High-fat diet (HFD) causes obesity and insulin resistance and down-regulates Glut4 expression selectively in adipose tissue1
. To investigate whether adipose ChREBP expression plays a role in the insulin resistance resulting from adipose-Glut4 down-regulation, we subjected mice to HFD. On chow, AG4OX mice are obese ( and ). AG4OX do not gain more weight on HFD and their degree of obesity on both diets is similar to HFD-fed WT (
Supplementary Fig. 9
). HFD induces insulin resistance in both genotypes (Supplementary Table 3
). In WT, HFD causes a diabetic GTT () but not in AG4OX mice. The enhanced glucose tolerance in AG4OX mice does not result from changes in serum metabolites typically associated with insulin sensitivity. AG4OX mice have higher non-esterified fatty acids and triglycerides, and lower leptin levels on both diets (Supplementary Table 3
), and lower adiponectin levels22
on chow compared to WT. This pattern is thought to contribute to insulin resistance2,3,23
and not to enhanced insulin sensitivity.
ChREBP is regulated in mouse and human adipose tissue in pathologic conditions
HFD reduces expression of adipose tissue DNL enzymes24,25
which correlates with insulin resistance7–10
. Therefore, we examined whether downregulation of adipose-ChREBP and DNL contributes to HFD-induced insulin resistance. HFD in WT mice reduces DNL in subcutaneous but not perigonadal adipose tissue, (). In AG4OX, HFD markedly reduces DNL in WAT, but DNL remains 2–7-fold higher than in WT on HFD. This intermediate rate of DNL parallels the change in glucose tolerance in AG4OX mice on HFD (). Changes in glucose incorporation into newly synthesized fatty acids (Supplementary Fig. 10
) are similar to the changes in total (from all substrates) DNL () in WAT. In liver, DNL is not higher in AG4OX on either diet (). HFD inhibits adipose-DNL independently of adipose tissue glucose uptake in AG4OX mice since Glut4 protein (Supplementary Fig. 11
) and glucose transport remain elevated26
To investigate the mechanisms underlying HFD-induced changes in adipose-DNL, we examined the expression of lipogenic transcription factors and DNL enzymes. In WT, HFD diminishes ChREBP but not SREBP-1c expression in WAT () as reported25,27
. In AG4OX WAT, HFD markedly decreases ChREBP and also SREBP-1c expression but ChREBP remains elevated compared to WT on HFD. Nevertheless, HFD reduces the degree of ChREBP induction in AG4OX adipose tissue despite persistent Glut4 overexpression (Supplementary Fig. 11
). Thus, HFD may regulate ChREBP expression in part independently of Glut4 expression and glucose flux, consistent with HFD effects on DNL ().
Many DNL enzymes that are up-regulated in WAT of AG4OX on chow are down-regulated in HFD WT and AG4OX mice (
Supplementary Fig. 12
) but remain higher in AG4OX WAT compared to WT in a pattern paralleling ChREBP expression () and DNL (). In WT WAT, ChREBP down-regulation most likely accounts for HFD-induced down-regulation of DNL gene expression since WAT SREBP-1c expression is not altered. Furthermore, whole body genetic ablation of SREBP-1c does not diminish expression of these genes in WAT28
In contrast, both ChREBP and SREBP-1c expression are down-regulated in WAT in AG4OX mice on HFD compared to chow (). Persistent modest elevation of ChREBP in WAT of AG4OX on HFD compared to WT on HFD () is likely responsible for the increased DNL which contributes to improved glucose tolerance in AG4OX ().
LXRs are unlikely to contribute to down-regulation of ChREBP expression, DNL, or DNL enzyme expression in WT mice on HFD because expression of LXRs and canonical LXR targets are either unchanged or modestly increased (Supplementary Fig. 13
). Mlx (Max like protein X) is an obligate dimerization partner for ChREBP transcriptional activity 29
. Neither HFD nor Glut4 overexpression alters Mlx expression in WAT (Supplementary Fig. 13
We next sought to determine whether adipose-ChREBP might contribute to regulating insulin sensitivity and glucose homeostasis in humans. In 123 non-diabetic individuals with normal glucose tolerance and widely ranging body mass index (Supplementary Table 4
), ChREBP expression in subcutaneous WAT correlates strongly with insulin sensitivity measured during a euglycemic-hyperinsulinemic clamp procedure (). Adipose-ChREBP correlates with Glut4 (), consistent with a role for ChREBP in the beneficial effects of adipose-Glut4 expression on glucose homeostasis.
Most, but not all, obese people are insulin resistant. To determine whether adipose-ChREBP expression could have a role in regulating insulin sensitivity in obese people, we also investigated adipose-ChREBP expression in non-diabetic obese individuals with widely ranging insulin-sensitivity (Supplementary Table 4
). Adipose-ChREBP expression was directly associated with insulin-stimulated glucose uptake during a clamp, independent of BMI (). Surprisingly, ChREBP and Glut4 expression did not correlate in these obese individuals (). Thus, adipose-ChREBP expression may have beneficial effects on insulin sensitivity, even among obese subjects, and this can be independent of Glut4 expression.