Stress mRNA metabolism in canavanine-treated chicken embryo cells

Heat shock protein 70 binds its own messenger ribonucleic acid as part of a gene expression self-limiting mechanism

Expression of heat shock proteins is a cellular response to a variety of stressors. HSP70, the major stress-induced heat shock protein, is involved in repair and protection after the insult. However, the prolonged presence of this protein is detrimental. Consequently, Hsp70 expression must be tightly regulated. We have previously shown an increase in the degradation of Hsp70 messenger ribonucleic acid (mRNA) paralleling the accumulation of HSP70. Incubation of cells with transcriptional and translational inhibitors after heat shock resulted in a significant reduction in Hsp70 mRNA degradation. These observations suggest that newly synthesized, stress-induced factors might be involved in the decay of Hsp70 mRNA. We found that HSP70 binds directly to Hsp70 mRNA, as demonstrated by immunoprecipitation. This observation was confirmed by RNA gel-shift assays. These results are evidence for a novel and likely direct interaction between HSP70 and Hsp70 mRNA in cells after stress. This interaction may be part of a self-limiting mechanism to reduce HSP70 production, thus avoiding potential toxic effects of this protein in the absence of stress.

Induction and Regulation of Heat-Shock Gene Expression by an Amino Acid Analog in Soybean Seedlings

The effect of the proline analog azetidine-2-carboxylic acid (Aze) on the induction and the regulation of heat-shock (HS) mRNA accumulation and heat-shock protein (HSP) synthesis in soybean (Glycine max) seedlings was studied. Treatment with Aze elicited an HS-like response at the normal growth temperature, 28[deg]C, with seven of nine HS cDNA clones tested. Two cDNA clones, Gm-Hsp22.5 and pFS2033, share 78% identity; however, transcripts hybridizing to GmHsp22.5 but not pFS2033 accumulated with Aze treatment at 28[deg]C. Substantial incorporation of radioactive amino acid into high molecular weight HSPs but not low molecular weight HSPs was observed in vivo during Aze treatment at 28[deg]C. Low molecular weight HSPs were detected using antibodies raised against an abundant member of low molecular weight class I HSPs, indicating that low molecular weight HSPs were synthesized at normal growth temperatures during Aze treatment despite a lack of substantial in vivo radioactive amino acid incorporation. In summary, Aze treatment induced accumulation of most but not all HS mRNAs and HSPs in soybean seedlings; the observations presented here suggest differential regulation among various HS genes at the transcriptional and posttranscriptional levels.

Heat-shock-induced protein synthesis is responsible for the switch-off of hsp70 transcription in Tetrahymena

We had previously described that new RNA synthesis is required for expression of the heat shock protein HSP70. Here, we find that the HSP70 mRNA decreases its levels under stress conditions, heat shock (HS) or arsenite (As), and that its levels start to decline at the same time as maximal HSPs synthesis (including HSP70) occurs. This suggests that regulation of the hsp70 gene is mainly exerted at the transcriptional level. Accumulation of the HSP70 mRNA in cells stressed in presence of cycloheximide (CHX), indicates that (a) protein(s) non-existent before stress, possibly HSP70 itself (which is shown here to be relatively stable), is involved in negatively regulating hsp70 expression. Since degradation of the HSP70 mRNA is also shown to occur in cells heat-shocked under CHX, as seen from decay of its levels upon addition of actinomycin D (AMD), the protein(s) must repress hsp70 expression at the transcriptional level. Other conditions that affect normal protein synthesis, namely the translation inhibitor puromycin and the arginine-analog canavanine (shown here to be stress inducers in Tetrahymena pyriformis), also cause a delay in transcription-arrest of the HSP70 mRNA. Under severe stress conditions of HS (36 degrees C) or As (350 microM), the levels of HSP70 mRNA are higher than under mild stress conditions, however, no significant difference is seen in the pattern of HSP70 mRNA decay.

Regulation of chicken Hsp70 and Hsp90 family gene expression by transforming growth factor-beta 1

Transforming growth factor-beta 1 (TGF beta) is a regulator of protein synthesis in cultured chicken embryo cells (CEC). Preceding a gradual increase in overall protein synthesis, members of the Hsp70 family (Hsp70, Hsc70, and Grp78) and the Hsp90 family (90-2 and 90-3) of molecular chaperones are induced rapidly and represent a new class of TGF beta-inducible proteins (I.M. Takenaka and L.E. Hightower, J. Cell. Physiol., 152:568-577, 1992). Herein, 32P-labeled cDNA probes encoding Hsc70 and Hsp90 were used to show that levels of the corresponding mRNAs increased as a fraction of total RNA and in polysomes within five hours of treatment of CEC with TGF beta. This cytokine did not increase rates of hsc70 and hsp90 gene transcription as measured by run-on transcription assays of isolated nuclei. However, the Hsp RNA inductions were inhibited by dactinomycin, indicating a requirement for newly synthesized RNA. Both Hsc70 and Hsp90 mRNAs had relatively short half-lives, measured by Northern blot analyses of dactinomycin chases, which were not altered substantially in TGF beta-treated cells. In contrast, Hsp mRNA half-lives increased in heat shocked CEC exposed to dactinomycin during recovery, revealing a difference in regulation of these genes in stressed cells compared with TGF beta-treated cells. Our results support the conclusion that hsc70 and hsp90 gene expression is regulated posttranscriptionally in TGF beta-treated CEC, and the mechanism likely involves a nuclear event such as increasing the half-lives of nuclear RNA transcripts, processing, or transport into the cytoplasm.

