activation, whereas PI3K deficiency only partially blocked activation of PLCgamma2. Moreover, whereas PI3K-/- PLCgamma2-/- double deficiency did not affect hematopoiesis, it resulted in embryonic lethality. PI3K-/- PLCgamma2-/- fetal liver cells transplanted into B-cell null JAK3-/- mice failed to restore development of peripheral B cells and failed to progress through early B-cell development at the pro-B- to pre-B-cell transition, a more severe phenotype than was observed with either PI3K or PLCgamma2 single-deficiency B cells. Consistent with this finding, BCR signaling was more severely impaired in the absence of both PI3K and PLCgamma2 genes than in the absence of either one alone. Taken together, these results demonstrate that whereas PI3K functions upstream of PLCgamma2, activation of PLCgamma2 can occur independently of PI3K and that PI3K and PLCgamma2 also have distinct functions in BCR signal transduction.

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.

Tec Kinases Mediate Sustained Calcium Influx via Site-specific Tyrosine Phosphorylation of the Phospholipase Cγ Src Homology 2-Src Homology 3 Linker

Tyrosine phosphorylation of phospholipase Cgamma2 (PLCgamma2) is a crucial activation switch that initiates and maintains intracellular calcium mobilization in response to B cell antigen receptor (BCR) engagement. Although members from three distinct families of non-receptor tyrosine kinases can phosphorylate PLCgamma in vitro, the specific kinase(s) controlling BCR-dependent PLCgamma activation in vivo remains unknown. Bruton's tyrosine kinase (Btk)-deficient human B cells exhibit diminished inositol 1,4,5-trisphosphate production and calcium signaling despite a normal inducible level of total PLCgamma2 tyrosine phosphorylation. This suggested that Btk might modify a critical subset of residues essential for PLCgamma2 activity. To evaluate this hypothesis, we generated site-specific phosphotyrosine antibodies recognizing four putative regulatory residues within PLCgamma2. Whereas all four sites were rapidly modified in response to BCR engagement in normal B cells, Btk-deficient B cells exhibited a marked reduction in phosphorylation of the Src homology 2 (SH2)-SH3 linker region sites, Tyr(753) and Tyr(759). Phosphorylation of both sites was restored by expression of Tec, but not Syk, family kinases. In contrast, phosphorylation of the PLCgamma2 carboxyl-terminal sites, Tyr(1197) and Tyr(1217), was unaffected by the absence of functional Btk. Together, these data support a model whereby Btk/Tec kinases control sustained calcium signaling via site-specific phosphorylation of key residues within the PLCgamma2 SH2-SH3 linker.

Activation of EGF receptor kinase by L1-mediated homophilic cell interactions

Neural cell adhesion molecules (CAMs) are important players during neurogenesis and neurite outgrowth as well as axonal fasciculation and pathfinding. Some of these developmental processes entail the activation of cellular signaling cascades. Pharmacological and genetic evidence indicates that the neurite outgrowth-promoting activity of L1-type CAMs is at least in part mediated by the stimulation of neuronal receptor tyrosine kinases (RTKs), especially FGF and EGF receptors. It has long been suspected that neural CAMs might physically interact with RTKs, but their activation by specific cell adhesion events has not been directly demonstrated. Here we report that gain-of-function conditions of the Drosophila L1-type CAM Neuroglian result in profound sensory axon pathfinding defects in the developing Drosophila wing. This phenotype can be suppressed by decreasing the normal gene dosage of the Drosophila EGF receptor gene. Furthermore, in Drosophila S2 cells, cell adhesion mediated by human L1-CAM results in the specific activation of human EGF tyrosine kinase at cell contact sites and EGF receptors engage in a physical interaction with L1-CAM molecules. Thus L1-type CAMs are able to promote the adhesion-dependent activation of EGF receptor signaling in vitro and in vivo.

Regulation of EGF-induced phospholipase C-gamma1 translocation and activation by its SH2 and PH domains

Translocation of phospholipase C-gamma1 is essential for its function in response to growth factors. However, in spite of recent progress, the phospholipase C-gamma1 translocation pattern and the molecular mechanism of the translocation are far from fully understood. Contradictory results were reported as to which domain, PH or SH2, controls the epidermal growth factor-induced translocation of phospholipase C-gamma1. In this communication, we studied epidermal growth factor-induced translocation of phospholipase C-gamma1 by using comprehensive approaches including biochemistry, indirect fluorescence and live fluorescence imaging. We provided original evidence demonstrating that: (i) endogenous phospholipase C-gamma1, similar to YFP-tagged phospholipase C-gamma1, translocated to endosomes following its initial translocation from cytosol to the plasma membrane in response to epidermal growth factor; (ii) phospholipase C-gamma1 remained phosphorylated in endosomes, but phospholipase C-gamma1 activity is not required for its translocation, which suggests a signaling role for phospholipase C-gamma1 in endosomes; (iii) the PH domain was not required for the initial translocation of phospholipase C-gamma1 from cytosol to the plasma membrane, but it stabilizes phospholipase C-gamma1 in the membrane at a later time; (iv) the function of the phospholipase C-gamma1 PH domain in stabilizing phospholipase C-gamma1 membrane association is very important in maintaining the activity of phospholipase C-gamma1; and (v) the role of the PH domain in phospholipase C-gamma1 membrane association and activation is dependent on PI3K activity. We conclude that the phospholipase C-gamma1 SH2 and PH domains coordinate to determine epidermal growth factor-induced translocation and activation of phospholipase C-gamma1.

