One pathological feature of chronic cyclosporin A (Cs-A) nephrotoxicity is striped interstitial fibrosis, the pathogenesis of which is not completely understood. We have previously reported that Cs-A directly stimulates the synthesis of various collagens in a cultured murine proximal tubular cell line (MCT cells). In addition, we have recently observed that Cs-A stimulates the synthesis of transforming growth factor beta (TGF-beta) in MCT cells. To this end, we investigated whether Cs-A directly stimulates transcription of TGF-beta in this cell line. A single dose of 2,000 ng/ml Cs-A stimulates the expression of TGF-beta1 transcripts in MCT cells. This stimulation is abrogated in the presence of 150 and 250 ng/ml actinomycin D without significantly influencing basal mRNA expression, suggesting that the increase in mRNA is due to stimulated transcriptional activity. Furthermore, nuclear runoff experiments demonstrate that Cs-A directly stimulates the transcriptional activity of the TGF-beta1 gene in MCT cells. The results of transient transfection of reporter gene constructs containing regulatory elements of the murine TGF-beta1 promoter into MCT cells and subsequent treatment with Cs-A further suggest direct effects of Cs-A on TGF-beta1 gene transcription. This is the first report demonstrating Cs-A-mediated TGF-beta1 gene transcription in renal cells. Considering the well-known profibrogenic effects of TGF-beta, our results suggest a novel mechanism of Cs-A nephrotoxicity.