Distinct ADAM metalloproteinases regulate G protein-coupled receptor-induced cell proliferation and survival

New insight into the role of the ADAM protease family in breast carcinoma progression

Protease and adhesion molecules play a very emphasized role in the occurrence or progression of metastasis in many types of cancers. In this context, a molecule that contains both protease and adhesion functions play a crucial role in metastasis. ADAMs (a disintegrin and metalloprotease) are molecules with this special characteristic. Recently, a lot of attention has been attracted to various ADAM molecules and researchers have tried to elucidate the role of ADAMs in breast cancer occurrence and progression. Disrupting ADAMs protease and adhesion capabilities can lead to the discovery of worthy therapeutic targets in breast cancer treatment. In this review, we intend to discuss the mechanism of action of various ADAM molecules, their relation to pathogenic processes of breast cancer, and their potential as possible targets for breast cancer treatment.

Identification of membrane proteins regulated by ADAM15 by SUSPECS proteomics

ADAM15 is a member of the disintegrin-metalloproteinase family of sheddases, which plays a role in several biological processes including cartilage homeostasis. In contrast with well-characterized ADAMs, such as the canonical sheddases ADAM17 and ADAM10, little is known about substrates of ADAM15 or how the enzyme exerts its biological functions. Herein, we used "surface-spanning enrichment with click-sugars (SUSPECS)" proteomics to identify ADAM15 substrates and/or proteins regulated by the proteinase at the cell surface of chondrocyte-like cells. Silencing of ADAM15 by siRNAs significantly altered membrane levels of 13 proteins, all previously not known to be regulated by ADAM15. We used orthogonal techniques to validate ADAM15 effects on 3 of these proteins which have known roles in cartilage homeostasis. This confirmed that ADAM15-silencing increased cell surface levels of the programmed cell death 1 ligand 2 (PDCD1LG2) and reduced cell surface levels of vasorin and the sulfate transporter SLC26A2 through an unknown post-translational mechanism. The increase of PDCD1LG2 by ADAM15 knockdown, a single-pass type I transmembrane protein, suggested it could be a proteinase substrate. However, shed PDCD1LG2 could not be detected even by a data-independent acquisition mass spectrometry, a highly sensitive method for identification and quantification of proteins in complex protein samples, suggesting that ADAM15 regulates PDCD1LG2 membrane levels by a mechanism different from ectodomain shedding.

Bombesin, endothelin, neurotensin and pituitary adenylate cyclase activating polypeptide cause tyrosine phosphorylation of receptor tyrosine kinases

Numerous peptides including bombesin (BB), endothelin (ET), neurotensin (NTS) and pituitary adenylate cyclase-activating polypeptide (PACAP) are growth factors for lung cancer cells. The peptides bind to G protein-coupled receptors (GPCRs) resulting in elevated cAMP and/or phosphatidylinositol (PI) turnover. In contrast, growth factors such as epidermal growth factor (EGF) or neuregulin (NRG)-1 bind to receptor tyrosine kinases (RTKs) such as the EGFR or HER3, increasing tyrosine kinase activity, resulting in the phosphorylation of protein substrates such as PI3K or phospholipase (PL)C. Peptide GPCRs can transactivate numerous RTKs, especially members of the EGFR/HER family resulting in increased phosphorylation of ERK, leading to cellular proliferation or increased phosphorylation of AKT, leading to cellular survival. GRCR antagonists and tyrosine kinase inhibitors are useful agents to prevent RTK transactivation and inhibit proliferation of cancer cells.

Interleukin 13 (IL-13)-regulated expression of the chondroprotective metalloproteinase ADAM15 is reduced in aging cartilage

Objective

The adamalysin metalloproteinase 15 (ADAM15) has been shown to protect against development of osteoarthritis in mice. Here, we have investigated factors that control ADAM15 levels in cartilage.

Design

Secretomes from wild-type and Adam15^−/− chondrocytes were compared by label-free quantitative mass spectrometry. mRNA was isolated from murine knee joints, either with or without surgical induction of osteoarthritis on male C57BL/6 mice, and the expression of Adam15 and other related genes quantified by RT-qPCR. ADAM15 in human normal and osteoarthritic cartilage was investigated similarly and by fluorescent immunohistochemistry. Cultured HTB94 chondrosarcoma cells were treated with various anabolic and catabolic stimuli, and ADAM15 mRNA and protein levels evaluated.

Results

There were no significant differences in the secretomes of chondrocytes from WT and Adam15^−/− cartilage. Expression of ADAM15 was not altered in either human or murine osteoarthritic cartilage relative to disease-free controls. However, expression of ADAM15 was markedly reduced upon aging in both species, to the extent that expression in joints of 18-month-old mice was 45-fold lower than in that 4.5-month-old animals. IL-13 increased expression of ADAM15 in HTB94 ​cells by 2.5-fold, while modulators of senescence and autophagy pathways had no effect. Expression of Il13 in the joint was reduced with aging, suggesting this cytokine may control ADAM15 levels in the joint.

Conclusion

Expression of the chondroprotective metalloproteinase ADAM15 is reduced in aging human and murine joints, possibly due to a concomitant reduction in IL-13 expression. We thus propose IL-13 as a novel factor contributing to increased osteoarthritis risk upon aging.

A rational roadmap for SARS-CoV-2/COVID-19 pharmacotherapeutic research and development: IUPHAR Review 29

In this review, we identify opportunities for drug discovery in the treatment of COVID-19 and, in so doing, provide a rational roadmap whereby pharmacology and pharmacologists can mitigate against the global pandemic. We assess the scope for targeting key host and viral targets in the mid-term, by first screening these targets against drugs already licensed, an agenda for drug repurposing, which should allow rapid translation to clinical trials. A simultaneous, multi-pronged approach using conventional drug discovery methods aimed at discovering novel chemical and biological means of targeting a short list of host and viral entities which should extend the arsenal of anti-SARS-CoV-2 agents. This longer term strategy would provide a deeper pool of drug choices for future-proofing against acquired drug resistance. Second, there will be further viral threats, which will inevitably evade existing vaccines. This will require a coherent therapeutic strategy which pharmacology and pharmacologists are best placed to provide. LINKED ARTICLES: This article is part of a themed issue on The Pharmacology of COVID-19. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v177.21/issuetoc.

Deregulated Lysophosphatidic Acid Metabolism and Signaling in Liver Cancer

Liver cancer is one of the leading causes of death worldwide due to late diagnosis and scarcity of treatment options. The major risk factor for liver cancer is cirrhosis with the underlying causes of cirrhosis being viral infection (hepatitis B or C), metabolic deregulation (Non-alcoholic fatty liver disease (NAFLD) in the presence of obesity and diabetes), alcohol or cholestatic disorders. Lysophosphatidic acid (LPA) is a bioactive phospholipid with numerous effects, most of them compatible with the hallmarks of cancer (proliferation, migration, invasion, survival, evasion of apoptosis, deregulated metabolism, neoangiogenesis, etc.). Autotaxin (ATX) is the enzyme responsible for the bulk of extracellular LPA production, and together with LPA signaling is involved in chronic inflammatory diseases, fibrosis and cancer. This review discusses the most important findings and the mechanisms related to ATX/LPA/LPAR involvement on metabolic, viral and cholestatic liver disorders and their progression to liver cancer in the context of human patients and mouse models. It focuses on the role of ATX/LPA in NAFLD development and its progression to liver cancer as NAFLD has an increasing incidence which is associated with the increasing incidence of liver cancer. Bearing in mind that adipose tissue accounts for the largest amount of LPA production, many studies have implicated LPA in adipose tissue metabolism and inflammation, liver steatosis, insulin resistance, glucose intolerance and lipogenesis. At the same time, LPA and ATX play crucial roles in fibrotic diseases. Given that hepatocellular carcinoma (HCC) is usually developed on the background of liver fibrosis, therapies that both delay the progression of fibrosis and prevent its development to malignancy would be very promising. Therefore, ATX/LPA signaling appears as an attractive therapeutic target as evidenced by the fact that it is involved in both liver fibrosis progression and liver cancer development.

