Biological relevance of Hsp90-binding immunophilins in cancer development and treatment.

Immunophilins are a family of intracellular receptors for immunosuppressive drugs. Those immunophilins that are related to immunosuppression are the smallest proteins of the family, i.e., FKBP12 and CyPA, whereas the other members of the family have higher molecular weight because the show additional domains to the drug-binding site. Among these extra domains, the TPR-domain is perhaps the most relevant because it permits the interaction of high molecular weight immunophilins with the 90-kDa heat-shock protein, Hsp90. This essential molecular chaperone regulates the biological function of several protein-kinases, oncogenes, protein phosphatases, transcription factors and cofactors . Hsp90-binding immunophilins where first characterized due to their association with steroid receptors. They regulate the cytoplasmic transport and the subcellular localization of these and other Hsp90 client proteins, as well as transcriptional activity, cell proliferation, cell differentiation and apoptosis. Hsp90-binding immunophilins are frequently overexpressed in several types of cancers and play a key role in cell survival. In this article we analyze the most important biological actions of the best characterized Hsp90-binding immunophilins in both steroid receptor function and cancer development and discuss the potential use of these immunophilins for therapeutic purposes as potential targets of specific small molecules.

Regulation of NF-κB signalling cascade by immunophilins.

The fine regulation of signalling cascades is a key event required to maintain the appropriate functional properties of a cell when a given stimulus triggers specific biological responses. In this sense, cumulative experimental evidence during the last years has shown that high molecular weight immunophilins possess a fundamental importance in the regulation of many of these processes. It was first discovered that TPR-domain immunophilins such as FKBP51 and FKBP52 play a cardinal role, usually in an antagonistic fashion, in the regulation of several members of the steroid receptor family via its interaction with the heat-shock protein of 90-kDa, Hsp90. These Hsp90-associated cochaperones form a functional unit with the molecular chaperone influencing ligand binding capacity, receptor trafficking, and hormone-dependent transcriptional activity. Recently, it was demonstrated that the same immunophilins are also able to regulate the NF-kB signalling cascade in an Hsp90 independent manner. In this article we analize these properties and discuss the relevance of this novel regulatory pathway in the context of the pleiotropic actions managed by NF-kB in several cell types and tissues.