Clinical studies have defined the core 'genetic blueprint' of a cancer cell, but this information does not necessarily predict the cancer phenotype. Signalling hubs that mediate such phenotype have been identified largely using OMICS platforms that measure dynamic molecular changes within the cancer cell landscape. The pro-oncogenic protein anterior gradient 2 (AGR2) is a case in point; AGR2 has been shown using a range of expression platforms to be involved in asthma, inflammatory bowel disease, cell transformation, cancer drug resistance and metastatic growth. AGR2 protein is also highly overexpressed in a diverse range of human cancers and can be secreted and detected in extracellular fluids, thus representing a compelling pro-oncogenic signalling intermediate in human cancer. AGR2 belongs to the protein disulphide isomerase family with all the key features of an endoplasmic reticulum-resident protein-this gives clues into how it might function as an oncoprotein through the regulation of protein folding, maturation and secretion that can drive metastatic cell growth. In this review, we will describe the known aspects of AGR2 molecular biology, including gene structure and regulation, emerging protein interaction networks and how its subcellular localization mediates its biological functions. We will finally review the cases of AGR2 expression in human cancers, the pathophysiological consequences of AGR2 overexpression, its potential role as a tumour biomarker that predicts the response to therapy and how the AGR2 pathway might form the basis for drug discovery programmes aimed at targeting protein folding/maturation pathways that mediate secretion and metastasis.
Overexpression of RasG12V in primary cells induces a permanent growth arrest called oncogene-induced senescence (OIS) that serves as a fail-safe mechanism against malignant transformation. We have performed a genomewide small interfering RNA (siRNA) screen and a microRNA (miRNA) screen to identify mediators of OIS and show that siRNA-mediated knockdown of p21Waf1/Cip1 rescues from Ras G12V-induced senescence in human mammary epithelial cells (HMECs). Moreover, we isolated a total of 28 miRNAs that prevented Ras G12V -induced growth arrest, among which all of the miR-106b family members were present. In addition, we obtained a number of hits, miR-130b, miR-302a, miR-302b, miR302c, miR-302d, miR-512-3p and miR-515-3p with seed sequences very similar to miR-106b family members. We show that overexpression of all these miRNAs rescues HMECs from Ras . Our results establish an important role for the cell cycle inhibitor p21Waf1/Cip1 in growth control of HMECs and extend the repertoire of miRNAs that modulate the activity of this tumour suppressor.
The extracellular matrix (ECM) plays an instrumental role in determining the spatial orientation of epithelial polarity and the formation of lumens in glandular tissues during morphogenesis. Here, we show that the Endoplasmic Reticulum (ER)-resident protein anterior gradient-2 (AGR2), a soluble protein-disulfide isomerase involved in ER protein folding and quality control, is secreted and interacts with the ECM. Extracellular AGR2 (eAGR2) is a microenvironmental regulator of epithelial tissue architecture, which plays a role in the preneoplastic phenotype and contributes to epithelial tumorigenicity. Indeed, eAGR2, is secreted as a functionally active protein independently of its thioredoxin-like domain (CXXS) and of its ER-retention domain (KTEL), and is sufficient, by itself, to promote the acquisition of invasive and metastatic features. Therefore, we conclude that eAGR2 plays an extracellular role independent of its ER function and we elucidate this gain-of-function as a novel and unexpected critical ECM microenvironmental pro-oncogenic regulator of epithelial morphogenesis and tumorigenesis.DOI: http://dx.doi.org/10.7554/eLife.13887.001
Anterior gradient 2 ( AGR 2) is a dimeric protein disulfide isomerase family member involved in the regulation of protein quality control in the endoplasmic reticulum ( ER ). Mouse AGR 2 deletion increases intestinal inflammation and promotes the development of inflammatory bowel disease ( IBD ). Although these biological effects are well established, the underlying molecular mechanisms of AGR 2 function toward inflammation remain poorly defined. Here, using a protein–protein interaction screen to identify cellular regulators of AGR 2 dimerization, we unveiled specific enhancers, including TMED 2, and inhibitors of AGR 2 dimerization, that control AGR 2 functions. We demonstrate that modulation of AGR 2 dimer formation, whether enhancing or inhibiting the process, yields pro‐inflammatory phenotypes, through either autophagy‐dependent processes or secretion of AGR 2, respectively. We also demonstrate that in IBD and specifically in Crohn's disease, the levels of AGR 2 dimerization modulators are selectively deregulated, and this correlates with severity of disease. Our study demonstrates that AGR 2 dimers act as sensors of ER homeostasis which are disrupted upon ER stress and promote the secretion of AGR 2 monomers. The latter might represent systemic alarm signals for pro‐inflammatory responses.