ADAM15 mediates upregulation of Claudin-1 expression in breast cancer cells

A Disintegrin and Metalloproteinase-15 (ADAM15) is a transmembrane protein involved in protein ectodomain shedding, cell adhesion and signalling. We previously cloned and characterised alternatively spliced variants of ADAM15 that differ in their intracellular domains and demonstrated correlation of the expression of specific variants with breast cancer prognosis. In this study we have created isogenic cell panels (MDA-MB-231 and MCF-7) expressing five ADAM15 variants including wild-type and catalytically inactive forms. The expression of ADAM15 isoforms in MDA-MB-231 cells led to cell clustering to varying degree, without changes in EMT markers vimentin, slug and E-cadherin. Analysis of tight junction molecules revealed ADAM15 isoform specific, catalytic function dependent upregulation of Claudin-1. The expression of ADAM15A, and to a lesser degree of C and E isoforms led to an increase in Claudin-1 expression in MDA-MB-231 cells, while ADAM15B had no effect. In MCF-7 cells, ADAM15E was the principal variant inducing Claudin-1 expression. Sh-RNA mediated down-regulation of ADAM15 in ADAM15 over-expressing cells reduced Claudin-1 levels. Additionally, downregulation of endogenous ADAM15 expression in T47D cells by shRNA reduced endogenous Claudin-1 expression confirming a role for ADAM15 in regulating Claudin-1 expression. The PI3K/Akt/mTOR pathway was involved in regulating Claudin-1 expression downstream of ADAM15. Immunofluorescence analysis of MDA-MB-231 ADAM15A expressing cells showed Claudin-1 at cell-cell junctions, in the cytoplasm and nuclei. ADAM15 co-localised with Claudin-1 and ZO1 at cell-cell junctions. Immunoprecipitation analysis demonstrated complex formation between ADAM15 and ZO1/ZO2. These findings highlight the importance of ADAM15 Intra Cellular Domain-mediated interactions in regulating substrate selection and breast cancer cell phenotype.

The metalloproteinase ADAM15 is upregulated by shear stress and promotes survival of endothelial cells

Reduced shear stress resulting from disturbed blood flow can impair endothelial integrity and drive the development of vascular inflammatory lesions. Metalloproteinases of the ADAM family have been implicated in the regulation of cell survival and inflammatory responses. Here we investigate the mechanism and function of ADAM15 upregulation in primary flow cultured endothelial cells. Transcriptomic analysis indicated that within the ADAM family ADAM15 mRNA is most prominently upregulated (4-fold) when endothelial cells are exposed to physiologic shear stress. This induction was confirmed in venous, arterial and microvascular endothelial cells and is associated with increased presence of ADAM15 protein in the cell lysates (5.6-fold) and on the surface (3.1-fold). The ADAM15 promoter contains several consensus sites for the transcription factor KLF2 which is also upregulated by shear stress. Induction of endothelial KLF2 by simvastatin treatment is associated with ADAM15 upregulation (1.8-fold) which is suppressed by counteracting simvastatin with geranylgeranyl pyrophosphate. KLF2 overexpression promotes ADAM15 expression (2.1-fold) under static conditions whereas KLF2 siRNA knockdown prevents ADAM15 induction by shear stress. Functionally, ADAM15 promotes survival of endothelial cells challenged by growth factor depletion or TNF stimulation as shown by ADAM15 shRNA knockdown (1.6-fold). Exposure to shear stress increases endothelial survival while additional knockdown of ADAM15 reduces survival (6.7-fold) under flow conditions. Thus, physiologic shear stress resulting from laminar flow promotes KLF2 induced ADAM15 expression which contributes to endothelial survival. The absence of ADAM15 at low shear stress or static conditions may therefore lead to increased endothelial damage and promote vascular inflammation.

Enhancer RNA and NFκB-dependent P300 regulation of ADAMDEC1

We observed increased expression of ADAMDEC1 RNA in monocytes from patients with systemic lupus erythematosus. The precise role of ADAMDEC1 is uncertain and uniquely among metalloproteinases it utilizes a zinc-coordinating aspartic acid residue which allows it to escape inhibition by tissue inhibitor of metalloprotease-3 (TIMP-3). A closely related gene encodes the protein ADAM28, which is not up-regulated in lupus. We leveraged the ability to look at both gene's promoters and enhancers simultaneously. ADAMDEC1 was up-regulated by LPS while ADAM28 was not upregulated in the short term. We identified MAP kinases and NFκB as critical cell pathways regulating the expression of ADAMDEC1. These same pathways were implicated in driving the expression of the ADAMDEC1 upstream enhancer RNAs. We demonstrated that binding of the enhancer RNAs produced from the upstream enhancer were critically important and that p300 bound to both the RNA from the enhancer and the DNA at the enhancer. P300 binding to the enhancer was dependent on NFκB. These data define the critical pathways regulating the expression of ADAMDEC1 and extend our knowledge of the roles of enhancer RNAs and mechanistically links p300 and enhancer RNAs.

Glutamate-dependent ectodomain shedding of neuregulin-1 type II precursors in rat forebrain neurons

The neurotrophic factor neuregulin 1 (NRG1) regulates neuronal development, glial differentiation, and excitatory synapse maturation. NRG1 is synthesized as a membrane-anchored precursor and is then liberated by proteolytic processing or exocytosis. Mature NRG1 then binds to its receptors expressed by neighboring neurons or glial cells. However, the molecular mechanisms that govern this process in the nervous system are not defined in detail. Here we prepared neuron-enriched and glia-enriched cultures from embryonic rat neocortex to investigate the role of neurotransmitters that regulate the liberation/release of NRG1 from the membrane of neurons or glial cells. Using a two-site enzyme immunoassay to detect soluble NRG1, we show that, of various neurotransmitters, glutamate was the most potent inducer of NRG1 release in neuron-enriched cultures. NRG1 release in glia-enriched cultures was relatively limited. Furthermore, among glutamate receptor agonists, N-Methyl-D-Aspartate (NMDA) and kainate (KA), but not AMPA or tACPD, mimicked the effects of glutamate. Similar findings were acquired from analysis of the hippocampus of rats with KA-induced seizures. To evaluate the contribution of members of a disintegrin and metalloproteinase (ADAM) families to NRG1 release, we transfected primary cultures of neurons with cDNA vectors encoding NRG1 types I, II, or III precursors, each tagged with the alkaline phosphatase reporter. Analysis of alkaline phosphatase activity revealed that the NRG1 type II precursor was subjected to tumor necrosis factor-α-converting enzyme (TACE) / a Disintegrin And Metalloproteinase 17 (ADAM17) -dependent ectodomain shedding in a protein kinase C-dependent manner. These results suggest that glutamatergic neurotransmission positively regulates the ectodomain shedding of NRG1 type II precursors and liberates the active NRG1 domain in an activity-dependent manner.

SheddomeDB: the ectodomain shedding database for membrane-bound shed markers

BACKGROUND

A number of membrane-anchored proteins are known to be released from cell surface via ectodomain shedding. The cleavage and release of membrane proteins has been shown to modulate various cellular processes and disease pathologies. Numerous studies revealed that cell membrane molecules of diverse functional groups are subjected to proteolytic cleavage, and the released soluble form of proteins may modulate various signaling processes. Therefore, in addition to the secreted protein markers that undergo secretion through the secretory pathway, the shed membrane proteins may comprise an additional resource of noninvasive and accessible biomarkers. In this context, identifying the membrane-bound proteins that will be shed has become important in the discovery of clinically noninvasive biomarkers. Nevertheless, a data repository for biological and clinical researchers to review the shedding information, which is experimentally validated, for membrane-bound protein shed markers is still lacking.

