Characterization of type I 5α-reductase activity in DU145 human prostatic adenocarcinoma cells

The conversion of testosterone (T) to dihydrotestosterone (DHT) has been demonstrated to be catalysed by at least two isoforms of human steroid 5α-reductase, designated types I and II. Type II 5α-reductase expression predominates in human accessory sex tissues, localized to the fibromuscular stromal compartment. The type I isoform predominates in skin, prostatic epithelia and, to a lesser extent, in prostatic fibromuscular stroma. The significance of the type I isoform to prostatic cellular growth and function remains undefined. In cultured DU145 cells, we evaluated the metabolism of [¹⁴C]-T and demonstrated the time-dependent formation of [¹⁴C]-DHT. Oxidative metabolism (conversion of [¹⁴C]-T to [¹⁴C]-androstenedione) and the formation of conjugated androgen metabolites occurred at a relatively low rate in the DU145 cells. Using human type I 5α-reductase cDNA, Northern blot analysis of DU145 cell mRNA revealed high levels of type I isoform expression. Analogous probing of the DU145 cells with a human 5α-reductase II cDNA failed to reveal expression of the type II isoform. The expression of functional type I activity has been confirmed pharmacologically using isoform-selective 5α-reductase inhibitors. Reductive metabolism of [³H]-T in the DU145 cells was inhibited in a concentration-dependent manner by LY306089, a potent non-steroidal type I-selective inhibitor (IC₅₀ = 10.0 nM). SKF105657, a steroidal type II-specific inhibitor was distinctly less active at inhibiting [³H]-DHT formation. LY306089 was a non-competitive inhibitor of type I 5α-reductase in DU145 cellular homogenates with an apparent K(i) value of 4.0 nM. These studies have identified and pharmacologically defined type I 5α-reductase activity in an androgen-insensitive prostatic cancer cell line and provide the basis for additional investigations into the significance of type I 5α-reductase to human prostatic pathophysiology.