Increased androgen receptor activity and altered c-myc expression in prostate cancer cells after long-term androgen deprivation.
Proliferation of LNCaP 104-S cells, a clonal subline of the human prostate cancer cell line, was very slow in androgen-depleted medium but increased 10-13-fold in the presence of 0.1 nM of a synthetic androgen, R1881. This induction of proliferation was diminished at higher concentrations of R1881, indicating the biphasic nature of the androgen effect. After 20-30 passages in androgen-depleted medium, these cells progressed to 104-I cells, which exhibited much lower proliferative sensitivity to 0.1 nM R1881. After another 20-30 passages, LNCaP 104-I cells gave rise to 104-R cells, which proliferated rapidly without additional androgen. Proliferation of 104-R cells was induced 2-fold by 0.01 nM R1881 but was repressed by 0.1 nM R1881 and above. Thus, androgen induction and repression of proliferation could be seen at lower concentrations of androgen as the cells progressed. During the transition of 104-S cells to 104-R cells, the androgen receptor mRNA level increased 2.5-fold whereas the androgen receptor protein level increased 15-fold in the absence of androgen. Androgen receptor transcriptional activity, measured by androgen induction of prostate-specific antigen mRNA and chloramphenicol acetyltransferase activity in transfected cells, increased up to 20-fold during the progression. LNCaP cells, therefore, appear to be able to adapt to reduced androgen availability by increasing their sensitivity to androgen, raising questions concerning the therapeutic strategies used against prostate cancer. Androgen induction of c-myc expression in 104-R cells occurred at a 10-fold lower concentration (0.01 nM) than in 104-S cells (0.1 nM). In all stages, cell proliferation and c-myc expression were repressed by androgen at a high concentration (20 nM), but the repression of cell proliferation was blocked by retroviral overexpression of c-myc.