Analysis of stress granule assembly in Schizosaccharomyces pombe.

Stress granules (SGs) are cytoplasmic aggregates of RNA and proteins in eukaryotic cells that are rapidly induced in response to environmental stress, but are not seen in cells growing under favorable conditions. SGs have been primarily studied in mammalian cells. The existence of SGs in the fission yeast and the distantly related budding yeast was demonstrated only recently. In both species, they contain many orthologs of the proteins seen in mammalian SGs. In this study, we have characterized these proteins and determined their involvement in the assembly of fission yeast SGs, in particular, the homolog of human G3BP proteins. G3BP interacts with the deubiquitinating protease USP10 and plays an important role in the assembly of SGs. We have also identified Ubp3, an ortholog of USP10, as an interaction partner of the fission yeast G3BP-like protein Nxt3 and required for its stability. Under thermal stress, like their human orthologs, both Nxt3 and Ubp3 rapidly relocalize to cytoplasmic foci that contain the SG marker poly(A)-binding protein Pabp. However, in contrast to G3BP1 and USP10, neither deletion nor overexpression of nxt3(+) or ubp3(+) affected the assembly of fission yeast SGs as judged by the relocalization of Pabp. Similar results were observed in mutants defective in orthologs of SG components that are known to affect SG assembly in human and in budding yeast, such as ataxia-2 and TIA-like proteins. Together, our data indicate that despite similar protein compositions, the underlying molecular mechanisms for the assembly of SGs could be distinct between species.

Vgl1, a multi-KH domain protein, is a novel component of the fission yeast stress granules required for cell survival under thermal stress.

Multiple KH-domain proteins, collectively known as vigilins, are evolutionarily highly conserved proteins that are present in eukaryotic organisms from yeast to metazoa. Proposed roles for vigilins include chromosome segregation, messenger RNA (mRNA) metabolism, translation and tRNA transport. As a step toward understanding its biological function, we have identified the fission yeast vigilin, designated Vgl1, and have investigated its role in cellular response to environmental stress. Unlike its counterpart in Saccharomyces cerevisiae, we found no indication that Vgl1 is required for the maintenance of cell ploidy in Schizosaccharomyces pombe. Instead, Vgl1 is required for cell survival under thermal stress, and vgl1Δ mutants lose their viability more rapidly than wild-type cells when incubated at high temperature. As for Scp160 in S. cerevisiae, Vgl1 bound polysomes accumulated at endoplasmic reticulum (ER) but in a microtubule-independent manner. Under thermal stress, Vgl1 is rapidly relocalized from the ER to cytoplasmic foci that are distinct from P-bodies but contain stress granule markers such as poly(A)-binding protein and components of the translation initiation factor eIF3. Together, these observations demonstrated in S. pombe the presence of RNA granules with similar composition as mammalian stress granules and identified Vgl1 as a novel component that required for cell survival under thermal stress.

Schizosaccharomyces pombe possesses two paralogous valyl-tRNA synthetase genes of mitochondrial origin.

Previous studies showed that VAS1 of Saccharomyces cerevisiae encodes both cytosolic and mitochondrial forms of valyl-tRNA synthetase (ValRS) through alternative initiation of translation. We show herein that except for Schizosaccharomyces pombe, all yeast species studied contained a single ValRS gene encoding both forms, and all of the mature protein forms deduced from those genes possessed an N-terminal appended domain (Ad) that was absent from their bacterial relatives. In contrast, S. pombe contained two distinct nuclear ValRS genes, one encoding the mitochondrial form and the other its cytosolic counterpart. Although the cytosolic form closely resembles other yeast ValRS sequences (approximately 60% identity), the mitochondrial form exhibits significant divergence from others (approximately 35% identity). Both genes are active and essential for the survival of the yeast. Most conspicuously, the mitochondrial form lacks the characteristic Ad. A phylogenetic analysis further suggested that both forms of S. pombe ValRS are of mitochondrial origin, and the mitochondrial form is ancestral to the cytoplasmic form.

Mitochondrial apoptosis and FAK signaling disruption by a novel histone deacetylase inhibitor, HTPB, in antitumor and antimetastatic mouse models.

