GTMB 7 - Gene Therapy & Molecular Biology

Latchman: Protective effect of heat shock proteinsresponse to stress. Interestingly, however, many of theseproteins are also synthesised by normal unstressed cellswith their synthesis being further enhanced upon exposureto stress. For example, hsp90 is one of the most abundantproteins in unstressed cells, constituting approximately 1%of the total protein in mammalian cells even prior toexposure to stress. This has led to the idea that thefunction of the hsps is one which is required in normalcells but is needed to an even greater extent in stressedcells. This idea is in accordance with the detailedfunctional studies of individual hsps which, as shown inTable 1, have indicated that a number of them have a rolein ensuring the correct protein folding of other proteinswithin the cells, acting as so-called “molecularchaperones” (for review see Ellis, 1990). Thus, forexample, hsp90 associates with the steroid receptors, suchas the glucocorticoid receptor and keeps them in aninactive form located in the cytoplasm prior to exposure tosteroid. Upon steroid treatment, hsp90 dissociates from thereceptor which then can move to the nucleus and activatesteroid responsive genes.Clearly, correct protein folding is of importance innormal cells but factors which aid this process will berequired at higher level in stressed cells, when forexample, stimuli such as elevated temperature result in anincreased level of denatured or partially denaturedproteins. This idea is also in agreement with findingswhich indicate that hsps can be induced by treatment ofcells with amino acid analogues, which again wouldinduce the formation of abnormally folded proteins (Liand Laszlo, 1985).Both in normal cells and in stressed cells there willalso be a need to degrade proteins which have becomeabnormally folded and cannot be rescued by the action ofchaperone proteins. It is therefore of interest that ubiquitinwhich plays a critical role in protein turnover by beinglinked to proteins marked for degradation, is also inducedby elevated temperature and is therefore a heat shockprotein (see Table 1). A further link between the hsps andprotein degradation is provided by the observation thatinhibition of hsp70 synthesis enhances the cell deathwhich is induced by inhibiting the proteosome whichmediates the degradation of ubiquitinated proteins(Robertson et al, 1999).The idea of the hsps as proteins which are ofimportance in normal cells but which assume a greatersignificance in stressed cells, leads logically to the ideathat the induction of these proteins by a stressful stimulusis of itself important in assisting the cell to protect itselffrom stress. In turn, this leads to the idea that the priorinduction of the hsps by a mild stress or by some othernon-stressful procedure, would be protective againstsubsequent more severe stress. This idea obviously hasconsiderable medical importance and has therefore beenintensively investigated.Over the years, this work has effectively proceededin three stages. Firstly, the demonstration that exposure tomildly stressful stimuli which can induce hsp expression,can in turn protect cells against exposure to a more severestress. Clearly, such findings implicate the hsps as beingprotective but do not prove this, since the protective effectcould be due to some other action of the mildly stressfultreatment, other than its ability to induce the hsps. Thisidea leads directly to the second stage of theseinvestigations, namely, the use of gene constructs to overexpressthe hsps in cultured cells and then demonstrate aprotective effect against subsequent exposure to stress.Finally, more recently, these experiments in cultured cellshave been complemented by experiments over-expressingthe hsps in an intact animal and again demonstrating aprotective effect. In subsequent sections of this review, Iwill discuss these three stages of work on the protectiveeffect of heat shock proteins, focusing on studies involvingneuronal or cardiac cells in culture or in the intact brainand heart, because of the key medical importance of theseorgans.Table 1. Major eukaryotic hspsFamily Members ProkaryoticHomologueFunctionalRoleCommentsHsp90Hsp100,Hsp90Grp94C62.5 (E. coli)Maintenance ofproteins such assteroid receptor.Src. in aninactive formuntil appropriateDrosophila andyeast homologues ofhsp90 are knownas hsp83Hsp70Grp78 (= Bip)Hsp72, Hsp73Hsx70dna K (E. coli)Protein foldingand unfolding:assembly ofmultimericcomplexesHsx - 70 only inprimatesHsp65Hsp65gro EL (E. coli)Mycobacterial65 kd antigenProtein foldingand unfolding:organelletranslocationMajor antigen ofmany bacteria andparasites which infectman246