Loss of TSC2 confers resistance to ceramide and nutrient deprivation

Abstract

Nutrient stress that produces quiescence and catabolism in normal cells is lethal to cancer cells because oncogenic mutations constitutively drive anabolism. One driver of biosynthesis in cancer cells is the mTORC1 signaling complex. Activating mTORC1 by deleting its negative regulator TSC2 leads to hypersensitivity to glucose deprivation. We have previously shown that ceramide kills cells in part by triggering nutrient transporter loss and restricting access to extracellular amino acids and glucose suggesting that TSC2-deficient cells would be hypersensitive to ceramide. However, murine embryonic fibroblasts (MEFs) lacking TSC2 were highly resistant to ceramide-induced death. Consistent with the observation that ceramide limits access to both amino acids and glucose, TSC2−/− MEFs also had a survival advantage when extracellular amino acids and glucose were both reduced. As TSC2−/− MEFs were resistant to nutrient stress despite sustained mTORC1 activity, we assessed whether mTORC1 signaling might be beneficial under these conditions. In low amino acid and glucose medium and following ceramide-induced nutrient transporter loss, elevated mTORC1 activity significantly enhanced the adaptive up-regulation of new transporter proteins for amino acids and glucose. Strikingly, the introduction of oncogenic Ras abrogated the survival advantage of TSC2−/− MEFs upon ceramide treatment most likely by increasing nutrient demand. These results suggest that, in the absence of oncogene-driven biosynthetic demand, mTORC1 dependent translation facilitates the adaptive cellular response to nutrient stress.

FTY720-induced endocytosis of yeast and human amino acid transporters is preceded by reduction of their inherent activity and TORC1 inhibition

“…The upstream mechanisms controlling LAT1-4F2hc endocytosis in response to FTY720 seem more complex than in yeast. A contribution of mTORC1 inhibition is likely, as murine embryonic fibroblasts (MEFs) show much less pronounced FTY720-elicited endocytosis of 4F2hc when mTORC1 is hyperactive 50 . Earlier studies, however, have shown that mTORC1 inhibition by rapamycin is not sufficient to induce 4F2hc endocytosis 51 , and we have similarly failed to detect significant LAT1 internalization in rapamycin-treated HeLa cells (data not shown).…”

Amino Acid Sensing by mTORC1: Intracellular Transporters Mark the Spot

“…Cells are often starved of amino acids and serum, and then specific amino acids are added back, or alternatively one amino acid is removed in the absence of serum ( Sancak et al., 2008 ). Other groups have used dialyzed serum to mirror physiological conditions more closely ( Averous et al., 2014 , Guenther et al., 2014 ). These types of starvation experiments highlight requirements for particular amino acids but do not clarify whether mTORC1 is sensing different levels of specific amino acids or merely responding to amino acid presence or absence.…”

Sphingolipids inhibit endosomal recycling of nutrient transporters by inactivating ARF6

“…Phytosphingosine starves heat-stressed yeast into growth arrest by triggering the internalization of several nutrient permeases, including the tryptophan transporters TAT1 and TAT2, the uracil transporter FUR4 and the general amino acid permease GAP1 (Bultynck et al, 2006;Chung et al, 2001;Dickson et al, 1997;Skrzypek et al, 1998;Welsch et al, 2004). In mammalian cells, ceramide produced in response to a variety of stresses antagonizes growth and proliferation, in part by downregulating nutrient transporter proteins and reducing amino acid and glucose import (Guenther et al, 2008(Guenther et al, , 2014Hannun and Obeid, 2008;Summers et al, 1998). Attributing cellular phenotypes to specific endogenous sphingolipids is complicated by the fact that a large number of enzymes rapidly metabolize sphingolipids into derivatives that can have different, and even opposing, effects on cells (Hannun and Obeid, 2008).…”