Mutations in the PI3K/PTEN/TSC2 Pathway Contribute to Mammalian Target of Rapamycin Activity and Increased Translation under Hypoxic Conditions

Abstract

Decreased oxygen causes a rapid inhibition of mRNA translation. An important regulatory mechanism of translational repression under hypoxic conditions involves inhibition of the mammalian target of rapamycin (mTOR). mTOR is a target of the phosphatase and tensin homologue detected on chromosome 10 (PTEN)/phosphatidylinositol 3-kinase/AKT/ TSC2 pathway, a pathway that is frequently mutated in human cancers. Although hypoxia has been shown to inhibit mTOR activity, we show here that the hypoxia-induced inhibition of mTOR activity is attenuated in cells lacking TSC2 or PTEN, resulting in a higher translation rate even under hypoxic conditions. Comparison of mTOR inhibition by hypoxia alone or in combination with rapamycin showed that prolonged exposure to hypoxia was required to fully inhibit mTOR activity even in wild-type cells. Increased mTOR activity and protein synthesis did not translate into enhanced cell proliferation rates. However, lack of TSC2 resulted in a survival advantage when cells were exposed to hypoxia. Protection against hypoxia-induced cell death due to TSC2 deficiency is rapamycin-resistant, suggesting that TSC2 affects an apoptotic pathway. Tumors derived from TSC2 wild-type cells exhibited a growth delay compared with TSC2-deficient tumors, indicating that enhanced mTOR activity is advantageous in the initial phase of tumor growth. Therefore, failure to inhibit mTOR under oxygen-limiting conditions can be affected by upstream activating mutations and increases the survival and growth of hypoxic tumor cells.

Akt1 Activation Can Augment Hypoxia-Inducible Factor-1α Expression by Increasing Protein Translation through a Mammalian Target of Rapamycin–Independent Pathway

“…Decreased phosphorylation of components of the mTOR signaling pathway, including S6, p70 S6K , 4E-BP1, and mTOR itself, in response to hypoxia has been shown by others (14, 46, 47). Kaper et al have recently proposed that loss of PTEN leads to activation of the PI3K/Akt/mTOR pathway and attenuates the hypoxia-induced inhibition of mTOR activity (48). This may be true, but we found that U87MG and PC3 cells, both of which have PTEN mutation, still showed significant decrease in S6 phosphorylation in response to hypoxia.…”

“Translating” Tumor Hypoxia: Unfolded Protein Response (UPR)–Dependent and UPR-Independent Pathways

“…They found that moderate hypoxia failed to inhibit mTOR or translation, which was interpreted as an indication that translational control during moderate hypoxia may be lost during cellular transformation. This finding is also supported by a recent publication by Kaper et al who showed that inhibition of mTOR under severe hypoxia (<0.02%) may also be cell type specific (78). Mutations in the phosphatidylinositol 3-kinase/PTEN/TSC2 pathway seemed to affect the ability of severe hypoxia to inhibit mTOR.…”

“…As discussed earlier, two recent reports suggest that cancer cells may display a differential ability to inhibit mTOR under hypoxia and to inhibit translation (30,78). This may be explained by the fact that several genes that participate in signaling pathways upstream of mTOR are frequently altered in cancer.…”

Stress and mTORture signaling

“…Accordingly, lack of TSC function in low-oxygen conditions results in higher translation rates, increased proliferation and/or increased survival (Brugarolas et al, 2004;Kaper et al, 2006;Liu et al, 2006). The hypoxia-inducible RTP801/REDD1/2 proteins and their Drosophila homologs Scylla and Charybdis have been identified as negative growth modulators upstream of TSC1/2.…”

“…Most likely, mTOR activity is not completely inhibited in most of the hypoxia studies performed, warranting residual mTOR activity sufficient to impact on HIF-a (Kaper et al, 2006). The beneficial effects of mTOR inhibitors may be partly related to the negative effects on HIF-1 and its associated downstream signaling.…”