Angiotensin II stimulates ERK via two pathways in epithelial cells: protein kinase C suppresses a G-protein coupled receptor-EGF receptor transactivation pathway

Autoantibody Signaling in Pemphigus Vulgaris: Development of an Integrated Model

Pemphigus vulgaris (PV) is an autoimmune skin blistering disease effecting both cutaneous and mucosal epithelia. Blister formation in PV is known to result from the binding of autoantibodies (autoAbs) to keratinocyte antigens. The primary antigenic targets of pathogenic autoAbs are known to be desmoglein 3, and to a lesser extent, desmoglein 1, cadherin family proteins that partially comprise the desmosome, a protein structure responsible for maintaining cell adhesion, although additional autoAbs, whose role in blister formation is still unclear, are also known to be present in PV patients. Nevertheless, there remain large gaps in knowledge concerning the precise mechanisms through which autoAb binding induces blister formation. Consequently, the primary therapeutic interventions for PV focus on systemic immunosuppression, whose side effects represent a significant health risk to patients. In an effort to identify novel, disease-specific therapeutic targets, a multitude of studies attempting to elucidate the pathogenic mechanisms downstream of autoAb binding, have led to significant advancements in the understanding of autoAb-mediated blister formation. Despite this enhanced characterization of disease processes, a satisfactory explanation of autoAb-induced acantholysis still does not exist. Here, we carefully review the literature investigating the pathogenic disease mechanisms in PV and, taking into account the full scope of results from these studies, provide a novel, comprehensive theory of blister formation in PV.

Attenuation of ligand-induced activation of angiotensin II type 1 receptor signaling by the type 2 receptor via protein kinase C

Angiotensin II (AII) type 2 receptor (AT2R) negatively regulates type 1 receptor (AT1R) signaling. However, the precise molecular mechanism of AT2R-mediated AT1R inhibition remains poorly understood. Here, we characterized the local and functional interaction of AT2R with AT1R. AT2R colocalized and formed a complex with AT1R at the plasma membrane, even in the absence of AII. Upon AII stimulation, the spatial arrangement of the complex was modulated, as confirmed by Förster resonance energy transfer (FRET) analysis, followed by AT2R internalization along with AT1R. AT2R internalization was specifically observed only in the presence of AT1R; AT2R alone could not be internalized. The AT1R-specific inhibitor losartan completely inhibited both the conformational change and the internalization of AT2R with AT1R, whereas the AT2R-specific inhibitor PD123319 partially hindered these phenomena, demonstrating that the activation of both receptors was indispensable for these effects. In addition, treatment with the protein kinase C (PKC) inhibitors inhibited the ligand-dependent accumulation of AT2R but not that of AT1R in the endosomes. A mutation in the putative phosphorylation sites of AT2R also abrogated the co-internalization of ATR2 with AT1R and the inhibitory effect of ATR2 on AT1R. These data suggest that AT2R inhibits ligand-induced AT1R signaling through the PKC-dependent pathway.

Reconstitution of the ERG Gene Expression Network Reveals New Biomarkers and Therapeutic Targets in ERG Positive Prostate Tumors

BACKGROUND

Despite a growing number of studies evaluating cancer of prostate (CaP) specific gene alterations, oncogenic activation of the ETS Related Gene (ERG) by gene fusions remains the most validated cancer gene alteration in CaP. Prevalent gene fusions have been described between the ERG gene and promoter upstream sequences of androgen-inducible genes, predominantly TMPRSS2 (transmembrane protease serine 2). Despite the extensive evaluations of ERG genomic rearrangements, fusion transcripts and the ERG oncoprotein, the prognostic value of ERG remains to be better understood. Using gene expression dataset from matched prostate tumor and normal epithelial cells from an 80 GeneChip experiment examining 40 tumors and their matching normal pairs in 40 patients with known ERG status, we conducted a cancer signaling-focused functional analysis of prostatic carcinoma representing moderate and aggressive cancers stratified by ERG expression.

RESULTS

In the present study of matched pairs of laser capture microdissected normal epithelial cells and well-to-moderately differentiated tumor epithelial cells with known ERG gene expression status from 20 patients with localized prostate cancer, we have discovered novel ERG associated biochemical networks.

CONCLUSIONS

Using causal network reconstruction methods, we have identified three major signaling pathways related to MAPK/PI3K cascade that may indeed contribute synergistically to the ERG dependent tumor development. Moreover, the key components of these pathways have potential as biomarkers and therapeutic target for ERG positive prostate tumors.

A pathophysiologic role for epidermal growth factor receptor in pemphigus acantholysis

The pemphigus family of autoimmune bullous disorders is characterized by autoantibody binding to desmoglein 1 and/or 3 (dsg1/dsg3). In this study we show that EGF receptor (EGFR) is activated following pemphigus vulgaris (PV) IgG treatment of primary human keratinocytes and that EGFR activation is downstream of p38 mitogen-activated protein kinase (p38). Inhibition of EGFR blocked PV IgG-triggered dsg3 endocytosis, keratin intermediate filament retraction, and loss of cell-cell adhesion in vitro. Significantly, inhibiting EGFR prevented PV IgG-induced blister formation in the passive transfer mouse model of pemphigus. These data demonstrate cross-talk between dsg3 and EGFR, that this cross-talk is regulated by p38, and that EGFR is a potential therapeutic target for pemphigus. Small-molecule inhibitors and monoclonal antibodies directed against EGFR are currently used to treat several types of solid tumors. This study provides the experimental rationale for investigating the use of EGFR inhibitors in pemphigus.

13-cis-Retinoic acid specific down-regulation of angiotensin type 1 receptor in rat liver epithelial and aortic smooth muscle cells

Transcriptional repression through cis- and trans-acting factors enabling an alternate approach to control angiotensin type 1 receptor (AT1 or AGTR1 as listed in the MGI database) expression has not been studied. In previous investigations, treatment with retinoic acid was found to be associated with enhanced insulin sensitivity. In our previous study, expression of AT1 was found to be inversely correlated with intracellular glucose concentrations. Therefore, we hypothesized that 13-cis-retinoic acid (13cRA), an antioxidant, enhances insulin-sensitive glucose-mediated down-regulation of the AT1. In this study, we used continuously passaged rat liver epithelial cells. Our study shows that cells exposed to 13cRA specifically down-regulated the AT1 protein in a dose- and time-dependent manner, independently of any change in receptor affinity. Down-regulation of the AT1 expression leads to reduced AngII-mediated intracellular calcium release, a hallmark of receptor-mediated intracellular signaling. Similarly with receptor down-regulation, we observed a significant reduction in AT1 mRNA; however, the AT1 down-regulation was independent of insulin-sensitive glucose uptake and retinoic acid receptor activation (RAR/RXR). Treatment with 13cRA resulted in phosphorylation of p42/p44 MAP kinases in these cells. Subsequent studies using MEK inhibitor PD98059 prevented 13cRA-mediated AT1 down-regulation and restored AngII-mediated intracellular calcium response. Furthermore, 13cRA-mediated inhibitory effects on AT1 were validated in primary rat aortic smooth muscle cells. In summary, our results demonstrate for the first time that 13cRA has a glucose- and RAR/RXR-independent mechanism for transcriptional inhibition of AT1, suggesting its therapeutic potential in systems in which AT1 expression is deregulated in insulin-sensitive and -insensitive tissues.

Hypoxia-induced up-regulation of aquaporin-1 protein in prostate cancer cells in a p38-dependent manner

BACKGROUND/AIMS

Aquaporin-1 (AQP1) is a glycoprotein that mediates osmotic water transport, its expression has been found to correlate with tumour stage in some tumours. However, the mechanism by which AQP1 protein expression is regulated in tumor cells remains to be fully elucidated. We hypothesized that hypoxia might play an important role in AQP1 induction during tumorigenesis and at the late stages of tumor development.

METHODS

Isotonic and serum-free hypoxic models were used to investigate AQP1 expression in PC-3M human prostate cancer cells.

RESULTS

AQP1 expression was up-regulated by density-induced pericellular hypoxia and cobalt(II) chloride (CoCl(2))-induced hypoxia at the transcriptional level. Moreover, phosphorylation of p38 mitogen-activated protein kinase (MAPK) was induced by density-induced pericellular hypoxia and CoCl(2)-induced hypoxia, specific inhibitors of p38 MAPK could concentration-dependently block those effects of hypoxia on AQP1 expression. Intracellular calcium ion (Ca(2+)) and protein kinase C (PKC) were shown to be responsible for the activation of p38 MAPK pathway. In addition, AQP1 induction in dense cultures was dependent on lowered oxygen (O(2)) tension. In high cell density culture, certain secretory proteins might induce AQP1 expression indirectly.

