© 2000 American Heart Association, Inc.
Editorials |
Stressing Rac, Ras, and Downstream Heat Shock Protein 70
From the Department of Pathology, University of Washington School of Medicine, Seattle, Wash.
Correspondence to Karin E. Bornfeldt, Department of Pathology, Box 357470, University of Washington School of Medicine, Seattle, WA 98195-7470. E-mail bornf{at}u.washington.edu
Key Words: G protein • mechanical strain • mitogen-activated protein kinases • smooth muscle • stress-activated protein kinase
 |
Introduction |
|---|
 Top Introduction Mechanical Forces Induce hsp70...What Signaling Pathways Lead...Future DirectionsReferences |
|---|
Stress proteins (also known as heat shock proteins) regulate fundamental cellular processes, such as protein folding, protein sorting, protein degradation, assembly of proteins into larger complexes, and resolubilization of aggregates.1 2 3 4 Heat shock proteins (hsps) are divided into 6 subfamilies based on molecular mass: large hsps (110 to 100 kDa), hsp90, hsp70, hsp60, hsp40, and small hsps (30 to 18 kDa).5 Members of the hsp70 group are abundant in eukaryotic cells, and most research to date has focused on stress-inducible hsp70. hsp70 is induced by a number of stress stimuli, and its expression is regulated at the transcriptional level by heat shock transcription factor 1 (HSF1). HSF1, which normally exists as a monomer, trimerizes after activation and binds to a specific DNA recognition sequence (heat shock element) in the hsp70 promoter. HSF1 seems to require phosphorylation for full activity; however, the identity of the putative HSF1-activating kinase remains to be elucidated. It has been shown that HSF1 can be phosphorylated by extracellular signal–regulated protein kinases (ERKs) of the mitogen-activated protein kinase (MAPK) family in a Ras-dependent manner; however, this represses rather than stimulates transcriptional activity.6 7 8 It is also possible that HSF1 is activated by mechanisms that do not rely on phosphorylation, or that dephosphorylation of inhibitory phosphorylation sites stimulates the transcriptional activity of HSF1.
Although the first evidence of existence of hsps dates back to the early 1960s,9 hsps have recently received substantial attention in the field of cardiovascular research after several important and interesting observations. hsp70 has been shown to protect cells from apoptosis and necrosis induced by various stimuli, induce cross-tolerance of stressed cells exposed to a different deleterious stimulus,10 and protect cardiomyocytes from ischemia in vitro and in vivo.1 Furthermore, induction of hsp70 has been demonstrated in the rat arterial wall subjected to acute hypertension,11 human arteries subjected to balloon angioplasty,12 and advanced lesions of atherosclerosis.13 Expression of another hsp, hsp47, has also recently been shown to be induced in the fibrous cap of human lesions of atherosclerosis, where it may regulate collagen secretion and plaque stability.14
|
Mechanical Forces Induce hsp70 Expression in Smooth Muscle Cells |
|---|
|
TopIntroductionMechanical Forces Induce hsp70...What Signaling Pathways Lead...Future DirectionsReferences |
|---|
In this issue of Circulation Research, Xu et al15 report that mechanical forces (cyclic strain stress) activate HSF1 and lead to subsequent hsp70 accumulation in cultured rat arterial smooth muscle cells (SMCs). This study is one in a series by this group on the effects of mechanical forces on SMCs. The finding that mechanical forces can increase hsp70 expression in cells is not novel per se. For example, hydrostatic pressure induces accumulation of hsp70 in human chondrocytic cells,16 and stretch upregulates hsp70 expression in rabbit hearts.17 What makes the study by Xu et al interesting is the finding that the small G protein Rac, and to a lesser extent Ras, is required for HSF1 activation and hsp70 expression. This observation is based on overexpression studies using dominant-inhibitory mutants of Rac and Ras, in which Ser17 has been mutated to an Asn. Xu et al additionally show that although ERK, c-Jun NH2-terminal protein kinase/stress-activated protein kinase (JNK/SAPK), and p38 MAPK are all activated after mechanical stress in these cells, inhibition of ERK or p38 MAPK using chemical inhibitors does not block HSF1-DNA binding. Thus, Rac and Ras seem to be required for cyclic strain stress–induced HSF1 activation in a manner independent of ERK and p38 MAPK.