CAIR-1/BAG-3 forms an EGF-regulated ternary complex with phospholipase C-gamma and Hsp70/Hsc70

CAIR-1/BAG-3 forms an EGF-regulated ternary complex with Hsp70/Hsc70 and latent phospholipase C-gamma (PLC-gamma). The expression of CAIR-1, CAI stressed-1, was induced in A2058 human melanoma cells by continuous exposure to CAI, an inhibitor of nonvoltage-gated calcium influx. CAIR-1 sequence is identical, save 2 amino acids, to BAG-3 also cloned recently as Bis, a member of the bcl-2-associated athanogene family. We show that CAIR-1/BAG-3 binds to Hsp70/Hsc70 in intact cells and this binding is increased by short term exposure to CAI (P<0.007). CAIR-1/BAG-3 is phosphorylated in vivo in the absence of stimulation. Basal phosphorylation is inhibited by treatment with d-erythrosphingosine (d-ES), a broad inhibitor of the protein kinase C family. CAIR-1/BAG-3 contains several PXXP SH3 binding domains leading to the hypothesis that it is a partner protein of phospholipase C-gamma. PLC-gamma is bound to CAIR-1/BAG-3 in unstimulated cells. It is increased by CAI or d-ES (P=0.05) treatment, and abrogated by EGF (r2=0.99); d-ES treatment blocks the EGF-mediated dissociation. We show that CAIR-1/BAG-3 binds to PLC-gamma and Hsp70/Hsc70 through separate and distinct domains. Hsp70/Hsc70 binds to the BAG domain of BAGs-1 and -3. CAIR-1/BAG-3 from control and EGF-treated cell lysates bound selectively to the SH3 domain of PLC-gamma, but not its N-SH2 or C-SH2 domains. Confirming the SH3 interaction, PLC-gamma was pulled down by CAIR-1/BAG-3 PXXP-GST fusions, but GST-PXXP constructs confronted with lysates from EGF-treated cells did not bind PLC-gamma as was seen in intact cells. Hsp70/Hsc70 was brought down by the PLC-gamma SH3 construct equally from native and EGF-treated cells, but did not bind the PXXP construct under either condition. We propose that CAIR-1/BAG-3 may act as a multifunctional signaling protein linking the Hsp70/Hsc70 pathway with those necessary for activation of the EGF receptor tyrosine kinase signaling pathways.

Increased expression of Galpha(q/11) and of phospholipase-Cbeta1/4 in differentiated human NT2-N neurons: enhancement of phosphoinositide hydrolysis

The CNS is enriched in phosphoinositide-specific phospholipase C (PLC) and in the G proteins linked to its activation. Although the regional distributions of these signaling components within the brain have been determined, neither their cell type-specific localizations (i.e., neuronal versus glial) nor the functional significance of their high expression has been definitively established. In this study, we have examined the expression of phosphoinositide signaling proteins in human NT2-N cells, a well characterized model system for CNS neurons. Retinoic acid-mediated differentiation of NT2 precursor cells to the neuronal phenotype resulted in five- to 15-fold increases in the expression of PLC-beta1, PLC-beta4, and Galpha(q/11) (the prime G protein activator of these isozymes). In contrast, the expression of PLC-beta3 and PLC-gamma1 was markedly reduced following neuronal differentiation. Similar alterations in cell morphology and in the expression of PLC-beta1, PLC-beta3, and Galpha(q/11) expression were observed when NT2 cells were differentiated with berberine, a compound structurally unrelated to retinoic acid. NT2-N neurons exhibited a significantly higher rate of phosphoinositide hydrolysis than NT2 precursor cells in response to direct activation of either G proteins or PLC. These results indicate that neuronal differentiation of NT2 cells is associated with dramatic changes in the expression of proteins of the phosphoinositide signaling system and that, accordingly, differentiated NT2-N neurons possess an increased ability to hydrolyze inositol lipids.

