Abstract #1735: Rapamycin inhibits hyperactivation of mTORC1 signaling in a novel xenograft model of tuberous sclerosis complex skin tumors

Tumors in patients with tuberous sclerosis complex (TSC) exhibit biallelic mutations in either the TSC1 or TSC2 tumor suppressor gene, resulting in hyperactivation of the mTORC1 signaling pathway. Rapamycin inhibits mTORC1 signaling and represents a potential medical treatment for TSC. Earlier, we showed that rapamycin inhibited the in vitro proliferation of skin tumor cells more than that of normal fibroblasts from TSC patients. In this study, we assessed the effects of rapamycin on TSC skin tumors in our newly developed xenograft mouse model. Fibroblast-like cells were grown from skin tumors and normal-appearing skin from each patient. Cells from one TSC patient were selected for use in the animal model since the tumor-derived cells contained a second-hit point mutation in TSC2 and no detectable TSC2 protein. Normal fibroblasts or tumor cells were incorporated into collagen gels, the surface was overlaid with normal human neonatal keratinocytes, and these constructs were grafted to the backs of nude mice. Beginning 5 weeks after grafting, rapamycin (2mg/kg) or vehicle was administered by intraperitoneal injection on alternate days for 12 weeks. Mice were sacrificed 24 hours after the last injection for analysis of the grafts by immunohistochemistry. Grafts containing tumor cells showed greater activation of the mTORC1 signaling pathway than did normal cells, as shown by staining for phosphorylated ribosomal protein S6. Phosphorylation of ribosomal protein S6 was markedly lower in grafts of mice treated with rapamycin. The content of human cells in grafts, determined by staining for a pan-human HLA class I antibody, tended to be fewer in tumor grafts treated with rapamycin than in vehicle control. Tumor cells were still present in the rapamycin treated grafts, however, as indicated by DNA analysis of tumor stroma. Therefore, although rapamycin decreased hyperactivation of the mTORC1 pathway and slowed proliferation of TSC skin tumor cells in vivo, it did not completely eliminate tumor cells during 12-week treatment in a xenograft model. The persistence of two-hit cells may portend tumor regrowth after discontinuing rapamycin, and points toward the need for combination therapy. This work was supported by R21 CA122963 from the National Cancer Institute, NIH for Dr. Darling. Dr. Moss was supported by the Intramural Research Program, NIH, NHLBI.

Citation Information: In: Proc Am Assoc Cancer Res; 2009 Apr 18-22; Denver, CO. Philadelphia (PA): AACR; 2009. Abstract nr 1735.