Metabolic reprogramming in cancer cells by oncogenes and tumor suppressors

Tumor Metastasis
Highlights
  • Cancer cells show alterations in bioenergetics and biosynthetic pathways
  • p53 loss renders cancer cells unable to cope with serine deficiency
  • Oncogenic KRAS manipulates glucose and glutamine metabolism
  • Targeting metabolic pathways may hold promise for cancer therapy
Author: Allison Bierly, PhD
Art Direction: Ken Mattiuz
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Recent developments have extended our knowledge about metabolic changes in cancer cells. The tumor suppressor p53 directs a program that helps normal tumor cells cope with serine starvation, but p53-null tumors fail to make the switch, suggesting a potential therapeutic strategy. Kras, an oncogene with a well-appreciated role in pancreatic cancer, promotes enhanced glucose uptake and flux into glycosylation and ribose synthesis pathways, and also manipulates glutamine metabolism via gene expression changes. Understanding the metabolic changes that take place in cancer will be invaluable in identifying new targets for treatment.
Summary
Introduction
Metabolic reprogramming by p53
Metabolic reprogramming by Kras
Conclusions
Back to topSummary
Recent developments have extended our knowledge about metabolic changes in cancer cells. The tumor suppressor p53 directs a program that helps normal tumor cells cope with serine starvation, but p53-null tumors fail to make the switch, suggesting a potential therapeutic strategy. Kras, an oncogene with a well-appreciated role in pancreatic cancer, promotes enhanced glucose uptake and flux into glycosylation and ribose synthesis pathways, and also manipulates glutamine metabolism via gene expression changes. Understanding the metabolic changes that take place in cancer will be invaluable in identifying new targets for treatment.

Back to topIntroduction
Cells produce energy and building blocks to sustain growth and proliferation through several important metabolic pathways. Glycolysis generates pyruvate for conversion to acetyl-CoA to feed into the tricarboxylic acid (TCA) cycle as well as various intermediates for biosynthetic pathways, and glutaminolysis yields alpha-ketoglutarate for use in the TCA cycle. Oxidative phosphorylation in the mitochondria produces high levels of ATP, approximately 36 per glucose molecule, and is preferred by non-proliferating cells in aerobic conditions. The TCA cycle produces NADH and succinate for the electron transport chain and also generates intermediates for biosynthesis, such as citrate, which feeds into the fatty acid synthesis pathway. Biosynthetic pathways generate nucleotides, proteins, and lipids, all of which are required for production of new daughter cells during proliferation.

Metabolic processes are often altered to meet the demands of abnormally proliferating cancer cells. For example, the Warburg effect observes that cancer cells increase glucose uptake and the rate of glycolysis even under normoxic conditions, termed aerobic glycolysis (see figure, Changes in cancer metabolism and the Warburg effect). Recently, it has been suggested that making greater use of glycolysis helps cancer cells support proliferation due to the generation of glycolytic intermediates that feed into biosynthetic pathways necessary for producing new cells, such as nucleotide, amino acid, and lipid synthesis pathways, as well as NADPH generation to maintain redox balance (1-2).

Discovery and characterization of specific metabolic aberrations in cancer cells continues to progress, and changes in energy metabolism are considered an "emerging hallmark" of cancer (5). Several oncogenes and tumor suppressors are intimately involved in modulating glycolysis, glutamine metabolism, and various biosynthetic pathways. For example, the PI3K pathway promotes increased uptake of glucose (4), and oncogenic Kras also enhances glucose import and expression of key glycolytic enzymes (6). Both p53 and c-Myc have been implicated in regulation of glutamine via modulation of glutaminase expression (7).

Recently, c-Myc was also shown to increase proline biosynthesis from glutamine and inhibit its catabolic enzyme POX/PRODH via a mechanism involving miR-23b*, sidestepping the ROS-generating, pro-apoptotic effects of POX/PRODH activity (8). Recent studies continue to highlight the importance of metabolic manipulation in cancer cells, and how knowledge about bioenergetic and biosynthetic changes could be exploited to stop tumor cells in their tracks.

Back to topMetabolic reprogramming by p53                                                                                                        
p53, a tumor suppressor and transcription factor, responds to stress conditions by halting the cell cycle, initiating DNA repair, or causing cell death. Its effects on energy metabolism and biosynthesis are also substantial, including induction of SCO2, which is required for mitochondrial respiration, and TIGAR, which inhibits glycolysis and diverts glucose to the pentose phosphate pathway (1, 9). A newly described metabolic role for p53 in cancer cells has suggested an intriguing possibility for therapy against p53-null tumors. Maddocks et al. showed that HCT116 tumors in mice, and especially those lacking p53, showed impaired growth on removal of the non-essential amino acid serine from the diet. Since p53-null cells produced less serine than normal cells under serine starvation, the team deduced that p53 must be playing a role in helping cells adapt to low-serine conditions (10).

