Search

Abstract While a definitive mechanism‐of‐action remains to be identified, recent findings indicate that ghrelin, particularly the unacylated form (UnAG), stimulates skeletal muscle fatty acid oxidation. The biological importance of UnAG‐mediated increases in fat oxidation remains unclear, as UnAG peaks in the circulation before mealtimes, and decreases rapidly during the postprandial situation before increases in postabsorptive circulating lipids. Therefore, we aimed to determine if the UnAG‐mediated stimulation of fat oxidation would persist long enough to affect the oxidation of meal‐derived fatty acids, and if UnAG stimulated the translocation of fatty acid transporters to the sarcolemma as a mechanism‐of‐action. In isolated soleus muscle strips from male rats, short‐term pre‐treatment with UnAG elicited a persisting stimulus on fatty acid oxidation 2 h after the removal of UnAG. UnAG also caused an immediate phosphorylation of AMPK, but not an increase in plasma membrane FAT/CD36 or FABPpm. There was also no increase in AMPK signaling or increased FAT/CD36 or FABPpm content at the plasma membrane at 2 h which might explain the sustained increase in fatty acid oxidation. These findings confirm UnAG as a stimulator of fatty acid oxidation and provide evidence that UnAG may influence the handling of postprandial lipids. The underlying mechanisms are not known.

TBC1D4 (AS160) and TBC1D1 are Rab GTPase-activating proteins that play a key role in the regulation of glucose and possibly the transport of long chain fatty acids (LCFAs) into muscle and fat cells. Knockdown (KD) of TBC1D4 increased CD36/SR-B2 and FABPpm protein expressions in L6 myotubes, whereas in murine cardiomyocytes, TBC1D4 deficiency led to a redistribution of CD36/SR-B2 to the sarcolemma. In our study, we investigated the previously unexplored role of both Rab-GAPs in LCFAs uptake in human adipocytes differentiated from the ADMSCs of subcutaneous and visceral adipose tissue origin. To this end we performed a single- and double-knockdown of the proteins (TBC1D1 and TBC1D4). Herein, we provide evidence that AS160 mediates fatty acid entry into the adipocytes derived from ADMSCs. TBC1D4 KD resulted in quite a few alterations to the cellular phenotype, the most obvious of which was the shift of the CD36/SR-B2 transport protein to the plasma membrane. The above translated into an increased uptake of saturated long-chain fatty acid. Interestingly, we observed a tissue-specific pattern, with more pronounced changes present in the adipocytes derived from subADMSCs. Altogether, our data show that in human adipocytes, TBC1D4, but not TBC1D1, deficiency increases LCFAs transport via CD36/SR-B2 translocation.

PGC-1α coactivator plays a decisive role in the maintenance of lipid balance via engagement in numerous metabolic processes (i.e., Krebs cycle, β-oxidation, oxidative phosphorylation and electron transport chain). It constitutes a link between fatty acids import and their complete oxidation or conversion into bioactive fractions through the coordination of both the expression and subcellular relocation of the proteins involved in fatty acid transmembrane movement. Studies on cell lines and/or animal models highlighted the existence of an upregulation of the total and mitochondrial FAT/CD36, FABPpm and FATPs content in skeletal muscle in response to PGC-1α stimulation. On the other hand, the association between PGC-1α level or activity and the fatty acids transport in the heart and adipocytes is still elusive. So far, the effects of PGC-1α on the total and sarcolemmal expression of FAT/CD36, FATP1, and FABPpm in cardiomyocytes have been shown to vary in relation to the type of PPAR that was coactivated. In brown adipose tissue (BAT) PGC-1α knockdown was linked with a decreased level of lipid metabolizing enzymes and fatty acid transporters (FAT/CD36, FABP3), whereas the results obtained for white adipose tissue (WAT) remain contradictory. Furthermore, dysregulation in lipid turnover is often associated with insulin intolerance, which suggests the coactivator's potential role as a therapeutic target.

