Targeting the PI3K/mTOR Pathway in Murine Endocrine Cell Lines

Abstract

The mammalian target of rapamycin (mTOR) inhibitors, such as rapalogues, are a promising new tool for the treatment of metastatic gastroenteropancreatic endocrine tumors. However, their mechanisms of action remain to be established. We used two murine intestinal endocrine tumoral cell lines, STC-1 and GLUTag, to evaluate the antitumor effects of rapamycin in vitro and in vivo in a preclinical model of liver endocrine metastases. In vitro, rapamycin inhibited the proliferation of cells in the basal state and after stimulation by insulin-like growth factor-1. Simultaneously, p70S6 kinase and 4EBP1 phosphorylation was inhibited. In vivo, rapamycin substantially inhibited the intrahepatic growth of STC-1 cells, irrespectively of the timing of its administration and even when the treatment was administered after cell intrahepatic engraftment. In addition, treated animals had significantly prolonged survival (mean survival time: 47.7 days in treated animals versus 31.8 days in controls) and better clinical status. Rapamycin treatment was associated with a significant decrease in mitotic index and in intratumoral vascular density within STC-1 tumors. Furthermore, the antitumoral effect obtained after treatment with a combination of rapamycin and phosphatidylinositol 3-kinase (PI3K) inhibitor LY294002 was more significant than with rapamycin alone in both cell lines. Our results suggest that the antitumor efficacy of rapamycin in neuroendocrine tumors results from a combination of antiproliferative and antiangiogenic effects. Interestingly, a more potent antitumor efficiency could be obtained by simultaneously targeting several levels of the PI3K/ mTOR pathway.

Antitumor Effect of Everolimus in Preclinical Models of High-Grade Gastroenteropancreatic Neuroendocrine Carcinomas

“…The effect of everolimus on the development of STC-1- and GluTag-derived intrahepatic tumor nodules was assessed in the experimental model of intrahepatic dissemination after intrasplenic injection of tumor cells [18,19]. Everolimus treatment strongly inhibited tumor development in vivo.…”

“…Our data suggest that this important reduction in tumor mass is due to a decrease in proliferation rather to an increase in apoptosis, since we found a significant decrease in Ki67 index between control and treated animals, but no difference in the percentage of cells expressing active caspase 3. Additional effects of everolimus treatment may be also involved, such as the antiangiogenic effect we previously observed with another mTOR inhibitor, rapamycin, in a comparable preclinical model [19]. …”

“…The xenografting procedure was as previously described [18,19]. Briefly, 50 µl of a solution containing tumor cells adjusted to a final concentration of 5·10 7 cells/ml were injected into the spleen, from where they disseminated into the liver through the portal vein to form intrahepatic tumor nodules.…”

Angiogenesis in Neuroendocrine Tumors: Therapeutic Applications

“…The role of the potential anti-angiogenic properties of everolimus remains to be evaluated in the clinical setting. There is, however, strong evidence for an anti-angiogenic in vivo effect of rapamycin and other rapalogues, as shown by the results obtained using various experimental models [55], including NET models [68]: the administration of mTOR inhibitors to tumor-bearing animals resulted not only in tumor growth inhibition but also in decreased angiogenesis. …”

Molecular challenges of neuroendocrine tumors (Review)

“…Authors of this paper claim that the antitumor efficacy of rapamycin in neuroendocrine tumors results from a ombination of antiproliferative (92) and antiangiogenic effects.…”