RESULTS

In this study, the database SheddomeDB was developed to integrate publicly available data of the shed membrane proteins. A comprehensive literature survey was performed to collect the membrane proteins that were verified to be cleaved or released in the supernatant by immunological-based validation experiments. From 436 studies on shedding, 401 validated shed membrane proteins were included, among which 199 shed membrane proteins have not been annotated or validated yet by existing cleavage databases. SheddomeDB attempted to provide a comprehensive shedding report, including the regulation of shedding machinery and the related function or diseases involved in the shedding events. In addition, our published tool ShedP was embedded into SheddomeDB to support researchers for predicting the shedding event on unknown or unrecorded membrane proteins.

CONCLUSIONS

To the best of our knowledge, SheddomeDB is the first database for the identification of experimentally validated shed membrane proteins and currently may provide the most number of membrane proteins for reviewing the shedding information. The database included membrane-bound shed markers associated with numerous cellular processes and diseases, and some of these markers are potential novel markers because they are not annotated or validated yet in other databases. SheddomeDB may provide a useful resource for discovering membrane-bound shed markers. The interactive web of SheddomeDB is publicly available at http://bal.ym.edu.tw/SheddomeDB/ .

Overexpression of the A Disintegrin and Metalloproteinase ADAM15 is linked to a Small but Highly Aggressive Subset of Prostate Cancers

The A Disintegrin and Metalloproteinase (ADAM) family of endopeptidases plays a role in many solid cancers and includes promising targets for anticancer therapies. Deregulation of ADAM15 has been linked to tumor aggressiveness and cell line studies suggest that ADAM15 overexpression may also be implicated in prostate cancer. To evaluate the impact of ADAM15 expression and its relationship with key genomic alterations, a tissue microarray containing 12,427 prostate cancers was analyzed by immunohistochemistry. ADAM15 expression was compared to phenotype, prognosis and molecular features including TMPRSS2:ERG fusion and frequent deletions involving PTEN, 3p, 5q and 6q. Normal prostate epithelium did not show ADAM15 staining. In prostate cancers, negative, weak, moderate, and strong ADAM15 staining was found in 87.7%, 3.7%, 5.6%, and 3.0% of 9826 interpretable tumors. Strong ADAM15 staining was linked to high Gleason grade, advanced pathological tumor stage, positive nodal stage and resection margin. ADAM15 overexpression was also associated with TMPRSS2:ERG fusions and PTEN deletions (P<.0001) but unrelated to deletions of 3p, 5q and 6q. In univariate analysis, high ADAM15 expression was strongly linked to PSA recurrence (P<.0001). However, in multivariate analyses this association was only maintained if the analysis was limited to preoperatively available parameters in ERG-negative cancers. The results of our study demonstrate that ADAM15 is strongly up regulated in a small but highly aggressive fraction of prostate cancers. In these tumors, ADAM15 may represent a suitable drug target. In a preoperative scenario, ADAM15 expression measurement may assist prognosis assessment, either alone or in combination with other markers.

Fluorescent substrates for ADAM15 useful for assaying and high throughput screening

A disintegrin and metalloproteinase 15 (ADAM15), also known as metargidin, plays important roles in regulating inflammation, wound healing, neovascularization, and is an attractive drug target. Fluorescence resonance energy transfer (FRET)-based peptide substrates were tested to identify candidate reagents for high throughput screening and detection of ADAM15 in biological samples. ADAM15 exhibits a unique and diverse activity profile compared to other metalloproteinases. Two FRET substrates, Dabcyl-Gly-Pro-Leu-Gly-Met-Arg-Gly-Lys(FAM)-NH2 (PEPDAB011) and Dabcyl-Ala-Pro-Arg-Trp-Ile-Gln-Asp-Lys(FAM)-NH2 (PEPDAB017), which also detect activities of several matrix metalloproteinases (MMPs -2, -9, and -13), were efficiently cleaved by ADAM15 with specificity constants of 5800 M^-1 s^-1 and 4300 M^-1 s^-1, respectively. Additionally, ADAM15 efficiently processed Dabcyl-Leu-Arg-Glu-Gln-Gln-Arg-Leu-Lys-Ser-Lys(FAM)-NH2 (PEPDAB022), which is based on a physiological CD23 cleavage site, with a specificity constant (k_cat/K_m) of 5200 M^-1 s^-1. PEPDAB022 was used to screen the ability of known metalloproteinase inhibitors including TAPI-2, marimastat, GI-254023, and the Tissue Inhibitor of Metalloproteinases(TIMPs) 1 and 3 to block ADAM15 activity. Even though ADAM15 exhibits similar substrate preferences to other metalloproteinases, many broad spectrum inhibitors failed to block ADAM15 activity at concentrations as high as 50 μM. Thus, a clear need exists to develop potent and selective ADAM15 inhibitors, and the FRET substrates described herein should aid future research efforts towards this aim.

Methyl Protodioscin, a Steroidal Saponin, Inhibits Neointima Formation in Vitro and in Vivo

Restenosis (or neointimal hyperplasia) remains a clinical limitation of percutaneous coronary angioplasty. Abnormal proliferation and migration of vascular smooth muscle cells (VSMCs) are known to be involved in the development of restenosis. The present study aimed to investigate the ability and molecular mechanisms of methyl protodioscin (1), a steroidal saponin isolated from the root of Dioscorea nipponica, to inhibit neointimal formation. Our study demonstrated that 1 markedly inhibited the growth and migration of VSMCs (A7r5 cells). A cytometric analysis suggested that 1 induced growth inhibition by arresting VSMCs at the G1 phase of the cell cycle. A rat carotid artery balloon injury model indicated that neointima formation of the balloon-injured vessel was markedly reduced after extravascular administration of 1. Compound 1 decreased the expression levels of ADAM15 (a disintegrin and metalloprotease 15) and its downstream signaling pathways in the VSMCs. Moreover, the expressions and activities of matrix metalloproteinases (MMP-2 and MMP-9) were also suppressed by 1 in a concentration-dependent manner. Additionally, the molecular mechanisms appear to be mediated, in part, through the downregulation of ADAM15, FAK, ERK, and PI3K/Akt.

ADAM15 Is Functionally Associated with the Metastatic Progression of Human Bladder Cancer

ADAM15 is a member of a family of catalytically active disintegrin membrane metalloproteinases that function as molecular signaling switches, shed membrane bound growth factors and/or cleave and inactivate cell adhesion molecules. Aberrant metalloproteinase function of ADAM15 may contribute to tumor progression through the release of growth factors or disruption of cell adhesion. In this study, we utilized human bladder cancer tissues and cell lines to evaluate the expression and function of ADAM15 in the progression of human bladder cancer. Examination of genome and transcriptome databases revealed that ADAM15 ranked in the top 5% of amplified genes and its mRNA was significantly overexpressed in invasive and metastatic bladder cancer compared to noninvasive disease. Immunostaining of a bladder tumor tissue array designed to evaluate disease progression revealed increased ADAM15 immunoreactivity associated with increasing cancer stage and exhibited significantly stronger staining in metastatic samples. About half of the invasive tumors and the majority of the metastatic cases exhibited high ADAM15 staining index, while all low grade and noninvasive cases exhibited negative or low staining. The knockdown of ADAM15 mRNA expression significantly inhibited bladder tumor cell migration and reduced the invasive capacity of bladder tumor cells through Matrigel^TM and monolayers of vascular endothelium. The knockdown of ADAM15 in a human xenograft model of bladder cancer inhibited tumor growth by 45% compared to controls. Structural modeling of the catalytic domain led to the design of a novel ADAM15-specific sulfonamide inhibitor that demonstrated bioactivity and significantly reduced the viability of bladder cancer cells in vitro and in human bladder cancer xenografts. Taken together, the results revealed an undescribed role of ADAM15 in the invasion of human bladder cancer and suggested that the ADAM15 catalytic domain may represent a viable therapeutic target in patients with advanced disease.

