Transmodulation of epidermal growth factor receptor function by cyclic AMP-dependent protein kinase

A Genome-scale CRISPR Screen Identifies the ERBB and mTOR Signaling Networks as Key Determinants of Response to PI3K Inhibition in Pancreatic Cancer

KRAS mutation is a key driver of pancreatic cancer and PI3K pathway activity is an additional requirement for Kras-induced tumorigenesis. Clinical trials of PI3K pathway inhibitors in pancreatic cancer have shown limited responses. Understanding the molecular basis for this lack of efficacy may direct future treatment strategies with emerging PI3K inhibitors. We sought new therapeutic approaches that synergize with PI3K inhibitors through pooled CRISPR modifier genetic screening and a drug combination screen. ERBB family receptor tyrosine kinase signaling and mTOR signaling were key modifiers of sensitivity to alpelisib and pictilisib. Inhibition of the ERBB family or mTOR was synergistic with PI3K inhibition in spheroid, stromal cocultures. Near-complete loss of ribosomal S6 phosphorylation was associated with synergy. Genetic alterations in the ERBB-PI3K signaling axis were associated with decreased survival of patients with pancreatic cancer. Suppression of the PI3K/mTOR axis is potentiated by dual PI3K and ERBB family or mTOR inhibition. Surprisingly, despite the presence of oncogenic KRAS, thought to bestow independence from receptor tyrosine kinase signaling, inhibition of the ERBB family blocks downstream pathway activation and synergizes with PI3K inhibitors. Further exploration of these therapeutic combinations is warranted for the treatment of pancreatic cancer.

Vasoactive intestinal peptide induces proliferation of human hepatocytes

OBJECTIVES

Proliferation of hepatocytes in vitro can be stimulated by growth factors such as epidermal growth factor (EGF), but the role of vasoactive intestinal peptide (VIP) remains unclear. We have investigated the effect of VIP on maintenance and proliferation of human hepatocytes.

MATERIALS AND METHODS

Human hepatocytes were isolated from liver specimens obtained from patients undergoing liver surgery. Treatment with VIP or EGF was started 24 h after plating and continued for 3 or 5 d. DNA replication was investigated by Bromodeoxyuridine (BrdU) incorporation and cell viability detected by MTT assay. Cell lysate was analysed by western blotting and RT-PCR. Urea and albumin secretion into the culture supernatants were measured.

RESULTS

VIP increased DNA replication in hepatocytes in a dose-dependant manner, with a peak response at day 3 of treatment. VIP treatment was associated with an increase in mRNA expression of antigen identified by monoclonal antibody Ki-67 (MKI-67) and Histone Cluster 3 (H3) genes. Western blotting analysis showed that VIP can induce a PKA/B-Raf dependant phosphorylation of extracellular signal-regulated kinases (ERK). Although EGF can maintain hepatocyte functions up to day 5, no marked efffect was found with VIP.

CONCLUSIONS

VIP induces proliferation of human hepatocytes with little or no effect on hepatocyte differentiation. Further investigation of the role of VIP is required to determine if it may ultimately support therapeutic approaches of liver disease.

The anti-diabetic drug exenatide, a glucagon-like peptide-1 receptor agonist, counteracts hepatocarcinogenesis through cAMP-PKA-EGFR-STAT3 axis

Epidemiological studies have demonstrated a close association of type 2 diabetes and hepatocellular carcinoma (HCC). Exenatide (Ex-4), a potent diabetes drug targeting glucagon-like peptide-1 receptor (GLP-1R), is protective against non-alcoholic fatty liver disease (NAFLD). However, the Ex-4 function and GLP-1R status have yet been explored in HCC. Herein we investigated the effect of Ex-4 in diethylnitrosamine (DEN)-treated mice consuming control or high-fat high-carbohydrate diet. Administration of Ex-4 significantly improved obesity-induced hyperglycemia and hyperlipidemia and reduced HCC multiplicity in obese DEN-treated mice, in which suppressed proliferation and induced apoptosis were confined to tumor cells. The tumor suppression effects of Ex-4 were associated with high expression of GLP-1R and activation of cyclic AMP (cAMP) and protein kinase A (PKA). Importantly, Ex-4 also downregulated epidermal growth factor receptor (EGFR) and signal transducer and activator of transcription 3 (STAT3), which lie downstream of cAMP-PKA signaling, resulting in suppression of multiple STAT3-targeted genes including c-Myc, cyclin D1, survivin, Bcl-2 and Bcl-xl. The growth inhibitory effects of Ex-4 were consistent in GLP-1R-abundant hepatoma cell lines and xenograft mouse model, wherein both PKA and EGFR had obligatory roles in mediating Ex-4 functions. In addition, Ex-4 also effectively suppressed inflammatory and fibrotic phenotypes in mice fed with methionine-choline-deficient (MCD) diet and choline-deficient ethionine-supplemented (CDE) diet, respectively. In summary, Ex-4 elicits protective functions against NAFLD and obesity-associated HCC through cAMP-PKA-EGFR-STAT3 signaling, suggesting its administration as a novel approach to reduce HCC risk in diabetic patients.

Temporal profiling of lapatinib-suppressed phosphorylation signals in EGFR/HER2 pathways

Lapatinib is a clinically potent kinase inhibitor for breast cancer patients because of its outstanding selectivity for epidermal growth factor receptor (EGFR) and EGFR2 (also known as HER2). However, there is only limited information about the in vivo effects of lapatinib on EGFR/HER2 and downstream signaling targets. Here, we profiled the lapatinib-induced time- and dose-dependent phosphorylation dynamics in SKBR3 breast cancer cells by means of quantitative phosphoproteomics. Among 4953 identified phosphopeptides from 1548 proteins, a small proportion (5-7%) was regulated at least twofold by 1-10 μm lapatinib. We obtained a comprehensive phosphorylation map of 21 sites on EGFR/HER2, including nine novel sites on HER2. Among them, serine/threonine phosphosites located in a small region of HER2 (amino acid residues 1049-1083) were up-regulated by the drug, whereas all other sites were down-regulated. We show that cAMP-dependent protein kinase is involved in phosphorylation of this particular region of HER2 and regulates HER2 tyrosine kinase activity. Computational analyses of quantitative phosphoproteome data indicated for the first time that protein-protein networks related to cytoskeletal organization and transcriptional/translational regulation, such as RNP complexes (i.e. hnRNP, snRNP, telomerase, ribosome), are linked to EGFR/HER2 signaling networks. To our knowledge, this is the first report to profile the temporal response of phosphorylation dynamics to a kinase inhibitor. The results provide new insights into EGFR/HER2 regulation through region-specific phosphorylation, as well as a global view of the cellular signaling networks associated with the anti-breast cancer action of lapatinib.

Cetuximab and biomarkers in non-small-cell lung carcinoma

Cancer progression is a highly complex process that is driven by a constellation of deregulated signaling pathways and key molecular events. In non-small-cell lung cancer (NSCLC), as in several other cancer types, the epidermal growth factor receptor (EGFR) and its downstream signaling components represent a key axis that has been found not only to trigger cancer progression but also to support advanced disease leading to metastasis. Two major therapeutic approaches comprising monoclonal antibodies and small molecule tyrosine kinase inhibitors have so far been used to target this pathway, with a combination of positive, negative, and inconsequential results, as judged by patient survival indices. Since these drugs are expensive and not all patients derive benefits from taking them, it has become both pertinent and paramount to identify biomarkers that can predict not only beneficial response but also resistance. This review focuses on the chimeric monoclonal antibody, cetuximab, its application in the treatment of NSCLC, and the biomarkers that may guide its use in the clinical setting. A special emphasis is placed on the EGFR, including its structural and mechanistic attributes.

