Serpentine receptors and STAT activation: more than one way to twin a STAT

Cloning and gene expression of signal transducers and activators of transcription (STAT) homologue provide new insights into the immune response and nucleus graft of the pearl oyster Pinctada fucata

The signal transducers and activators of the transcription (STAT) family play an important role in regulatory and cellular functions by regulating the expression of a variety of genes, including cytokines and growth factors. In the present study, a Pinctada fucata STAT protein, termed PfSTAT, was described. The deduced amino acid sequence of PfSTAT contains the conserved STAT_bind domain and the SH2 domain, and the additional Bin/Amphiphysin/Rvs (BAR) domain, but does not have STAT_alpha and STAT_int domains. Multiple sequence alignments revealed that PfSTAT showed relatively low identity with vertebrate and other invertebrate STATs, and phylogenetic analysis indicated that the evolution of STAT may have been more complex and ancient. Gene expression analysis revealed that PfSTAT is involved in the immune response to polyinosinic-polycytidylic acid (poly I:C) stimulation and in the nucleus insertion operation. This study contributes to a better understanding of PfSTAT in protecting the pearl oyster from disease or injury caused by grafting.

Dictyostelium cell dynamics

This unit describes culturing and imaging of D. discoideum amoebae to study fundamental cellular responses, such as motility and directed migration. The system displays powerful molecular genetics that can be used to link structural determinants of proteins with in vivo cellular functions.

Identification and characterization of five-transmembrane isoforms of human vasoactive intestinal peptide and pituitary adenylate cyclase-activating polypeptide receptors

The seven-transmembrane (7TM) G-protein-coupled neuroendocrine receptors VPAC1 (HGNC approved gene symbol VIPR1) and VPAC2 (HGNC approved gene symbol VIPR2) are expressed in different tissues and involved in the regulation of important biological functions. We now report the identification and characterization of novel five-transmembrane(5TM) forms of both human VPAC1 and human VPAC2. These alternatively spliced variant mRNAs result from the skipping of exons 10/11, spanning the third intracellular loop, the fourth extracellular loop, and the transmembrane regions 6 and 7, producing in-frame 5TM receptors predicted to lack a G-protein-binding motif. RT-PCR showed that these 5TM receptors are differentially expressed in transformed and normal cells. Translation of the 5TM protein was demonstrated by transfection and expression in CHO cells. Following agonist stimulation, differential signaling of the 7TM versus 5TM forms was shown both for the activation of adenylate cyclase and for tyrosine phosphorylation. The identification of these splice variants in various cells and their expression and differential signal transduction compared to the 7TM form suggest that these novel receptors have biological relevance.

C. elegans STAT: evolution of a regulatory switch

STAT transcription factors have been implicated in many biological processes, particularly host immune defense and development. Here we characterize a STAT orthologue from the nematode, C. elegans. We show that this protein, termed STA-1, is structurally and functionally related to other vertebrate and invertebrate STAT proteins, recognizing a cis DNA element conserved through phylogeny. Unexpectedly, STA-1 lacks the conserved amino-terminal oligomerization domain found in vertebrate and other invertebrate STAT proteins, a feature also lacking in orthologues from a distantly related nematode species and from the slime mold, Dictyostelium discoideum. This absence suggests that a primordial STAT protein lacked this domain, which was accreted later in evolution to provide further regulatory control of STAT signaling. Derivation of null mutants demonstrated that STA-1 is not required for nematode viability, despite its widespread expression in multiple tissues of the worm. However, mutant STA-1 proteins that lack functional coiled-coil and DNA binding domains could still be activated and accumulated in the nucleus, suggesting that DNA binding is not a necessary prerequisite for nuclear retention of activated STAT proteins. Our results shed new light on the evolution and function of the STAT signaling pathway and on the structural requirements for STAT activation.