Epidermal growth factor signaling pathway influences retinoid metabolism by reduction of retinyl ester hydrolase activities in normal and malignant keratinocytes

The effects of EGFR signaling on retinol metabolism were evaluated in the squamous cell carcinoma cell lines defective in LRAT. In a 24-h incubation, the presence of EGF resulted in a 20-25% increase in retinyl ester accumulation. Assessment of retinol esterification and retinyl ester utilization (hydrolysis), in cell cultures and in cell homogenates, revealed that the increase in retinyl ester mass was the result of a reduction in retinyl ester hydrolysis. When grown in the absence of EGF, the cultures used about 40% of their retinyl esters, compared to about 21% in cultures grown with EGF. This effect of EGF was blocked by an EGF receptor-neutralizing antibody, an EGF receptor tyrosine-kinase inhibitor (PD153035), and a specific inhibitor of MEK kinase influencing the mitogen-activated protein kinase (MAPK) cascade (PD98059). Both transcription and translation were required, suggesting that signaling from the EGF receptor through the MAPK cascade controls the expression of modulators or inhibitors of the retinyl ester hydrolase(s). Thus EGFR signaling can alter the intracellular concentration of retinol by suppressing the access to the retinyl ester pool. Similar EGF effects were seen in cultures of normal keratinocytes.

Neurotrophin-induced activation of casein kinase 2 in rat hippocampal slices

Casein kinase 2 is present in the brain, including the hippocampus. It is associated with long-term potentiation and is known to be involved in phosphorylation of proteins potentially important for neuroplasticity, but regulation of its activity in neuronal cells is not yet known. In the present work, it was found that brain-derived neurotrophic factor and neurotrophin-4 control the activity of casein kinase 2 in hippocampal slices of adult rat. It is shown that: (i) treatment of slices for 4 h with the neurotrophins results in a five-fold increase in the activity of cytosolic casein kinase 2; (ii) this effect does not require protein synthesis. In addition, using calcium chelators, phospholipase inhibitors and protein kinase inhibitors, evidence is provided that: (i) neurotrophin-induced activation of casein kinase 2 is dependent on the availability of intracellular calcium due to stimulation of phospholipase C; (ii) both a tyrosine kinase(s) and a serine/threonine kinase(s) convey the signal of calcium. Since there is now accumulating evidence for involvement of brain-derived neurotrophic factor, intracellular calcium, tyrosine kinases and serine/threonine kinases in the regulation of synaptic plasticity, it is suggested that the signalling cascade detected here might contribute to control of synaptic strength in the hippocampus.

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.

Epidermal growth factor receptor-stimulated intestinal epithelial cell migration requires phospholipase C activity

BACKGROUND & AIMS

Ulceration of intestinal mucosa is rapidly followed by enterocyte migration via restitution. The aim of this study was to investigate signaling mechanisms of epidermal growth factor (EGF) receptor-stimulated monolayer restitution in a mouse intestinal epithelial cell line.

METHODS

EGF-stimulated cell migration was determined using a wound model in the presence of agonists and/or antagonists of tyrosine kinase, phospholipase C, phosphatidylinositol 3-kinase, or protein kinase C. The tyrosine phosphorylation state of the EGF receptor, phosphatidylinositol phospholipase C gamma1 (PLCgamma1), focal adhesion kinase, and cellular lysates was determined by immunodetection.

RESULTS

EGF stimulated cell migration twofold at 4, 8, and 24 hours. Inhibition of EGF receptor tyrosine kinase activity, phospholipase C, or phosphatidylinositol 3-kinase attenuated EGF-induced intestinal cell migration. Pretreatment of cells with phorbol 12-myristate 13-acetate, known to down-regulate protein kinase C expression, blocked EGF-induced cell migration. Increased tyrosine phosphorylation of the EGF receptor and PLCgamma1 was detected within 5 minutes after wounding.

CONCLUSIONS

EGF-stimulated intestinal cell migration requires intact EGF receptor tyrosine kinase, phospholipase, and protein kinase C activities. PLCgamma1 may be a key regulatory molecule in the initial EGF receptor signal transduction pathway of EGF-stimulated cell migration.

Requirement for phospholipase C-gamma1 enzymatic activity in growth factor-induced mitogenesis

The cytoplasmic regions of the receptors for epidermal growth factor (EGF) and platelet-derived growth factor (PDGF) bind and activate phospholipase C-gamma1 (PLC-gamma1) and other signaling proteins in response to ligand binding outside the cell. Receptor binding by PLC-gamma1 is a function of its SH2 domains and is required for growth factor-induced cell cycle progression into the S phase. Microinjection into MDCK epithelial cells and NIH 3T3 fibroblasts of a polypeptide corresponding to the noncatalytic SH2-SH2-SH3 domains of PLC-gamma1 (PLC-gamma1 SH2-SH2-SH3) blocked growth factor-induced S-phase entry. Treatment of cells with diacylglycerol (DAG) or DAG and microinjected inositol-1,4,5-triphosphate (IP3), the products of activated PLC-gamma1, did not stimulate cellular DNA synthesis by themselves but did suppress the inhibitory effects of the PLC-gamma1 SH2-SH2-SH3 polypeptide but not the cell cycle block imposed by inhibition of the adapter protein Grb2 or p21 Ras. Two c-fos serum response element (SRE)-chloramphenicol acetyltransferase (CAT) reporter plasmids, a wild-type version, wtSRE-CAT, and a mutant, pm18, were used to investigate the function of PLC-gamma1 in EGF- and PDGF-induced mitogenesis. wtSRE-CAT responds to both protein kinase C (PKC)-dependent and -independent signals, while the mutant, pm18, responds only to PKC-independent signals. Microinjection of the dominant-negative PLC-gamma1 SH2-SH2-SH3 polypeptide greatly reduced the responses of wtSRE-CAT to EGF stimulation in MDCK cells and to PDGF stimulation in NIH 3T3 cells but had no effect on the responses of mutant pm18. These results indicate that in addition to Grb2-mediated activation of Ras, PLC-gamma1-mediated DAG production is required for EGF- and PDGF-induced S-phase entry and gene expression, possibly through activation of PKC.

