Bone morphogenetic proteins (BMPs) are multifunctional cytokines, which play a key role in vascular development and remodelling. Heme oxygenase-1 (HO-1), the rate-limiting enzyme in heme catabolism, has been shown to be protective against vascular and lung injury. In a microarray study, we identified HO-1 as a major target of BMP4 signalling in human pulmonary artery smooth muscle cells (PASMCs), and confirmed the induction of HO-1 mRNA and protein by RT-PCR and Western blotting, respectively. Immunoblotting demonstrated that incubation of PASMCs with BMP4 rapidly phosphorylated Smad1/5 and activated the mitogen activated protein kinases, p38^MAPK and ERK1/2, in PASMCs, but not JNK. Using pathway selective inhibitors, the induction of HO-1 mRNA and protein was shown to be dependent on activation of p38^MAPK. Induction was independent of Smad1/5 signaling since HO-1 mRNA and protein induction was intact in PASMCs harbouring a mutation in the kinase domain of BMP type II receptor, with disrupted Smad signalling. In addition, adenoviral transfection of kinase deficient BMPR-II also failed to inhibit BMP4-induced HO-1 expression. In functional studies, the HO-1 inhibitor, ZnPP-IX, partly reversed the growth inhibitory effects of BMP4 and overexpression of HO-1 in PASMCs inhibited serum-stimulated [³H]-thymidine incorporation. Taken together these findings show that HO-1 is an important Smad-independent target of BMP signalling in vascular smooth muscle. Inhibition of HO-1 function or expression will further increase the proproliferative capacity of BMPR-II-deficient PASMCs and may thus represent a potential second hit necessary for disease manifestation.