Abstract: Brite adipocytes are inducible energy-dissipating cells expressing UCP1 which appear within white adipose tissue of healthy adult individuals. Recruitment of these cells represents a potential strategy to fight obesity and associated diseases. Using human Multipotent Adipose-Derived Stem cells, able to convert into brite adipocytes, we show that arachidonic acid strongly inhibits brite adipocyte formation via a cyclooxygenase pathway leading to secretion of PGE2 and PGF2α. Both prostaglandins induce an oscillatory Ca++ signaling coupled to ERK pathway and trigger a decrease in UCP1 expression and in oxygen consumption without altering mitochondriogenesis. In mice fed a standard diet supplemented with ω6 arachidonic acid, PGF2α and PGE2 amounts are increased in subcutaneous white adipose tissue and associated with a decrease in the recruitment of brite adipocytes. Our results suggest that dietary excess of ω6 polyunsaturated fatty acids present in Western diets, may also favor obesity by preventing the “browning” process to take place.
Alex’s Notes: The two most well-known types of adipose tissue is white (WAT) and brown (BAT), but there is a third type that can be seen as an intermediate of the two. The fat is called brite or beige adipose tissue, and can be thought of as BAT within WAT; that is, it is fat that acts similarly to BAT but is formed from WAT. And yes, this fat does exist in adult humans, and in fact one of the strongest methods to stimulate its creation is exercise via the release of the hormone irisin from skeletal muscle.
The above shouldn’t be so surprising, as brite fat cells are virtually absent in the obese. But a lack of exercise cannot be the only factor. Diet and lifestyle habits no doubt play a critical role as well, and the authors of the current paper sought to analyze the interaction of the long-chained omega-6 fatty acid, arachidonic acid (ARA), on brite development. This fatty acid is the main precursor for inflammation through prostaglandins and while necessary for life and proper development, can cause issues when in excess. Of course, being in excess and in too great a ratio relative to the omega-3s is central in the Western diet and disease.
The researchers performed both in vitro and in vivo testing. First, they obtained hMADS-3 (Human multipotent adipose-derived stem cells) cells from the fat pads of a four month old male. These cells are able to become WAT, BAT, or brite depending on the stimulus and timing of that stimulus while they are developing. All cells were treated with rosiglitazone between days 3-9 in order to produce WAT, but the brite adipose tissue group was treated again with rosiglitazone between days 14-17 to stimulate conversion of the forming WAT into brite. It was during this last change phase (days 14-17) that the brite cells were incubated with or without ARA to see how it interacts with brite development.
The second experiment involved ten-week-old C57Bl/6JRccHsd female mice that were fed for 4 weeks with ARA- or oleic acid (OA)-supplemented (11g/kg) standard chow diet, and underwent chronic beta-adrenergic receptor stimulation during the last week of the diet by daily intra-abdominal injections of CL316,243 – a selective β3-adrenoceptor agonist (1 mg/kg/day).
What did the in vitro testing tell us?
If you recall from our previous discussion on BAT, a characteristic feature of this and brite adipose tissue is the expression of uncoupling protein 1 (UCP1), which leads to wasteful mitochondrial respiration in effort to produce heat. It was shown that incubation with ARA reduced UCP1 gene expression and protein levels by about 71% and 40%, respectively, compared to the control (non-ARA-treated brite). It should be noted that even in the ARA treated group UCP1 gene expression was 10-fold greater than in WAT, demonstrating why brite is considered an intermediate between WAT and BAT. Interestingly, these effects were not seen with linoleic acid, the short-chained and most common dietary omega-6 fatty acid.
The above inhibition of UCP1 was accompanied by a significant reduction in the basal oxygen consumption and cytochrome c oxidase activity of the cells, both reflecting the reduced UCP1 activity of the ARA cells. Additionally, ARA inhibited the expression of adiponectin (ADPQ; a hormone that promotes insulin sensitivity, fat burning, and weight loss) and fatty acid binding protein 4 (FABP4) without affecting overall adipogenesis.
It appears that these effects are mediated by COX-1 and COX-2 activity, as both were greater in the ARA treated cells (the gene expression of COX-2 was 20-fold greater than the control), and inhibition of COX-2 by indomethacin and celecoxib were able to reverse the ARA inhibitory effect on UCP1, ADPQ, & FABP4 gene expression. It is through the COX pathway that ARA is converted into PGH2 and various downstream prostaglandins. In order to determine whether these prostaglandins were indeed responsible of the ARA inhibitory effect on UCP1 expression, specific prostaglandin receptor ligands were used, and it was found that these were able to inhibit, in a dose dependent manner, UCP1 expression at the mRNA and protein levels in a manner similar to that seen with ARA.
In summary, the in vitro data strongly suggests that,
“Inhibition of the conversion of white to brite adipocytes by ARA is a multi-step process involving the synthesis and secretion of prostaglandins PGF2α and PGE2… leading in turn to a lower PPARγ activity characterized by a decreased expression of PPARγ-target genes, such as ADPQ, FABP4 and UCP1.”
And what about the mice?
Apparently, the ARA or OA diets neither induced weight gain nor modified leptin levels, but mice fed the ARA-supplemented diet display a defective induction of “browning.” In accordance with the in vitro data, the ARA mice displayed nearly 40% lower levels of UCP1 gene expression and enhanced COX-2 expression, as determined by molecular analysis of subcutaneous WAT. On this note, it is intriguing that mice fed a high-fat diet with inhibition of COX-1 and COX-2 are protected from weight gain partly through increased recruitment of brite adipose tissue. It is also interesting to speculate about the fact that NSAIDs work through COX-2 inhibition.
Now, the ARA in the mice diets was 1.1% of the wet weight, or 11g/kg of bodyweight. After conversion to the human equivalent dose (HED), this becomes 0.89g/kg. For a 70kg person that would be 62 grams of ARA daily. Clearly this is an absolutely unrealistic intake for humans, especially when you consider that the richest source of ARA is boiled beef kidney with 0.37g per 100g. That means you would need to eat over 16 kg of boiled beef kidney daily.
So don’t go freaking out about the ARA in your eggs, but do keep this information in mind, as there is nothing to say that the effects don’t occur at lower amounts, and even less to say that it is a dose-dependent relationship. In the meantime, go exercise and get your brite on!