title: Decreased heat shock protein 27 expression and altered autophagy in human cells harboring A8344G mitochondrial DNA mutation.
abstract: Mitochondrial DNA (mtDNA) mutations are responsible for human neuromuscular diseases caused by mitochondrial dysfunction. Myoclonus epilepsy associated with ragged-red fibers (MERRF) is a maternally inherited mitochondrial encephalomyopathy with various syndromes involving both muscular and nervous systems. The most common mutation in MERRF syndrome, A8344G mutation in mtDNA, has been associated with severe defects in protein synthesis. This defect impairs assembly of complexes in electron transport chain and results in decreased respiratory function of mitochondria. In this study, we showed a significant decrease of the heat shock protein 27 (Hsp27) in lymphoblastoid cells derived from a MERRF patient and in cybrid cells harboring MERRF A8344G mutation. However, normal cytoplasmic distributions of Hsp27 and normal heat shock responses were observed in both wild type and mutant cybrids. Furthermore, overexpression of wild type Hsp27 in mutant MERRF cybrids significantly decreased cell death under staurosporine (STS) treatment, suggesting a protective function of Hsp27 in cells harboring the A8344G mutation of mtDNA. Meanwhile, reverse transcriptase PCR showed no difference in the mRNA level between normal and mutant cybrids, indicating that alterations may occur at the protein level. Evidenced by the decreased levels of Hsp27 upon treatment with proteasome inhibitor, starvation and rapamycin and the accumulation of Hsp27 upon lysosomal inhibitor treatment; Hsp27 may be degraded by the autophagic pathway. In addition, the increased formation of LC3-II and autophagosomes was found in MERRF cybrids under the basal condition, indicating a constitutively-activated autophagic pathway. It may explain, at least partially, the faster turnover of Hsp27 in MERRF cybrids. This study provides information for us to understand that Hsp27 is degraded through the autophagic pathway and that Hsp27 may have a protective role in MERRF cells. Regulating Hsp27 and the autophagic pathway might help develop therapeutic solutions for treatment of MERRF syndrome in the future.
authors.affiliation: Department of Life Science, TungHai University, Taichung, Taiwan, Republic of China.
authors.forename: Chin-Yi
authors.initials: CY
authors.lastname: Chen
authors.forename: Hsueh-Fu
authors.initials: HF
authors.lastname: Chen
authors.forename: Siao-Jhen
authors.initials: SJ
authors.lastname: Gi
authors.forename: Tang-Hao
authors.initials: TH
authors.lastname: Chi
authors.forename: Che-Kun
authors.initials: CK
authors.lastname: Cheng
authors.forename: Chi-Fu
authors.initials: CF
authors.lastname: Hsu
authors.forename: Yi-Shing
authors.initials: YS
authors.lastname: Ma
authors.forename: Yau-Huei
authors.initials: YH
authors.lastname: Wei
authors.forename: Chin-Shan
authors.initials: CS
authors.lastname: Liu
authors.forename: Mingli
authors.initials: M
authors.lastname: Hsieh
mesh_terms: D017209:Apoptosis; D001343:Autophagy / Q000235:genetics*; D000071192:Autophagy-Related Protein 12; D000071187:Autophagy-Related Protein 5; D053148:Caspase 3 / Q000378:metabolism; D002478:Cells, Cultured; D004272:DNA, Mitochondrial / Q000235:genetics*; D015536:Down-Regulation; D004789:Enzyme Activation; D055551:HSP27 Heat-Shock Proteins / Q000235:genetics / Q000378:metabolism*; D018840:HSP70 Heat-Shock Proteins / Q000378:metabolism; D018841:HSP90 Heat-Shock Proteins / Q000378:metabolism; D006360:Heat-Shock Proteins; D006801:Humans; D017243:MERRF Syndrome / Q000235:genetics; D008869:Microtubule-Associated Proteins / Q000235:genetics / Q000378:metabolism; D018832:Molecular Chaperones; D017354:Point Mutation; D059748:Proteolysis; D025841:Small Ubiquitin-Related Modifier Proteins / Q000235:genetics / Q000378:metabolism; D013312:Stress, Physiological
chemical_list: C466624:ATG12 protein, human; C502778:ATG5 protein, human; D000071192:Autophagy-Related Protein 12; D000071187:Autophagy-Related Protein 5; D004272:DNA, Mitochondrial; D055551:HSP27 Heat-Shock Proteins; D018840:HSP70 Heat-Shock Proteins; D018841:HSP90 Heat-Shock Proteins; C057725:HSPB1 protein, human; D006360:Heat-Shock Proteins; D008869:Microtubule-Associated Proteins; D018832:Molecular Chaperones; D025841:Small Ubiquitin-Related Modifier Proteins; D053148:Caspase 3