The Tumour Suppressor Fhit Protein Activates C-Raf Ubiquitination and Degradation in Human Melanoma Cells by Interacting with Hsp90

Fhit protein expression is reduced in the majority of human tumors; moreover, its restoration both triggers apoptosis of cancer cells and suppresses tumor formation in a large number of preclinical models of cancers. In the following study, we observed that Fhit expression is significantly reduced in human melanoma cells, and their in vivo growth is blocked by a recombinant adenovirus carrying the gene. Importantly, we found here that Fhit physically interacts with Hsp90. Since Hsp90 is a chaperone with a crucial function in the conformational maturation and stabilization of C-Raf, we also investigated whether Fhit could interfere with the Hsp90/C-Raf protein complex in melanoma. Interestingly, the administration of the Hsp90 inhibitor 17-AAG, in combination with Fhit protein overexpression in melanoma cells, reacts synergistically to increase C-Raf ubiquitination and degradation. These data reveal Hsp90 as a novel interactor of Fhit and suggest that activity restoration could represent a helpful strategy for suppressing the oncogenic C-Raf pathway in the therapy of human melanoma.