1-(Carboxymethylthio)tetradecane attenuates PDGF- and TPA-induced c-fos mRNA expression and increases the formation of phosphatidylethanolamine with a shift from less to more polar molecular species in C3H/10T1/2 cells

1-(Carboxymethylthio)tetradecane caused C3H/10T1/2Cl8 and C3H/10T1/2Cl16 to incorporate 10 times more [32P]Pi into diacylphosphatidylethanolamine than control. This 3-thia fatty acid caused a shift in incorporation of 32P-radioactivity into phosphatidylethanolamine species from species with long to species with short HPLC elution times. The increase in 32P-labeling was parallelled by a change in the apparent mass of phosphatidylethanolamine to a higher proportion of molecular species with short elution times than with long elution times. 1-(Carboxymethylthio)tetradecane caused loss of molecular species containing stearoyl groups. These results indicate that culturing the cells with 1-(carboxymethylthio)tetradecane causes looser packing and an increase in fluidity of the diacylphosphatidylethanolamine molecules in the membranes. 12-O-tetradecanoyl phorbol-13-acetate (TPA), platelet-derived growth factor (PDGF)-BB, or PDGF-AA stimulation of 1-(carboxymethylthio)tetradecane-treated cells resulted in decreased maximal levels of c-fos mRNA expression, indicating attenuation of signal transduction. Compared to cells not treated, the levels of both PDGF-alpha and PDGF-beta receptors were lower while GTPase-activating protein and phospholipase C-gamma levels were not altered in C3H/10T1/2Cl8 and C3H/10T1/2Cl16 cells cultured in the presence of 1-(carboxymethylthio)tetradecane. Our data demonstrate that 1-(carboxymethylthio)tetradecane-mediated changes in phospholipid structure and composition may affect PDGF- and TPA-mediated c-fos gene regulation in fibroblasts.

Rapid stimulation of amyloid precursor protein release by epidermal growth factor: role of protein kinase C

The amyloid precursor protein (APP) of Alzheimer's disease is a transmembrane protein that is cleaved by an uncharacterized enzyme known as alpha-secretase within its extracellular/intraluminal domain after the activation of guanine nucleotide-binding protein-coupled receptors linked to phosphoinositide hydrolysis. The secretory process results in the release of large soluble derivatives of APP (APPs), and, when elicited by muscarinic receptor activation, exhibits both protein kinase C (PKC)-dependent and tyrosine phosphorylation-dependent components [Slack, Breu, Petryniak, Srivastava and Wurtman (1995) J. Biol. Chem. 270, 8337-8344]. In this report we examine the regulation of the release of APPs by epidermal growth factor (EGF) receptors, which possess intrinsic tyrosine kinase activity, and are coupled to a variety of effectors including phosphoinositide-specific phospholipase Cgamma. In A431 cells, EGF caused time-dependent and dose-dependent increases in the formation of inositol phosphates in cultures prelabelled with myo--3H-inositol, and in the release of APPs into the culture medium; the two responses exhibited similar time courses and EC50 values for EGF. Concomitant with these effects, there were concentration-dependent (3-300 ng/ml) increases in the phosphorylation of tyrosine residues in several proteins, including the EGF receptor itself. The specific PKC antagonist GF 109203X decreased the effect of EGF by approx. 35% at a concentration that abolished the stimulation of the release of APPs by the PKC activator PMA. Tyrphostin AG 1478, an inhibitor of EGF receptor tyrosine kinase, abolished the EGF-induced release of APPs. These results demonstrate that in A431 cells, activation of the EGF receptor stimulates alpha-secretase activity by a mechanism that is partly dependent on PKC activity.

