mTORC1 Hyperactivity Inhibits Serum Deprivation-Induced Apoptosis via Increased Hexokinase II and GLUT1 Expression, Sustained Mcl-1 Expression, and Glycogen Synthase Kinase 3β Inhibition

Abstract

The current concept is that Tsc-deficient cells are sensitized to apoptosis due to the inhibition of Akt activity by the negative feedback mechanism induced by the hyperactive mTORC1. Unexpectedly, however, we found that Tsc1/2-deficient cells exhibit increased resistance to serum deprivation-induced apoptosis. mTORC1 hyperactivity contributes to the apoptotic resistance of serum-deprived Tsc1/2-deficient cells in part by increasing the growth factor-independent expression of hexokinase II (HKII) and GLUT1. mTORC1-mediated increase in hypoxiainducible factor 1␣ (HIF1␣) abundance, which occurs in the absence of serum in normoxic Tsc2-deficient cells, contributes to these changes. Increased HIF1␣ abundance in these cells is attributed to both an increased level and the sustained translation of HIF1␣ mRNA. Sustained glycogen synthase kinase 3␤ inhibition and Mcl-1 expression also contribute to the apoptotic resistance of Tsc2-deficient cells to serum deprivation. The inhibition of mTORC1 activity by either rapamycin or Raptor knockdown cannot resensitize these cells to serum deprivation-induced apoptosis because of elevated Akt activity that is an indirect consequence of mTORC1 inhibition. However, the increased HIF1␣ abundance and the maintenance of Mcl-1 protein expression in serum-deprived Tsc2 ؊/؊ cells are dependent largely on the hyperactive eIF4E in these cells. Consistently, the reduction of eIF4E levels abrogates the resistance of Tsc2 ؊/؊ cells to serum deprivation-induced apoptosis.

Stress Responses Affecting Homeostasis of the Alveolar Capillary Unit

“…These effects were related to the mTOR inhibitory activity of RTP801 (and are in accord with the effects of rapamycin in the lung) because a RTP801 mutant protein (variant RPAA), unable to bind to TSC-2 and displace the 14-3-3 adaptor (22), did not cause the lung pathologies seen with the wild-type construct (17). These findings confirm the report that blockade of mTOR, such as that achieved by rapamycin or RTP801 overexpression, can be proinflammatory (23)(24)(25). In line with these findings, HIF-1a, an inducer of RTP801 expression, activates NF-kB under conditions of hypoxia (26).…”

Hexokinase II integrates energy metabolism and cellular protection: Akting on mitochondria and TORCing to autophagy

“…Insulin treatment increases HK-II mRNA and protein in various cell types and the increase is blocked by inhibition of PI3K, an upstream kinase of Akt, as well as inhibition of mTORC1, suggesting Akt/mTORC1 contribution 44,[46][47][48][49][50][51] (Figure 2). Hyperactivity of mTORC1 is sufficient to increase HK-II expression 52 and a recent comprehensive and unbiased analysis also supports mTORC1-mediated HK-II upregulation and further demonstrated that mTORC1 signaling activates the genes encoding nearly every step of glycolysis. 53 Hypoxia-inducible factor 1 (HIF-1) is a transcription factor.…”

“…The α-subunit of HIF-1 (HIF-1α) is stabilized under hypoxic conditions, leading to the activation of a transcriptional program to adapt to the lack of oxygen. The HK-II promoter has a consensus motif for HIF-1 54,55 and HK-II expression is enhanced by hypoxia, [54][55][56][57] providing cellular protection 52,56 as well as a mechanism for elevated glycolysis in tumors. [58][59][60]61 HIF-1α expression is also under the control of the PI3K/Akt/ mTORC1 pathway, 53,[62][63][64][65] linking Akt/mTOR activation and HK-II upregulation.…”

“…Indeed HIF-1α expression is demonstrated to be responsible for mTORC1-induced HK-II upregulation. 52 microRNAs (miRNAs) are small, non-coding RNAs and post-transcriptional inhibitory regulators of gene expression. miR-143, a tumor suppressor miRNA, which is downregulated in many tumors, 66 targets HK-II mRNA.…”

Mnk earmarks eIF4E for cancer therapy

“…Notably, tuberous sclerosis complex (TSC)-deficient cells are resistant to cell death in the absence of growth factors, in part because of the high levels of Mcl-1 due to mTORC1 and eIF4E activation in these cells. Silencing eIF4E in these cells induces high levels of cell death (13). Thus, pharmacological inhibition of Mnks could be an attractive therapeutic approach for tuberous sclerosis complex.…”