CONCLUSION

These findings suggest that AQP1 could be induced by hypoxia at transcription level, and the regulation of AQP1 in PC-3M cells is dependent on calcium, PKC and p38 MAPK, as well as low oxygen tension.

Pharmacologic effects of 2-methoxyestradiol on angiotensin type 1 receptor down-regulation in rat liver epithelial and aortic smooth muscle cells

BACKGROUND

Delayed onset of cardiovascular disease (CVD) in female patients is not well understood, but could be due in part to the protective effect of estrogen before menopause. Experimental studies have identified the angiotensin type 1 receptor (AT1R) as a key factor in the progression of CVD.

OBJECTIVE

We examined the effects of the estrogen metabolite 2-methoxyestradiol (2ME2) on AT1R expression.

METHODS

Rat liver cells were exposed to 2ME2 for 24 hours, and angiotensin II (AngII) binding and AT1R mRNA expressions were assessed.

RESULTS

In the presence of 2ME2, cells exhibited significant down-regulation of AngII binding that was both dose and time dependent, independent of estrogen receptors (ERα/ERβ). Down-regulation of AngII binding was AT1R specific, with no change in receptor affinity. Under similar conditions, we observed lower expression of AT1R mRNA, significant inhibition of AngII-mediated increase in intracellular Ca(2+), and increased phosphorylation of ERK1/2. Pretreatment of cells with the MEK inhibitor PD98059 prevented 2ME2-induced ERK1/2 phosphorylation and down-regulation of AT1R expression, which suggests that the observed inhibitory effect is mediated through ERK1/2 signaling intermediates. Similar analyses in stably transfected CHO (Chinese hamster ovary) cell lines with a constitutively active cytomegalovirus promoter showed no change in AT1R expression, which suggests that 2ME2-mediated effects are through transcriptional regulation. The effects of 2ME2 on AT1R down-regulation through ERK1/2 were consistently reproduced in primary rat aortic smooth muscle cells.

CONCLUSIONS

Because AT1R has a critical role in the control of CVD, 2ME2-induced changes in receptor expression may provide beneficial effects to the cardiovascular and other systems.

Clinical science review article: understanding the implications of diabetes on the vascular system

Patients with diabetes comprise an extremely complex subset of patients for the vascular surgeon. Often, they have numerous comorbidities that can further complicate matters. The diabetic environment is highly complex and the interplay of various diseases makes this an extremely challenging condition to manage. Knowing the mechanisms by which diabetes inflicts adverse microscopic changes in the vasculature allows the clinician to anticipate problems and minimize the heightened risks observed in diabetic patients undergoing surgery. In this review, we will illustrate how diabetes affects the vasculature and how the molecular and cellular derangements that occur in diabetic environments lead to these pathophysiologic consequences.

Antimicrobial peptide of an anti-lipopolysaccharide factor modulates of the inflammatory response in RAW264.7 cells

In this study, to clarify the protective mechanism of a peptide from shrimp anti-lipopolysaccharide (LPS) factor (SALF) against endotoxin shock, we evaluated the effects of the SALF and LPS on the production and release of tumor necrosis factor (TNF)-alphain vitro using the RAW264.7 murine macrophage cell line. Stimulation by LPS induced the production of inflammatory cytokines, and the SALF was able to modulate TNF-alpha production in LPS-stimulated RAW264.7 cells. Microarray studies revealed a transcriptional profile which was assessed in the presence or absence of the SALF by a quantitative real-time polymerase chain reaction. Pretreatment with the SALF significantly downregulated the expression of nuclear factor (NF)-kappaB in the presence of LPS. In contrast, pretreatment with the SALF significantly elevated the expressions of Anp32a, CLU, and SLPI, which are considered to be immune-related genes in the presence of LPS. Inhibitor studies suggested that the SALF's modulation of LPS-induced TNF-alpha production involved a complex mechanism with mitogen-activated protein kinase kinase, calcium, and protein kinase C. The data from this study, which imply that the SALF can suppress TNF-alpha production, suggest a role for the SALF in the defense mechanism which can potentially be applied to mammals for endotoxin treatment.

Drug-based therapies for vascular disease in Marfan syndrome: from mouse models to human patients

Marfan syndrome is a congenital disorder of the connective tissue with a long history of clinical and basic science breakthroughs that have forged our understanding of vascular-disease pathogenesis. The biomedical importance of Marfan syndrome was recently underscored by the discovery that the underlying genetic lesion impairs both tissue integrity and transforming growth factor-beta regulation of cell behavior. This discovery has led to the successful implementation of the first pharmacological intervention in a connective-tissue disorder otherwise incurable by either gene-based or stem cell-based therapeutic strategies. More generally, information gathered from the study of Marfan syndrome pathogenesis has the potential to improve the clinical management of common acquired disorders of connective-tissue degeneration.

Cholinergic receptor pathways involved in apoptosis, cell proliferation and neuronal differentiation

Acetylcholine (ACh) has been shown to modulate neuronal differentiation during early development. Both muscarinic and nicotinic acetylcholine receptors (AChRs) regulate a wide variety of physiological responses, including apoptosis, cellular proliferation and neuronal differentiation. However, the intracellular mechanisms underlying these effects of AChR signaling are not fully understood. It is known that activation of AChRs increase cellular proliferation and neurogenesis and that regulation of intracellular calcium through AChRs may underlie the many functions of ACh. Intriguingly, activation of diverse signaling molecules such as Ras-mitogen-activated protein kinase, phosphatidylinositol 3-kinase-Akt, protein kinase C and c-Src is modulated by AChRs. Here we discuss the roles of ACh in neuronal differentiation, cell proliferation and apoptosis. We also discuss the pathways involved in these processes, as well as the effects of novel endogenous AChRs agonists and strategies to enhance neuronal-differentiation of stem and neural progenitor cells. Further understanding of the intracellular mechanisms underlying AChR signaling may provide insights for novel therapeutic strategies, as abnormal AChR activity is present in many diseases.

GABA-B(1) receptors are coupled to the ERK1/2 MAP kinase pathway in the absence of GABA-B(2) subunits

In the current model of gamma-aminobutyric acid (GABA) B receptor function, there is a requirement for GABA-B(1/2) heterodimerisation for targetting to the cell surface. However, different lines of evidence suggest that the GABA-B(1) subunit can form a functional receptor in the absence of GABA-B(2). We observed coupling of endogenous GABA-B(1) receptors in the DI-TNC1 glial cell line to the ERK pathway in response to baclofen even though these cells do not express GABA-B(2). GABA-B(1A) receptors were also able to mediate a rapid, transient, and dose-dependent activation of the ERK1/2 MAP kinase pathway when transfected alone into HEK 293 cells. The response was abolished by G(i/o) and MEK inhibition, potentiated by inhibitors of phospholipase C and protein kinase C and did not involve PI-3-kinase activity. Finally, using bioluminescence resonance energy transfer and co-immunoprecipitation, we show the existence of homodimeric GABA-B(1A) receptors in transfected HEK293 cells. Altogether, our observations show that GABA-B(1A) receptors are able to activate the ERK1/2 pathway despite the absence of surface targetting partner GABA-B(2) in both HEK 293 cells and the DI-TNC1 cell line.

Functional angiotensin II type 2 receptors inhibit growth factor signaling in LNCaP and PC3 prostate cancer cell lines

BACKGROUND

There is clear evidence of a tissue-based renin-angiotensin system in the prostate and studies to date suggest that AT(1)-receptor blocking drugs inhibit the growth of some prostate cancer cell lines and delay the development of prostate cancer. The present studies examine the action of Ang II in two prostate cancer cell lines and report the presence of functional AT(2)-receptors that regulate the actions of growth factors.

METHODS

Immunohistochemistry was used to identify the presence of Ang II and QPCR techniques to examine AT(1)- and AT(2)-receptor mRNA expression in androgen-dependent (LNCaP) and independent (PC3) cell lines. The effects of AT(1)- and AT(2)-receptor activation upon EGF-induced DNA synthesis and ERK2 phosphorylation in these cells were also examined.

RESULTS

Functional AT(2)-receptors together with Ang II were identified in both cell lines and stimulation of these receptors inhibited EGF-induced DNA synthesis and ERK2 phosphorylation. AT(1)-receptors, although present in both cell lines, were only functional in LNCaP cells where activation stimulated DNA synthesis.

CONCLUSIONS

Functional AT(2)-receptors are present and have the capacity to inhibit EGF-induced prostate cancer cell growth in LNCaP and fast growing androgen-independent PC3 cell lines, whereas functional AT(1)-receptors are found only in LNCaP cells where their activation stimulates DNA synthesis.

