GIV/girdin binds exocyst subunit-Exo70 and regulates exocytosis of GLUT4 storage vesicles

Baicalein inhibits hepatitis B virus through the coiled coil domain containing protein 88A (CCDC88A)-dependent autophagy pathway

BACKGROUND

Chronic infection with the hepatitis B virus (HBV) represents a significant global health concern. Baicalein, a naturally occurring flavone derived from the roots of Scutellaria baicalensis Georgi, has exhibited both anti-inflammatory and antiviral activities. S. baicalensis is extensively utilized in traditional Chinese medicine for the treatment of various liver disorders, including hepatitis. However, the specific anti-HBV effects of baicalein have not been fully elucidated.

PURPOSE

This study aimed to investigate the inhibitory effects of baicalein on HBV and to elucidate its underlying mechanisms.

MATERIALS AND METHODS

The levels of hepatitis B surface antigen (HBsAg) and hepatitis B e antigen (HBeAg) were measured using enzyme-linked immunosorbent assay (ELISA) kits. Quantification of HBV DNA was performed using quantitative real-time polymerase chain reaction (qRT-PCR). Western blot analysis was conducted to evaluate proteins involved in autophagy, lysosomal acidification, and autophagy-related signaling pathways. Immunofluorescence microscopy was utilized to assess autophagic flux and lysosomal acidification.

RESULTS

Baicalein demonstrated significant inhibition of HBsAg, HBeAg, and HBV-DNA secretion in both in vivo and in vitro environments. Subsequent investigations revealed that baicalein disrupted the intracellular trafficking of the hepatitis B virus by inhibiting the CCDC88A-AKT-mTOR (Coiled coil domain containing protein 88A- protein kinase B-mammalian target of rapamycin) signaling pathway. Additionally, baicalein induced autophagy in HepG2 (Human hepatocellular carcinoma cell line 2) and HepG2.215 cell models. The anti-hepatitis B antigen effect of baicalein was partially attenuated when both early and late stages of autophagy were inhibited. A significant correlation was identified between the phosphorylation of AMPKα and the enhanced autophagy observed in baicalein-treated cells.

CONCLUSIONS

This study elucidates a novel mechanism by which baicalein inhibits the hepatitis B virus (HBV). Specifically, baicalein exerts its antiviral effects by activating autophagy and suppressing the CCDC88A-AKT-mTOR signaling pathway.

Knockdown of Girdin Induced Apoptosis of Glioblastoma Cells via the Mitochondrion Signaling Pathway

Glioblastoma is the most common primary brain tumor with poor survival rate and without effective treatment strategy. However, the influence of Girdin on human glioblastoma and the underlying molecular mechanisms have yet to be uncovered. We mainly investigated the role of Girdin in glioblastoma cells apoptosis. First, we examined Girdin expression in 90 glioma patients by using immunohistochemistry (IHC) and analyzed its association with patients' prognosis. The results showed that the expression of Girdin was positively associated with the histological grade of glioma, and glioma patients with high Girdin expression had a poor prognosis. Next, proliferation assay, Colony formation assay and Cell Counting Kit-8 (CCK-8) results showed that knockdown of Girdin suppressed proliferation and reduced cell survival rate. Flow cytometry and DAPI staining results showed that knockdown of Girdin induced apoptosis in LN229 cells. Western blot results suggested that reduction of Gridin increased the level of Cytochrome C (Cyt-C) and Bad while decreased the expression of Bcl-2 and p-AKT. Moreover, subcutaneous mouse xenograft model was used to validate the role of Girdin in glioblastoma apoptosis. Consistently, in vivo assays showed that knockdown of Girdin inhibited the growth of the grafted tumor and increased the level of Cyt-C and Bad. These findings demonstrated that knockdown of Girdin may induce Bad expression and reduce Bcl-2 expression by inhibiting the activation of AKT, leading to the release of Cyt-C from mitochondria, thereby promoting glioblastoma cells apoptosis.

CCDC88A/GIV promotes HBV replication and progeny secretion via enhancing endosomal trafficking and blocking autophagic degradation

