Prenyltransferase inhibitor radiosensitization of pancreatic ductal carcinoma (PaCa) cells

Farnesyltransferase inhibitors (FTIs) radiosensitize tumor cell lines expressing activated H-Ras. K-Ras however remains active after FTI treatment due to prenylation by geranylgeranyltransferase. Up to 90% of pancreatic carcinomas (PaCa) are mutant in K-ras. We hypothesized that combined FTI and geranylgeranyltransferase inhibitor (GGTI) treatment could radiosensitize PaCa cells. Nine human PaCa lines (7 K-ras-mutant, 2 ras-wt) and transgenic mouse pancreatic ductal cells (PDC) expressing wt-ras or mutant K-ras were tested in clonogenic assays with combined FTI-A +/- GGTI-B (Merck and Co Inc.). Inhibition of PI3- kinase (with LY294002) or inhibition of MEK1/2 (with U0126) served to assess the significance of the PI3-kinase and MAPK to radiation survival in these cells. H- and K-Ras prenylation status and changes in phosphorylation of AKT and MAPK were monitored as were changes in cell cycle distribution. FTI+GGTI treatment achieved inhibition of K-Ras prenylation in all PaCa cell lines. This treatment radiosensitized the K-ras mutant cell lines AsPC-1, Capan-2, MiaPaCa-2 and PSN-1, PancM, but not Capan-1 or the ras-wt cell lines (BxPC-3, HS766T, PDC-wt). L-778,123, a dual action inhibitor, sensitized all K-ras mutant cells. Surprisingly, PancM, Panc-1, MiaPaCa-2 and PDC K-Ras cells were radiosensitized by FTI treatment alone. R11577, another FTI without GGTI activity, also sensitized Panc-1 and MiaPaCa-2 and additionally AsPC-1 cells. Radiosensitization was also achieved after treatment with LY294002 in all PaCa lines expressing mutant-K-ras and the ras-wt line BxPC-3 overexpressing Akt2. However these lines were not sensitized by U0126. FTI+GGTI sensitize K-ras mt PaCa cell lines to radiation. PI3-kinase signaling but not MAPK signaling appears to contribute to radiation survival in PaCa cells. Radiosensitization of certain PaCa cells by FTI alone indicates that alternate pathways or farnesylated targets other than K-Ras may also be involved in radiation survival of a subset of pancreatic tumor cells