The lymphatic system is important to various biological processes such as clearing harmful metabolites, collecting dietary lipids, and controlling the amount of water in one’s system. Additionally, there are roles that the lymphatic system has that controls cancer metastasis or organ failure. To better understand how these roles are executed, it is important to look at the factors that promote these activities and the essential metabolites.
As a molecule travels through metabolic pathways, one can quickly see that the resulting molecule looks entirely different than the molecule that started. One very important intermediate in various metabolic pathways is Acetyl-CoA. Acetyl-CoA is the product of many metabolic reactions while being used in numerous metabolic pathways. Since Acetyl-CoA is constantly forming and be used, it is important to understand its sources and the roles Acetyl-CoA has.
Fatty acids can be β-oxidized into Acetyl-CoA. This process starts with the fatty acid being linked to co-enzyme A in the mitochondria and turns the Acetyl-CoA into a fatty acyl CoA. Once this is complete another enzyme, carnitine palmitoyl-transferase 1 (CPT1), reacts with the fatty acyl CoA and metabolizes it into fatty acyl-carnite. The benefit of fatty acyl-carnite is that is it capable of being transported into the matrix of the mitochondria where it can be further metabolized.
After the fatty acyl-carnite is transported, carnitine palmitoyl-transferase 2 (CPT2) reverses the work CPT1 does and turns the molecule back to fatty acyl-CoA. Since the fatty acyl-CoA is now in the matrix of the mitochondria, it can be β-oxidized into acetyl-CoA. To preserve the function of acetyl-CoA, it is mixed with oxaloacetate to form citrate which can pass through the mitochondria membrane. Most importantly, citrate can be transported into the cytosol or other areas of the cell where it acts as a source of acetyl-CoA that is necessary for other reactions, even reactions that can lead to differentiation.
Wong et al explored the differentiation of different lymphatic cells. The Wong lab knew that different cues or factors largely controlled the differentiation of lymphatic endothelial cells from venous endothelia cells. Wong et al knew that CPT1A, CPT1B, and CPT1C had a role in this differentiation. While carrying out experiments where CPT1A, CPT1B, and CPT1C were being measured in lymphatic endothelial cells, they group noticed that there was also an increased level of fatty acid β-oxidation (FAO).
Since the high levels of FAO were unusual, the Wong group decided to investigate. The first experiment they conducted was overexpressing PROX1, a gene induces the differentiation. From this experiment, the data suggested that the overexpression caused the lymphatic differentiation and an increased level of FAO. This led the group to explore the role FAO has on the differentiation.
After realizing that FAO may have a role in the differentiation of endothelial venous cells to lymphatic endothelial cells, they knocked down CPT1A to see how the relationship between differentiation, FAO levels, and CPT1A. After doing the knockdown, Wong et al. saw that there was low levels of FAO and differentiation did not occur. This occurred because CPT1A was unable to make FAO occur therefore insinuating that FAO has a role on differentiation.
Since it was unexpected that this metabolite has a role on differentiation, the group tried to determine the mechanism behind how this occurs. To determine the mechanism, the group looked at p300 since it has a relationship between acetyl-CoA levels and histone acetylation. To determine if p300 was more active in higher levels of FAO, the group used a ChIP-seq experiment. They found that there were higher levels of histone acetylation when there was more FAO. Additionally, this increased histone acetylation increased the expression of key differentiation genes like H3K39ac and VEGFR3.
In conclusion, Wong et al. was able to see that acetyl-CoA created through fatty acid β-oxidation. These oxidized acetyl-CoA molecules were then able to interact with p300. This interaction led to the increased acetylation of histones leading to the increased expression of key differentiation genes. Lastly, the group explained some translation applications of this work. The key application would be to treat injury-induced lymphangiogenesis. The group hypothesized that adding acetate would create excess acetyl-CoA which will treat the defect in differentiation of the lymphatic endothelial cells. This treatment would allow one of the causes of the symptoms of injury-induced lymphangiogenesis to disappear allowing the patient to have some relief.