Nuclear mTOR acts as a transcriptional integrator of the androgen signaling pathway in prostate cancer

Abstract

Androgen receptor (AR) signaling reprograms cellular metabolism to support prostate cancer (PCa) growth and survival. Another key regulator of cellular metabolism is mTOR, a kinase found in diverse protein complexes and cellular localizations, including the nucleus. However, whether nuclear mTOR plays a role in PCa progression and participates in direct transcriptional cross-talk with the AR is unknown. Here, via the intersection of gene expression, genomic, and metabolic studies, we reveal the existence of a nuclear mTOR-AR transcriptional axis integral to the metabolic rewiring of PCa cells. Androgens reprogram mTOR-chromatin associations in an AR-dependent manner in which activation of mTOR-dependent metabolic gene networks is essential for androgeninduced aerobic glycolysis and mitochondrial respiration. In models of castration-resistant PCa cells, mTOR was capable of transcriptionally regulating metabolic gene programs in the absence of androgens, highlighting a potential novel castration resistance mechanism to sustain cell metabolism even without a functional AR. Remarkably, we demonstrate that increased mTOR nuclear localization is indicative of poor prognosis in patients, with the highest levels detected in castration-resistant PCa tumors and metastases. Identification of a functional mTOR targeted multigene signature robustly discriminates between normal prostate tissues, primary tumors, and hormone refractory metastatic samples but is also predictive of cancer recurrence. This study thus underscores a paradigm shift from AR to nuclear mTOR as being the master transcriptional regulator of metabolism in PCa.

Canonical signaling and nuclear activity of mTOR—a teamwork effort to regulate metabolism and cell growth

“…) . Interestingly, treatment of prostate cancer cells with the inhibitor of Pol II transcriptional activity α‐amanitin, which has no effect on classic cytoplasmic mTOR signaling, abrogated the metabolic reprogramming associated with the transcriptional function of nmTOR observed in these cells . The recruitment of mTOR at Pol II‐dependent promoters near transcription start sites observed in ChIP‐sequencing experiments suggests that mTOR could directly interact with and influence basic transcription machinery.…”

“…Far more reaching, analyses of ChIP‐sequencing datasets revealed that mTOR directly binds to thousands of regulatory regions of Pol II‐transcribed genes in both mouse liver and human prostate cancer cells, including but certainly not restricted to ribosomal protein‐encoding genes (Fig. ) . Interestingly, treatment of prostate cancer cells with the inhibitor of Pol II transcriptional activity α‐amanitin, which has no effect on classic cytoplasmic mTOR signaling, abrogated the metabolic reprogramming associated with the transcriptional function of nmTOR observed in these cells .…”

“…Common names for the gene products are shown for each target gene. Data were obtained either from mouse liver (left panels) or human LNC aP prostate cancer cells (right panels).…”

“…Indeed, evidence has been accumulating for almost two decades that mTOR and some components of mTORC1 and mTORC2 complexes such as Raptor, Rictor, RHEB, SIN1, and S6K are indeed found in the nucleus , and more recently that nmTOR directly controls gene transcription. The physical presence and/or activity of mTOR in the nucleus has been reported by numerous groups using a wide range of techniques in organisms ranging from yeast to human , including primary tissues such as mouse liver and human tumors of various origins . In particular, the recent genome‐wide localization of nmTOR at specific loci by chromatin immunoprecipitation coupled with deep sequencing (ChIP‐seq) assays , the subcellular localization of mTOR‐tagged enhanced green fluorescent protein (EGFP) using lifetime imaging microscopy , and the dynamic visualization of mTORC1 activity in living cells using a fluorescence resonance energy transfer (FRET)‐based activity biosensor , have provided not only further evidence of mTOR nuclear localization but solidified the notion that nmTOR is an authentic participant of the mTOR signaling pathway.…”

“…It is now well documented that manipulating mTOR signaling, whether by genetic or pharmacological means, influences the expression of sizable gene networks involved in cell growth and proliferation, most particularly metabolic genes . Gene regulation by mTOR signaling could be occurring via two major mechanisms: either indirectly by modulating the epigenome as well as the activity of transcription factors, including the three RNA polymerases, or directly by nmTOR acting as a transcription factor or cofactor on chromatin.…”