Thrombin activation of human platelets dissociates a complex containing gelsolin and actin from phosphatidylinositide-specific phospholipase Cgamma1

We have examined the association of two cytoskeleton proteins, gelsolin and actin, with phosphatidylinositide-specific phospholipase Cgamma1 (PLCgamma1) in resting and thrombin-stimulated human platelets. In unstimulated platelets, gelsolin, actin and PLCgamma1 were immunoprecipitated as a complex by a polyclonal antibody to PLCgamma1. The association of gelsolin and actin was specific for PLCgamma1 because immunoprecipitates of PLCs beta2, beta3, gamma2 and delta1, which are also expressed in human platelets, did not contain detectable gelsolin or actin. Activation with thrombin resulted in platelet aggregation and the dissociation of gelsolin and actin from PLCgamma1. Inhibition of thrombin-induced platelet aggregation blocked the dissociation of gelsolin and actin from PLCgamma1. After stimulation with thrombin, PLCgamma1 activity in immunoprecipitates was increased 2-3-fold. This elevation in PLCgamma1 activity in response to thrombin activation was not observed when platelet aggregation was blocked. Although PLCgamma1 is tyrosine phosphorylated in response to many agonists, we could not detect, by Western analysis with anti-phosphotyrosine antibodies, tyrosine phosphorylation of PLCgamma1 immunoprecipitated from thrombin-stimulated platelets. These results demonstrate that PLCgamma1 is associated with gelsolin and actin in resting platelets, and that thrombin-induced platelet aggregation results in the dissociation of PLCgamma1 from gelsolin and actin, and the stimulation of PLCgamma1 activity.

Subsets of epidermal growth factor receptors during activation and endocytosis

Mutation of the autophosphorylation sites of receptor protein-tyrosine kinases alters ligand dependent internalization and down-regulation, indicating a critical role for these sites in receptor processing. Currently, no differences in receptor processing based on an individual autophosphorylation site have been defined. By using a glutathione S-transferase fusion protein containing the src homology 2 domains of phospholipase C-gamma1 to specifically recognize tyrosine 992 on the EGF receptor (Tyr(P)992), we have found differences in this subpopulation of receptors. Following EGF stimulation, the number of Tyr(P)992 receptors increased 2-fold over receptors identified by an antibody that recognizes activated EGF receptors (alpha-Act. EGFR) in A431 cells. Confocal fluorescence microscopy showed that Tyr(P)992 receptors underwent endocytosis at a slower rate and did not rapidly concentrate in juxtanuclear bodies. Tyr(P)992 receptors were associated with more SOS, Ras-GTPase activating protein, phosphatidylinositol 3-kinase, and SHPTP2/syp, but less Grb2, than receptors in the general population, and these receptors were more heavily phosphorylated than the general population of active receptors. These findings suggest that autophosphorylation status is relevant to the endocytosis, degradation, and effector molecule interaction of individual EGF receptors. Further investigations based on phosphorylation status should provide new insights into how receptor protein-tyrosine kinase signaling is regulated.

Hormone signalling via G-protein: regulation of phosphatidylinositol 4,5-bisphosphate hydrolysis by G q

Heterotrimeric GTP-dependent regulatory proteins (G-proteins) mediate modulation by many cell surface receptors. Activation of the G-proteins promotes dissociation of their α and βγ subunits. The similarity of behaviour of βγ subunits derived from a variety of G-proteins has led to their use as affinity reagents for the analysis of the more unique a subunits. The evolution of these uses is presented. One of the more insightful results was the isolation of a new class of G -protein α subunits (the α q subfamily) which have been shown to regulate the activity of a phospholipase C (PLC) specific for phosphatidylinositols. The experimental evidence leading to this conclusion is discussed. The activation by α q increases the apparent V max of the β isoform of phosphatidylinositol-specific phospholipase C (PLCβ) and can be modulated by βγ subunits.

Localization and turnover of phosphatidylinositol 4,5-bisphosphate in caveolin-enriched membrane domains

Caveolae are small, plasma membrane invaginations that have been implicated in cell signaling. In A431 cells, approximately half of the total cellular phosphatidylinositol 4,5-bisphosphate (PtdIns 4, 5-P2) was found to be localized in low density, Triton-insoluble membrane domains enriched in caveolin. Treatment of cells with either epidermal growth factor or bradykinin for 5 min at 37 degrees C resulted in approximately a 50% decrease in this caveolar PtdIns 4,5-P2 with no change in the levels of plasma membrane PtdIns 4,5-P2. These data suggest that the PtdIns 4,5-P2 present in cells is largely compartmentalized and that the caveolar PtdIns 4,5-P2 is subject to hydrolysis by hormone-stimulated phospholipase C. As growth factor receptors, seven transmembrane domain receptors, heterotrimeric G proteins, and the inositol trisphosphate receptor have all been shown to be enriched in caveolae, these findings suggest that both the generation and response to inositol trisphosphate is highly compartmentalized within the cell.

