SIRT1 and chronic obstructive pulmonary disease: Observational studies have consistently documented reduced histone deacetylase activity in COPD, notably decreased HDAC2 expression and lowered class III deacetylase SIRT1 levels in lung and peripheral tissues, changes that are mechanistically linked to heightened inflammatory signaling, oxidative stress and corticosteroid resistance in patients [42,150,151,152].