Direct modulation of the protein kinase A catalytic subunit α by growth factor receptor tyrosine kinases

The cyclic-AMP-dependent protein kinase A (PKA) regulates processes such as cell proliferation and migration following activation of growth factor receptor tyrosine kinases (RTKs), yet the signaling mechanisms that link PKA with growth factor receptors remain largely undefined. Here we report that RTKs can directly modulate the function of the catalytic subunit of PKA (PKA-C) through post-translational modification. In vitro kinase assays revealed that both the epidermal growth factor and platelet derived growth factor receptors (EGFR and PDGFR, respectively) tyrosine phosphorylate PKA-C. Mass spectrometry identified tyrosine 330 (Y330) as a receptor-mediated phosphorylation site and mutation of Y330 to phenylalanine (Y330F) all but abolished the RTK-mediated phosphorylation of PKA-C in vitro. Y330 resides within a conserved region at the C-terminal tail of PKA-C that allosterically regulates enzymatic activity. Therefore, the effect of phosphorylation at Y330 on the activity of PKA-C was investigated. The K(m) for a peptide substrate was markedly decreased when PKA-C subunits were tyrosine phosphorylated by the receptors as compared to un-phosphorylated controls. Importantly, tyrosine-phosphorylated PKA-C subunits were detected in cells stimulated with EGF, PDGF, and Fibroblast growth factor 2 (FGF2) and in fibroblasts undergoing PDGF-mediated chemotaxis. These results demonstrate a direct, functional interaction between RTKs and PKA-C and identify tyrosine phosphorylation as a novel mechanism for regulating PKA activity.

MC1R expression in HaCaT keratinocytes inhibits UVA-induced ROS production via NADPH oxidase- and cAMP-dependent mechanisms

Ultraviolet A (UVA) radiations are responsible for deleterious effects, mainly due to reactive oxygen species (ROS) production. Alpha-melanocyte stimulating hormone (α-MSH) binds to melanocortin-1 receptor (MC1R) in melanocytes to stimulate pigmentation and modulate cutaneous inflammatory responses. MC1R may be induced in keratinocytes after UV exposure. To investigate the effect of MC1R signaling on UVA-induced ROS (UVA-ROS) production, we generated HaCaT cells that stably express human MC1R (HaCaT-MC1R) or the Arg151Cys (R(151)C) non-functional variant (HaCaT-R(151)C). We then assessed ROS production immediately after UVA exposure and found that: (1) UVA-ROS production was strongly reduced in HaCaT-MC1R but not in HaCaT-R(151)C cells compared to parental HaCaT cells; (2) this inhibitory effect was further amplified by incubation of HaCaT-MC1R cells with α-MSH before UVA exposure; (3) protein kinase A (PKA)-dependent NoxA1 phosphorylation was increased in HaCaT-MC1R compared to HaCaT and HaCaT-R(151)C cells. Inhibition of PKA in HaCaT-MC1R cells resulted in a marked increase of ROS production after UVA irradiation; (4) the ability of HaCaT-MC1R cells to produce UVA-ROS was restored by inhibiting epidermal growth factor receptor (EGFR) or extracellular signal-regulated kinases (ERK) activity before UVA exposure. Our findings suggest that constitutive activity of MC1R in keratinocytes may reduce UVA-induced oxidative stress via EGFR and cAMP-dependent mechanisms.

EGF receptor activation by GPCRs: an universal pathway reveals different versions

About one decade ago has been demonstrated that G protein-coupled receptors (GPCRs) are able to utilize the epidermal growth factor (EGF) receptor (EGFR) as signalling intermediate. Thereby GPCRs are enabled to regulate cell growth, differentiation, and migration. A molecular mechanism for this process has been proposed that involves the activation of a distinct set of metalloproteases and the subsequent generation and release of particular members of the EGF peptide family which in turn activate the EGFR in an autocrine/paracrine manner. This model that allows GPCRs direct access to the signalling network of the EGFR family has emerged as a valid concept in a variety of cell types including cancer cells. The present review briefly summarizes the current knowledge but will be focussed on the ligand-dependency of EGFR transactivation. Several alternative mechanisms and novel aspects will be introduced. Using the example of head and neck squamous carcinoma, the potency of EGFR transactivation as a therapeutical target will be discussed.

Phosphatidic acid induces ligand-independent epidermal growth factor receptor endocytic traffic through PDE4 activation

Endocytic traffic can control cell surface versus intracellular distribution of empty/inactive EGFR, an thus its accessibility to external stimuli, through a pathway involving down regulation of PKA activity mediated by PA signaling towards PDE4. This novel control mechanism can trans-modulate EGFR function by heterologous stimuli of PLD.

Endocytosis modulates EGFR function by compartmentalizing and attenuating or enhancing its ligand-induced signaling. Here we show that it can also control the cell surface versus intracellular distribution of empty/inactive EGFR. Our previous observation that PKA inhibitors induce EGFR internalization prompted us to test phosphatidic acid (PA) generated by phospholipase D (PLD) as an endogenous down-regulator of PKA activity, which activates rolipram-sensitive type 4 phosphodiesterases (PDE4) that degrade cAMP. We found that inhibition of PA hydrolysis by propranolol, in the absence of ligand, provokes internalization of inactive (neither tyrosine-phosphorylated nor ubiquitinated) EGFR, accompanied by a transient increase in PA levels and PDE4s activity. This EGFR internalization is mimicked by PA micelles and is strongly counteracted by PLD2 silencing, rolipram or forskolin treatment, and PKA overexpression. Accelerated EGFR endocytosis seems to be mediated by clathrin-dependent and -independent pathways, leading to receptor accumulation in juxtanuclear recycling endosomes, also due to a decreased recycling. Internalized EGFR can remain intracellular without degradation for several hours or return rapidly to the cell surface upon discontinuation of the stimulus. This novel regulatory mechanism of EGFR, also novel function of signaling PA, can transmodulate receptor accessibility in response to heterologous stimuli.

cAMP-GEF cytoprotection by Src tyrosine kinase activation of phosphoinositide-3-kinase p110 beta/alpha in rat hepatocytes

Cyclic AMP protects against hepatocyte apoptosis by a protein kinase A-independent cAMP-GEF/phosphoinositide-3-kinase (PI3K)/Akt signaling pathway. However, the signaling pathway coupling cAMP-GEF with PI3K is unknown. The aim of this study was to investigate the role of Src tyrosine kinases (Src-TYK) and PI3K-p110 isoforms in this pathway. Studies were done in rat hepatocytes using the hydrophobic bile acid glycochenodeoxycholic acid (GCDC) to induce apoptosis. cAMP-binding guanine nucleotide exchange factors (cAMP-GEFs) were selectively activated by using 4-(4-chloro-phenylthio)-2'-O-methyladenosine-3'-5'-cyclic monophosphate (CPT-2-Me-cAMP), which sequentially phosphorylated Src-TYK (within 1 min) followed by Akt (within 5 min). The Src inhibitors PP2 and SU6656 inhibited basal and CPT-2-Me-cAMP-mediated Src and Akt phosphorylation. These inhibitors had no effect on CPT-2-Me-cAMP-mediated activation of Rap GTPases. CPT-2-Me-cAMP induced transient Src dependent autophosphorylation of the epidermal growth factor receptor (EGFR). Inhibition of the EGFR with AG 1478 partially inhibited the ability of CPT-2-Me to phosphorylate Akt. Whereas PP2 completely abolished the protective effect of CPT-2-Me-cAMP in GCDC induced apoptosis, AG 1478 partially inhibited the cytoprotective effect. CPT-2-Me-cAMP treatment resulted in Src-dependent activation of the p110 beta and alpha subunits of PI3K, but only the latter was sensitive to inhibition with AG 1478. In conclusion, activation of cAMP-GEFs results in phosphorylation of Src-TYK and Akt and activation of the p110 beta/alpha subunits of PI3K. Maximal cAMP-GEF-mediated Akt phosphorylation as well as protection from bile acid-induced apoptosis requires activation of Src-TYK and the EGFR. These studies support the existence of two pathways: cAMP-GEF/Rap/Src/PI3Kbeta/Akt and cAMP-GEF/Rap/Src/EGFR/PI3Kalpha/Akt, both of which are necessary for maximal cytoprotective effect of cAMP-GEFs in hepatocytes.

