The platelet-derived growth factor beta receptor triggers multiple cytoplasmic signaling cascades that arrive at the nucleus as distinguishable inputs

Pediatric glioma immune profiling identifies TIM3 as a therapeutic target in BRAF fusion pilocytic astrocytoma

Despite being the leading cause of cancer-related childhood mortality, pediatric gliomas have been relatively understudied, and the repurposing of immunotherapies has not been successful. Whole-transcriptome sequencing, single-cell sequencing, and sequential multiplex immunofluorescence were used to identify an immunotherapeutic strategy that could be applied to multiple preclinical glioma models. MAPK-driven pediatric gliomas have a higher IFN signature relative to other molecular subgroups. Single-cell sequencing identified an activated and cytotoxic microglia (MG) population designated MG-Act in BRAF-fused, MAPK-activated pilocytic astrocytoma (PA), but not in high-grade gliomas or normal brain. T cell immunoglobulin and mucin domain 3 (TIM3) was expressed on MG-Act and on the myeloid cells lining the tumor vasculature but not normal brain vasculature. TIM3 expression became upregulated on immune cells in the PA microenvironment, and anti-TIM3 reprogrammed ex vivo immune cells from human PAs to a proinflammatory cytotoxic phenotype. In a genetically engineered murine model of MAPK-driven, low-grade gliomas, anti-TIM3 treatment increased median survival over IgG- and anti-PD-1-treated mice. Single-cell RNA-Seq data during the therapeutic window of anti-TIM3 revealed enrichment of the MG-Act population. The therapeutic activity of anti-TIM3 was abrogated in mice on the CX3CR1 MG-KO background. These data support the use of anti-TIM3 in clinical trials of pediatric low-grade, MAPK-driven gliomas.

The role of platelet-derived growth factor BB signaling pathway in the regulation of stem and progenitor Leydig cell proliferation and steroidogenesis in male rats

Platelet-derived growth factor BB (BB) regulates cell proliferation and function. However, the roles of BB on proliferation and function of Leydig stem (LSCs) and progenitor cells (LPCs) and the underlying signaling pathways remain unclear. This study aimed to analyze the roles of PI3K and MAPK pathways in the regulation of proliferation-related and steroidogenesis-related gene expression in rat LSCs/LPCs. In this experiment, BB receptor antagonist, tyrosine kinase inhibitor IV (PKI), the PI3K inhibitor, LY294002, and the MEK inhibitor, U0126, were used to measure the effects of these pathways on the expression of cell cycle-related genes (Ccnd1 and Cdkn1b) and steroidogenesis-related genes (Star, Cyp11a1, Hsd3b1, Cyp17a1, and Srd5a1), as well as Leydig cell maturation gene Pdgfra [1]. These results showed that BB (10 ng/mL)-stimulated EdU-incorporation into LSCs and BB-mediated inhibition on its differentiation was mediated through the activation of its receptor, PDGFRB, as well as MAPK and PI3K pathways. The results of LPC experiment also showed that LY294002 and U0126 decreased BB (10 ng/mL)-upregulated Ccnd1 expression while only U0126 reversed BB (10 ng/mL)-downregulated Cdkn1b expression. U0126 significantly reversed BB (10 ng/mL)-mediated downregulation of Cyp11a1, Hsd3b1, and Cyp17a1 expression. On the other hand, LY294002 reversed the expression of Cyp17a1 and Abca1. In conclusion, BB-mediated induction of proliferation and suppression of steroidogenesis of LSCs/LPCs are dependent on the activation of both MAPK and PI3K pathways, which show distinct regulation of gene expression.

The Role of G Protein-Coupled Receptor Kinase 6 Regulation in Inflammation and Pain

The G protein-coupled receptor kinase 6 is associated with inflammation and pathological pain. Impairment of GRK6 expression was described in chronic inflammatory diseases such as rheumatoid arthritis and this was shown to be accompanied by an imbalance of downstream signaling pathways. Here, we discuss novel aspects of GRK6 interaction and its impact upon hyperalgesia and inflammatory processes. In this review, we compile important findings concerning GRK6 regulation for a better pathophysiological understanding of the intracellular interaction in the context of inflammation and show clinical implications-for example, the identification of possible therapy goals in the treatment of chronic inflammatory hyperalgesia.