The ADAM15 ectodomain is shed from secretory exosomes

We demonstrated previously that a disintegrin and metalloproteinase 15 (ADAM15) is released into the extracellular space as an exosomal component, and that ADAM15-rich exosomes have tumor suppressive functions. However, the suppressive mechanism of ADAM15-rich exosomes remains unclear. In this study, we show that the ADAM15 ectodomain is cleaved from released exosomes. This shedding process of the ADAM15 ectodomain was dramatically enhanced in conditioned ovarian cancer cell medium. Proteolytic cleavage was completely blocked by phenylmethylsulfonyl fluoride, indicating that a serine protease is responsible for exosomal ADAM15 shedding. Experimental evidence indicates that the ADAM15 ectodomain itself has comparable functions with those of ADAM15-rich exosomes, which effectively inhibit vitronectininduced cancer cell migration and activation of the MEK/extracellular regulated kinase signaling pathway. We present a tumor suppressive mechanism for ADAM15 exosomes and provide insight into the functional significance of exosomes that generate tumor-inhibitory factors. [BMB Reports 2015; 48(5): 277-282]

International Union of Basic and Clinical Pharmacology. XCIX. Angiotensin Receptors: Interpreters of Pathophysiological Angiotensinergic Stimuli [corrected]

The renin angiotensin system (RAS) produced hormone peptides regulate many vital body functions. Dysfunctional signaling by receptors for RAS peptides leads to pathologic states. Nearly half of humanity today would likely benefit from modern drugs targeting these receptors. The receptors for RAS peptides consist of three G-protein-coupled receptors—the angiotensin II type 1 receptor (AT1 receptor), the angiotensin II type 2 receptor (AT2 receptor), the MAS receptor—and a type II trans-membrane zinc protein—the candidate angiotensin IV receptor (AngIV binding site). The prorenin receptor is a relatively new contender for consideration, but is not included here because the role of prorenin receptor as an independent endocrine mediator is presently unclear. The full spectrum of biologic characteristics of these receptors is still evolving, but there is evidence establishing unique roles of each receptor in cardiovascular, hemodynamic, neurologic, renal, and endothelial functions, as well as in cell proliferation, survival, matrix-cell interaction, and inflammation. Therapeutic agents targeted to these receptors are either in active use in clinical intervention of major common diseases or under evaluation for repurposing in many other disorders. Broad-spectrum influence these receptors produce in complex pathophysiological context in our body highlights their role as precise interpreters of distinctive angiotensinergic peptide cues. This review article summarizes findings published in the last 15 years on the structure, pharmacology, signaling, physiology, and disease states related to angiotensin receptors. We also discuss the challenges the pharmacologist presently faces in formally accepting newer members as established angiotensin receptors and emphasize necessary future developments.

Cholinergic transactivation of the EGFR in HaCaT keratinocytes stimulates a flotillin-1 dependent MAPK-mediated transcriptional response

Acetylcholine and its receptors regulate numerous cellular processes in keratinocytes and other non-neuronal cells. Muscarinic acetylcholine receptors are capable of transactivating the epidermal growth factor receptor (EGFR) and, downstream thereof, the mitogen-activated protein kinase (MAPK) cascade, which in turn regulates transcription of genes involved in cell proliferation and migration. We here show that cholinergic stimulation of human HaCaT keratinocytes results in increased transcription of matrix metalloproteinase MMP-3 as well as several ligands of the epidermal growth factor family. Since both metalloproteinases and the said ligands are involved in the transactivation of the EGFR, this transcriptional upregulation may provide a positive feed-forward loop for EGFR/MAPK activation. We here also show that the cholinergic EGFR and MAPK activation and the upregulation of MMP-3 and EGF-like ligands are dependent on the expression of flotillin-1 which we have previously shown to be a regulator of MAPK signaling.

Niacin activates the PI3K/Akt cascade via PKC- and EGFR-transactivation-dependent pathways through hydroxyl-carboxylic acid receptor 2

Niacin has been demonstrated to activate a PI3K/Akt signaling cascade to prevent brain damage after stroke and UV-induced skin damage; however, the underlying molecular mechanisms for HCA₂-induced Akt activation remain to be elucidated. Using CHO-K1 cells stably expressing HCA₂ and A431 cells, a human epidermoid cell line with high levels of endogenous expression of functional HCA₂ receptors, we first demonstrated that niacin induced a robust Akt phosphorylation at both Thr³⁰⁸ and Ser⁴⁷³ in a time-dependent fashion, with a maximal activation at 5 min and a subsequent reduction to baseline by 30 min through HCA₂, and that the activation was significantly blocked by pertussis toxin. The HCA₂-mediated activation of Akt was also significantly inhibited by the PKC inhibitors GF109203x and Go6983 in both cell lines, by the PDGFR-selective inhibitor tyrphostin A9 in CHO-HCA₂ cells and by the MMP inhibitor GM6001 and EGFR-specific inhibitor AG1478 in A431 cells. These results suggest that the PKC pathway and PDGFR/EGFR transactivation pathway play important roles in HCA₂-mediated Akt activation. Further investigation indicated that PI3K and the G_βγ subunit were likely to play an essential role in HCA₂-induced Akt activation. Moreover, Immunobloting analyses using an antibody that recognizes p70S6K1 phosphorylated at Thr³⁸⁹ showed that niacin evoked p70S6K1 activation via the PI3K/Akt pathway. The results of our study provide new insight into the signaling pathways involved in HCA₂ activation.

G protein-coupled receptors as oncogenic signals in glioma: emerging therapeutic avenues

Gliomas are the most common malignant intracranial tumors. Newly developed targeted therapies for these cancers aim to inhibit oncogenic signals, many of which emanate from receptor tyrosine kinases, including the epidermal growth factor receptor (EGFR) and the vascular endothelial growth factor receptor (VEGFR). Unfortunately, the first-generation treatments targeting these oncogenic signals provide little survival benefit in both mouse xenograft models and human patients. The search for new treatment options has uncovered several G protein-coupled receptor (GPCR) candidates and generated a growing interest in this class of proteins as alternative therapeutic targets for the treatment of various cancers, including glioblastoma multiforme (GBM). GPCRs constitute a large family of membrane receptors that influence oncogenic pathways through canonical and non-canonical signaling. Accordingly, evidence indicates that GPCRs display a unique ability to crosstalk with receptor tyrosine kinases, making them important molecular components controlling tumorigenesis. This review summarizes the current research on GPCR functionality in gliomas and explores the potential of modulating these receptors to treat this devastating disease.

Targeting the sheddase activity of ADAM17 by an anti-ADAM17 antibody D1(A12) inhibits head and neck squamous cell carcinoma cell proliferation and motility via blockage of bradykinin induced HERs transactivation

A disintegrin and metalloproteinase 17 (ADAM17) regulates key cellular processes including proliferation and migration through the shedding of a diverse array of substrates such as epidermal growth factor receptor (EGFR) ligands. ADAM17 is implicated in the pathogenesis of many diseases including rheumatoid arthritis and cancers such as head and neck squamous cell carcinoma (HNSCC). As a central mediator of cellular events, overexpressed EGFR is a validated molecular target in HNSCC. However, EGFR inhibition constantly leads to tumour resistance. One possible mechanism of resistance is the activation of alternative EGFR family receptors and downstream pathways via the release of their ligands. Here, we report that treating human HNSCC cells in vitro with a human anti-ADAM17 inhibitory antibody, D1(A12), suppresses proliferation and motility in the absence or presence of the EGFR tyrosine kinase inhibitor (TKI) gefitinib. Treatment with D1(A12) decreases both the endogenous and the bradykinin (BK)-stimulated shedding of HER ligands, accompanied by a reduction in the phosphorylation of HER receptors and downstream signalling pathways including STAT3, AKT and ERK. Knockdown of ADAM17, but not ADAM10, also suppresses HNSCC cell proliferation and migration. Furthermore, we show that heregulin (HRG) and heparin-binding epidermal growth factor like growth factor (HB-EGF) predominantly participate in proliferation and migration, respectively. Taken together, these results demonstrate that D1(A12)-mediated inhibition of cell proliferation, motility, phosphorylation of HER receptors and downstream signalling is achieved via reduced shedding of ADAM17 ligands. These findings underscore the importance of ADAM17 and suggest that D1(A12) might be an effective targeted agent for treating EGFR TKI-resistant HNSCC.