Activation of tyrosine kinases by reactive oxygen species in vascular smooth muscle cells: significance and involvement of EGF receptor transactivation by angiotensin II

Enhanced production of reactive oxygen species (ROS) such as H(2)O(2) and a failure in ROS removal by scavenging systems are hallmarks of several cardiovascular diseases such as atherosclerosis and hypertension. ROS act as second messengers that play a prominent role in intracellular signaling and cellular function. In vascular smooth muscle cells (VSMCs), a vascular pathogen, angiotensin II, appears to initiate growth-promoting signal transduction through ROS-sensitive tyrosine kinases. However, the precise mechanisms by which tyrosine kinases are activated by ROS remain unclear. In this review, the current knowledge that suggests how certain tyrosine kinases are activated by ROS, along with their functional significance in VSMCs, will be discussed. Recent findings suggest that transactivation of the epidermal growth factor receptor by ROS requires metalloprotease-dependent heparin-binding epidermal growth factor-like growth factor production, whereas other ROS-sensitive tyrosine kinases such as PYK2, JAK2, and platelet-derived growth factor receptor require activation of protein kinase C-delta. Each of these ROS-sensitive kinases could mediate specific signaling critical for pathophysiological responses. Detailed analysis of the mechanism of cross-talk and the downstream function of these various tyrosine kinases will yield new therapeutic interventions for cardiovascular disease.

STAT proteins: from normal control of cellular events to tumorigenesis

Signal transducers and activators of transcription (STAT) proteins comprise a family of transcription factors latent in the cytoplasm that participate in normal cellular events, such as differentiation, proliferation, cell survival, apoptosis, and angiogenesis following cytokine, growth factor, and hormone signaling. STATs are activated by tyrosine phosphorylation, which is normally a transient and tightly regulates process. Nevertheless, several constitutively activated STATs have been observed in a wide number of human cancer cell lines and primary tumors, including blood malignancies and solid neoplasias. STATs can be divided into two groups according to their specific functions. One is made up of STAT2, STAT4, and STAT6, which are activated by a small number of cytokines and play a distinct role in the development of T-cells and in IFNgamma signaling. The other group includes STAT1, STAT3, and STAT5, activated in different tissues by means of a series of ligands and involved in IFN signaling, development of the mammary gland, response to GH, and embriogenesis. This latter group of STATS plays an important role in controlling cell-cycle progression and apoptosis and thus contributes to oncogenesis. Although an increased expression of STAT1 has been observed in many human neoplasias, this molecule can be considered a potential tumor suppressor, since it plays an important role in growth arrest and in promoting apoptosis. On the other hand, STAT3 and 5 are considered as oncogenes, since they bring about the activation of cyclin D1, c-Myc, and bcl-xl expression, and are involved in promoting cell-cycle progression, cellular transformation, and in preventing apoptosis.

Identification of nucleocytoplasmic cycling as a remote sensor in cellular signaling by databased modeling

Considerable progress has been made in identifying the molecular composition of complex signaling networks controlling cell proliferation, differentiation, and survival. However, to discover general building principles and predict the dynamic behavior of signaling networks, it is necessary to develop quantitative models based on experimental observations. Here we report a mathematical model of the core module of the Janus family of kinases (JAK)-signal transducer and activator of transcription (STAT) signaling pathway based on time-resolved measurements of receptor and STAT5 phosphorylation. Applying the fitted model, we can determine the quantitative behavior of STAT5 populations not accessible to experimental measurement. By in silico investigations, we identify the parameters of nuclear shuttling as the most sensitive to perturbations and verify experimentally the model prediction that inhibition of nuclear export results in a reduced transcriptional yield. The model reveals that STAT5 undergoes rapid nucleocytoplasmic cycles, continuously coupling receptor activation and target gene transcription, thereby forming a remote sensor between nucleus and receptor. Thus, dynamic modeling of signaling pathways can promote functional understanding at the systems level.

