Proliferating cell nuclear antigen (PCNA) expression is required for DNA replication. Because androgens are critical for prostate cell proliferation, we investigated the effects of androgen on PCNA expression in the prostatic cancer cell line LNCaP. Flow cytometric analysis was used to measure cellular DNA content with dual labeling of PCNA. Semiconfluent LNCaP cells were grown in serum-free medium containing varying concentrations of the synthetic androgen mibolerone and processed for either fluorescence-activated cell sorting or Western analysis. Supplementation of serum-free medium with androgens resulted in dose-dependent changes in PCNA immunoreactivity, with maximum stimulation (2-fold) being achieved at 48 h with 10-9m mibolerone. Non-androgenic steroids did not change PCNA immunoreactivity compared with untreated controls, and the antiandrogen, casodex, inhibited the mibolerone-stimulated increase in PCNA immunoreactivity, suggesting that the androgenic induction of PCNA is mediated through the androgen receptor. The presence of a non-consensus androgen response element in the promoter region of the PCNA gene led us to investigate whether androgen responsiveness of the PCNA gene in LNCaP cells might be mediated at the transcriptional level. No change in steady-state mRNA for PCNA with androgen administration was observed. However, an investigation of the androgenic regulation of PCNA protein stability indicated that androgen treatment increased the half-life of 35S-labeled PCNA protein. In addition, polysome run-off translation assays demonstrated an increase in PCNA protein after a 6-h stimulation of LNCaP cells with 10-9m mibolerone. These data suggest that androgen induction of prostate cell proliferation may be mediated, at least in part, through PCNA at the posttranscriptional level.