Heat shock and light activation of aChlamydomonas HSP70 gene are mediated by independent regulatory pathways

Induction ofHSP70 heat shock genes by light has been demonstrated inChlamydomonas. Our aim was to establish whether this induction by light is mediated by the heat stress sensing pathway or by an independent signal chain. Inhibitors of cytoplasmic protein synthesis revealed an initial difference. Cycloheximide and other inhibitors of protein synthesis preventedHSP70A induction upon illumination but not during heat stress. Analysis ofHSP70A induction in cells that had differentiated into gametes revealed a second difference. While heat shock resulted in elevatedHSP70A mRNA levels, light was no longer able to serve as an inducer in gametes. To identify the regulatory sequences that mediate the response of theHSP70A gene to either heat stress or light we introduced a series of progressive 5 truncations into its promoter sequence. Analyses of the levels of mRNA transcribed from these deletion constructs showed that in most of them the responses to heat shock and light were similar, suggesting that light induction is mediated by a light-activated heat shock factor. However, we show that theHSP70A promoter also containscis-acting sequences involved in light induction that do not participate in induction by heat stress. Together, these results provide evidence for a regulation ofHSP70A gene expression by light through a heat shock-independent signal pathway.     

A new assay for promoter analysis in Chlamydomonas reveals roles for heat shock elements and the TATA box in HSP70A promoter-mediated activation of transgene expression

The aim of this work was to identify cis-regulatory sequences within the Chlamydomonas HSP70A promoter that mediate its stimulatory effect on the expression of downstream promoters. For this, we deleted/mutated the HSP70A promoter and, using a new assay, quantified its stimulatory effect. Our results indicate that the effect is mediated largely by heat shock elements and the TATA box.     

Light-inducible geneHSP70B encodes a chloroplast-localized heat shock protein inChlamydomonas reinhardtii

The nuclear heat shock geneHSP70B ofChlamydomonas reinhardtii is inducible by heat stress and light. Induction by either environmental cue resulted in a transient elevation in HSP70B protein. Here we describe the organization and nucleotide sequence of theHSP70B gene. The deduced protein exhibits a distinctly higher homology to prokaryotic HSP70s than to those of eukaryotes, including the cytosolic HSP70A ofChlamydomonas reinhardtii. The HSP70B protein, as previously demonstrated by in vitro translation, is synthesized with a cleavable presequence. Using an HSP70B-specific antibody, this heat shock protein was localized to the chloroplast by cell fractionation experiments. A stromal location was suggested by the presence of a conserved sequence motif used for cleavage of presequences by a signal peptidase of the stroma. Amino acid alignments of HSP70 proteins from various organisms and different cellular compartments allowed the identification of sequence motifs, which are diagnostic for HSP70s of chloroplasts and cyanobacteria.     

PP2A Mediated AMPK Inhibition Promotes HSP70 Expression in Heat Shock Response

Background

Under stress, AMP-activated protein kinase (AMPK) plays a central role in energy balance, and the heat shock response is a protective mechanism for cell survival. The relationship between AMPK activity and heat shock protein (HSP) expression under stress is unclear.

Methodology/Principal Findings

We found that heat stress induced dephosphorylation of AMPKα subunit (AMPKα) in various cell types from human and rodent. In HepG2 cells, the dephosphorylation of AMPKα under heat stress in turn caused dephosphorylation of acetyl-CoA carboxylase and upregulation of phosphoenolpyruvate carboxykinase, two downstream targets of AMPK, confirming the inhibition of AMPK activity by heat stress. Treatment of HepG2 cells with phosphatase 2A (PP2A) inhibitor okadaic acid or inhibition of PP2A expression by RNA interference efficiently reversed heat stress-induced AMPKα dephosphorylation, suggesting that heat stress inhibited AMPK through activation of PP2A. Heat stress- and other HSP inducer (CdCl₂, celastrol, MG132)-induced HSP70 expression could be inhibited by AICAR, an AMPK specific activator. Inhibition of AMPKα expression by RNA interference reversed the inhibitory effect of AICAR on HSP70 expression under heat stress. These results indicate that AMPK inhibition under stress contribute to HSP70 expression. Mechanistic studies showed that activation of AMPK by AICAR had no effect on heat stress-induced HSF1 nuclear translocation, phosphorylation and binding with heat response element in the promoter region of HSP70 gene, but significantly decreased HSP70 mRNA stability.

