Nox4 mediates TGF-β1-induced retinoblastoma protein phosphorylation, proliferation, and hypertrophy in human airway smooth muscle cells

Transforming growth factor-beta 1 ( TGF-beta 1) plays a pivotal role in increasing airway smooth muscle mass in severe asthma by inducing proliferation and hypertrophy of human airway smooth muscle. The mechanism( s) for these effects of TGF-beta 1 have not been fully elucidated. In this study, we demonstrate that TGF-beta 1 is a potent inducer of expression of the nonphagocyte NAD( P) H oxidase catalytic homolog Nox4, diphenylene iodonium- inhibitable reactive oxygen species production, proliferation, and hypertrophy in cultured human airway smooth muscle cells. By confocal microscopy, TGF-beta 1- induced Nox4 was localized with the endoplasmic reticulum and the nucleus, implying a role for Nox4 in regulation of both the cell cycle and protein synthesis. Consistent with this hypothesis, TGF-beta 1 increased retinoblastoma protein phosphorylation at both Ser807/ 811 and Ser780. Silencing Nox4 prevented TGF-beta 1- mediated retinoblastoma protein phosphorylation, proliferation, and cell hypertrophy. TGF-beta 1 also increased phosphorylation of eukaryotic translation initiation factor 4E binding protein- 1 at Thr37/ 46, and this was likewise blocked by silencing Nox4. This is the first report to suggest a functional role for Nox4 in cell cycle transition and to demonstrate that Nox4 influences the pathobiochemistry of asthma by generating reactive oxygen species that promote TGF-beta 1- induced proliferation and hypertrophy of human airway smooth muscle.