Mice genetically engineered to lack the ability to make the amino acid cysteine, and fed a cysteine-free diet, lost 30 percent of their body weight in just one week, a new study shows.
Published online May 21 in Nature, the work found that cysteine depletion disrupts the normal metabolic pathways used by mammalian cells to convert food into energy, forcing the animals to rapidly burn fat stores in a futile attempt to meet energy demands.
Led by researchers at NYU Grossman School of Medicine, the study reveals key details about how cells process fuels like carbohydrates and fats (metabolism), and how cysteine depletion affects tissues. Experiments showed that lowering cysteine levels caused a drop in levels of the small molecule called coenzyme A (CoA), which rendered inefficient mechanisms that convert carbohydrates and fats into energy.
Despite CoA being involved in more than 100 intermediate metabolic reactions and serving as a partner (cofactor) for 4% of all enzymes in the body, scientists had previously been unable to study its function directly. This is because mice with defective CoA synthesis typically do not survive beyond three weeks of age. The current findings detail, for the first time, how CoA shapes metabolism in adult mice.
"Our surprising findings reveal that low cysteine levels triggered rapid fat loss in our study mice by activating a network of interconnected biological pathways," said co-senior study author Evgeny Nudler, PhD , the Julie Wilson Anderson Professor in the Department of Biochemistry and Molecular Pharmacology at NYU Grossman School of Medicine, and an investigator with the Howard Hughes Medical Institute. "While driving weight loss in the clinic remains a key future mission, we are most excited for the moment about the profound, fundamental aspects of metabolism revealed in this study," added Dr. Nudler.
The current finding does not immediately suggest a new approach to weight loss, the authors caution, as cysteine is found in nearly all foods. Achieving a truly cysteine-free diet would require patients to consume a specially formulated solution that would be challenging for most. Moreover, because cysteine is involved in numerous cellular pathways, eliminating it – such as through a drug that inhibits cysteine production – could make organs more vulnerable to everyday toxins, including medications.
That said, the study authors say it is worth considering that fruits, vegetables, and legumes contain much lower levels of cysteine and its precursor, the sulfur-containing amino acid methionine, than red meat. While earlier studies have linked low sulfur amino acid intake to health benefits, this study clarifies that these benefits are due to cysteine depletion specifically, and not methionine restriction.
"Given that achieving maximum cysteine deprivation weight loss in the mice was dependent on both diet and deletion of the gene, moving forward we can now restore cysteine production genetically in specific cells or tissues and determine the role of each in the dramatic weight loss we observed," said co-senior author Dan L. Littman, MD, PhD , the Helen L. and Martin S. Kimmel Professor of Molecular Immunology in the Department of Pathology , and a professor in the Department of Cell Biology , at NYU Grossman School of Medicine. "We hope in the future to hijack parts of this process to induce a similar weight loss in humans but without completely removing cysteine," added Dr. Littman, who is also an investigator with the Howard Hughes Medical Institute.
Overlapping Mechanisms
The study is the first to examine the effects of removing cysteine, or any of the nine of the essential amino acids, which must be obtained through diet and are required for building proteins that make up most of the body's enzymes, tissues, and signaling molecules. The findings revealed that eliminating cysteine from the mammalian body led to far greater weight loss than the removal of any other essential amino acid.
Specifically, cysteine deprivation disrupted oxidative phosphorylation, the main process for producing adenosine triphosphate (ATP), the molecule that serves as cells' energy currency. Oxidative phosphorylation is known to be tightly dependent on CoA. As a result, sugar-derived intermediate molecules (carbon skeletons) such as pyruvate, orotate, citrate, and α-ketoglutarate were no longer used efficiently, and were instead lost in the urine. In response, the body turned to stored lipids (fats) to make energy.
Further, the team found that cysteine restriction activates both the integrated stress response (ISR), a signaling network that restores cellular balance after stress, and the oxidative stress response (OSR), which is triggered by higher levels of reactive oxygen species (ROS) following depletion of glutathione, the body's primary antioxidant. ROS can oxidize (take away electrons from) and damage sensitive cell parts like DNA.
Remarkably, this simultaneous activation of ISR and OSR—previously observed only in cancer cells—was shown to occur in normal tissues in mice in the cysteine-restriction group, with the two stress responses reinforcing each other. The study also shows that ISR and OSR, acting independently of CoA depletion, increase production of the stress hormone GDF15, which contributes to food aversion and degradation of acetyl-CoA-carboxylase, a key enzyme in lipid synthesis. This increased weight loss further in the study mice by preventing the replenishment of their fat stores.
