27 - 30 May 2005

PATHWAY-TARGETED THERAPY AND MOLECULAR CHARACTERIZATION OF TUMOR RESISTANCE IN A GENETICALLY DEFINED MOUSE MODEL OF OVARIAN CARCINOMA Deyin Xing and Sandra Orsulic Harvard Medical School and Massachusetts General Hospital Charlestown, MA 02129 USA

Advanced ovarian cancers typically show numerous genetic alterations and chromosomal abnormalities. However, it is difficult to discern which of these alterations are required for the maintenance of ovarian cancers, and thus could be used for targeted therapy. The phosphatidylinositol 3-kinase (PI3K)/Akt pathway is commonly activated in human ovarian carcinomas. The mammalian target of rapamycin (mTOR) plays a key regulatory function downstream of the PI3K/Akt pathway.

It has been shown that the inhibition of mTOR with immunosuppressive macrolide rapamycin blocks oncogenic transformation induced by either PI3K or Akt, indicating that the mTOR function is obligatory for the oncogenic effects of PI3K or Akt. Studies on preclinical mouse models and cell lines have shown that the anti-tumor effects of mTOR inhibitors correlate with heightened Akt activity, and that the introduction of constitutively active Akt into mTOR inhibitor-resistant cell lines significantly increases sensitivity to mTOR inhibitors. However, several recent studies have revealed the necessity of combined therapeutic approaches in the treatment of tumors with an activated Akt pathway, suggesting that tumors with greater complexity develop resistance to rapamycin.

Cell lines and tumors with defined genetic alterations provide ideal systems in which to test the molecular mechanisms of tumor sensitivity to pathway-targeted therapy. We tested the susceptibility of ovarian cancer to the mTOR inhibitor rapamycin by using mouse ovarian cell lines that contain various combinations of defined genetic alterations. Using both in vitro and in vivo approaches, we investigated the effects of rapamycin on cell proliferation and apoptosis, as well as on tumor angiogenesis and the accumulation of peritoneal ascites in the presence or absence of an activated Akt oncogene. We demonstrated that rapamycin treatment effectively inhibits ascites formation and the growth of cell lines and tumors that rely on Akt signaling for proliferation and tumor maintenance. However, tumors in which Akt signaling was not the requisite driving force in proliferation were resistant to rapamycin.

The introduction of activated Akt to the rapamycin-resistant cells did not render the cells susceptible to rapamycin if they were able to utilize alternative pathways for survival and proliferation. Accordingly, we demonstrated that rapamycin-sensitive tumors become resistant to rapamycin when presented with alternative survival pathways such as K-ras and Her-2/neu.

These results provide molecular evidence that mTOR inhibitors may be effective in a subset of tumors that depend on Akt activity for survival, however, not effective in all tumors that exhibit Akt activation. We anticipate that mouse ovarian cancer cell lines with defined genetic alterations will be useful for the optimization of rationally designed cancer therapy that targets specific signaling pathways.

List of abstracts from the 3rd International Conference on the Female Reproductive Tract