The Jak/STAT pathway in model organisms: emerging roles in cell movement

The JAK/STAT pathway was originally identified in mammals. Studies of this pathway in the mouse have revealed that JAK/STAT signaling plays a central role during hematopoeisis and other developmental processes. The role of JAK/STAT signaling in blood appears to be conserved throughout evolution, as it is also required during fly hematopoeisis. Studies in Dictyostelium, Drosophila, and zebrafish have shown that the JAK/STAT pathway is also required in an unusually broad set of developmental decisions, including cell proliferation, cell fate determination, cell migration, planar polarity, convergent extension, and immunity. There is increasing evidence that the versatility of this pathway relies on its cooperation with other signal transduction pathways. In this review, we discuss the components of the JAK/STAT pathway in model organisms and what is known about its requirement in cellular and developmental processes. In particular, we emphasize recent insights into the role that this pathway plays in the control of cell movement.

To interfere and to anti-interfere: the interplay between hepatitis C virus and interferon

As popular strategies used by numerous viruses, interception of interferon (IFN) signaling and inhibition of IFN-induced antiviral functions allow viruses to evade the host immune response and set up successful infections. Hepatitis C virus (HCV), the leading cause of chronic liver disease worldwide and a major public health hazard, causes persistent infection in the majority of infected individuals. IFN-based therapies, currently the only ones available for HCV infection, have been unable to eliminate viral infection in the majority of patients, and many studies suggest that HCV possesses mechanisms to antagonize the IFN-induced antiviral response. Multiple viral, host, and IFN-associated factors have been implicated in the interplay between HCV and IFN. Two viral proteins, NS5A and E2, became the focus of much attention and extensive study because of their abilities to inhibit IFN-induced, double-stranded RNA-activated protein kinase (PKR), a major mediator of the IFN-induced biologic response, and to perturb the IFN signaling pathway. In this review, we discuss the significance of the interferon sensitivity determining region (ISDR) within NS5A, which has been the subject of intense debates. In addition, we discuss the potential mechanisms by which NS5A interferes with IFN signaling and the current working models. Further understanding of the molecular mechanisms underlying the interaction between HCV and IFN will likely facilitate improvement of current IFN-based therapies and development of novel treatments for the HCV pandemic. Future HCV research will benefit from both the development of efficient, convenient model systems for viral propagation, and the utilization of high throughput, genomic-scale approaches.

Analysis of G-protein-coupled receptor dimerization following chemokine signaling

An abundance of information has been generated in recent decades on the signaling events triggered through G-protein-coupled receptors (GPCRs). Nonetheless, the structural changes at the cell surface that provoke receptor activation are only now beginning to be understood. It is becoming clear that receptors are not isolated entities that are activated following ligand binding, but that they interact with other molecules already present or recruited to the vicinity, which results in a wide variety of new signaling possibilities. Understanding receptor interactions with relatives and/or friends on the cell surface is thus critical. The most important point is to determine which of these interactions are "casual" and which give rise to functional consequences.

Chemokine signaling and functional responses: the role of receptor dimerization and TK pathway activation

A broad array of biological responses, including cell polarization, movement, immune and inflammatory responses, and prevention of HIV-1 infection, are triggered by the chemokines, a family of structurally related chemoattractant proteins that bind to specific seven-transmembrane receptors linked to G proteins. Here we discuss one of the early signaling pathways activated by chemokines, the JAK/STAT pathway. Through this pathway, and possibly in conjunction with other signaling pathways, the chemokines promote changes in cellular morphology, collectively known as polarization, required for chemotactic responses. The polarized cell expresses the chemokine receptors at the leading cell edge, to which they are conveyed by rafts, a cholesterol-enriched membrane fraction fundamental to the lateral organization of the plasma membrane. Finally, the mechanisms through which the chemokines promote their effect are discussed in the context of the prevention of HIV-1 infection.

Signaling at zero G: G-protein-independent functions for 7-TM receptors

Eukaryotic cells, whether free-living, single-celled microbes or components of complex metazoa, can sense environmental cues through specialized seven-transmembrane (7-TM) receptors (also called heptahelical or G-protein-coupled receptors). 7-TM receptors detect "inputs" such as light, peptide hormones, neurotransmitters, pheromones, odorants, morphogens and chemoattractants, linking extracellular stimuli to intracellular signaling networks via heterotrimeric G proteins. Recently, this obligatory paradigm has been challenged. A growing body of evidence indicates that 7-TM receptors can also transmit extracellular signals through mechanisms that function independently of G-protein coupling. This review discusses pathways and protein interactions for 7-TM receptors signaling "at zero G" in Dictyostelium and mammalian cells.