Involvement of ErbB2 in the signaling pathway leading to cell cycle progression from a truncated epidermal growth factor receptor lacking the C-terminal autophosphorylation sites

To investigate the mechanisms underlying the enhanced mitogenic activity of the truncated epidermal growth factor receptor (EGFR) lacking the C-terminal autophosphorylation sites (Delta973-EGFR), we studied the intracellular signaling pathways in NR6 cells expressing human wild type EGFR and Delta973-EGFR. Microinjection of dominant/negative p21ras(N17) completely inhibited EGF-induced DNA synthesis in both cell types. EGF stimulated Shc phosphorylation as well as the formation of wild type EGFR.Shc complexes. In contrast, EGF stimulated Shc phosphorylation without formation of Delta973-EGFR.Shc complexes. Tyrosine-phosphorylated Shc formed complexes with Grb2.Sos, and microinjection of anti-Shc antibody and Shc-SH2 GST fusion protein inhibited EGF stimulation of DNA synthesis in both cell lines. EGF markedly increased ErbB2 tyrosine phosphorylation in wild type EGFR cells. In Delta973-EGFR cells, ErbB2 was tyrosine phosphorylated in the basal state and EGFR stimulated further phosphorylation of ErbB2. In addition to ErbB2, additional proteins were tyrosine phosphorylated in Delta973-EGFR cells, mostly in the molecular mass range of 120 170 kDa. Taken together with our findings indicating coupling of ErbB2 to Shc, these data suggest the importance of an alternative signaling pathway in Delta973-EGFR cells mediated by the formation of heterodimeric structures between the truncated EGFR and ErbB2, followed by coupling through Shc to Grb2.Sos and the p21ras pathway, ultimately leading to mitogenesis.

SH2 domain protein interaction and possibilities for pharmacological intervention

SH2 domain containing proteins play a key role in the process of intracellular transmission of signalling events initiated at the cell surface. As a pre-requisite in the fulfillment of this function, these proteins bind to a variety of phospho-tyrosine (pY) containing target molecules. Delineation of these binding sites as essentially short linear peptides (both structurally and functionally) has led to the suggestion that the activity of these signalling complexes may be manipulated by the development of relatively simple peptide reagents. This review examines the range of possibilities open on this approach and the extent to which positive results have already been realised.

Mitogenic signals and transforming potential of Nyk, a newly identified neural cell adhesion molecule-related receptor tyrosine kinase

Nyk/Mer is a recently identified receptor tyrosine kinase with neural cell adhesion molecule-like structure (two immunoglobulin G-like domains and two fibronectin III-like domains) in its extracellular region and belongs to the Ufo/Axl family of receptors. The ligand for Nyk/Mer is presently unknown, as are the signal transduction pathways mediated by this receptor. We constructed and expressed a chimeric receptor (Fms-Nyk) composed of the extracellular domain of the human colony-stimulating factor 1 receptor (Fms) and the transmembrane and cytoplasmic domains of human Nyk/Mer in NIH 3T3 fibroblasts in order to investigate the mitogenic signaling and biochemical properties of Nyk/Mer. Colony-stimulating factor 1 stimulation of the Fms-Nyk chimeric receptor in transfected NIH 3T3 fibroblasts leads to a transformed phenotype and generates a proliferative response in the absence of other growth factors. We show that phospholipase C gamma, phosphatidylinositol 3-kinase/p70 S6 kinase, Shc, Grb2, Raf-1, and mitogen-activated protein kinase are downstream components of the Nyk/Mer signal transduction pathways. In addition, Nyk/Mer weakly activates p90rsk, while stress-activated protein kinase, Ras GTPase-activating protein (GAP), and GAP-associated p62 and p190 proteins are not activated or tyrosine phosphorylated by Nyk/Mer. An analysis comparing the Nyk/Mer signal cascade with that of the epidermal growth factor receptor indicates substrate preferences by these two receptors. Our results provide a detailed description of the Nyk/Mer signaling pathways. Given the structural similarity between the Ufo/Axl family receptors, some of the information may also be applied to other members of this receptor tyrosine kinase family.

Immunoprecipitation of a phospholipase D activity with antiphosphotyrosine antibodies

When granulocyte-macrophage colony-stimulating factor (GM-CSF)-treated human neutrophils were challenged with the chemotactic factor fMet-Leu-Phe, it was possible to detect a time-dependent increase in the hydrolytic (as measured by the production of phosphatidic acid, PA) and the transphosphatidylation (as measured by the production of phosphatidylethanol, PEt) activities of phospholipase D in intact cells prelabeled with a radioactive fatty acid. Both activities were inhibited by preincubation of cells with genistein. Appropriate conditions were developed to test the PLD transphosphatidylation activity against exogenous phosphatidylcholine (PCho) in an in vitro system. As in intact cells, increased PLD activity could be detected in cell lysates obtained from fMet-Leu-Phe-treated cells compared with controls. When lysates were immunoprecipitated with antiphosphotyrosine antibodies, a PLD activity was found only in immune complexes that were prepared from fMet-Leu-Phe-treated cells. Conversely, no activity was found in lysates immunoprecipitated with an irrelevant antibody (GTPase-activating protein, GAP) that nevertheless was able to recognize a tyrosylphosphorylated form of GAP, as demonstrated by western blotting. These data suggest that a PCho-PLD, or a tightly bound protein, is tyrosine phosphorylated during cell activation.