Capacitative calcium entry contributes to the differential transactivation of the epidermal growth factor receptor in response to thiazolidinediones

Thiazolidinediones (TZDs) are synthetic ligands for the peroxisome proliferator-activated receptor gamma (PPARgamma) but also elicit PPARgamma-independent effects, most notably activation of mitogen-activated protein kinases (MAPKs). Ciglitazone rapidly activates extracellular signal-regulated kinase (Erk) MAPK, an event requiring c-Src kinase-dependent epidermal growth factor receptor (EGFR) transactivation, whereas troglitazone only weakly activates Erk and does not induce EGFR transactivation; the mechanism underlying this difference remains unclear. In this study, both ciglitazone and troglitazone increased Src activation. Similar effects were observed with Delta2-derivatives of each TZD, compounds that bind PPARgamma but do not lead to its activation, further indicating a PPARgamma-independent mechanism. Neither EGFR kinase nor Pyk2 inhibition prevented Src activation; however, inhibition of Src kinase activity prevented Pyk2 activation. Intracellular calcium chelation blocks TZD-induced Pyk2 activation; here, Src activation by both TZDs and ciglitazone-induced EGFR transactivation were prevented by calcium chelation. Accordingly, both TZDs increased calcium concentrations from intracellular stores; however, only ciglitazone produced a secondary calcium influx in the presence of extracellular calcium. Removal of extracellular calcium or inhibition of capacitative calcium entry by 2-APB prevented ciglitazone-induced EGFR transactivation and Erk activation but did not affect upstream kinase signaling pathways. These results demonstrate that upstream kinases (i.e., Src and Pyk2) are required but not sufficient for EGFR transactivation by TZDs. Moreover, influx of extracellular calcium through capacitative calcium entry may be an unrecognized component that provides a mechanism for the differential induction of EGFR transactivation by these compounds.

Protein kinase Calpha mediates feedback inhibition of EGF receptor transactivation induced by Gq-coupled receptor agonists

While a great deal of attention has been focused on G-protein-coupled receptor (GPCR)-induced epidermal growth factor receptor (EGFR) transactivation, it has been known for many years that the tyrosine kinase activity of the EGFR is inhibited in cells treated with tumor-promoting phorbol esters, a process termed EGFR transmodulation. Because many GPCR agonists that elicit EGFR transactivation also stimulate the Gq/phospholipase C (PLC)/protein kinase C (PKC) pathway, we hypothesized that PKC-mediated inhibition of EGFR transactivation operates physiologically as a feedback loop that regulates the intensity and/or duration of GPCR-elicited EGFR transactivation. In support of this hypothesis, we found that treatment of intestinal epithelial IEC-18 cells with the PKC inhibitors GF 109203X or Ro 31-8220 or chronic exposure of these cells to phorbol-12,13-dibutyrate (PDB) to downregulate PKCs, markedly enhanced the increase in EGFR tyrosine phosphorylation induced by angiotensin II or vasopressin in these cells. Similarly, PKC inhibition enhanced EGFR transactivation in human colonic epithelial T84 cells stimulated with carbachol, as well as in bombesin-stimulated Rat-1 fibroblasts stably transfected with the bombesin receptor. Furthermore, cell treatment with inhibitors with greater specificity towards PKCalpha, including Gö6976, Ro 31-7549 or Ro 32-0432, also increased GPCR-induced EGFR transactivation in IEC-18, T84 and Rat-1 cells. Transfection of siRNAs targeting PKCalpha also enhanced bombesin-induced EGFR tyrosine phosphorylation in Rat-1 cells. Thus, multiple lines of evidence support the hypothesis that conventional PKC isoforms, especially PKCalpha, mediate feedback inhibition of GPCR-induced EGFR transactivation.

ANG II-stimulated DNA synthesis is mediated by ANG II receptor-dependent Ca2+/PKC as well as EGF receptor-dependent PI3K/Akt/mTOR/p70S6K1 signal pathways in mouse embryonic stem cells

Effect of angiotensin II (ANG II) on mouse embryonic stem (ES) cell proliferation was examined. ANG II increased [(3)H] thymidine incorporation in a time- (>4 h) and dose- (>10(-9) M) dependent manner. The ANG II-induced increase in [(3)H] thymidine incorporation was blocked by inhibition of ANG II type 1 (AT(1)) receptor but not by ANG II type 2 (AT(2)) receptor, and AT(1) receptor was expressed. ANG II increased inositol phosphates formation and [Ca(2+)](i), and translocated PKC alpha, delta, and zeta to the membrane fraction. Consequently, the inhibition of PLC/PKC suppressed ANG II-induced increase in [(3)H] thymidine incorporation. The inhibition of EGF receptor kinase or tyrosine kinase prevented ANG II-induced increase in [(3)H] thymidine incorporation. ANG II phosphorylated EGF receptor and increased Akt, mTOR, and p70S6K1 phosphorylation blocked by AG 1478 (EGF receptor kinase blocker). ANG II-induced increase in [(3)H] thymidine incorporation was blocked by the inhibition of p44/42 MAPKs but not by p38 MAPK inhibition. Indeed, ANG II phosphorylated p44/42 MAPKs, which was prevented by the inhibition of the PKC and AT(1) receptor. ANG II increased c-fos, c-jun, and c-myc levels. ANG II also increased the protein levels of cyclin D1, cyclin E, cyclin-dependent kinase (CDK) 2, and CDK4 but decreased the p21(cip1/waf1) and p27(kip1), CDK inhibitory proteins. These proteins were blocked by the inhibition of AT(1) receptor, PLC/PKC, p44/42 MAPKs, EGF receptor, or tyrosine kinase. In conclusion, ANG II-stimulated DNA synthesis is mediated by ANG II receptor-dependent Ca(2+)/PKC and EGF receptor-dependent PI3K/Akt/mTOR/p70S6K1 signal pathways in mouse ES cells.

ANG II-induced cell proliferation is dually mediated by c-Src/Yes/Fyn-regulated ERK1/2 activation in the cytoplasm and PKCζ-controlled ERK1/2 activity within the nucleus

High-affinity binding of angiotensin II (ANG II) to the ANG II type 1 receptor (AT1R) results in the activation of ERK1/2 mitogen-activated protein kinases (MAPK). However, the precise mechanism of ANG II-induced ERK1/2 activation has not been fully characterized. Here, we investigated the signaling events leading to ANG II-induced ERK1/2 activation using a c-Src/Yes/Fyn tyrosine kinase-deficient mouse embryonic fibroblast (MEF) cell line stably transfected with the AT1R (SYF/AT1). ERK1/2 activation was reduced by ∼50% within these cells compared with wild-type controls (WT/AT1). The remaining ∼50% of intracellular ERK1/2 activation was dependent upon heterotrimeric G protein and protein kinase C zeta (PKCζ) activation. Therefore, ANG II-induced ERK1/2 activation occurs via two independent mechanisms. We next investigated whether a loss of either c-Src/Yes/Fyn or PKCζ signaling affected ERK1/2 nuclear translocation and cell proliferation in response to ANG II. ANG II-induced cell proliferation was markedly reduced in SYF/AT1cells compared with WT/AT1cells ( P < 0.01), but interestingly, ERK2 nuclear translocation was normal. ANG II-induced nuclear translocation of ERK2 was blocked via pretreatment of WT/AT1cells with a PKCζ pseudosubstrate. ANG II-induced cell proliferation was significantly reduced in PKCζ pseudosubstrate-treated WT/AT1cells ( P < 0.01) and was completely blocked in SYF/AT1cells treated with this same compound. Thus ANG II-induced cell proliferation appears to be regulated by both ERK1/2-driven nuclear and cytoplasmic events. In response to ANG II, the ability of ERK1/2 to remain within the cytoplasm or translocate into the nucleus is controlled by c-Src/Yes/Fyn or heterotrimeric G protein/PKCζ signaling, respectively.

NADPH oxidase-derived reactive oxygen species in cardiac pathophysiology

Chronic heart failure, secondary to left ventricular hypertrophy or myocardial infarction, is a condition with increasing morbidity and mortality. Although the mechanisms underlying the development and progression of this condition remain a subject of intense interest, there is now growing evidence that redox-sensitive pathways play an important role. This article focuses on the involvement of reactive oxygen species derived from a family of superoxide-generating enzymes, termed NADPH oxidases (NOXs), in the pathophysiology of ventricular hypertrophy, the accompanying interstitial fibrosis and subsequent heart failure. In particular, the apparent ability of the different NADPH oxidase isoforms to define the response of a cell to a range of physiological and pathophysiological stimuli is reviewed. If confirmed, these data would suggest that independently targeting different members of the NOX family may hold the potential for therapeutic intervention in the treatment of cardiac disease.