Tyrosine phosphorylation-independent nuclear translocation of a dictyostelium STAT in response to DIF signaling

We describe a Dictyostelium STAT, Dd-STATc, which regulates the speed of early development and the timing of terminal differentiation. Dd-STATc also functions as a repressor, which directs graded expression of the ecmA gene in different prestalk cell populations. Developing Dictyostelium cells produce a chlorinated hexaphenone, DIF, which directs prestalk cell differentiation. Dd-STATc is tyrosine phosphorylated, dimerizes, and translocates to the nucleus when cells are exposed to DIF. Surprisingly, however, SH2 domain-phosphotyrosine interaction is not necessary for the DIF-induced nuclear translocation of Dd-STATc. In this respect, Dd-STATc activation resembles several recently described, noncanonical mammalian STAT signaling processes. We show instead that DIF mediates nuclear translocation via sequences located in the divergent, N-terminal half of the Dd-STATc molecule.

Protein tyrosine phosphatase PTP1 negatively regulates Dictyostelium STATa and is required for proper cell-type proportioning

The protein tyrosine phosphatase PTP1, which mediates reversible phosphorylation on tyrosine, has been shown to play an important regulatory role during Dictyostelium development. Mutants lacking PTP1 develop more rapidly than normal, while strains that overexpress PTP1 display aberrant morphology. However, the signalling pathways involved have not been characterised. In reexamining these strains, we have found that there is an inverse correlation between levels of PTP1 activity, the extent of tyrosine phosphorylation on Dictyostelium STATa after treatment with cAMP, and the proportion of the slug population exhibiting STATa nuclear enrichment in vivo. This suggests that PTP1 acts to attenuate the tyrosine phosphorylation of STATa and downstream STATa-mediated pathways. Consistent with this, we show that when PTP1 is overexpressed, there is increased expression of a prestalk cell marker at the slug posterior, a phenocopy of STATa null slugs. In ptp1 null strains, STATa tyrosine phosphorylation and nuclear enrichment in the slug anterior is increased. There is also a change in the prestalk to prespore cell ratio. Synergy experiments suggest that this is due to a cell-autonomous defect in forming the subset of prespore cells that are located in the anterior prespore region.

STAT signalling in cell proliferation and in development

Recent advances in STAT signalling research include a better understanding of the roles of mammalian STAT proteins in cell proliferation and apoptosis, and of non-mammalian STAT proteins in morphogenesis. Two different ways in which STAT signalling pathways can interface with Smad signalling pathways significantly increasing combinatorial signalling possibilities, have also been described.

The roles of the Drosophila JAK/STAT pathway

The JAK/STAT signal transduction pathway has been conserved throughout evolution such that true structural and functional homologues of components originally identified in vertebrate systems are also present in the model genetic system Drosophila melanogaster. In addition to roles during larval hematopoiesis reminiscent of the requirement for this pathway in mammalian systems, the JAK/STAT pathway in Drosophila is also involved in a number of other developmental events. Recent data has demonstrated further roles for the JAK/STAT pathway in the establishment of sexual identity via the early embryonic expression of Sex lethal, the segmentation of the embryo via the control of pair rule genes including even skipped and the establishment of polarity within the adult compound eye via a mechanism that includes the four jointed gene. Use of the powerful genetics in the model organism Drosophila may identify new components of the JAK/STAT pathway, define new roles for this pathway, and provide insights into the function of this signal transduction system. Here we review the roles of STAT and its associated signaling pathway during both embryonic and adult stages of Drosophila development and discuss future prospects for the identification and characterization of novel pathway components and targets. Oncogene (2000).