Conclusions/Significance

These results demonstrate that during heat shock response, PP2A mediated AMPK inhibition upregulates HSP70 expression at least partially through stabilizing its mRNA, which suggests a novel mechanism for HSP induction under stress.     

Molecular cloning and expression of two HSP70 genes in the Wuchang bream (Megalobrama amblycephala Yih)

Two complementary deoxyribonucleic acid (cDNA) clones encoding heat shock cognate 70 (HSC70) and inducible heat shock protein 70 (HSP70) were isolated from the liver of Wuchang bream (Megalobrama amblycephala Y.) using RT-PCR and rapid amplification of cDNA ends (RACE). They were named Ma-HSC70 and Ma-HSP70, respectively. The cDNAs were 2336 and 2224 bp in length [not including poly (A)] and contained 1950 and 1932 bp open reading frames (ORFs), respectively. The ORFs encoded proteins of 649 and 643 amino acids with predicted molecular weights of 71.24 and 70.52 kDa, and theoretical isoelectric points of 5.25 and 5.30, respectively. Genomic DNA structure analysis revealed that Ma-HSC70 gene contained seven introns with all introns conforming to the GT/AG rule whereas Ma-HSP70 gene did not contain any intron in the coding region. Amino acid sequence analysis indicated that both Ma-HSC70 and Ma-HSP70 contained three signature sequences of HSP70 family, two partial overlapping bipartite nuclear localization signal sequences (NLS) and cytoplasmic characteristic motif (EEVD). Homology analysis revealed that Ma-HSC70 shared more than 93.0% identity with the known HSC70s of other vertebrates, while Ma-HSP70 shared more than 85.0% identity with the known HSP70s of other vertebrates, and Ma-HSC70 and Ma-HSP70 shared 86.5% identity. Bioinformatics analysis indicated that the proteins encoded by Ma-HSC70 and Ma-HSP70 genes were hydrophilic, rich in B cells antigenic sites, without any signal peptide or transmembrane region. The two proteins also contained many protein kinase C phosphorylation sites, N-myristoylation sites, casein kinase II phosphorylation sites, and N-glycosylation sites, predicting that they could play essential roles in protein folding, translocation, intracellular localization, signal transduction and regulation. The predominant secondary structures of the two proteins were α-helix and random coil. Fluorescent real-time quantitative RT-PCR was used to study the effects of heat shock (34 °C), crowding stress (100 g L^?1) and challenge with bacteria Aeromonas hydrophila on the mRNA expression of the two HSP70s in Wuchang bream liver. The results indicated that, during 24 h stress, Ma-HSC70 mRNA expression decreased at first and then rose to the level before stress under heat shock and crowding stress, but Ma-HSP70 mRNA expression increased at first and then decreased under heat stress, and appeared to increase continuously under crowding stress. After bacterial challenge, the mRNA levels of both Ma-HSC70 and Ma-HSP70 increased at first and then decreased. The cloning and expression analysis of the two HSP70s provide theoretical basis to further study the mechanism of anti-adverseness and expression characteristics under stress conditions of Wuchang bream.     

Analysis of Chromatin Structure in the Control Regions of the <Emphasis Type="Italic">Chlamydomonas HSP70A</Emphasis> and <Emphasis Type="Italic">RBCS2</Emphasis> Genes

