Asthma is characterized by aberrant airway smooth muscle (ASM) proliferation, which increases the thickness of the ASM layer within the airway wall and exacerbates airway obstruction during asthma attacks. The mechanisms that drive ASM proliferation in asthma are not entirely elucidated. Ten-eleven translocation methylcytosine dioxygenase (TET) is an enzyme that participates in the regulation of DNA methylation by catalyzing the hydroxylation of 5-methylcytosine (5-mC) to 5-hydroxymethylcytosine (5-hmC). The generation of 5-hmC disinhibits the gene silencing effect of 5-mC. In this study, TET1 activity and protein were enhanced in asthmatic human ASM cell cultures. Moreover, the concentration of 5-hmC was higher in asthmatic ASM cells than in nonasthmatic ASM cells. Knockdown (KD) of TET1, but not TET2, reduced the concentration of 5-hmC in asthmatic cells. Because the cytoskeletal protein nestin controls cell proliferation by modulating mTOR, we evaluated the effects of TET1 KD on this pathway. TET1 KD reduced nestin expression in ASM cells. In addition, TET1 inhibition alleviated the platelet-derived growth factor-induced phosphorylation of p70S6K, 4E-BP, S6, and Akt. TET1 inhibition also attenuated the proliferation of ASM cells. Taken together, these results suggest that TET1 drives ASM proliferation via the nestin-mTOR axis.
Keywords: DNA methylation; airway smooth muscle; cytoskeletal protein; signal transduction.