Heat Shock Proteins
Heat shock or stress proteins are ubiquitous biomolecules highly conserved in both eukaryotes and prokaryotes. HSPs have been shown to act as molecular chaperones and have cytoprotective effects that are strong (Schmitt et al., 2007, p. 15). HSPs have been classified into five categories in mammals. These classes include small HSPs, HSP60, HSP70, HSP90, and HSP100 per their molecular size from the smallest to the largest. HSPs can be expressed constitutively or inductively regulated with different targets of subcellular locations. Once expressed, HSPs have significant roles in the cells. According to Li and Srivastava (2004, p. 1), chaperones help in protein folding and unfolding, sorting and transporting proteins to their correct subcellular compartment. Moreover, chaperones play roles in complex multiprotein assembly, signaling and controlling the cell cycle and in cell protection against apoptosis or stress. HSPs play role in the presentation of antigens. Extracellular HSPs also act as stimulants in which case they stimulate immune system cells that present antigens.
Working of HSPs
- HSPs as chaperones
Most HSP molecules are chaperones. Chaperons refer to proteins that can bind to either stabilize or destabilize other protein molecule conformers (Csermely & Yahara, 2003, p. 68). The cycle of binding and releasing of the protein molecules by the chaperones is controlled, and it determines their in vivo fate. Chaperones help in preventing improper folding of proteins. Moreover, they bind to incorrectly folded proteins and facilitate their refolding. Few chaperones have catalytic properties in the folding process. A few other chaperones can desegregate loose protein aggregates.
The working of chaperones depends on their role and their classification. However, most chaperones are capable of binding and hydrolyzing ATP to acquire the energy needed in their working. According to Csermely and Yahara, (2003, p. 68), binding of ATP on the chaperons induces a conformational change. The conformational change is essential in the process of folding protein molecules. Co-chaperons help chaperons working. Co-chaperons regulate the process of ATP hydrolysis. Co-chaperons also assist in regulating the cycle of binding and dissociating target molecules on the chaperons. Moreover, co-chaperons assist in directing target molecules or a chaperone complex to their cellular destination.
- HSPs as signal transducers
HSPs are crucial in signal transduction. The role of HSPs in signal transduction is observed in their roles such as in protection against apoptosis and oxidative stress. Moreover, HSPs also play a crucial role in inhibiting the stress kinases while others are necessary for activation of cell cycle modulators.
This section seeks to explore literature on HSPs primarily HSP27 and HSP72. The study of these HSPSs is subjective in their relation to breast cancer. Moreover, interactions of the two forms of HSPs with microtubules are also considered. Lastly, the paper examines inhibitors of HSF1 exploring how they inhibit HSP molecules. Finally, the paper discusses possible implications of inhibitory action of HSF1 on HSP molecules on therapy of breast cancer.
Heat shock protein 27 and heat shock protein 72
- HSP27
HSP27 is classified a small HSP. Schmitt et al. (2007, p. 16) explain that HSP27’S size ranges from 15 to 30 kDa and is possible of forming oligomers of about 1000kDa. Oligomerization of HSP27 depends on its phosphorylation and exposure to stress and is a dynamic process. Phosphorylation of HSP27 process is reversible. It is catalyzed by MAPKAP kinases upon exposure to such as mitogens, differentiating agents and inflammatory cytokines. HSP27 does not have ATPase activity. It plays a crucial role in providing protection against aggregation of proteins. Moreover, when overexpressed, HSP27 offers protection against apoptosis through cytochrome c release inhibition.
Interaction of HSP27 with microtubules
HSP27 contains numerous cytoplasmic regions characterized by polymerization of actin filaments. HSP27 is responsible for maintaining the integrity and guarding microtubules (Garlick & Robertson, 2007, p. 347). HSP27 modulates microtubule stability in a manner that depends on phosphorylation. According g to Liang and MacRae (1997, p. 1435), HSP27 increases the occurrence of microtubules subsequent to its phosphorylation. Presence of high amounts of HSP27 facilitates reoccurrence of microtubules after treatment. HSP27 exhibits the ability to prevent fragmentation of actin and microtubules by oxidative stress. Schmitt et al. (2007, p. 14) explains that phosphorylation of HSP27 results to stabilization of microtubules and consequently. Moreover, Williams, Rahimtula and Mearow (2005, p. 1) claims that HSP27 has the capability to protect microtubules from collapsing induced by Ph and heat shock.
HSP27 and breast cancer
According to Williams, Rahimtula and Mearow (2005, p. 1), tumor cells results in loss of function of p53. p53 is a protein that suppresses tumor development and proliferation. HSPs play a crucial role in enabling survival and maintenance of tumor cells. Straume et al. (2012, p. 8699) accounts for a rise in expression of HSP27 in the case of breast cancer. The exact role of HSP27 in breast tumors is still unclear. However, it is speculated that HSP27 plays a crucial role in the maintenance of tumor cells growth (Williams, Rahimtula & Mearow, 2005, p. 1). In this case, HSP27 prevents programmed cell death and is associated with high level of aggressiveness of the tumor.
Williams, Rahimtula and Mearow (2005, p. 1) explains that HSP27 causes poor prognosis of breast cancer. Moreover, HSP27 result to the acquisition of phenotypes that are drug resistant. Moreover, HSP27 causes resistance in breast cancer chemotherapy. According to Straume et al. (2012, p. 8702), expression of low levels of HSP27 is crucial in breast cancer therapy. It helps in reducing the aggressiveness of breast tumors and hence improving their survival.