Erbb2 up-regulation of ADAM12 expression accelerates skin cancer progression

Solar ultraviolet (UV) radiation can cause severe damage to the skin and is the primary cause of most skin cancer. UV radiation causes DNA damage leading to mutations and also activates the Erbb2/HER2 receptor through indirect mechanisms involving reactive oxygen species. We hypothesized that Erbb2 activation accelerates the malignant progression of UV-induced skin cancer. Following the induction of benign squamous papillomas by UV exposure of v-ras(Ha) transgenic Tg.AC mice, mice were treated topically with the Erbb2 inhibitor AG825 and tumor progression monitored. AG825 treatment reduced tumor volume, increased tumor regression, and delayed the development of malignant squamous cell carcinoma (SCC). Progression to malignancy was associated with increased Erbb2 and ADAM12 (A Disintegin And Metalloproteinase 12) transcripts and protein, while inhibition of Erbb2 blocked the increase in ADAM12 message upon malignant progression. Similarly, human SCC and SCC cell lines had increased ADAM12 protein and transcripts when compared to normal controls. To determine whether Erbb2 up-regulation of ADAM12 contributed to malignant progression of skin cancer, Erbb2 expression was modulated in cultured SCC cells using forced over-expression or siRNA targeting, demonstrating up-regulation of ADAM12 by Erbb2. Furthermore, ADAM12 transfection or siRNA targeting revealed that ADAM12 increased both the migration and invasion of cutaneous SCC cells. Collectively, these results suggest Erbb2 up-regulation of ADAM12 as a novel mechanism contributing to the malignant progression of UV-induced skin cancer. Inhibition of Erbb2/HER2 reduced tumor burden, increased tumor regression, and delayed the progression of benign skin tumors to malignant SCC in UV-exposed mice. Inhibition of Erbb2 suppressed the increase in metalloproteinase ADAM12 expression in skin tumors, which in turn increased migration and tumor cell invasiveness.

Regulation of receptor tyrosine kinase ligand processing

A primary mode of regulating receptor tyrosine kinase (RTK) signaling is to control access of ligand to its receptor. Many RTK ligands are synthesized as transmembrane proteins. Frequently, the active ligand must be released from the membrane by proteolysis before signaling can occur. Here, we discuss RTK ligand shedding and describe the proteases that catalyze it in flies and mammals. We focus principally on the control of EGF receptor ligand shedding, but also refer to ligands of other RTKs. Two prominent themes emerge. First, control by regulated trafficking and cellular compartmentalization of the proteases and their ligand substrates plays a key role in shedding. Second, many external signals converge on the shedding proteases and their control machinery. Proteases therefore act as regulatory hubs that integrate information that the cell receives and translate it into precise outgoing signals. The activation of signaling by proteases is therefore an essential element of the cellular communication machinery.

Angiotensin II induced proteolytic cleavage of myocardial ACE2 is mediated by TACE/ADAM-17: a positive feedback mechanism in the RAS

Angiotensin converting enzyme (ACE) 2 is a key negative regulator of the renin-angiotensin system where it metabolizes angiotensin (Ang) II into Ang 1-7. We hypothesize that Ang II suppresses ACE2 by increasing TNF-α converting enzyme (TACE) activity and ACE2 cleavage. Ang II infusion (1.5 mg/kg/day) in wild-type mice for 2 weeks resulted in substantial decrease in myocardial ACE2 protein levels and activity with corresponding increase in plasma ACE2 activity, prevented by AT1R blockade. Ang II resulted in AT1R-mediated increase in myocardial TACE expression and activity, and membrane translocation of TACE. Ang II treatment in Huh7 cells exhibited AT1R-dependent metalloproteinase mediated shedding of ACE2 while transfection with siTACE prevented shedding of ACE2; cardiomyocyte-specific deletion of TACE also prevented shedding of ACE2. Reactive oxygen species played a key role since p47(phox)KO mice were resistant to Ang II-induced TACE phosphorylation and activation with preservation of myocardial ACE2 which dampened Ang II-induced cardiac dysfunction and hypertrophy. In conclusion, Ang II induces ACE2 shedding by promoting TACE activity as a positive feedback mechanism whereby Ang II facilitates the loss of its negative regulator, ACE2. In HF, elevated plasma ACE2 activity likely represents loss of the protective effects of ACE2 in the heart.

Inhibition of Src kinase blocks high glucose-induced EGFR transactivation and collagen synthesis in mesangial cells and prevents diabetic nephropathy in mice

Chronic exposure to high glucose leads to diabetic nephropathy characterized by increased mesangial matrix protein (e.g., collagen) accumulation. Altered cell signaling and gene expression accompanied by oxidative stress have been documented. The contribution of the tyrosine kinase, c-Src (Src), which is sensitive to oxidative stress, was examined. Cultured rat mesangial cells were exposed to high glucose (25 mmol/L) in the presence and absence of Src inhibitors (PP2, SU6656), Src small interfering RNA (siRNA), and the tumor necrosis factor-α-converting enzyme (TACE) inhibitor, TAPI-2. Src was investigated in vivo by administration of PP2 to streptozotocin (STZ)-induced diabetic DBA2/J mice. High glucose stimulated Src, TACE, epidermal growth factor receptor (EGFR), mitogen-activated protein kinases (MAPKs), extracellular signal-regulated kinase (ERK1/2, p38), and collagen IV accumulation in mesangial cells. PP2 and SU6656 blocked high glucose-stimulated phosphorylation of Src Tyr-416, EGFR, and MAPKs. These inhibitors and Src knockdown by siRNA, as well as TAPI-2, also abrogated high glucose-induced phosphorylation of these targets and collagen IV accumulation. In STZ-diabetic mice, albuminuria, increased Src pTyr-416, TACE activation, ERK and EGFR phosphorylation, glomerular collagen accumulation, and podocyte loss were inhibited by PP2. These data indicate a role for Src in a high glucose-Src-TACE-heparin-binding epidermal growth factor-EGFR-MAPK-signaling pathway to collagen accumulation. Thus, Src may provide a novel therapeutic target for diabetic nephropathy.

The cell surface proteome of Entamoeba histolytica

Surface molecules are of major importance for host-parasite interactions. During Entamoeba histolytica infections, these interactions are predicted to be of prime importance for tissue invasion, induction of colitis and liver abscess formation. To date, however, little is known about the molecules involved in these processes, with only about 20 proteins or protein families found exposed on the E. histolytica surface. We have therefore analyzed the complete surface proteome of E. histolytica. Using cell surface biotinylation and mass spectrometry, 693 putative surface-associated proteins were identified. In silico analysis predicted that ∼26% of these proteins are membrane-associated, as they contain transmembrane domains and/or signal sequences, as well as sites of palmitoylation, myristoylation, or prenylation. An additional 25% of the identified proteins likely represent nonclassical secreted proteins. Surprisingly, no membrane-association sites could be predicted for the remaining 49% of the identified proteins. To verify surface localization, 23 proteins were randomly selected and analyzed by immunofluorescence microscopy. Of these 23 proteins, 20 (87%) showed definite surface localization. These findings indicate that a far greater number of E. histolytica proteins than previously supposed are surface-associated, a phenomenon that may be based on the high membrane turnover of E. histolytica.

