Abstract B140: Profiling the metabolic dependencies of PIK3CA mutant cancers.
Authors: Foster Rebecca , Rigby Sue , Grooby Suzanne , Feltell Ruth , Christopherson Cindy , Chang Monica , Sninsky John , Kwok Shirley , Torrance Chris 
Published: June 2012

Targeting tumor metabolism is becoming a major new area of pharmaceutical endeavor. Consequently, a systematic search to define whether there are specific energy source dependencies in tumors, and how these might be dictated by upstream driving genetic mutations, is now required. This is particularly relevant given the potential ability of cancer cells to switch between metabolic pathways. The PI3K-Akt-mTOR signaling pathway has a seminal role in regulating diverse cellular processes including cell proliferation and survival. This pathway has also been associated with metabolic dysregulation, particularly gluconeogenesis, largely through effects on Akt and mTOR.

We have used isogenic cell models, wherein homologous recombination is used to specifically delete endogenous mutant PI3KCA alleles or introduce PIK3CA activating mutations in human cell lines. By performing rtPCR on candidate metabolic genes, we have found gene expression signatures indicative of a consistent up-regulation of glycolysis in PIK3CA mutant cells. Interestingly, the genes up- and down-regulated varied between isogenic models suggesting that the primary node of regulation is not the same between models. In MCF10A breast epithelial ‘knock-in’ lines, hexokinase 2 (HK2) is the primary target of up-regulation. However, in HCT116 colon cancer cells that are heterozygous for the H1047R PIK3CA mutation, knocking-out mutant PIK3CA resulted in a marked reduction of Glut1. Altered expression of asparagine synthetase (ASNS) and glutaminase (GLS2) was also observed in both models, suggestive of a potential increased dependency on glutaminolysis, while up-regulation of the fatty acid transporter SLC27A2 was observed in HCT116 PIK3CA mutant cells.

To determine whether any of these metabolic expression changes impart hard-wired nutrient dependencies, we next performed nutrient switching experiments. These demonstrated that growth of PIK3CA mutant cells is highly dependent on glucose, whereas glutamine dependency is independent of PIK3CA status. In addition, the glucose dependency exhibited by PIK3CA mutant cells could not be overridden by supplementation with other nutrients such as alternative monosaccharides, fatty acids or aspartic acid. The extent of glucose dependency of the PIK3CA mutant cells suggested that targeted disruption of the glycolytic pathway should inhibit growth, even in the presence of glucose. The use of hexokinase 2 siRNA to disrupt glycolysis did indeed result in anti-proliferative effects that were selective for the PIK3CA mutant cells.

In conclusion, we have demonstrated that glycolysis-associated genes are up-regulated in PIK3CA mutant cells and that the shift to a glycolytic phenotype does not appear to be controlled by a single rate limiting molecule. Despite suggestions that cancer cells are adept at utilizing alternative nutrient sources, this study has demonstrated that PIK3CA mutant cell are not able to compensate for glucose withdrawal. This specific dependence on glucose for growth, identified using precise isogenic models, was also present across a wider panel of cancer cell lines harboring endogenous PIK3CA mutations. Understanding the metabolic dependencies of PIK3CA mutant cancers will provide critical information for the design of effective therapies.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2011 Nov 12-16; San Francisco, CA. Philadelphia (PA): AACR; Mol Cancer Ther 2011;10(11 Suppl):Abstract nr B140.