Cellular redox homeoHstasis is precisely balanced by generation and elimination of reactive oxygen species (ROS). ROS are not only capable of causing oxidation of proteins, lipids and DNA to damage cells but can also act as signaling molecules to modulate transcription factors and epigenetic pathways that determine cell survival and death. Hsp70 proteins are central hubs for p70 is the key chaperone in protein quality control and the central hub of the cellular proteostasis and are important factors to ameliorate damage from different kinds of stress including oxidative stress. Hsp70 members often participate in different cellular signaling pathways via their clients and cochaperones. ROS can directly cause oxidative cysteine modifications of network, participating in numerous cellular processes by interacting with different clients. Hsp70 members to alter their structure and chaperone activity, resulting in changes in the interactions between Hsp70 and their clients or cochaperones, which can then transfer redox signals to Hsp70-related signaling pathways. On the other hand, ROS also activate some redox-related signaling pathways to indirectly modulateis tightly related to redox homeostasis in several ways, including functional regulation of Hsp70 activity and expression. Pocaused by post-translational modifications including phosphorylation together with elevated Hsp70 (especially cysteine modifications), induced expression can expand the capacity n of Hsp70 to deal with ROS-damaged proteins and support antioxidant enzymes. Knowledge about the response and role of Hsp70 in redox homcaused by oxidative stress, Hsp70-dependent proteostasis will facilitate our understanding of the cellular knock-on effects of inhibitors targeting Hsp70 and the mechanisms of under oxidative stress and redox-related diseases and agingsignaling pathways involving Hsp70.
Cellular Processes | Critical Signaling Molecules Modified by ROS |
---|---|
proliferation and survival | mitogen-activated protein kinases (MAPKs), phosphatidylinositol-4,5-bisphosphate 3-kinase (PI3K), phosphatase and tensin homolog deleted on chromosome ten (PTEN) and protein tyrosine phosphatases |
redox homeostasis | thioredoxin, peroxiredoxin, redox factor-1 (Ref-1) and Kelch-like ECH-associated protein 1 (Keap1)/nuclear factor erythroid 2-related factor 2 (Nrf2) |
mitochondrial oxidative stress and aging | p66Shc |
iron homeostasis | iron response element–iron regulatory protein (IRE-IRP) containing iron–sulfur cluster |
DNA damage response | ataxia telangiectasia mutated (ATM) |
Cellular redox homeostasis is precisely balanced by generation and elimination of reactive oxygen species (ROS). ROS are not only capable of causing oxidation of proteins, lipids and DNA to damage cells but can also act as signaling molecules to modulate transcription factors and epigenetic pathways that determine cell survival and death. Hsp70 proteins are central hubs for proteostasis and are important factors to ameliorate damage from different kinds of stress including oxidative stress. Hsp70 members often participate in different cellular signaling pathways via their clients and cochaperones. ROS can directly cause oxidative cysteine modifications of Hsp70 members to alter their structure and chaperone activity, resulting in changes in the interactions between Hsp70 and their clients or cochaperones, which can then transfer redox signals to Hsp70-related signaling pathways. On the other hand, ROS also activate some redox-related signaling pathways to indirectly modulate Hsp70 activity and expression. Post-translational modifications including phosphorylation together with elevated Hsp70 expression can expand the capacity of Hsp70 to deal with ROS-damaged proteins and support antioxidant enzymes. Knowledge about the response and role of Hsp70 in redox homeostasis will facilitate our understanding of the cellular knock-on effects of inhibitors targeting Hsp70 and the mechanisms of redox-related diseases and aging.