Epithelial wounds induce differential phosphorylation changes in response to purinergic and EGF receptor activation

Protein phosphorylation is a dynamic post-translational modification. Mass spectrometry-based quantitation was performed to determine the phosphoproteome profile of epithelial cells in response to injury, nucleotide, or epidermal growth factor. Phosphotyrosine enrichment used immunoprecipitation and immobilized metal affinity chromatography. Nucleotides released after scratch wounding activate purinergic receptors, leading to a distinct phosphorylation profile on epidermal growth factor receptor (EGFR) compared with its natural ligand. ATP induced a 2- to 15-fold phosphorylation increase over control on EGFR Y974, Y1086, and Y1148, with minimal phosphorylation intensity on EGFR Y1173 compared with the level measured in response to epidermal growth factor. Differential phosphorylation induced by epidermal growth factor or ATP was site specific on Src, Shc, phospholipase Cγ, protein kinase C, focal adhesion kinase, paxillin, and mitogen-activated protein kinases 1, 12, and 13. After wounding, the P2Y2 receptor mRNA expression increased, and after knockdown, migration and Ca(2+) mobilization were impaired. To examine phosphorylation mediated by P2Y2, cells were cultured in media containing stable isotope-labeled amino acids, the receptor was knocked down, and the cells were stimulated. Mass spectrometry-based comparison of the phosphorylation profiles of control versus transfected cells revealed a 50-fold decrease in phosphorylation of EGFR Y974 and 1086, with no decrease in Y1173 phosphorylation. A similarfold decrease in Src Y421 and Y446 and paxillin Y118 was detected, indicating the far-reaching importance of the P2Y2 receptor in mediating migration.

Unravelling the molecular complexity of GPCR-mediated EGFR transactivation using functional genomics approaches

To influence physiology and pathophysiology, G protein-coupled receptors (GPCRs) have evolved to appropriate additional signalling modalities, such as activation of adjacent membrane receptors. Epidermal growth factor receptors (EGFRs) mediate growth and remodelling actions of GPCRs, although the precise network of gene products and molecular cascades linking GPCRs to EGFRs (termed EGFR transactivation) remains incomplete. In this review, we describe the current view of GPCR-EGFR transactivation, identifying the established models of receptor cross-talk. We consider the limitations in our current knowledge, and propose that recent advances in molecular and cell biology technology, including functional genomics approaches, will allow a renewed focus of efforts to understand the mechanism underlying EGFR transactivation. Using an unbiased approach for identification of the molecules required for GPCR-mediated EGFR transactivation will provide a contemporary and more complete representation from which to extrapolate therapeutic control in diseases from cardiovascular remodelling to cancer.

Stimulation of α1a adrenergic receptors induces cellular proliferation or antiproliferative hypertrophy dependent solely on agonist concentration

Stimulation of α_1aAdrenergic Receptors (ARs) is known to have anti-proliferative and hypertrophic effects; however, some studies also suggests this receptor can increase cell proliferation. Surprisingly, we find the α_1aAR expressed in rat-1 fibroblasts can produce either phenotype, depending exclusively on agonist concentration. Stimulation of the α_1aAR by high dose phenylephrine (>10⁻⁷ M) induces an antiproliferative, hypertrophic response accompanied by robust and extended p38 activation. Inhibition of p38 with SB203580 prevented the antiproliferative response, while inhibition of Erk or Jnk had no effect. In stark contrast, stimulation of the α_1aAR with low dose phenylephrine (∼10⁻⁸ M) induced an Erk-dependent increase in cellular proliferation. Agonist-induced Erk phosphorylation was preceded by rapid FGFR and EGFR transactivation; however, only EGFR inhibition blocked Erk activation and proliferation. The general matrix metalloprotease inhibitor, GM6001, blocked agonist induced Erk activation within seconds, strongly suggesting EGFR activation involved extracellular triple membrane pass signaling. Erk activation required little Ca²⁺ release and was blocked by PLCβ or PKC inhibition but not by intracellular Ca²⁺ chelation, suggesting Ca²⁺ independent activation of novel PKC isoforms. In contrast, Ca²⁺ release was essential for PI3K/Akt activation, which was acutely maximal at non-proliferative doses of agonist. Remarkably, our data suggests EGFR transactivation leading to Erk induced proliferation has the lowest activation threshold of any α_1aAR response. The ability of α_1aARs to induce proliferation are discussed in light of evidence suggesting antagonistic growth responses reflect native α_1aAR function.

GPR87 mediates lysophosphatidic acid-induced colony dispersal in A431 cells

We have previously reported that an orphan G protein-coupled receptor GPR87 was activated by lysophosphatidic acid (LPA) and that it induced an increase in the intracellular Ca(2+) levels in the CHO cells genetically engineered to express GPR87-Gα16 fusion protein. Because the Ca(2+) response was blocked by the LPA receptor antagonist Ki16425, GPR87 was suggested to be a putative LPA receptor. However, further studies are required to confirm whether GPR87 is an LPA receptor. A previous study showed that colonies of A431 cells treated with LPA showed rapid and synchronized dissociation. Because A431 cells have been shown to express GPR87, we used these cells to examine whether GPR87 acted as an LPA receptor. When A431 cells were treated with gpr87-specific siRNA, the expression of GPR87 was decreased and LPA-induced colony dispersal was significantly reduced. Treatment of the cells with lpa1 siRNA had an additive effect in decrease in the colony dispersal. Studies on the LPA-mediated signaling pathway in A431 cells indicated that transactivation of the epidermal growth factor receptor (EGFR) by LPA led to cell scattering. PD153035, an inhibitor of tyrosine-kinase of EGFR, and BB94, an inhibitor of metalloprotease which produces a ligand for EGFR, significantly prevented the LPA-induced scattering of A431 cells pretreated with lpa1 or gpr87-siRNA. These results strongly suggested that GPR87 acts as an LPA receptor and induces colony dispersal via the transactivation of EGFR in A431 cells.

Roles of proteolysis in regulation of GPCR function

The enzymatic activity of peptidases must be tightly regulated to prevent uncontrolled hydrolysis of peptide bonds, which could have devastating effects on biological systems. Peptidases are often generated as inactive propeptidases, secreted with endogenous inhibitors, or they are compartmentalized. Propeptidases become active after proteolytic removal of N-terminal activation peptides by other peptidases. Some peptidases only become active towards substrates only at certain pHs, thus confining activity to specific compartments or conditions. This review discusses the different roles proteolysis plays in regulating GPCRs. At the cell-surface, certain GPCRs are regulated by the hydrolytic inactivation of bioactive peptides by membrane-anchored peptidases, which prevent signalling. Conversely, cell-surface peptidases can also generate bioactive peptides, which directly activate GPCRs. Alternatively, cell-surface peptidases activated by GPCRs, can generate bioactive peptides to cause transactivation of receptor tyrosine kinases, thereby promoting signalling. Certain peptidases can signal directly to cells, by cleaving GPCR to initiate intracellular signalling cascades. Intracellular peptidases also regulate GPCRs; lysosomal peptidases destroy GPCRs in lysosomes to permanently terminate signalling and mediate down-regulation; endosomal peptidases cleave internalized peptide agonists to regulate GPCR recycling, resensitization and signalling; and soluble intracellular peptidases also participate in GPCR function by regulating the ubiquitination state of GPCRs, thereby altering GPCR signalling and fate. Although the use of peptidase inhibitors has already brought success in the treatment of diseases such as hypertension, the discovery of new regulatory mechanisms involving proteolysis that control GPCRs may provide additional targets to modulate dysregulated GPCR signalling in disease.

Human breast cancer-associated fibroblasts enhance cancer cell proliferation through increased TGF-α cleavage by ADAM17

We demonstrate here increased expression of ADAM17 protein in cancer-associated fibroblasts (CAFs) extracted from human breast carcinomas compared with donor-matched normal fibroblasts, and TGF-α secretion positively correlates with ADAM17 expression in these cells. In SK-BR-3 cells co-cultured with CAFs, CAF-secreted TGF-α promotes cell proliferation by activation of EGFR, Akt, and ERK, but it does not promote cell migration. Furthermore, anti-TGF-α neutralizing antibodies antagonize the CAF-dependent increase in proliferation and activation of EGFR, Akt and ERK. Thus, pharmacologic inhibition of ADAM17 and TGF-α may have therapeutic potential for the treatment of breast cancer when fibroblast-directed therapy is considered.

