Novel Insights into Hypoxia Induced AKT Signaling

Hypoxia is a common feature of most tumors and is a product of rapid cell growth and poor vascularization¹. When oxygen availability is low in the tumor environment, the hypoxia inducing transcription factors (HIFs) regulate a variety of signaling programs that can affect the balance between tumor cell apoptosis² and autophagy³.  In normoxia, HIFs are bound by the von Hippel-Lindau protein (VHL) in the cytosol where it is degraded by the proteasome, however, under hypoxia HIFs are translocated to the nucleus where they activate survival signals. Additionally, HIF mediated signaling can increase the metastatic capabilities of tumor cells and facilitate the pro-metastatic phenotypes of epithelial to mesenchymal transition (EMT), alter tumor cell metabolism, increase vascularization and angiogenesis, as well as suppress immune reactivity¹. Together these HIF regulated pathways help cancer cells to thrive and activate pro-tumorigenic, pro-metastatic, and pro-survival signals.  Therefore, targeting this pathway could provide clinical benefits, however, efficient targeted therapies have remained elusive.

Receptor tyrosine kinases and AKT signaling have been heavily implicated in hypoxia driven signaling cascades and recently, Chae et al discovered a novel link between mitochondrial AKT and hypoxia mediated reprograming⁴. After profiling the mitochondrial phosphoproteome of cancer cell lines, a distinct AKT signature was differentially regulated in hypoxic conditions compared to normoxia.  With the use of 1D proteomics and liquid chromatography mass-spectrometry the authors identified Thr346 of pyruvate dehydrogenase kinase (PDK1) as a direct target of AKT under hypoxia. While over 84 AKT substrates were differently expressed in hypoxia including the mitochondrial proteins, UGP2 and SLC2A1, as well as the extracellular matrix remodeler P4HA1, and the metabolic regulator Acot9, the authors focused on PDK1 due to its known prominence in the tumor hypoxia response.

Immunohistochemistry-Paraffin: PDK-1 Antibody (4A11) [NBP2-22171] - Immunohistochemical analysis of paraffin-embedded breast cancer tissues (left) and brain tissues (right) using PDK1 mouse mAb with DAB staining.

The authors went on to show that a pool of AKT is recruited to the mitochondrial membrane where it can phosphorylate mitochondrial PDK1. Interestingly, they show this mechanism is independent of HIF1α, an important finding as PDK1 is a direct target of HIF1α and mediates the hypoxia induced metabolic switch⁵.

The mitochondrial AKT-PDK1 axis is maintained in-vivo where it facilitates tumor metabolic reprograming towards glycolysis and maintains cell proliferation when oxygen is restricted.  This pool of AKT antagonizes hypoxia induced apoptosis and autophagy, assayed by caspase 3 cleavage and LC3B-II cleavage, respectively. 

Importantly these findings translate to the clinic, where AKT mediated phosphorylation of PDK1 in the mitochondria increases with tumor severity in a cohort of 116 glioma patients. The dramatic decrease in overall survival observed in patients with elevated phospho-PDK1 levels suggests that therapies targeting the mitochondrial AKT-PDK1 axis may be efficacious in the clinic. As their study was limited to one tumor type, their data does not address the possibility that this mechanism could be context dependent. This is especially relevant given that targeted therapies against this pathway are currently moving forward in clinical trials.  Nonetheless, this study is one of the first to illuminate the role of the AKT pathway in promoting tumor growth in hypoxic conditions.

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By Christina Towers, PhD

References

1. Wilson, W. R. & Hay, M. P. Targeting hypoxia in cancer therapy. Nature reviews. Cancer 11, 393-410, doi:10.1038/nrc3064 (2011).
2. Erler, J. T. et al. Hypoxia-mediated down-regulation of Bid and Bax in tumors occurs via hypoxia-inducible factor 1-dependent and -independent mechanisms and contributes to drug resistance. Mol Cell Biol 24, 2875-2889 (2004).
3. Rouschop, K. M. et al. The unfolded protein response protects human tumor cells during hypoxia through regulation of the autophagy genes MAP1LC3B and ATG5. J Clin Invest 120, 127-141, doi:10.1172/JCI40027 (2010).
4. Chae, Y. C. et al. Mitochondrial Akt Regulation of Hypoxic Tumor Reprogramming. Cancer Cell 30, 257-272, doi:10.1016/j.ccell.2016.07.004 (2016).
5. A novel gene containing a trinucleotide repeat that is expanded and unstable on Huntington's disease chromosomes. The Huntington's Disease Collaborative Research Group. Cell 72, 971-983 (1993).