Androgen receptor as a licensing factor for DNA replication in androgen-sensitive prostate cancer cells

Androgen receptor (AR) protein expression and function are critical for survival and proliferation of androgen-sensitive (AS) prostate cancer cells. Besides its ability to function as a transcription factor, experimental observations suggest that AIR becomes a licensing factor for DNA replication in AS prostate cancer cells and thus must be degraded during each cell cycle in these cells to allow reinitiation of DNA replication in the next cell cycle. This possibility was tested by using the AS human prostate cancer cell lines, LNCaP, CWR22Rv1, and LAPC-4. These studies demonstrated that AIR levels fluctuate both within and between various phases of the cell cycle in each of these AS lines. Consistent with its licensing ability, AIR is degraded during mitosis via a proteasome-dependent pathway in these AS prostate cancer cells. In contrast, proteasome-dependent degradation of AIR during mitosis is not observed in AR-expressing but androgen-insensitive human prostate stromal cells, in which AIR does not function as a licensing factor for DNA replication. To evaluate mitotic degradation of AIR in vivo, the same series of human AS prostate cancers growing as xenografts in nude mice and malignant tissues obtained directly from prostate cancer patients were evaluated by dual Ki-67 and AIR immunohistochemistry for AR expression in mitosis. These results document that AIR is also down-regulated during mitosis in vivo. Thus, AS prostate cancer cells do not express AR protein during mitosis, either in vitro or in vivo, consistent with AIR functioning as a licensing factor for DNA replication in AS prostate cancer cells.