Phloretin, an apple polyphenol, modulates the pathogenicity of chronic obstructive pulmonary disease associated bacteria

Chronic obstructive pulmonary disease (COPD) is the fourth leading cause of death worldwide and is a significant public health burden. Acute exacerbations of COPD (AECOPD) are sudden worsening of symptoms that can induce significant morbidity and disability and can accelerate lung function decline. Bacterial infections are a primary risk factor for AECOPD. A diet rich in apples reduces symptoms and incidence of COPD due to their polyphenolic content. We examined if phloretin, an apple polyphenol, could exhibit antimicrobial and immunomodulatory activity against the primary pathogens isolated from persons experiencing AECOPD: nontypeable Haemophilus influenzae (NTHi), Moraxella catarrhalis, Streptococcus pneumoniae, and Pseudomonas aeruginosa. Phloretin exhibited broad bacteriostatic and anti-biofilm activity against the COPD pathogens and inhibited bacterial growth, adherence, and invasion of respiratory epithelial cells. Additionally, phloretin enhanced bacterial clearance and attenuated neutrophilia in the lung of mice administered a phloretin supplemented diet prior to NTHi exposure. Furthermore, we investigated whether phloretin could inhibit the cellular signaling involved in COPD pathogen-induced mucus overproduction. Mucus contributes to the pathology of AECOPD, as overproduction and insufficient clearance results in mucus obstruction of the airways. We found that NTHi stimulated Toll-like 4 (TLR4) receptor in respiratory epithelial cells, which induced NADPH oxidase 4 (NOX4) production of reactive oxygen species (ROS). ROS can activate the epidermal growth like receptor (EGFR), a major regulator of mucus production in the airway, by modifying cysteine residues in its active site or through the modification of kinases (e.g. SRC) that can transphosphorylate the receptor. ROS generated by NTHi-induced NOX4 resulted in the phosphorylation of EGFR and subsequent activation of the mitogen-activated protein kinase pathway. This resulted in the downstream transcript and protein production of mucin 5AC, a mucus marker. Phloretin attenuated pulmonary mucus hypersecretion through inhibition of TLR4-NOX4 signaling and the downstream EGFR-MAPK signaling cascade. Phloretin is therefore a promising candidate for relieving and preventing the development of bacterial-induced AECOPD