meso-dihydroguaiaretic acid (DGA), naturally occurring in plants such as Machilus thunbergii and Myristica fragrans, exhibits a neuroprotective effect and also exerts cytotoxicity to certain cancer cells. Activated hepatic stellate cells (HSCs) play an important role in liver fibrogenesis through the production of transforming growth factor beta1 (TGF-beta1) after injuries. TGF-beta1 mediates the deposition of extracellular matrix and the inhibition of collagenase activity in the liver. This study has investigated the inhibitory effect of DGA on the activation of rat HSCs in culture and TGF-beta1 production from HSCs. The level of alpha-smooth muscle actin (alpha-SMA), a representative marker of stellate cell transdifferentiation, was decreased upon treatment of activated HSCs with DGA (1 - 10 microM). Immunoblot analysis revealed that DGA inhibited the expression of TGF-beta1 in activated HSCs. Consistently, DGA down-regulated the transactivation of the TGF-beta1 promoter linked to the luciferase reporter gene in HSCs. Promoter deletion analysis revealed that the region located between -731 bp and -323 bp in the TGF-beta1 promoter, which is comprised of AP-1 response elements, conferred the inhibition of TGF-beta1 expression by DGA. DGA also inhibited AP-1-mediated gene transactivation in HSCs to a comparable extent, indicating that down-regulation of the TGFbeta1 gene by DGA might result from its inhibition of AP-1 activity. We found in addition that DGA inhibited DNA synthesis in HSCs stimulated by platelet-derived growth factor. The data provide evidence that DGA directly inhibits activation of HSCs and down-regulates TGF-beta1 gene expression through inhibition of AP-1 activity.