Robert M. Tanguay

Heat shock induces in cells the development of a transient state of thermotolerance thought to result from the induction of heat shock proteins. To assess directly whether a transient overexpression of one of these proteins, HSP27, can contribute to increased cellular resistance, mouse NIH/3T3 cells were cotransfected with a plasmid containing the Chinese hamster HSP27 gene under the control of the metallothionein promoter and a plasmid containing the neo gene. Stable transfectant cell lines were selected for resistance to the antibiotic G418. Analyses of several stable transfectant cell lines indicated that expression of Chinese hamster HSP27 could be selectively induced by exposure to 3 microM CdCl2, a concentration that had no effect on the induction of the endogenous heat shock proteins (HSP). In clone 15, the level of HSP27 increased steadily during the first day of exposure to CdCl2, from a concentration of 1 microgram/mg of total protein to 7 micrograms/mg. After withdrawal of CdCl2, the level of HSP27 returned to normal within the next 5 days. Accumulation of the Chinese hamster HSP27 was accompanied by a progressive development of thermoresistance that attained a level approaching heat shock-induced thermotolerance. After CdCl2 removal, thermal resistance and HSP27 decayed in a coordinated manner. In control cells transfected with the neo gene only, increased thermoresistance was not induced by 3 microM CdCl2; in these cells, an exposure to 20 microM CdCl2 was required to induce a level of thermoresistance comparable to that induced by 3 microM CdCl2 in clone 15. Elevated expression of HSP27 was accompanied by an increased stability of stress fibers during hyperthermia. The protein also partially prevented actin depolymerization during acute exposure to cytochalasin D and reduced cytotoxicity and growth inhibition of chronic exposures to the drug. The results indicated that accumulation of HSP27, as it occurs after a mild heat shock or other inducing treatments, is sufficient for acquisition of thermotolerance that may result in part from a stabilization of actin filaments.

Morris hepatoma 7777 cells, heat conditioned at 43 degrees for 0.5 hr, become gradually thermoresistant during an incubation at 37 degrees as judged by their ability to form colonies following a second heat challenge. Pulse incorporation of [35S]methionine into proteins at various times after the conditioning treatment and subsequent fractionation of the proteins by polyacrylamide gel electrophoresis indicate that the gradual putative modifications occurring at the cellular level and leading to the thermotolerance state are accompanied by an elevated synthesis above the normal level of a small set of polypeptides with apparent molecular weights of 27,000, 65,000, 68,000, 70,000, 89,000, and 107,000. Both thermotolerance development and protein induction are completed after a 6- to 8-hr period. At the end of this period, thermotolerance is at its maximum level and heat shock protein synthesis is returned to normal. This acquired thermal resistance eventually disappears between 60 and 80 hr following conditioning treatment. In a parallel manner, the heat shock-induced proteins synthesized during the first 4 hr following the conditioning treatment are maintained in the cells at a high level for several hr but become undetectable by 82 hr. The results provide strong circumstantial evidence that heat shock proteins are involved in the acquisition, maintenance, and decay of thermotolerance.

Heat shock proteins (Hsp) are involved in protein folding, transport and stress resistance. Studies reporting an increased mRNA level of hsp genes in aged Drosophila suggest that expression of Hsp might be beneficial in preventing damages induced by aging. Because oxidative damage is often observed in aged organisms and mitochondria are sensitive to reactive oxygen species, we tested the hypothesis that increased levels of a small Hsp localized in mitochondria, Hsp22 of Drosophila melanogaster, could protect mitochondrial proteins and influence the aging process. We demonstrate that a ubiquitous or a targeted expression of Hsp22 within motorneurons increases the mean life span by more than 30%. Hsp22 shows beneficial effects on early-aging events since the premortality phase displays the same increase as the mean lifespan. Moreover, flies expressing Hsp22 in their motorneurons maintain their locomotor activity longer as assessed by a negative geotaxis assay. The motorneurons-targeted expression of Hsp22 also significantly increases flies' resistance to oxidative injuries induced by paraquat (up to 35%) and thermal stress (39% at 30 degrees C and 23% at 37 degrees C). These observations establish Hsp22 as a key player in cell-protection mechanisms against oxidative injuries and aging in Drosophila and corroborate the pivotal role of mitochondria in the process of aging.