Editors' ChoiceCellular Metabolism

How inhibition of TORC1 stimulates gene expression

Sci. Signal.  12 May 2015:
Vol. 8, Issue 376, pp. ec124
DOI: 10.1126/scisignal.aac5127

Under nutrient replete conditions, TORC1 (target of rapamycin complex 1) promotes cell growth by globally stimulating anabolic processes such as ribosome biogenesis, translational initiation, and synthesis of lipids and nucleotides. Inhibition of TORC1 represses the transcription of genes involved in these anabolic processes and stimulates the expression of genes predicted to help cells adapt to low-nutrient conditions. Tiebe et al. found that REPTOR (repressed by TOR) and REPTOR-BP (REPTOR binding partner) were required to induce the expression of most of the genes that were transcriptionally upregulated by rapamycin treatment of cultured Drosophila melanogaster S2 cells. Whereas REPTOR-BP was constitutively nuclear, REPTOR was mostly cytoplasmic under basal conditions and translocated to the nucleus upon inhibition of TORC1. REPTOR-BP bound to the promoters of endogenous target genes independently of TORC1 activity and recruited REPTOR to target genes upon TORC1 inhibition. REPTOR acted as a transcriptional transactivator independently of REPTOR-BP. A TORC1 component coimmunoprecipitated with REPTOR, and two serine residues in REPTOR were phosphorylated when TORC1 was active but rapidly dephosphorylated upon rapamycin treatment. Mutating these serine residues to alanine caused constitutive nuclear localization of REPTOR. Two 14-3-3 proteins coimmunoprecipitated with REPTOR from untreated S2 cells, but not from rapamycin-treated cells, and protein phosphatase 2A (PP2A) was required for the nuclear translocation of REPTOR. These observations suggest that 14-3-3 proteins mediate the retention of phosphorylated REPTOR in the cytoplasm when TORC1 is active and that PP2A-mediated dephosphorylation allows REPTOR to translocate into the nucleus upon TORC1 inhibition. Transcriptional responses induced by rapamycin treatment in vivo required REPTOR and REPTOR-BP. Knocking out REPTOR or REPTOR-BP increased mortality in response to starvation in both larvae and adult flies, and adult knockouts had reduced triacylglycerides and glycogen stores. Thus, TORC1 inhibition causes REPTOR and REPTOR-BP to cooperatively stimulate the expression of genes required to survive nutrient stress. Many REPTOR target genes also contained binding sites for FOXO, a transcription factor that stimulates gene expression in response to reduced insulin signaling, and REPTOR interacted genetically with FOXO, consistent with coordinate regulation of the physiological response to starvation by insulin and TORC1 signaling (see commentary by Stocker).

M. Tiebe, M. Lutz, A. De La Garza, T. Buechling, M. Boutros, A. A. Teleman, REPTOR and REPTOR-BP regulate organismal metabolism and transcription downstream of TORC1. Dev. Cell 33, 272–284 (2015).[PubMed]

H. Stocker, Stress relief downstream of TOR. Dev. Cell 33, 245–246 (2015). [PubMed]