Almut Schulze

The role of the PI3K/Akt pathway in cell growth and transformation

The Ras signalling pathway is frequently activated in human cancer. Two of the major downstream effector pathways of Ras involve the Raf kinases and the lipid kinase phosphatidyl-inositol 3-kinase (PI3K). Activation of PI3K leads to the generation of phosphatidyl-inositol 3-phosphate (PIP3) and subsequent activation of protein kinase B/Akt, a serine/threonine kinase implicated in cell proliferation and survival as well as metabolic control ¹.

Akt has numerous cellular targets including the FOXO transcription factors that are involved in cell proliferation, apoptosis and stress response ^{2, 3}. Akt also activates mTORC1, a multi-protein complex that senses the availability of nutrients and regulates protein synthesis and cell growth downstream of the PI3K/Akt pathway ⁴.

We have shown that Akt activates the transcription factor SREBP (sterol regulatory element binding protein) by inducing nuclear accumulation of its active form. SREBPs regulate expression of genes involved in fatty acid and cholesterol biosynthesis. We have shown that Akt induces expression of fatty acids synthase (FAS) the rate limiting enzyme in fatty acid biosynthesis through activation of SREBP ⁵.

Many cancer cells rely on glycolysis as a main source of ATP production, even under conditions in which oxygen is not limiting. This phenotype, termed "aerobic glycolysis" or "Warburg effect" has been identified as a feature of cancer cells ^{6, 7}. Increased glycolysis allows faster energy production, albeit less efficient, and increases hypoxia tolerance of tumour cells. In addition, glucose uptake and increased glycolysis provides cells with metabolites for the biosynthesis of macromolecules, which are required for growth and proliferation.

Several activated oncogenes including Ras induce a metabolic phenotype reminiscent of the Warburg effect and the importance of metabolic adaptations for cancer cell survival is increasingly recognised ⁸. Furthermore, many tumours show increased expression of FAS and enhanced fatty acid synthesis ⁹ and FAS has been described as a "metabolic oncogene" in prostate cancer ¹⁰. However, the role of FAS in cancer development is not fully understood.

The aim of the proposed project is to increase our understanding of the role of lipid and cholesterol biosynthesis in cell growth and transformation. We plan to use RNAi strategies and/or chemical inhibitors to interfere with expression/activity of components of metabolic pathways and their regulators to investigate their involvement in cell growth, proliferation, cell transformation and survival. We have already identified a number of enzymes that show specific importance in different human cancer cells. We plan to analyse these enzymes in greater detail and study their potential regulation by Akt as well as their de-regulation in human cancer.

The proposed project will involve a number of molecular biology techniques including tissue culture, siRNA transfection, stable silencing using retroviral delivery of short hairpin constructs, determination of cell mass and apoptosis, analysis of cell volume, gene expression analysis as well as cell transformation assays. It is also planned to perform metabolic profiling of cancer cell lines and to analyse expression of specific genes in human cancer tissue by in situ hybridisation and/or immunohistochemistry.

The results of this study will increase our understanding of the regulation of metabolic processes by cellular signalling pathways and their involvement in the regulation of cell growth and transformation.

References

1. Testa JR and Tsichlis PN. AKT signaling in normal and malignant cells. Oncogene 2005; 24: 7391-7393.
2. Manning BD and Cantley LC. AKT/PKB signaling: navigating downstream. Cell 2007; 129: 1261-1274.
3. van der Horst A and Burgering BM. Stressing the role of FoxO proteins in lifespan and disease. Nat Rev Mol Cell Biol 2007; 8: 440-450.
4. Sarbassov DD, Ali SM and Sabatini DM. Growing roles for the mTOR pathway. Curr Opin Cell Biol 2005; 17: 596-603.
5. Porstmann T, et al. PKB/Akt induces transcription of enzymes involved in cholesterol and fatty acid biosynthesis via activation of SREBP. Oncogene 2005; 24: 6465-6481.
6. Bui T and Thompson CB. Cancer's sweet tooth. Cancer Cell 2006; 9: 419-420.
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8. Swinnen JV, et al. Overexpression of fatty acid synthase is an early and common event in the development of prostate cancer. Int J Cancer2002; 98: 19-22.
9. Baron A, Migita T, Tang D and Loda M. Fatty acid synthase: a metabolic oncogene in prostate cancer? J Cell Biochem 2004; 91: 47-53.