Effect of amino acid analogs on the development of thermotolerance and on thermotolerant cells

Exposure of HA-1 Chinese hamster fibroblasts to amino acid analogs has been shown to have a heat-sensitizing effect as well as inducing the heat shock response (Li and Laszlo, 1985a). In this study, we have examined the effect of amino acid analogs on the development of thermotolerance after a brief heat shock or exposure to sodium arsenite and the effect of amino acid analogs on cells that are already thermotolerant. Exposure of HA-1 cells to amino acid analogs inhibited the development of thermotolerance following a mild heat shock or treatment with sodium arsenite. However, cells that were already thermotolerant were resistant to the sensitizing action of amino acid analogs. The refractoriness of thermotolerant cells to amino acid analog treatment developed in parallel with thermotolerance. The uptake of the arginine analog, canavanine, and its incorporation into proteins was not altered in the thermotolerant cells. Furthermore, another biological consequence of exposure to amino acid analogs, sensitization to ionizing radiation, also was not altered in the thermotolerant cells. The inhibition of the development of thermotolerance by amino acid analogs and the refractoriness of thermotolerant cells to the heat-sensitizing action of amino acid analogs lend further support the role of heat-shock proteins in the phenomenon of thermotolerance.

Inhibition of heat shock (stress) protein induction by deuterium oxide and glycerol: additional support for the abnormal protein hypothesis of induction

The patterns of radioactively labeled proteins from cultured chicken embryo cells stressed in the presence of either D2O or glycerol were analyzed by using one-dimensional polyacrylamide gel electrophoresis. These hyperthermic protectors blocked the induction of stress proteins during a 1-hour heat shock at 44 degrees C. The inhibitory effect of glycerol but not D2O on the induction of heat shock proteins could be overcome by increased temperature. By using transcriptional run-on assays of isolated nuclei and cDNA probes to detect hsp70- and hsp88-specific RNA transcripts, it was shown that the D2O and glycerol blocks occurred at or before transcriptional activation of the hsp70 and hsp88 genes. After heat-stressed cells were returned to 37 degrees C and the protectors were removed, heat shock proteins were inducible by a second heating. This result and the fact that the chemical stressor sodium arsenite induced stress proteins in glycerol medium indicated that the treatments did not irreversibly inhibit the induction pathways and that the stress response could be triggered even in the presence of glycerol by a stressor other than heat. In principle then, cells incurring thermal damage during a 1-hour heat shock at 44 degrees C in D2O or glycerol medium should be competent to respond by inducing heat shock proteins during a subsequent recovery period at 37 degrees C in normal medium. We found that heat shock proteins were not induced in recovering cells, suggesting that glycerol and D2O protected heat-sensitive targets from thermal damage. Evidence that the heat-sensitive target(s) is likely to be a protein(s) is summarized. During heat shocks of up to 3 hours duration, neither D2O nor glycerol significantly altered hsp23 gene activity, a constitutively expressed chicken heat shock gene whose RNA transcripts and protein products are induced by stabilization (increased half-life). During a 2-hour heat shock, glycerol treatment blocked the heat-induced stabilization of hsp23 RNA and proteins; however, D2O treatment only blocked RNA transcript stabilization, effectively uncoupling the hsp23 protein stabilization pathway from hsp23 RNA stabilization and transcriptional activation of hsp70 and hsp88 genes.

Accumulation of heat shock protein 70 RNA and its relationship to protein synthesis after heat shock in mammalian cells

Heat stress induces a set of heat shock proteins (hsps) in a wide variety of species. In response to either a mild (5 min X 45 degrees C) or severe (30 min X 45 degrees C) heat shock, the timing of expression of the hsps and the recovery of general protein synthesis in rat embryonic fibroblasts was dependent on the duration of the hyperthermic exposure. Synthesis of mRNA coding for an hsp of Mr approximately equal to 70,000 (hsp 70) followed immediately after the mild heat shock but was delayed after the severe heat shock. Appearance of the hsps paralleled the synthesis and decay of RNA and was indicative that new RNA synthesis was required for hsp 70 expression. Inhibition of protein synthesis by cycloheximide after the mild heat shock increased the maximal accumulation of hsp 70 encoding mRNA but did not prevent the subsequent decrease in this mRNA species. These results suggest that mammalian cells control the expression of hsp 70 primarily at the level of transcription, and that the normal pattern of hsp 70 mRNA turnover after an inducing heat stress is not dependent on new protein synthesis.

