Extracellular Roles for the Molecular Chaperone,HSP90

Heat shock proteins (hsps) are versatile molecular chaperones that are responsiblefor many cellular functions including proper folding, oligomeric assembly, activation,and transport of proteins. Most of the known roles for hsps involve intracellular proteinsand processes. Mounting evidence suggests that hsps are present and function in theextracellular space. Hsp90 alpha was recently found on the surface and in conditionedmedia of HT-1080 fibrosarcoma cells. Here it acts as a molecular chaperone that assistsin the activation of matrix metalloproteinase-2 (MMP2), leading to increased tumorinvasiveness. Few other extracellular substrates of hsp90 have been identified, butseveral independent observations of extracellular hsp90 suggest that this protein may beimportant for both normal physiology and disease states. Hsp90 typically works in acomplex of associated proteins, and some of these proteins have also been observedextracellularly. Here we show that some of these components, including hsp90organizing protein (hop) and p23, are also found in HT-1080 conditioned mediasupporting the notion that hsp90 complexes function in invasiveness. These findingssuggest a wide-ranging phenomenon of extracellular molecular chaperoning that couldhave implications for biological processes and disease.