Diverse roles of tumor-stromal PDGFB-to-PDGFRβ signaling in breast cancer growth and metastasis

Over the last couple of decades, it has become increasingly apparent that the tumor microenvironment (TME) mediates every step of cancer progression and solid tumors are only able to metastasize with a permissive TME. This intricate interaction of cancer cells with their surrounding TME, or stroma, is becoming more understood with an ever greater knowledge of tumor-stromal signaling pairs such as platelet-derived growth factors (PDGF) and their cognate receptors. We and others have focused our research efforts on understanding how tumor-derived PDGFB activates platelet-derived growth factor receptor beta (PDGFRβ) signaling specifically in the breast cancer TME. In this chapter, we broadly discuss PDGF and PDGFR expression patterns and signaling in normal physiology and breast cancer. We then detail the expansive roles played by the PDGFB-to-PDGFRβ signaling pathway in modulating breast tumor growth and metastasis with a focus on specific cellular populations within the TME, which are responsive to tumor-derived PDGFB. Given the increasingly appreciated importance of PDGFB-to-PDGFRβ signaling in breast cancer progression, specifically in promoting metastasis, we end by discussing how therapeutic targeting of PDGFB-to-PDGFRβ signaling holds great promise for improving current breast cancer treatment strategies.

Resistance Mechanisms to Anti-angiogenic Therapies in Cancer

Tumor growth and metastasis rely on tumor vascular network for the adequate supply of oxygen and nutrients. Tumor angiogenesis relies on a highly complex program of growth factor signaling, endothelial cell (EC) proliferation, extracellular matrix (ECM) remodeling, and stromal cell interactions. Numerous pro-angiogenic drivers have been identified, the most important of which is the vascular endothelial growth factor (VEGF). The importance of pro-angiogenic inducers in tumor growth, invasion and extravasation make them an excellent therapeutic target in several types of cancers. Hence, the number of anti-angiogenic agents developed for cancer treatment has risen over the past decade, with at least eighty drugs being investigated in preclinical studies and phase I-III clinical trials. To date, the most common approaches to the inhibition of the VEGF axis include the blockade of VEGF receptors (VEGFRs) or ligands by neutralizing antibodies, as well as the inhibition of receptor tyrosine kinase (RTK) enzymes. Despite promising preclinical results, anti-angiogenic monotherapies led only to mild clinical benefits. The minimal benefits could be secondary to primary or acquired resistance, through the activation of alternative mechanisms that sustain tumor vascularization and growth. Mechanisms of resistance are categorized into VEGF-dependent alterations, non-VEGF pathways and stromal cell interactions. Thus, complementary approaches such as the combination of these inhibitors with agents targeting alternative mechanisms of blood vessel formation are urgently needed. This review provides an updated overview on the pathophysiology of angiogenesis during tumor growth. It also sheds light on the different pro-angiogenic and anti-angiogenic agents that have been developed to date. Finally, it highlights the preclinical evidence for mechanisms of angiogenic resistance and suggests novel therapeutic approaches that might be exploited with the ultimate aim of overcoming resistance and improving clinical outcomes for patients with cancer.

Regulator of G-protein signaling 5 regulates the shift from perivascular to parenchymal pericytes in the chronic phase after stroke

Poststroke recovery requires multiple repair mechanisms, including vascular remodeling and blood-brain barrier (BBB) restoration. Brain pericytes are essential for BBB repair and angiogenesis after stroke, but they also give rise to scar-forming platelet-derived growth factor receptor β (PDGFR-β)–expressing cells. However, many of the molecular mechanisms underlying this pericyte response after stroke still remain unknown. Regulator of G-protein signaling 5 (RGS5) has been associated with pericyte detachment from the vascular wall, but whether it regulates pericyte function and vascular stabilization in the chronic phase of stroke is not known. Using RGS5–knockout (KO) mice, we study how loss of RGS5 affects the pericyte response and vascular remodeling in a stroke model at 7 d after ischemia. Loss of RGS5 leads to a shift toward an increase in the number of perivascular pericytes and reduction in the density of parenchymal PDGFR-β–expressing cells associated with normalized PDGFR-β activation after stroke. The redistribution of pericytes resulted in higher pericyte coverage, increased vascular density, preservation of vessel lengths, and a significant reduction in vascular leakage in RGS5-KO mice compared with controls. Our study demonstrates RGS5 in pericytes as an important target to enhance vascular remodeling.—Roth, M., Gaceb, A., Enström, A., Padel, T., Genové, G., Özen, I., Paul, G. Regulator of G-protein signaling 5 regulates the shift from perivascular to parenchymal pericytes in the chronic phase after stroke.

Resistance to Anti-Angiogenic Therapy in Cancer-Alterations to Anti-VEGF Pathway

Anti-angiogenic therapy is one of the promising strategies for many types of solid cancers. Bevacizumab (Avastin), a recombinant humanized monoclonal antibody of vascular endothelial growth factor (VEGF) A, was approved for the first time as an anti-angiogenic drug for the treatment of metastatic colorectal cancer (CRC) by the Food and Drug Administration (FDA) in 2004. In addition, the other VEGF pathway inhibitors including small molecule tyrosine kinase inhibitors (sunitinib, sorafenib, and pazopanib), a soluble VEGF decoy receptor (aflibercept), and a humanized monoclonal antibody of VEGF receptor 2 (VEGFR2) (ramucirumab) have been approved for cancer therapy. Although many types of VEGF pathway inhibitors can improve survival in most cancer patients, some patients have little or no beneficial effect from them. The primary or acquired resistance towards many oncological drugs, including anti-VEGF inhibitors, is a common problem in cancer treatment. This review summarizes the proposed alternative mechanisms of angiogenesis other than the VEGF pathway. These mechanisms are involved in the development of resistance to anti-VEGF therapies in cancer patients.