Manipulating the angiotensin system--new approaches to the treatment of solid tumours

Angiotensin II (Ang II), a main effector peptide in the renin-angiotensin system (RAS), plays a fundamental role as a vasoconstrictor in controlling cardiovascular function and renal homeostasis. Ang II also acts as a growth promoter or angiogenic factor via type 1 angiotensin II receptors (AT1Rs) in certain tumour cell lines. Recent studies have shown the activation of the local RAS in various tumour tissues, including the abundant generation of Ang II by angiotensin-converting enzyme (ACE) and the upregulation of AT1R expression. Thus, considerable attention has been paid to the role of the RAS in cancer and its blockade as a new approach to the treatment of cancer. There is increasing evidence that the Ang II-AT1R system is involved in tumour growth, angiogenesis and metastasis in experimental models, suggesting the therapeutic potential of an ACE inhibitor and AT1R blocker, both of which have been used as antihypertensive drugs. In addition, specific Ang II-degrading enzymes are expressed in tumours and play a regulatory role in cell proliferation and invasion. This review focuses on the role of the Ang II-AT1R system in solid tumours, particularly in the progression of gynaecological cancer, and presents the clinical potential of manipulating the angiotensin system as a novel and promising strategy for cancer treatment.

NADPH oxidases in cardiovascular health and disease

Increased oxidative stress plays an important role in the pathophysiology of cardiovascular diseases such as hypertension, atherosclerosis, diabetes, cardiac hypertrophy, heart failure, and ischemia-reperfusion. Although several sources of reactive oxygen species (ROS) may be involved, a family of NADPH oxidases appears to be especially important for redox signaling and may be amenable to specific therapeutic targeting. These include the prototypic Nox2 isoform-based NADPH oxidase, which was first characterized in neutrophils, as well as other NADPH oxidases such as Nox1 and Nox4. These Nox isoforms are expressed in a cell- and tissue-specific fashion, are subject to independent activation and regulation, and may subserve distinct functions. This article reviews the potential roles of NADPH oxidases in both cardiovascular physiological processes (such as the regulation of vascular tone and oxygen sensing) and pathophysiological processes such as endothelial dysfunction, inflammation, hypertrophy, apoptosis, migration, angiogenesis, and vascular and cardiac remodeling. The complexity of regulation of NADPH oxidases in these conditions may provide the possibility of targeted therapeutic manipulation in a cell-, tissue- and/or pathway-specific manner at appropriate points in the disease process.

Differential regulation of Pyk2 phosphorylation at Tyr-402 and Tyr-580 in intestinal epithelial cells: roles of calcium, Src, Rho kinase, and the cytoskeleton

The calcium-dependent proline-rich tyrosine kinase Pyk2 is activated by tyrosine phosphorylation, associates with focal adhesion proteins, and has been linked to proliferative and migratory responses in a variety of mesenchymal and epithelial cell types. Full Pyk2 activation requires phosphorylation at functionally distinct sites, including autophosphorylation site Tyr-402 and catalytic domain site Tyr-580, though the mechanisms involved are unclear. The pathways mediating Pyk2 phosphorylation at Tyr-402 and Tyr-580 were therefore investigated. Both sites were rapidly and transiently phosphorylated following cell stimulation by Ang II or LPA. However, only Tyr-580 phosphorylation was rapidly enhanced by intracellular Ca(2+) release, or inhibited by Ca(2+) depletion. Conversely, Tyr-402 phosphorylation was highly sensitive to inhibition of actin stress fibers, or of Rho kinase (ROK), an upstream regulator of stress fiber assembly. Ang II also induced a delayed (30-60 min) secondary phosphorylation peak occurring at Tyr-402 alone. Unlike the homologous focal adhesion kinase (FAK), Pyk2 phosphorylation was sensitive neither to the Src inhibitor PP2, nor to truncation of its N-terminal region, which contains a putative autoinhibitory FERM domain. These results better define the mechanisms involved in Pyk2 activation, demonstrating that autophosphorylation is ROK- and stress fiber-dependent, while transphosphorylation within the kinase domain is Ca(2+)-dependent and Src-independent in intestinal epithelial cells. This contrasts with the tight sequential coupling of phosphorylation seen in FAK activation, and further underlines the differences between these closely related kinases.

Direct involvement of G protein alpha(q/11) subunit in regulation of muscarinic receptor-mediated sAPPalpha release

The G(q/11) protein-coupled receptors, such as muscarinic (M1 & M3) receptors, have been shown to regulate the release of a soluble amyloid precursor protein (sAPPalpha) produced from alpha-secretase processing. However, there is no direct evidence for the precise characteristics of G proteins, and the signaling mechanism for the regulation of G(q/11) protein-coupled receptor-mediated sAPPalpha release is not clearly understood. This study examined whether the muscarinic receptor-mediated release of sAPPalpha is directly regulated by Galpha(q/11) proteins. The HEK293 cells were transiently cotransfected with muscarinic M3 receptors and a dominant-negative minigene construct of the G protein alpha subunit. The sAPPalpha release in the media was measured using an antibody specific for sAPP. The sAPPalpha release enhancement induced by muscarinic receptor stimulation was decreased by a G(q/11) minigene construct, whereas it was not blocked by a control minigene construct (the Galpha carboxy peptide in random order, Galpha(q)R) or Galpha(i) constructs. This indicated a direct role of the Galpha(q/11) protein in the regulation of muscarinic M3 receptor-mediated sAPPalpha release. We also investigated whether the transactivation of the epidermal growth factor receptor (EGFR) by a muscarinic agonist could regulate the sAPPalpha release in SH-SY5Y cells. Pretreatment of a specific EGFR kinase inhibitor, tyrophostin AG1478 (250 nM), blocked the EGF-stimulated sAPPalpha release, but did not block the oxoM-stimulated sAPPalpha release. This demonstrated that the transactivation of the EGFR by muscarinic receptor activation was not involved in the muscarinic receptor-mediated sAPPalpha release.

As2O3-induced c-Src/EGFR/ERK signaling is via Sp1 binding sites to stimulate p21WAF1/CIP1 expression in human epidermoid carcinoma A431 cells

Arsenic has been effectively used to treat acute promyelocytic leukemia, and can induce cell cycle arrest or apoptosis in human solid tumors. Previously, we have demonstrated that As2O3 can induce p21WAF1/CIP1 (p21) expression in A431 cells and then due to cellular cytotoxicity. Presently, we have clarified these signaling events and compared them with EGF. Using reporter assay, RT-PCR and Western blotting, we show that c-Src activation might be a prerequisite for As2O3-induced EGFR/Ras/Raf/ERK signaling. Furthermore, with the aids of 5'-deletion and site-directed mutagenesis, we demonstrate that Sp1 binding sites, ranging from -64 to -84 bp, are essential for As2O3- or EGF-regulated p21 expression. Finally, our experiments utilizing cycloheximide prompt the suggestion that the stability of mRNA or protein also contributes to As2O3- or EGF-induced p21 expression. Taken together, we conclude that the Sp1 binding sites are required for As2O3-induced p21 gene transcription through c-Src/EGFR/Ras/Raf/ERK pathway. Furthermore, post-transcriptional or post-translational stabilization mechanism is also essential for As2O3-induced p21 expression. EGF-induced p21 expression may involve similar mechanisms as those that operate in the As2O3-mediated reactions in A431 cells.

Nuclear compartmentalization of FAK and FRNK in cardiac myocytes

Focal adhesion kinase (FAK) and FAK-related non-kinase (FRNK) accumulate in the nucleus of cardiac myocytes during hypertensive hypertrophy. Nuclear FAK and FRNK are phosphorylated on different serines and form distinct bright spots. The subnuclear distribution of serine-phosphorylated FAK and FRNK was examined in this study by double labeling with fibrillarin, a component of nucleoli, and Sam68, a constituent of Sam68 nuclear bodies. We also investigated the role of protein kinase C (PKC)-mediated phosphorylation of FAK and FRNK on nuclear translocation. PKC activation by 12-O-tetradecanoylphorbol 13-acetate treatment increased serine phosphorylation of FAK and FRNK. Specifically, FAK was phosphorylated on serine 722 but not serine 910. On the other hand, FRNK was phosphorylated on serine 217, the equivalent site of FAK serine 910, but not serine 30, the homologous site of FAK serine 722. Serine-phosphorylated FAK and FRNK redistributed into the nucleus and formed distinct patterns. FAK with phosphorylation on serine 722 colocalized with Sam68 but not fibrillarin. On the contrary, FRNK phosphorylated on 217 coexisted with fibrillarin but not Sam68. Immunoprecipitation also confirmed that FAK associated with Sam68 and FRNK interacted with fibrillarin, respectively. These results suggest that FAK and FRNK target different nuclear subdomains by their association with distinct nuclear proteins.

