Heat shock proteins (HSPs) are a highly conserved group of proteins that are constitutively expressed and function as molecular chaperones, aiding in protein folding and preventing the accumulation of misfolded proteins. In the arterial wall, HSPs have a protective role under normal physiologic conditions. In disease states, however, HSPs expressed on the vascular endothelial cell surface can act as targets for detrimental autoimmunity due to their highly conserved sequences. Developing therapeutic strategies for atherosclerosis based on HSPs is challenged by the need to balance such physiologic and pathologic roles of these proteins. This paper summarizes the role of HSPs in normal vascular wall processes as well as in the development and progression of atherosclerosis. The potential implications of HSPs in clinical therapies for atherosclerosis are also discussed.
Heat shock proteins (HSPs) were first discovered as being expressed in response to increased temperature, as the name suggests [
Although the diverse roles of HSPs in normal arterial physiology as well as in atherosclerosis have been discussed in prior reviews, the body of literature at both the basic science and clinical levels has expanded exponentially in this field in recent years [
We performed our literature search using MEDLINE, with no limits regarding date of publication. The search terms used were “heat shock proteins” and “atherosclerosis”. Limits included articles in English only.
The arterial wall is undoubtedly a dynamic structure that continually responds to stresses in its environment [
Functions of heat shock proteins.
Heat shock protein molecular weight (kilodaltons) | Function | Pathologic associations |
---|---|---|
60 | Protein folding | Atherosclerosis |
Protein unfolding | Rheumatoid arthritis | |
Polypeptide assembly | Systemic sclerosis | |
Protein translocation across membranes | Schizophrenia | |
Diabetes mellitus | ||
| ||
10 | Cofactor for HSP 60 | Cardiovascular disease |
| ||
27 | Competes for uptake with lipids | Atherosclerosis |
Estrogen receptor- |
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| ||
70 | Protein folding | Atherosclerosis |
Protein unfolding | Leprosy | |
Degradation of misfolded or denatured proteins | Tuberculosis | |
Assembly of new proteins | ||
Translocation of proteins across membranes | ||
| ||
90 | Molecular chaperone involved in protein folding and activation | Atherosclerosis |
Systemic lupus erythematosus |
HSP60, for instance, has been shown to have roles in polypeptide assembly and protein translocation across membranes, in addition to protein folding [
HSP60 is the most relevant and well-studied HSP subtype with regards to autoimmunity and development of atherosclerosis. Indeed, earlier studies formulated the hypothesis that T-cell-mediated and humoral immune responses to HSP60 in endothelial regions subject to hemodynamic stress were the initiating event in atherosclerosis [
The HSP60 family also includes HSP65, which is the mycobacterial homologue of mammalian HSP60 [
Under normal conditions, HSP60 is not expressed on the vascular endothelial cell surface. However, under stressed conditions including the traditional risk factors for atherosclerosis, mitochondrial HSP60 is translocated to the cytoplasm and then to the cell surface (Figure
Concept of autoimmunity towards heat shock protein 60 and the development of atherosclerosis. Under normal conditions, heat shock protein 60 is located intracellularly and is not expressed on the vascular endothelial cell surface. Under stressed conditions, heat shock protein 60 and various adhesion molecules are upregulated and expressed on the cell surface. This leads to inflammation and the development of atherosclerosis.
With regards to immune cell subtypes, CD4+ T cells are present in the highest concentration in the earliest phases of atherosclerosis. Furthermore, the strongest T-cell reactions against HSP60 are found in intralesional T cells, which display an oligoclonally restricted receptor phenotype, as compared to extralesional peripheral T cells which have weaker reactions to HSP60 and display polyclonal phenotypes [
Logistic regression analysis for the impact of various risk factors on high vascular intima-media thickness in a study of 141 young (17- or 18-year old) white males (see [
Risk factor | Odds ratio (95% CI) |
|
---|---|---|
Cigarette smoking | 3.58 (1.34–9.54) | 0.0108 |
High-density lipoprotein level | 0.56 (0.36–0.89) | 0.0144 |
Alcohol consumption | 0.51 (0.30–0.87) | 0.0133 |
Diastolic blood pressure | 1.61 (1.03–2.52) | 0.0374 |
Maximum expiratory flow at 50% vital capacity | 0.52 (0.33–0.82) | 0.0047 |
HSP60 stimulation index | 2.18 (1.32–3.60) | 0.0023 |
HSP60 antibody titer | 1.52 (1.00–2.31) | 0.0514 |
Odds ratios were calculated based on a 1 standard deviation unit change in the given variable.