|
What Signaling Pathways Lead to hsp70 Induction After Cyclic Strain Stress? |
|---|
|
TopIntroductionMechanical Forces Induce hsp70...What Signaling Pathways Lead...Future DirectionsReferences |
|---|
What downstream signaling pathway may be responsible for the Rac- and Ras-induced hsp70 transcription in SMCs? G proteins (or GTP-binding proteins) are active when bound to GTP and inactive when bound to GDP. Activation of G proteins is induced by guanine-nucleotide exchange factors (GEFs) that accelerate release of GDP from the G proteins. The active GTP-bound G proteins then activate several downstream target enzymes (effectors). Ras has 3 major effectors, namely, the Raf serine and threonine protein kinases that act upstream of ERKs, lipid and protein kinase phosphatidylinositol 3-kinase (PI3K), and Ral-GDS, a GEF for the small G protein Ral.18 19 Because ERK is unlikely to activate HSF1,7 15 a probable mediator of cyclic strain stress–induced hsp70 expression seems to be PI3K. PI3K can be directly activated by Ras,20 and the dominant-inhibitory Ras (N17) mutant inhibits PI3K activity.21 Furthermore, other studies have shown that Rac can be activated by a PI3K-dependent pathway.22 Thus, Ras can activate Rac through PI3K. Rac, in turn, activates the p21-activated protein kinases (PAKs), which are known to induce JNK/SAPK and p38 activation.23 Perhaps the most likely signaling pathway regulating hsp70 expression by cyclic strain stress is Ras
PI3KRacPAK?HSF1hsp70 transcription (see
Figure
). Based on the results of Xu et al,15 it cannot be
ruled out that the JNK/SAPK pathway mediates the effects of PAK on
HSF1, although there is indirect evidence to suggest that JNK
phosphorylation of HSF1 may repress its
activity.7 On the other hand, oxidative stress–induced
hsp70 expression is inhibited by a dominant-inhibitory JNK1
mutant,24 supporting a role for JNK in the pathway leading
to hsp70 expression. PAKs also activate downstream signaling
targets unrelated to JNK/SAPK.25 26 Interestingly, PAK was
recently shown to protect cells from apoptosis,27
which suggests that this pathway could mediate cell survival. Another
possibility is that the small G protein Ral, which is activated
by Ras,19 affects HSF1 and hsp70 expression through an
unknown pathway.
|
A word of caution is warranted concerning studies in which the major conclusions are based on cells overexpressing dominant-negative mutants of small G proteins. Interpretation of the results is hampered by the fact that small G proteins often share GEFs, and dominant-negative mutants can interfere with activation of other G protein family members through binding to GEFs. Thus, it is possible that in the study by Xu et al,15 overexpression of N17 Ras and N17 Rac resulted in inhibition of other small G proteins in addition to Ras and Rac. For example, R-Ras, which preferentially activates the PI3K pathway,28 is likely to be inhibited by N17 Ras because this mutant forms a nonproductive complex with exchange factors used by both Ras and R-Ras.29
|
Future Directions |
|---|
|
TopIntroductionMechanical Forces Induce hsp70...What Signaling Pathways Lead...Future DirectionsReferences |
|---|
Induction of hsp70 has been shown to protect various cell types against apoptosis and necrosis, enhancing cell survival after exposure to cytokines, endotoxin, oxidative stress, ischemia, starvation, and other harmful stimuli.10 Interestingly, hearts from transgenic mice that overexpress human hsp70 are protected against ischemia, have increased recovery of ATP stores compared with littermates without the transgene, and have a reduced infarct size after ischemia and reperfusion.1 In the study by Xu et al,15 it is shown that cyclic strain stress increases SMC survival after exposure to oxidative stress, an effect that may be mediated by hsp70. Thus, induction of hsp70 may be beneficial in many different types of cardiovascular disease. However, many questions remain to be answered. For example, what Ras- and Rac-dependent signaling pathway activates HSF1 in SMCs, and does the same signaling pathway induce HSF1 in other cell types and after exposure of cells to other stress stimuli? How does hsp70 protect cells against apoptosis? Is there functional specificity among members of the hsp family? What is the physiological relevance of hsp70 induction in vivo, and does it occur in humans with hypertension and other types of cardiovascular disease? We are likely to see more exciting advances in this area of cardiovascular research in the years to come.
|
Footnotes |
|---|
The opinions expressed in this editorial are not necessarily those of the editors or of the American Heart Association.
|
References |
|---|
|
TopIntroductionMechanical Forces Induce hsp70...What Signaling Pathways Lead...Future DirectionsReferences |
|---|
1. Benjamin IJ, McMillan DR. Stress (heat shock) proteins: molecular chaperones in cardiovascular biology and disease. Circ Res. 1998;83:117–132.
