Aberration of the enzymatic activity of Fhit tumor suppressor protein enhances cancer cell death upon photodynamic therapy similarly to that driven by wild-type Fhit

The tumor suppressor Fhit protein lost in many human pre-malignant tissues, possesses diadenosine triphosphate activity regulated by a photosensitizer, protoporphyrin IX (PpIX) in vitro. Interestingly, when exogenously restored, the protein suppresses the growth of human cervical carcinoma HeLa cells which is further enhanced by PpIX. Additionally, Fhit production enhances the overall response of cells to PpIX-mediated photodynamic reaction. In the present study, we have estimated, for the first time, the biological activity of two Fhit mutated forms exhibiting aberrant Ap3A hydrolase activity in vitro which emphasizes the recent findings that hydrolysis of Ap3A is not necessary for Fhit tumor suppression function. Using several biophysical methods we revealed the dynamic nature of mutant Fhit-PpIX complexes in vitro which support our previous hypothesis that Fhit-Ap3A-PpIX might be a signaling molecule driving apoptosis in cancer cells. Moreover, according to our findings, substitution at histidine94 in Fhit active site induces the vulnerability of HeLa cells to PpIX-PDT in a similar manner to that caused by wild-type Fhit protein. These results support the view that inhibition of Fhit hydrolase activity might be a crucial element in a Fhit-driven cancer cells death.