Death by chaperone: HSP90, HSP70 or both?

HSP70 family members are highly conserved proteins that function as molecular chaperones. Their principle role is to aid protein folding and promote the correct cellular localisations of their respective substrates. The function of HSP70 isoforms can be exhibited independently or with the HSP90 chaperone system in which HSP70 is important for substrate recruitment. In addition to their chaperone role, HSP70 isoforms promote cell survival by inhibiting apoptosis at multiple points within both the intrinsic and extrinsic cell death pathways. Consistent with this cytoprotective function, increased expression of HSP70 isoforms is commonly associated with the malignant phenotype. We recently reported that dual silencing of the major constitutive (HSC70) and inducible (HSP72) isoforms of HSP70 in cancer cells could phenocopy the effects of a pharmacologic HSP90 inhibitor to induce proteasome-dependent degradation of HSP90 client proteins CRAF, CDK4 and ERBB2. This was accompanied by a G1 cell cycle arrest and extensive apoptosis which was not seen in non-tumorigenic human cell lines. Here we discuss the possible implications of our research for the development of HSP70 family modulators which offer not only the possibility of inhibiting HSP70 activity but also the simultaneous inhibition of HSP90, resulting in extensive tumour-specific apoptosis.