De novo lipogenesis in the differentiating human adipocyte can provide all fatty acids necessary for maturation

Abstract

The primary products of de novo lipogenesis (DNL) are saturated fatty acids, which confer adverse cellular effects. Human adipocytes differentiated with no exogenous fat accumulated triacylglycerol (TG) in lipid droplets and differentiated normally. TG composition showed the products of DNL (saturated fatty acids from 12:0 to 18:0) together with unsaturated fatty acids (particularly 16:1n-7 and 18:1n-9) produced by elongation/desaturation. There was parallel upregulation of expression of genes involved in DNL and in fatty acid elongation and desaturation, suggesting coordinated control of expression. Enzyme products (desaturation ratios, elongation ratios, and total pathway flux) were also correlated with mRNA levels. We used (13)C-labeled substrates to study the pathway of DNL. Glucose (5 mM or 17.5 mM in the medium) provided less than half the carbon used for DNL (42% and 47%, respectively). Glutamine (2 mM) provided 9-10%, depending upon glucose concentration. In contrast, glucose provided most (72%) of the carbon of TG-glycerol. Pathway analysis using mass isotopomer distribution analysis (MIDA) revealed that the pathway for conversion of glucose to palmitate is complex. DNL in human fat cells is tightly coupled with further modification of fatty acids to produce a range of saturated and unsaturated fatty acids consistent with normal maturation.

Fig. 1.

Photomicrograph of adipocyte after 14 days of differentiation with no exogenous fat source. Human adipocytes differentiated (A) in the absence of fatty acids and (B) with a combination of different exogenous fatty acids (0.2 mM palmitate + 0.2 mM oleate).

Fig. 2.

TG and PL content of cells during differentiation and fatty acid composition. A: TG content during differentiation; n = 5, P = 0.06 for effect of time. B: PL content during differentiation; n = 5, P = 0.16. C: TG fatty acid composition, n = 5 for days 0-10, n = 74 at day 14, main effects of day (P = 0.04), fatty acid (P < 0.001), and day × fatty acid interaction (P < 0.001). D: PL fatty acid composition, n = 5, day × fatty acid interaction, P < 0.001. All statistics by repeated measures ANOVA.

Fig. 3.

Time course for mRNA expression of genes involved in DNL during differentiation. For each gene, mRNA values are plotted for days 0, 3, 6, 9, 12, and 14 of differentiation (n = 4).

Fig. 4.

Fatty acid ratios in TG and gene expression. A: Relationship between ratios reflecting desaturation; r_s = 0.72, P < 0.001. B: Relationship between ratios reflecting elongation; r_s = 0.84, P < 0.001. C: Relationship between flux through SCD over 14 days and SCD mRNA; r_s = 0.68, P = 0.008. D: Relationship between flux through ELOVL6 over 14 days and ELOVL6 mRNA; r_s = 0.67, P = 0.009.

Fig. 5.

Mass isotopomer spectra for 16:0 in TG with different labeled substrates. A: Labeling from [1-¹³C]acetate. B: Labeling from [U-¹³C]glucose. C: Labeling from [U-¹³C]glutamine. B and C: Black, differentiation in 5 mM glucose; Gray, differentiation in 17.5 mM glucose.