Purpose: Long non-coding RNAs (lncRNAs) may act as oncogenes in small-cell lung cancer (SCLC). Exosomes containing lncRNAs released from cancer-associated fibroblasts (CAF) accelerate tumorigenesis and confer chemoresistance. This study aimed to explore the action mechanism of the CAF-derived lncRNA maternally expressed gene 3 (MEG3) on cisplatin (DDP) chemoresistance and cell processes in SCLC. Materials and Methods: Quantitative real-time PCR was conducted to determine the expression levels of MEG3, miR-15a-5p, and CCNE1. Cell viability and metastasis were measured by 3-(4, 5-dimethyl-2-thiazolyl)-2, 5-diphenyl-2-h-tetrazolium bromide and invasion assays, respectively. A xenograft tumor model was developed to confirm the effect of MEG3 overexpression on SCLC progression in vivo. Relationships between miR-15a-5p and MEG3/CCNE1 were predicted using StarBase software and validated by dual luciferase reporter assay. Western blotting was used to determine protein levels. A co-culture model was established to explore the effects of exosomes on MEG3 expression in SCLC cell lines. Results: MEG3 was overexpressed in SCLC tissues and cells. MEG3 silencing significantly repressed cell viability and metastasis in SCLC. High expression of MEG3 was observed in CAF-derived conditioned medium (CM) and exosomes, and promoted chemoresistance and cancer progression. Additionally, MEG3 was found to serve as a sponge of miR-15a-5p to mediate CCNE1 expression. Overexpression of miR-15a-5p and knockout of CCNE1 reversed the effects of MEG3 overexpression on cell viability and metastasis. Conclusion MEG3 lncRNA released from CAF-derived exosomes promotes DDP chemoresistance via regulation of a miR-15a-5p/CCNE1 axis. These findings may provide insight into SCLC therapy.

Mitophagy is an essential intracellular process that eliminates dysfunctional mitochondria and maintains cellular homeostasis. Mitophagy is regulated by the post-translational modification of mitophagy receptors. Fun14 domain-containing protein 1 (FUNDC1) was reported to be a new receptor for hypoxia-induced mitophagy in mammalian cells and interact with microtubule-associated protein light chain 3 beta (LC3B) through its LC3 interaction region (LIR). Moreover, the phosphorylation modification of FUNDC1 affects its binding affinity for LC3B and regulates selective mitophagy. However, the structural basis of this regulation mechanism remains unclear. Here, we present the crystal structure of LC3B in complex with a FUNDC1 LIR peptide phosphorylated at Ser17 (pS), demonstrating the key residues of LC3B for the specific recognition of the phosphorylated or dephosphorylated FUNDC1. Intriguingly, the side chain of LC3B Lys49 shifts remarkably and forms a hydrogen bond and electrostatic interaction with the phosphate group of FUNDC1 pS. Alternatively, phosphorylated Tyr18 (pY) and Ser13 (pS) in FUNDC1 significantly obstruct their interaction with the hydrophobic pocket and Arg10 of LC3B, respectively. Structural observations are further validated by mutation and isothermal titration calorimetry (ITC) assays. Therefore, our structural and biochemical results reveal a working model for the specific recognition of FUNDC1 by LC3B and imply that the reversible phosphorylation modification of mitophagy receptors may be a switch for selective mitophagy.
Crystallography, X-Ray ; Membrane Proteins ; chemistry ; metabolism ; Microtubule-Associated Proteins ; chemistry ; metabolism ; Mitochondrial Degradation ; Mitochondrial Proteins ; chemistry ; metabolism ; Peptides ; chemistry ; metabolism ; Phosphorylation ; Protein Structure, Quaternary