MTOR and chronic obstructive pulmonary disease: ,118 A reduction of SIRT1 expression has been described in serum, peripheral lungs, airway epithelial cells, and circulating PBMC of COPD patients.119, 120, 121 This reduction of SIRT expression is induced by oxidative stress through the inhibition of the phosphatase and tensin homolog from chromosome 10 (PTEN), which leads to the activation of PI3K–mTOR pathway.122