Dawn A. Parsell

18 Heat Shock Proteins and Stress Tolerance

I. INTRODUCTION The capacity of different individuals of the same species to survive short exposures to extreme temperatures (thermotolerance) varies over a remarkable range, commonly over several orders of magnitude. Both differences in growth conditions and differences in genetic background contribute. Although the contributions of genetic background are just beginning to be deciphered, the effects of growth conditions have been the subject of detailed and intense scrutiny. Nutrient availability, oxygen tension, diurnal rhythms, and a host of other variables exert highly reproducible effects on thermotolerance. By far the most closely studied phenomenon, however, is the tolerance afforded by short pretreatments at moderately elevated temperature. When whole organisms or cultured cells are given such pretreatments, their resistance to killing by extreme heat increases dramatically. This increased resistance, known as induced thermotolerance (Fig. 1), is observed in virtually every organism studied. Remarkably, mild heat pretreatments elicit resistance not just to high temperatures, but to an extraordinary variety of other stresses. In addition, exposure to other mild stresses elicits protection not just against higher doses of those particular stresses, but against high temperatures as well. Such tolerance-inducing treatments generally also induce the synthesis of a small number of proteins known as the heat shock proteins (hsps; Fig. 2) (Li and Laszlo 1985; Lindquist 1986; Nagao et al. 1986; Subjeck and Shyy 1986; Sanchez and Lindquist 1990; Nover 1991; Sanchez et al. 1992). Historically, many observations have suggested that hsps play a vital part in induced tolerance. For example, it is striking that hsps...

Heat-shock proteins Hsp104 and Hsp70 reactivate mRNA splicing after heat inactivation.

BACKGROUND The heat-shock protein Hsp104 plays a crucial role in the survival of cells exposed to high temperatures and other severe stresses, but its specific functions and the biological pathways on which it operates have been unclear. Indeed, very little is known about the specific cellular processes in which any of the heat-shock proteins acts to affect thermotolerance. One essential process that is particularly sensitive to heat in many organisms is the splicing of intervening sequences from mRNA precursors. RESULTS We have examined the role of Hsp104 in the repair of splicing after disruption by heat shock. When splicing in the budding yeast Saccharomyces cerevisiae was disrupted by a brief heat shock, it recovered much more rapidly in wild-type strains than in strains containing hsp104 mutations. Constitutive expression of Hsp104 promoted the recovery of heat-damaged splicing in the absence of other protein synthesis, but did not protect splicing from the initial disruption, suggesting that Hsp104 functions to repair splicing after heat damage rather than to prevent the initial damage. A modest reduction in the recovery of splicing after heat shock in an hsp70 mutant suggested that Hsp70 may also function in the repair of splicing. The roles of Hsp104 and Hsp70 were confirmed by the ability of the purified proteins to restore splicing in extracts that had been heat-inactivated in vitro. Together, these two proteins were able to restore splicing to a greater degree than could be accomplished by an optimal concentration of either protein alone. CONCLUSIONS Our findings provide the first demonstration of the roles of heat-shock proteins in a biological process that is known to be particularly sensitive to heat in vivo. The results support previous genetic arguments that the Hsp104 and Hsp70 proteins have different, but related, functions in protecting cells from the toxic effects of high temperatures. Because Hsp104 and Hsp70 are able to function in vitro, after the heat-damaged substrate or substrates have been generated, neither protein is required to bind to its target(s) during heating in order to effect repair.

Relaxin Binds to and Elicits a Response from Cells of the Human Monocytic Cell Line, THP-1

Relaxin is a 6-kDa peptide of the insulin family that is present at increased levels in the circulation during pregnancy. Its functions at that time are thought to include maintenance of myometrial quiescence, regulation of plasma volume, and release of neuropeptides, such as oxytocin and vasopressin. The protein also promotes connective tissue remodeling, which allows cervical ripening and separation of the pelvic symphysis in various mammalian species. In this report, we provide evidence for a novel target of relaxin, the human monocytic cell line, THP-1. Relaxin bound with high affinity (Kd = 102 pM) to a specific receptor on THP-1 cells. Receptor density was low (∼275 receptors/cell), but binding of relaxin triggered intracelluar signaling events. Receptor density was not modulated by pretreatment with estrogen, progesterone, or a number of other agents known to induce differentiation of THP-1 cells. Cross-linking studies showed radiolabeled relaxin bound primarily to cell surface proteins with an apparent molecular mass of >200 kDa. Other members of the insulin-like family of proteins (insulin, insulin-like growth factors I and II, and relaxin-like factor) were unable to displace the binding of relaxin to THP-1 cells, suggesting that a distinct receptor for relaxin exists on this monocyte/macrophage cell line.