Further experiments bore this out, showing that tumor cells switch from aerobic glycolysis to the TCA cycle upon serine starvation, increasing oxygen consumption. However, p53-positive tumor cells manage to bring the glycolytic flux to the TCA cycle under control in 6-18 hours. They respond to serine-starvation-induced decreases in GMP by upregulating p21 and going into brief G1 arrest, reducing the number of cells in S-phase, for a period of about 48 hours. During this time, they cut off serine support to nucleotide synthesis, instead directing their remaining serine stores to replenishment of glutathione levels, which drop upon serine starvation. The antioxidant staves off damage from ROS generated by the brief upregulation in oxygen consumption. Therefore, by restoring GSH levels instead of maintaining nucleotide synthesis, p53 partially maintains cell proliferation in the face of serine starvation.
 
By contrast, p53-null cells never make this critical transition. Oxygen consumption remains high, G1 arrest is not efficient, and the cells continue pumping serine into the nucleotide synthesis pathway instead of generating more GSH. Rising ROS levels are not countered by GSH, therefore, and the p53-null cells fail to proliferate. The implications for therapy against p53-null tumors are exciting and potentially far-reaching, as p53 loss is a common feature of many different types of cancers. The authors caution that other genetic changes could certainly counteract sensitivity to serine depletion, however, so more work will need to be done to determine if targeting serine is a workable method to combat these tumors.

Back to topMetabolic reprogramming by Kras                                                                         
The Kras oncogene is mutated in 17-25% of cancers (11), and recent studies have revealed multiple roles for this oncogene in the regulation of cancer cell metabolism. In pancreatic ductal adenocarcinoma cells specifically, Kras directs several important metabolic effects that enhance the growth of these tumors, including effects on glucose uptake and flux, and alteration of glutamine catabolism. Both of the studies detailed below provide solid leads for investigators searching to find ways to control PDAC, a particularly lethal disease in dire need of new therapeutic options.

Regarding Kras's role in glucose metabolism and biosynthesis, Ying et al. demonstrated that in an inducible, tissue-specific model of Kras expression in PDAC tumors, Kras appeared to be involved in driving the expression of genes in several biosynthesis pathways, including genes involved in steroid and pyrimidine biosynthesis (6). Kras also promoted glycolytic flux at both the gene expression and protein level, as measured by glucose uptake, expression of glycolytic rate-limiting enzymes, presence of glycolytic intermediates, and lactate production. Additionally, Kras upregulated Gfpt1, the first enzyme in the hexosamine biosynthesis pathway (HBP), significantly affecting glycosylation.

Glucose, more abundant due to Kras activity, was directed into the non-oxidative arm of the pentose phosphate pathway. Flux into the non-oxidative PPP increased ribose-5-phosphate production for incorporation into DNA and RNA while leaving the oxidative arm of the pathway, which is involved in redox balance via NADPH generation, untouched. Blocking this pathway diminished Kras-dependent tumorigenesis, indicating that Kras's role in ribose biosynthesis is critical to maintaining PDAC tumors. Furthermore, knockdown experiments clarified the involved signaling pathways, showing that both MAPK and Myc are involved in Kras's effects on the HBP and PPP (6).

Kras is also substantially involved in manipulating glutamine metabolism in PDAC. Son et al. showed that oncogenic Kras modulates the expression of key genes to switch to a glutamine-utilizing transaminase pathway that is required by PDAC cells, but not normal cells, for growth and maintenance of redox balance. (12). Glutamine is required for PDAC growth, but while GLUD1 is the traditional metabolism pathway utilized by many other cancer cells, the team's results showed that PDAC cells needed both alpha-KG and NEAA to rescue growth upon glutamine deprivation, suggesting that PDAC cells used transaminases for glutamine metabolism instead. Indeed, knocking down the aspartate transaminase GOT1 inhibited PDAC growth and had substantial effects on levels of aspartate, oxaloacetate, and reduced glutathione.

Further characterization of this pathway revealed that GOT1 kicks off a series of reactions converting glutamine-derived aspartate to oxaloacetate (OAA) to malate to NADPH. Thus, GOT1's role in glutamine metabolism ultimately results in the generation of the primary cellular antioxidant, keeping reactive oxygen species under control. Oncogenic Kras directs this process by controlling gene expression of GLUD1, which it suppresses, and GOT1, which it upregulates, leading to the dominance of the GOT1 pathway in PDAC cells (12).

Back to topConclusions
Our ever-growing understanding of alterations in cancer cell metabolism is yielding a number of therapeutic possibilities. Some targets are well known and are already being addressed by clinical trials; for example, metformin, which is well-known as a diabetes drug and activator of AMPK, is being tested for use in cancer, as is the pyruvate mimetic dichloroacetate (13). Other potential targets are still emerging, such as the serine starvation pathway described above. As metabolic influences in cancer cells continue to be unveiled, more targets will no doubt be revealed.

Tracing metabolic changes in cancer will require comprehensive gene expression analysis of metabolism pathways, and assessment of mutations in tumor suppressors and oncogenes will also have a part to play in the story. QIAGEN provides tools to aid in this search, including metabolism-focused gene expression PCR arrays, cancer-focused targeted exon enrichment panels for next-generation sequencing, and PCR arrays for assessing somatic mutation and copy number alteration status.

Back to topReferences
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