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Amino acid transporter ASCT2 expression regulates mammalian Target‐of‐Rapamycin (mTOR) growth and survival signaling in human hepatoma cells
Author(s) -
Onan Marie C.,
Fuchs Bryan C.,
Bode Barrie P.
Publication year - 2006
Publication title -
the faseb journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.709
H-Index - 277
eISSN - 1530-6860
pISSN - 0892-6638
DOI - 10.1096/fasebj.20.5.a841
Subject(s) - mtorc1 , pi3k/akt/mtor pathway , mtorc2 , gene silencing , protein kinase b , microbiology and biotechnology , phosphorylation , chemistry , signal transduction , small interfering rna , amino acid transporter , biology , cancer research , transfection , transporter , biochemistry , gene
Targeted silencing of amino acid transporter ASCT2 elicits apoptosis in SK‐Hep human hepatoma cells within 48h by mechanisms that transcend its role in amino acid delivery. To more closely investigate the mechanistic reliance of cancerous cells on this transporter, we examined early consequences of ASCT2 silencing on growth and survival signaling that presages apoptosis. Induced ASCT2 antisense RNA in SK‐Hep1 cells led to >90% decrease in ASCT2 mRNA expression by 6h, followed by inhibition of mTOR/Raptor (mTOR complex 1 (mTORC1)) signaling by 8h, as determined by diminished P70S6K1 and 4EBP1 phosphorylation and protein synthesis rates. Decreased mTORC1 signaling occurred prior to a detectable reduction in ASCT2 activity, but coincided with a 33% decline in total cellular ASCT2 protein levels. At 12h after ASCT2 silencing, a 50% decrease was observed in both transporter protein and activity levels, while mTOR/Rictor (mTOR complex 2(mTORC2)) and Akt survival signaling were enhanced, indexed by increased phosphorylation of Akt/PKB on serine 473 and of Bad on serine 136, respectively. Conversely, protein kinase C activation by phorbol esters diminished mTORC2 signaling and reduced plasma membrane localized ASCT2 protein, conceivably by enhanced endocytosis or decreased transporter trafficking to the membrane. Our findings suggest that expression of ASCT2, a transporter enhanced in several human cancers, is linked to mTOR signaling, establishing a mechanistic link between amino acid transporter expression and growth regulation. (Supported by 1R15CA108519‐01A1 from the National Cancer Institute to BPB)