The protein phosphatase 2A regulatory subunits B'beta and B'delta mediate sustained TrkA neurotrophin receptor autophosphorylation and neuronal differentiation

Nerve growth factor (NGF) is critical for the differentiation and maintenance of neurons in the peripheral and central nervous system. Sustained autophosphorylation of the TrkA receptor tyrosine kinase and long-lasting activation of downstream kinase cascades are hallmarks of NGF signaling, yet our knowledge of the molecular mechanisms underlying prolonged TrkA activity is incomplete. Protein phosphatase 2A (PP2A) is a heterotrimeric Ser/Thr phosphatase composed of a scaffolding, catalytic, and regulatory subunit (B, B', and B" gene families). Here, we employ a combination of pharmacological inhibitors, regulatory subunit overexpression, PP2A scaffold subunit exchange, and RNA interference to show that PP2A containing B' family regulatory subunits participates in sustained NGF signaling in PC12 cells. Specifically, two neuron-enriched regulatory subunits, B'beta and B'delta, recruit PP2A into a complex with TrkA to dephosphorylate the NGF receptor on Ser/Thr residues and to potentiate its intrinsic Tyr kinase activity. Acting at the receptor level, PP2A/ B'beta and B'delta enhance NGF (but not epidermal growth factor or fibroblast growth factor) signaling through the Akt and Ras-mitogen-activated protein kinase cascades and promote neuritogenesis and differentiation of PC12 cells. Thus, select PP2A heterotrimers oppose desensitization of the TrkA receptor tyrosine kinase, perhaps through dephosphorylation of inhibitory Ser/Thr phosphorylation sites on the receptor itself, to maintain neurotrophin-mediated developmental and survival signaling.

Cross-talk between G protein-coupled and epidermal growth factor receptors regulates gonadotropin-mediated steroidogenesis in Leydig cells

Gonadal steroid production is stimulated by gonadotropin binding to G protein-coupled receptors (GPCRs). Although GPCR-mediated increases in intracellular cAMP are known regulators of steroidogenesis, the roles of other signaling pathways in mediating steroid production are not well characterized. Recent studies suggest that luteinizing hormone (LH) receptor activation leads to trans-activation of epidermal growth factor (EGF) receptors in the testes and ovary. This pathway is critical for LH-induced steroid production in ovarian follicles, probably through matrix metalloproteinase (MMP)-mediated release of EGF receptor (EGFR) binding ectodomains. Here we examined LH and EGF receptor cross-talk in testicular steroidogenesis using mouse MLTC-1 Leydig cells. We demonstrated that, similar to the ovary, trans-activation of the EGF receptor was critical for gonadotropin-induced steroid production in Leydig cells. LH-induced increases in cAMP and cAMP-dependent protein kinase (PKA) activity mediated trans-activation of the EGF receptor and subsequent mitogen-activated protein kinase (MAPK) activation, ultimately leading to StAR phosphorylation and mitochondrial translocation. Steroidogenesis in Leydig cells was unaffected by MMP inhibitors, suggesting that cAMP and PKA trans-activated EGF receptors in an intracellular fashion. Interestingly, although cAMP was always needed for steroidogenesis, the EGFR/MAPK pathway was activated and necessary only for early (30-60 min), but not late (120 min or more), LH-induced steroidogenesis in vitro. In contrast, 36-h EGF receptor inhibition in vivo significantly reduced serum testosterone levels in male mice, demonstrating the physiologic importance of this cross-talk. These results suggest that GPCR-EGF receptor cross-talk is a conserved regulator of gonadotropin-induced steroidogenesis in the gonads, although the mechanisms of EGF receptor trans-activation may vary.

Epidermal growth factor receptor levels are reduced in mice with targeted disruption of the protein kinase A catalytic subunit

BACKGROUND

Epidermal Growth Factor Receptor (EGFR) is a key target molecule in current treatment of several neoplastic diseases. Hence, in order to develop and improve current drugs targeting EGFR signalling, an accurate understanding of how this signalling pathway is regulated is required. It has recently been demonstrated that inhibition of cAMP-dependent protein kinase (PKA) induces a ligand-independent internalization of EGFR. Cyclic-AMP-dependent protein kinase consists of a regulatory dimer bound to two catalytic subunits.

RESULTS

We have investigated the effect on EGFR levels after ablating the two catalytic subunits, Calpha and Cbeta in two different models. The first model used targeted disruption of either Calpha or Cbeta in mice whereas the second model used Calpha and Cbeta RNA interference in HeLa cells. In both models we observed a significant reduction of EGFR expression at the protein but not mRNA level.

CONCLUSION

Our results suggest that PKA may represent a target that when manipulated can maintain EGFR protein levels at the single cell level as well as in intact animals.

Epidermal growth factor receptors: function modulation by phosphorylation and glycosylation interplay

Post-translational modifications (PTMs) of proteins induce structural and functional changes that are most often transitory and difficult to follow and investigate in vivo. In silico prediction procedures for PTMs are very valuable to foresee and define such transitory changes responsible for the multifunctionality of proteins. Epidermal growth factor receptor (EGFR) is such a multifunctional transmembrane protein with intrinsic tyrosine kinase activity that is regulated primarily by ligand-stimulated transphosphorylation of dimerized receptors. In human EGFR, potential phosphorylation sites on Ser, Thr and Tyr residues including five autophosphorylation sites on Tyr were investigated using in silico procedures. In addition to phosphorylation, O-GlcNAc modifications and interplay between these two modifications was also predicted. The interplay of phosphorylation and O-GlcNAc modification on same or neighboring Ser/Thr residues is termed as Yin Yang hypothesis and the interplay sites are named as Yin Yang sites. Amongst these modification sites, one residue is localized in the juxtamembrane (Thr 654) and two are found in the catalytic domain (Ser 1046/1047) of the EGFR. We propose that, when EGFR is O-GlcNAc modified on Thr 654, EGFR may be transferred from early to late endosomes, whereas when EGFR is O-GlcNAc modified on Ser 1046/1047 desensitization of the receptor may be prevented. These findings suggest a complex interplay between phosphorylation and O-GlcNAc modification resulting in modulation of EGFR's functionality.

Cyclin D1 overexpression induces epidermal growth factor-independent resistance to apoptosis linked to BCL-2 in human A431 carcinoma

Overexpression of EGF receptors and constitutive cyclin D1 expression are frequently associated with human squamous carcinomas. We have now investigated whether these parameters influence susceptibility to okadaic acid induced cell death in EGF-receptor overexpressing mutant p53 A431 human carcinoma. Exposure of these cells to 20 nM okadaic acid induced apoptosis-associated caspase 3 activation, DNA fragmentation, cleavage of Poly ADP-Ribose Polymerase (PARP), p53-independent expression of pro-apoptotic bax, and loss of proliferation-promoting cyclin D1. All these alterations were antagonized by concurrent addition of exogenous EGF. Ectopic overexpression of the cyclin D1 gene in A431 carcinoma conferred resistance to 20 nM okadaic acid irrespective of exogenous EGF, associated with a parallel induction of anti-apoptotic bcl-2. Treatment with a subtoxic concentration of a bispecific bcl-2/bcl xL antisense oligonucleotide cooperated with okadaic acid to down-regulate bcl-2 and sensitize cyclin D1-overexpressing cells to okadaic acid. Although EGF protects EGF-R proficient epithelial cells from diverse apoptotic stimuli through Mcl-1, this is the first report demonstrating that cyclin D1 overexpression provides an EGF independent protection from okadaic acid-induced cell death through induction of bcl-2. We also show that this anti-apoptotic effect of cyclin D1 overexpression, can be partly antagonized with antisense strategies that down-regulate anti-apoptotic bcl-2 family members.