BACKGROUND: Compound targeting histone deacetylase (HDAC) represents a new era in molecular cancer therapeutics. However, effective HDAC inhibitors for the treatment of solid tumors remain to be developed. METHODOLOGY/PRINCIPAL FINDINGS: Here, we propose a novel HDAC inhibitor, N-Hydroxy-4-(4-phenylbutyryl-amino) benzamide (HTPB), as a potential chemotherapeutic drug for solid tumors. The HDAC inhibition of HTPB was confirmed using HDAC activity assay. The antiproliferative and anti-migratory mechanisms of HTPB were investigated by cell proliferation, flow cytometry, DNA ladder, caspase activity, Rho activity, F-actin polymerization, and gelatin-zymography for matrix metalloproteinases (MMPs). Mice with tumor xenograft and experimental metastasis model were used to evaluate effects on tumor growth and metastasis. Our results indicated that HTPB was a pan-HDAC inhibitor in suppressing cell viability specifically of lung cancer cells but not of the normal lung cells. Upon HTPB treatment, cell cycle arrest was induced and subsequently led to mitochondria-mediated apoptosis. HTPB disrupted F-actin dynamics via downregulating RhoA activity. Moreover, HTPB inhibited activity of MMP2 and MMP9, reduced integrin-ß1/focal adhesion complex formation and decreased pericellular poly-fibronectin assemblies. Finally, intraperitoneal injection or oral administration of HTPB efficiently inhibited A549 xenograft tumor growth in vivo without side effects. HTPB delayed lung metastasis of 4T1 mouse breast cancer cells. Acetylation of histone and non-histone proteins, induction of apoptotic-related proteins and de-phosphorylation of focal adhesion kinase were confirmed in treated mice. CONCLUSIONS/SIGNIFICANCE: These results suggested that intrinsic apoptotic pathway may involve in anti-tumor growth effects of HTPB in lung cancer cells. HTPB significantly suppresses tumor metastasis partly through inhibition of integrin-ß1/FAK/MMP/RhoA/F-actin pathways. We have provided convincing preclinical evidence that HTPB is a potent HDAC targeted inhibitor and is thus a promising candidate for lung cancer chemotherapy.

A novel histone deacetylase inhibitor exhibits antitumor activity via apoptosis induction, F-actin disruption and gene acetylation in lung cancer.

BACKGROUND: Lung cancer is the leading cause of cancer mortality worldwide, yet the therapeutic strategy for advanced non-small cell lung cancer (NSCLC) is limitedly effective. In addition, validated histone deacetylase (HDAC) inhibitors for the treatment of solid tumors remain to be developed. Here, we propose a novel HDAC inhibitor, OSU-HDAC-44, as a chemotherapeutic drug for NSCLC. METHODOLOGY/PRINCIPAL FINDINGS: The cytotoxicity effect of OSU-HDAC-44 was examined in three human NSCLC cell lines including A549 (p53 wild-type), H1299 (p53 null), and CL1-1 (p53 mutant). The antiproliferative mechanisms of OSU-HDAC-44 were investigated by flow cytometric cell cycle analysis, apoptosis assays and genome-wide chromatin-immunoprecipitation-on-chip (ChIP-on-chip) analysis. Mice with established A549 tumor xenograft were treated with OSU-HDAC-44 or vehicle control and were used to evaluate effects on tumor growth, cytokinesis inhibition and apoptosis. OSU-HDAC-44 was a pan-HDAC inhibitor and exhibits 3-4 times more effectiveness than suberoylanilide hydroxamic acid (SAHA) in suppressing cell viability in various NSCLC cell lines. Upon OSU-HDAC-44 treatment, cytokinesis was inhibited and subsequently led to mitochondria-mediated apoptosis. The cytokinesis inhibition resulted from OSU-HDAC-44-mediated degradation of mitosis and cytokinesis regulators Auroroa B and survivin. The deregulation of F-actin dynamics induced by OSU-HDAC-44 was associated with reduction in RhoA activity resulting from srGAP1 induction. ChIP-on-chip analysis revealed that OSU-HDAC-44 induced chromatin loosening and facilitated transcription of genes involved in crucial signaling pathways such as apoptosis, axon guidance and protein ubiquitination. Finally, OSU-HDAC-44 efficiently inhibited A549 xenograft tumor growth and induced acetylation of histone and non-histone proteins and apoptosis in vivo. CONCLUSIONS/SIGNIFICANCE: OSU-HDAC-44 significantly suppresses tumor growth via induction of cytokinesis defect and intrinsic apoptosis in preclinical models of NSCLC. Our data provide compelling evidence that OSU-HDAC-44 is a potent HDAC targeted inhibitor and can be tested for NSCLC chemotherapy.