Regulation of heat shock protein HSP on apoptosis
Apoptosis is a common phenomenon in the process of removing senescent and damaged cells during biological development or under normal physiological conditions. The occurrence of apoptosis is induced by a variety of extracellular or intracellular stimuli, and heat shock proteins (heat shock proteins) are one of the regulators of apoptosis. Cells adapt to environmental stress by regulating their own defense system, and rely on their own genetic mechanisms or regulate their own state to resist stress, or actively induce cell death, depending on the degree of stress. There are two forms of cell death: apoptosis and cell necrosis.
Apoptosis: An energy-dependent, ubiquitous physiological process regulated by proteins expressed by some evolutionarily conserved genes. Heat shock protein (HSP) is considered to be one of the most conserved protein families in biological evolution. Studies have shown that HSP is closely related to apoptosis, especially HSP70 and HSP27 for heat shock stress, oxidative stress and ionization. Apoptosis caused by radiation or the like has a protective effect.
1. Heat shock protein and apoptosis
Apoptosis is a cell-controlled death of a gene. It refers to a specific procedure (self-genetic mechanism) in which nucleated cells initiate cells under certain physiological or pathological conditions, and causes the cells to naturally die by activating endogenous DNA endonucleases. Apoptosis is characterized by specific morphological and physiological changes: morphological changes include cell shrinkage, pore formation in the plasma membrane, nuclear condensation, and division of the cell into many ultrastructured cell bodies surrounded by membranes, called apoptotic The apoptotic body; and physiological changes include the cleavage of nuclear DNA, and the activation of caspases (cysteine ​​as-partate-specific protease) to produce partially digested protein hydrolysates. Apoptosis is a highly precise process that first responds to initial stimuli and then undergoes a series of cascades under the action of some kinases and regulatory factors.
The process of apoptosis can be roughly divided into three stages: (1) the initial stage (or signal induction stage), including activation of cell surface death receptors, mainly tumor necrosis factor (TNF) family members; (2) signal transduction The lead phase (or preparation phase), including the caspases sensor and specific kinase/phosphatase activation; (3) the execution phase (or death phase), primarily the activation of the caspases effector. Regulation of apoptosis occurs primarily at the intersection of signal transduction pathways.
Expression of heat shock proteins is one of the most important responses at the molecular level after high temperature stress. Depending on the molecular mass, the heat shock proteins can be divided into five conserved families: HSP100, HSP90, HSP70, HSP60 and sHSPs (relative molecular mass of about 1.6 x 104-4.2 x 104). Heat shock proteins are an important regulatory protein in cells. Many physiological functions of cells, such as cell proliferation and cell cycle maintenance, are related to heat shock proteins. Another important physiological function of heat shock proteins is to protect cells against environmental stress. It is generally believed that heat shock proteins can protect cells under stress conditions from damage by preventing denaturation of proteins and promoting renaturation of denatured proteins. Recent studies have shown that important members of the heat shock protein family, such as HSP90, HSP70, HSP27, etc., directly affect apoptosis by interfering with apoptosis signal transduction pathways, and they play an important role as key factors in apoptosis regulation. effect.
2. The regulation of heat shock protein on caspase-dependent cell apoptosis
Caspases are a class of proteolytic enzymes that exist in the form of inactive procaspases in cells and cause apoptosis after use as an intracellular specific substrate. Thornberry and Lazebnik [1] reported about 14 types of caspases, which can be divided into three groups: initiation, excitation and effect caspases. Activation of caspases is a cascade of responses in different apoptotic pathways. Through the caspase cascade, cells can amplify and integrate the apoptotic signal step by step.
Heat shock proteins play a very complex role in the regulation of caspase-dependent apoptosis. On the one hand, due to the protective function of heat shock proteins on cells, they inhibit apoptosis; on the other hand, they directly promote cell apoptosis as a key signaling protein chaperone. We will explain how heat shock proteins play a dual role in regulating apoptosis through different initiation sites of the apoptotic signaling pathway.