An integrated approach to infer cross-talks between intracellular protein transport and signaling pathways

Background

The endomembrane system, known as secretory pathway, is responsible for the synthesis and transport of protein molecules in cells. Therefore, genes involved in the secretory pathway are essential for the cellular development and function. Recent scientific investigations show that ER and Golgi apparatus may provide a convenient drug target for cancer therapy. On the other hand, it is known that abundantly expressed genes in different cellular organelles share interconnected pathways and co-regulate each other activities. The cross-talks among these genes play an important role in signaling pathways, associated to the regulation of intracellular protein transport.

Results

In the present study, we device an integrated approach to understand these complex interactions. We analyze gene perturbation expression profiles, reconstruct a directed gene interaction network and decipher the regulatory interactions among genes involved in protein transport signaling. In particular, we focus on expression signatures of genes involved in the secretory pathway of MCF7 breast cancer cell line. Furthermore, network biology analysis delineates these gene-centric cross-talks at the level of specific modules/sub-networks, corresponding to different signaling pathways.

Conclusions

We elucidate the regulatory connections between genes constituting signaling pathways such as PI3K-Akt, Ras, Rap1, calcium, JAK-STAT, EFGR and FGFR signaling. Interestingly, we determine some key regulatory cross-talks between signaling pathways (PI3K-Akt signaling and Ras signaling pathway) and intracellular protein transport.

Electronic supplementary material

The online version of this article (10.1186/s12859-018-2036-2) contains supplementary material, which is available to authorized users.

Platelet gel: a new therapeutic tool with great potential

Chronic wounds, such as diabetic foot ulcers, represent a serious clinical problem for patients and clinicians. Management of these wounds has a strong economic impact worldwide. Complications resulting from injuries are a frequent cause of morbidity and mortality. Chronic wounds lead to infections, painful dressings and prolonged hospitalisation. This results in poor patient Quality of Life and in high healthcare costs. Platelet concentrates (PC) are defined as autologous or allogeneic platelet derivatives with a platelet concentration higher than baseline. PC are widely used in different areas of Regenerative Medicine in order to enhance wound healing processes; they include platelet-rich plasma (PRP), platelet gel (PG), platelet-rich fibrin (PRF), serum eye drops (E-S), and PRP eye drops (E-PRP). This review highlights the use of platelet-rich plasma (PRP) and platelet gel (PG) preparation for clinical use.

Human Serum Eye Drops in Eye Alterations: An Insight and a Critical Analysis

Human serum contains a physiological plethora of bioactive elements naturally released by activated platelets which might have a significant effect on the regeneration of corneal layers by stimulating the cell growth. This mechanism supported the use of human serum eye drops in some ocular diseases associated with dystrophic changes and alterations of the tear film, such as persistent corneal epithelial defects and dry eye syndrome. We focused our effort on potential benefits and limitations of the use of human serum eye drops when conventional therapies failed. We reviewed the recent literature by reporting published studies from 2010 to 2014. Despite the limited evaluated study populations, most of the clinical studies have confirmed that serum eye drop therapy is effective in corneal healing by reducing ocular symptom, particularly during the short-term follow-up. In addition, three recent published studies have shown the efficacy of the serum eye drop therapy in comparison to traditional ones in intractable patients. Besides, reported ongoing clinical studies confirmed the open debate regarding the use of biologic tools for cornea regeneration. Results from these studies might open novel challenges and perspectives in the therapy of such refractory patients.

RNA-Seq analysis of high NaCl-induced gene expression

High extracellular NaCl is known to change expression of numerous genes, many of which are regulated by the osmoprotective transcription factor nuclear factor of activated T cells-5 (NFAT5). In the present study we employed RNA-Seq to provide a comprehensive, unbiased account of genes regulated by high NaCl in mouse embryonic fibroblast cells (MEFs). To identify genes regulated by NFAT5 we compared wild-type MEFs (WT-MEFs) to MEFs in which mutation of the NFAT5 gene inhibits its transcriptional activity (Null-MEFs). In WT-MEFs adding NaCl to raise osmolality from 300 to 500 mosmol/kg for 24 h increases expression of 167 genes and reduces expression of 412. Raising osmolality through multiple passages (adapted cells) increases expression of 196 genes and reduces expression of 528. In Null-MEFs, after 24 h of high NaCl, expression of 217 genes increase and 428 decrease, while in adapted Null-MEFs 143 increase and 622 decrease. Fewer than 10% of genes are regulated in common between WT- and null-MEFs, indicating a profound difference in regulation of high-NaCl induced genes induced by NFAT5 compared with those induced in the absence of NFAT5. Based on our findings we suggest a mechanism for this phenomenon, which had previously been unexplained. The NFAT5 DNA-binding motif (osmotic response element) is overrepresented in the vicinity of genes that NFAT5 upregulates, but not genes that it downregulates. We used Gene Ontology and manual curation to determine the function of the genes targeted by NFAT5, revealing many novel consequences of NFAT5 transcriptional activity.