EGF-induced increase in diacylglycerol, choline release, and DNA synthesis is extracellular calcium dependent

Previous studies have demonstrated a strict extracellular Ca2+ dependence for the G0 to G1 and G1 to S transition in growth factor-treated T51B rat liver cells that is associated with increased levels of protein kinase C activity. Consequently, we have examined these cells for changes in phospholipid-derived second messengers in response to epidermal growth factor (EGF) and thrombin in order to determine which signals are generated during the initiation of the G0 to G1 transition. Thrombin is coupled to a phosphoinositide hydrolyzing phospholipase C, as we have found a rapid Ca(2+)-independent increase in the levels of inositol 1,4,5-trisphosphate (Ins[1,4,5]P3), inositol 1,4-bisphosphate (Ins[1,4]P2), and inositol 4-monophosphate (Ins[4]P), as well as a concomitant, transient elevation in diacylglycerol. No changes in either intracellular or extracellular choline metabolites, or an increase in DNA synthesis, were found in response to thrombin. By contrast, treatment of T51B cells with EGF results in a slower, more prolonged extracellular Ca(2+)-dependent increase in both [3H]-glycerol radiolabeled diacyl-glycerol, and diacylglycerol mass, an increase in choline release into the extracellular medium, and eventually a substantial DNA synthesis. We were, however, unable to detect any changes in phosphatidylinositol (PtdIns) turnover, either by accumulation of inositol phosphates or by changes in phospholipids in response to EGF. These results indicate that DNA synthesis can readily occur in the absence of stimulated PtdIns turnover, and that PtdIns turnover is not sufficient in itself or necessary to induce DNA synthesis and is not necessary for a Ca(2+)-dependent increase in diacylglycerol. Moreover, we have demonstrated that the extracellular Ca(2+)-dependent increase in diacylglycerol levels in response to EGF is associated with an increase in extracellular choline release, which is indicative of an activation of a phosphatidylcholine-linked phospholipase D. These results suggest that diacylglycerol sources other than PtdIns's may be important in the extracellular Ca(2+)-dependent regulation of EGF-mediated cell replication.

EGF and TPA stimulate de novo synthesis of PGHS-1 and PGHS-2 through different signal transduction pathways

Epidermal growth factor (EGF) as well as phorbol 12-myristate 13-acetate (TPA) stimulate de novo synthesis of PGHS (prostaglandin H synthase)-1 and PGHS-2 mRNA, resulting in increased production of PGE2 in rat tracheal epithelial cells (RTE, EGV-6 cells). Stimulation of PGE2 production by TPA is more potent than that by EGF. Staurosporine and H-7, protein kinase C (PKC) inhibitors, suppressed the increase of mRNA and PGE2 levels caused by TPA, but not that caused by EGF. On the other hand, methyl 2,5-dihydroxycinnamate, a tyrosine kinase inhibitor (TKI), suppressed the increase of mRNA and PGE2 levels caused by EGF, but not that caused by TPA. These results indicate that EGF stimulates de novo synthesis of PGHS-1 and PGHS-2 mRNA through a signal transduction pathway which is independent from PKC-associated mechanisms but dependent upon the tyrosine kinase activity of the EGF 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.

Convergence of angiotensin II and platelet-derived growth factor receptor signaling cascades in vascular smooth muscle cells

Signaling cascades elicited by angiotensin II resemble those characteristic of growth factor stimulation. In this report, we demonstrate that angiotensin II converges with platelet-derived growth factor (PDGF) beta-receptor signaling cascades, independent of PDGF. Stimulation of smooth muscle cells with angiotensin II resulted in tyrosine phosphorylation on Shc proteins and subsequent complex formation between Shc and growth factor receptor binding protein-2 (GRB2). A 180-kDa protein co-precipitating with Shc.GRB2 complexes also demonstrated increased phosphorylation in response to angiotensin II. Immunoblot analyses and proteolytic digests failed to distinguish this 180-kDa protein from authentic PDGF beta-receptors. Corresponding with Shc and PDGF receptor phosphorylation induced by angiotensin II was the recruitment and phosphorylation of c-Src. Autocrine release of platelet-derived growth factor failed to account for Shc complex formation at the PDGF receptor following angiotensin II treatment, and a specific angiotensin II type I receptor antagonist, losartan, abolished the response. These results support a novel model for cross-talk between the G-protein-linked angiotensin II receptor and the PDGF receptor tyrosine kinase in vascular smooth muscle cells. Communication with the PDGF receptor may account for the ability of angiotensin II to elicit responses typical of growth factor signal transduction.

Role of heterotrimeric G-proteins in epidermal growth factor signalling

Since in 1986 it was reported that a pertussis toxin-sensitive substrate was involved in the Ca2+ signal induced by epidermal growth factor (EGF) in rat hepatocytes, much evidence accumulated to implicate heterotrimeric G-proteins in EGF action. EGF can also induce a cyclic AMP signal, but while the generation of a Ca2+ signal appears to be quite general in EGF action, the increase in cyclic AMP occurs only in few cell types. In non-transformed cell types these effects appear to involve G-proteins. EGF not only induces cell proliferation but also interacts with hormones in the short-term control of cell function in quiescent cells. Most of the known interactions are on cyclic AMP mediated hormone effects, and in many cases, the interaction between EGF and hormones involves G-proteins. Here we review the evidence accumulated in recent years that implicate G-proteins in EGF action. An understanding of the mechanisms involved may reveal new mechanisms of G-protein regulation and will contribute to our knowledge of EGF function and signal transduction.