Subcellular localization and biological actions of activated RSK1 are determined by its interactions with subunits of cyclic AMP-dependent protein kinase

Cyclic AMP (cAMP)-dependent protein kinase (PKA) and ribosomal S6 kinase 1 (RSK1) share several cellular proteins as substrates. However, to date no other similarities between the two kinases or interactions between them have been reported. Here, we describe novel interactions between subunits of PKA and RSK1 that are dependent upon the activation state of RSK1 and determine its subcellular distribution and biological actions. Inactive RSK1 interacts with the type I regulatory subunit (RI) of PKA. Conversely, active RSK1 interacts with the catalytic subunit of PKA (PKAc). Binding of RSK1 to RI decreases the interactions between RI and PKAc, while the binding of active RSK1 to PKAc increases interactions between PKAc and RI and decreases the ability of cAMP to stimulate PKA. The RSK1/PKA subunit interactions ensure the colocalization of RSK1 with A-kinase PKA anchoring proteins (AKAPs). Disruption of the interactions between PKA and AKAPs decreases the nuclear accumulation of active RSK1 and, thus, increases its cytosolic content. This subcellular redistribution of active RSK1 is manifested by increased phosphorylation of its cytosolic substrates tuberous sclerosis complex 2 and BAD by epidermal growth factor along with decreased cellular apoptosis.

Regulation of hepatocyte cell cycle progression and differentiation by type I collagen structure

Cell behavior is strongly influenced by the extracellular matrix (ECM) to which cells adhere. Both chemical determinants within ECM molecules and mechanical properties of the ECM network regulate cellular response, including proliferation, differentiation, and apoptosis. Type I collagen is the most abundant ECM protein in the body with a complex structure that can be altered in vivo by proteolysis, cross-linking, and other processes. Because of collagen's complex and dynamic nature, it is important to define the changes in cell response to different collagen structures and its underlying mechanisms. This chapter reviews current knowledge of potential mechanisms by which type I collagen affects cell behavior, and it presents data that elucidate specific intracellular signaling pathways by which changes in type I collagen structure differentially regulate hepatocyte cell cycle progression and differentiation. A network of polymerized fibrillar type I collagen (collagen gel) induces a highly differentiated but growth-arrested phenotype in primary hepatocytes, whereas a film of monomeric collagen adsorbed to a rigid dish promotes cell cycle progression and dedifferentiation. Studies presented here demonstrate that protein kinase A (PKA) activity is significantly elevated in hepatocytes on type I collagen gel relative to collagen film, and inhibition of this elevated PKA activity can promote hepatocyte cell cycle progression on collagen gel. Additional studies are presented that examine changes in hepatocyte cell cycle progression and differentiation in response to increased rigidity of polymerized collagen gel by fiber cross-linking. Potential mechanisms underlying these cellular responses and their implications are discussed.

Metalloprotease-dependent amphiregulin release mediates tumor necrosis factor-α-induced IL-8 secretion in the human airway epithelial cell line NCI-H292

Tumor necrosis factor-alpha (TNF-alpha) is a potent multifunctional cytokine that plays a central role in the pathogenesis of many inflammatory diseases. Interleukin-8 (IL-8) is a principle neutrophil chemoattractant and activator in humans. The alveolar macrophage-derived TNF-alpha initiates lung inflammation through its ability to stimulate IL-8 synthesis in airway epithelial cells. Since recent studies demonstrated that the stimulation of epidermal growth factor receptor (EGFR) could induce IL-8 secretion, the involvement of EGFR in TNF-alpha-induced IL-8 secretion in airway epithelium-like NCI-H292 cells was investigated in this study. TNF-alpha and epidermal growth factor (EGF) stimulated IL-8 secretion in a time- and concentration-dependent manner. Inhibition of the EGFR by either an anti-EGFR neutralizing antibody or by its specific inhibitor AG1478 (1 microM) blocked TNF-alpha-induced IL-8 secretion. In addition, TNF-alpha stimulated tyrosine phosphorylation of the EGFR within 5 min after stimulation. Further, TNF-alpha-induced IL-8 secretion was completely inhibited by the neutralizing antibody against amphiregulin (AR), an EGFR ligand, suggesting that TNF-alpha-induced IL-8 secretion was mediated by the AR-EGFR pathway. Furthermore, TNF-alpha stimulated the release of AR in a concentration-dependent manner. Finally, both AR and IL-8 release-induced by TNF-alpha were eliminated by pretreatment with either GM6001, a broad-spectrum inhibitor for metalloprotease, or TAPI-1, relatively selective inhibitor for TNF-alpha converting enzyme (TACE). These findings indicate that metalloprotease-mediated AR shedding and subsequent activation of EGFR play a critical role in TNF-alpha-induced IL-8 secretion from the human airway epithelium-like NCI-H292 cells, and that TACE is one of the most possible candidates for metalloprotease responsible for TNF-alpha-induced AR shedding.

Type I collagen structure regulates cell morphology and EGF signaling in primary rat hepatocytes through cAMP-dependent protein kinase A

Adhesion to type 1 collagen elicits different responses dependent on whether the collagen is in fibrillar (gel) or monomeric form (film). Hepatocytes adherent to collagen film spread and proliferate, whereas those adherent to collagen gel remain rounded and growth arrested. To explore the role of potential intracellular inhibitory signals responsible for collagen gel-mediated growth arrest, cAMP-dependent protein kinase A (PKA) was examined in hepatocytes adherent to collagen film or gel. PKA activity was higher in hepatocytes on collagen gel than on film during G1 of the hepatocyte cell cycle. Inhibition of PKA using H89 increased cell spreading on collagen gel in an EGF-dependent manner, whereas activation of PKA using 8-Br-cAMP decreased cell spreading on collagen film. PKA inhibition also restored ERK activation, cyclin D1 expression and G1-S progression on collagen gel, but had no effect on cells adherent to collagen film. Analysis of EGF receptor phosphorylation revealed that adhesion to collagen gel alters tyrosine phosphorylation of the EGF receptor, leading to reduced phosphorylation of tyrosine residue 845, which was increased by inhibition of PKA. These results demonstrate that fibrillar type 1 collagen can actively disrupt cell cycle progression by inhibiting specific signals from the EGF receptor through a PKA-dependent pathway.

cAMP inhibits CSF-1-stimulated tyrosine phosphorylation but augments CSF-1R-mediated macrophage differentiation and ERK activation

Macrophage colony stimulating factor (M-CSF) or CSF-1 controls the development of the macrophage lineage through its receptor tyrosine kinase, c-Fms. cAMP has been shown to influence proliferation and differentiation in many cell types, including macrophages. In addition, modulation of cellular ERK activity often occurs when cAMP levels are raised. We have shown previously that agents that increase cellular cAMP inhibited CSF-1-dependent proliferation in murine bone marrow-derived macrophages (BMM) which was associated with an enhanced extracellular signal-regulated kinase (ERK) activity. We report here that increasing cAMP levels, by addition of either 8-bromo cAMP (8BrcAMP) or prostaglandin E(1) (PGE1), can induce macrophage differentiation in M1 myeloid cells engineered to express the CSF-1 receptor (M1/WT cells) and can potentiate CSF-1-induced differentiation in the same cells. The enhanced CSF-1-dependent differentiation induced by raising cAMP levels correlated with enhanced ERK activity. Thus, elevated cAMP can promote either CSF-1-induced differentiation or inhibit CSF-1-induced proliferation depending on the cellular context. The mitogen-activated protein kinase/extracellular signal-related protein kinase kinase (MEK) inhibitor, PD98059, inhibited both the cAMP- and the CSF-1R-dependent macrophage differentiation of M1/WT cells suggesting that ERK activity might be important for differentiation in the M1/WT cells. Surprisingly, addition of 8BrcAMP or PGE1 to either CSF-1-treated M1/WT or BMM cells suppressed the CSF-1R-dependent tyrosine phosphorylation of cellular substrates, including that of the CSF-1R itself. It appears that there are at least two CSF-1-dependent pathway(s), one MEK/ERK dependent pathway and another controlling the bulk of the tyrosine phosphorylation, and that cAMP can modulate signalling through both of these pathways.

