Browsing by Subject "Glucose"

Background

Methods and findings

Conclusions

Normal cells require input from extrinsic growth factors to control proliferation and survival. Recent studies have demonstrated that these same extrinsic signals also regulate cellular metabolism to ensure that metabolism adequately supports the demands of cell function, proliferation, and cell survival. The PI3K/Akt pathway is downstream of many growth factors and can promote both glucose metabolism and cell survival. Aberrant activation of the PI3K/Akt pathway is common in cancer, and its activation can contribute to the growth factor independence that is a hallmark of neoplastic cells. Metabolic demand is high in stimulated and leukemic T cells, and activation of Akt can increase glucose metabolism to meet these requirements. There is great interest in targeting the unique metabolism of cancer cells for cancer therapy, thus making an understanding of the interaction of metabolism and cell death essential.

Akt is also anti-apoptotic and can inhibit cell death by regulating members of the Bcl-2 family. Interestingly, the ability of Akt to prevent cell death is inextricably linked to its metabolic function. Several recent studies have demonstrated that glucose metabolism can affect Bcl-2 to family members to promote cell survival, but the role of Akt-dependent glucose metabolism in the regulation of Bcl-2 family members is not understood. Using a model of growth factor withdrawal-induced apoptosis, we show that Akt prevents cell death by maintaining glucose metabolism to regulate the Bcl-2 family members Puma and Mcl-1, and demonstrate the importance of this pathway in the survival of stimulated T lymphocytes and leukemia.

After growth factor withdrawal, active Akt suppressed Puma induction in abundant glucose, but Puma was rapidly upregulated in glucose-deficient conditions and was necessary and sufficient to promote efficient cell death. Importantly, glucose was not uniquely required, as provision of alternative mitochondrial fuels allowed Akt to suppress Puma and maintain survival. This metabolic regulation of Puma was mediated through partially p53-dependent transcriptional induction as well as control of Puma protein stability.

In addition to inhibiting Puma expression, active Akt prevented the loss of Mcl-1 after growth factor withdrawal by sustaining Mcl-1 protein synthesis in a glucose-dependent manner. Mcl-1 was essential for preventing Bim-induced apoptosis, as Akt could not inhibit Bim induction after growth factor deprivation. Slowing of Mcl-1 synthesis by inhibiting glucose metabolism reversed Mcl-1-mediated resistance of leukemic cells to the Bcl-2 inhibitor ABT-737. Importantly, Akt and glucose-reliant Mcl-1 expression required mTOR-dependent phosphorylation of 4EBP, and treatment with mTOR inhibitors also reversed ABT-737 resistance.

Together, this study demonstrates that Akt promotes cell survival by preventing metabolic checkpoints that stimulate Puma expression and stability and inhibit Mcl-1 synthesis, advancing our understanding of the links between metabolism and cell death. These studies highlight the importance of cellular metabolism--including a potential role for the alternative sugar fructose--in cancer cell survival that may provide a mechanistic understanding to drive development of metabolism-targeted cancer therapies.

Healthy cells require input from growth factor signaling pathways to maintain cell metabolism and survival. Growth factor deprivation induces a loss of glucose metabolism that contributes to cell death in this context, and we have previously shown that maintenance of glycolysis after growth factor deprivation suppresses the activation of p53 and the induction of the pro-apoptotic protein Puma to prevent cell death. However, it has remained unclear how cell metabolism regulates p53 activation and whether this increased glycolysis promotes cell survival in the face of additional types of cell stress. To examine these questions, we have utilized a system in which stable overexpression of the glucose transporter Glut1 and hexokinase 1 in hematopoietic cells drives growth-factor independent glycolysis. This system allows us to examine the effects of glucose metabolism in the absence of other signaling events activated downstream of growth factor receptors. Here, we demonstrate that elevated glucose metabolism, characteristic of cancer cells, can suppress PKCδ-dependent p53 activation to maintain cell survival after growth factor withdrawal. In contrast, DNA damage-induced p53 activation was PKCδ-independent and was not metabolically sensitive. Both stresses required p53 serine 18 phosphorylation for maximal activity but led to unique patterns of p53 target gene expression, demonstrating distinct activation and response pathways for p53 that were differentially regulated by metabolism.

Unlike the growth factor-dependence of normal cells, cancer cells can maintain growth factor-independent glycolysis and survival and often demonstrate dramatically increased rates of glucose uptake and glycolysis, in part to meet the metabolic demands associated with cell proliferation. Given the ability of elevated glucose metabolism to suppress p53 activity in the context of metabolic stress, we examined the effect of increased glucose uptake on leukemogenesis using a mAkt-driven model of leukemia and adoptive transfer experiments. We show here that elevated glucose uptake promoted leukemogenesis in vivo, perhaps through suppression of p53 transcriptional activity. During the process of leukemogenesis, cancerous cells can acquire growth factor independent control over metabolism and survival through expression of oncogenic kinases, such as BCR-Abl. While targeted kinase inhibition can promote cancer cell death, therapeutic resistance develops frequently and further mechanistic understanding regarding these therapies is needed. Kinase inhibition targets the necessary survival signals within cancerous cells and may activate similar cell death pathways to those initiated by growth factor deprivation. As we have demonstrated that loss of metabolism promotes cell death after growth factor withdrawal, we investigated whether cell metabolism played a role in the induction of apoptosis after treatment of BCR-Abl-expressing cells with the tyrosine kinase inhibitor imatinib. Consistent with oncogenic kinases acting to replace growth factors, treatment of BCR-Abl-expressing cells with imatinib led to reduced metabolism and p53- and Puma-dependent cell death. Accordingly, maintenance of glucose uptake inhibited p53 activation and promoted imatinib resistance, while inhibition of glycolysis enhanced imatinib sensitivity in BCR-Abl-expressing cells with wild type p53 but had little effect on p53 null cells. Together, these data demonstrate that distinct pathways regulate p53 after DNA damage and metabolic stress and that inhibition of glucose metabolism may enhance the efficacy of and overcome resistance to targeted molecular cancer therapies.

Background

Results

Conclusions