Impact of hepatic lipid accumulation and composition on glucose tolerance and insulin sensitivity: a longitudinal study in male and female mice
Background and aims: Metabolic disruptions characterized by high hepatic lipid content (HLC) are associated with impairments in whole body glucose homeostasis. To gain insight on the role of hepatic lipids in the metabolic performance in the absence artificial metabolic stresses we measured non- invasively and longitudinally the HLC and profile in mice during adult devel - opment by Magnetic Resonance (MR) Spectroscopy in vivo . In parallel, mice were challenged with insulin and glucose tolerance tests. Materials and methods: Male (N=10) and female (N=10) C57Bl/6J mice were studied at 3 (3Mo), 7 (7Mo) and 10 months (10Mo) of age. Mice were scanned in a 14.1 T magnet with a 1 H quadrature surface coil over the abdo - men. Localized 1 H spectra were acquired from a 8 μl volume with stimulated echo acquisition mode sequence and the HLC expressed as the percent of to - tal 1 H MR signal, with corrections for spin-spin relaxation effects. Additional spectra were acquired from the same volume with suppression of the water signal to enable the detection and quantification of all the lipid protons. The lipid profile was characterized by the following indices: saturated component (SC); unsaturated fatty acyl chains (UFA); mean number of double bonds per fatty acyl chain (ndb/FA), mean number of poly-unsaturated double bonds per fatty acyl chain (PUdb/FA) and per UFA (PUdb/UFA); mean chain length (MCL). OGTTs (1.5 g/Kg) and i.p. insulin tolerance tests (ITTs) were per - formed after a 6h-fast. Plasma insulin was determined by ELISA and insulin sensitivity estimated with the quantitative insulin check index (QUICKI) as the inverse of the log 10 sum of fasting insulin (μIU/ml) and fasting glucose (mg/dl). Data are expressed as mean ± SEM. Statistical significance was ac - cepted for a P < 0.05 (one-way ANOVA with Newman-Keuls post test) and correlations assessed by the Pearson r coefficient. Results: In males, the HLC at 3Mo was 1.35 ± 0.15%, increasing to 3.06 ± 0.38% at 7Mo, not different from 2.70 ± 0.31% at 10Mo. Females had higher HLC at 3Mo (2.63 ± 0.19%) but no further changes henceforward (2.31 ± 0.20% at 7Mo; 2.36 ± 0.20% at10 Mo). In males, the SC and MCL of hepatic lipids increased with age, with a trend for decreased PUdb/FA and PUdb/ UFA with no changes in ndb/FA or UFA content. Females showed the same trends. Glycemia 3h-post ITT and 2h-post OGTT was lower in females, while QUICKI was higer. These scores were preserved until 10Mo in females. In males, glycemia 2-h post OGTT increased with age and the area above the curve (AAC) for the ITT decreased. In males, but not females, higher body weight correlated with hepatic lipid accumulation ( r = 0.7); worse ITT scores correlated with higher body weight ( r = -0.6) and HLC ( r = -0.7) and lower Pudb/UFA ( r = 0.5); worse OGTT scores correlated with higher HLC ( r = 0.4). Conclusion: In male mice, loss of insulin sensitivity correlated with weight gain, HL accumulation and lower poly-unsaturation. Glucose intolerance was specifically associated with HLC, suggesting a deleterious effect of lipids on the adaptation of hepatic metabolism to the fed state. This behaviour was not observed in females even if they showed similar HLC. In fact, the poly- unsaturation of HL in females didn’t change with HLC, suggesting a positive effect of PUFA on preserving the hepatic metabolic performance.