The role of B cells in the development of atherosclerosis is less clearly understood. Several studies have demonstrated progression of atherosclerotic disease with B-cell depletion, whereas other studies demonstrated reduction in the disease with B-cell depletion [
In addition to being expressed on the endothelial cell surface, HSP60 can be shed into the circulation in a soluble form under stressed conditions. A study of 826 human patients found that levels of soluble HSP60 were significantly elevated in patients with carotid atherosclerosis [
Soluble heat shock proteins and their association with cardiovascular diseases.
Soluble heat shock protein subtype | Number of patients | Cardiovascular disease | Study finding | Reference |
---|---|---|---|---|
HSP70 | 24 | Acute myocardial infarction | Soluble HSP70 is released into the circulation after an acute myocardial infarction | [ |
HSP60 | 684 | Carotid atherosclerosis | Levels of soluble HSP60 are associated with early carotid atherosclerosis | [ |
HSP70 | 52 cases |
Acute myocardial infarction | Levels of soluble HSP70 are associated with progression of heart failure after acute myocardial infarction | [ |
HSP60, HSP72 | 88 cases |
Idiopathic left ventricular dysfunction | Levels of soluble HSP60 and HSP72 correlate with severity of cardiac and microvascular dysfunction in patients with idiopathic left ventricular dysfunction | [ |
HSP70 | 167 | Congestive heart failure | Levels of soluble HSP70 are associated with severity of heart failure in patients with congestive heart failure | [ |
HSP60 | 1003 cases |
Coronary artery disease | Levels of soluble HSP60 correlate with the presence of coronary artery disease | [ |
Similar to soluble HSP60, prior studies have also demonstrated elevated serum antibody levels to HSP65, which is the mycobacterial homolog of human HSP60 [
Although soluble HSP60 and antibodies to HSP60/65 have been shown to be elevated in human patients with atherosclerosis, it is unclear whether this simply represents an association or whether a causal relationship exists. Administration of a murine monoclonal antibody (II-13) to amino acid residues 288 to 366 of HSP60 induced atherosclerosis in apolipoprotein E-deficient mice [
A study of 120 normocholesterolemic rabbits found that those immunized with recombinant mycobacterial HSP65 had increased atherosclerosis [
HSP10 is an important cofactor for HSP60 [
Emerging data has implicated HSP27 in the pathogenesis of atherosclerosis. A study of human atherosclerotic plaques revealed an increase in expression of HSP27 in normal-appearing vessel adjacent to the plaque, with decreased levels in the plaque itself [
Another study similarly found that HSP27 release was significantly decreased in atherosclerotic plaques [
HSP27 may also offer protection from atherosclerosis due to its role in plaque stability. A proteomic analysis of stable versus unstable human carotid artery atherosclerotic plaques found reduced levels of HSP27 in unstable lesions [
In addition, HSP27 has been demonstrated
In an apolipoprotein E-deficient animal model, overexpression of human HSP27 resulted in a 35% reduction in aortic atherosclerosis in female, but not male, mice [
In early atherosclerosis, dendritic cells exclusively overexpress HSP70 as well as HLA-DR and CD1d, the latter being a unique molecule used in lipid antigen presentation [
Another study found that oxidized low-density lipoprotein stimulated the expression of HSP70 and that supernatants from oxidized low-density lipoprotein-treated macrophages could induce both IL-1
HSP70 may also have anti-inflammatory roles. In one immunization study, a peptide sequence of myobacterial HSP70 was found to induce the production of IL-10 by peptide-specific T cells, a phenomenon that was also seen with T cells responsive to the whole HSP70 protein [
HSP70 may also be implicated in the calcification of blood vessels. HSP70 was found to enhance bone morphogenetic protein-4-induced proliferation in endothelial cells and to enhance bone morphogenetic protein-induced calcium deposition in vascular cells [
A study of human carotid atherosclerosis demonstrated overexpression of HSP90 in both plaque and serum as compared to healthy controls [
