Integrated stress response inhibition restores hsa-miR-145-5p levels after IFN-β stimulation in salivary gland epithelial cells. Association between cellular stress and miRNA biogenesis in Sjögren's disease

Abstract

Labial salivary glands (LSG) from Sjögren's disease (SjD) patients are characterized by increased levels of pro-inflammatory cytokines, such as type I interferons (IFN-I). These LSG also show activation of the integrated stress response (ISR) with overexpression of protein kinase R (PKR), a known IFN-stimulated gene. In vitro, IFN-I stimulation reproduces the downregulation of hsa-miR-145-5p, which is associated with TLR4 overexpression observed in LSG of SjD patients. MicroRNA levels depend on its biogenesis, which is a multi-step process involving several protein complexes. It is not known whether altered miRNA biogenesis is associated with the activation of the ISR induced by IFN-I in LSG from SjD. The aim of this study was to characterize the expression and localization of components of the miRNA biogenesis machinery in LSG of SjD patients, to assess the effect of pro-inflammatory cytokines on these components, and to test whether inhibition of the IFN-β-induced ISR restores the levels of hsa-miR-145-5p. In LSG from 12 SjD patients and 11 non-SjD sicca controls, we determined mRNA fold changes, relative protein levels, and the localization of the ISR and miRNA biogenesis machinery components by RT-qPCR, Western blot, and immunofluorescence, respectively. Pro-inflammatory cytokines, the ISR inhibitor ISRIB, and the PKR inhibitor C16 were used for in vitro assays. In LSG from SjD patients, PKR and its activator PACT colocalized in the cytoplasm, whereas the PKR inhibitor TRBP was observed in the nuclei. IFN-β activates PKR, increases p-eIF2α and ATF4 levels, and increases PACT and AGO2 detection in stress granules. C16 inhibits PKR phosphorylation but increases ATF4 by activating GCN2. ISRIB restores levels of hsa-miR-145-5p and its target TLR4 mRNA upon IFN-β stimulation. These findings suggest an association between inflammation, cellular stress, and miRNA biogenesis, where modulation of the ISR emerges as a potential strategy to restore cellular homeostasis in LSG from SjD patients.