HSP90 proteins critically involved in the modulation of several cell signaling pathways; however, the expression and even the functions of HSP90 may be altered under pathological conditions
[28]. Indeed, HSP90 expression is affected by indoxyl sulfate, a uremic toxin that accumulates in the body during CKD progression. Milanesi et al. demonstrated that indoxyl sulfate induces HSP90 expression in kidney fibroblasts (NRK-49F cells)
[29]. However, in these cells selective HSP90 inhibition reverses the inductive effect of indoxyl sulfate on monocyte chemoattractant protein-1 (MCP-1), α-smooth muscle actin, collagen I, and transforming growth factor-β (TGF-β) expression, thus indicating that HSP90 contributes to kidney inflammation and fibrosis at the cellular level
[30]. In vivo, the authors also observed an increase in HSP90 expression in the kidneys of mice treated with indoxyl sulfate
[29]. Furthermore, clinical studies have demonstrated that the HSP90α isoform is present at elevated serum levels in pediatric patients with CKD compared to levels in the control group
[31].
According to another study, serum levels of HSP60 were associated with the risk of death and readmission in patients with acute heart failure
[32]. In an experimental model, it was demonstrated that extracellular HSP60 induces apoptosis in cardiac myocytes via TLR4
[33]. A previous study from our group observed the involvement of HSP60 in cardiomyocyte hypertrophy and its association with inflammation and TLR4 activation. In this study, primary culture of cardiomyocytes treated with HSP60 showed hypertrophy, increase on complement system components, C3 and factor B as well as an increase in IL-6 and TNF-α expression
[34].
It is also established that cardiac myocytes can release HSP60 in exosomes; however, the role of HSP60 in intercellular communication through extracellular vesicles remains unclear
[35]. Additionally, studies have demonstrated that atherosclerotic lesions exhibit increased HSP60 expression
[36]. HSP60 can also regulate important cellular mechanisms such as VSMC migration and proliferation that could contribute to atherosclerosis and endothelial damage
[37].
Despite its importance in the cardiovascular system, few studies have investigated the role of HSP60 in kidney diseases. Fang et al. demonstrated that HSP60 is a target of miR-382 that reduces its expression in renal cells and contributes, at least in part, to renal tubulointerstitial fibrosis that is related to CKD progression
[38]. In diabetic nephropathy, HSP60 may also be involved in renal tubular cell dysfunction
[39][40].
In general, the TLR2/4 pathway is the one that interacts with both organs and innate immune system. This conversation between heart–kidneys axis and HSPs depends on different molecular mechanisms of action. In general, HSP27, 60 or 70 couple to TLRs activating IKKγ or MAPK/p38 pathways. In the nucleus, NF-kB is responsible for the inflammatory gene expression while p38 activates the apoptotic genes
[41][42]. This inflammatory response is observed in many cardiac injuries and kidney diseases, in addition of CRS itself
[27]. Not only can this inflammation be induced by HSPs in CRS, but also the fibrosis observed during the syndrome
[18][43]. The interaction of HSP90 with the receptor of TGF-β has been described to stimulate fibrosis by SMAD2/3 in renal tissue and can also promote fibroses in heart
[44]. Last, but no less important, stress factors (free radicals, hypoxia, environmental factors, etc.) caused by CRS can directly induce an increase on HSPs expression by HSF1 phosphorylation. The activation of HSF1 has already been studied to cause cardiac dysfunction
[45] and cause more apoptosis during renal injury
[46].
Therefore, the cardiovascular and renal systems are strongly linked and present a complex relationship in which HSPs may be relevant in the pathological processes that affect these two systems. However, our knowledge regarding the role of HSPs in CRS is incomplete, and further studies are required. Based on the complexity of the relationship between cardiovascular and renal diseases, understanding the pathophysiological mechanisms involved in this process, including the possible role of HSPs, may be relevant for the development of new therapeutic strategies for CRS.