Autocrine, paracrine and juxtacrine signaling by EGFR ligands

Receptor and cytoplasmic protein tyrosine kinases play prominent roles in the control of a range of cellular processes during embryonic development and in the regulation of many metabolic and physiological processes in a variety of tissues and organs. The epidermal growth factor receptor (EGFR) is a well-known and versatile signal transducer that has been highly conserved during evolution. It functions in a wide range of cellular processes, including cell fate determination, proliferation, cell migration and apoptosis. The number of ligands that can activate the EGF receptor has increased during evolution. These ligands are synthesized as membrane-anchored precursor forms that are later shed by metalloproteinase-dependent cleavage to generate soluble ligands. In certain circumstances the membrane anchored isoforms as well as soluble growth factors may also act as biologically active ligands; therefore depending on the circumstances these ligands may induce juxtacrine, autocrine, paracrine and/or endocrine signaling. In this review, we discuss the different ways that EGFR ligands can activate the receptor and the possible biological implications.

PKC-δ-dependent pathways contribute to PDGF-stimulated ERK1/2 activation in vascular smooth muscle

Platelet-derived growth factor (PDGF) is an important regulator of vascular smooth muscle (VSM) cell growth and migration and has been identified as a key mediator of neointima formation resulting from vascular injury. PDGF exerts its effects, in part, through activation of ERK1/2. Previously, we reported that PKC-δ, specifically compared with PKC-α, mediated phorbol ester- and ATP-dependent activation of ERK1/2 in VSM cells. The purpose of this study was to determine whether PKC-δ was involved in PDGF-dependent activation of ERK1/2 in VSM cells. The addition of PDGF resulted in the activation, and Src family kinase-dependent tyrosine phosphorylation, of PKC-δ. Treatment with rottlerin (0.1–10 μM), a selective PKC-δ inhibitor, or adenoviral overexpression of kinase-negative PKC-δ significantly attenuated PDGF-induced activation of ERK1/2. The effects of the PKC-δ inhibitors decreased with increasing concentrations of activator PDGF. Interestingly, treatment with Gö6976 (0.1–3 μM), a selective inhibitor of cPKCs, or adenoviral overexpression of kinase-negative PKC-α also inhibited PDGF-stimulated ERK1/2. Furthermore, inhibition of cPKC activity with Gö6976 or overexpression of kinase-negative PKC-α attenuated PKC-δ activation and tyrosine phosphorylation in response to PDGF. These studies indicate involvement of both PKC-δ and PKC-α isozymes in PDGF-stimulated signaling in VSM and suggest an unexpected role for PKC-α in the regulation of PKC-δ activity.

G alpha 12/13- and reactive oxygen species-dependent activation of c-Jun NH2-terminal kinase and p38 mitogen-activated protein kinase by angiotensin receptor stimulation in rat neonatal cardiomyocytes

In the present study, we examined signal transduction mechanism of reactive oxygen species (ROS) production and the role of ROS in angiotensin II-induced activation of mitogen-activated protein kinases (MAPKs) in rat neonatal cardiomyocytes. Among three MAPKs, c-Jun NH(2)-terminal kinase (JNK) and p38 MAPK required ROS production for activation, as an NADPH oxidase inhibitor, diphenyleneiodonium, inhibited the activation. The angiotensin II-induced activation of JNK and p38 MAPK was also inhibited by the expression of the Galpha(12/13)-specific regulator of G protein signaling (RGS) domain, a specific inhibitor of Galpha(12/13), but not by an RGS domain specific for Galpha(q). Constitutively active Galpha(12)- or Galpha(13)-induced activation of JNK and p38 MAPK, but not extracellular signal-regulated kinase (ERK), was inhibited by diphenyleneiodonium. Angiotensin II receptor stimulation rapidly activated Galpha(13), which was completely inhibited by the Galpha(12/13)-specific RGS domain. Furthermore, the Galpha(12/13)-specific but not the Galpha(q)-specific RGS domain inhibited angiotensin II-induced ROS production. Dominant negative Rac inhibited angiotensin II-stimulated ROS production, JNK activation, and p38 MAPK activation but did not affect ERK activation. Rac activation was mediated by Rho and Rho kinase, because Rac activation was inhibited by C3 toxin and a Rho kinase inhibitor, Y27632. Furthermore, angiotensin II-induced Rho activation was inhibited by Galpha(12/13)-specific RGS domain but not dominant negative Rac. An inhibitor of epidermal growth factor receptor kinase AG1478 did not affect angiotensin II-induced JNK activation cascade. These results suggest that Galpha(12/13)-mediated ROS production through Rho and Rac is essential for JNK and p38 MAPK activation.

Role of Ras/PKCζ/MEK/ERK1/2 signaling pathway in angiotensin II-induced vascular smooth muscle cell proliferation

The role of protein kinase C (PKC) and its cross talk with extracellular signal-regulated kinase (ERK) cascade in angiotensin II (AngII)-elicited vascular smooth muscle cell (VSMC) proliferation are still unclear. In this study, the PKC pathway of AngII to activate ERK1/2 and induce cell proliferation was investigated in rat aortic smooth muscle cells. The proliferation of VSMCs was tested by [3H]-thymidine incorporation assay. Phosphorylated and non-phosphorylated PKCzeta, ERK1/2, Elk-1, and mitogen-activated ERK-activating kinase (MEK) were estimated by Western blot analysis. The interactions of signal molecules were examined by immunoprecipitation. AngII-induced VSMC proliferation and activation of ERK1/2 and nuclear transcription factor Elk-1 were all down-regulated by PKC non-specific inhibitor (staurosporine) and PKCzeta pseudosubstrate inhibitor (PS-PKCzeta). Dominant negative Ras transfection into VSMCs decreased AngII-induced PKCzeta and ERK1/2 phosphorylation. AngII stimulated the association of PKCzeta with Ras. AngII-induced MEK phosphorylation was inhibited by PKCzeta pseudosubstrate inhibitor and the PKCzeta-MEK complex was detected by immunoprecipitation. These results suggest that PKCzeta isoform is involved in VSMC proliferation and Elk-1 activation. AngII can activate ERK1/2 by Ras/PKCzeta/MEK pathway, which may be one of the important signal transduction pathways in AngII-induced VSMC proliferation.

Peroxisome Proliferator-activated Receptor γ-independent Activation of p38 MAPK by Thiazolidinediones Involves Calcium/Calmodulin-dependent Protein Kinase II and Protein Kinase R

The thiazolidinediones (TZDs) are synthetic peroxisome proliferator-activated receptor gamma (PPARgamma) ligands that promote increased insulin sensitivity in type II diabetic patients. In addition to their ability to improve glucose homeostasis, TZDs also exert anti-proliferative effects by a mechanism that is unclear. Our laboratory has shown that two TZDs, ciglitazone and troglitazone, rapidly induce calcium-dependent p38 mitogen-activated protein kinase (MAPK) phosphorylation in liver epithelial cells. Here, we further characterize the mechanism responsible for p38 MAPK activation by PPARgamma ligands and correlate this with the induction of endoplasmic reticulum (ER) stress. Specifically, we show that TZDs rapidly activate the ER stress-responsive pancreatic eukaryotic initiation factor 2alpha (eIF2alpha) kinase or PKR (double-stranded RNA-activated protein kinase)-like endoplasmic reticulum kinase/pancreatic eIF2alpha kinase, and that activation of these kinases is correlated with subsequent eIF2alpha phosphorylation. Interestingly, PPARgamma ligands not only activated calcium/calmodulin-dependent kinase II (CaMKII) 2-fold over control, but the selective CaMKII inhibitor, KN-93, attenuated MKK3/6 and p38 as well as PKR and eIF2alpha phosphorylation. Although CaMKII was not affected by inhibition of PKR with 2-aminopurine, phosphorylation of MKK3/6 and p38 as well as eIF2alpha were significantly reduced. Collectively, these data provide evidence that CaMKII is a regulator of PKR-dependent p38 and eIF2alpha phosphorylation in response to ER calcium depletion by TZDs. Furthermore, using structural derivatives of TZDs that lack PPARgamma ligand-binding activity as well as a PPARgamma antagonist, we show that activation of these kinase signaling pathways is PPARgamma-independent.

