We have compared the influence of insulin-like growth factor 1 (IGF-1) on pituitary gene expression in the rat cell lines GH4C1 and GH3. Incubation with IGF-1 increased PRL messenger RNA (mRNA) levels in GH4C1 cells by 4- to 5-fold but decreased the levels of PRL transcripts in GH3 cells. In addition, the levels of GH-mRNA that were not affected by IGF-1 in GH4C1 cells were significantly inhibited by the growth factor in GH3 cells. IGF-1 also decreased PRL and GH-mRNA response to T3, retinoic acid, and Fk in GH3 cells. Stability of PRL or GH transcripts was not altered by IGF-1 in GH3 cells, suggesting that the inhibitory effect is exerted at a transcriptional level. The pituitary-specific transcription factor GHF-1/Pit-1 activates both the GH and PRL promoters. As analyzed by Western blot, IGF-1 did not alter GHF-1/Pit-1 protein levels in GH4C1 cells but reduced the levels of the transcription factor in GH3 cells. This decrease is secondary to a reduction of GHF-1/Pit-1 transcripts in IGF-1-treated GH3 cells. Thus, a different effect of IGF-1 on the expression of GHF-1/Pit-1 in GH3 and GH4C1 cells is likely involved in the different regulation of GH and PRL gene in both cell types. IGF-1 increases the activity of the PRL promoter in transient transfection assays in GH4C1 cells by a Ras-dependent mechanism. Expression of oncogenic Ras(Val12) mimics the effect of IGF-1, and the dominant negative Ras(Asn17) blocks IGF-1-mediated stimulation of the PRL promoter in GH4C1 cells. Although IGF-1 did not stimulate the PRL promoter in GH3 cells, Ras(Val12) strongly activated the promoter in these cells. Hence, the machinery to activate Ras-dependent signaling is intact in GH3 cells. Moreover, IGF-1 stimulates the mitogen-activated protein kinase in GH3 cells, showing that the components linking the IGF-1 receptor to Ras are also active. These results suggest that, in addition to the Ras/mitogen-activated protein kinase pathway, IGF-1 could activate a different pathway and that the combination of both is required to elicit PRL gene expression by the growth factor. This second pathway may be defective in GH3 cells that respond to Ras but not to IGF-1.