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 of a chicken small heat shock (stress) protein: evidence of multilevel posttranscriptional regulation

A novel form of regulation of expression of a vertebrate heat shock gene is described. A cDNA clone encoding human Hsp27 was shown to specifically recognize chicken Hsp23 RNA by Northern (RNA) blot analysis and hybrid-select translation. This probe was then used to measure chicken hsp23 gene activity in control and heat-stressed cells. The hsp23 gene(s) was transcriptionally active in non-heat-stressed cells, and its rate of transcription did not increase significantly upon heat shock. Cytoplasmic Hsp23 mRNA, which was metabolically very stable in nonstressed cells, underwent a fourfold increase in amount after a 1-h heat shock, resulting in a twofold increase in Hsp23 mRNA in polysomes. Hsp23 mRNA was relatively abundant and translationally active even in non-heat-shocked cells. Taken together, these data implicated posttranscriptional nuclear events as an important control point for induction of Hsp23 RNA transcripts. The protein half-life of Hsp23 increased from approximately 2 h in control cultures to 13 h in heat-shocked cells, revealing a second major control point. Hsp23 which was synthesized prior to heat shock also increased in stability and contributed to the overall accumulation of Hsp23 in heat-shocked cells. Cycloheximide had no effect on this change in Hsp23 half-life, while dactinomycin blocked the stabilization of Hsp23, suggesting a need for newly synthesized RNA. These data indicated that stabilization of Hsp23 protein and posttranscriptional nuclear events resulting in increased production of Hsp23 mRNA were primarily responsible for a 13-fold increase in the accumulation of newly synthesized Hsp23 after 1 h of heat shock. The regulation of the hsp23 gene is discussed in comparison with several other posttranscriptionally regulated genes, including the proto-oncogene c-fos, the developmentally regulated chicken delta-crystallin gene, and regulation of cellular gene expression by the proto-oncogene c-myc.

Induction of a chicken small heat shock (stress) protein: evidence of multilevel posttranscriptional regulation

A novel form of regulation of expression of a vertebrate heat shock gene is described. A cDNA clone encoding human Hsp27 was shown to specifically recognize chicken Hsp23 RNA by Northern (RNA) blot analysis and hybrid-select translation. This probe was then used to measure chicken hsp23 gene activity in control and heat-stressed cells. The hsp23 gene(s) was transcriptionally active in non-heat-stressed cells, and its rate of transcription did not increase significantly upon heat shock. Cytoplasmic Hsp23 mRNA, which was metabolically very stable in nonstressed cells, underwent a fourfold increase in amount after a 1-h heat shock, resulting in a twofold increase in Hsp23 mRNA in polysomes. Hsp23 mRNA was relatively abundant and translationally active even in non-heat-shocked cells. Taken together, these data implicated posttranscriptional nuclear events as an important control point for induction of Hsp23 RNA transcripts. The protein half-life of Hsp23 increased from approximately 2 h in control cultures to 13 h in heat-shocked cells, revealing a second major control point. Hsp23 which was synthesized prior to heat shock also increased in stability and contributed to the overall accumulation of Hsp23 in heat-shocked cells. Cycloheximide had no effect on this change in Hsp23 half-life, while dactinomycin blocked the stabilization of Hsp23, suggesting a need for newly synthesized RNA. These data indicated that stabilization of Hsp23 protein and posttranscriptional nuclear events resulting in increased production of Hsp23 mRNA were primarily responsible for a 13-fold increase in the accumulation of newly synthesized Hsp23 after 1 h of heat shock. The regulation of the hsp23 gene is discussed in comparison with several other posttranscriptionally regulated genes, including the proto-oncogene c-fos, the developmentally regulated chicken delta-crystallin gene, and regulation of cellular gene expression by the proto-oncogene c-myc.

Posttranscriptional regulation of hsp70 expression in human cells: effects of heat shock, inhibition of protein synthesis, and adenovirus infection on translation and mRNA stability

We have examined the posttranscriptional regulation of hsp70 gene expression in two human cell lines, HeLa and 293 cells, which constitutively express high levels of HSP70. HSP70 mRNA translates with high efficiency in both control and heat-shocked cells. Therefore, heat shock is not required for the efficient translation of HSP70 mRNA. Rather, the main effect of heat shock on translation is to suppress the translatability of non-heat shock mRNAs. Heat shock, however, has a marked effect on the stability of HSP70 mRNA; in non-heat-shocked cells the half-life of HSP70 mRNA is approximately 50 min, and its stability increases at least 10-fold upon heat shock. Moreover, HSP70 mRNA is more stable in cells treated with protein synthesis inhibitors, suggesting that a heat shock-sensitive labile protein regulates its turnover. An additional effect on posttranscriptional regulation of hsp70 expression can be found in adenovirus-infected cells, in which HSP70 mRNA levels decline precipititously late during infection although hsp70 transcription continues unabated.

Posttranscriptional regulation of hsp70 expression in human cells: effects of heat shock, inhibition of protein synthesis, and adenovirus infection on translation and mRNA stability

We have examined the posttranscriptional regulation of hsp70 gene expression in two human cell lines, HeLa and 293 cells, which constitutively express high levels of HSP70. HSP70 mRNA translates with high efficiency in both control and heat-shocked cells. Therefore, heat shock is not required for the efficient translation of HSP70 mRNA. Rather, the main effect of heat shock on translation is to suppress the translatability of non-heat shock mRNAs. Heat shock, however, has a marked effect on the stability of HSP70 mRNA; in non-heat-shocked cells the half-life of HSP70 mRNA is approximately 50 min, and its stability increases at least 10-fold upon heat shock. Moreover, HSP70 mRNA is more stable in cells treated with protein synthesis inhibitors, suggesting that a heat shock-sensitive labile protein regulates its turnover. An additional effect on posttranscriptional regulation of hsp70 expression can be found in adenovirus-infected cells, in which HSP70 mRNA levels decline precipititously late during infection although hsp70 transcription continues unabated.

