The bitter end: T2R bitter receptor agonists elevate nuclear calcium and induce apoptosis in non-ciliated airway epithelial cells

Abstract

Bitter taste receptors (T2Rs) localize to airway motile cilia and initiate innate immune responses in retaliation to bacterial quorum sensing molecules (acyl-homoserine lactones and quinolones). Activation of T2Rs leads to calcium-driven NO production that increases cilia beating and directly kills bacteria. Several airway diseases, including chronic rhinosinusitis, COPD, and cystic fibrosis, are characterized by epithelial remodeling, including loss of motile cilia and/or squamous metaplasia. To understand the function of T2Rs within the altered landscape of airway disease, we studied T2Rs in non-ciliated airway cell lines and primary cells de-differentiated to a squamous phenotype. In differentiated cells, T2Rs localize to cilia, however in de-differentiated, non-ciliated cells they localize to the nucleus. Cilia and nuclear import utilize many shared proteins, thus in the absence of motile cilia some T2Rs may target to the nucleus. T2R agonists selectively elevated both nuclear and mitochondrial calcium through a G-protein-coupled receptor, phospholipase C, and InsP3 receptor-dependent mechanism. Additionally, T2R agonists decreased nuclear cAMP, increased nitric oxide, and increased cGMP, consistent with T2R signaling. Furthermore, exposure to T2R agonists led to nuclear calcium-induced mitochondrial depolarization and caspase activation. T2R agonists induced apoptosis in primary bronchial and nasal cells differentiated at air-liquid interface but then induced to a squamous phenotype by apical submersion. Air-exposed well-differentiated cells did not die. This T2R-induced apoptosis may be a last-resort defense against infection, possibly when bacteria have breached the epithelial barrier and reach non-ciliated cells below. However, it may also increase susceptibility of de-differentiated or remodeled epithelia to damage by bacterial metabolites. Moreover, the T2R-activated apoptosis pathway occurs in airway cancer cells. T2Rs may thus contribute to microbiome-tumor cell crosstalk in airway cancers. T2R agonists may also be useful topical therapeutics (e.g., delivered by nasal rinse or nebulizer) for activating airway cancer cell apoptosis without killing surrounding differentiated tissue.