Heat shock proteins and cancer: intracellular chaperones or extracellular signalling ligands?

Heat shock proteins (HSPs) are found at elevated concentrations in tumour cells, and this increase reflects the proteotoxic stress experienced by the cells due to expanding levels of the mutated oncoproteins that drive tumorigenesis. The protection of oncogenic proteins by HSPs offers a window of vulnerability in tumour metabolism that has been exploited using Hsp90-targeting drugs. Such compounds have been shown to cause inhibition and degradation of a wide range of proteins essential for oncogenesis. Recently, Hsp90 has also been shown to be secreted by tumour cells and to interact in autocrine or paracrine manners with the surfaces of adjacent cells, leading to increased growth and metastasis. Future studies will address a number of key questions associated with these findings, including the relative importance of intracellular versus extracellular HSPs in tumorigenesis, as well as overcoming potential problems with normal tissue toxicity associated with Hsp90 drugs. Targeting individual members of HSP families and inactivating extracellular HSPs may be desirable future approaches that offer increased selectivity in targeting HSPs in cancer. This article is part of the theme issue 'Heat shock proteins as modulators and therapeutic targets of chronic disease: an integrated perspective'.