Hsp70 can counteract oxidative stress damage to proteins by inhibiting aggregation and/or facilitating degradation of oxidized proteins, as well as inducing the expression of key antioxidant enzymes such as SOD1 and CAT and adjusting the activity of antioxidant enzymes [15, 89-92]. Overexpression of HSP70 in the muscle of mice antagonizes the aging-related increase in protein cysteine modifications including carbonylation, oxidation and formation of disulfide bonds [93]. Overexpression of mitochondrial HSP70/Hsp75 in rat brain protects mitochondria and results in the marked reduction in free radical generation [94]. Under oxidative stress, HspA1A and HspA8 can mediate the C-terminus of Hsp70-interacting protein (CHIP, also known as STUB1) to recognize and ubiquitinate ROS-stressed peroxisomes, resulting in their turnover by autophagy [95]. Overexpression of HSP70 suppresses ROS production from mitochondria in human lung microvascular endothelial cells (HLMVEC) exposed to bacterial toxins [96]. The HSPA1B rs1061581 polymorphism (1267 A/G), which is linked to the risk of developing multiple sclerosis, is related to ROS levels and has a role in the variation in HSPA1B expression levels under oxidative stimulus [97].Hsp70 can buffer different kinds of stress including oxidative stress to contribute to redox homeostasis. Hsp70 members are important antioxidative components in eliminating damaged oxidized proteins and support antioxidative enzymes. Under oxidative stress, Hsp70 members will inevitably undergo oxidization, resulting in changes in their structure and chaperone activity. However, cysteine modifications often downregulate the chaperone activity of Hsp70, which is disadvantageous for dealing with protein damage due to oxidation. From this point of view, cysteine residues in Hsp70 are not conducive towards Hsp70 exerting its antioxidative functions. At the same time, only very few Hsp70 members do not contain any cysteine residues, and the number of cysteine residues in Hsp70 increases with evolution, suggesting important roles of cysteine residues in Hsp70. Hsp70 members can sense redox by cysteine modifications, then transfer redox signals by modulating the interaction between Hsp70 members and their clients. Hsp70 members are involved in cellular signaling due to the fact that their clients include key molecules in signaling pathways, including Hsf1 and Akt1. Cochaperones also guide Hsp70 members within different cellular processes, such as CHIP, acting as a link between Hsp70 and the ubiquitin–proteasome system (UPS) for degradation of Hsp70 clients. Thus, cysteine modifications of Hsp70 also change the fate of the clients by modifying the cooperation between Hsp70 and cochaperones. Therefore, cysteine modifications of Hsp70 may act to amplify the transfer of redox signaling to a broader range of proteins, achieving greater antioxidative potential in a shorter time and restoring the cellular environment by multiple pathways. As a consequence, Hsp70 proteins are expressed much more rapidly through different signaling pathways, and the newly synthesized Hsp70 proteins are not oxidized and have normal chaperone activity to deal with the damaged proteins that have accumulated during oxidative stress. Thus, combining an in-depth understanding of the basic principles of Hsp70 systems (including the relationship between the structure and function of Hsp70, the interaction between Hsp70 and its cochaperones, and the interaction between Hsp70 and its clients) and the extensive exploration of redox-related signaling pathways involving Hsp70 will lead to a fuller understanding of Hsp70 function in redox homeostasis.
Oxidative stress can activate the HSR and Kelch-like ECH-associated protein 1 (Keap1)/Nrf2 pathway to upregulate expression of HSP70, and the increased protein level of Hsp70 protects cells against oxidative-stress-related damage [15, 98, 99]. However, different types of oxidative stress differ in their capacity to induce the HSR and elevate expression of HSP70, and the same oxidative stimulus may lead to distinct stress responses in different cells [100-102]. Oxidative-stress induced by 15-deoxy-Δ12,14-prostaglandin J2 (15d-PGJ2) enhances HSP70 expression to exert anti-inflammatory effects [103]. Under mild oxidative stress conditions, expression of HSPA1A/B is upregulated in the mammalian neuronal cell line HT22 [90, 104]. In the early phase after oxidative stress, HspA1A/B binds to partially unfolded oxidized proteins to prevent their aggregation [104]. Then, HspA1A/B facilitates the degradation of bound oxidized proteins by 20S proteasomes [104]. Expression of HspA8 is not affected by oxidative stress, and HspA8 has no effect on removal of proteins damaged by oxidation by the 20S proteasome [104]. Thus, HspA1A/B but not HspA8 mediates degradation of damaged proteins by the UPS under oxidative stress conditions [104], while HspA8-facilitated CMA is also activated by oxidative stress to facilitate degradation of oxidized proteins [105, 106].
