Activating transcription factor 4-mediated alanine-serine-cysteine transporter 2 promotes colorectal cancer proliferation

BACKGROUNDMetabolic reprogramming of glutamine (Gln) supports rapid proliferation of cancer cells.As a high-affinity Gln transporter, alanine-serine-cysteine transporter 2 (ASCT2) mediates Gln uptake and couples Gln efflux with leucine (Leu) uptake via the SLC7A5/SLC3A2 (L-type amino acid transporter 1) complex, thereby activating the mechanistic target of rapamycin complex 1 (mTORC1)-S6 kinase 1 signaling axis to regulate cell growth and protein translation.However, the precise mechanisms linking ASCT2 to mTORC1 activation remain incompletely defined in colorectal cancer (CRC).Activating transcription factor 4 (ATF4) facilitates amino acid utilization by upregulating adaptive genes.Our previous studies showed that URB1 promotes ATF4 expression under mTORC1-activated conditions.On this basis, we investigated whether ATF4 feedback-regulates ASCT2 in CRC, a mechanism that, if operative, may constitute a proposed ATF4-ASCT2-mTORC1 positive feedback loop. AIMTo determine whether ATF4 feedback-regulates ASCT2 expression in CRC and to explore the functional linkage among ATF4, ASCT2, and mTORC1 signaling. METHODSQuantitative reverse transcription PCR and immunohistochemistry revealed significantly higher expression levels of ASCT2, L-type amino acid transporter 1, and ATF4 in tumor tissues than in matched normal tissues. RESULTSCorrelation analyses showed significant associations among these factors in CRC.Functional assays and tumorigenesis experiments in nude mice demonstrated that Gln deprivation and ASCT2 knockdown significantly inhibited CRC cell growth in vitro and in vivo.Metabolite analyses in HT29 cells further showed that Gln