Selective Trafficking of Fatty Acids in the Liver: Add Them2 to the List of Influencers
Authors: Henry N. Ginsberg
Journal: Hepatology (2019)

Potential conflict of interest: Nothing to report. See Article on Page 496. Nonalcoholic fatty liver disease (NAFLD) occurs in 25%‐40% of the U.S. and European populations, with rates up to 70% in individuals with diabetes mellitus. The majority of people with NAFLD also have hypertriglyceridemia, characterized by increased secretion of very low density lipoproteins (VLDLs) carrying triglycerides (TGs).1 This raises several questions: Why would there be both increased secretion of VLDLs and increased hepatic fat content? If VLDL TG secretion cannot keep up with TG synthesis, why isn't there a compensatory increase in hepatic fatty acid oxidation? What determines whether TG synthesized within the bilayer membrane of the endoplasmic reticulum (ER) moves to the cytoplasm as lipid droplets (LDs) or into the ER lumen, where it can be incorporated into VLDL? Do the sources of hepatic TG (uptake of plasma fatty acids [FAs], uptake of chylomicron and VLDL remnants, hepatic de novo lipogenesis [DNL]) determine the partitioning of TG between LD formation, VLDL secretion, or oxidation? The assembly and secretion of VLDL have been characterized in great detail, revealing the complexity of this process.2 There have been many studies on LD biology, with increased understanding of the physical, molecular, and biochemical processes involved in the formation of nascent LDs from the ER membrane3 as well as the regulation of LD‐TG lipolysis.4 FAs are central to these processes and are also key substrates for oxidation by mitochondria. Prior work has demonstrated that targeting of FAs to different fates, e.g., secretion as VLDL TG, storage in LDs, and oxidation, is determined by their origin and by compartmental trafficking in hepatocytes.5 There are several FA transporters that facilitate uptake of plasma FA; there are multiple FA binding proteins that move FAs through the cytosol; there are multiple acyl‐coenzyme A (CoA) synthetases that activate FA; and there are several glycerol‐3‐phosphate acyltransferases (GPATs), acylglycerolphophate acyltransferase (AGPATs), lipins, and diacylglycerol acyltransferases (DGATs) that link FAs to glycerol to make TG.6 Not nearly as well studied are a large superfamily of acyl‐thiosterases (called Thems or Acots) that catalyze the hydrolysis of FA‐acyl‐CoA to FA and Coenzyme A, thus reversing the activation of FA.7 In this issue of hepatology, Alves‐Bezerra and colleagues identify a role for thiosterase superfamily member 2 (Them2 or Acot 13) in VLDL secretion.8 Using a liver‐specific knockout of Them2, the authors found that despite 2‐3‐fold increases in concentrations of fatty acyl‐CoAs (accompanied by reductions in free FA levels), there were no differences in hepatic lipid levels of TG between knockout and control mice. This result is indicative of the many pathways available for use of FA acyl‐CoAs. Additionally, reduced VLDL secretion in Them2 knockout mice without changes in hepatic lipids suggested compartmentalization of the sources of TG for VLDL secretion in these mice. Further support for targeting of FA from different sources to unique metabolic pathways was the demonstration that although synthesis and secretion of TG derived from acetate was unaffected by loss of Them2 activity, those parameters were significantly reduced when palmitate was the precursor. Thus, the loss of Them2 thioesterase activity did not affect the quantity of acetyl‐CoA available for FA synthesis through DNL, but it did significantly reduce the availability of palmitoyl‐CoA available for TG synthesis. These results are consistent with our demonstration many years ago that although increasing plasma FA uptake into mouse livers in vivo stimulated apolipoprotein B (apoB) but not TG secretion, delivery of the same quantity of FA by uptake of remnant‐like particles by the liver increased secretion of both apoB and TG.9 Source‐specific trafficking of FA has also been demonstrated for FA use as substrates for the two DGAT enzymes.10 Lastly, several groups have shown that increasing hepatic DNL results in secretion of more TG without changes in apoB secretion. When the authors measured TG secretion rates in vivo using Tyloxapol, they observed an approximately 20%‐25% reduction in the Them2 knockout mice on both diets. Plasma apoB100 levels were reduced about 40% on chow and tended to be reduced on high fat; apoB48 levels were unaffected by loss of Them2. The lower levels of B100 suggest reduced secretion. It is not unusual in our experience to see much less effect of metabolic perturbations on apoB48 secretion compared with apoB100 secretion; the basis for this is unclear, although apoB48 is less susceptible to proteasomal degradation in the ER. Of note, the absolute rates of TG secretion in both groups of mice were very similar on chow versus high‐fat diets, despite a 10‐fold difference in liver TG between the two diet groups. Do these results conform to the acetate and palmitate labeling experiments described above? Plasma FAs are the major source of VLDL TG, so the significant reduction in palmitate incorporation into secreted TG in hepatocytes from Them2 knockout mice would have predicted reduced VLDL TG secretion and, based on our published data,9 reduced apoB secretion as well. The lack of effect of Them2 loss of function on DNL‐derived TG would be compatible with the marked increase in hepatic TG on the high‐fat diet, when obesity and insulin resistance stimulate DNL. The authors did not, however, find evidence for increased DNL based on examination of the expression of key lipogeneic genes and proteins. The high‐fat diet would have likely raised plasma FA uptake by the liver (there were no plasma FA levels provided on the high‐fat diet), and the absolute increase in uptake could have more than compensated for reduced incorporation of those FAs into TGs in the Them2 knockout mice. Increased hepatic uptake of TG‐enriched remnants on the high‐fat diet could have also contributed to hepatic fat content. These suggested explanations, however, point to the specific effect of loss of Them2 thioesterase activity on the incorporation of plasma‐derived FA into VLDL TG, as the authors note at the end of their discussion. With all of this said, why would a biochemical reaction that deactivates FAs in wild‐type mice with intact hepatic Them2 play a role in maintaining the assembly and secretion of VLDL? Alves‐Bezerra et al. provide some ideas: One that appeals to me is the idea that in states of overnutrition in humans, when FA flux to the liver and hepatic DNL are increased, Them2 targets a portion of the exogenous FA for deactivation followed by reactivation by an Long‐chain acylCoA synthetase (ACSL) that is linked to the synthesis of TG destined for incorporation into VLDL and secretion (see their Fig. 8). Although an appealing model, there are obviously many steps that require further validation, including the identity of the ACSL, more evidence that DGAT1, which the authors focus on, is specifically or at least preferentially linked to VLDL secretion, and whether there are specific GPATs, AGPATs, and lipins in this VLDL‐targeting pathway. Finally, future studies are needed to determine if natural variants in Them2 that reduce expression might show linkage to lower TG levels and/or higher levels of hepatic fat in humans.