Guangming Daily, Beijing, March 18 (Reporter Jin Haotian) The research group of Zhu Jian from the School of Life Sciences of Peking University and the Peking University-Tsinghua Joint Center for Life Sciences recently published an online publication in Molecular Cell A research paper titled “Dietary essential amino acids induce ubiquitination to improve fatty liver disease”. The research group used Drosophila larvae crimson cells as a research system and found that insufficient intake of essential amino acids is the “culprit” of fatty liver disease in malnourished people.
In most people’s opinion, fatty liver disease is a kind of “wealthy disease” caused by excess nutrition. In fact, there are also many patients with fatty liver disease who are thin, accounting for 20% of the total number of patients with fatty liver disease. As early as 1929, paediatrician Cicely Williams discovered that many infants along the coast of Ghana would suffer from malnutrition as a result of early weaning, and although these sick children were emaciated, they tended to suffer from severe fatty liver disease. In addition to malnourished children, patients with anorexia nervosa also suffer from fatty liver disease. What is even more surprising is that the incidence of fatty liver disease in people who rely on fasting to lose weight quickly is also significantly higher than that of healthy people.
Why both overnutrition obese people and undernourished lean people have fatty liver disease? “When we feel lost, fruit flies can always point us in the right direction.” As Michael Young, winner of the 2017 Nobel Prize in Physiology or Medicine, said, “It is the fruit fly that keeps us on the right path time and time again. Even if The giant wheel of evolution is rolling forward, and we can still capture the shadow of mammals and even humans from Drosophila.” Whole genome sequencing results show that the degree of homology between Drosophila melanogaster and the human genome is as high as 60%, covering 77% of humans Genetic disease causative gene is an ideal disease model. Drosophila larval magenta cells are highly similar to human hepatocytes, and also accumulate a large number of lipid droplets under the stimulation of starvation, simulating human fatty liver disease, and providing an ideal entry point for understanding the clinical manifestations of fatty liver disease caused by malnutrition. Based on the above-mentioned homologous characteristics and the strong advantages of Drosophila genetics, magenta cells provide an ideal research system for the study of the molecular mechanism of fatty liver disease caused by malnutrition.
Based on this, Jian Zhu’s group found that when the essential amino acids are deficient, the E3 ubiquitin ligase Ubr1 in hepatocytes is inactivated and cannot catalyze the polyubiquitination and degradation of the lipid droplet protection protein Plin2. . Elevated levels of Plin2 protein inhibit the breakdown of liver fat, resulting in fatty liver disease.
Zhu Jian’s research group introduced that Ubr1 is an important essential amino acid receptor in hepatocytes, but unlike known amino acid binding proteins, it can distinguish between essential amino acids and non-essential amino acids. Seven essential amino acids (arginine, lysine, histidine, leucine, isoleucine, tryptophan, and phenylalanine) can directly bind and activate Ubr1, enabling it to catalyze the ubiquitination of its substrate Plin2. peptide degradation, thereby alleviating fat accumulation in hepatocytes. As two branched-chain amino acids, leucine and isoleucine had the strongest ability to activate Ubr1. The addition of leucine or isoleucine to the food significantly increased the resistance of Drosophila larvae to fat accumulation in red cells under starvation conditions. Therefore, for malnourished people, adequate supplementation of essential amino acids, especially branched-chain amino acids, has important guiding significance for the prevention of fatty liver disease.
As the most common chronic liver disease in the world, fatty liver disease is a serious threat to human life and health. Although a lot of human, material and financial resources have been expended, there is still no drug for the treatment of fatty liver disease approved for marketing. So, can the treatment of fatty liver disease be achieved by activating Ubr1?
Zhu Jian’s group found that the activation of Ubr1 by essential amino acids depends on the release of its autoinhibitory effect. By deactivating Ubr1 autoinhibition, they constructed a persistently activated form of Ubr1 mutants. In the absence of amino acid binding, the persistently activated form of Ubr1 mutants can also efficiently catalyze the ubiquitination and degradation of Plin2, significantly ameliorating fatty liver disease caused by insufficient amino acid intake. It is worth mentioning that through the improvement of the isolated intein technology, they achieved the recombinant expression of the persistently activated form of Ubr1 mutant protein in mouse liver. Over a 2-week period, expression of a persistently activated form of Ubr1 mutant significantly ameliorated fatty liver disease caused by obesity or a high-fat diet, reducing liver fat content by more than 40% in mice. Therefore, activation of Ubr1 can be used as a potential therapeutic strategy for fatty liver disease, and lipid-soluble small molecules that can specifically bind to and activate Ubr1 are expected to become specific drugs for the treatment of fatty liver disease.
“Guangming Daily” (04 edition on March 19, 2022)
Source: Guangming.com – “Guangming Daily”