We have used DNaseI and micrococcal nuclease sensitivity assays to determine the chromatin structures in the control regions of the Chlamydomonas reinhardtii HSP70A and RBCS2 genes. Both genes appear to be organized into nucleosome arrays, which exhibit shorter nucleosome repeat lengths than bulk chromatin. In HSP70A we have identified up to four confined DNaseI hypersensitive sites, three of them localize to the promoter region, a fourth one to the fourth intron. Three hypersensitive sites map close to putative heat shock elements, one close to a CCAAT-box. All hypersensitive sites are located to internucleosomal linkers. Alternative nucleosome positions at half-nucleosomal phasing were constitutively detected in the HSP70A promoter region, indicating local chromatin remodelling. Upon heat shock, dramatic changes in the nucleosome structure of HSP70A were detected that particularly affected the promoter, but also a region within the fourth intron. In contrast, light induction entailed no change in HSP70A chromatin. In the RBCS2 control region we identified a strong DNaseI hypersensitive site that maps close to a CCAAT-box. This site forms the boundary of a nucleosome array with a region of ~700 bp apparently devoid of nucleosomes. This study demonstrates that chromatin structure may be determined readily at fairly high resolution in Chlamydomonas, suggesting this organism as a well-suited model for studying the role of chromatin structure on gene expression in photosynthetic eukaryotes.     

Differential regulation of HSC70, HSP70, HSP90 alpha, and HSP90 beta mRNA expression by mitogen activation and heat shock in human lymphocytes

A subset of heat shock proteins, HSP90 alpha, HSP90 beta, and a member of the HSP70 family, HSC70, shows enhanced synthesis following mitogenic activation as well as heat shock in human peripheral blood mononuclear cells. In this study, we have examined expression of mRNA for these proteins, including the major 70-kDa heat shock protein, HSP70, in mononuclear cells following either heat shock or mitogenic activation with phytohemagglutinin (PHA), ionomycin, and the phorbol ester, tetradecanoyl phorbol acetate. The results demonstrate that the kinetics of mRNA expression of these four genes generally parallel the kinetics of enhanced protein synthesis seen following either heat shock or mitogen activation and provide clear evidence that mitogen-induced synthesis of HSC70 and HSP90 is due to increased mRNA levels and not simply to enhanced translation of preexisting mRNA. Although most previous studies have focused on cell cycle regulation of HSP70 mRNA, we found that HSP70 mRNA was only slightly and transiently induced by PHA activation, while HSC70 is the predominant 70-kDa heat shock protein homologue induced by mitogens. Similarly, HSP90 alpha appears more inducible by heat shock than mitogens while the opposite is true for HSP90 beta. These results suggest that, although HSP70 and HSC70 have been shown to contain similar promoter regions, additional regulatory mechanisms which result in differential expression to a given stimulus must exist. They clearly demonstrate that human lymphocytes are an important model system for determining mechanisms for regulation of heat shock protein synthesis in unstressed cells. Finally, based on kinetics of mRNA expression, the results are consistent with the hypothesis that HSC70 and HSP90 gene expression are driven by an IL-2/IL-2 receptor-dependent pathway in human T cells.     

Development of rapid and highly sensitive HSPA1A promoter-driven luciferase reporter system for assessing oxidative stress associated with low-dose photodynamic therapy

Photodynamic therapy (PDT) is a regulatory-approved modality for treating a variety of malignant tumors. It induces tumor tissue damage via photosensitizer-mediated oxidative cytotoxicity. The heat shock protein 70 (HSP70-1) is a stress protein encoded by the HSPA1A gene and is significantly induced by oxidative stress associated with PDT. The aim of this study was to identify the functional region of the HSPA1A promoter that responds to PDT-induced oxidative stress and uses the stress responsiveness of HSPA1A expression to establish a rapid and cost-effective photocytotoxic assessment bioassay to evaluate the photodynamic potential of photosensitizers. By constructing luciferase vectors with a variety of hspa1a promoter fractions and examining their relative luciferase activity, we demonstrated that the DNA sequence from −218 to +87 of the HSPA1A gene could be used as a functional promoter to detect the PDT-induced oxidative stress. The maximal relative luciferase activity level of HSPA1A (HSP70-1) induced by hypericin-PDT was nearly nine times that of the control. Our results suggest that the novel reporter gene assay using a functional region of the HSP70A1A promoter has significant advantages for the detection of photoactivity in terms of both speed and sensitivity, when compared with a cell viability test based on ATP quantification and ROS levels. Furthermore, phthalocyanine zinc and methylene blue both induced significantly elevated levels of relative luciferase activity in a dose-dependent manner.