MUC5AC and chronic obstructive pulmonary disease: Additionally, the expression levels of Muc5ac and Mmp12, which are highly involved in COPD (Caramori et al., 2009; Hunninghake et al., 2009; Churg et al., 2012; Li and Ye, 2020), were also increased after ENDS exposure (Figures 1D,F).