2. Jensen RE, Johnson AE. Protein translocation: is Hsp70 pulling my chain? Curr Biol. 1999;9:R779–R782.[Medline] [Order article via Infotrieve]
3. Clark JI, Muchowski PJ. Small heat-shock proteins and their potential role in human disease. Curr Opin Struct Biol.. 2000;10:52–59.[Medline] [Order article via Infotrieve]
4. Feldman DE, Frydman J. Protein folding in vivo: the importance of molecular chaperones. Curr Opin Struct Biol.. 2000;10:26–33.[Medline] [Order article via Infotrieve]
5. Lindquist S, Craig EA. The heat shock proteins. Annu Rev Genet. 1988;22:631–677.[Medline] [Order article via Infotrieve]
6.
Knauf U, Newton EM, Kyriakis J, Kingston RE.
Repression of human heat shock factor 1 activity at control temperature
by phosphorylation. Genes Dev. 1996;10:2782–2793.
7. Kim U, Nueda A, Meng YH, Dynan WS, Mivechi NF. Analysis of the phosphorylation of human heat shock transcription factor-1 by MAP kinase family members. J Cell Biochem. 1997;67:43–54.[Medline] [Order article via Infotrieve]
8.
Chu B, Zhong R, Soncin F, Stevenson MA, Calderwood SK.
Transcriptional activity of heat shock factor 1 at 37°C is repressed
through phosphorylation on two distinct serine residues
by glycogen synthase kinase 3
and protein kinases C and
C
. J Biol Chem. 1998;273:18640–18646.
9. Ritossa F. A new puffing pattern induced by temperature shock and DNP in Drosophila. Experimentia. 1962;18:571–573.
10. Jäättelä M. Heat shock proteins as cellular lifeguards. Trends Mol Med. 1999;31:261–271.
11.
Xu Q, Fawcett TW, Udelsman R, Holbrook NJ. Activation
of heat shock transcription factor 1 in rat aorta in response to high
blood pressure. Hypertension. 1996;28:53–57.
12. Kirby LB, Mondy JS, Brophy CM. Balloon angioplasty induces heat shock protein 70 in human blood vessels. Ann Vasc Surg. 1999;13:475–479.[Medline] [Order article via Infotrieve]
13.
Johnson AD, Berberian PA, Tytell M, Bond MG.
Differential distribution of 70-kD heat shock protein in
atherosclerosis: its potential role in
arterial SMC survival. Arterioscler Thromb Vasc
Biol. 1995;15:27–36.
14.
Rocnik E, Chow LH, Pickering G. Heat shock protein 47
is expressed in fibrous regions of human atheroma and is
regulated by growth factors and oxidized low-density lipoprotein.
Circulation. 2000;101:1229–1233.
15. Xu Q, Schett G, Li C, Hu Y, Wick G. Mechanical stress–induced heat shock protein 70 expression in vascular smooth muscle cells is regulated by Rac and Ras small G proteins but not mitogen-activated protein kinases. Circ Res. 2000;86:1124–1130.
16.
Kaarniranta K, Elo M, Sironen R, Lammi MJ, Goldring MB,
Eriksson JE, Sistonen L, Helminen HJ. Hsp70 accumulation in
chondrocytic cells exposed to high continuous hydrostatic pressure
coincides with mRNA stabilization rather than transcriptional
activation. Proc Natl Acad Sci U S A. 1998;95:2319–2324.
17. Knowlton AA, Eberli FR, Brecher P, Romo GM, Owen A, Apstein CS. A single myocardial stretch or decreased systolic fiber shortening stimulates the expression of heat shock protein 70 in the isolated, perfused rabbit heart. J Clin Invest. 1991;88:2018–2025.