The receptor tyrosine kinase Ror2 associates with and is activated by casein kinase Iepsilon

Ror2, a member of the mammalian Ror family of receptor tyrosine kinases, plays important roles in developmental morphogenesis, although the mechanism underlying activation of Ror2 remains largely elusive. We show that when expressed in mammalian cells, Ror2 associates with casein kinase Iepsilon (CKIepsilon), a crucial regulator of Wnt signaling. This association occurs primarily via the cytoplasmic C-terminal proline-rich domain of Ror2. We also show that Ror2 is phosphorylated by CKIepsilon on serine/threonine residues, in its C-terminal serine/threonine-rich 2 domain, resulting in autophosphorylation of Ror2 on tyrosine residues. Furthermore, it was found that association of Ror2 with CKIepsilon is required for its serine/threonine phosphorylation by CKIepsilon. Site-directed mutagenesis of tyrosine residues in Ror2 reveals that the sites of phosphorylation are contained among the five tyrosine residues in the proline-rich domain but not among the four tyrosine residues in the tyrosine kinase domain. Moreover, we show that in mammalian cells, CKIepsilon-mediated phosphorylation of Ror2 on serine/threonine and tyrosine residues is followed by the tyrosine phosphorylation of G protein-coupled receptor kinase 2, a kinase with a developmental expression pattern that is remarkably similar to that of Ror2. Intriguingly, a mutant of Ror2 lacking five tyrosine residues, including the autophosphorylation sites, fails to tyrosine phosphorylate G protein-coupled receptor kinase 2. This indicates that autophosphorylation of Ror2 is required for full activation of its tyrosine kinase activity. These findings demonstrate a novel role for CKIepsilon in the regulation of Ror2 tyrosine kinase.

Optimal lysophosphatidic acid-induced DNA synthesis and cell migration but not survival require intact autophosphorylation sites of the epidermal growth factor receptor

Lysophosphatidic acid (LPA)-elicited transphosphorylation of receptor tyrosine kinases has been implicated in mediating extracellular signal-regulated kinase (ERK) 1/2 activation, which is necessary for LPA-induced cell proliferation, migration, and survival. B82L cells lack epidermal growth factor receptor (EGFR) but express LPA(1-3), platelet-derived growth factor (PDGF), ErbB2, and insulin-like growth factor receptor transcripts, yet LPA caused no detectable transphosphorylation of these receptor tyrosine kinases. LPA equally protected B82L cells, or transfectants expressing EGFR, the kinase dead EGFR(K721A), EGFR(Y5F) receptor mutant, which lacks five autophosphorylation sites, or EGFR(Y845F), which lacks the Src phosphorylation site from tumor necrosis factor-alpha-induced apoptosis. In contrast, LPA-elicited DNA synthesis and migration were augmented in cells expressing EGFR, EGFR(K721A), or EGFR(Y845F), but not EGFR(Y5F), although the PDGF responses were indistinguishable. LPA-induced transphosphorylation of the EGFR, ErbB2, or PDGF receptor was not required for its antiapoptotic effect. EGFR with or without intrinsic kinase activity or without the Src-phosphorylation site augmented, but was not required for, LPA-elicited cell proliferation or migration. In B82L cells, augmentation of these two LPA responses required intact autophosphorylation sites because among the four EGFR mutants, only cells expressing the EGFR(Y5F) mutant showed no enhancement. In EGFR(Y5F)-expressing cells, LPA failed to elicit tyrosine phosphorylation of Src homologous and collagen protein (SHC) and caused only a modest increase in ERK1/2 phosphorylation similar to that in wild-type B82L cells. The present data pinpoint the lack of importance of the intrinsic kinase activity in contrast to the importance of autophosphorylation sites of the EGFR for SHC phosphorylation in the enhancement of select ERK1/2-dependent LPA responses.

Critical role for protein phosphatase 2A heterotrimers in mammalian cell survival

The predominant forms of protein phosphatase 2A (PP2A), one of the major Ser/Thr phosphatases, are dimers of catalytic (C) and scaffolding (A) subunits and trimers with an additional variable regulatory subunit. In mammals, catalytic and scaffolding subunits are encoded by two genes each (alpha/beta), whereas three gene families (B, B', and B'') with a total of 12 genes contribute PP2A regulatory subunits. We generated stable PC12 cell lines in which the major scaffolding Aalpha subunit can be knocked down by inducible RNA interference (RNAi) to study its role in cell viability. Aalpha RNAi decreased total PP2A activity as well as protein levels of C, B, and B' but not B'' subunits. Inhibitor experiments indicate that monomeric C and B subunits are degraded by the proteosome. Knock-down of Aalpha triggered cell death by redundant apoptotic and non-apoptotic mechanisms because the inhibition of RNAi-associated caspase activation failed to stall cell death. PP2A holoenzymes positively regulate survival kinase signaling, because RNAi reduced basal and epidermal growth factor-stimulated Akt phosphorylation. RNAi-resistant Aalpha cDNAs rescued RNAi-induced loss of the C subunit, and Aalpha point mutants prevented regulatory subunit degradation as predicted from each mutant's binding specificity. In transient, stable, and stable-inducible rescue experiments, both wild-type Abeta and Aalpha mutants capable of binding to at least one family of regulatory subunits were able to delay Aalpha RNAi-induced death of PC12 cells. However, only the expression of wild-type Aalpha restored viability completely. Thus, heterotrimeric PP2A holoenzymes containing the Aalpha subunit and members of all three regulatory subunit families are necessary for mammalian cell viability.

Single Transmembrane Spanning Heterotrimeric G Protein-Coupled Receptors and Their Signaling Cascades

Heptahelical of serpentine receptors such as the adrenergic receptors are well known to mediate their actions via heterotrimeric GTP-binding proteins. Likewise, receptors that traverse the cell membrane once have been shown to mediate their biological actions by activating several different mechanisms including stimulation of their intrinsic tyrosine kinase activities or the kinase activities of other proteins. Some of these single transmembrane receptors have an intrinsic guanylyl cyclase activity and can stimulate the cyclic GMP second messenger system; however, over the last few years, several studies have shown the involvement of heterotrimeric GTP-binding proteins in mediating signals that eventually culminate in the biological actions of single transmembrane spanning receptors and proteins. These receptors include the receptor tyrosine kinases that mediate the actions of growth factors such as epidermal growth factor, insulin, insulin-like growth factor as well as receptors for atrial natiuretic hormone or the zona pellucida protein (ZP3) and integrins. In this review, the significance of the coupling of the single transmembrane spanning receptors to G proteins has been highlighted by providing several examples of the concept that signaling via these receptors may involve the activation of multiple signaling cascades.

ErbB2 and EGFR are downmodulated during the differentiation of 3T3-L1 preadipocytes

The expression of receptors belonging to the epidermal growth factor receptor subfamily has been largely studied these last years in epithelial cells mainly as involved in cell proliferation and malignant progression. Although much work has focused on the role of these growth factor receptors in the differentiation of a variety of tissues, there is little information in regards to normal stromal cells. We investigated erbB2 expression in the murine fibroblast cell line Swiss 3T3L1, which naturally or hormonally induced undergoes adipocyte differentiation. We found that the Swiss 3T3-L1 fibroblasts express erbB2, in addition to EGFR, and in a quantity comparable to or even greater than the breast cancer cell line T47D. Proliferating cells increased erbB2 and EGFR levels when reaching confluence up to 4- and 10-fold, respectively. This expression showed a significant decrease when growth-arrested cells were stimulated to differentiate with dexamethasone and isobutyl-methylxanthine. Differentiated cells presented a decreased expression of both erbB2 and EGFR regardless of whether the cells were hormonally or spontaneously differentiated. EGF stimulation of serum-starved cells increased erbB2 tyrosine phosphorylation and retarded erbB2 migration in SDS-PAGE, suggesting receptor association and activation. Heregulin-alpha1 and -beta1, two EGF related factors, had no effect on erbB2 or EGFR phosphorylation. Although 3T3-L1 cells expressed heregulin, its specific receptors, erbB3 and erbB4, were not found. This is the first time in which erbB2 is reported to be expressed in an adipocytic cell line which does not depend on non EGF family growth factors (thyroid hormone, growth hormone, etc.) to accomplish adipose differentiation. Since erbB2 and EGFR expression were downmodulated as differentiation progressed it is conceivable that a mechanism of switching from a mitogenic to a differentiating signaling pathway may be involved, through regulation of the expression of these growth factor receptors.