Hepatitis B virus (HBV) particles are thought to be secreted from hepatocytes through multivesicular bodies (MVBs); however, the cellular trafficking mechanisms prior to this process remain elusive. It has been reported that CCDC88A/GIV expression, which is involved in multiple aspects of vesicular trafficking, changes dynamically at different phases of chronic HBV infection. In this study, we focused on the role of CCDC88A/GIV in HBV replication. In the liver tissues of chronically HBV-infected patients, HBV infection significantly enhanced CCDC88A/GIV expression, and increased endoplasmic reticulum (ER) stress and autophagosome formation without changing endosome formation. Additionally, colocalization of SHBsAg with early endosomes (~30.2%) far exceeded that with autophagosomes (~3.2%). In hepatoma cells, CCDC88A/GIV and its downstream proteins, DNM2 (dynamin 2; a CCDC88A/GIV effector), CLTC and RAB5A significantly enhanced HBV replication and endosome formation but inhibited autophagosome formation. Blocking endocytosis disrupted HBsAg trafficking to endosomes and caused its accumulation in the ER lumen, which triggered ER stress to initiate the unfolded protein response (UPR). Therefore, HBsAg trafficking into autophagosomes was increased, and the lysosomal activity and maturation, which was inhibited by HBV infection, were restored. Meanwhile, core particles were prevented from entering MVBs. CCDC88A/GIV and its other effector, GNAI3, decreased autophagic flux by enhancing the insulin-induced AKT-MTOR pathway, thereby inhibiting HBV antigens autophagic degradation. In conclusion, CCDC88A/GIV enhanced HBV replication by increasing endosomal trafficking and reducing autophagic degradation of HBV antigens, suggesting that CCDC88A/GIV-mediated endosomal trafficking plays an important role in HBV replication and progeny secretion.Abbreviations: ACTB: actin beta; AO: acridine orange; ATF6: activating transcription factor 6; CCDC88A/GIV: coiled-coil domain containing 88A; CLTC: clathrin heavy chain; CQ: chloroquine; DAPI: 4ʹ,6-diamidino-2-phenylindole; DNM2: dynamin 2; ER: endoplasmic reticulum; ERN1: endoplasmic reticulum to nucleus signaling 1; EIF2A: eukaryotic translation initiation factor 2A; FBS: fetal bovine serum; GNAI3: G protein subunit alpha i3; HBV: hepatitis B virus; HBV RIs: HBV replication intermediates; HBcAg: HBV core protein; HBsAg: HBV surface antigen; MAP1LC3B/LC3B: microtubule associated protein 1 light chain 3 beta; MVBs: multivesicular bodies; MTOR: mechanistic target of rapamycin kinase; PDI: protein disulfide isomerase; PHH: primary human hepatocyte; pSM2: a HBV replication-competent plasmid; HSPA5/BIP: heat shock protein family A (Hsp70) member 5; SQSTM1/p62: sequestosome 1; siRNA: small interfering RNA; SEM: standard error of the mean; UPR: unfolded protein response

Autocrine insulin pathway signaling regulates actin dynamics in cell wound repair

Cells are exposed to frequent mechanical and/or chemical stressors that can compromise the integrity of the plasma membrane and underlying cortical cytoskeleton. The molecular mechanisms driving the immediate repair response launched to restore the cell cortex and circumvent cell death are largely unknown. Using microarrays and drug-inhibition studies to assess gene expression, we find that initiation of cell wound repair in the Drosophila model is dependent on translation, whereas transcription is required for subsequent steps. We identified 253 genes whose expression is up-regulated (80) or down-regulated (173) in response to laser wounding. A subset of these genes were validated using RNAi knockdowns and exhibit aberrant actomyosin ring assembly and/or actin remodeling defects. Strikingly, we find that the canonical insulin signaling pathway controls actin dynamics through the actin regulators Girdin and Chickadee (profilin), and its disruption leads to abnormal wound repair. Our results provide new insight for understanding how cell wound repair proceeds in healthy individuals and those with diseases involving wound healing deficiencies.

GIV/Girdin and Exo70 Collaboratively Regulate the Mammalian Polarized Exocytic Machinery

Polarized exocytosis is a fundamental process by which membranes and cargo proteins are delivered to the cell surface with precise spatial control. Although the need for the octameric exocyst complex is conserved from yeast to humans, what imparts spatial control is known only in yeast, i.e., a polarity scaffold called Bem1p. We demonstrate here that the mammalian scaffold protein, GIV/Girdin, fulfills the key criteria and functions of its yeast counterpart Bem1p; both bind Exo70 proteins via similar short-linear interaction motifs, and each prefers its evolutionary counterpart. Selective disruption of the GIV⋅Exo-70 interaction derails the delivery of the metalloprotease MT1-MMP to invadosomes and impairs collagen degradation and haptotaxis through basement membrane matrix. GIV's interacting partners reveal other components of polarized exocytosis in mammals. Findings expose how the exocytic functions aid GIV's pro-metastatic functions and how signal integration via GIV may represent an evolutionary advancement of the exocytic process in mammals.

Biochemical, Biophysical and Cellular Techniques to Study the Guanine Nucleotide Exchange Factor, GIV/Girdin

Canonical signal transduction via heterotrimeric G proteins is spatiotemporally restricted, i.e., triggered exclusively at the plasma membrane, only by agonist activation of G protein-coupled receptors via a finite process that is terminated within a few hundred milliseconds. Recently, a rapidly emerging paradigm has revealed a noncanonical pathway for activation of heterotrimeric G proteins via the nonreceptor guanidine-nucleotide exchange factor, GIV/Girdin. Biochemical, biophysical, and functional studies evaluating this pathway have unraveled its unique properties and distinctive spatiotemporal features. As in the case of any new pathway/paradigm, these studies first required an in-depth optimization of tools/techniques and protocols, governed by rationale and fundamentals unique to the pathway, and more specifically to the large multimodular GIV protein. Here we provide the most up-to-date overview of protocols that have generated most of what we know today about noncanonical G protein activation by GIV and its relevance in health and disease. © 2016 by John Wiley & Sons, Inc.

