Gam1-associated alterations of drug responsiveness through activation of apoptosis.

An early gene product, Gam1, encoded by the avian adenovirus CELO, is an inhibitory protein for the sumoylation machinery, which has been implicated in regulating a variety of cellular pathways. In this study, we found that Gam1 effectively suppressed both constitutive and inducible sumoylation and caused significant cell growth inhibition. This Gam1-mediated cell growth inhibition was associated with induction of apoptosis. In particular, Gam1 induced caspase-3 activity as detected by immunostaining and Western blot. Of interest, like the Ubc9 dominant-negative mutant, Gam1 also sensitized cells to DNA-damaging agents such as topotecan and doxorubicin and non-DNA-damaging agents such as paclitaxel and vincristine. Taken together, our findings suggest that activation of the caspase pathways is at least in part responsible for the increased apoptosis in Gam1-expressing cells and, thus, contributes to the growth inhibition and enhanced chemosensitivity.