Platelet derivatives in regenerative medicine: an update

Prior preclinical and clinical studies support the use of platelet-derived products for the treatment of soft and hard tissue lesions. These regenerative effects are controlled by autocrine and paracrine biomolecules including growth factors and cytokines contained in platelet alpha granules. Each growth factor is involved in a phase of the healing process, such as inflammation, collagen synthesis, tissue granulation, and angiogenesis collectively promoting tissue restitution. Platelet derivatives have been prepared as platelet-rich plasma, platelet gel, platelet-rich fibrin, and platelet eye drops. These products vary in their structure, growth factors, composition, and cytokine concentrations. Here, we review the current use of platelet-derived biological products focusing on the rationale for their use and the main requirements for their preparation. Variation in the apparent therapeutic efficacy may have resulted from a lack of reproducible, standardized protocols for preparation. Despite several individual studies showing favorable treatment effects, some randomized controlled trials as well as meta-analyses have found no constant clinical benefit from the application of platelet-derived products for prevention of tissue lesions. Recently, 3 published studies in dentistry showed an improvement in bone density. Seven published studies showed positive results in joint regeneration. Five published studies demonstrated an improvement in the wound healing, and an improvement of eye epithelial healing was observed in 2 reports. Currently, at least 14 ongoing clinical trials in phase 3 or 4 have been designed with large groups of treated patients (n > 100). Because the rationale of the therapy with platelet-derived compounds is still debated, a definitive insight can be acquired only when these large randomized trials will be completed.

Crenolanib, a PDGFR inhibitor, suppresses lung cancer cell proliferation and inhibits tumor growth in vivo

Platelet-derived growth factor (PDGF) and its receptors (PDGFR), including PDGFRα and PDGFRβ, play important roles in tumorigenesis, tumor progression, and the regulation of stromal cell function. Constitutive activation of PDGFR signaling, gene rearrangement, and activating mutations of PDGFR have been identified in various types of human tumors and malignancies. PDGFRα and PDGFRβ belong to the family of type III receptor tyrosine kinases and, upon stimulation, activate downstream signaling cascades. Crenolanib is a specific tyrosine kinase inhibitor that targets and inhibits the kinase activity of PDGFR and the FMS-related tyrosine kinase 3. Its clinical efficacy in several human tumors is currently under investigation in Phase II clinical trials. In this study, we examined the potential role of crenolanib in the treatment of non-small-cell lung cancer (NSCLC). Using A549 cells as a model system, we have shown that crenolanib is capable of suppressing proliferation and inducing apoptosis in a dose-dependent manner. Crenolanib-treated cells have reduced migratory activity in response to inducers of chemotaxis. Furthermore, the in vivo antitumor activity of crenolanib was confirmed in an NSCLC xenograft tumor model. Injection of crenolanib significantly inhibited the growth of tumor mass by inducing apoptosis in tumor cells. Our results provide strong evidence supporting the use of crenolanib as a potential therapeutic agent in treating NSCLC. This work sets a foundation for further development of targeted and personalized therapeutics for lung cancer.

Imatinib mesylate for the treatment of pulmonary arterial hypertension

INTRODUCTION

Despite recent advances, pulmonary arterial hypertension (PAH) remains a devastating disease which harbors a poor prognosis. Novel therapeutic approaches directly targeting pulmonary vascular remodeling are warranted.

AREAS COVERED

This review delineates the current limitations in the management of PAH and focuses on a novel, anti-proliferative therapeutic concept. It will help readers understand the mechanisms of receptor tyrosine kinase signaling, with a special focus on platelet-derived growth factor (PDGF) receptors and their role in the pathobiology of PAH. Furthermore, it provides a comprehensive summary regarding the rationale, efficacy and safety of the tyrosine kinase inhibitor imatinib mesylate , which potently inhibits the PDGF receptor, as an additional treatment option in PAH.

EXPERT OPINION

PDGF is a potent mitogen for pulmonary vascular smooth muscle cells and represents an important mediator of pulmonary vascular remodeling. Imatinib mesylate, a compound that inhibits the Bcr-Abl kinase and was developed for the treatment of chronic myeloid leukemia, also targets PDGF receptors. Both experimental and clinical data indicate that it reverses the vascular remodeling process even when it is fully established. Results from Phase II and III clinical trials suggest potent and prolonged efficacy in patients with severe PAH (i.e., pulmonary vascular resistance > 800 dynes*s*cm(-5)). Future studies should evaluate the long-term clinical efficacy and safety of imatinib, including patients with less impaired hemodynamics. Based on the current knowledge, this compound is likely to become an additional treatment option for patients with PAH and has the potential to at least partially correct the pathology of the disease.

