Switch from antagonist to agonist of the androgen receptor blocker bicalutamide is associated with prostate tumour progression in a new model system

Abstract

Advanced prostate cancer is treated by androgen ablation and/or androgen receptor (AR) antagonists. In order to investigate the mechanisms relevant to the development of therapy-resistant tumours, we established a new tumour model which closely resembles the situation in patients who receive androgen ablation therapy. Androgen-sensitive LNCaP cells were kept in androgen-depleted medium for 87 passages. The new LNCaP cell subline established in this manner, LNCaP-abl, displayed a hypersensitive biphasic proliferative response to androgen until passage 75. Maximal proliferation of LNCaP-abl cells was achieved at 0.001 n M of the synthetic androgen methyltrienolone (R1881), whereas 0.01 n M of this compound induced the same effect in parental cells. At later passages (> 75), androgen exerted an inhibitory effect on growth of LNCaP-abl cells. The non-steroidal anti-androgen bicalutamide stimulated proliferation of LNCaP-abl cells. AR protein expression in LNCaP-abl cells increased approximately fourfold. The basal AR transcriptional activity was 30-fold higher in LNCaP-abl than in LNCaP cells. R1881 stimulated reporter gene activity in LNCaP-abl cells even at 0.01 n M , whereas 0.1 n M of R1881 was needed for induction of the same level of reporter gene activity in LNCaP cells. Bicalutamide that acts as a pure antagonist in parental LNCaP cells showed agonistic effects on AR transactivation activity in LNCaP-abl cells and was not able to block the effects of androgen in these cells. The non-steroidal AR blocker hydroxyflutamide exerted stimulatory effects on AR activity in both LNCaP and LNCaP-abl cells; however, the induction of reporter gene activity by hydroxyflutamide was 2.4- to 4-fold higher in the LNCaP-abl subline. The changes in AR activity were associated neither with a new alteration in AR cDNA sequence nor with amplification of the AR gene. Growth of LNCaP-abl xenografts in nude mice was stimulated by bicalutamide and repressed by testosterone. In conclusion, our results show for the first time that the non-steroidal anti-androgen bicalutamide acquires agonistic properties during long-term androgen ablation. These findings may have repercussions on the natural course of prostate cancer with androgen deprivation and on strategies of therapeutic intervention. © 1999 Cancer Research Campaign

Risks and Benefits of Hormonal Manipulation as Monotherapy or Adjuvant Treatment in Localised Prostate Cancer

“…This conversion from antagonist to agonist was associated with alterations in the recruitment of coactivators and corepressors to the promoters of AR target genes. These findings have also been reported by Culig and co-workers [17][18][19]. A recent publication has reported on the x-ray crystal structure of a mutant on the AR ligand binding domain which confers agonist activity to bicalutamide [20].…”

Inhibition of LNCaP prostate cancer cells by means of androgen receptor antisense oligonucleotides

“…Another promising result was the observation that the long-term androgen-ablated LNCaP subline, LNCaPabl, which was recently established in our lab, 14 was significantly growth inhibited by antisense AR treatment. Although these cells express increased levels of AR, are hypersensitive to androgen, and have a higher AR transactivation activity, they respond to antisense AR treatment with significant growth retardation in comparison with mock-treated controls.…”

“…14 The AR protein level is increased ϳ3-fold in these cells. They respond in a hypersensitive manner to androgen stimulation and, in contrast to parental LNCaP cells, grow in castrated mice.…”

“…In contrast to parental LNCaP cells, LNCaP-abl cells also grow in castrated nude mice. 14 We tested the efficacy of AR antisense treatment in LNCaP-abl cells, as these cells are a model for prostate cancer after long-term androgen ablation.…”

“…12 When long-term androgen ablation therapy is simulated in the LNCaP tumor model, these cells also up-regulate the AR at the mRNA and protein level and become hypersensitive to small concentrations of androgens. 13,14 Another important fact is the occurrence of AR gene mutations that can result in a promiscuous receptor with a broad hormone binding and transactivation spectrum. 15,16 Activation of mutated AR has been shown for steroid hormones such as estrogens, progestins, adrenal androgens, and dihydrotestosterone metabolites, and even for the nonsteroidal anti-androgens flutamide and nilutamide.…”