The most widely studied pathway underlying agonist-promoted internalization of G protein-coupled receptors (GPCRs) involves -arrestin and clathrin-coated pits. However, both -arrestin-and clathrin-independent processes have also been reported. Classically, the endocytic routes are characterized using pharmacological inhibitors and various dominant negative mutants, resulting sometimes in conflicting results and interpretational difficulties. Here, taking advantage of the fact that -arrestin binding to the 2 subunit of the clathrin adaptor AP-2 (2-adaptin) is needed for the -arrestin-mediated targeting of GPCRs to clathrin-coated pits, we developed a bioluminescence resonance energy transfer-based approach directly assessing the molecular steps involved in the endocytosis of GPCRs in living cells. For 10 of the 12 receptors tested, including some that were previously suggested to internalize via clathrinindependent pathways, agonist stimulation promoted -arrestin 1 and 2 interaction with 2-adaptin, indicating a -arrestinand clathrin-dependent endocytic process. Detailed analyses of -arrestin interactions with both the receptor and 2-adaptin also allowed us to demonstrate that recruitment of -arrestins to the receptor and the ensuing conformational changes are the leading events preceding AP-2 engagement and subsequent clathrin-mediated endocytosis. Among the receptors tested, only the endothelin A and B receptors failed to promote interaction between -arrestins and 2-adaptin. However, both receptors recruited -arrestins upon agonist stimulation, suggesting a -arrestin-dependent but clathrin-independent route of internalization for these two receptors. In addition to providing a new tool to dissect the molecular events involved in GPCR endocytosis, the bioluminescence resonance energy transfer-based -arrestin/2-adaptin interaction assay represents a novel biosensor to assess receptor activation. G protein-coupled receptors (GPCRs)7 are seven transmembrane domain receptors that constitute the largest family of cell surface proteins involved in signal transduction. In humans, it is estimated that GPCRs are encoded by ϳ800 distinct genes that control a variety of important physiological responses (1). Following agonist binding, GPCRs undergo conformational changes that regulate the activity of downstream effector systems to mediate various cellular responses. The extent and duration of GPCR signaling is tightly regulated by mechanisms that terminate the initial signaling and later re-establish the capacity of the receptors to respond to new agonist exposure. The removal of GPCRs from the cell surface, also known as internalization or endocytosis, plays an important role in these processes (2, 3). For most GPCRs, rapid feedback desensitization is initiated by G protein-coupled receptor kinases that phosphorylate agonist-occupied GPCRs to create high affinity binding sites for -arrestins, which in turn uncouple the receptor from its cognate G protein (reviewed in Ref. 4). -Arrestins also target rece...
Beta-arrestins are multifunctional adapters involved in the internalization and signaling of G protein-coupled receptors (GPCRs). They target receptors to clathrin-coated pits (CCPs) through binding with clathrin and clathrin adapter 2 (AP-2) complex. They also act as transducers of signaling by recruiting c-Src kinase to certain GPCRs. Here we sought to determine whether c-Src regulates the recruitment of AP-2 to beta-arrestin and the angiotensin II (Ang II) type 1 receptor (AT1R) during internalization. We show that the agonist stimulation of native AT1R in vascular smooth muscle cells (VSMCs) induces the formation of an endogenous complex containing c-Src, beta-arrestins and AP-2. In vitro studies using coimmunoprecipitation experiments and a yeast three-hybrid assay reveal that c-Src stabilizes the agonist-independent association between beta-arrestin2 and the beta-subunit of AP-2 independently of the kinase activity of c-Src. However, although c-Src expression promoted the rapid dissociation of AP-2 from both beta-arrestin and AT1R after receptor stimulation, a kinase-inactive mutant of c-Src failed to induce the dissociation of AP-2 from the agonist-occupied receptor. Thus, the consequence of c-Src in regulating the dissociation of AP-2 from the receptor was also examined on the internalization of AT1R by depleting c-Src in human embryonic kidney (HEK) 293 cells using a small interfering RNA strategy. Experiments in c-Src depleted cells reveal that AT1R remained mostly colocalized with AP-2 at the plasma membrane after Ang II stimulation, consistent with the observed delay in receptor internalization. Moreover, coimmunoprecipitation experiments in c-Src depleted HEK 293 cells and VSMCs showed an increased association of AP-2 to the agonist-occupied AT1R and beta-arrestin, respectively. Together, our results support a role for c-Src in regulating the dissociation of AP-2 from agonist-occupied AT1R and beta-arrestin during the clathrin-mediated internalization of receptors and suggest a novel function for c-Src kinase in the internalization of AT1R.
In a genome-wide siRNA analysis of p16(INK4a) (p16) modulators, we identify the Hedgehog (Hh) pathway component SUFU and formally demonstrate that Hh signaling promotes mitogenesis by suppression of p16. A fragment of the Hh-responsive GLI2 transcription factor directly binds and inhibits the p16 promoter and senescence is associated with the loss of nuclear GLI2. Hh components partially reside in the primary cilium (PC), and the small fraction of cells in mass culture that elaborate a PC have the lowest expression of p16. Suppression of p16 is effected by both PC-dependent and -independent routes, and ablation of p16 renders cells insensitive to an Hh inhibitor and increases PC formation. These results directly link a well-established developmental mitogenic pathway with a key tumor suppressor and contribute to the molecular understanding of replicative senescence, Hh-mediated oncogenesis, and potentially the role of p16 in aging.
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