AHR and chronic obstructive pulmonary disease: Characterization of the molecular pathways associated with our waterpipe smoking-dependent differential methylated gene set showed significant enrichment of pathways previously associated with cigarette smoking and COPD in the SAE, including aryl hydrocarbon receptor signaling, xenobiotic metabolism signaling and G-protein coupled receptor signaling, further supporting the concept that waterpipe smoking has harmful effects on lung biology [24, 25].