Modulation of Elk-dependent-transcription by Gene33

Gene33 is a cytoplasmic protein expressed in many cell types, including those of renal and hepatic origin. Its expression is regulated by a large number of mitogenic and stressful stimuli, both in cultured cells and in vivo. Gene33 protein possesses binding domains for ErbB receptors, 14-3-3 proteins, SH-3 domains, and GTP bound Cdc42, suggesting that it may play a role in signal transduction. Indeed, these regions of Gene33 have been reported to modulate signaling through the ERK, JNK, and NFkappaB pathways. In the present work, epitope-tagged full-length and truncation mutants, as well as wild-type Gene33, were overexpressed in 293 cells. The expression of these proteins was compared to the level of endogenous Gene33 by Western blot using a newly developed polyclonal antibody. As proxies for activity of the ERK and JNK pathways, Elk- and c-Jun-dependent transcription were measured by a luciferase reporter gene. Moderate expression levels of full-length Gene33 caused a twofold increase in Elk-dependent transcription, while at higher levels, c-Jun-dependent transcription was partially inhibited. The C-terminal half of Gene33 significantly increased both Elk- and c-Jun-dependent transcription when expressed at approximately threefold above control levels. This effect on Elk-dependent transcription was lost at higher levels of Gene33 expression. In contrast, higher levels of the C-terminal half of Gene33 caused a progressively greater effect on c-Jun-dependent transcription. These findings suggest that Gene33 may increase ERK activity, and that the C-terminal half of Gene33 may act less specifically in the absence of the N-terminal half, inducing JNK activity.

Mechanisms of vascular angiotensin II surface receptor regulation by epidermal growth factor

We investigated mechanisms by which epidermal growth factor (EGF) reduces angiotensin II (AngII) surface receptor density and stimulated actions in vascular smooth muscle cells (VSMC). EGF downregulated specific AngII radioligand binding in intact cultured rat aortic smooth muscle cells but not in cell membranes and also inhibited AngII-stimulated contractions of aortic segments. Inhibitors of cAMP-dependent kinases, PI-3 kinase, MAP kinase, cyclooxygenase, and calmodulin did not prevent EGF-mediated downregulation of AngII receptor binding, whereas the EGF receptor kinase inhibitor AG1478 did. Total cell AngII AT1a receptor protein content of EGF-treated and untreated cells, measured by immunoblotting, did not differ. Actinomycin D or cytochalasin D, which interacts with the cytoskeleton, but not the protein synthesis inhibitor cycloheximide, prevented EGF from downregulating AngII receptor binding. Consistently, EGF inhibited AngII-stimulated formation of inositol phosphates in the presence of cycloheximide but not in the presence of actinomycin D or cytochalasin D. In conclusion, EGF needs an intact signal transduction pathway to downregulate AngII surface receptor binding, possibly by altering cellular location of the receptors.

Thromboxane A2 receptor signaling inhibits vascular endothelial growth factor-induced endothelial cell differentiation and migration

Vascular endothelial growth factor (VEGF) is an important patho-physiological mediator of angiogenesis. VEGF-induced endothelial cell (EC) migration and angiogenesis often occur in complicated environments containing multiple agents capable of modifying the response. Thromboxane (TX) A2 is released from multiple cell types and is a prime mediator of pathogenesis of many vascular diseases. Human EC express both TXA2 receptor (TP) isoforms; however, the effects of individual TP isoforms on VEGF-induced EC migration and angogenesis are unknown. We report here that the TXA2 mimetic [1S-(1alpha, 2beta(5Z), 3alpha(1E, 3R), 4alpha]-7-[3-(3-hydroxy-4-(4'-iodophenoxy)-1-butenyl)-7-oxab icyclo-[2.2.1]heptan-2yl]-5'-heptenoic acid (IBOP) (100 nmol/L) is a potent antagonist (IC50 30 nmol/L) of VEGF-induced EC migration and differentiation. TPbeta, but not TPalpha, expression is required for the inhibition of VEGF-induced migration and angiogenesis. IBOP costimulation suppressed nitric oxide (NO) release from VEGF-treated EC through decreased activation of Akt, eNOS, and PDK1. TPbeta costimulation also ablated the increase in focal adhesion formation in response to VEGF. This mechanism was characterized by decreased recruitment of focal adhesion kinase (FAK) and vinculin to the alpha(v)beta3 integrin and reduced FAK and Src activation in response to VEGF. Addition of NO donors together with transfection of a constitutively active Src construct could circumvent the blockade of VEGF-induced migration by TP; however, neither intervention alone was sufficient. Thus, TP stimulation appears to limit angiogenesis, at least in part, by inhibiting the pro-angiogenic cytokine VEGF. These data further support a role for antagonism of TP activation in enhancing the angiogenic response in tissues exposed to elevated TXA2 levels in which revascularization is important.

Src kinase mediates angiotensin II-dependent increase in pulmonary endothelial nitric oxide synthase

We have previously demonstrated that angiotensin II (Ang II) stimulates nitric oxide (NO) production in bovine pulmonary artery endothelial cells (BPAECs) by increasing NO synthase (NOS) expression via the type 2 receptor. The purpose of this study was to identify the Ang II-dependent signaling pathway that mediates this increase in endothelial NOS (eNOS). The Ang II-dependent increase in eNOS expression is prevented when BPAECs are pretreated with the tyrosine kinase inhibitors, herbimycin A and 4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-D]pyrimidine, which also blocked Ang II-dependent mitogen-activated protein kinase (MAPK) kinase/extracellular-regulated protein kinase (MEK)-1 and MAPK phosphorylation, suggesting that Src is upstream of MAPK in this pathway. Transfection of BPAECs with an Src dominant negative mutant cDNA prevented the Ang II-dependent Src activation and increase in eNOS protein expression. PD98059, a MEK-1 inhibitor, prevented the Ang II-dependent phosphorylation of extracellular-regulated protein kinases 1 and 2 and increase in eNOS expression. Neither AG1478, an epidermal growth factor receptor kinase inhibitor, nor AG1295, a platelet derived growth factor receptor kinase inhibitor, had any effect on Ang II-stimulated Src activity, MAPK activation, or eNOS expression. Pertussis toxin prevented the Ang II-dependent increase in Src activity, MAPK activation, and eNOS expression. These data suggest that Ang II stimulates Src tyrosine kinase via a pertussis toxin-sensitive pathway, which in turn activates the MAPK pathway, resulting in increased eNOS protein expression in BPAECs.

Thromboxane A 2 Receptor Agonists Antagonize the Proangiogenic Effects of Fibroblast Growth Factor-2

Thromboxane (TX) A 2 is released from multiple cell types and is a prime mediator of the pathogenesis of many vascular events, including angiogenesis. Endothelial cells express TXA 2 receptors (TP) but the effects of TP stimulation on angiogenesis remain controversial. In this study, we show that stimulation of endothelial cell TP impairs ligand-induced FGF receptor internalization and consequently abrogates FGF-2-induced endothelial cell migration in vitro and angiogenesis in vivo. Prevention of FGF-2-induced angiogenesis was associated with expression of the TPβ isoform. The deficit in FGFR1 internalization was mediated through activation of TPβ preventing the FGF-2-mediated decrease in p53 expression, thus enhancing thrombospondin-1 (TSP-1) release from EC and reducing FGFR1 internalization. Once released TSP-1 interacted with the α v β 3 integrin on the EC surface. On stimulation, FGFR1 and α v β 3 were found to associate in a complex. We determined that complex formation was important for receptor internalization as conditions that inhibit FGFR1 internalization, such as inappropriate ligation of α v β 3 by either TSP-1 or a neutralizing antibody, disrupted the complex. These results establish a novel role for isoform specific regulation of angiogenesis by TP, provide the first functional significance for the existence of two TP isoforms in humans, and clarify the mechanism by which TP signaling regulates FGFR1 kinetics and signaling.

Heparin-binding epidermal growth factor cleavage mediates zinc-induced epidermal growth factor receptor phosphorylation

We have previously shown that exposure to zinc ions can activate epidermal growth factor (EGF) receptor (EGFR) signaling in murine fibroblasts and A431 cells through a mechanism involving Src kinase. While studying the effects of zinc ions in normal human bronchial epithelial cell, we uncovered evidence for an additional mechanism of Zn(2+)-induced EGFR activation. Exposure to Zn(2+) induced phosphorylation of EGFR at tyrosine 1068, a major autophosphorylation site, in a dose- and time-dependent fashion. This effect of Zn(2+) on EGFR was significantly blocked with an antibody against the ligand-binding domain of the receptor. Neutralizing antibodies against EGFR ligands revealed the involvement of heparin-binding EGF (HB-EGF) in Zn(2+)-induced EGFR phosphorylation. This observation was further supported by immunoblots showing elevated levels of HB-EGF released by Zn(2+)-exposed cells. Zymography showed the existence of matrix metalloproteinase-3 in Zn(2+)-challenged cells. Incubation with a specific matrix metalloproteinase-3 inhibitor suppressed Zn(2+)-induced EGFR phosphorylation as well as HB-EGF release. Therefore, these data support an autocrine or paracrine mechanism whereby Zn(2+) induces EGFR phosphorylation through the extracellular release of EGFR ligands, which may be mediated by metalloproteinases.