Effects of cycloheximide on thermotolerance expression, heat shock protein synthesis, and heat shock protein mRNA accumulation in rat fibroblasts

A single hyperthermic exposure can render cells transiently resistant to subsequent high temperature stresses. Treatment of rat embryonic fibroblasts with cycloheximide for 6 h after a 20-min interval at 45 degrees C inhibits protein synthesis, including heat shock protein (hsp) synthesis, and results in an accumulation of hsp 70 mRNA, but has no effect on subsequent survival responses to 45 degrees C hyperthermia. hsp 70 mRNA levels decreased within 1 h after removal of cycloheximide but then appeared to stabilize during the next 2 h (3 h after drug removal and 9 h after heat shock). hsp 70 mRNA accumulation could be further increased by a second heat shock at 45 degrees C for 20 min 6 h after the first hyperthermic exposure in cycloheximide-treated cells. Both normal protein and hsp synthesis appeared increased during the 6-h interval after hyperthermia in cultures which received two exposures to 45 degrees C for 20 min compared with those which received only one treatment. No increased hsp synthesis was observed in cultures treated with cycloheximide, even though hsp 70 mRNA levels appeared elevated. These data indicate that, although heat shock induces the accumulation of hsp 70 mRNA in both normal and thermotolerant cells, neither general protein synthesis nor hsp synthesis is required during the interval between two hyperthermic stresses for Rat-1 cells to express either thermotolerance (survival resistance) or resistance to heat shock-induced inhibition of protein synthesis.

A major collagen-binding protein of chick embryo fibroblasts is a novel heat shock protein

Heat shock proteins of chick embryo fibroblasts were analyzed on SDS polyacrylamide gradient gels and were found to include not only three previously well-characterized proteins of 25,000, 73,000, and 89,000 D, but also a 47,000-D protein. Two-dimensional gel electrophoresis revealed that this protein was unusually basic (pI = 9.0) and corresponded to a recently characterized, major gelatin- and collagen-binding protein. The induction of synthesis of this 47,000-D membrane glycoprotein after heat stress of fibroblasts was particularly apparent in preparations isolated by gelatin-affinity chromatography. Regulation of this 47,000-D phosphoprotein was more sensitive than that of three major heat shock proteins in that a substantial stimulation of synthesis occurred at even 42 degrees C, as well as at higher temperature. Phosphorylation of the 47,000-D protein was not altered after heat shock. These studies establish this phosphorylated membrane glycoprotein as a member of the heat shock/stress protein family, and they add collagen binding to the unexpectedly diverse spectrum of biochemical activities induced by exposure of cells to stress.

Effects of cycloheximide on thermotolerance expression, heat shock protein synthesis, and heat shock protein mRNA accumulation in rat fibroblasts

A single hyperthermic exposure can render cells transiently resistant to subsequent high temperature stresses. Treatment of rat embryonic fibroblasts with cycloheximide for 6 h after a 20-min interval at 45 degrees C inhibits protein synthesis, including heat shock protein (hsp) synthesis, and results in an accumulation of hsp 70 mRNA, but has no effect on subsequent survival responses to 45 degrees C hyperthermia. hsp 70 mRNA levels decreased within 1 h after removal of cycloheximide but then appeared to stabilize during the next 2 h (3 h after drug removal and 9 h after heat shock). hsp 70 mRNA accumulation could be further increased by a second heat shock at 45 degrees C for 20 min 6 h after the first hyperthermic exposure in cycloheximide-treated cells. Both normal protein and hsp synthesis appeared increased during the 6-h interval after hyperthermia in cultures which received two exposures to 45 degrees C for 20 min compared with those which received only one treatment. No increased hsp synthesis was observed in cultures treated with cycloheximide, even though hsp 70 mRNA levels appeared elevated. These data indicate that, although heat shock induces the accumulation of hsp 70 mRNA in both normal and thermotolerant cells, neither general protein synthesis nor hsp synthesis is required during the interval between two hyperthermic stresses for Rat-1 cells to express either thermotolerance (survival resistance) or resistance to heat shock-induced inhibition of protein synthesis.

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.

Cloning of the chick hsp 90 cDNA in expression vector

A cDNA clone for the 90kDa heat-shock protein, which we have recently identified as a component of steroid hormone receptors in their heteromeric 8S form, was isolated by direct immunological screening of a chicken smooth muscle cDNA expression library, prepared in the expression plasmids pUC8 and pUC9. Using polyclonal and monoclonal antibodies against the 90kDa protein a colony was identified that reacted with both antibodies. Plasmid 9.11 (p9.11, approximately 1100 base pair insert) was found to hybrid-select mRNA for the 90kDa heat-shock protein. Northern blot analysis revealed that RNA isolated from various chicken tissues contain a single transcript of approximately 3 Kb hybridizing to a [32P]labelled cDNA probe made from p9.11. Heat-shock treatment of chick embryonic fibroblasts resulted in increased steady-state levels of a 3 Kb transcript in both poly A+ and poly A- RNA fractions. Southern blot analysis of chicken genomic DNA indicated that the cDNA hybridizes to a single copy sequence. Sequence data show that the p9.11 cDNA displays a high degree of homology with the 5' portion of yeast heat shock protein 90 cDNA.