EGFR and beta1 integrins utilize different signaling pathways to activate Akt

Akt, also called PKB, is a serine/threonine kinase that plays a major role in cell survival. It can be activated by several cellular receptors, including integrins and growth factor receptors, in PI3K-dependent manners. In this study, we analyzed the two current models for Akt activation upon beta1 integrin-mediated adhesion: via focal adhesion kinase and via transactivation of the EGF receptor. Distinct differences in the pathways leading to phosphorylation and activation of Akt from stimulated beta1 integrins and EGF receptor were observed, including opposing sensitivity to the tyrosine kinase inhibitors PP2 and Gefitinib. Using knockout cells and integrin mutant cells, we show that beta1 integrins can induce phosphorylation of Akt at Ser473 and Thr308 and Akt kinase activity independently of the EGF receptor activity, focal adhesion kinase, and the Src family members. In contrast to stimulation with EGF, beta1 integrin-mediated adhesion did not induce Akt tyrosine phosphorylation. Moreover, tyrosine phosphorylation of Akt was found not to be required for its catalytic activity. The results identify a previously unrecognized mechanism by which beta1 integrins activate the PI3K/Akt pathway.

Angiotensin II stimulates phosphorylation of an ectodomain-truncated platelet-derived growth factor receptor-beta and its binding to class IA PI3K in vascular smooth muscle cells

PI3K (phosphoinositide 3-kinase) activity is involved in Ang (angiotensin) II-stimulated VSMC (vascular smooth muscle cell) growth and hypertrophy. In the present study, we demonstrate that the inhibition of PI3K in VSMCs by expression of a dominant-negative p85alpha mutant lacking the p110-binding domain (Deltap85), or by treatment of cells with LY294002, inhibited Ang II-stimulated PAI-1 (plasminogen activator inhibitor-1) mRNA expression. Using a GST (glutathione S-transferase) fusion protein containing the p85 N-terminal SH2 (Src homology 2) domain as 'bait' followed by MS/MS (tandem MS), we identified a 70 kDa fragment of the p70 PDGFR-beta (platelet-derived growth factor receptor-beta) as a signalling adapter that is phosphorylated and recruits the p85 subunit of PI3K after Ang II stimulation of AT1 (Ang II subtype 1) receptors on VSMCs. This fragment of the PDGFR-beta, which has a truncation of its extracellular domain, accounted for approx. 15% of the total PDGFR-beta detected in VSMCs with an antibody against its cytoplasmic domain. Stimulation of VSMCs with Ang II increased tyrosine-phosphorylation of p70 PDGFR-beta at Tyr751 and Tyr1021 and increased its binding to p85. PDGF also induced phosphorylation of p70 PDGFR-beta, a response inhibited by the PDGF tyrosine kinase selective inhibitor, AG1296. By contrast, Ang II-induced phosphorylation of the 70 kDa receptor was not affected by AG1296. Ang II-stimulated phosphorylation of the p70 PDGFR-beta was blocked by the AT1 receptor antagonist, candesartan (CV 11974) and was partially inhibited by PP2 {4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine}, an Src family kinase inhibitor. Our result suggests that the p70 PDGFR-beta functions as an adapter that recruits PI3K to the membrane upon AT1 receptor stimulation.

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

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

Phosphotyrosine-binding domains in signal transduction

Protein phosphorylation provides molecular control of complex physiological events within cells. In many cases, phosphorylation on specific amino acids directly controls the assembly of multi-protein complexes by recruiting phospho-specific binding modules. Here, the function, structure, and cell biology of phosphotyrosine-binding domains is discussed.

Erythropoietin activates two distinct signaling pathways required for the initiation and the elongation of c-myc

Erythropoietin (Epo) stimulation of erythroid cells results in the activation of several kinases and a rapid induction of c-myc expression. Protein kinase C is necessary for Epo up-regulation of c-myc by promoting elongation at the 3'-end of exon 1. PKCepsilon mediates this signal. We now show that Epo triggers two signaling pathways to c-myc. Epo rapidly up-regulated Myc protein in BaF3-EpoR cells. The phosphatidylinositol 3-kinase (PI3K) inhibitor LY294002 blocked Myc up-regulation in a concentration-dependent manner but had no effect on the Epo-induced phosphorylation of ERK1 and ERK2. LY294002 also had no effect on Epo up-regulation of c-fos. MEK1 inhibitor PD98059 blocked both the c-myc and the c-fos responses to Epo. PD98059 and the PKC inhibitor H7 also blocked the phosphorylation of ERK1 and ERK2. PD98059 but not LY294002 inhibited Epo induction of ERK1 and ERK2 phosphorylation in normal erythroid cells. LY294002 blocked transcription of c-myc at exon 1. PD98059 had no effect on transcription from exon 1 but, rather, blocked Epo-induced c-myc elongation at the 3'-end of exon 1. These results identify two Epo signaling pathways to c-myc, one of which is PI3K-dependent operating on transcriptional initiation, whereas the other is mitogen-activated protein kinase-dependent operating on elongation.

