So what gives? What is the nature of the interaction between PUFAs and other dietary components or metabolic states that produces inflammation?
In an earlier blog post I identified the enzyme systems upregulated in NAFLD as those of the microsomal ethanol oxidase system (MEOS) and also showed that the evolutionary function of the MEOS is to degrade PUFA, rather than alcohol which is a latecomer to our diets.
But what activates the MEOS when alcohol does not? How, for example, does fructose send PUFAs down this pathway, and how does this promote inflammation?
I found a clue in this hepatitis C editorial by Jenny Heathcote on a study in which weight loss improved liver function. This is some quite brilliant speculation.
Here is the description of fatty liver due to insulin resistance (HCV causes IR by a pharmacological action of its core protein):
In peripheral tissues, insulin normally downregulates the hormone sensitive lipase (HSL) enzyme responsible for hydrolysis of stored triglycerides from free fatty acids within adipocytes. In patients who are insulin resistant, this enzyme is no longer suppressed. In addition, counterregulatory hormones such as catecholamines, glucagon, and growth hormone are increased in response to increased circulating insulin levels. These counterregulatory hormones stimulate HSL to hydrolyse more triglycerides into free fatty acids, the end result being an increased flux of dietary and stored free fatty acids away from the adipose tissues and towards the liver. Unfortunately, Hickman et al did not measure free fatty acid levels before or after the weight reduction programme. Within the liver, insulin upregulates esterification of free fatty acids to triglycerides. Once the triglycerides are formed, insulin downregulates the secretory pathways, thus favouring increased storage of triglycerides in the cytosolic pool. Furthermore, free fatty acids can themselves upregulate the esterification pathway. The net result is a positive feedback cycle contributing to an ever increasing amount of free fatty acids and triglycerides in the liver. Thus portal hyperinsulinaemia leads to hepatic steatosis.
And here is the description of how hepatic steatosis influences PUFA disposal:
These studies have suggested that the presence of fat in patients with hepatitis C is associated with markers of progressive liver disease in that fat was associated with increased stellate cell activation, but the mechanism by which this takes place is uncertain. It is possible that this occurs secondary to saturation of beta oxidation pathways within mitochondria which then leads to free fatty acids becoming more available to intracellular microsomes where they undergo lipid peroxidation. There are three main products of microsomal lipid peroxidation: malondialdehyde, 4-hydroxynonenal, and hydrogen peroxide. Malondialdehyde has been shown to activate stellate cells to produce fibrin, and may be responsible at least in part for liver fibrosis in patients with non-alcoholic steatohepatitis.
Malondialdehyde (MDA) and 4-HNE are unsaturated products of PUFA, and H2O2 is also a step in the MEOS disposal of PUFA, requiring catalase for its reduction to H2O + O.
We can see how this relates to the "essential" role that PUFA plays in the development of alcoholic liver disease; not only can the liver become fatty from the conversion of alcohol to triglycerides, but also the disposal of excess ethanol through the MEOS has upregulated this enzyme system (hepatic CYP2E1 is upregulated 10-fold by ethanol); to add insult to injury, the liver's ability to dispose of excess fat via beta oxidation is impaired by the depletion of NAD+ during the conversion of ethanol to fat.
But another clue was supplied by Tucker Goodrich, the PUFA ninja, who found a rodent study showing that 4-HNE and 9-ONE could themselves be cleared if beta-oxidation pathways were upregulated enough, that is, by a ketogenic diet.
Our results showed that livers from rats fed ketogenic diet or high fat mix diet had high ω-6 polyunsaturated fatty acid concentrations and markers of oxidative stress. However, high concentrations of HNE (1.6 ± 0.5 nmol/g) and ONE (0.9 ± 0.2 nmol/g) were only found in livers from rats fed the high fat mix diet. Livers from rats fed the ketogenic diet had low HNE (0.8 ± 0.1 nmol/g) and ONE (0.4 ± 0.07 nmol/g), similar to rats fed the standard diet. A possible explanation is that the predominant pathway of HNE catabolism (i.e. beta oxidation) is activated in the liver by the ketogenic diet. This is consistent with a 10 fold decrease in malonyl-CoA in livers from rats fed a ketogenic diet compared to a standard diet. The accelerated catabolism of HNE lowers HNE and HNE analog concentrations in livers from rats fed the ketogenic diet. On the other hand, rats fed the high fat mix diet had high rates of lipid synthesis and low rates of fatty acid oxidation, resulting in the slowing down of the catabolic disposal of HNE and HNE analogs. Thus, decreased HNE catabolism by a high fat mix diet induces high concentrations of HNE and HNE analogs. The results of the present work suggested a potential causal relationship to metabolic syndrome induced by western diets (i.e. high fat mix), as well as the effects of the ketogenic diet on the catabolism of lipid peroxidation products in liver.
So - any state in which beta-oxidation is inhibited, but fat is present, will see PUFA shunted into the microsome - essentially the MEOS - and a high production of damaging peroxides and aldehydes. This also happens when mice are fed a ketogenic diet, but the aldehydes can be disposed of by beta-oxidation.
Note that the high fat (non-keto) diet in the mouse study was the Surwit diet relatively low in PUFA and MUFA (coconut and soy oil), overloading beta-oxidation with a mixture of ~50% saturated fat and 22.5% sucrose. Don't try this at home, kids.
For reasons of time I haven't gone into every possible ramification such as the role of peroxisomal oxidation in PUFA disposal, the proper function of the MEOS (making and disposing of eicosanoids), the hormetic effect on antioxidant systems of low level HNE production, and the difference between liver and other fat-burning tissues (i.e. is this relevant to heart disease if the same thing happens in muscle, macrophages, or endothelial cells? Magic 8 ball says very probably).
However, here's a model that allows us to predict and explain the likely role of PUFA in inflammatory diseases at a metabolic level. Especially, for now, liver diseases.
* FFQ epidemiology studies are notoriously inaccurate at capturing intakes of calories (and protein, which often looks wonky in epidemiology). They really can't tell you in what context PUFA is being consumed, and in any case it's hard to see how deep frying oil in food can really be measured - do you even know what your chips (fries) are cooked in and in what part of the FFQ would you put this information?
Structural and Functional Changes in Human Insulin Induced by the Lipid Peroxidation Byproducts 4-Hydroxy-2-nonenal and 4-Hydroxy-2-hexenal
 Browning JD, Baker JA, Rogers T et al. Short-term weight loss and hepatic triglyceride reduction: evidence of a metabolic advantage with dietary carbohydrate restriction. Am J Clin Nutr. 2011 May; 93(5): 1048–1052.
 Heathcote J. Weighty issues in hepatitis C. Gut. 2002;51(1):7-8.
 Li Q, Tomcik K, Zhang S, Puchowicz MA, Zhang G-F. Dietary-regulation of catabolic disposal of 4-hydroxynonenal analogs in rat liver. Free radical biology & medicine. 2012;52(6):1043-1053. doi:10.1016/j.freeradbiomed.2011.12.022