To support the high energetic demands of reproduction, female mammals display plasticity in many physiological processes, such as the lipid transport system. Lipids support the energy demands of females during reproduction, and energy and structural demands of the developing offspring via the placenta in utero or milk during the suckling period. We hypothesized that key proteins supporting lipid transport in reproductive females will increase during pregnancy and lactation, but drop to non-reproductive levels shortly after reproduction has ended. We compared the relative protein levels of liver-type cytosolic fatty acid transporter (L-FABPc), plasma membrane fatty acid transporter (FABPpm), fatty acid translocase (FAT/CD36) in the liver, a key site of lipid storage and synthesis, and free fatty acid transporter albumin and triglyceride transporter [represented by apolipoprotein B (apoB)] levels in serum in reproductive Sprague-Dawley rats during late pregnancy, peak-lactation, and 1-week post-lactation as well as in non-reproductive rats. We found that all lipid transporter levels were greater in pregnant rats compared to non-reproductive rats. Lactating rats also showed higher levels of FAT/CD36 and FABPpm than non-reproductive rats. Moreover, all fat transporters also dropped back to non-reproductive levels during post-lactation except for FAT/CD36. These results indicate that fat uptake and transport capacities in liver cells are elevated during late gestation and lactation. Liver lipid secretion is up-regulated during gestation but not during lactation. These data supported the plasticity of lipid transport capacities in liver and blood during reproductive stages.

While statins significantly reduce cholesterol levels and thereby reduce the risk of cardiovascular disease, the development of myopathy with statin use is a significant clinical side-effect. Recent guidelines recommend increasing inclusion criteria for statin treatment in diabetic individuals; however, the impact of statins on skeletal muscle health in those with diabetes (who already suffer from impairments in muscle health) is ill-defined. Here we investigate the effects of Fluvastatin treatment on muscle health in wild-type and streptozotocin (STZ)-induced diabetic mice. Wild-type and STZ-diabetic mice received diet enriched with 600 mg/kg Fluvastatin or control chow for 24 days. Muscle morphology, intra and extracellular lipid levels, and lipid transporter content was investigated. Our findings indicate that short-term Fluvastatin administration induced a myopathy that was not exacerbated by the presence of STZ-induced diabetes. Fluvastatin significantly increased ectopic lipid deposition within the muscle of STZ-diabetic animals, findings that were not seen with diabetes or statin treatment alone. Consistent with this observation, only Fluvastatin-treated diabetic mice downregulated protein expression of lipid transporters FAT/CD36 and FABPpm in their skeletal muscle. No differences in FAT/CD36 or FABPpm mRNA content were observed. Altered lipid compartmentalization resultant of a downregulation in lipid transporter content in STZ-induced diabetic skeletal muscle was apparent in the current investigation. Given the association between ectopic lipid deposition in skeletal muscle and the development of insulin-resistance, our findings highlight the necessity for more thorough investigations into the impact of statins in humans with diabetes.

Giant sarcolemmal vesicles were isolated from rat heart and hindlimb muscles for a) characterization of long-chain fatty acid transport in the absence of metabolism and b) comparison of fatty acid transport protein expression with fatty acid transport. Giant vesicles contained cytosolic fatty acid binding protein. Palmitate uptake was completely divorced from its metabolism. All palmitate taken up was recovered in the intravesicular cytosol as unesterified FA. Palmitate uptake by heart vesicles exhibited a Km of 9.7 nm, similar to that of muscle (Km = 9.7 nm). Vmax (2.7 pmol/mg protein/s) in heart was 8-fold higher than in muscle (0.34 pmol/mg protein/s). Palmitate uptake was inhibited in heart (55–80%) and muscle (31–50%) by trypsin, phloretin, sulfo-N-succinimidyloleate (SSO), or a polyclonal antiserum against the 40 kDa plasma membrane fatty acid binding protein (FABPpm). Palmitate uptake by heart and by red and white muscle vesicles correlated well with the expression of fatty acid translocase (FAT/CD36) and fatty acid binding protein FABPpm, which may act in concert. The expression of fatty acid transport protein (FATP), was 10-fold lower in heart vesicles than in white muscle vesicles. It is concluded that long-chain fatty acid uptake by heart and muscle vesicles is largely protein-mediated, involving FAT/CD36 and FABPpm. The role of FATP in muscle and heart remains uncertain.—Luiken, J. J. F. P., L. P. Turcotte, and A. Bonen. Protein-mediated palmitate uptake and expression of fatty acid transport proteins in heart giant vesicles. J. Lipid Res. 1999. 40: 1007–1016.