G protein-coupled receptor agonist-stimulated expression of ATF3/LRF-1 and c-myc and comitogenic effects in hepatocytes do not require EGF receptor transactivation

Several agonists acting on G protein-coupled receptors (GPCR) enhance the mitogenic effect of epidermal growth factor (EGF) in rat hepatocytes, through mechanisms that have only partially been clarified. Results in various cells have led to the idea that a major mechanism for GPCR-mediated stimulation of cell growth is transactivation of receptor tyrosine kinases, particularly the EGF receptor (EGFR), leading to rapid phosphorylation of the EGFR and activation of downstream signaling pathways. In the present study cultured rat hepatocytes were exposed to various GPCR agonists, including vasopressin, angiotensin II (Ang.II), norepinephrine, or prostaglandin F(2 alpha) (PGF(2 alpha)). None of these agents increased the phosphorylation of the EGFR or the docking protein Shc. Furthermore, we examined the effect of the GPCR agonists on the expression of two early response genes believed to be involved in growth activation. The GPCR agonists increased the mRNA expression of c-myc, and also of activating transcription factor 3 (ATF3)/liver regeneration factor-1 (LRF-1), which is a novel finding. Finally, the selective EGFR inhibitor AG1478 did not suppress the activation of extracellular signal-regulated kinase 1/2 (ERK1/2) or the induction of c-myc or ATF3/LRF-1 by the GPCR agonists, and did not prevent the comitogenic effects induced by these agents, while it blocked the effect of EGF on these responses. The results suggest that GPCR agonists induce expression of ATF3/LRF-1 and c-myc and exert comitogenic effects through mechanisms that do not require EGFR transactivation.

Arsenite and arsenate activate extracellular signal-regulated kinases 1/2 by an epidermal growth factor receptor-mediated pathway in normal human keratinocytes

BACKGROUND

Inorganic arsenic is an environmental contaminant and is associated with the increased risk of human skin cancer. Arsenic has been reported to activate or inhibit a variety of cellular signalling pathways which has effects on cell growth, differentiation and apoptosis. However, the molecular mechanisms of these arsenic-induced biological effects are not completely understood.

OBJECTIVES

To understand the molecular basis for the mode of action of arsenicals, we examined the effect of arsenite and arsenate on the activation of mitogen-activated protein kinases (MAPK) and the upstream signalling cascade in normal human epidermal keratinocytes (NHEK).

METHODS

NHEK were exposed to arsenite or arsenate. Western blot analysis was performed to determine the activation of extracellular signal-regulated kinases (ERK) 1/2, c-jun N-terminal kinases (JNK), p38, and MAPK or ERK kinases (MEK) 1/2. Epidermal growth factor receptor (EGFR) tyrosine phosphorylation and recruitment of its adaptor proteins, Shc and Grb2, to EGFR were detected by immunoprecipitation and Western blot analysis.

RESULTS

Both arsenicals activated ERK1/2, which are most highly activated in response to mitogenic stimulation, in addition to JNK and p38, which show greater activation in response to cellular stresses. The kinetics of ERK1/2 activation differed from those of JNK and p38 activation. Both arsenicals transiently activated ERK1/2 prior to JNK and p38 activation. MEK1/2, upstream kinases of ERK1/2, were also activated by arsenicals with similar time kinetics to that of ERK1/2 activation. To investigate a signalling pathway leading to activation of MEK1/2-ERK1/2, we examined the tyrosine phosphorylation of EGFR and Shc adapter protein. Both arsenicals stimulated tyrosine phosphorylation of EGFR and Shc. After arsenical treatment, Shc immunoprecipitates contained coprecipitated EGFR and Grb2, suggesting that both arsenicals induce the assembly of EGFR-Shc-Grb2 complexes. Both the EGFR inhibitor tyrphostin AG1478 and anti-EGFR blocking antibody markedly attenuated ERK1/2 activation induced by arsenicals, but did not affect JNK and p38 activation.

CONCLUSIONS

Our data indicate that both arsenite and arsenate activate the EGFR-Shc-Grb2-MEK1/2-ERK1/2 signalling cascade in NHEK.

PKC-delta and CaMKII-delta 2 mediate ATP-dependent activation of ERK1/2 in vascular smooth muscle

ATP, a purinergic receptor agonist, has been shown to be involved in vascular smooth muscle (VSM) cell DNA synthesis and cell proliferation during embryonic and postnatal development, after injury, and in atherosclerosis. One mechanism that ATP utilizes to regulate cellular function is through activation of ERK1/2. In the present study, we provide evidence that ATP-dependent activation of ERK1/2 in VSM cells utilizes specific isoforms of the multifunctional serine/threonine kinases, PKC, and Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) as intermediates. Selective inhibition of PKC-delta activity with rottlerin, or adenoviral overexpression of kinase-negative PKC-delta, attenuated the ATP- and phorbol 12,13-dibutyrate (PDBu)-stimulated ERK1/2 activation. Inhibition of PKC-alpha activity with Gö-6976, or adenoviral overexpression of kinase-negative PKC-alpha, was ineffective. Alternatively, treatment with KN-93, a selective inhibitor of CaMKII activation, or adenoviral overexpression of kinase-negative CaMKII-delta(2), inhibited ATP-dependent activation of ERK1/2 but had no effect on PDBu- or PDGF-stimulated ERK1/2. In addition, adenoviral overexpression of dominant-negative ras (Ad.HA-Ras(N17)) partially inhibited the ATP- and PDBu-induced activation of ERK1/2 and blocked ionomycin- and EGF-stimulated ERK1/2, and inhibition of tyrosine kinases with AG-1478, an EGFR inhibitor, or the src family kinase inhibitor PP2 attenuated ATP-stimulated ERK1/2 activation. Taken together, these data indicate that PKC-delta and CaMKII-delta(2) coordinately mediate ATP-dependent transactivation of EGF receptor, resulting in increased ERK1/2 activity in VSM cells.

Dependence of peroxisome proliferator-activated receptor ligand-induced mitogen-activated protein kinase signaling on epidermal growth factor receptor transactivation

Peroxisome proliferator-activated receptors (PPARs) are nuclear hormone receptors that function as ligand-activated transcription factors regulating lipid metabolism and homeostasis. In addition to their ability to regulate PPAR-mediated gene transcription, PPARalpha and gamma ligands have recently been shown to induce activation of mitogen-activated protein kinases (MAPKs), which in turn phosphorylate PPARs, thereby affecting transcriptional activity. However, the mechanism for PPAR ligand-dependent MAPK activation is unclear. In the current study, we demonstrate that various PPARalpha (nafenopin) and gamma (ciglitazone and troglitazone) agonists rapidly induced extracellular signal-regulated kinase (Erk) and/or p38 phosphorylation in rat liver epithelial cells (GN4). The selective epidermal growth factor receptor (EGFR) kinase inhibitors, PD153035 and ZD1839 (Iressa), abolished PPARalpha and gamma agonist-dependent Erk activation. Consistent with this, PPAR agonists increased tyrosine autophosphorylation of the EGFR as well as phosphorylation at a putative Src-specific site, Tyr845. Experiments with the Src inhibitor, PP2, and the antioxidant N-acetyl-L-cysteine revealed critical roles for Src and reactive oxygen species as upstream mediators of EGFR transactivation in response to PPAR ligands. Moreover, PPARalpha and gamma ligands increased Src autophosphorylation as well as kinase activity. EGFR phosphorylation, in turn, led to Ras-dependent Erk activation. In contrast, p38 activation by PPARalpha and gamma ligands occurred independently of Src, oxidative stress, the EGFR, and Ras. Interestingly, PPARalpha and gamma agonists caused rapid activation of proline-rich tyrosine kinase or Pyk2; Pyk2 as well as p38 phosphorylation was reduced by intracellular Ca2+ chelation without an observable effect on EGFR and Erk activation, suggesting a possible role for Pyk2 as an upstream activator of p38. In summary, PPARalpha and gamma ligands activate two distinct signaling cascades in GN4 cells leading to MAPK activation.