Human antigen R-mediated mRNA stabilization is required for ultraviolet B-induced autoinduction of amphiregulin in keratinocytes

All members of the EGF family are produced as transmembrane precursors that are proteolytically processed into soluble forms by disintegrin and metalloproteinases (ADAMs) for autocrine/paracrine pathways. In turn, the ligand-activated EGF receptor (EGFR) induces the expression of EGF family members, so-called "autoinduction." However, it is not well understood how this autoinduction occurs. In this study, we investigated the molecular mechanism of the autoinduction of amphiregulin (AREG), a member of the EGF family. We found that ultraviolet B (UVB) exposure increased the AREG mRNA level by stabilization of its mRNA in a human immortalized keratinocyte cell line, HaCaT. The 3' UTR of AREG mRNA was responsible for binding to an mRNA-binding protein, human antigen R (HuR), and the interaction between AREG mRNA and HuR was enhanced by UVB. Inducible knockdown of HuR expression significantly decreased AREG mRNA stability. Interestingly, treatment of HaCaT cells with an EGFR inhibitor, an EGFR neutralizing antibody, or an ADAM inhibitor destabilized AREG mRNA. In the case of ADAM inhibition, administration of soluble AREG restored the mRNA level, indicating that the stabilization occurs in a shedding-dependent manner of EGFR ligands. The HuR dependence of AREG mRNA and protein expression was also confirmed in human primary keratinocytes. Taken together, we propose a novel mechanism by which HuR regulates the stability of AREG mRNA in keratinocytes after UVB exposure and suggest that targeting of HuR functions might be crucial for understanding skin cancers caused by aberrant EGF family member-EGFR signaling.

Activated human hydroxy-carboxylic acid receptor-3 signals to MAP kinase cascades via the PLC-dependent PKC and MMP-mediated EGFR pathways

BACKGROUND AND PURPOSE 3-Hydroxy-octanoate, recently identified as a ligand for, the orphan GPCR, HCA(3), is of particular interest given its ability to treat lipid disorders and atherosclerosis. Here we demonstrate the pathway of HCA(3)-mediated activation of ERK1/2. EXPERIMENTAL APPROACH Using CHO-K1 cells stably expressing HCA(3) receptors and A431 cells, a human epidermoid cell line with high levels of endogenous expression of functional HCA(3) receptors, HCA(3)-mediated activation of ERK1/2 was measured by Western blot. KEY RESULTS HCA(3)-mediated activation of ERK1/2 was rapid, peaking at 5 min, and was Pertussis toxin sensitive. Our data, obtained by time course analyses in combination with different kinase inhibitors, demonstrated that on agonist stimulation, HCA(3) receptors evoked ERK1/2 activation via two distinct pathways, the PLC/PKC pathway at early time points (≤ 2 min) and the MMP/ epidermal growth factor receptor (EGFR) transactivation pathway with a maximum response at 5 min. Furthermore, our present results also indicated that the βγ-subunits of the G(i) protein play a critical role in HCA(3)-activated ERK1/2 phosphorylation, whereas β-arrestins and Src were not required for ERK1/2 activation. CONCLUSIONS AND IMPLICATIONS We have described the molecular mechanisms underlying the coupling of human HCA(3) receptors to the ERK1/2 MAP kinase pathway in CHO-K1 and A431 cells, which implicate the G(i) protein-initiated, PLC/PKC -and platelet-derived growth factor receptor/EGFR transactivation-dependent pathways. These observations may provide new insights into the pharmacological effects and the physiological functions modulated by the HCA(3)-mediated activation of ERK1/2.

The relationship between the MMP system, adrenoceptors and phosphoprotein phosphatases

The MMPs and their inhibitors [tissue inhibitor of MMPs (TIMPs)] form the mainstay of extracellular matrix homeostasis. They are expressed in response to numerous stimuli including cytokines and GPCR activation. This review highlights the importance of adrenoceptors and phosphoprotein phosphatases (PPP) in regulating MMPs in the cardiovascular system, which may help explain some of the beneficial effects of targeting the adrenoceptor system in tissue remodelling and will establish emerging crosstalk between these three systems. Although α- and β-adrenoceptor activation increases MMP but decreases TIMP expression, MMPs are implicated in the growth stimulatory effects of adrenoceptor activation through transactivation of epidermal growth factor receptor. Furthermore, they have recently been found to catalyse the proteolysis of β-adrenoceptors and modulate vascular tone. While the mechanisms underpinning these effects are not well defined, reversible protein phosphorylation by kinases and phosphatases may be key. In particular, PPP (Ser/Thr phosphatases) are not only critical in resensitization and internalization of adrenoceptors but also modulate MMP expression. The interrelationship is complex as isoprenaline (ISO) inhibits okadaic acid [phosphoprotein phosphatase type 1/phosphoprotein phosphatase type 2A (PP2A) inhibitor]-mediated MMP expression. While this may be simply due to its ability to transiently increase PP2A activity, there is evidence for MMP-9 that ISO prevents okadaic acid-mediated expression of MMP-9 through a β-arrestin, NF-κB-dependent pathway, which is abolished by knock-down of PP2A. It is essential that crosstalk between MMPs, adrenoceptors and PPP are investigated further as it will provide important insight into how adrenoceptors modulate cardiovascular remodelling, and may identify new targets for pharmacological manipulation of the MMP system.

Mechanisms involved in PGE2-induced transactivation of the epidermal growth factor receptor in MH1C1 hepatocarcinoma cells

Background

It is important to understand the mechanisms by which the cells integrate signals from different receptors. Several lines of evidence implicate epidermal growth factor (EGF) receptor (EGFR) in the pathophysiology of hepatocarcinomas. Data also suggest a role of prostaglandins in some of these tumours, through their receptors of the G protein-coupled receptor (GPCR) family. In this study we have investigated mechanisms of interaction between signalling from prostaglandin receptors and EGFR in hepatocarcinoma cells.

Methods

The rat hepatocarcinoma cell line MH₁C₁ and normal rat hepatocytes in primary culture were stimulated with EGF or prostaglandin E₂ (PGE₂) and in some experiments also PGF_2α. DNA synthesis was determined by incorporation of radiolabelled thymidine into DNA, phosphorylation of proteins in signalling pathways was assessed by Western blotting, mRNA expression of prostaglandin receptors was determined using qRT-PCR, accumulation of inositol phosphates was measured by incorporation of radiolabelled inositol, and cAMP was determined by radioimmunoassay.

Results

In the MH₁C₁ hepatocarcinoma cells, stimulation with PGE₂ or PGF_2α caused phosphorylation of the EGFR, Akt, and ERK, which could be blocked by the EGFR tyrosine kinase inhibitor gefitinib. This did not occur in primary hepatocytes. qRT-PCR revealed expression of EP1, EP4, and FP receptor mRNA in MH₁C₁ cells. PGE₂ stimulated accumulation of inositol phosphates but not cAMP in these cells, suggesting signalling via PLCβ. While pretreatment with EP1 and EP4 receptor antagonists did not inhibit the effect of PGE₂, pretreatment with an FP receptor antagonist blocked the phosphorylation of EGFR, Akt and ERK. Further studies suggested that the PGE₂-induced signal was mediated via Ca²⁺ release and not PKC activation, and that it proceeded through Src and shedding of membrane-bound EGFR ligand precursors by proteinases of the ADAM family.

Conclusion

The results indicate that in MH₁C₁ cells, unlike normal hepatocytes, PGE₂ activates the MEK/ERK and PI3K/Akt pathways by transactivation of the EGFR, thus diversifying the GPCR-mediated signal. The data also suggest that the underlying mechanisms in these cells involve FP receptors, PLCβ, Ca²⁺, Src, and proteinase-mediated release of membrane-associated EGFR ligand(s).

