Altered phosphorylation of topoisomerase I following overexpression in an ovarian cancer cell line.

There is a growing interest regarding the use of camptothecins (CPTs) for the management of ovarian cancer. Since topoisomerase I has been established as a prime target of these drugs in other experimental models, it was important to determine whether sensitivity to CPTs in ovarian cancer cells is also correlated with the cellular level of this enzyme. Despite the 7-fold increase in topoisomerase expression achieved by adenovirus-mediated expression, the sensitivity to a CPT derivative (topotecan), was not improved compared with control cells harboring an endogenous level of the enzyme. This observation is in accordance with the similar level of topoisomerase I activity found in control and overexpressing cells and suggests that these cells may efficiently regulate the enzyme activity. Indeed, topoisomerase I overexpressing cells are characterized by a lack of alkaline phosphatase sensitivity and elimination of the hyperphosphorylated form of the protein. Taken together, these observations strongly suggest that an alteration in the phosphorylation state of topoisomerase I could limit its activity and prevent improvement of CPT response in ovarian cancer cells. In addition, a limited extent of topoisomerase I phosphorylating activity was found in nuclear extract of OVCAR-3 cells. Hence, providing enhancement in topoisomerase I expression may not result in improvement of CPT response in ovarian cancer cells because of an efficient control of the phosphorylation state of the enzyme.