Cancer Caught in the Act

Glucose Deprivation Contributes to the Development of KRAS Pathway Mutations in Tumor Cells

Yun, et. al. Science 325: 1555-1559 (2009).

Out of approximately 20,000 human genes, why are the usual suspects (p53, Rb, RAS, BRAF, etc.) the ones that are most commonly mutated in cancer? What selective pressure drives their mutation in so many different types of tumors? In a recent Science paper, Yun, et. al. provide evidence that KRAS and BRAF mutations can arise in tumor cell lines grown in a low glucose environment, and that constitutive KRAS- or BRAF-dependent upregulation of glucose transporter 1 (GLUT1) is specifically required for the survival of mutant cells in such inhospitable conditions. Acquisition of such a trait would be especially useful in a blood-starved, oxygen-deprived tumor, as it would allievate the cells' dependence on oxygen for aerobic ATP production. Alternatively, this could also potentially provide a mechanism for a phenomenon observed in many cancer cells known as the Warburg Effect, where cells rely on increased glucose uptake and glycolysis to generate ATP even if oxygen is available ("aerobic glycolysis"). This is an inefficient way to generate ATP, though it has been proposed that this benefits rapidly proliferating cancer cells because glucose can instead go towards building the amino acids, fatty acids, and nucleotides required for new cells.

KRAS and BRAF mutations are never found in the same tumor cell, so Yun, et. al. hypothesized that a common set of genes would be deregulated following mutation of either gene, pointing to the signaling pathways that conspire to give mutant cells a selective growth advantage. Yun, et. al. first compare the expression profiles of several tumor cell lines with mutant KRAS or BRAF to that of otherwise identical cell lines whose mutant KRAS or BRAF allele has been corrected. (Tumor cell lines have frequently acquired many mutations, but the authors try to negate any potential effects of non-KRAS or BRAF mutations by using multiple pairs of cell lines that should be exactly the same except for a single allele of a single gene, or isogenic.) The only gene of interest commonly upregulated in all the mutant cell lines was GLUT1, or glucose transporter 1. The authors show that GLUT1 overexpression is specific to tumor cell lines with mutant KRAS or BRAF, and that KRAS and BRAF mutant cell lines exhibit GLUT1-dependent increased glucose uptake and lactate production consistent with an increased rate of glycolysis. This characteristic of cells with KRAS and BRAF mutations may also have therapeutic implications, as the authors shows a glycolysis inhibitor is selectively toxic to cells harboring mutant KRAS or BRAF alleles.

If GLUT1 overexpression contributes to increased glucose uptake and glycolysis, does its overexpression permit cells to survive in low-glucose environments? The authors show that this is indeed the case, first by demonstrating that cell lines with mutant KRAS or BRAF (and therefore increased GLUT1 expression) could outgrow cell lines with wild-type alleles in low-glucose conditions. Second, the authors grew cell lines with wild-type KRAS and BRAF alleles for multiple generations in low-glucose conditions, and found that over 75% of the surviving cells had obtained stable overexpression of GLUT1 that persisted even after cells were returned to normal conditions. Even more striking was their observation that 4.4% of the wild-type KRAS cells had obtained KRAS mutations, compared to zero when the wild-type KRAS cell lines were grown in normal glucose conditions.

My only complaint with this paper is the title, I think it overstates their findings, as glucose deprivation can cause acquisition of a KRAS mutation but only in 4.4% of surviving tumor cells, and fails to mention their major finding, that KRAS and BRAF mutant tumor cell lines commonly overexpress GLUT1, and have characteristic GLUT1-dependent increases in glucose uptake and rate of glycolysis. GLUT1 was such an intriguing hit, not only because of its obvious links to glucose metabolism, but because its overexpression had previously been shown in several types of cancer and associated with poor prognosis (the KRAS and BRAF status of these tumors isn't mentioned, however). This suggests that a broad spectrum of tumors commonly acquire the trait of increased glycolysis in response to glucose-poor growth conditions by upregulating GLUT1, either via mutation of KRAS, BRAF, or some as yet unidentified gene, or by mutation of GLUT1 itself. Using a clever, unbiased, isogenic tumor cell line system, the authors of this paper caught cancer cells in the act of acquiring the traits needed to survive in harsh environments.

Other references:
Vander Heiden, et. al. Understanding the Warburg Effect: The Metabolic Requirements of Cell Proliferation. Science 324 (5930): 1029-1033 (2009).