Gene expression profile of endothelin-1-induced growth in glomerular mesangial cells

Endothelin (ET)-1 is a vasoconstrictor and mitogen involved in vascular remodeling. Changes in gene expression that underlie control of cell growth by ET-1 remain poorly characterized. To identify pathways of growth control we used microarrays to analyze ET-1-regulated gene expression in human mesangial cells, an important ET-1 vascular target cell in vivo. Statistical assessment of differential expression (significance analysis of microarrays) revealed upregulated transcripts for growth factors [heparin-binding epidermal growth factor (EGF)-like growth factor (HB-EGF), fibroblast growth factor (FGF), interleukin (IL)-6] and downregulated transcripts for genes that inhibit growth (BAX, p27KIP1, DAD1). Consistent with the gene expression profile, quantitative RT-PCR and Western blotting confirmed induction of HB-EGF by ET-1. To test a functional role for HB-EGF in ET-1 signaling, we showed that exogenous HB-EGF stimulated phosphorylation of ErbB1 and growth of mesangial cells. ET-1-induced proliferation was blocked by an ErbB1 receptor-selective kinase inhibitor and by a specific ErbB1 receptor-neutralizing antibody. Proliferation in response to ET-1 was also inhibited by neutralizing antisera against human HB-EGF. Together, these results provide data for modeling ET-1 pathways for growth control and suggest a specific role for HB-EGF gene induction in mesangial cell growth in response to ET-1.

A Role for Angiotensin II in the Activation of Extracellular Signal-Regulated Kinases in the Liver During Hemorrhagic Shock

Hemorrhagic shock (HS) is a complex process that initiates a global stress response. However, the earliest signaling pathways responsible for initiating this response remain unidentified. We have investigated the involvement of the extracellular signal-regulated kinases (ERK 1/2; also known as p42/44) and their activation in the liver by angiotensin II in the early signal transduction after HS. Hemorrhage of mice to 25 mmHg for 30 min was associated with the activation of ERK 1/2 in the liver, and this was accompanied by a 6.7-fold elevation of circulating angiotensin II levels. Similar results were obtained in rats. Both the angiotensin II levels and ERK 1/2 phosphorylation were suppressed by administration of an angiotensin-converting enzyme inhibitor peptide. Plasma from shocked rats, but not shocked rats treated with the angiotensin-converting enzyme inhibitor, increased ERK 1/2 phosphorylation in cultured hepatocytes. Together, these data suggest that angiotensin II is an important stimulus for ERK 1/2 activation in the liver during HS.

PKC-zeta is required for angiotensin II-induced activation of ERK and synthesis of C-FOS in MCF-7 cells

We examined the signalling pathways responsible for the Ang II induction of growth in MCF-7 human breast cancer cells. Ang II in MCF-7 cells induced: (a) the translocation from the cytosol to membrane and nucleus of atypical protein kinase C-zeta (PKC-zeta) but not of PKC-alpha, -delta, - epsilon and -eta; (b) the expression of c-fos mRNA and protein; (c) the phosphorylation of the extracellular signal-regulated protein kinases 1 and 2 (ERK1/2). All these effects were due to the activation of the Ang II type I receptor (AT1) since they were blocked by the AT1 antagonist losartan. The Ang II-stimulated ERK1/2 phosphorylation was blocked by (a) high doses of staurosporine, inhibitor of PKC-zeta, and by a synthetic myristoylated peptide with sequences based on the endogenous PKC-zeta pseudosubstrate region (zeta-PS); (b) PD098059, a mitogen-activated protein kinase kinase inhibitor (MAPKK/MEK); and, moreover, (c) the inhibitors of phosphoinositide 3-kinases (PI3K), LY294002 and wortmannin, thus indicating that PI3K may act upstream of ERK1/2. The Ang II-evoked c-fos induction was blocked only by high doses of staurosporine and by zeta-PS whilst PD098059, LY294002 and wortmannin were ineffective, thus indicating that c-fos induction is not due to ERK1/2 activity. When the epidermal growth factor-receptor (EGFR) tyrosine kinase activity was inhibited by the use of its inhibitor AG1478, Ang II was still able to induce ERK1/2 phosphorylation and c-fos expression, therefore proving that the transactivation of EGFR was not required for these Ang II effects in MCF-7 cells. The previously reported proliferation of MCF-7 cells induced by Ang II was blocked by PD098059 and by wortmannin in a dose-dependent manner, thereby indicating that in MCF-7 cells the PI3K and ERK pathways mediate the mitogenic signalling of AT1. Our results suggest that in MCF-7 cells Ang II activates multiple signalling pathways involving PKC-zeta, PI3K and MAPK; of these pathways only PKC-zeta appears responsible for the induction of c-fos.

Angiotensin II activates extracellular signal regulated kinases via protein kinase C and epidermal growth factor receptor in breast cancer cells

Angiotensin II (Ang II) induces, through AT1, intracellular Ca(2+) increase in both normal and cancerous breast cells in primary culture (Greco et al., 2002 Cell Calcium 2:1-10). We here show that Ang II stimulated, in a dose-dependent manner, the 24 h-proliferation of breast cancer cells in primary culture, induced translocation of protein kinase C (PKC)-alpha, -beta1/2, and delta (but not -epsilon, -eta, -theta, -zeta, and -iota), and phosphorylated extracellular-regulated kinases 1 and 2 (ERK1/2). The proliferative effects of Ang II were blocked by the AT1 antagonist, losartan. Also epidermal growth factor (EGF) had mitogenic effects on serum-starved breast cancer cells since induced cell proliferation after 24 h and phosphorylation of ERK1/2. The Ang II-induced proliferation of breast cancer cells was reduced by (a) Gö6976, an inhibitor of conventional PKC-alpha and -beta1, (b) AG1478, an inhibitor of the tyrosine kinase of the EGF receptor (EGFR), and (c) downregulation of 1,2-diacylglycerol-sensitive PKCs achieved by phorbol 12-myristate 13-acetate (PMA). A complete inhibition of the Ang II-induced cell proliferation was achieved using the inhibitor of the mitogen activated protein kinase kinase (MAPKK or MEK), PD098059, or using Gö6976 together with AG1478. These results indicate that in human primary cultured breast cancer cells AT1 regulates mitogenic signaling pathways by two simultaneous mechanisms, one involving conventional PKCs and the other EGFR transactivation.

Extracellular signal-regulated kinase 1/2 activation by myometrial oxytocin receptor involves Galpha(q)Gbetagamma and epidermal growth factor receptor tyrosine kinase activation

The mechanisms by which oxytocin (OT) stimulates extracellular signal-regulated kinase 1/2 (ERK1/2) are only partially understood. OT receptor (OTR) signals predominantly through Galpha(q), but ERK1/2 phosphorylation (ERK1/2-P) in PHM1 myometrial cells was not eliminated by inhibition of downstream effectors such as phospholipase C or protein kinase C. Inconsistent with a Galpha(i)-coupled response, pertussis toxin inhibition of OT-induced ERK1/2-P was reversed by the protein kinase A inhibitors Rp-cAMPS and KT5720. Consistent with an inhibitory role for protein kinase A, pertussis toxin pretreatment raised cellular cAMP and 8-(4-chlorophenylthio)-cAMP inhibited OT-induced ERK1/2-P. Attenuation of the OT response by the Gbetagamma scavenger carboxyl terminus of the beta-adrenergic receptor kinase implicated a Gbetagamma-mediated pathway. In both COSM6 cells overexpressing OTR (OTR-COSM6) and in PHM1 cells, the epidermal growth factor receptor (EGFR) tyrosine kinase inhibitor AG1478 markedly reduced OT-induced ERK1/2-P, whereas the platelet-derived growth factor receptor tyrosine kinase inhibitor AG1296 had no effect. Furthermore, OT increased EGFR tyrosine phosphorylation in OTR-COSM6 cells, which was inhibited by AG1478 or EGTA plus thapsigargin pretreatment. AG1478 did not affect inositol 1,4,5-triphosphate production by OT or protein kinase C-stimulated ERK1/2-P but completely blocked ionomycin-induced ERK1/2-P and EGFR tyrosine phosphorylation. In both OTR-COSM6 and PHM1 cells, EGTA reduced OT-stimulated ERK1/2-P; no ERK1/2-P was observed when intracellular calcium increases were blocked by pretreatment with thapsigargin plus EGTA. These data are consistent with activation of a Gbetagamma-mediated pathway as a consequence of Galpha(q) activation in myometrium and OTR-COSM6 cells that results in increased ERK1/2-P. This pathway involves both EGFR activation and an influence of calcium.