A. Apoptosis signals originating from the plasma membrane and regulation of HSP:
The major player in the initial stage of apoptosis on the plasma membrane is the death receptor, such as the Fas (Apo-1/CD95) receptor family. The Fas receptor is a member of the TNF receptor superfamily and is widely expressed in a variety of cells, tissues (mainly immune cells). The receptor is a transmembrane protein comprising a cysteine-rich extracellular domain and an intracellular death domain. When the Fas receptor binds to its antibody or cognate ligand, it can induce phosphorylation of some proteins and trigger apoptosis. FADD (Fas-death domain junction protein) and the adaptor protein Daxx, two downstream gamma proteins, mediate two independent apoptotic signal transduction pathways, respectively. Members of the HSP family play an important role in the Fas "death receptor" pathway. The localization of the adaptor protein Daxx from the nucleus to the cytoplasm is a necessary step in promoting apoptosis. Phosphorylated HSP27 blocks the interaction between Daxx and apoptosis signaling regulator kinase 1 (ASK1). This form of transport is prevented, thereby inhibiting Fas-mediated apoptosis. HSP90 assists in the transduction of apoptotic signaling from the protoplast membrane, promoting the activation of death domain kinases and receptor-associated proteins, making cells susceptible to TNF-induced apoptosis. In addition, the HSP90 isoform, HSP75/TRAP-1, interacts with the TNF receptor and may promote TNF signaling. In the late stages of the implementation phase, apoptosis is characterized by significant changes in cell morphology, including membrane rupture to form vesicular structures, and phosphatidylserine (PS) exposure to the surface of the membrane. HSP is involved in the formation of membrane vesicles: Some scholars believe that HSP27 acts as a cap-binding protein and F-actin to form the HSP27-F-actin loop, which acts as a dividing membrane vesicle. The asymmetry of cell loss of protoplasmic membrane phospholipids leads to the exposure of PS to the outer layer of the plasma membrane. The role of HSP in the surface of PS is unclear, but the surfaced PS can be recognized by the immune system, among which HSP90 analogues - Grp94 assisted in the identification.
B. Apoptosis signal originating from cytosol and regulation of HSP
In cytosol, stress kinase is an important component in regulating apoptosis signaling pathways. Mitogen-acti-vated protein kinases (MAPKs) are important signaling systems for eukaryotic cell-mediated cellular responses, and c-Jun N-terminal kinase (JNK) is one of them. , related to the modification of some molecules in the apoptotic pathway, such as JNK-mediated phosphorylation of the anti-apoptotic members of the Bcl-2 family, Bcl-2 and Bcl-XL, can offset some of their functions. It promotes the release of cytochrome c from the mitochondria, and cytochrome c is released into the cytoplasm as a stress sensor, which can trigger the cell to perform an apoptosis program. Another stress kinase, ASK1, is a mitogen-activated protein kinase kinase kinase (MAPKKK) that phosphorylates JNK and triggers apoptotic responses. HSP70 has a general inhibitory effect in the stress kinase pathway. HSP72 is a direct inhibitor of ASK1: in vitro experiments have shown that HSP72 interacts with ASK1 kinase, and HSP72 expression inhibits H2O2-mediated ASK1 activation and subsequent apoptosis of NIH 3T3 cells. CHIP (the C-terminal structure of HSP70-interacting protein) attenuates ASK1-dependent apoptosis in oxidative stress by inhibiting the activity of ASK1 [11]. HSP72 also interacts directly with the polypeptide binding domain of JNK, and it has been proved by antisense RNA that the accumulation of HSP72 leads to a decrease in the expression level of JNK [12]. HSP105α, another member of the HSP70 family, is constitutively expressed in neuronal PC12 cells and also protects cells against stress-induced apoptosis by inhibiting JNK activity. HSP90 is a protein essential for many kinase folding and activation. Dissociation of the HSP90-Raf1 complex induces activation of JNK and triggers the occurrence of apoptosis in rod-shaped cells and B-lymphocytes.
