The peripheral protein quality control (QC) system removes non-native membrane proteins, including ΔF508-CFTR, the most common CFTR mutant in cystic fibrosis (CF), from the plasma membrane (PM) for lysosomal degradation by ubiquitination. It remains unclear how unfolded membrane proteins are recognized and targeted for ubiquitination and how they are removed from the apical PM. Using comprehensive siRNA screens, we identified RFFL, an E3 ubiquitin (Ub) ligase that directly and selectively recognizes unfolded ΔF508-CFTR through its disordered regions. RFFL retrieves the unfolded CFTR from the PM for lysosomal degradation by chaperone-independent K63-linked poly-ubiquitination. RFFL ablation enhanced the functional expression of cell-surface ΔF508-CFTR in the presence of folding corrector molecules, and this effect was further improved by inhibiting the Hsc70-dependent ubiquitination machinery. We propose that multiple peripheral QC mechanisms evolved to dispose of non-native PM proteins and to preserve cellular proteostasis, even at the cost of eliminating partially functional polypeptides.

Glucocorticoids are among the most commonly used anti-inflammatory agents. Despite the enormous efforts in elucidating the glucocorticoid-mediated anti-inflammatory actions, how glucocorticoids tightly control overactive inflammatory response is not fully understood. Here we show that glucocorticoids suppress bacteria-induced inflammation by enhancing IRAK-M, a central negative regulator of Toll-like receptor signalling. The ability of glucocorticoids to suppress pulmonary inflammation induced by non-typeable Haemophilus influenzae is significantly attenuated in IRAK-M-deficient mice. Glucocorticoids improve the survival rate after a lethal non-typeable Haemophilus influenzae infection in wild-type mice, but not in IRAK-M-deficient mice. Moreover, we show that glucocorticoids and non-typeable Haemophilus influenzae synergistically upregulate IRAK-M expression via mutually and synergistically enhancing p65 and glucocorticoid receptor binding to the IRAK-M promoter. Together, our studies unveil a mechanism by which glucocorticoids tightly control the inflammatory response and host defense via the induction of IRAK-M and may lead to further development of anti-inflammatory therapeutic strategies.

Phosphodiesterase 4B (PDE4B) plays a key role in regulating inflammation. Roflumilast, a phosphodiesterase (PDE)4-selective inhibitor, has recently been approved for treating severe chronic obstructive pulmonary disease (COPD) patients with exacerbation. However, there is also clinical evidence suggesting the development of tachyphylaxis or tolerance on repeated dosing of roflumilast and the possible contribution of PDE4B up-regulation, which could be counterproductive for suppressing inflammation. Thus, understanding how PDE4B is up-regulated in the context of the complex pathogenesis and medications of COPD may help improve the efficacy and possibly ameliorate the tolerance of roflumilast. Here we show that roflumilast synergizes with nontypeable Haemophilus influenzae (NTHi), a major bacterial cause of COPD exacerbation, to up-regulate PDE4B2 expression in human airway epithelial cells in vitro and in vivo. Up-regulated PDE4B2 contributes to the induction of certain important chemokines in both enzymatic activity-dependent and activity-independent manners. We also found that protein kinase A catalytic subunit β (PKA-Cβ) and nuclear factor-κB (NF-κB) p65 subunit were required for the synergistic induction of PDE4B2. PKA-Cβ phosphorylates p65 in a cAMP-dependent manner. Moreover, Ser276 of p65 is critical for mediating the PKA-Cβ-induced p65 phosphorylation and the synergistic induction of PDE4B2. Collectively, our data unveil a previously unidentified mechanism underlying synergistic up-regulation of PDE4B2 via a cross-talk between PKA-Cβ and p65 and may help develop new therapeutic strategies to improve the efficacy of PDE4 inhibitor.PDE4B | Roflumilast | nontypeable Haemophilus influenzae | PKA-Cβ | p65

Airway diseases such as asthma and chronic obstructive pulmonary disease (COPD) are characterized by excessive inflammation and are exacerbated by nontypeable Haemophilus influenzae (NTHi). Airway epithelial cells mount the initial innate immune responses to invading pathogens and thus modulate inflammation. While inflammation is necessary to eliminate a pathogen, excessive inflammation can cause damage to the host tissue. Therefore, the inflammatory response must be tightly regulated and deciphering the signaling pathways involved in this response will enhance our understanding of the regulation of the host inflammatory response. NTHi binds to TLR2 and signal propagation requires the adaptor molecule myeloid differentiation factor 88 (MyD88). An alternative spliced form of MyD88 is called MyD88 short (MyD88s) and has been identified in macrophages and embryonic cell lines as a negative regulator of inflammation. However, the role of MyD88s in NTHi-induced inflammation in airway epithelial cells remains unknown. Here we show that NTHi induces MyD88s expression and MyD88s is a negative regulator of inflammation in airway epithelial cells. We further demonstrate that MyD88s is positively regulated by IKKβ and CREB and negatively regulated by ERK1/2 signaling pathways. Taken together these data indicate that airway inflammation is controlled in a negative feedback manner involving MyD88s and suggest that airway epithelial cells are essential to maintain immune homeostasis.

Phosphodiesterase 4B (PDE4B) plays an important role in inflammation. Recently we have reported that roflumilast as a PDE4-selective inhibitor, synergizes with nontypeable Haemophilus influenzae (NTHi) to up-regulate PDE4B expression in vitro and in vivo. Clinical evidence and our previous results suggest that synergistic induction of PDE4B could be counterproductive for suppressing inflammation or may contribute to tolerance to roflumilast. We thus investigated if dexamethasone inhibits the synergistic induction of PDE4B by roflumilast and NTHi as well as inflammation. Here, dexamethasone markedly suppressed the synergistic induction of PDE4B in human lung epithelial cells and in vivo. We also found that dexamethasone further suppressed NTHi-induced inflammatory response in vitro and in vivo. Moreover, Compound A, as a dissociating non-steroidal glucocorticoid receptor (GR) ligand, inhibited the synergistic induction of PDE4B, thereby suggesting the requirement of dexamethasone-mediated GR activation in the suppression of PDE4B expression. Taken together, our data suggest that dexamethasone may help attenuate inflammation and tolerance through suppressing the PDE4B expression in chronic obstructive pulmonary disease (COPD) patients using roflumilast.