Introduction:
COPD is associated with cellular senescence and increased levels of oxidative stress. Downregulation of PTEN activates the PI3K pathway and upregulates miR-34a in COPD. PTEN is a target of miR-21 and is also upregulated in COPD. However, whether miR-21 affects PTEN expression under oxidative stress conditions and in COPD remains unclear. Thus, this is assessed using small airway epithelial cells (SAEC).

Methods:
SAEC were isolated from resected lung tissue from non-smokers (NS) and COPD subjects. Cells were treated with miRNA mimics, antagomirs, H₂O₂ (50 ?M) or a PI3K? inhibitor, PIK75 (10 ?M). MiR-21, miR-34a and PTEN expression were measured by RT-qPCR and western blots.

Results:
In COPD SAEC (n=6), PTEN mRNA expression was significantly reduced by 88% and miR-21 was significantly elevated by 5.3-fold compared to NS cells (n=6). In NS SAEC (n=6), a miR-21 mimic significantly suppressed PTEN mRNA (54%) and protein (76%) expression whereas an antagomir significantly elevated levels of PTEN mRNA by 2.4-fold and protein by 1.8-fold. Similarly, in COPD cells (n=6), miR-21 mimic significantly inhibited PTEN both mRNA and protein levels by 35% whereas an antagomir significantly increased PTEN mRNA by 2.4-fold and protein by 1.8-fold. Under oxidative stress conditions, H₂O₂ significantly upregulated miR-21 (2.6-fold) and miR-34a (3.7-fold) in NS cells (n=3) while downregulated PTEN mRNA by 28%. PIK75 inhibitor reduced H₂O₂ induced miR-34a expression by 67% in NS cells (n=3) and had no effect on miR-21 and PTEN mRNA expression.

Conclusion:
This suggests miR-21 could be a key regulator of PTEN/PI3K pathway and the deficiency of PTEN expression and activity in COPD can be restored by miR-21 antagomir.