Phospholipase C-gamma1 is required for the induction of immediate early genes by platelet-derived growth factor

To explore the functional role of phospholipase C-gamma1 (PLC-gamma1) in the induction of immediate early genes (IEGs), we have examined the influence of Plcg1 gene disruption on the expression of 14 IEG mRNAs induced by platelet-derived growth factor (PDGF). Plcg1-null embryos were used to produce immortalized fibroblasts genetically deficient in PLC-gamma1 (Null cells), and retroviral infection of those cells was used to derive PLC-gamma1 re-expressing cells (Null+ cells). In terms of PDGF activation of PDGF receptor tyrosine phosphorylation as well as the mitogen-activated protein kinases Erk1 and Erk2, Null and Null+ cells responded equivalently. However, the PDGF-dependent expression of all IEG mRNAs was diminished in cells lacking PLC-gamma1. The expression of FIC, COX-2, KC, JE, and c-fos mRNAs were most strongly compromised, as the stimulation of these genes was reduced by more than 90% in cells lacking PLC-gamma1. The combination of PMA and ionomycin, downstream analogs of PLC activation, did provoke expression of mRNAs for these IEGs in the Null cells. We conclude that PLC-gamma1 is necessary for the maximal expression of many PDGF-induced IEGs and is essential for significant induction of at least five IEGs.

SRF-dependent gene expression is required for PI3-kinase-regulated cell proliferation

Recent evidence indicates that phosphatidylinositol 3-kinase (PI3K) is a central regulator of mitosis, apoptosis and oncogenesis. Nevertheless, the mechanisms by which PI3K regulates proliferation are not well characterized. Mitogens stimulate entry into the cell cycle by inducing the expression of immediate early genes (IEGs) that in turn trigger the expression of G(1) cyclins. Here we describe a novel PI3K- regulated transcriptional cascade that is critical for mitogen regulation of the IEG, c-fos. We show that PI3K activates gene expression by transactivating SRF-dependent transcription independently of the previously described Rho and ETS TCF pathways. PI3K-stimulated cell cycle progression requires transactivation of SRF and expression of dominant- negative PI3K blocks mitogen-stimulated cell cycle progression. Furthermore, dominant-interfering SRF mutants attenuate mitogen-stimulated cell cycle progression, but are without effect on MEK-stimulated cell cycle entry. Moreover, expression of constitutively active SRF is sufficient for cell cycle entry. Thus, we delineate a novel SRF-dependent mitogenic cascade that is critical for PI3K- and growth factor-mediated cell cycle progression.

Phospholipase C-gamma as a signal-transducing element

A ubiquitous signaling event in hormonal responses is the phospholipase C (PLC)-catalyzed hydrolysis of phosphatidylinositol 4, 5-bisphosphate to produce the metabolite second messenger molecules inositol 1,4,5-trisphosphate and diacylglycerol. The former provokes a transient increase in intracellular free Ca(2+), while the latter serves as a direct activator of protein kinase C. In tyrosine kinase-dependent signaling pathways this reaction is mediated by the PLC-gamma isozymes. These are direct substrates of many tyrosine kinases in a wide variety of cell types. The mechanism of PLC-gamma activation involves its association with and phosphorylation by receptor and non-receptor tyrosine kinases, as well as interaction with specialized adaptor molecules and, perhaps, other second messenger molecules. However, the biochemistry of PLC-gamma is at a more advanced state than a clear understanding of exactly how this signaling element functions in the generation of a mitogenic response.

Platelet-derived growth factor-induced activation of sphingosine kinase requires phosphorylation of the PDGF receptor tyrosine residue responsible for binding of PLCgamma

