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A2208 - mTORC1 Controls Cellular Senescence in Alveolar Epithelial Type II Cells
Author Block: R. S. Summer1, H. Shaghaghi1, D. Schriner1, X. Hong2, D. Sales1, V. Desai3, A. Bhushan3, M. Ramirez1, F. Romero1; 1Center for Translational Medicine and The Jane and Leonard Korman Institute, Thomas Jefferson University, Philadelphia, PA, United States, 2Medical Research Center, North China University of Science and Technology, Thangshan City, China, 3Department of Pharmaceutical Sciences, Thomas Jefferson University, Philadelphia, PA, United States.
Rationale: Cellular senescence is a physiological state in which cells lose their capacity to proliferate but maintain a high metabolic rate and secrete a multitude of bioactive factors. While cellular senescence of alveolar epithelial type II (AE2) cells is believed to contribute to the development of idiopathic pulmonary fibrosis (IPF) the molecular mechanisms regulating the induction of cellular senescence remain unknown. Objectives: To determine whether the mTOR pathway, which controls both anabolic and metabolic processes, contributes to the induction of cellular senescence in AE2 cells. Methods: Senescent AE2 cells were obtained from the lungs of 18 month-old C57BL/6 mice. Cellular senescence was induced in rat alveolar epithelial L2 cells by exposing cells to bleomycin (50 μg/ml) for 7 consecutive days. Transcript and/or protein levels for various cellular senescence markers (IL-6, TNF-α, MCP-1, TGFβ, p53, p21 and γ-H2AX) and markers of mTOR activation (mTORC1 and p70 S6 kinase) were measured in mouse and rat tissues. mTORC1 activity was inhibited by exposing L2 cells to rapamycin (20 μM) for 24h. Oxygen consumption was measured in L2 cells using the Seahorse XFp Bioanalyzer. Cellular senescence and mTORC1 activity were assessed in control and IPF lung tissues. Results: Markers of cellular senescence were significantly increased in AE2 cells from older mice and IPF lung tissues compared to mice 8 weeks of age. These changes were associated with an upregulation in mTOR activity, as judged by an increase in levels of phosphorylated mTORC1 and pS6K proteins. Similarly, we found that rat L2 cells exposed to prolonged genotoxic stress expressed higher levels of cellular senescence markers and exhibited an increase in mTOR activation. The induction of cellular senescence in L2 cells was also associated with an increase in mitochondrial oxygen consumption and ROS production. Moreover, blocking mTORC activity with rapamycin effectively reduced mitochondrial oxygen consumption and ROS production and inhibited the induction of cellular senescence. Conclusions: This study suggests that activation of mTORC1 is important for driving AE2 cells toward a cellular senescence phenotype. We postulate that blocking mTORC1 activity could prove effective for decreasing cellular senescence in the IPF lung.