Hsp70 and redox homeostasis are linked by multiple signaling pathways including inositol-4,5-bisphosphate 3-kinase (PI3K)/protein kinase B (PKB, Akt)/Hsp70, Janus kinase 2 (JAK2)/signal transducer and activator of transcription 3 (STAT3)/Hsp70, and Keap1/Nrf2/Hsf1/Hsp70. Expression or activity of Hsp70 is often modulated by the related signaling pathways [107-109].
SOD2 protects cells against mitochondrial oxidative damage [110]. HspA1A can bind to SOD2 to prevent its aggregation and control its CHIP-mediated degradation or import it into mitochondria [107, 111]. CHIP cooperates with HspA1A by binding to the C-terminus EEVD motif and the SBDα of HspA1A [112]. PI3K/Akt signaling is crucial for receiving input from the cell membrane and mitochondria [113]. Akt1 is the downstream kinase of PI3K and a client of HspA1A. Akt1 binds to the SBDα of HspA1A and phosphorylates the C-terminus Ser631 of HspA1A, resulting in decoupling of HspA1A and CHIP and inhibition of the degradation of SOD2 [107]. Then, the import of SOD2 into mitochondria is promoted. As a negative feedback loop, the resulting decrease in oxidative stress activation permits phosphatase 2C (PP2C) to dephosphorylate HspA1A to promote degradation of SOD2 [107]. Thus, Hsp70 phosphorylation involved in the PI3K/Akt/Hsp70 signaling pathway contributes to redox homeostasis.
The oncogenic signaling pathway JAK2/STAT3 plays crucial roles in regulating apoptosis, proliferation, differentiation, and the inflammatory response and participates in the occurrence and development of various tumors [114]. AG490 is an inhibitor of the JAK2/STAT3 signaling pathway [115]. It was found that H2O2-induced time-dependent increase in Hsp70 protein expression in vascular smooth muscle cells is inhibited by pretreatment with AG-490, suggesting that ROS can regulate HSP70 expression via the JAK2/STAT3 signaling pathway [116]. It was also found that both HSP70 RNAi and AG490 can increase ROS levels in Burkitt’s lymphoma Raji cells and reduce the activity of SOD and GPX, suggesting that inhibiting Hsp70 or the JAK2/STAT3 pathway may induce oxidative stress of Raji cells [108]. Therefore, the JAK2/STAT3 signaling pathway may contribute to redox homeostasis by modulating the expression of HSP70.
The Keap1/Nrf2 pathway is a thiol-based sensor–effector for maintaining redox homeostasis [16]. Keap1 acts as a cysteine-thiol-rich sensor of redox status, and Nrf2 is a transcription factor for some cytoprotective genes including some enzymatic antioxidants and Hsp70 [16]. The Keap1/Nrf2 complex facilitates degradation of Nrf2 and represses the activity of Nrf2 [16]. ROS or electrophiles can cause cysteine modification of Keap1, which contains multiple active thiols, to weaken the interaction between Keap1 and Nrf2 and promote the entrance of Nrf2 into the cell nucleus and activation of Nrf2 [16, 117]. Hsf1 is an important transcription factor for genes related to HSR. Hsp90, Hsp70 and Hsp40 cooperate to inactivate Hsf1 by binding to it and preventing it from entering the cell nucleus, and they are also transcription targets of Hsf1, forming a negative feedback regulatory loop [118, 119]. Although the mechanism by which Hsf1 is activated has not been fully elucidated, activation of Hsf1 is thought to be related to stress damage of proteins and modification of HSPs or Hsf1 [117, 120]. Both Nrf2 and Hsf1 are critical for adaptation and survival, and there is crosstalk between the two pathways through overlapping transcriptional targets including HSP70 [99]. Some reports indicate that the Keap1/Nrf2 pathway is activated before the heat shock response upon oxidative or electrophilic stress [99, 121, 122]. When Hsf1 is mutated and therefore unable to activate the HSR, Nrf2 can be activated by heat shock to cause delayed upregulation of HSP70, suggesting the two pathways may compensate for each other [123]. Oxidative stress resulting from a lack of methionine induces HSPA1A expression through Nrf2 activation but not Hsf1 activation in HEK293 cells [109]. Both Hsf1 and Nrf2 pathways contribute to redox homeostasis and cooperate to promote a more reducing cellular environment.