| Activation of the mTOR pathway is an important and early event in lung tumorigenesis, and therapies that target mTOR could be effective in the treatment or prevention of lung cancer. The prototypic mechanism of mTOR regulation in cells is through activation by the PI3K/Akt pathway, but mTOR receives input from multiple signaling pathways. In particular, the LKB1/AMPK pathway negatively regulates mTOR. To validate mTOR as a therapeutic target in lung cancer, we evaluated three drugs that inhibit the mTOR pathway, namely, the lipid-based Akt inhibitors phosphatidylinositol ether lipid analogues (PIAs), the AMPK activator metformin, and the mTOR inhibitor rapamycin. PIAs activated AMPK independently of Akt in NSCLC cells, which contributed to the cytotoxicity of these compounds and their ability to inhibit mTOR. AMPK activation by PIAs also occurred independently of the tumor suppressor LKB1, but required another upstream kinase of AMPK, CaMKKbeta. Treatment of LKB1-mutant NSCLC xenografts with PIA decreased tumor volume by ∼50%. These studies suggest that PIAs might have utility in the treatment of LKB1-mutant lung cancers, which are characterized by aberrant activation of the mTOR pathway. Another AMPK activator, metformin, was also effective in preventing tumor growth in a mouse model of tobacco carcinogen-induced lung tumorigenesis. Administration of the tobacco carcinogen NNK induces K-Ras mutations that activate the mTOR pathway and promote lung tumorigenesis in A/J mice. Metformin decreased tumor burden in this model by as much as 72%. The ability of metformin to prevent NNK-induced lung tumorigenesis was likely due to indirect effects on tumor cells. Although metformin inhibited the mTOR pathway in lung tissue and tumors, this occurred independently of AMPK activation and was associated with decreases in the phosphorylation of IGF-1R/IR and Akt. These data suggest that metformin might prevent NNK-induced lung tumorigenesis by decreasing the response of lung tissue to insulin or IGF-1. Inhibitors of the mTOR pathway might also prevent tobacco carcinogen-induced lung tumorigenesis by affecting the tumor microenvironment. Administration of NNK increased lung-associated Foxp3+ regulatory T cells (Treg) weeks prior to tumor development in A/J mice, and both metformin and rapamycin prevented this increase. Treg are a subset of CD4+ T cells that can limit the development of an effective immune response against cancer. Using a variety of techniques, we demonstrated that Foxp3+ Treg are required for K-Ras driven lung tumorigenesis. Therefore, the fact that metformin and rapamycin decrease lung- and tumor-associated Treg could contribute to their abilities to prevent NNKinduced lung tumorigenesis. Collectively, our studies demonstrate that inhibitors of the mTOR pathway are effective in both the treatment and prevention of lung cancer. Also, because metformin and rapamycin are both FDA-approved drugs, our studies could provide rationale for clinical prevention trials with these agents in patients at high risk to develop lung cancer. |