Sphingosine-1-phosphate, a sphingolipid metabolite, is involved in the mitogenic response of platelet-derived growth factor (PDGF) and is formed by activation of sphingosine kinase. We examined the effect of PDGF on sphingosine kinase activation in TRMP cells expressing wild-type or various mutant betaPDGF receptors. Sphingosine kinase was stimulated by PDGF in cells expressing wild-type receptors but not in cells expressing kinase-inactive receptors (R634). Cells expressing mutated PDGF receptors with phenylalanine substitutions at five major tyrosine phosphorylation sites 740/751/771/1009/1021 (F5 mutants), which are unable to associate with PLCgamma, phosphatidylinositol 3-kinase, Ras GTPase-activating protein, or protein tyrosine phosphatase SHP-2, not only failed to increase DNA synthesis in response to PDGF but also did not activate sphingosine kinase. Moreover, mutation of tyrosine-1021 of the PDGF receptor to phenylalanine, which impairs its association with PLCgamma, abrogated PDGF-induced activation of sphingosine kinase. In contrast, PDGF was still able to stimulate sphingosine kinase in cells expressing the PDGF receptor mutated at tyrosines 740/751 and 1009, responsible for binding of phosphatidylinositol 3-kinase and SHP-2, respectively. In agreement, PDGF did not stimulate sphingosine kinase activity in F5 receptor 'add-back' mutants in which association with the Ras GTPase-activating protein, phosphatidylinositol 3-kinase, or SHP-2 was individually restored. However, a mutant PDGF receptor that was able to bind PLCgamma (tyrosine-1021), but not other signaling proteins, restored sphingosine kinase sensitivity to PDGF. These data indicate that the tyrosine residue responsible for binding of PLCgamma is required for PDGF-induced activation of sphingosine kinase. Moreover, calcium mobilization downstream of PLCgamma, but not protein kinase C activation, appears to be required for stimulation of sphingosine kinase by PDGF.-Olivera, A., Edsall, J., Poulton, S., Kazlauskas, A., Spiegel, S. Platelet-derived growth factor-induced activation of sphingosine kinase requires phosphorylation of the PDGF receptor tyrosine residue responsible for binding of PLCgamma.

Serine phosphorylation of the ligand-activated beta-platelet-derived growth factor receptor by casein kinase I-gamma2 inhibits the receptor's autophosphorylating activity

Platelet-derived growth factor (PDGF) receptors (PDGFRs) are membrane protein-tyrosine kinases that, upon activation, become tyrosine-phosphorylated and associate with numerous SH2 domain-containing molecules involved in mediating signal transduction. In Rat-2 fibroblasts, we have characterized the phosphorylation of the beta-PDGFR following its activation by PDGF. In contrast to tyrosine phosphorylation, which was transient and returned to near basal levels by 30 min, PDGF-stimulated Ser/Thr phosphorylation of the beta-PDGFR was increased by 5 min and remained elevated after 30 min. In vivo, after 5 min of PDGF stimulation, serine phosphorylation of the beta-PDGFR was greatly reduced by CKI-7, a specific inhibitor of casein kinase I (CKI). In vitro, recombinant CKI-gamma2 phosphorylated the ligand-activated beta-PDGFR on serine residues in a CKI-7-sensitive manner and resulted in a marked inhibition of the receptor's autophosphorylating activity. Furthermore, in Rat-2 fibroblasts, expression of hemagglutinin epitope-tagged active CKI-gamma2 resulted in a dramatic decrease in the tyrosine phosphorylation state of the beta-PDGFR in response to PDGF, consistent with receptor inactivation. Our data suggest that upon PDGF stimulation, CKI-gamma2 is activated and/or translocated in proximity to the beta-PDGFR, whereby it phosphorylates the beta-PDGFR on serine residues and negatively regulates its tyrosine kinase activity, leading to receptor inactivation.

Diverse signaling pathways activated by growth factor receptors induce broadly overlapping, rather than independent, sets of genes

We sought to explore the relationship between receptor tyrosine kinase (RTK) activated signaling pathways and the transcriptional induction of immediate early genes (IEGs). Using global expression monitoring, we identified 66 fibroblast IEGs induced by platelet-derived growth factor beta receptor (PDGFRbeta) signaling. Mutant receptors lacking binding sites for activation of the PLCgamma, PI3K, SHP2, and RasGAP pathways still retain partial ability to induce 64 of these IEGs. Removal of the Grb2-binding site further broadly reduces induction. These results suggest that the diverse pathways exert broadly overlapping effects on IEG induction. Interestingly, a mutant receptor that restores the RasGAP-binding site promotes induction of an independent group of genes, normally induced by interferons. Finally, we compare the PDGFRbeta and fibroblast growth factor receptor 1; each induces essentially identical IEGs in fibroblasts.

PDGF induces an early and a late wave of PI 3-kinase activity, and only the late wave is required for progression through G1

BACKGROUND

Platelet-derived growth factor (PDGF) triggers cytoskeletal rearrangements and chemotaxis within minutes. These events are at least in part due to the activation of phosphoinositide (PI) 3-kinase; there is good temporal correlation between these events and the accumulation of 3-phosphorylated products of the kinase. Prolonged and continuous PDGF exposure results in S-phase entry many hours after the initial burst of activity. Although early signals appear responsible for the early responses, they may not fully account for later responses, such as cell-cycle progression.

RESULTS

We assessed when PI 3-kinase products accumulate in PDGF-stimulated cells. In addition to the previously identified early accumulation of products, we detected a second, prolonged wave of accumulation 3-7 hours after stimulation. To determine the relative contribution of each phase to PDGF-dependent DNA synthesis, we first developed an assay in which synthetic 3-phosphorylated lipids were used to rescue DNA synthesis in cells expressing a PDGF-receptor mutant. The lipids rescued DNA synthesis only when added 2-6 hours after PDGF. In addition, PI 3-kinase inhibitors failed to block PDGF-dependent DNA synthesis if added during the first wave of PI 3-kinase activity, but adding them later, in G1 phase, prevented PDGF-dependent cell-cycle progression.