Gs protein-coupled receptor agonists induce transactivation of the epidermal growth factor receptor in T84 cells: implications for epithelial secretory responses

We have previously shown that Gq protein-coupled receptor (GqPCR) agonists stimulate epidermal growth factor receptor (EGFr) transactivation and activation of mitogen-activated protein kinases (MAPK) in colonic epithelial cells. This constitutes a mechanism by which Cl- secretory responses to GqPCR agonists are limited. In the present study we examined a possible role for the EGFr in regulating Cl- secretion stimulated by agonists that act through GsPCRs. All experiments were performed using monolayers of T84 colonic epithelial cells grown on permeable supports. Protein phosphorylation and protein-protein interactions were analyzed by immunoprecipitation and Western blotting. Cl- secretion was measured as changes in short-circuit current (DeltaIsc) across voltage-clamped T84 cells. The GsPCR agonist, vasoactive intestinal polypeptide (VIP; 100 nM), rapidly stimulated EGFr phosphorylation in T84 cells. This effect was mimicked by a cell-permeant analog of cAMP, Bt2cAMP/AM (3 microM), and was attenuated by the protein kinase A (PKA) inhibitor, H-89 (20 microM). The EGFr inhibitor, tyrphostin AG1478 (1 microM), inhibited both Bt2cAMP/AM-stimulated EGFr phosphorylation and Isc responses. VIP and Bt2cAMP/AM both stimulated ERK MAPK phosphorylation and recruitment of the p85 subunit of phosphatidylinositol 3-kinase (PI3K) to the EGFr in a tyrphostin AG1478-sensitive manner. The PI3K inhibitor, wortmannin (50 nM), but not the ERK inhibitor, PD 98059 (20 microM), attenuated Bt2cAMP/AM-stimulated secretory responses. We conclude that GsPCR agonists rapidly transactivate the EGFr in T84 cells by a signaling pathway involving cAMP and PKA. Through a mechanism that likely involves PI3K, transactivation of the EGFr is required for the full expression of cAMP-dependent Cl- secretory responses.

FK506 potentiates NGF-induced neurite outgrowth via the Ras/Raf/MAP kinase pathway

Nerve growth factor (NGF) and other members of the neurotrophin family are critical for the survival and differentiation of neurons within the peripheral and central nervous systems. Neurophilin ligands, including FK506, potentiate NGF-induced neurite outgrowth in several experimental models, although the mechanism of this potentiation is unclear. Therefore, we tested which signaling pathways were involved in FK506-potentiated neurite outgrowth in SH-SY5Y neuroblastoma cells using specific pharmacological inhibitors of various signaling molecules. Inhibitors of Ras (lovastatin), Raf (GW5074), or MAP kinase (PD98059 and U0126) blocked FK506 activity, as did inhibitors of phospholipase C (U73122) and phosphatidylinositol 3' kinase (LY294002). Protein kinase C inhibitors (Go6983 and Ro31-8220) slightly but significantly inhibited neurite outgrowth, whereas inhibitors of p38 MAPK (SB203580) or c-Jun N-terminal kinase (SP600125) had no effect. These data suggest that FK506 potentiates neurite outgrowth through the Ras/Raf/MAP kinase signaling pathway downstream of phospholipase C and phosphatidylinositol 3' kinase.

Regulation of glial glutamate transporter expression by growth factors

Injuries to the brain result in the decline of glial glutamate transporter expression within hours and a recovery after several days. One consequence of this disturbed expression seems to consist in the temporary accumulation of toxic extracellular glutamate levels followed by secondary neuronal cell death. Whereas evidence exists that the decline in glutamate transporter expression results from a loss of neuronal PACAP influences on astroglia, the mechanism(s) inducing the reexpression of glial glutamate transporters is presently unknown. We now demonstrate that the injury-induced growth factors EGF, TGFalpha, FGF-2, and PDGF all promote the expression of the glutamate transporters GLT-1 and/or GLAST in cultured cortical astroglia. In contrast, similar stimulatory influences were absent with GDNF and BDNF, growth factors not affected by brain injuries. The effects of EGF, TGFalpha, FGF-2, and PDGF on glial glutamate transport were only partly redundant and involved distinctly different signaling pathways. Unlike EGF, TGFalpha, and FGF-2, PDGF promoted GLT-1, but not GLAST expression and further failed to increase the maximal velocity of sodium-dependent glutamate uptake. Moreover, FGF-2 only affected glial glutamate transport when the RAF-MEK-ERK signaling pathway was concomitantly inhibited with PD98059. Depending on the extracellular growth factor and glutamate transporter subtype, the observed stimulatory effects required the activation of PKA, PKC, and/or AKT. We suggest that after brain injury, reactive processes may limit secondary neuronal cell death by promoting glial glutamate transport. The detailed knowledge of these compensatory mechanisms will eventually allow us to therapeutically interfere with glutamate-associated neuronal cell death in the brain.

Protein kinase A intersects SRC signaling in membrane microdomains

Regulation of Src kinase activity is tightly coupled to the phosphorylation status of the C-terminal regulatory tyrosine Tyr(527), which, when phosphorylated by Csk, represses Src. Here, we demonstrate that activation of Csk through a prostaglandin E(2)-cAMP-protein kinase A (PKA) pathway inhibits Src. This inhibitory pathway is operative in detergent-resistant membrane fractions where cAMP-elevating agents activate Csk, resulting in a concomitant decrease in Src activity. The inhibitory effect on Src depends on a detergent-resistant membrane-anchored Csk and co-localization of all components of the inhibitory pathway in membrane microdomains. Furthermore, epidermal growth factor-induced activation of Src and phosphorylation of the Src substrates Cbl and focal adhesion kinase are inhibited by activation of the cAMP-PKA-Csk pathway. We propose a novel mechanism whereby G protein-coupled receptors inhibit Src signaling by activation of Csk in a cAMP-PKA-dependent manner.

Regulation of epidermal growth factor receptor internalization by G protein-coupled receptors

The epidermal growth factor (EGF) receptor (EGFR) plays a central role in regulating cell proliferation, differentiation, and migration. Cellular responses to EGF are dependent upon the amount of EGFR present on the cell surface. Stimulation with EGF induces sequestration of the receptor from the plasma membrane and its subsequent downregulation. Recently, internalization of the EGFR was also shown to be required for mitogenic signaling via the activation of MAP kinases. Therefore, mechanisms regulating internalization of the EGFR represent an important facet for the control of cellular response. Here, we demonstrate that EGFR is removed from the cell surface not only following stimulation with EGF, but also in response to stimulation of G protein-coupled lysophosphatidic acid (LPA) and beta2 adrenergic (beta2AR) receptors. Using a FLAG epitope-tagged EGFR to quantitate receptor internalization, we show that incubation with EGF, LPA, or isoproterenol (ISO) causes the time-dependent loss of cell surface EGFR. Internalization of EGFR by these ligands involves the tyrosine kinase activity of the receptor itself and c-Src, as well as the GTPase activity of dynamin. Unexpectedly, we find that internalization of the EGFR by EGF is dependent upon Gbetagamma and beta-arrestin proteins; expression of minigenes encoding the carboxyl terminii of the G protein-coupled receptor kinase 2, or beta-arrestin1, attenuates LPA-, ISO-, and EGF-mediated internalization of EGFR. Thus, G protein-coupled receptors can control the function of the EGFR by regulating its endocytosis.

