Breast Cancer Recycles Toxic Waste to Restock Nitrogen for Growth

Scientists may have hit upon a new therapeutic strategy against breast cancer with the finding that breast tumor cells recycle the ammonia that is generated as a byproduct of normal cell metabolism and use the toxic waste as a source of nitrogen to fuel their growth. The studies, by researchers at Harvard Medical School (HMS) and the Broad Institute of MIT and Harvard, showed that while cultured breast cancer cells proliferated and thrived in the presence of ammonia, inhibiting ammonia metabolism slowed breast cancer cell proliferation in vitro and held back tumor growth in experimental mice.

“Classically, ammonia was thought to be metabolic waste that must be cleared due to its high toxicity,” commented Marcia Haigis, Ph.D., associate professor of cell biology at HMS, who headed the research. “We found that not only was ammonia not toxic for breast cancer cells, it could be used to feed tumors by serving as a source for the building blocks that tumors need to grow.”

Jessica Spinelli, first author of the team’s published paper in Science (“Metabolic Recycling of Ammonia via Glutamate Dehydrogenase Supports Breast Cancer Biomass”) added, “we found that repressing ammonia metabolism stunts tumor growth in mice. Therefore, inhibition of ammonia assimilation or ammonia production may be rational strategies for therapy design.

Ammonia is a byproduct of cellular metabolism, which is removed from the cells and carried away in blood vessels to the liver, from where it cleared through the urea cycle. Rapidly growing and proliferating cells, such as cancer cells, need a ready supply of nutrients, but they also generate an excess of metabolic waste, including ammonia, which is exported into the environment surrounding the cells. Tumors are poorly vascularized, however, which means that ammonia tends to build up in the tumor microenvironment.

The researchers showed that rather than being adversely affected by the presence of high levels of ammonia, cultured breast cancer cells exhibited increased growth rates, doubling seven hours faster than cells grown without ammonia. Exposing 3D cultures to ammonia also promoted cell proliferation and led to 50% increases in the cell-surface area of clusters, when compared with control cultures.

Ammonia is produced from the breakdown of glutamine and asparagine. To investigate how tumors handle high levels of ammonia, the team labeled the nitrogen on glutamine and followed the fate of the labeled ammonia that was generated when glutamine was metabolized by the cells.

As they traced the labeled ammonia, the team analyzed more than 200 cellular metabolites in breast cancer cells and in human xenografts transplanted into mice. They discovered that cancer cells efficiently recycled the ammonia produced and incorporated it primarily into the amino acid glutamate and its derivatives. The findings indicated that around 20% of the cellular glutamate pool contained recycled nitrogen.

Further studies in mice bearing human breast tumors showed that the ammonia that accumulated in the tumor microenvironment was used directly by the cancer cells for amino acid synthesis.

Ammonia assimilation by tumor cells in vitro and in vivo required the enzyme glutamate dehydrogenase (GDH). Further in vitro studies showed that GDH-depleted cancer cells weren’t able to benefit from the otherwise growth-boosting effects of exposure to ammonia, and the depletion of GDH in xenografts decreased tumor growth in experimental mice.

The ability to reassimilate ammonia into metabolic pathways is essential for cancer cells, the authors suggest. “Since ammonia transport is mediated by diffusion, elevated ammonia in the microenvironment leads to its accumulation inside of tumor cells. Therefore, the ability to reassimilate this ammonia into metabolic pathways is critical in this context,” they stress. “Ammonia is not simply a metabolic waste product, and it can be recycled to support the high demand for amino acid synthesis in rapidly proliferating cells.”

The researchers’ results indicate that that it may be worth re-evaluating the biological role of ammonia, and the potential to use nutrient deprivation as a means to blocking tumor growth. Haigis’ team is now investigating ammonia metabolism as a possible therapeutic avenue.