The bipolar
androgen therapy (BAT) to treat
prostate cancer (PCa) includes cycles of supraphysiological
androgen levels (SAL) under
androgen-deprivation
therapy (ADT). We showed previously that SAL induces cellular senescence in
androgen-sensitive PCa cells and in ex vivo-treated patient PCa
tumor samples. Here, we analyzed the underlying molecular pathway and reveal that SAL induces cellular senescence in both,
castration-sensitive (CSPC) LNCaP and
castration-resistant PCa (CRPC) C4-2 cells through the cell cycle inhibitor p15INK4b and increased phosphorylation of AKT. Treatment with the AKT inhibitor (AKTi) potently inhibited SAL-induced expression of p15INK4b and cellular senescence in both cell lines. Proximity-
ligation assays (PLA) combined with high-resolution
laser-scanning microscopy indicate that SAL promotes interaction of endogenous
androgen receptor (AR) with AKT in the cytoplasm as well as in the nucleus detectable after three days. Transcriptome sequencing (
RNA-seq) comparing the SAL-induced transcriptomes of LNCaP with C4-2 cells as well as with AKTi-treated cell transcriptomes revealed landscapes for cell senescence. Interestingly, one of the identified genes is the lncRNASAT1. SAL treatment of native patient
tumor samples ex vivo upregulates lncRNASAT1. In PCa
tumor tissues, lncRNASAT1 is downregulated compared with nontumor tissues of the same patients. Knockdown indicates that the lncRNASAT1 is crucial for SAL-induced
cancer-cell senescence as an upstream factor for pAKT and for p15INK4b. Further, knockdown of lncRNASAT1 enhances cell proliferation by SAL, suggesting that lncRNASAT1 serves as a
tumor suppressor at SAL. Interestingly, immunoprecipitation of AR detected lncRNASAT1 as an AR-interacting partner that regulates AR target-gene expression. Similarly,
RNA-ChIP experiments revealed the interaction of AR with lncRNASAT1 on
chromatin. Thus, we identified a novel AR-lncRNASAT1-AKT-p15INK4b signaling axis to mediate SAL-induced cellular senescence.