CONCLUSIONS

PDGF induces distinct waves of PI 3-kinase activity. The second wave is required for PDGF-dependent DNA synthesis, whereas the initial wave is not. One of the ways in which cells use PI 3-kinase to mediate distinct cellular responses seems to be by regulating when its products accumulate.

Disruption of gap junctional communication by the platelet-derived growth factor is mediated via multiple signaling pathways

The platelet-derived growth factor (PDGF) mediates its cellular functions via activation of its receptor tyrosine kinase followed by the recruitment and activation of several signaling molecules. These signaling molecules then initiate specific signaling cascades, finally resulting in distinct physiological effects. To delineate the PDGF signaling pathway responsible for the disruption of gap junctional communication (GJC), wild-type PDGF receptor beta (PDGFRbeta) and a series of PDGFRbeta mutants were expressed in T51B rat liver epithelial cells. In cells expressing wild-type PDGFRbeta, PDGF induced disruption of GJC and phosphorylation of a gap junctional protein, connexin-43 (Cx43), which required activation of mitogen-activated protein kinase, although involvement of additional factors was also evident. In the F5 mutant lacking binding sites for phosphatidylinositol 3-kinase, GTPase-activating protein, SHP-2, and phospholipase Cgamma1 (PLCgamma1), PDGF induced mitogen-activated protein kinase, but failed to affect GJC or Cx43, indicating involvement of additional signals presumably initiated by one or more of the mutated binding sites. Examination of the single-site mutants revealed that PDGF effects were not mediated via a single signaling component. This was confirmed by the "add-back" mutants, which showed that restoration of either SHP-2 or PLCgamma1 binding was sufficient to propagate the GJC inhibitory actions of PDGF. Further analysis showed that activation of PLCgamma1 is involved in Cx43 phosphorylation, which surprisingly failed to correlate with GJC blockade. The results of our study demonstrate that PDGF-induced disruption of GJC can be mediated by multiple signaling pathways and requires participation of multiple components.

Evidence for distinct signaling properties and biological responses induced by the PDGF receptor alpha and beta subtypes

Platelet-derived growth factor (PDGF) acts as a potent mitogen, chemoattractant and survival factor for mesenchymal cells. In addition to its importance in mammalian development, PDGF plays a critical role in physiological repair mechanisms and in the pathogenesis of various proliferative diseases. The biological effects of PDGF are initiated via two related receptor tyrosine kinases, termed alpha and betaPDGF receptors. Recent observations provide increasing evidence for distinct roles of the two PDGF receptor subtypes in both embryogenesis and disease formation. Moreover, characterization of the signal relay mechanisms indicates, that the alpha and betaPDGF receptors are not identical in their ability to bind intracellular effector molecules. Furthermore, the two PDGF receptors initiate overlapping, yet distinct signal transduction pathways. These differences may account for some of the variabilities in biological responses resulting from activation of these two receptors.

A role for Src in signal relay by the platelet-derived growth factor alpha receptor

Previous studies have shown that Src is required for platelet-derived growth factor (PDGF)-dependent cell cycle progression in fibroblasts. Since fibroblasts usually express both PDGF receptors (PDGFRs), these findings suggested that Src was mandatory for signal relay by both the alpha and betaPDGFRs. In this study, we have focused on the role of Src in signal relay by the alphaPDGFR. In response to stimulation with PDGF-AA, which selectively engages the alphaPDGFR, Src family members (Src) associated with the alphaPDGFR and Src kinase were activated. A mutant receptor, in which tyrosines 572 and 574 were replaced with phenylalanine (F72/74), failed to efficiently associate with Src or activate Src. The wild type (WT) and F72/74 receptors induced the expression of c-myc and c-fos to comparable levels. Furthermore, an equivalent extent of PDGF-dependent soft agar growth was observed in cells expressing the WT or the F72/74 alphaPDGFR. Comparing the ability of these two receptors to initiate tyrosine phosphorylation of signaling molecules indicated that both receptors mediated phosphorylation of the receptor itself, phospholipase Cgamma 1, and SHP-2 to similar levels. In contrast, the F72/74 receptor triggered phosphorylation of Shc to 1 and 20% of the WT levels for the 55- and 46-kDa Shc isoforms, respectively. These findings indicate that after exposure of cells to PDGF-AA, Src stably associates with the alphaPDGFR, and Src activity is increased. Furthermore, Src is required for the PDGF-dependent phosphorylation of signaling molecules such as Shc. Finally, activation of Src during the G0/G1 transition does not appear to be required for latter cell cycle events such as induction of c-myc or cell proliferation.