Engagement of β-arrestin by transactivated insulin-like growth factor receptor is needed for V2 vasopressin receptor-stimulated ERK1/2 activation

G protein-coupled receptors (GPCRs) have been shown to activate the mitogen-activated protein kinases, ERK1/2, through both G protein-dependent and -independent mechanisms. Here, we describe a G protein-independent mechanism that unravels an unanticipated role for β-arrestins. Stimulation of the V2 vasopressin receptor (V2R) in cultured cells or in vivo in rat kidney medullar collecting ducts led to the activation of ERK1/2 through the metalloproteinase-mediated shedding of a factor activating the insulin-like growth factor receptor (IGFR). This process was found to be both Src- and β-arrestin-dependent. Whereas Src was found to act upstream of the metalloproteinase activation and be required for the release of the IGFR-activating factor, β-arrestins were found to act downstream of the IGFR transactivation. Unexpectedly, the engagement of β-arrestins by the IGFR but not by the V2R was needed to promote the vasopressin-stimulated ERK1/2 activation, indicating that a pool of β-arrestins distinct from those β-arrestins recruited to the V2R acts downstream of the receptor tyrosine kinase to activate ERK1/2. Such a dual site of action for β-arrestins helps explain the pleiotropic actions of this scaffolding protein. Given the role that V2R-stimulated ERK1/2 plays in kidney cell proliferation, this transactivation mechanism may have important implications for renal pathophysiology. Still, the role of β-arrestins downstream of a transactivation event is not limited to the V2R, because we observed a similar involvement for an unrelated GPCR (the platelet-activating factor receptor), indicating that it may be a general mechanism shared among GPCRs.

Expression patterns of ADAMs in the developing chicken lens

In the present study the expression patterns of ADAM (a disintegrin and metalloprotease) genes in the chicken developing lens were analyzed. Using in situ hybridization, we found that seven members of the ADAM family including ADAM9, ADAM10, ADAM12, ADAM13, ADAM17, ADAM22, and ADAM23 are expressed in the developing embryonic lens. From embryonic incubation day (E) 2 to E3, most of the ADAMs investigated here are expressed in the lens placode and lens vesicle. From E5 to E7, all seven ADAMs, but predominantly ADAM9 and ADAM10, are throughly expressed in the central epithelium, as well as in the proliferating lens epithelium and the equatorial lens epithelium. From E9 to E14, expression of ADAM9, ADAM10, and ADAM17 decreases moderately in these regions. ADAM12 and ADAM13 are weakly expressed in the central epithelium and the lens epithelium, and are not detectable from E14 onward. ADAM22 and ADAM23 are expressed in the central epithelium, the lens epithelium and the equatorial lens epithelium at E5 and decrease gradually afterwards in the same regions. At E16, only weak ADAM9, ADAM10 and ADAM17 signals are found in the anterior lens epithelium. The changing spatiotemporal expression of the seven ADAMs suggests a regulatory role for these molecules during chicken lens development.

Histamine may induce airway remodeling through release of epidermal growth factor receptor ligands from bronchial epithelial cells

Asthma is a chronic inflammatory disease that is associated with airway remodeling, including hyperplasia of airway epithelial cells and airway smooth muscle cells, and goblet cell differentiation. We wished to address the potential role of histamine, a key biogenic amine involved in allergic reactions, in airway remodeling through the epidermal growth factor receptor (EGFR) pathway. Here, we demonstrate that histamine releases 2 EGFR ligands, amphiregulin and heparin-binding epidermal growth factor-like growth factor (HB-EGF), from airway epithelial cells. Amphiregulin and HB-EGF were expressed in airway epithelium of patients with asthma. Histamine up-regulated their mRNA expression (amphiregulin 3.2-fold, P<0.001; HB-EGF 2.3-fold, P<0.05) and triggered their release (amphiregulin EC(50) 0.50 μM, 31.2 ± 2.7 pg/ml with 10 μM histamine, P<0.01; HB-EGF EC(50) 0.54 μM, 78.5 ± 1.8 pg/ml with 10 μM histamine, P<0.001) compared to vehicle control (amphiregulin 19.3 ± 0.9 pg/ml; HB-EGF 60.2 ± 1.0 pg/ml), in airway epithelial cells. Histamine increased EGFR phosphorylation (2.1-fold by Western blot analysis) and induced goblet cell differentiation (CLCA1 up-regulation by real-time qPCR) in normal human bronchial epithelial (NHBE) cells. Moreover, amphiregulin and HB-EGF caused proliferation and migration of both NHBE cells and human airway smooth muscle cells. These results suggest that histamine may induce airway remodeling via the epithelial-derived EGFR ligands amphiregulin and HB-EGF.

Differential expression of the ADAMs in developing chicken retina

The expression patterns of the seven members of the ADAM (a disintegrin and metalloprotease) family, ADAM9, ADAM10, ADAM12, ADAM13, ADAM17, ADAM22, and ADAM23 were analyzed in the developing chicken retina by in situ hybridization and immunohistochemistry. Results show that each individual ADAM is expressed and regulated spatiotemporally in the developing retinal layers. ADAM9, ADAM10 and ADAM17 are widely expressed in the differential layers of the retina throughout the whole embryonic period, while ADAM12 and ADAM13 are mainly expressed in the ganglion cell layer at a later stage. ADAM22 and ADAM23 are restricted to the inner nuclear layer and the ganglion cell layer at a later stage. Furthermore, ADAM10 protein is co-expressed with the four members of the classic cadherins, N-cadherin, R-cadherin, cadherin-6B and cadherin-7 in distinct retinal layers. Therefore, the differential expression of the investigated ADAMs in the developing retina suggests the contribution of them to the retina development.

In vivo invasion of head and neck squamous cell carcinoma cells does not require macrophages

Invasion of tumor cells into the local stroma is an important component in cancer progression. Here we report studies of the in vivo invasion of head and neck squamous cell carcinoma (HNSCC) cells in response to applied gradients of a growth factor [epidermal growth factor (EGF)] and a chemokine (CXCL12), using orthotopic floor-of-mouth models. Analysis of the invading cells indicated that >75% of them were tumor cells, about 15% macrophages, and <10% were unidentified. Surprisingly, although macrophages invaded together with tumor cells, macrophage contributions were not required for HNSCC invasion. CXCL12-induced in vivo invasion of HNSCC cells was also observed and found to occur via a unidirectional transactivation of epidermal growth factor receptor (EGFR) through CXCR4. Inhibition of tumor necrosis factor-α-converting enzyme using TNF-α protease inhibitor-2 selectively inhibited CXCL12-induced invasion but not EGF-induced invasion, consistent with CXCL12 activation of EGFR via release of EGFR ligands.

TNF-α converting enzyme-mediated ErbB4 transactivation by TNF promotes colonic epithelial cell survival

Disruption of intestinal epithelial homeostasis, including enhanced apoptosis, is a hallmark of inflammatory bowel disease (IBD). We have recently shown that tumor necrosis factor (TNF) increases the kinase activity of ErbB4, a member of the epidermal growth factor receptor family that is elevated in mucosa of IBD patients and that promotes colon epithelial cell survival. In this study, we tested the hypothesis that TNF transactivates ErbB4 through TNF-α converting enzyme (TACE)-mediated ligand release and that this transactivation is necessary to protect colonic epithelial cells from cytokine-induced apoptosis. Using neutralizing antibodies, we show that heparin-binding EGF-like growth factor (HB-EGF) is required for ErbB4 phosphorylation in response to TNF. Pharmacological or genetic inhibition of the metalloprotease TACE, which mediates HB-EGF release from cells, blocked TNF-induced ErbB4 activation. MEK, but not Src or p38, was also required for transactivation. TACE activity and ligand binding were required for ErbB4-mediated antiapoptotic signaling; whereas mouse colon epithelial cells expressing ErbB4 were resistant to TNF-induced apoptosis, TACE inhibition or blockade of ErbB4 ligand binding reversed the survival advantage. We conclude that TNF transactivates ErbB4 through TACE-dependent HB-EGF release, thus protecting colon epithelial cells from cytokine-induced apoptosis. These findings have important implications for understanding how ErbB4 protects the colon from apoptosis-induced tissue injury in inflammatory conditions such as IBD.

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.