Palmitoleic acid is elevated in fatty liver disease and reflects hepatic lipogenesis

Abstract

Background: Biochemical evidence has linked the coordinate control of fatty acid (FA) synthesis with the activity of stearoyl-CoA desaturase-1 (SCD1). The ratio of 16:1n-7 to 16:0 [SCD1₁₆] in plasma triacylglycerol FA has been used as an index to reflect liver SCD1₁₆ activity and has been proposed as a biomarker of FA synthesis, although this use has not been validated by comparison with isotopically measured de novo lipogenesis (DNL(Meas)).

Objective: We investigated plasma lipid 16:1n-7 and FA indexes of elongation and desaturation in relation to lipogenesis.

Design: In this cross-sectional investigation of metabolism, 24 overweight adults, who were likely to have elevated DNL, consumed D2O for 10 d and had liver fat (LF) measured by magnetic resonance spectroscopy. Very-low-density lipoprotein (VLDL)-triacylglycerols and plasma free FA [nonesterified fatty acids (NEFAs)] were analyzed by using gas chromatography for the FA composition (molar percentage) and gas chromatography-mass spectrometry and gas chromatography-combustion isotope ratio mass spectrometry for deuterium enrichment.

Results: In all subjects, VLDL-triacylglycerol 16:1n-7 was significantly (P < 0.01) related to DNL(Meas) (r = 0.56), liver fat (r = 0.53), and adipose insulin resistance (r = 0.56); similar positive relations were shown with the SCD1₁₆ index, and the pattern in NEFAs echoed that of VLDL-triacylglycerols. Compared with subjects with low LF (3.1 ± 2.7%; n = 11), subjects with high LF (18.4 ± 3.6%; n = 13) exhibited a 45% higher VLDL-triacylglycerol 16:1n-7 molar percentage (P < 0.01), 16% of subjects had lower 18:2n-6 (P = 0.01), and 27% of subjects had higher DNL as assessed by using a published DNL index (ratio of 16:0 to 18:2n-6; P = 0.03), which was isotopically confirmed by DNL(Meas) (increased 2.5-fold; P < 0.01). Compared with 16:0 in the diet, the low amount of dietary 16:1n-7 in VLDL-triacylglycerols corresponded to a stronger signal of elevated DNL.

Conclusion: The current data provide support for the use of the VLDL-triacylglycerol 16:1n-7 molar percentage as a biomarker for elevated liver fat when isotope use is not feasible; however, larger-scale confirmatory studies are needed.

Trial registration: ClinicalTrials.gov NCT01371396.

Keywords: VLDL triacylglycerol; biomarker; lipogenesis; palmitoleic fatty acid; stearoyl-CoA desaturase.

FIGURE 1

Flowchart of study participants throughout screening and recruitment.

FIGURE 2

VLDL-TG fatty acid composition and indicators of lipid enzyme activities. Data are means ± SDs for individual fatty acids in VLDL-TG from subjects with low liver fat (gray bars; n = 11) and high liver fat (black bars; n = 12) (A) and fatty acid ratios used as putative biomarkers in the literature for the activities of ELOVL6, which elongates palmitate (16:0) to stearate (18:0), and SCD1, which can desaturate palmitate (16:0) to palmitoleate (16:1n–7) or stearate (18:0) to oleate (18:1n–9) (B). *Difference between liver fat groups, P < 0.05 (unpaired 2-tailed t test). ELOVL6, elongation of very-long-chain fatty acid protein 6; HighLF, high liver fat; LowLF, low liver fat; mol%, molar percentage; SCD1, stearoyl-CoA desaturase-1; SCD1₍₁₆₎, ratio of 16:1n–7 to 16:0; SCD1₍₁₈₎, ratio of 18:1n–9 to 18:0; TG, triacylglycerol.

FIGURE 3

Comparison of DNL_Meas with fatty acid indices of DNL. Data are means ± SDs from subjects with low liver fat (gray bars; n = 11) and high liver fat (black bars; n = 12 for VLDL-TG data and n = 13 for NEFA data). In the literature, the molar ratio of 16:0 to 18:2n–6 is used as a DNL_Index. In this figure, the index is compared with DNL_Meas by using deuterated water administration and fatty acid analysis by GC-MS. * P value <0.05 for differences between liver fat groups, analyzed by unpaired 2-tailed t test. DNL, de novo lipogenesis; DNL_Index, index of de novo lipogenesis; DNL_Meas, isotopically measured de novo lipogenesis; GC-MS, gas chromatography–mass spectrometry; NEFA, nonesterified fatty acid; TG, triacylglycerol.

FIGURE 4

NEFA fatty acid composition and indicators of lipid enzyme activities. Data are means ± SDs for individual fatty acids in plasma NEFAs from subjects with LowLF (gray bars; n = 11) and HighLF (black bars; n = 13) (A) and fatty acid ratios used as putative biomarkers in the literature for activities of ELOVL6, which elongates palmitate (16:0) to stearate (18:0), and SCD1, which can desaturate palmitate (16:0) to palmitoleate (16:1n–7) or stearate (18:0) to oleate (18:1n–9). *Difference between liver fat groups, P < 0.05 (unpaired 2-tailed t test). ELOVL6, elongation of very-long-chain fatty acid protein 6; HighLF, high liver fat; LowLF, low liver fat; mol%, molar percentage; NEFA, nonesterified fatty acid; SCD1, stearoyl-CoA desaturase-1; SCD1₍₁₆₎, ratio of 16:1n–7 to 16:0; SCD1₍₁₈₎, ratio of 18:1n–9 to 18:0; TG, triacylglycerol.

FIGURE 5

Relations between DNL and the fatty acid compositions of VLDL-TG, NEFAs, and the diet. Pearson's correlations in subjects with low liver fat (gray circles; n = 11) and high liver fat (black circles; n = 12 for VLDL-TG data and n = 13 for NEFA data). Data reflect the positive association between DNL_Meas and VLDL-TG 16:1n–7 (A) but a lack of association with VLDL-TG 16:0 (B) as well as the DNL_Index (C). Instead, VLDL-TG 16:0 was associated with the dietary content of 16:0 (D), whereas this relation was not consistent for 16:1n–7 (E). Finally, the proportions of 16:1n–7 in VLDL-TGs and NEFAs were highly correlated with each other (F). Note interruption in y-axes in panels B and D. DNL_Index, index of de novo lipogenesis; DNL_Meas, isotopically measured de novo lipogenesis; mol%, molar percentage; NEFA, nonesterified fatty acid; TG, triacylglycerol.