Autophagy response against protection and pathology; balance for better or for worse in Cystic Fibrosis

Introduction: Cystic fibrosis (CF) patients are commonly associated with increased bacterial infection of the airways usually with P. aeruginosa and S. aureus leading to increased mortality and morbidity. However, reduced innate immune responses were reported. The underlying molecular details linking protection against immunity and destructive pathology in CF are becoming more interesting. It is most likely that during infection, there is a crosstalk between the interaction of TLR4 with pathogen-associated molecular patterns and the immune cells thereby activating the anti-inflammatory pathways and cytoprotective pathways. One such cytoprotective pathway which is activated upon infection in CF is autophagy. In CF, it is important that the defective Cystic Fibrosis Transmembrane Regulator (CFTR) undergoes autophagy in order to reduce the stress inside the cell. Here, we aimed to study (a) hypoxia induced autophagy, (b) defective CFTR induced unfolded protein response (UPR) mediated autophagy, and (c) the interplay between UPR and HIF mediated autophagy in understanding the reduced innate immune responses in CF.

Methods: Immunofluorescence and immunoblotting were performed using the defective CFTR cell line (CFBE41o-), its corrected counterpart (corr-CFBE41o-) and normal human bronchial epithelial cell line (16HBE14o-) as control to examine the expression of TLR4, unfolded protein response (UPR), autophagy, hypoxia and anti-inflammatory pathways upon LPS stimulation.

Results: We observed significant differences in autophagy cargo assisting proteins LC3 II and Beclin-I in CFBE41o- when compared to its corrected counterpart corr-CFBE41o- and 16HBE14o-. Increase in LPS concentrations directly influenced the autophagy proteins like Beclin-1 and LC3 II proteins. Increased stabilization of HIF1α was observed in CFBE41o- as compared to 16HBE14o- under hypoxic conditions suggesting a probable role of hypoxia in CF.

Conclusion: Our results hint towards a role of autophagy in regulating TLR4 surface expression in cystic fibrosis. This nonselective engulfment of TLR4 by the autophagosomes might be helpful in preventing the hyper-inflammatory state in CF, thereby interfering in innate immune responses of the cell. Further studies are required to understand the complex mechanisms involved.