Cell density-dependent regulation of PLC gamma 1 tyrosine phosphorylation and catalytic activity in an intestinal cell line (IEC-6)

Administration of epidermal growth factor (EGF) to rats has been shown to induce both mitogenic and nonmitogenic responses in the intestine. The mechanisms to describe a multiplicity of hormonal responses within a single tissue are unclear but likely involve selectivity among receptor substrates. A nontransformed rat jejunal crypt intestinal epithelial cell line (IEC-6) was studied to determine if the regulation of receptor tyrosine kinase substrates is affected by cell population physiology. EGF stimulated a rapid increase in inositol trisphosphate in confluent but not subconfluent cells. Similarly, treatment of confluent IEC-6 cells with EGF provoked a significant increase in the hydrolysis of PtdIns 4,5-P2 by immunoisolated PLC gamma 1. The tyrosine phosphorylation state of PLC gamma 1 and the association of PLC gamma 1 with the EGF receptor were increased by EGF in confluent cells only. In contrast, the autophosphorylation state of the EGF receptor and the tyrosine phosphorylation state of another SH2-containing EGF receptor substrate SHC were increased by EGF regardless of cell density. Western blot analysis revealed equal protein expression of PLC gamma 1 in confluent and subconfluent cells. EGF receptor protein expression and ligand binding capacity were slightly increased in confluent compared to subconfluent cells. EGF regulation of PLC gamma 1, therefore, is regulated by physiological factors dependent on cell density in IEC-6 cells.

A minor tyrosine phosphorylation site located within the CAIN domain plays a critical role in regulating tissue-specific transformation by erbB kinase

Avian c-erbB encodes a protein that is homologous to the human epidermal growth factor receptor. Truncation of the amino-terminal, ligand-binding domain of this receptor results in an oncogene product which is a potent inducing agent for erythroleukemias but not fibrosarcomas in chickens. Here we show that mutation of a single tyrosine residue, p5, in the carboxyl terminus of the erbB oncogene product allows it to become sarcomagenic in vivo and to transform fibroblasts in vitro. Mutations of other autophosphorylation sites do not generate comparable effects. The increased transforming activity of the p5 mutant is accompanied by an elevated level of mitogen-activated protein kinase phosphorylation. By analogy to the human epidermal growth factor receptor, p5 is a minor autophosphorylation site and is located in a domain known to be involved in regulating calcium influx and receptor internalization (CAIN domain). This area of the erbB product has been found to be repeatedly deleted in various sarcomagenic avian erythroblastosis virus isolates. We precisely deleted the CAIN domain and also made point mutations of the acidic residues within the CAIN domain. In both cases, fibroblast-transforming potential is activated. We interpret these data to mean that p5 and its surrounding region negatively regulate fibroblast-transforming and sarcomagenic potential. To our knowledge, this represents the first point mutation of an autophosphorylation site that activates erbB oncogenicity.

Cellular proliferation and lipid metabolism: importance of lipoxygenases in modulating epidermal growth factor-dependent mitogenesis

In this article we have reviewed and discussed the results of our investigation of lipid metabolites as modulators of epidermal growth factor (EGF) signaling pathways. We have studied epidermal growth factor-dependent mitogenesis in BALB/c 3T3 and Syrian hamster embryo (SHE) cells in culture. We observed that EGF stimulates the formation of prostaglandins in BALB/c 3T3 cells and their formation appears to be necessary for EGF dependent mitogenesis. EGF did not stimulate PGE2 formation in SHE cells and in fact, exogenously added PGE2 inhibited mitogenesis. In both cell lines, EGF stimulated the formation of lipoxygenase-derived 13(S)-hydroxyoctadecadienoic acid (13-HODE) and inhibition of 13-HODE formation attenuated mitogenesis. The addition of 13-(S)-HODE enhanced EGF-dependent mitogenesis but when added alone, the compound was not mitogenic. Other metabolites, including lipoxygenase metabolites of arachidonic acid, were either weak simulators of EGF-dependent mitogenesis or essentially inactive. The 13(S)-HODE appears to be formed by an apparently unique lipoxygenase that is regulated by the tyrosine kinase activity of the EGF receptor. The mechanisms by which lipids, particularly the lipoxygenase-derived linoleic acid metabolites, modulate the EGF signaling pathways leading to cell proliferation is discussed. The possible significance of lipoxygenase and prostaglandin H synthase-dependent metabolism of unsaturated fatty acids in breast and colon cancer is also discussed.