Hierarchical phosphorylation of HOXB13 by mTOR dictates its activity and oncogenic function in prostate cancer [RNAseq_HOXB13]

Dysregulation of mTOR signaling plays a critical role in promoting prostate cancer (PCa) growth. HOXB13, a homeodomain transcription factor, is known to influence the androgen response and PCa development. Recently, HOXB13 was found to complex with mTOR on chromatin. However, the functional crosstalk between HOXB13 and mTOR remains elusive. We now report that mTOR directly interacts with and hierarchically phosphorylates HOXB13 at threonine 8 and 41 then serine 31 to promote its destabilization by the E3 ligase SKP2 while enhancing its oncogenic properties. Expression of HOXB13 harboring phosphomimetic mutations at the mTOR-targeted sites stimulates PCa cellular growth both in vitro and in murine xenografts. Transcriptional profiling studies revealed a phospho-HOXB13-dependent gene signature capable of robustly discriminating between normal prostate tissues, primary and metastatic PCa samples. This work uncovers a previously unanticipated molecular cascade by which mTOR directly phosphorylates HOXB13 to dictate a specific gene program with oncogenic implications in PCa. Triplicate RNA-seq experiments in LNCaP cells with shRNA-mediated knockdown of HOXB13 rescued with either WT HOXB13 , HOXB13 3A phospho mutant (T8, T41 and S31 mutations to alanine) or empty vector (EV) conrol treated with 10nM R1881 or vehicle (EtOH) control for 24h.