C. Apoptosis signal originating from the nucleus and regulation of HSP
The cleavage of DNA in the nucleus is a biochemical feature of apoptosis, which is due to the activation of specific endonucleases to cleave chromatin into shorter DNA fragments. However, DNA damage may also be an early signal transduction event in an apoptotic cascade. It is thought that when DNA is broken, histone H1.2 is transported to the cytoplasm by activating Bcl-2 protein-Bak. Promotes the release of cytochrome c. HSP plays a major role in protecting cells from DNA damage caused by various stress stimuli. After stress, most HSPs are transported to the nucleus. HSP27 and HSP70 family members play a protective role in DNA damage caused by oxidative stress. Overexpression of HSP25 can reduce oxidative DNA damage caused by TNF-α treatment. . A small part of HSP90 is also transported to the nucleus after stress, and HSP90 tightly binds to histones, which induces compression aggregation of chromatin, thereby avoiding DNA damage. A rather specific form of DNA damage is accompanied by a shortening of telomeres. When the critical length of the telomere region at the end of the chromosome is about 7 kb, the cells enter the senescence state, and it is possible to further cause apoptosis. Telomerase is responsible for the synthesis and maintenance of the length of the telomere region. HSP90 directly interacts with telomerase to assist in the development of its enzyme activity. It has been shown that HSP90 can enhance the activity of telomerase in prostate cancer cells.
D. Apoptosis signal originating from mitochondria and regulation of HSP
Mitochondria are central coordinators of apoptotic events, and many apoptotic signal transduction pathways are clustered in mitochondria, triggering leakage of the mitochondrial membrane. The mitochondria-mediated apoptotic signaling pathway is characterized by the formation of apoptotic bodies: the release of cytochrome c from mitochondria drives the assembly of caspase-activated complexes with larger molecular mass into apoptotic bodies. Apoptosis protease activating factor-1 (Apaf-1), which accumulates in the presence of dATP and caspase-9, promotes apoptosis-executing protein caspase -3 self-cleavage activation. Studies have shown that HSP family members play an important role in the regulation of mitochondria-mediated apoptosis pathways. Experts demonstrated by immunoprecipitation experiments that HSP27 chelate with cytochrome c and caspase-3 zymogen, preventing the correct formation of apoptotic bodies and the function of their functions. In fact, HSP27 binds to cytochrome c released from mitochondria into the cytosol, preventing cytochrome c-mediated interaction between Apaf-1 and caspase-9 zymogen, specifically interfering with caspase-dependent apoptosis. Mitochondrial pathway. These results demonstrate that expression of HSP27 maintains mitochondrial function during apoptotic activation. Mitochondrial HSP60 and its synergistic chaperone, HSP10, interact with caspase-3 zymogen to induce the release of cytochrome c, thereby promoting the apoptosis of Jurkat cells. In contrast, some scholars have shown that cytosolic HSP60 can chelate with a variety of anti-apoptotic molecules (such as Bax or Bak) to help them perform their functions, indicating that HSP60 plays a dual role in the process of apoptosis regulation. .
In vitro, experiments with purified HSP70 demonstrated that it inhibited the activation of cytochrome c/dATP-mediated caspase, but did not inhibit the oligomerization of Apaf-1, which was directly associated with Apaf-1. The formation of functional apoptotic bodies is prevented and the process of apoptosis is inhibited. In contrast, overexpression of the HSP70 homolog, HSP105α, in mouse embryonic F9 cells induced apoptosis. HSP90 binds directly to Apaf-1, preventing the formation of apoptotic body complexes. In addition, the active cysteine ​​on the surface of HSP90 can reduce the level of cytochrome c, indicating that HSP90 plays an important role in regulating the redox balance of cells.
Mitochondria is the original site of reactive oxygen species (ROS) and is the main source of ROS in vivo. In a normal cell, there is usually a balance between pro-oxidative and antioxidant pathways. When stimulated by stress, the redox environment is unbalanced, leading to the accumulation of ROS. As a messenger of cell damage, ROS can cause general damage to cells through oxidation of membrane lipids, proteins and DNA. Excessive production of ROS is associated with many forms of apoptosis and necrosis. There is a correlation between the production of ROS and the induced expression of HSP. Small molecule heat shock proteins increase the level of reduced glutathione (GSH) by enhancing the activity of glucose-6-phosphate dehydrogenase and slightly activating glutathione reductase and glutathione transferase. Under the stimulation of various oxidative stress, GSH is discharged into the extracellular environment and is considered to be a molecular switch that triggers apoptosis. Members of the HSP70 family play a similar role as small molecule heat shock proteins in resisting oxidative stress. For example, expression of HSP70 in mouse tissue cells can counteract heat-induced apoptosis by inhibiting the induction of ROS.
