5-Lipoxygenase (5-LO) expression is strongly induced by transforming growth factor-beta (TGFbeta) and 1alpha,25-dihydroxyvitamin D(3) in Mono Mac 6 cells. Since Smads have been described as downstream effectors of TGFbeta, we have investigated the role of the TGFbeta/Smad signalling system in the regulation of 5-LO gene expression. The rapid induction of 5-LO mRNA, determined with real-time quantitative RT-PCR, suggests that 5-LO is a primary TGFbeta target gene. In reporter gene assays with plasmids containing the 5-LO promoter plus different parts of the gene, Smads3/4 mediate a prominent upregulation of reporter activity that strongly depends on the coding sequence and to a lesser extent on the 3'-UTR and introns J-M. Deletion studies revealed the most profound decrease of inducibility by Smads3/4 when exons 10-14 are deleted. Sequence analysis and deletion studies indicate the existence of up to four Smad binding elements and at least one TGFbeta responsive element far downstream of the transcriptional start site.