RNA-binding protein HuR inhibits the expression of sirtuin-1 in patients with COPD

Background: COPD is characterized by the accelerated ageing of the lung, induced by elevated levels of oxidative stress. SIRT1, a putative anti-ageing molecule, is reduced at both the protein and mRNA level in COPD patients, but the mechanism by which this occurs is not fully elucidated. Aims: To identify the role of HuR, a RNA stability protein which binds the 3'UTR of a target gene, in the regulation of SIRT1 in response to oxidative stress. Methods: Protein levels were determined by Western blotting and mRNA expression by qPCR. siRNA was used to silence gene and protein expression. Results: HuR was significantly reduced at the mRNA and protein level in peripheral lung samples taken from COPD patients (P<0.001, n=23) compared to non-disease controls (n=12); expression correlated with decreased FEV1% predicted (P<0.05). HuR (P<0.001) and SIRT1 (P<0.01) were significantly decreased at the mRNA level in small airway epithelial cells from COPD patients (n=10) compared to non-smoking controls (n=6). HuR was also decreased at the mRNA level in mice exposed to cigarette smoke for 3 days compared to those exposed to air (n=5, P<0.05). Oxidative stress with H₂O₂ (100µM) reduced HuR at both the mRNA and protein (n=4, P<0.05) in bronchial epithelial cells (BEAS2B). Gene silencing of HuR led to a significant decrease in the mRNA and protein expression of SIRT1 (n=4, P<0.01) whilst increasing the mRNA expression of cellular senescence markers p21 and p16 (n=6, P<0.01). Conclusions: Our results indicate that HuR expression is inhibited by oxidative stress, suggesting a potential mechanism by which SIRT1 is reduced in COPD patients, leading to the accelerated ageing phenotype.