TGF-β₁ stimulates HO-1 via the p38 mitogen-activated protein kinase in A549 pulmonary epithelial cells

In lung injury and progressive lung diseases, the multifunctional cytokine transforming growth factor-β₁ (TGF-β₁) modulates inflammatory responses and wound repair. Heme oxygenase-1 (HO-1) is a stress-inducible protein that has been demonstrated to confer cytoprotection against oxidative injury and provide a vital function in maintaining tissue homeostasis. Here we report that TGF-β₁ is a potent inducer of HO-1 and examined the signaling pathway by which TGF-β₁ regulates HO-1 expression in human lung epithelial cells (A549). TGF-β₁ (1-5 ng/ml) treatment resulted in a marked time-dependent induction of HO-1 mRNA in A549 cells, followed by corresponding increases in HO-1 protein and HO enzymatic activity. Actinomycin D and cycloheximide inhibited TGF-β₁-responsive HO-1 mRNA expression, indicating a requirement for transcription and de novo protein synthesis. Furthermore, TGF-β₁ rapidly activated the p38 mitogen-activated protein kinase (p38 MAPK) pathway in A549 cells. A chemical inhibitor of p38 MAPK (SB-203580) abolished TGF-β₁-inducible HO-1 mRNA expression. Both SB-203580 and expression of a dominant-negative mutant of p38 MAPK inhibited TGF-β₁-induced ho-1 gene activation, as assayed by luciferase activity of an ho-1 enhancer/luciferase fusion construct (pMHO1luc-33+SX2). These studies demonstrate the critical intermediacy of the p38 MAPK pathway in the regulation of HO-1 expression by TGF-β₁.