The androgen receptor—lncRNA SAT1- AKT-p15 axis mediates androgen-induced cellular senescence in prostate cancer cells

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 p15 INK4b and increased phosphorylation of AKT. Treatment with the AKT inhibitor (AKTi) potently inhibited SAL-induced expression of p15 INK4b 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 lncRNA SAT1 . SAL treatment of native patient tumor samples ex vivo upregulates lncRNA SAT1 . In PCa tumor tissues, lncRNA SAT1 is downregulated compared with nontumor tissues of the same patients. Knockdown indicates that the lncRNA SAT1 is crucial for SAL-induced cancer-cell senescence as an upstream factor for pAKT and for p15 INK4b . Further, knockdown of lncRNA SAT1 enhances cell proliferation by SAL, suggesting that lncRNA SAT1 serves as a tumor suppressor at SAL. Interestingly, immunoprecipitation of AR detected lncRNA SAT1 as an AR-interacting partner that regulates AR target-gene expression. Similarly, RNA-ChIP experiments revealed the interaction of AR with lncRNA SAT1 on chromatin. Thus, we identified a novel AR-lncRNA SAT1 -AKT-p15 INK4b signaling axis to mediate SAL-induced cellular senescence.