佐々木 桂奈江

Abstract The Golgi stress response is an important cytoprotective system that enhances Golgi function in response to cellular demand, while cells damaged by prolonged Golgi stress undergo cell death to ensure the survival of organisms. OSW-1, a natural compound with anticancer activity, acts as a potent inhibitor of OSBP that transports cholesterol and phosphatidylinositol-4-phosphate (PI4P) at contact sites between the endoplasmic reticulum and the Golgi apparatus. Previously, we reported that OSW-1 induces the Golgi stress response, resulting in Golgi stress-induced transcription and cell death. However, the underlying molecular mechanism has been unknown. To reveal the mechanism of a novel pathway of the Golgi stress response regulating transcriptional induction and cell death (the cholesterol pathway), we performed a genome-wide knockout screen and found that transcriptional induction as well as cell death induced by OSW-1 was repressed in HeLa cells deficient in factors involved in the PI4P metabolism, such as PITPNB and PI4KB genes. Our data indicate that OSW-1 induces Golgi stress-dependent transcriptional induction and cell death through dysregulation of the PI4P metabolism in the Golgi apparatus.

Abstract The Golgi stress response is a homeostatic mechanism that augments Golgi function when Golgi function becomes insufficient (Golgi stress). Glycosylation of the core proteins of proteoglycans is one of the important functions of the Golgi. If the production of core proteins is increased and the amount of glycosylation enzymes for proteoglycans becomes insufficient (PG-type Golgi stress), the proteoglycan pathway of the Golgi stress response is activated, resulting in the transcriptional induction of glycosylation enzymes, including NDST2, HS6ST1 and GLCE. The transcriptional induction of these glycosylation enzymes is regulated by the enhancer element, PGSE-A; however, transcription factors that induce transcription from PGSE-A have not yet been identified. We herein proposed KLF2 and KLF4 as candidate transcription factors for transcriptional induction from PGSE-A, and revealed that their expression was up-regulated in response to PG-type Golgi stress. These results suggest that KLF2 and KLF4 are important regulators of the proteoglycan pathways of the mammalian Golgi stress response.

OSW-1 is a plant-derived natural product proposed to selectively kill cancer cells by binding to members of the oxysterol binding protein family, thereby disrupting lipid/sterol homeostasis. However, how these protein-ligand interactions mediate cell death signaling has remained elusive. Here, we discovered that OSW-1 selectively activates the Golgi stress response leading to apoptosis, providing a mechanistic basis for the anticancer activity of OSW-1.

The Golgi stress response is a homeostatic mechanism that controls the capacity of the Golgi apparatus in accordance with cellular demands. When the capacity of the Golgi apparatus becomes insufficient (Golgi stress), transcription levels of Golgi-related genes encoding glycosylation enzymes, a Golgi structural protein, and components of vesicular transport are upregulated through a common cis-acting enhancer-the Golgi apparatus stress response element (GASE). Here, we identified the transcription factor MLX as a GASE-binding protein. MLX resides in the cytoplasm and does not bind to GASE in normal growth conditions, whereas MLX translocates into the nucleus and specifically binds to GASE in response to Golgi stress. Suppression of MLX expression increased transcriptional induction of target genes of the Golgi stress response, whereas overexpression of MLX reduced GASE-binding of TFE3 as well as transcriptional induction from GASE, suggesting that MLX is a transcriptional repressor of the mammalian Golgi stress response.

Organelle autoregulation is a homeostatic mechanism to regulate the capacity of each organelle according to cellular demands. The endoplasmic reticulum (ER) stress response increases the expression of ER chaperones and ER-associated degradation factors when the capacity of the ER becomes insufficient, e.g. during cellular differentiation or viral propagation, and which can be restored through increased synthesis of secretory or membrane proteins. In the Golgi stress response, insufficient organelle capacity is responded to by augmentation of glycosylation enzyme expression and vesicular transport components. The mitochondrial stress response upregulates mitochondrial chaperone and protease expression in the mitochondrial matrix and intermembrane space when unfolded proteins accumulate in the mitochondria. The lysosome stress response is activated during autophagy to enhance the function of the lysosome by transcriptional induction of lysosome genes including cathepsins. However, many of the molecular mechanisms of organelle autoregulation remain unclear. Here, we review recent discoveries in organelle autoregulation and their molecular mechanisms.

ALG-2, a 22-kDa penta-EF-hand protein, is involved in cell death, signal transduction, membrane trafficking, etc., by interacting with various proteins in mammalian cells in a Ca2+-dependent manner. Most known ALG-2-interacting proteins contain proline-rich regions in which either PPYPXnYP (type 1 motif) or PXPGF (type 2 motif) is commonly found. Previous X-ray crystal structural analysis of the complex between ALG-2 and an ALIX peptide revealed that the peptide binds to the two hydrophobic pockets. In the present study, we resolved the crystal structure of the complex between ALG-2 and a peptide of Sec31A (outer shell component of coat complex II, COPII; containing the type 2 motif) and found that the peptide binds to the third hydrophobic pocket (Pocket 3). While amino acid substitution of Phe(85), a Pocket 3 residue, with Ala abrogated the interaction with Sec31A, it did not affect the interaction with ALIX. On the other hand, amino acid substitution of Tyr(180), a Pocket 1 residue, with Ala caused loss of binding to ALIX, but maintained binding to Sec31A. We conclude that ALG-2 recognizes two types of motifs at different hydrophobic surfaces. Furthermore, based on the results of serial mutational analysis of the ALG-2-binding sites in Sec31A, the type 2 motif was newly defined.