Another study of human atherosclerotic plaques found that the expression of HSP90 was associated with plaque instability in advanced lesions [
The potential anti-inflammatory therapeutic benefits of HSP90 inhibitors have been demonstrated in other diseases as well. In a mouse model of systemic lupus erythematosus, HSP90 was found to have a potential role in regulating T-cell differentiation and activation, and its inhibition was associated with reduced inflammation [
There are several clinical implications related to HSPs and their role in atherosclerosis. One potential clinical application would be to exploit the presence of HSP antibodies for screening at-risk patients to detect significant atherosclerosis. A study of 750 human subjects demonstrated the correlation between HSP65 antibody titers and advanced carotid atherosclerotic lesions [
Screening patients based on titers could be a useful strategy for the detection of significant atherosclerosis, although the sensitivity and specificity of such a test are unknown, as are the cost implications. Such blood tests may have more utility in directing diagnostic workup for coronary artery disease, particularly in patients with marginal indications for further testing. In these intermediate risk patients, antibody titers could be used as further risk stratification, with those patients with high antibody levels undergoing further workup and those with undetectable or low levels continuing to undergo clinical surveillance.
HSPs may also have a role in prognosis [
In addition to its potential screening, diagnostic, and prognostic roles, there may be potential utility for HSPs in the treatment of atherosclerosis (Table
Summary of studies on potential heat shock protein-related treatments for various diseases.
Study | Disease | Subjects | Study summary and major findings |
---|---|---|---|
Maron et al. [ |
Atherosclerosis | Mice | Nasal vaccination with HSP65 resulted in a significant decrease in the size of atherosclerotic plaques, a reduced number of T cells, and an increased IL-10 expression |
Harats et al. [ |
Atherosclerosis | Mice | Oral tolerance induced with HSP65 led to a reduction in atherosclerosis |
Jun et al. [ |
Atherosclerosis | Rabbits | Vaccine targeting HSP65 and cholesterol ester transfer protein reduced low-density lipoprotein levels and atherosclerotic burden |
Ishii et al. [ |
Multiple myeloma | Human | The addition of an HSP90 inhibitor enhanced the antitumor activity of a proteasome inhibitor both |
Kaiser et al. [ |
Acute leukemia | Human | HSP70 inhibitor displayed antileukemic effects both alone and in combination with other antineoplastic agents |
There are clinical concerns with such vaccines given the high sequence homology between human and bacterial HSPs. Inducing tolerance may result in increased susceptibility to serious infections. Despite these concerns, there have been phase I and II trials in the realm of cancer that have demonstrated the safety of similar vaccines. In metastatic melanoma, for instance, vaccination with autologous tumor-derived HSP peptide complexes resulted in only mild toxicity in some patients, limited mostly to local erythema and induration at the injection site [
There have been several recent studies published regarding potential HSP-related therapies for atherosclerosis. One study examined the effect of a vaccine that targets HSP65 and cholesterol ester transfer protein simultaneously and found more protective IL-10, less adverse IFN-
There have also been several recent studies regarding the potential of HSP-related therapies in diseases other than atherosclerosis (Table
A growing body of evidence in both animal models and human subjects has implicated autoimmunity towards HSPs as a potential pathogenic mechanism for the development of atherosclerosis. Ongoing and future studies that further elucidate the mechanisms whereby HSPs, infection, and immune response pathways interact and lead to the common pathway of atherosclerosis will be essential to developing more specific and potentially safer novel therapies for this devastating disease process. A better understanding of the functions of HSPs in other pathologies such as cancer may also be useful in advancing our knowledge of the role of this important family of molecules in atherosclerosis and their potential therapeutic utility.