Induction of heat-shock proteins in the embryonic chicken lens

The 11-day embryonic chicken lens displays a rapid and vigorous response to a heat shock. Exposure of the lens to elevated temperature (45 degrees C) dramatically increased the synthesis of three proteins with subunit molecular weights of 89,000, 70,000 and 24,000. These lens heat-shock proteins are identical to the heat-shock proteins of cultured chicken embryo fibroblasts in electrophoretic mobility and immunological cross-reactivity. Maximum induction of lens heat-shock protein synthesis occurred upon incubation at 45 degrees C for about 1-1.5 hr and was blocked in the presence of actinomycin D. The functional half-lives at 37 degrees C of the mRNAs encoding the lens heat-shock proteins were about 3-5 hr. Synthesis of normal lens proteins continued throughout the heat shock, but delta-crystallin was unusual in that its synthesis initially rose upon heat shock, a behavior similar to that of a heat-shock protein. Proteins which co-migrated with each of the major lens heat-shock proteins were detected immunologically in non-stressed lens extracts. The endogenous lens protein, p24, which cross-reacted with antibodies raised against the small heat-shock protein (hsp 24) from chicken-embryo fibroblasts and co-migrated in one dimensional SDS gels with a major beta-crystallin was shown to be structurally similar to the chicken small heat shock protein but was distinct from the beta-crystallins.

Modulators of the eukaryotic heat shock response

A wide variety of agents other than heat have been reported to induce or suppress heat shock protein (hsp) synthesis in eukaryotic cells. Such agents, termed 'modulators', include inhibitors of respiration, low molecular weight nutrients, oxygen, hormones, sulfhydryl reagents, ionophores and amino acid analogues. The evidence for modulation is critically reviewed and common mechanisms of action that may relate modulation to induction by heat are discussed.

Sidestream cigarette smoke-exposure of mouse cells induces cell stress/heat shock-like proteins

Specific alterations in cellular protein synthesis have been identified in mouse L929 and B16 cells exposed to "passive" (sidestream) smoke freshly generated from unfiltered cigarettes. A decrease in both cell viability and protein synthesis was observed in monolayer cell cultures following exposure to increasing numbers (0-12) of puffs of sidestream smoke. With exposures that resulted in approximately 30% or higher loss in cell viability, there was an apparent induction of cell stress/heat shock-like polypeptides with approximate molecular weights of 88,000, 66,000 and 23,000. After exposure to higher numbers of puffs that led to a loss of cell viability of 80% or greater, a different set of polypeptides was synthesized, including a major new protein of 38,000 mol. wt and 2 other predominant proteins of 45,000 and 30,000 mol. wt. The same specific effects on cellular protein synthesis were also observed after exposure to a similar number of puffs of the gas phase of sidestream cigarette smoke (minus the particulate phase components).

Induction of pathogenesis-related proteins in tobacco leaves

The synthesis of pathogenesis-related proteins (PR proteins), induced in tobacco leaves in response to infection with tobacco mosaic virus or treatment with salicylic acid, was studied with in vivo pulse-labeling experiments. PR proteins synthesis began after a lag phase of about 8 hours in leaf discs treated with salicylic acid and after more than 18 hours in those infected with tobacco mosaic virus. In both cases, the synthesis declined rapidly after 50 hours. The results show that the accumulation of PR proteins results from de novo synthesis and not from degradation of preexisting precursors and that the induced synthesis is transient like other stress-inducible proteins. The proteins have a half-life of at least 50 hours. The induction of these PR proteins was not inhibited by either 25 micrograms per milliliter of actinomycin D or 200 micrograms per milliliter of alpha-amanitin, which completely inhibited the increase of peroxidase activity in tobacco mosaic virus-infected leaf and the induction of heat shock proteins in tobacco leaf discs. These findings indicate that the induction of PR proteins is not regulated by a transcriptional step but by a translational step.

Protein synthesis and proteolysis in immobilized cells of the cyanobacterium Nostoc commune UTEX 584 exposed to matric water stress

Cells of the cyanobacterium Nostoc commune UTEX 584 in exponential growth were subjected to acute water stress by immobilizing them on solid supports and drying them at a matric water potential (psi m) of -99.5 MPa. Cells which had been grown in the presence of Na235SO4 before immobilization and rapid drying continued to incorporate 35S into protein for 90 min. This incorporation was inhibited by chloramphenicol. No unique proteins appeared to be synthesized during this time. Upon further drying, the level of incorporation of 35S in protein began to decrease. In contrast, there was an apparent increase in the level of certain phycobiliprotein subunits in solubilized protein extracts of these cells. Extensive proteolysis was detected after prolonged desiccation (17 days) of the cells in the light, although they still remained intact. Phycobilisomes became dissociated in both light- and dark-stored desiccated material.