Effect of acute streptozotocin diabetes on fatty acid content and composition in different lipid fractions of rat skeletal muscle

Membrane Fatty Acid Transporters as Regulators of Lipid Metabolism: Implications for Metabolic Disease

Long-chain fatty acids and lipids serve a wide variety of functions in mammalian homeostasis, particularly in the formation and dynamic properties of biological membranes and as fuels for energy production in tissues such as heart and skeletal muscle. On the other hand, long-chain fatty acid metabolites may exert toxic effects on cellular functions and cause cell injury. Therefore, fatty acid uptake into the cell and intracellular handling need to be carefully controlled. In the last few years, our knowledge of the regulation of cellular fatty acid uptake has dramatically increased. Notably, fatty acid uptake was found to occur by a mechanism that resembles that of cellular glucose uptake. Thus, following an acute stimulus, particularly insulin or muscle contraction, specific fatty acid transporters translocate from intracellular stores to the plasma membrane to facilitate fatty acid uptake, just as these same stimuli recruit glucose transporters to increase glucose uptake. This regulatory mechanism is important to clear lipids from the circulation postprandially and to rapidly facilitate substrate provision when the metabolic demands of heart and muscle are increased by contractile activity. Studies in both humans and animal models have implicated fatty acid transporters in the pathogenesis of diseases such as the progression of obesity to insulin resistance and type 2 diabetes. As a result, membrane fatty acid transporters are now being regarded as a promising therapeutic target to redirect lipid fluxes in the body in an organ-specific fashion.

Liver fatty acid-binding protein targets fatty acids to the nucleus. Real time confocal and multiphoton fluorescence imaging in living cells

Although unesterified long chain fatty acids interact with peroxisome proliferator-activated receptors to initiate transcription within the nucleus, almost nothing is known regarding factors regulating long chain fatty acid distribution to the nucleus of living cells. The possibility that the liver fatty acid-binding protein (L-FABP) may function in this role was addressed in transfected L-cell fibroblasts overexpressing L-FABP using a series of fluorescent fatty acids differing in chain length and unsaturation. After 30 min of incubation, oxidation of BODIPY-, NBD-, and cis-parinaric acids was undetectable in L-cells. Likewise, L-cells very poorly esterified these fluorescent fatty acids in the following order: 0% BODIPY-C5, NBD-C6 (short chain length) < 0-3% NBD-C18, BODIPY-C16, cis-parinaric acid (long chain length) < 11% BODIPY-C12 (medium chain length). Real time confocal and multiphoton laser scanning microscopy (CLSM and MPLSM) showed that these fluorescent fatty acids were generally taken up in the following order: long chain (BODIPY-C16, NBD-C18) > medium chain (BODIPY-C12) short chain (BODIPY-C5, NBD-C6). The fluorescent fatty acids were imaged in the nucleus, primarily associated with the nuclear envelope, at levels about 2-3-fold lower than outside the nucleus. CLSM and MPLSM showed that L-FABP expression enhanced by 2-4-fold the initial rate and/or average maximal uptake of the long and medium chain but not the short chain fluorescent fatty acids in living cells. Furthermore, L-FABP expression increased the targeting of long and medium but not short chain fluorescent fatty acids to the nucleus by 2.9-4.4-fold and increased the proportion (i.e. nuclear:cytoplasm ratio) of medium and long chain but not short chain fatty acids by 2-3.6-fold. In summary, these results showed for the first time the presence of unesterified fatty acids in the nucleus of living cells and demonstrated that expression of a fatty acid-binding protein, L-FABP, specifically enhanced uptake and intracellular targeting of long and medium chain fatty acids to the nucleus.

Age-related changes in fatty acids in obese offspring of streptozotocin-induced diabetic rats

OBJECTIVE

The long-term effects of fetal hyperinsulinemia, time course of changes in liver and very-low-density lipoprotein (VLDL) lipid levels and fatty acid compositions were investigated in obese offspring of streptozotocin-induced mildly diabetic rats.

RESEARCH METHODS AND PROCEDURES

Mild hyperglycemia in pregnant rats was induced by intraperitoneal injection of streptozotocin on day 5 of gestation. Control pregnant rats were injected with citrate buffer. Liver and VLDL lipids and fatty acids were analyzed in offspring at different ages.

RESULTS

At birth, obese pups had higher VLDL triglyceride levels, saturated fatty acids, and C20:4n-6. They also had lower C18:2n-6 proportions in VLDL triglycerides, phospholipids, and cholesteryl esters than controls pups. In 1-month-old male and female obese rats, VLDL and liver lipid amounts were similar to those in their respective controls; however, high levels of C18:2n-6 and C20:4n-6 were noted in liver and VLDL lipids. At the age of 2 months, liver and VLDL triglyceride levels were higher in obese females than in control females. Fatty acid abnormalities seen in obese rats included low C18:3n-3 and high C22:6n-3 proportions in liver triglycerides and phospholipids. At the age of 3 months, obese rats, both males and females, compared with control animals, had higher VLDL and hepatic lipids with reduced C20:4n-6 levels and polyunsaturated/saturated fatty acids ratios in hepatic and VLDL triglycerides and phospholipids.

DISCUSSION

Fetal obesity, associated with alterations in VLDL lipid fatty acid composition, represents an important risk factor for adult obesity and diabetes.