Targeting GCN2 Regulation of Amino Acid Homeostasis in Prostate Cancer

Abstract

The Integrated Stress Response (ISR) plays a critical role in the adaptation and survival of tumor cells to exogenous and endogenous stresses. The ISR features four protein kinases (PERK, GCN2, PKR, and HRI), each activated by different stresses, that phosphorylate the eukaryotic translation initiation factor eIF2, resulting in repression of global protein synthesis. Paradoxically, eIF2 phosphorylation also enhances translation of select gene transcripts, including the transcription factor ATF4, which is central for ISR‐directed gene transcription. Therefore, the ISR directs translational and transcriptional control that is critical for cancer stress adaptation. Moreover, eIF2 phosphorylation and ATF4 have recently been suggested to play a role in prostate cancer (PCa) growth and survival; however, the specific function of ISR kinases, their mode of activation, and the mechanisms by which the ISR facilitate PCa progression are unknown.We discovered that GCN2 is activated in a range of PCa cell lines, contributing to enhanced eIF2 phosphorylation and ATF4 expression. Genetic or pharmacological inhibition of GCN2 reduces growth in androgen‐sensitive and castration‐resistant PCa cell lines in culture and cell line‐derived and patient‐derived xenograft mouse models in vivo. Induction of GCN2 is accompanied by limitations of select amino acids and accumulation of cognate tRNAs that are reported to be activators of GCN2.A transcriptome analysis of PCa cells treated with a specific GCN2 small molecular inhibitor indicates that GCN2 is critical for expression of SLCgenes involved in metabolite transport. We found that GCN2 inhibition decreases intracellular amino acid levels accounting for reduced growth in PCa cells. Using CRISPR‐based phenotypic screens and genome‐wide gene expression analyses of wild‐type and GCN2‐depleted PCa cells, we confirmed the importance of the transporter genes in PCa fitness. One transporter, SLC3A2 (4F2), is induced by GCN2 and is essential for PCa proliferation. SLC3A2 engages with many nutrient transporters, allowing for their localization to the plasma membrane. Importantly, expression of SLC3A2 reduced GCN2 activation and rescued decreased amino acid levels and growth inhibition due to loss of GCN2.Our results indicate that select amino acid limitations activate GCN2 in PCa, resulting in the upregulation of key amino acid transporters, including 4F2 (SLC3A2), which provide for nutrient import to facilitate protein synthesis and metabolism required for PCa progression. We conclude that GCN2 and the ISR are promising therapeutic targets for both androgen‐sensitive and castration‐resistant prostate cancer.