Hypoxia-inducible factor-1α (HIF-1α), a transcription factor that functions as a master regulator of oxygen homeostasis, has been implicated in fibrinogenesis. Here, we explore the role of HIF-1α in transforming growth factor-β (TGF-β) signaling by examining the effects of TGF-β₁ on the expression of plasminogen activator inhibitor-1 (PAI-1). Immunohistochemistry of lung tissue from a mouse bleomycin (BLM)-induced pulmonary fibrosis model revealed that expression of HIF-1α and PAI-1 was predominantly induced in alveolar macrophages. Real-time RT-PCR and ELISA analysis showed that PAI-1 mRNA and activated PAI-1 protein level were strongly induced 7 days after BLM instillation. Stimulation of cultured mouse alveolar macrophages (MH-S cells) with TGF-β₁ induced PAI-1 production, which was associated with HIF-1α protein accumulation. This accumulation of HIF-1α protein was inhibited by SB431542 (type I TGF-β receptor/ALK receptor inhibitor) but not by PD98059 (MEK1 inhibitor) and SB203580 (p38 MAP kinase inhibitor). Expression of prolyl-hydroxylase domain (PHD)-2, which is essential for HIF-1α degradation, was inhibited by TGF-β₁, and this decrease was abolished by SB431542. TGF-β₁ induction of PAI-1 mRNA and its protein expression were significantly attenuated by HIF-1α silencing. Transcriptome analysis by cDNA microarray of MH-S cells after HIF-1α silencing uncovered several pro-fibrotic genes whose regulation by TGF-β₁ required HIF-1α, including platelet-derived growth factor-A. Taken together, these findings expand our concept of the role of HIF-1α in pulmonary fibrosis in mediating the effects of TGF-β₁ on the expression of the pro-fibrotic genes in activated alveolar macrophages.