SMURF2-Mediated Ubiquitination and Degradation of HOXA5 Augments Endoplasmic Reticulum Stress in Alveolar Epithelial Type II Cells Induced by LPS

Acute lung injury (ALI) is a clinical syndrome characterized by pulmonary inflammatory and endoplasmic reticulum stress (ERS). Nevertheless, the involvement of Homeobox A5 (HOXA5) in ERS of alveolar epithelial type II (ATII) cells remains unclear. RT-qPCR and western blot were employed to detect the expression of HOXA5, SMAD-specific E3 ubiquitin protein Ligase 2 (SMURF2), Fatty-acid-binding protein 4 (FABP4), inflammatory factors, and ERS-related proteins. The MTT measured cell viability. Calcium Assay was employed to measure intraluminal CA²⁺ levels within the ER. Thioflavin T staining was used to assess misfolded or unfolded proteins in ERS. CM-H2DCFDA detected the levels of ROS, immunoprecipitation assay detected the ubiquitination level of HOXA5. Co-immunoprecipitation was utilized to confirm the interaction between HOXA5 and SMURF2. The binding relationship between HOXA5 and FABP4 promoter was investigated using dual luciferase reporter gene and ChIP assays. HOXA5 overexpression inhibited LPS-induced Apoptosis and ERS in ATII cells. SMURF2 promoted ubiquitination and degradation of HOXA5. SMURF2 exacerbated LPS-induced ERS by down-regulating HOXA5. HOXA5 suppressed the transcriptional activity of FABP4. FABP4 overexpression counteracted the effect of HOXA5 overexpression on LPS-induced ERS. The ubiquitination of HOXA5 mediated by SMURF2 enhanced ERS induced by LPS in ATII cells through facilitating the transcriptional activity of FABP4.

Acute lung injury; Endoplasmic reticulum stress; FABP4; HOXA5; LPS; SMURF2.