AMP-activated protein kinase fortifies epithelial tight junctions during energetic stress via its effector GIV/Girdin

Loss of epithelial polarity impacts organ development and function; it is also oncogenic. AMPK, a key sensor of metabolic stress stabilizes cell-cell junctions and maintains epithelial polarity; its activation by Metformin protects the epithelial barrier against stress and suppresses tumorigenesis. How AMPK protects the epithelium remains unknown. Here, we identify GIV/Girdin as a novel effector of AMPK, whose phosphorylation at a single site is both necessary and sufficient for strengthening mammalian epithelial tight junctions and preserving cell polarity and barrier function in the face of energetic stress. Expression of an oncogenic mutant of GIV (cataloged in TCGA) that cannot be phosphorylated by AMPK increased anchorage-independent growth of tumor cells and helped these cells to evade the tumor-suppressive action of Metformin. This work defines a fundamental homeostatic mechanism by which the AMPK-GIV axis reinforces cell junctions against stress-induced collapse and also provides mechanistic insight into the tumor-suppressive action of Metformin.

Prognostic impact of total and tyrosine phosphorylated GIV/Girdin in breast cancers

Gα-interacting vesicle-associated protein (GIV, aka Girdin) is a guanine exchange factor (GEF) for the trimeric G protein Gαi and a bona fide metastasis-related gene that serves as a platform for amplification of tyrosine-based signals via G-protein intermediates. Here we present the first exploratory biomarker study conducted on a cohort of 187 patients with breast cancer to evaluate the prognostic role of total GIV (tGIV) and tyrosine phosphorylated GIV (pYGIV) across the various molecular subtypes. A Kaplan-Meier analysis of recurrence-free survival showed that the presence of tGIV, either cytoplasmic or nuclear, carried poor prognosis, but that nuclear tGIV had a greater prognostic impact (P = 0.007 in early and P = 0.0048 in late clinical stages). Activated pYGIV in the cytoplasm had the greatest prognostic impact in late clinical stages (P = 0.006). Furthermore, we found that the prognostic impacts of cytoplasmic pYGIV and nuclear tGIV were additive (hazard ratio 19.0548; P = 0.0002). Surprisingly, this additive effect of nuclear tGIV/cytoplasmic pYGIV was observed in human epidermal growth factor receptor 2-positive tumors (hazard ratio 16.918; P = 0.0005) but not in triple-negative breast cancers. In triple-negative breast cancers, tGIV and cytoplasmic pYGIV had no prognostic impact; however, membrane-association of pYGIV carried a poor prognosis (P = 0.026). Both tGIV and pYGIV showed no correlation with clinical stage, tumor size, pathologic type, lymph node involvement, and BRCA1/2 status. We conclude that immunocytochemical detection of pYGIV and tGIV can serve as an effective prognosticator. On the basis of the differential prognostic impact of tGIV/pYGIV within each molecular subtype, we propose a diagnostic algorithm. Further studies on larger cohorts are essential to rigorously assess the effectiveness and robustness of this algorithm in prognosticating outcome among patients with breast cancer.-Dunkel, Y., Diao, K., Aznar, N., Swanson, L., Liu, L., Zhu, W., Mi, X.-Y., Ghosh, P. Prognostic impact of total and tyrosine phosphorylated GIV/Girdin in breast cancers.

Heterotrimeric G protein signaling via GIV/Girdin: Breaking the rules of engagement, space, and time

Canonical signal transduction via heterotrimeric G proteins is spatially and temporally restricted, that is, triggered exclusively at the plasma membrane (PM), only by agonist activation of G protein-coupled receptors (GPCRs) via a process that completes within a few hundred milliseconds. Recently, a rapidly emerging paradigm has revealed a non-canonical pathway for activation of heterotrimeric G proteins by the non-receptor guanidine-nucleotide exchange factor (GEF), GIV/Girdin. This pathway has distinctive temporal and spatial features and an unusual profile of receptor engagement: diverse classes of receptors, not just GPCRs can engage with GIV to trigger such activation. Such activation is spatially and temporally unrestricted, that is, can occur both at the PM and on internal membranes discontinuous with the PM, and can continue for prolonged periods of time. Here, we provide the most complete up-to-date review of the molecular mechanisms that govern the unique spatiotemporal aspects of non-canonical G protein activation by GIV and the relevance of this new paradigm in health and disease.

The untapped potential of tyrosine-based G protein signaling

Tyrosine-based and trimeric G protein-based signaling are the two most widely studied and distinct mechanisms for signal transduction in eukaryotes. How each of them relay signals across the plasma membrane independently of each other has been extensively characterized; however, an understanding of how they work together remained obscure. Recently, a rapidly emerging paradigm has revealed that tyrosine based signals are relayed via G proteins, and that the cross-talk between the two hubs are more robustly and sophisticatedly integrated than was previously imagined. More importantly, by straddling the two signaling hubs that are most frequently targeted for their therapeutic significance, the tyrosine-based G-protein signaling pathway has its own growing list of pathophysiologic importance, both as therapeutic target in a variety of disease states, and by paving the way for personalized medicine. The fundamental principles of this emerging paradigm and its pharmacologic potential are discussed.