A role of mTOR in cigarette smoke-induced corticosteroid insensitivity

Introduction The corticosteroid insensitivity is a major barrier to the treatment of severe asthma and COPD. It has been revealed that cigarette smoke induced inactivation of HDAC2 through activation of PI3K/Akt pathway, leading to corticosteroid insensitivity. Mammalian target of rapamycin (mTOR) is one of the most important effectors of PI3K/Akt pathway. It is a serine/threonine kinase, and implicated in disease states where growth is deregulated and homeostasis is compromised. Aims To determine the role of mTOR on the development of corticosteroid insensitivity in COPD and under oxidative stress. Methods mTOR activity was calculated by the phospho-p70s6k band intensity in Western blot analysis. Corticosteroid sensitivity was determined as the 50% inhibitory concentration of dexamethason on TNF-a-induced interleukin-8 release in U937 cells. Results mTOR activity was increased within 5minutes after cigarette smoke stimulation in U937 cells, then decreased gradually until below the baseline. mTOR inhibitor, rapamycin, totally suppressed this increase. The EC50 value of dexamethasone was 5.0nM and the Emax value was 62.8 +/- 6.4 %(n=4) in the intact cells, but after incubation with cigarette smoke extract, EC50 was increased to 47.9nM and Emax was decreased to 42.4 +/- 9.1 %, suggesting cigarette smoke decreased dexamethasone sensitivity almost by one-tenth. EC50-Dex was 12.9nM and Emax was 55.9 +/- 6.9 % in the presence of Rapamycin (20nM, 2hr) and rapamycin partially reversed this insensitivity. Conclusions Cigarrete smoke extract treatments increased mTORC1 activity in U937 cells, and rapamycin partially reversed corticosteroid insensitivity caused by cigarette smoke exposure.