18. Wittinghofer A, Herrmann C. Ras-effector interactions, the problem of specificity. FEBS Lett. 1995;369:52–56.[Medline] [Order article via Infotrieve]
19. Joneson T, Bar-Sagi D. Ras effectors and their role in mitogenesis and oncogenesis. J Mol Med. 1997;75:587–593.[Medline] [Order article via Infotrieve]
20. Rodriguez-Viciana P, Warne PH, Dhand R, Vanhaesebroeck B, Gout I, Fry MJ, Waterfield MD, Downward J. Phosphatidylinositol-3-OH kinase as a direct target of Ras. Nature. 1994;370:527–532.[Medline] [Order article via Infotrieve]
21.
Hu Q, Klippel A, Muslin AJ, Fantl WJ, Williams LT.
Ras-dependent induction of cellular responses by constitutively active
phosphatidylinositol-3 kinase. Science. 1995;268:100–102.
22. Hawkins PT, Eguinoa A, Qiu RG, Stokoe D, Cooke FT, Walters R, Wennström S, Claesson-Welsh L, Evans T, Symons M, Stephens L. PDGF stimulates an increase in GTP-Rac via activation of phosphoinositide 3-kinase. Curr Biol. 1995;5:393–403.[Medline] [Order article via Infotrieve]
23. Robinson MJ, Cobb MH. Mitogen-activated protein kinase pathways. Curr Opin Cell Biol. 1997;9:180–186.[Medline] [Order article via Infotrieve]
24.
Lee YJ, Corry PM. Metabolic oxidative
stress-induced HSP70 gene expression is mediated through SAPK pathway.
J Biol Chem. 1998;273:29857–29863.
25. Bagrodia S, Cerione RA. PAK to the future. Trends Cell Biol. 1999;9:350–355.[Medline] [Order article via Infotrieve]
26.
Joneson T, McDonough M, Bar-Sagi D, Van Aelst L. RAC
regulation of actin polymerization and proliferation by a pathway
distinct from Jun kinase. Science. 1996;274:1374–1376.
27.
Schurmann A, Mooney AF, Sanders LC, Sells MA, Wang HG,
Reed JC, Bokoch GM. p21-activated kinase 1
phosphorylates the death agonist bad and protects cells
from apoptosis. Mol Cell Biol.. 2000;20:453–461.
28. Marte BM, Rodriguez-Viciana P, Wennström S, Warne PH, Downward J. R-Ras can activate the phosphoinositide 3-kinase but not the MAP kinase arm of the Ras effector pathways. Curr Biol. 1997;7:63–70.[Medline] [Order article via Infotrieve]
29. Reuther GW, Der CJ. The Ras branch of small GTPases: Ras family members don’t fall far from the tree. Curr Opin Cell Biol.. 2000;12:157–165.[Medline] [Order article via Infotrieve]
30. Chou MM, Blenis J. The 70 kDa S6 kinase complexes with and is activated by the Rho family G proteins Cdc42 and Rac1. Cell. 1996;85:573–583.[Medline] [Order article via Infotrieve]
This article has been cited by other articles:
 |
|
 |
|
  X. Yin, N. J.D. Gower, H. A. Baylis, and K. Strange Inositol 1,4,5-Trisphosphate Signaling Regulates Rhythmic Contractile Activity of Myoepithelial Sheath Cells in Caenorhabditis elegans Mol. Biol. Cell, August 1, 2004; 15(8): 3938 - 3949. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 J. Zhou, T. Schmid, R. Frank, and B. Brune PI3K/Akt Is Required for Heat Shock Proteins to Protect Hypoxia-inducible Factor 1{alpha} from pVHL-independent Degradation J. Biol. Chem., April 2, 2004; 279(14): 13506 - 13513. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 F. Zhang, N. R. Hackett, G. Lam, J. Cheng, R. Pergolizzi, L. Luo, S. V. Shmelkov, J. Edelberg, R. G. Crystal, and S. Rafii Green fluorescent protein selectively induces HSP70-mediated up-regulation of COX-2 expression in endothelial cells Blood, September 15, 2003; 102(6): 2115 - 2121. [Abstract] [Full Text] [PDF]
|
|
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||