PIK3R3, an isoform of class IA phosphoinositide 3-kinase (PI3K), specifically interacts with cell proliferation regulators, such as retinoblastoma and proliferation cell nuclear antigen, to promote cell proliferation. However, the mechanisms behind the upstream signaling pathway of PIK3R3 remain unclear to date. This study showed that PIK3R3 expression was regulated by transforming growth factor- (TGF-) signaling and that PIK3R3 mediated the TGF--induced inhibition of lung adenocarcinoma cell proliferation. TGF- down-regulated PIK3R3 expression in lung adenocarcinoma cells. However, this TGF--induced inhibition of cell proliferation can be attenuated by PIK3R3 overexpression. In addition, TGF- can attenuate the transcriptional activity of NKX2.1, a transcription factor that binds to the promoter of PIK3R3. This result indicated that TGF- regulated PIK3R3 expression by targeting NKX2.1. We confirmed the correlation between NKX2.1 and PIK3R3 in clinical samples. Therefore, the TGF-/NKX2.1/PIK3R3 axis is crucial in the TGF--induced inhibition of cell proliferation, and the NKX2.1/PIK3R3 axis might become a target in TGF- receptor-repressed lung adenocarcinoma. (c) 2014 Wiley Periodicals, Inc.