The transcription factor ATF4 regulates the expression of genes involved in amino acid metabolism, redox homeostasis and ER stress responses, and it is overexpressed in human solid tumours, suggesting that it has an important function in tumour progression. Here, we report that inhibition of ATF4 expression blocked proliferation and survival of transformed cells, despite an initial activation of cytoprotective macroautophagy. Knockdown of ATF4 significantly reduced the levels of asparagine synthetase (ASNS) and overexpression of ASNS or supplementation of asparagine in trans, reversed the proliferation block and increased survival in ATF4 knockdown cells. Both amino acid and glucose deprivation, stresses found in solid tumours, activated the upstream eukaryotic initiation factor 2a (eIF2a) kinase GCN2 to upregulate ATF4 target genes involved in amino acid synthesis and transport. GCN2 activation/overexpression and increased phospho-eIF2a were observed in human and mouse tumours compared with normal tissues and abrogation of ATF4 or GCN2 expression significantly inhibited tumour growth in vivo. We conclude that the GCN2-eIF2a-ATF4 pathway is critical for maintaining metabolic homeostasis in tumour cells, making it a novel and attractive target for anti-tumour approaches.

Two hallmarks of clear cell renal cell carcinoma (ccRCC) are constitutive hypoxia inducible factor (HIF) signaling and abundant intracellular lipid droplets (LDs). However, regulation of lipid storage and its role in ccRCC are incompletely understood. Transcriptional profiling of primary ccRCC samples revealed that expression of the LD coat protein gene PLIN2 was elevated in tumors and correlated with HIF-2α, but not HIF-1α, activation. HIF-2α dependent PLIN2 expression promoted lipid storage, proliferation, and viability in xenograft tumors. Mechanistically, lipid storage maintained integrity of the endoplasmic reticulum (ER), which is functionally and physically associated with LDs. Specifically, PLIN2 dependent lipid storage suppressed cytotoxic ER stress responses that otherwise result from elevated protein synthetic activity characteristic of ccRCC cells. Thus, in addition to promoting ccRCC proliferation and anabolic metabolism, HIF-2α modulates lipid storage to sustain ER homeostasis, particularly under conditions of nutrient and oxygen limitation, thereby promoting tumor cell survival.