Background: ALG-2 is present both in the cytoplasm and nucleus, but little is known about its nuclear function. Results: ALG-2 interacts with the SR superfamily protein CHERP and accumulates at nuclear speckles in a Ca2+-dependent manner. Conclusion: ALG-2 and CHERP participate in alternative splicing. Significance: We propose a new role of ALG-2 and CHERP in post-transcriptional processing in the nucleus. The intracellular Ca2+ signaling pathway is important for the control of broad cellular processes from fertilization to cell death. ALG-2 is a Ca2+-binding protein that contains five serially repeated EF-hand motifs and interacts with various proteins in a Ca2+-dependent manner. Although ALG-2 is present both in the cytoplasm and in the nucleus, little is known about its nuclear function. Ca2+ homeostasis endoplasmic reticulum protein (CHERP) was first identified as an endoplasmic reticulum protein that regulates intracellular Ca2+ mobilization in human cells, but recent proteomics data suggest an association between CHERP and spliceosomes. Here, we report that CHERP, containing a Pro-rich region and a phosphorylated Ser/Arg-rich RS-like domain, is a novel Ca2+-dependent ALG-2-interactive target in the nucleus. Immunofluorescence microscopic analysis revealed localization of CHERP to the nucleoplasm with prominent accumulation at nuclear speckles, which are the sites of storage and modification for pre-mRNA splicing factors. Live cell time-lapse imaging showed that nuclear ALG-2 was recruited to the CHERP-localizing speckles upon Ca2+ mobilization. Results of co-immunoprecipitation assays revealed binding of CHERP to a phosphorylated form of RNA polymerase II. Knockdown of CHERP or ALG-2 in HT1080 cells resulted in generation of alternatively spliced isoforms of the inositol 1,4,5-trisphosphate receptor 1 (IP(3)R1) pre-mRNA that included exons 41 and 42 in addition to the major isoform lacking exons 40-42. Furthermore, binding between CHERP and IP(3)R1 RNA was detected by an RNA immunoprecipitation assay using a polyclonal antibody against CHERP. These results indicate that CHERP and ALG-2 participate in regulation of alternative splicing of IP(3)R1 pre-mRNA and provide new insights into post-transcriptional regulation of splicing variants in Ca2+ signaling pathways.

Many nonenzymatic cellular proteins exert their functions by interacting with other proteins or -macromolecules. Analysis of the physical interactions of proteins is an important step to understand their functions, and the information obtained is helpful for predicting the roles of the proteins in cells. Here we describe three biochemical and immunological methods for the detection of interactions between ALG-2 (a penta-EF-hand Ca(2+)-binding protein, also known as PDCD6) and its target proteins: (1) glutathione-S-transferase (GST) pulldown assay, (2) co-immunoprecipitation assay, and (3) Far Western blot analysis using biotinylated ALG-2. Dependency of Ca(2+) for interaction is examined by inclusion of CaCl(2) or EGTA in buffers used for binding assays.

ALG-2 (also named PDCD6) is a 22-kDa Ca2+-binding protein that belongs to the penta-EF-hand family including calpain small subunit and interacts with various proteins such as ALIX and Sec31A at their specific sites containing an ALG-2-binding motif (ABM) present in their respective Pro-rich region (PRR). In this study, to search for novel ALG-2-interacting proteins, we first performed in silico screening of ABM-containing PRRs in a human protein database. After selecting 17 sequences, we expressed the PRR or full-length proteins fused with green fluorescent protein (GFP) in HEK293T cells and analysed their abilities to bind to ALG-2 by Far-Western blotting using biotinylated ALG-2 as a probe. As a result, we found 10 positive new ALG-2-binding candidates with different degrees of binding ability. For further investigation, we selected PATL1 (alternatively designated Pat1b), a component of the P-body, which is a cytoplasmic non-membranous granule composed of translation-inactive mRNAs and proteins involved in mRNA decay. Interactions between endogenous PATL1 and ALG-2 proteins were demonstrated by a co-immunoprecipitation assay using their specific antibodies. Furthermore, in immunofluorescence microscopic analyses, PATL1 as well as DCP1A, a well-known P-body marker, co-localized with a subset of ALG-2. This is the first report showing interaction of ALG-2 with a P-body component.

The microRNA miR-122 and DDX6/Rck/p54, a microRNA effector, have been implicated in hepatitis C virus (HCV) replication. In this study, we demonstrated for the first time that HCV-JFH1 infection disrupted processing (P)-body formation of the microRNA effectors DDX6, Lsm1, Xrn1, PATL1, and Ago2, but not the decapping enzyme DCP2, and dynamically redistributed these microRNA effectors to the HCV production factory around lipid droplets in HuH-7-derived RSc cells. Notably, HCV-JFH1 infection also redistributed the stress granule components GTPase-activating protein (SH3 domain)-binding protein 1 (G3BP1), ataxin-2 (ATX2), and poly(A)-binding protein 1 (PABP1) to the HCV production factory. In this regard, we found that the P-body formation of DDX6 began to be disrupted at 36 h postinfection. Consistently, G3BP1 transiently formed stress granules at 36 h postinfection. We then observed the ringlike formation of DDX6 or G3BP1 and colocalization with HCV core after 48 h postinfection, suggesting that the disruption of P-body formation and the hijacking of P-body and stress granule components occur at a late step of HCV infection. Furthermore, HCV infection could suppress stress granule formation in response to heat shock or treatment with arsenite. Importantly, we demonstrate that the accumulation of HCV RNA was significantly suppressed in DDX6, Lsm1, ATX2, and PABP1 knockdown cells after the inoculation of HCV-JFH1, suggesting that the P-body and the stress granule components are required for the HCV life cycle. Altogether, HCV seems to hijack the P-body and the stress granule components for HCV replication.