Study Reveals How Production of Enzyme with Key Role in Cancer Is Regulated

glutaminase
Tumor cells have a high metabolic rate, exemplified here by the location of glutaminase C within an extensive web of mitochondria (in green). Mitochondria are "furnaces" that produce energy for cells. In blue, cell nuclei (credit: LNBio/CNPEM)

In an article published by the journal Molecular Cell, researchers at the National Bioscience Laboratory (LNBio) in Brazil describe molecular mechanisms that regulate production of the enzyme glutaminase C (GAC), which plays a key role in the metabolism of an essential “fuel” for rapidly proliferating tumor cells.

According to the authors, the discovery opens windows for novel targeted therapies against cancer.

The investigation was conducted by LNBio’s Tumor Metabolism Research Group in partnership with scientists affiliated with the University of Texas MD Anderson Cancer Institute in the United States. It was supported by FAPESP through regular grants awarded to Sandra Dias and Andre Ambrosio, as well as a doctoral research scholarship awarded to Douglas Meira.

“Glutaminase converts the amino acid glutamine into glutamate. This chemical reaction enables cells to use glutamine for fuel, just as they do with glucose, to produce energy and synthesize amino acids, nucleic acids, and other macromolecules cells need,” Ambrosio said. “Because tumor cells proliferate relentlessly, they need to double their biomass all the time, so it’s essential for them to consume glutamine.”

Various isoforms of glutaminase can be found in humans, Ambrosio explained. In addition to GAC, they include kidney-type glutaminase (KGA) and liver-type glutaminase (LGA). Their amino acid chains are slightly different, but they all act as catalysts for the same chemical reaction.

“Data from the literature show the isoform LGA has a role in the transmission of nervous stimuli in the brain, and KGA is important to kidney detox by removing ammonia,” Dias said. “The isoform GAC is mostly associated with the growth of tumors, so understanding how its expression is regulated has a direct impact on therapy.”

The discovery that GAC is the most important glutaminase isoform for supplying the metabolic needs of cancer cells was made by LNBio’s Tumor Metabolism Research Group and published in the journal Proceedings of the National Academy of Sciences of the United States of America (PNAS) in 2012.

“In that previous study, we showed that GAC displays far more significant catalytic activity than the other isoforms and that its expression is augmented in tumor cells,” Ambrosio said. “However, the reasons for this were unknown. In other words, more research was needed to find out how the reprogramming of cell metabolism favors overexpression of this enzyme in cancer.”

After the PNAS paper was published, the researchers at LNBio were contacted by George Calin, a physician and geneticist at the MD Anderson Cancer Institute. Calin’s research on the molecular mechanisms involved in the development of colorectal cancer had led him to the discovery that overexpression of a non-coding RNA gene called CCAT2 was common in this type of tumor (non-coding RNA does not contain information for protein synthesis).

“More specifically, he’d observed that an allelic variant of CCAT2, with a guanine (G) base instead of thymine (T), was associated with a higher risk of colorectal cancer,” Dias said. “He also noted that replacing this nucleotide led to metabolic alterations in the cell and raised the hypothesis that this non-coding RNA could be a regulator of glutaminase. That’s why he contacted us.”

Collaboration with seven countries

The meeting resulted in a collaboration involving researchers in five countries besides Brazil and the US – Croatia, Italy, Romania, Spain, and the Netherlands. Supervised by Calin and Dias, the group used several techniques to show that the RNA gene CCAT2 with the allelic variant interacts more strongly with a protein complex known as CFIm.

“These CFIm proteins act on the RNA transcribed by the gene that codes for the glutaminases GAC and KGA, and through a process we call alternative splicing they are capable of defining which of the two isoforms will be produced by the cell,” Dias said.

The results of the study prove, according to Dias, that the allelic variant of CCAT2 induces an increase in the expression of GAC to the detriment of KGA, favoring augmented consumption of glutamine by tumor cells and leading to faster proliferation and metastasis.

“Based on this knowledge, we can think of ways to inhibit expression of GAC,” she concluded. “Various studies have shown that GAC is a promising target. Right now, a compound is being clinically tested in Phase I trials for different types of tumors.”

The article “Allele-Specific Reprogramming of Cancer Metabolism by the Long Non-Coding RNA CCAT2” (doi: 10.1016/j.molcel.2016.01.015) can be read at sciencedirect.com/science/article/pii/S1097276516000162.