Protein kinase A mediates cAMP-induced tyrosine phosphorylation of the epidermal growth factor receptor

An increase in the intracellular cAMP concentration induces tyrosine phosphorylation of the epidermal growth factor receptor (EGFR) followed by activation of extracellular signal-regulated kinases 1/2 (ERK1/2). In this report we demonstrate that these effects of cAMP are mediated via activation of protein kinase A (PKA). Chemical inhibition of PKA suppressed forskolin-induced EGFR tyrosine phosphorylation and ERK1/2 activation in PC12 cells. Furthermore, forskolin failed to induce significant tyrosine phosphorylation of the EGFR and ERK1/2 activation in PKA-defective PC12 cells. Forskolin-induced EGFR tyrosine phosphorylation was also observed in A431 cells and in membranes isolated from these cells. Phosphoamino acid analysis indicated that the recombinant catalytic subunit of PKA elicited phosphorylation of the EGFR on both tyrosine and serine but not threonine residues in A431 membranes. Together, our data indicate that activation of PKA mediates the effects of cAMP on the EGFR and ERK1/2. While PKA may directly phosphorylate the EGFR on serine residues, PKA-induced tyrosine phosphorylation of the EGFR occurs by an indirect mechanism.

Effects of epidermal growth factor on epinephrine-stimulated heart function in rodents

Epidermal growth factor (EGF) interferes with beta-adrenergic receptor (beta-AR) signaling in adipocytes and hepatocytes, which leads to decreased lipolytic and glycogenolytic responses, respectively. We studied the effect of EGF on the heart. EGF interfered with the cAMP signal generated by beta-AR agonists in cardiac myocytes. In perfused hearts, EGF decreased inotropic and chronotropic responses to epinephrine but not to 8-(4-chlorophenylthio)adenosine 3',5'-cyclic monophosphate. Sustained epinephrine infusion induced heart contracture, which resulted in altered heart function as demonstrated by decreased inotropy and increased heart rate variability. EGF prevented all these alterations. In the whole animal (anesthetized mice), EGF administration reduced the rise in heart rate induced by a single epinephrine dose and the occurrence of Bezold-Jarisch reflex episodes induced by repeated doses. Sialoadenectomy enhanced the response to epinephrine, and EGF administration restored normal response. All these results suggest that, by interfering with beta-AR signaling, EGF protects the heart against the harmful effects of epinephrine.

Cooperation of protein kinase A and Ras/ERK signaling pathways is required for AP-1-mediated activation of fibroblast growth factor-inducible response element (FiRE)

Recent studies suggest a crucial role for protein kinase A (PKA) in the regulation of growth factor signaling. However, the effect of PKA on the transcription of growth factor-responsive genes has drawn far less attention. Here we have investigated the signaling mechanisms involved in the activation of an activator protein-1 (AP-1)-driven, growth factor-specific enhancer element, fibroblast growth factor-inducible response element (FiRE). The activation was found to be mediated by three phorbol 12-O-tetradecanoate-13-acetate-response element-related DNA elements of FiRE, including motif 4 and two distinct elements of motif 5 (referred to as M5-1 and M5-2). All three elements were required for full FiRE activity. Stimulation of cells with fibroblast growth factor-2 (FGF-2) induced the binding of AP-1 to motif 4 and M5-2, whereas M5-1 did not show detectable binding. The FGF-2-induced FiRE activation appeared to require cooperational function of the Ras/ERK and PKA pathways. Inhibition of either of the pathways abolished the binding of AP-1 complexes to motif 4 and motif 5 and the subsequent FiRE activation. By contrast, costimulation of cells with FGF-2 and the PKA activator 8-bromo-cyclic AMP increased the binding of AP-1 to FiRE and potentiated the level of transcriptional activity. The cooperational function of these two pathways was confirmed by experiments with cell lines stably expressing 4-hydroxytamoxifen-inducible oncogenic Raf-1 (DeltaRaf-1:ER[DD]). Noticeably, the induction systems showed variations with respect to regulation of AP-1-driven activation of FiRE. These differences were likely to originate from the ability of these two systems to induce the differential activation pattern of the Ras/ERK pathway.

Protein kinase A balances the growth factor-induced Ras/ERK signaling

Protein kinase A (PKA) has been proposed to regulate the signal transduction through the Ras/extracellular-regulated kinase (ERK) pathway. Here we demonstrate that when the PKA activity was inhibited prior to growth factor stimulus the signal flow through the Ras/ERK pathway was significantly increased. Furthermore, the data indicated that this PKA-mediated regulation was simultaneously targeted to the upstream kinase Raf-1 and to the ERK-specific phosphatase mitogen-activated protein kinase phosphatase-1 (MKP-1). Moreover, our data suggested that the level of PKA activity determined the transcription rate of mkp-1 gene, whereas the Ras/ERK signal was required to protect the MKP-1 protein against degradation. These results point to a tight regulatory relationship between PKA and the growth factor signaling, and further suggest an important role for basal PKA activity in such regulation. We propose that PKA adjusts the activity of the Ras/ERK pathway and maintains it within a physiologically appropriate level.

Novel mechanism for regulation of epidermal growth factor receptor endocytosis revealed by protein kinase A inhibition

Current models put forward that the epidermal growth factor receptor (EGFR) is efficiently internalized via clathrin-coated pits only in response to ligand-induced activation of its intrinsic tyrosine kinase and is subsequently directed into a lysosomal-proteasomal degradation pathway by mechanisms that include receptor tyrosine phosphorylation and ubiquitylation. Herein, we report a novel mechanism of EGFR internalization that does not require ligand binding, receptor kinase activity, or ubiquitylation and does not direct the receptor into a degradative pathway. Inhibition of basal protein kinase A (PKA) activity by H89 and the cell-permeable substrate peptide Myr-PKI induced internalization of 40-60% unoccupied, inactive EGFR, and its accumulation into early endosomes without affecting endocytosis of transferrin and mu-opioid receptors. This effect was abrogated by interfering with clathrin function. Thus, the predominant distribution of inactive EGFR at the plasma membrane is not simply by default but involves a PKA-dependent restrictive condition resulting in receptor avoidance of endocytosis until it is stimulated by ligand. Furthermore, PKA inhibition may contribute to ligand-induced EGFR endocytosis because epidermal growth factor inhibited 26% of PKA basal activity. On the other hand, H89 did not alter ligand-induced internalization of EGFR but doubled its half-time of down-regulation by retarding its segregation into degradative compartments, seemingly due to a delay in the receptor tyrosine phosphorylation and ubiquitylation. Our results reveal that PKA basal activity controls EGFR function at two levels: 1) residence time of inactive EGFR at the cell surface by a process of "endocytic evasion," modulating the accessibility of receptors to stimuli; and 2) sorting events leading to the down-regulation pathway of ligand-activated EGFR, determining the length of its intracellular signaling. They add a new dimension to the fine-tuning of EGFR function in response to cellular demands and cross talk with other signaling receptors.

