Triglyceride synthesis in epididymal adipose tissue: contribution of glucose and non-glucose carbon sources

Abstract

The obesity epidemic has generated interest in determining the contribution of various pathways to triglyceride synthesis, including an elucidation of the origin of triglyceride fatty acids and triglyceride glycerol. We hypothesized that a dietary intervention would demonstrate the importance of using glucose versus non-glucose carbon sources to synthesize triglycerides in white adipose tissue. C57BL/6J mice were fed either a low fat, high carbohydrate (HC) diet or a high fat, carbohydrate-free (CF) diet and maintained on 2H2O (to determine total triglyceride dynamics) or infused with [6,6-(2)H]glucose (to quantify the contribution of glucose to triglyceride glycerol). The 2H2O labeling data demonstrate that although de novo lipogenesis contributed approximately 80% versus approximately 5% to the pool of triglyceride palmitate in HC- versus CF-fed mice, the epididymal adipose tissue synthesized approximately 1.5-fold more triglyceride in CF- versus HC-fed mice, i.e. 37+/-5 versus 25+/-3 micromolxday(-1). The [6,6-(2)H]glucose labeling data demonstrate that approximately 69 and approximately 28% of triglyceride glycerol is synthesized from glucose in HC- versus CF-fed mice, respectively. Although these data are consistent with the notion that non-glucose carbon sources (e.g. glyceroneogenesis) can make substantial contributions to the synthesis of triglyceride glycerol (i.e. the absolute synthesis of triglyceride glycerol from non-glucose substrates increased from approximately 8 to approximately 26 micromolxday(-1) in HC- versus CF-fed mice), these observations suggest (i) the importance of nutritional status in affecting flux rates and (ii) the operation of a glycerol-glucose cycle.

FIGURE 1.

Distribution of triglyceride-bound fatty acids. Total lipids were extracted from epididymal fat pads, and the absolute quantity of various fatty acids was determined following hydrolysis (A, high carbohydrate diet; B, carbohydrate-free diet). Data were compared on day 3 and day 28 after administration of ²H₂O, n = 4 per day per group, *, p < 0.05 within a group.

FIGURE 2.

²H labeling of triglyceride palmitate. A demonstrates the incorporation of ²H-labeled palmitate in mice fed a high carbohydrate diet, n = four mice per day. B compares the ²H labeling profile of palmitate from mice fed a high carbohydrate versus a carbohydrate-free diet for 28 days, n = 4 mice per diet group.

FIGURE 3.

²H labeling of triglyceride glycerol. The total ²H labeling of triglyceride glycerol was determined (n = 4 per day per group), and the fractional rates of turnover were calculated by fitting the respective data to single exponential curves, i.e. 7.7 ± 0.2% versus 11.8 ± 1.2% newly made triglyceride per day (mean ± S.E.) in HC- versus CF-fed mice, respectively. The inset compares the absolute rates of triglyceride synthesis and breakdown, determined as described under “Experimental Procedures.”

FIGURE 4.

²H labeling of water, triglyceride glycerol and blood glucose. The total ²H labeling of water was determined and expressed as a multiple of the maximum possible n for each end product, i.e. 5 and 7 for triglyceride glycerol and blood glucose, respectively. Data are shown for samples collected on day 3 following the administration of ²H₂O. Note: A steady-state ²H labeling of water and blood glucose was observed between days 3 and 28 (data not shown).

FIGURE 5.

Glycerol-glucose cycling between adipose tissue and liver. Two modes of operation of a glucose-glycerol cycle can be envisioned; each is self-sustaining based on carbon balance. First, one molecule of glucose can contribute two molecules of α-glycerol-3-phosphate; this would require a source of ATP and NADH in adipose tissue. Second, one molecule of glucose can provide one molecule of α-glycerol-3-phosphate and one molecule of pyruvate; this would generate the necessary ATP and NADH that are required to run the cycle in adipose tissue.