Impact of Posttranslational Modifications on the Anticancer Activity of Hsp90 Inhibitors.

Molecular chaperones are essential for guarding proteins that are indispensable for normal cellular functions. Heat shock protein 90 (Hsp90) is a vital molecular chaperone in eukaryotes that participates in stabilizing and activating approximately 200 target proteins, called "clients," many of which are involved in signal transduction pathways. Cancer cells however utilize Hsp90 to chaperone an array of mutated and overexpressed oncoproteins to protect them from misfolding and degradation. Therefore, Hsp90 is an attractive target in cancer therapy. Hsp90 chaperone function relies on ATP binding and hydrolysis, which in turn guides its carefully orchestrated conformational changes. This chaperone cycle is fine-tuned by another group of proteins called co-chaperones. They are able to accelerate or decelerate the cycle, allowing Hsp90 to chaperone different clients. Posttranslational modifications (PTMs) can also regulate the chaperone cycle at an epigenetic level thereby tailoring Hsp90 function to suit a specific cell type or environmental condition. Recent evidence suggests that inhibition of the enzymes that catalyze the PTM of Hsp90 can act synergistically with Hsp90 inhibitors, providing a novel therapeutic strategy to enhance the efficacy of Hsp90 inhibitors in cancer cells.