Toll-like receptor 9 signaling promotes autophagy and apoptosis via divergent functions of the p38/JNK pathway in human salivary gland cells

Abstract

Abnormal signaling transduction in salivary gland cells is associated with the pathogenesis of Sjögren's syndrome (SS). Previously, we identified aberrant expression of toll-like receptor 9 (TLR9) in gland cells of SS patients and mouse models. In this study, we investigated the role of TLR9 and its downstream p38/mitogen-activated protein kinase (MAPK) and c‐Jun N‐terminal kinase (JNK) signaling in mediating apoptosis and autophagy in human salivary gland (HSG) cells. We selected either CpG-Odn, a classical TLR9 activator, or lentivirus-packaged TLR9 full-length cDNA to activate TLR9 signaling transduction. Activation of TLR9 signaling induced phosphorylation of its downstream protein kinases, p38/MAPK and JNK, in a time-dependent manner, and decreased HSG cell viability. Western blotting of LC3B-II and p62 in both normal and autophagic flux-administered conditions revealed elevated autophagy upon TLR9 activation. Observing the cell cytoplasm through transmission electron microscopy and mRFP-GFP-LC3B-tagged fluorescence confirmed an increased number of autophagosomes and autolysosomes in TLR9-activated cells. Bax/Bcl-2 ratio calculations, caspase-3 activity assays and Hoechst nuclear staining were utilized to confirm the involvement of apoptosis in TLR9 signaling activation. Furthermore, we selected SB239063, a p38/MAPK signaling inhibitor, and SP600125, a JNK inhibitor, to identify the functions of p38/MAPK and JNK in TLR9-mediated signaling transduction. Multiple approaches, including Western blotting assays, fluorescence assessments and caspase-3 activity measurements, confirmed that inhibition of p38/MAPK signaling ameliorated both autophagy and apoptosis in TLR9-activated HSG cells, whereas inhibition of JNK signaling attenuated apoptosis but failed to modulate autophagy in the models mentioned above. Our results indicate a divergent function of p38/MAPK and JNK in TLR9-mediated autophagy and apoptosis in salivary gland cells.