- HSP72
HSP72 is in the family of HSP70. Under normal conditions, it is expressed at low levels. Environmental stress such as anoxia, heavy metals and heat shock that result to misfolding of protein induce expression of HSP72. Similar to HSP27, HSP72 has strong cytoprotective properties. It also acts as a molecular chaperone. These properties have an inhibitory effect on apoptosis. HSP72 achieves apoptosis inhibition through various mechanisms. Such include apoptosome formation prevention and inhibition of cytochrome c release (Gupta et al., 2010, p. 1). Moreover, HSP72 inhibits apoptosis through suppression of JNK. JNK is protein kinase activated by stress and plays a role in the opening stress-induced pathway of apoptosis. HSP72 is capable of binding apoptosis-inducing factor (AIF). Upon successive binding to AIF, HSP72 prevents condensation of chromatin and hence cell death that results from the same.
Interaction of HSP72 with microtubules
Similarly, to HSP27, HSP72 also interact with the cytoskeletal system. Garlick and Robertson (2007, p. 347) explains that HSP72 bind to the microtubule system to regulate their assembly or their disassembly. Interaction of HSP72 with the microtubules is also dependent on their phosphorylation that occurs in the presence of stress. Stress causes HSP aggregates to disassemble and consequently become phosphorylated. The phosphorylated small HSP aggregates interact with actin directly or through their associated proteins. Such interaction of the HSP aggregates prevents destabilization and promotes reorganization of actin filaments.
HSP72 and breast cancer
Similarly, to HSP27, HSP72 also plays a crucial role in tumorigenesis. According to Wang et al. (2014, p. 1), HSP72 is capable of senescence in tumor cells through both p53-dependent and independent mechanisms. In the case of breast cancer, Conroy and Latchman (1996, p. 718) claim that HSP72 plays a role in determining the aggressiveness of the tumor. HSP72 is crucial in chaperoning c-Myc oncogene and products of p53 tumor suppressor gene. Consequently, it is highly expressed in the presence of breast tumors due to transformation and progression of tumor cells. p53 immune response in breast cancer is dependent on p53- HSP72 complexes whereby it is crucial in the antigenic presentation of p53 due to the occurrence of p53 and HSP72 complexes. Mutation of p53 gene cause conformational change on its protein, which consequently cause formation of complexes by HSP72.
HSF1 inhibitors and their interrelation with breast cancer
Heat shock factor 1 (HSF1) are biomolecules concerned with the regulation of expression of heat shock genes. HSF1 have two primary roles that include transcriptional and DNA-binding activities. HSF1 play role in the production of HSPs due to their interaction with them in a repressive manner. Whitesell and Lindquist (2009, p. 471) explain that HSF1 play a crucial role in transcription of inducible HSPs.
HSPs are highly expressed in cancerous cells. As earlier mentioned, they play role in the survival of the tumor cells. Downregulation of HSPs, therefore, can be crucial in cancer therapy (Schmitt et al., 2007, p. 19). Such inhibitors can target HSF1, which is essential for the transcription of HSP. Inhibition of HSF1 prevents activation of the stress induced HSPs and consequently interfering with cancer cells survival. Moreover, such inhibition of HSP results to mutant protein degradation, tumor growth prevention, and apoptosis activation. There are various inhibitors of HSF1 such as quercetin, triptolide. Emunin and NZ28. However, this section considers KNK37 and KRIBB11 inhibitors and their role in breast cancer therapy (Whitesell & Lindquist, 2009, p. 474).
KNK437 is one of the HSF12 inhibitors that indirectly suppresses the functioning of HSF1 (Fione et al., 2014, p.1). KNK437 inhibits accumulation of heat induced HSPs without causing toxicity. However, KNK437 has been shown to have poor potency in its use as a target for cancer therapy (Whitesell & Lindquist, 2009, p. 474). The working of KNK437 depends on its ability to reduce tolerance of p53. Moreover, KNK437 has the ability to increase sensitivity of the p53 to heat. The above named factors compromise the amount of p53 status that is responsible for regulating amount of HSP27. Consequently, the amount of HSP27 is reduced lowering the survival of cancerous cells. Moreover, HSP27 and HSP72 can be induced by heat. Ohnishi et al. (2004, p. 607) argue that KNK437 is able to suppress accumulation of heat induced HSP27 and HSP72.
KRIBB11 inhibitor directly binds to HSF1 suppressing their functioning (Filone et al., 2014, p.1; Yoon et al., 2011, p.1). Yoon et al. further explain that the working of this inhibitor depends on its ability to block positive transcription elongation factor b (p-TEFb) from being recruited on HSP72 promoter. This recruitment of p-TEFb is dependent on HSF1. Therefore, KRIBB11 comprises the working of HSF1. As earlier mentioned, HSF1 are crucial inhibitor works by recruitment of an elongation factor b on HSP72 that depends on HSF1 activity. Inhibition of HSF1 would, therefore, interfere with transcription of HSP72 consequently lowering its expression. According to Straume et al. (2012, p. 8702), low expression of HSP, in our case HSP72, results to less aggressiveness of breast cancer.
KNK437 and KRIB11 result to a decline in the level of HSPs, in our case HSP27 and HSP72 as discussed above. As earlier mentioned, HSPs are crucial in the maintenance of the survival of cancerous cells. Therefore, lowering their amounts offers a potential way of breast cancer therapy since it would help lower their aggressiveness. Low aggressiveness results to better survival chances of breast cancer patients. Moreover, downregulation of HSP27 and HSP72 has been shown to induce breast cancer dormancy.
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