Proteins in living organisms present a wide variety of different shapes and structures each specialised for their particular function and role in maintaining homoeostasis. As a wide and rough guide proteins can be divided into categories such as enzymes, antibodies, transport proteins and structural proteins. When a cell is subjected to raised temperatures, a group of proteins (aptly named Heat shock proteins or Hsp's first discovered by Ritossa, 1962) - present and essential under normal conditions (Borkovich et al., 1989) - increase in number to keep the cell functioning while experiencing unfavourable conditions. Hsp's are essential in a protective role and as chaperones aiding in the folding of proteins and limiting the dangerous aggregation of immature and non-native proteins. Hsp60 and Hsp70 act as general chaperones where as Hsp90's role is more specific to a select clientele of proteins. The exact function and role Hsp90 plays in the maturation of its client proteins is not yet fully understood but what is known is that Hsp90's role in acting as chaperone to a number of oncogenes is worthy of further research. If Hsp's act as chaperone to cancerous cells then they are providing the cell with a form of protection. Various studies (e.g. Blagosklonny, Fojo, Bhalla, Kim, Trepel, Figg, Rivera, and Neckers, 2001), have shown that by targeting the Hsp's then cancer therapy maybe more successful in eradicating the cancerous cells. Such methods make use of Hsp inhibitors, of which there is a variety, each with their own advantages and drawbacks.
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The role of the Hsp 90 family
The naming of Hsp's is based on molecular weight - eg. Hsp 90 weighs 90 kilodaltons. Hsp 90 is an umbrella term which refers to a family of proteins, each carrying out specific roles as: homodimers, molecular chaperones and assistants in direct protein folding and protein trafficking. Hsp 90 is ubiquitously expressed and integrally involved in cell signalling, proliferation and survival. Also known as stress proteins, Hsp's are considered an important part in cellular stress response. Infection, inflammation, toxins, hypoxia, starvation and exercise are all stressors which produce increased levels of Hsp's. The connection between cancer and Hsp's is one of dependence on the cancer cells part. Proteins in tumour cells depend on the Hsp 90's protein folding machinery for their stability, refolding and maturation. Cancerous cells rely on Hsp 90 more heavily than normal cells, which makes the use of Hsp 90 inhibition drugs (that corrupt signalling pathways) effective at controlling cancerous growth. To date five isoforms of HSP90 have been identified: Hsp90Î± and Hsp90Î² which share approximately 85% sequence identity at the protein level, GRP94 (in the endoplasmic recticulum), TRAP1 (in mitochondrial matrix) and Hsp90N. HSP90N is different from other Hsp isoforms because its N-terminal is much shorter, which means it does not contain the highly conserved ATPase domain.
Inhibitors of Hsp90
Hsp90 inhibitors interact specifically with a single molecular target causing inactivation, destabilization and degradation of Hsp90 client proteins. They bind to the ATP binding site. Although directed towards a specific molecular target, inhibitors simultaneously inhibit multiple signalling pathways, on which cancer cells depend for growth and survival. Hsp90 client proteins include: Telomerase, Mutated p53, Bcr-Ab1, Raf-1, Akt, HER2/Neu(Erb B2), Mutated B-Raf, Mutated EGF receptor, HIF-1alpha.
Hsp90 inhibitors can be used in cancer therapy as single agents where Hsp90 client protein chaperoning is necessary for cancer development or progression or to assist in enhancing a response to chemotherapy. Especially promising as a target for anti-cancer drugs is that many of Hsp90's client proteins are involved in signaling and chromatin-remodeling pathways. These are often the pathways disrupted by cancer. (Xaio, Lu, Ruden). Particularly advantageous is the use of these inhibitor drugs to destroy cancer cells that can overcome the inhibition of a single target or pathway. The identification of benzoquinone ansamycins as specific antagonists of the chaperone HSP90 (Neckers 1993) uncovered the importance of this protein for the growth and survival of cancer cells. This led directly to the first phase I clinical trial of an Hsp90 inhibitor as an anticancer agent.
Fig 1. Possible/ proposed uses of Hsp90 inhibitors in cancer therapy:
Hsp90 inhibitors as single agents in situations in which Hsp90 client protein is necessary for cancer development or progression. Hsp90 inhibitors used to enhance response to chemotherapy
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â€¢ Bcr-Abl-positive leukemias
â€¢ Other leukemias/lymphomas dependent on a chimeric or mutated protein (e.g. Fit-3-dependant leukemias, anaplastic large cell lymphoma dependent on NPM/ALK)
â€¢ Androgen or estrogen receptor dependent cancers (prostate and breast)
â€¢ Hormone independent breast and prostate cancers in which receptor mutation can be documented
â€¢ Clear cell renal carcinoma in which pVHL is either in active or deleted. â€¢ Taxol or doxorubicin in HER-2 or AKT over-expressing tumors (breast, ovarian, prostate and lung cancer)
â€¢ Gleevac in Bcr-Abl positive lukemias
â€¢ Proteasome inhibitor in multiple myeloma
(Isaacs, Xu & Neckers)
Discuss: tumour affinity, mutant protein accumulation in tumours and the effect it has on increased hsp's (most mutant proteins in cancer cells use Hsp90 to compensate for their structural instability).
Fig 2. Basic structure of Hsp90
The N-terminal domain contains an ATP-binding site that binds the natural products geldanamycin and radicicol. The middle domain is highly charged and has high affinity for co-chaperones and client proteins. C-terminal domain contains conserved pentapeptide sequence (MEEVD) which is recognised by co-chaperones. C-terminal nucleotide binding pocket has been shown to not only bind ATP, but cisplatin, novobiocin, epilgallocatechin-3-gallate (EGCG) and taxol.
Initial studies by Csermely and co-workers suggested a second ATP-binding site in the C-terminus of Hsp90. Unlike the N-terminal ATP binding site, there is no reported co-crystal structure of Hsp90 C-terminus bound to any inhibitor. (Donelly, Blagg)
The first known inhibitor of Hsp90 was identified by Neckers in 1993 as benzoquinone ansamycins. The discovery of Radicicol followed a few years later (Schutle et al. 1998, Roe et al, 1999). Before these findings geldnamycin and radicicol were thought to act as kinase inhibitors (Kwon et al .1992, Zhao et al 1995). Additional target based screening lead to novobiocin.