Aberrant activation of the androgen receptor (AR) may be one of the mechanisms which contribute to progression of prostatic carcinoma to an androgen-independent stage. We investigated effects of growth factors on stimulation of the AR-mediated gene transcription in human prostatic tumor cell lines. DU-145 cells, which do not contain endogenous AR, were cotransfected with an androgen-inducible chloramphenicol acetyltransferase (CAT) reporter gene and an AR expression vector. The reporter gene (CAT) was driven either by artificial promoters consisting of one or two androgen-responsive elements in front of a TATA box or by the promoter of the prostate-specific antigen (PSA) gene, a naturally occurring androgen-inducible promoter. Insulin-like growth factor-I (IGF-I), at a concentration of 50 ng/ml, stimulated AR-mediated reporter gene transcription to the same extent as the synthetic androgen methyltrienolone. This growth factor was effective irrespective of the nature of the androgen-inducible promoter. Keratinocyte growth factor (KGF) and epidermal growth factor (EGF), at concentrations of 50 ng/ml, activated CAT reporter gene transcription only in experiments in which the artificial promoter with two androgen-responsive elements was used. Insulin-like growth factor-II and basic fibroblast growth factor displayed no effect on AR-mediated gene transcription. None of the growth factors stimulated reporter gene activity in control experiments when added to cells cotransfected with the CAT gene and an empty expression vector. AR activation by IGF-I, KGF, and EGF was completely inhibited by the pure AR antagonist casodex, showing that these effects are AR mediated. Activation of endogenous AR by growth factors was studied in the LNCaP cell line by determination of PSA secretion. IGF-I, at a concentration of 50 ng/ml, increased the PSA level in the supernatant of this cell line 5-fold. Again, the IGF-I effect on PSA secretion was blocked by casodex. Our results provide evidence that IGF-I, KGF, and EGF directly activate the AR in the absence of androgens, which means that the androgen-signaling chain may be activated by growth factors in an androgen-depleted environment. These findings may have implications for endocrine therapy for metastatic prostatic carcinoma.