Transforming growth factor b1 (TGF-b1) belongs to a family of multifunctional modulatory proteins involved in cell growth, differentiation, development, and wound healing. Although the biological activities of TGF-b1 have been extensively studied, its regulation remains obscure. Here we report the effects of insulin-like growth factor-1 (IGF-1) on the expression of TGF-b1 by dermal fibroblasts and suggest a possible mechanism. An enzyme-linked immunosorbent assay (ELISA) specific for TGF-b revealed a greater than twofold increase (12.3 { 1.6 vs. 4.8 { 0.8 pg/10 4 cells, n Å 7, P õ 0.05) in the protein in conditioned medium obtained from IGF-1-treated cells compared to that from untreated controls. Similar results were obtained by the mink lung epithelial cell growth inhibition assay. The results of Northern analysis revealed a dosedependent increase in TGF-b1 mRNA in response to IGF-1 treatment. Using the optimum concentration of IGF-1 (100 ng/ml), a greater than twofold increase (25.43 { 5.7 vs. 12.13 { 4.5, P õ 0.05) in TGF-b1 mRNA was observed. This effect persisted for at least 48 h after IGF-1 was removed from the culture medium. Nuclear run-on assay showed that this stimulation was due, at least in part, to an increase in the rate of transcription of the TGF-b1 gene. Treatment of human dermal fibroblasts with IGF-1 caused a substantial increase in c-fos and c-jun mRNA expression within 30 and 60 min, respectively. In contrast to c-jun mRNA which was constitutively expressed by dermal fibroblasts, the expression of c-fos mRNA was transient and only detectable between 15 and 60 min. Greater than 58% of the increase in TGF-b1 caused by IGF-1 could be blocked by the addition of anti-TGF-b1 neutralizing antibody to the culture medium, suggesting that autoinduction of TGF-b1 may be involved. An increase in IGF-1-induced TGF-b1 should be important in many different physiological processes such as cellular proliferation, differentiation, and wound healing. These findings also suggest that induction of TGF-b1 mRNA and protein by IGF-1 may be a mechanism by which this cytokine is regulated in physiological and/or pathological conditions.