Regulation of extracellular signal-regulated kinase cascades by alpha- and beta-isoforms of the human thromboxane A(2) receptor

Thromboxane A(2) (TXA(2)) stimulates mitogenic growth of vascular smooth muscle. In humans, TXA(2) signals through two TXA(2) receptor (TP) isoforms, termed TPalpha and TPbeta. To investigate the mechanism of TXA(2)-mediated mitogenesis, regulation of extracellular signal-regulated kinase (ERK) signaling was examined in human embryonic kidney 293 cells stably overexpressing the individual TP isoforms. The TXA(2) mimetic 9,11-dideoxy-9alpha,11alpha-methano epoxy prostaglandin F(2alpha) (U46619) elicited concentration- and time-dependent activation of ERK1 and -2 through both TPs with maximal TPalpha- and TPbeta-mediated ERK activation observed after 10 and 5 min, respectively. U46619-mediated ERK activation was inhibited by the TP antagonist [1S-[1alpha,2beta-(5Z)-3beta,4alpha-]]-7-[3-[[2-(phenylamino)carbonyl]hydrazine] methyl]-7-oxabicyclo[-2,2,1-]hept-2yl]-5-heptenoic acid (SQ29,548), and by the mitogen-activated protein kinase kinase inhibitor 2'-amino-3'-methoxyflavone (PD 98059). Although ERK activation through TPalpha was dependent on 2-[1-(dimethylaminopropyl)-1H-indol-3-yl]-3-(1H-indol-3-yl)-maleimide (GF 109203X)-sensitive protein kinase (PK) Cs, ERK activation through TPbeta was only partially dependent on PKCs. ERK activation through both TPalpha and TPbeta was dependent on PKA and phosphoinositide 3-kinase (PI3K) class 1(A), but not class 1(B), and was modulated by Harvey-Ras, A-Raf, c-Raf, and Rap1B/B-Raf and also involved transactivation of the epidermal growth factor receptor. Additionally, PKB/Akt was activated through TPalpha and TPbeta in a PI3K-dependent manner. In conclusion, we have defined the key components of TXA(2)-mediated ERK signaling and have established that both TPalpha and TPbeta are involved. TXA(2)-mediated ERK activation through the TPs is a complex event involving PKC-, PKA-, and PI3K-dependent mechanisms in addition to transactivation of the EGF receptor. TPalpha and TPbeta mediate ERK activation through similar mechanisms, although the time frame for maximal ERK activation and PKC dependence differs.

Redox-sensitive transactivation of epidermal growth factor receptor by tumor necrosis factor confers the NF-kappa B activation

Cross-communication between different signaling systems allows the integration of the great diversity of stimuli that a cell receives under varying physiological situations. In this paper we have explored the possibility that tumor necrosis factor (TNF) receptor signal cross-talks with epidermal growth factor (EGF) receptor signal on the nuclear factor-kappa B (NF-kappa B) activation pathway. We have demonstrated that overexpression of the EGF receptor (EGFR) in NIH3T3 cells significantly enhances TNF-induced NF-kappa B-dependent luciferase activity even without EGF, that EGF treatment has a synergistic effect on the induction of the reporter activity, and that this enhancement is suppressed by AG1478, EGFR-specific tyrosine kinase inhibitor. We also have shown that TNF induces tyrosine phosphorylation and internalization of the overexpressed EGFR in NIH3T3 cells and the endogenously expressed EGFR in A431 cells and that the transactivation by TNF is suppressed by N-acetyl-l-cysteine or overexpression of an endogenous reducing molecule, thioredoxin, but not by phosphatidylinositol 3-kinase inhibitors and protein kinase C inhibitor. Taken together, this evidence strongly suggests that EGFR transactivation by TNF, which is regulated in a redox-dependent manner, is playing a pivotal role in TNF-induced NF-kappa B activation.

Heterodimerization of the epidermal-growth-factor (EGF) receptor and ErbB2 and the affinity of EGF binding are regulated by different mechanisms

When clathrin-dependent endocytosis is inhibited in HeLa cells by overexpression of a K44A (Lys(44)-->Ala) mutant of the GTPase dynamin, high-affinity binding of epidermal growth factor (EGF) to the EGF receptor (EGFR) is disrupted [Ringerike, Stang, Johannessen, Sandnes, Levy and Madshus (1998) J. Biol. Chem. 273, 16639-16642]. We now report that the effect of [K44A]dynamin on EGF binding was counteracted by incubation with the non-specific kinase inhibitor staurosporine (SSP), implying that a protein kinase is responsible for disrupted high-affinity binding of EGF upon overexpression of [K44A]dynamin. The effect of [K44A]dynamin on EGF binding was not due to altered phosphorylation of the EGFR, suggesting that the activated kinase is responsible for phosphorylation of a substrate other than EGFR. The number of EGFR molecules was increased in cells overexpressing [K44A]dynamin, while the number of proto-oncoprotein ErbB2 molecules was unaltered. EGF-induced receptor dimerization was not influenced by overexpression of [K44A]dynamin. ErbB2-EGFR heterodimer formation was found to be ligand-independent, and the number of heterodimers was not altered by overexpression of [K44A]dynamin. Neither SSP nor the phorbol ester PMA, which disrupts high-affinity EGF-EGFR interaction, had any effect on the EGFR homo- or hetero-dimerization. Furthermore, the EGF-induced tyrosine phosphorylation of ErbB2 was not affected by overexpression of [K44A]dynamin, implying that EGFR-ErbB2 dimers were fully functional. Our results strongly suggest that high-affinity binding of EGF and EGFR-ErbB2 heterodimerization are regulated by different mechanisms.

Tissue inhibitor of metalloproteinases-2 (TIMP-2) suppresses TKR-growth factor signaling independent of metalloproteinase inhibition

The tissue inhibitors of metalloproteinases (TIMPs) block matrix metalloproteinase (MMP)-mediated increases in cell proliferation, migration, and invasion that are associated with extracellular matrix (ECM) turnover. Here we demonstrate a direct role for TIMP-2 in regulating tyrosine kinase-type growth factor receptor activation. We show that TIMP-2 suppresses the mitogenic response to tyrosine kinase-type receptor growth factors in a fashion that is independent of MMP inhibition. The TIMP-2 suppression of mitogenesis is reversed by the adenylate cyclase inhibitor SQ22536, and implicates cAMP as the second messenger in these effects. TIMP-2 neither altered the release of transforming growth factor alpha from the cell surface, nor epidermal growth factor (EGF) binding to the cognate receptor, EGFR. TIMP-2 binds to the surface of A549 cells in a specific and saturable fashion (K(d) = 147 pm), that is not competed by the synthetic MMP inhibitor BB-94 and is independent of MT-1-MMP. TIMP-2 induces a decrease in phosphorylation of EGFR and a concomitant reduction in Grb-2 association. TIMP-2 prevents SH2-protein-tyrosine phosphatase-1 (SHP-1) dissociation from immunoprecipitable EGFR complex and a selective increase in total SHP-1 activity. These studies represent a new functional paradigm for TIMP-2 in which TIMP suppresses EGF-mediated mitogenic signaling by short-circuiting EGFR activation.

Antitumor activity and pharmacokinetics of a mixed-backbone antisense oligonucleotide targeted to the RIalpha subunit of protein kinase A after oral administration

Overexpression of the RIalpha subunit of cAMP-dependent protein kinase (PKA) has been demonstrated in various human cancers. PKA has been suggested as a potential target for cancer therapy. The goal of the present study was to evaluate an anti-PKA antisense oligonucleotide (mixed-backbone oligonucleotide) as a therapeutic approach to human cancer treatment. The identified oligonucleotide inhibited the growth of cell lines of human colon cancer (LS174T, DLD-1), leukemia (HL-60), breast cancer (MCF-7, MDA-MB-468), and lung cancer (A549) in a time-, concentration-, and sequence-dependent manner. In a dose-dependent manner, the oligonucleotide displayed in vivo antitumor activity in severe combined immunodeficient and nude mice bearing xenografts of human cancers of the colon (LS174T), breast (MDA-MB-468), and lung (A549). The routes of drug administration were intraperitoneal and oral. Synergistic effects were found when the antisense oligonucleotide was used in combination with the cancer chemotherapeutic agent cisplatin. The pharmacokinetics of the oligonucleotide after oral administration of (35)S-labeled oligonucleotide into tumor-bearing mice indicated an accumulation and retention of the oligonucleotide in tumor tissue. This study further provides a basis for clinical studies of the antisense oligonucleotide targeted to the RIalpha subunit of PKA (GEM 231) as a cancer therapeutic agent used alone or in combination with conventional chemotherapy.