E. Apoptosis signal originating from the endoplasmic reticulum and regulation of HSP
The endoplasmic reticulum (ER) is the main site for protein synthesis and transportation. Many stress conditions, including the destruction of intracellular homeostasis and changes in the oxidative environment in the ER cavity, can cause endoplasmic reticulum stress (ER stress) and lead to apoptosis. ER is involved at least in two mechanisms of apoptosis: unfolded protein response and disruption of Ca2+ signaling. Studies have shown that ER protease, caspase-7 and -12 are involved in ER stress-mediated apoptosis. The unfolded protein response induces activation of the CHOP/GADD153 transcription factor, which also promotes ER-induced apoptosis. ER stress induces the expression of the Glucose-regulated protein Grp78 in the endoplasmic reticulum. Grp78 is involved in the transmembrane transport of peptides and acts as an apoptosis regulator to protect host cells against ER stress-mediated apoptosis. In vitro experiments demonstrated that Grp78 inhibits cytochrome c-mediated activation of caspase and thereby inhibits caspase-mediated apoptosis. However, the role of heat shock proteins in the endoplasmic reticulum in ER stress-mediated apoptosis is unclear. In addition, a significant interaction between ER and mitochondria was found during apoptosis. This interaction involves the junction of two organelles, making it possible for pro-apoptotic and anti-apoptotic molecules such as Bcl-2 to regulate the flow of calcium ions at the junction. Two pro-apoptotic mitochondrial molecules, Bax and Bak, have been shown to localize to ER and promote caspase-12-dependent apoptosis. Whether ER heat shock proteins are involved in the regulation of ER/mitochondrial connections remains unclear.
3. The regulation of heat shock protein on caspase-independent cell apoptosis
The caspase-non-dependent apoptotic pathway mainly includes serine proteases, cathepsins, calcium-activated neutral proteases, and ceramide-mediated apoptosis. These pathways are not centered on the activation of caspase. Obviously, the various signal transduction pathways of apoptosis are interrelated, so the "caspase-independent" pathway may cross the caspase-dependent pathway. At present, there are few studies on the role of HSP in these pathways, and experimental evidence is limited. For example, HSP27 has been shown to co-precipitate with serine protease A in cytosolic lysates. HSP70 is expressed on the surface of tumor cells, and can induce apoptosis of tumor cells by binding and uptake of serine protease B. Furthermore, it was further demonstrated that HSP70 has an inhibitory effect on ceramide-mediated apoptosis. Further research is needed in this area.
4. Pleiotropic effects of heat shock proteins in the process of apoptosis
Death signals transmitted from death receptors or intracellular stress induce oligomerization and autoactivation of various apoptotic molecules, and HSP is widely involved in these processes. Highly dynamic interactions among members of the apoptotic cascade, such as receptor dimerization, recruitment of caspase zymogen/caspase in receptor complexes, dATP/cytochrome c/Apaf-1/caspase-9 The formation of complexes, etc., in most cases, is affected by HSP. As a central regulator of assembly, transport and folding of proteins, HSP plays a major regulatory role in apoptotic signal transduction events. However, their pro-apoptotic function is restricted and is usually overtaken by the protective effect of HSP on cells. This fine-tuning balance is not only the key to regulating cell death or survival, but also the two forms of cell death, the transition between apoptosis and cell necrosis. In short; HSP heat shock protein is involved in a variety of physiological and biochemical processes in the cell, and is an important protein that determines cell fate. They are the "central coordinators" of cells that respond quickly and ubiquitously to physical, chemical, and environmental stresses, acting together to regulate cell proliferation, survival, and death. HSP is an important regulator of apoptosis, participates in and regulates various events of apoptosis, and plays a role in anti-apoptosis or apoptosis. Of course, there is still a need for further development of heat shock proteins in certain fields. In-depth study.
Excerpt from Life Science 08-06-02
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