Smad4 effects on LAMB3 and LAMC2 promoter activities are conferred through AP1 and Sp1 sites. Normalized and averaged LAMB3 and LAMC2 promoter activity obtained from three experiments in Smad4-negative, Smad4-reexpressing and TGFβ treated Smad4-reexpressing SW480 colon carcinoma cells (a). Mutation of the promoter-proximal AP1 site reduced basal promoter as well as Smad4-dependent constitutive and TGFβ induced LAMB3 promoter activity (b). Mutation of the 3.5 kb AP1 site inactivated TGFβ responsiveness but did not alter constitutive expression levels (c). When both mutations (d) were combined with mutation of the Sp1 site (e) the Smad4 effect on the LAMB3 promoter activity was completely abolished (f). For the LAMC2 promoter a 0.8 kb fragment displayed increased constitutive activities in Smad4-positive cells and mediated Smad4-dependent TGFβ induction (a'). Both AP1 sites are involved in the Smad4 effect on constitutive and TGFβ induced expression levels (b'-d'). When combined with the mutated Sp1 site (e') all promoter activities were significantly suppressed (f').