The Role of the Carbohydrate Response Element-Binding Protein in Male Fructose-Fed Rats

Abstract

By 2030, nearly half of Americans will have nonalcoholic fatty liver disease. In part, this epidemic is fueled by the increasing consumption of caloric sweeteners coupled with an innate capacity to convert sugar into fat via hepatic de novo lipogenesis. In addition to serving as substrates, monosaccharides also increase the expression of key enzymes involved in de novo lipogenesis via the carbohydrate response element-binding protein (ChREBP). To determine whether ChREBP is a potential therapeutic target, we decreased hepatic expression of ChREBP with a specific antisense oligonucleotide (ASO) in male Sprague-Dawley rats fed either a high-fructose or high-fat diet. ChREBP ASO treatment decreased plasma triglyceride concentrations compared with control ASO treatment in both diet groups. The reduction was more pronounced in the fructose-fed group and attributed to decreased hepatic expression of ACC2, FAS, SCD1, and MTTP and a decrease in the rate of hepatic triglyceride secretion. This was associated with an increase in insulin-stimulated peripheral glucose uptake, as assessed by the hyperinsulinemic-euglycemic clamp. In contrast, ChREBP ASO did not alter hepatic lipid content or hepatic insulin sensitivity. Interestingly, fructose-fed rats treated with ChREBP ASO had increased plasma uric acid, alanine transaminase, and aspartate aminotransferase concentrations. This was associated with decreased expression of fructose aldolase and fructokinase, reminiscent of inherited disorders of fructose metabolism. In summary, these studies suggest that targeting ChREBP may prevent fructose-induced hypertriglyceridemia but without the improvements in hepatic steatosis and hepatic insulin responsiveness.

Fructose metabolism and metabolic disease

“…Activation of the lipogenic program is observed immediately after a single load of fructose and contributes to increased VLDL triglyceride secretion (114,115 (135)(136)(137). ChREBP knockdown using antisense oligonucleotides (ASOs) in fructose-fed rats reduced circulating triglyceride levels and confirmed a role for ChREBP in fructose-mediated dyslipidemia, although steatosis was unaffected (138). Consistent with this, GWAS have identified multiple common SNPs within the ChREBP locus associated with increased serum triglyceride and low HDL cholesterol levels (139,140).…”

“…However, whether steatosis itself can cause hepatic insulin resistance remains controversial (131,161). In addition to ChREBP's role in fructose-induced dyslipidemia, Erion et al demonstrated that ChREBP knockdown enhanced peripheral insulin sensitivity in high-fructose-fed rats (138). Whether the improvement in peripheral insulin sensitivity was directly related to the improvement in circulating lipid levels or adiposity is uncertain.…”

The pathogenesis of insulin resistance: integrating signaling pathways and substrate flux

“…Human subjects who consumed fructose-containing beverages, in comparison with those consuming isocaloric glucose-containing beverages, developed hyperlipidemia and insulin resistance despite gaining a similar amount of weight (113). Monosaccharides also enlist other transcription factors, including ChREBP (114,115), PPARγ coactivator 1-β (116), and liver X receptor (117), to activate lipogenesis. In summary, though insulin potentiates DNL by activating SREBP1c, substrate flux into the liver also impacts hepatic lipid synthesis.…”

Dietary carbohydrate and control of hepatic gene expression: mechanistic links from ATP and phosphate ester homeostasis to the carbohydrate-response element-binding protein

“…In support of the second possibility, gene microarrays in cells with ChREBP knockdown identified dihydrofolate reductase which is involved in purine biosynthesis in the down-regulated genes (70) , and genes involved in purine metabolism were also identified as ChREBP targets by chromatin immunoprecipitationsequencing (44) . A role for ChREBP in ATP homeostasis is also indicated by raised plasma uric acid levels in rats with partial ChREBP knock-down on a high-fructose diet (71) . Plasma levels of uric acid in man are elevated in association with insulin resistance (72) and NAFLD (73) and increased hepatic production of uric acid is inferred in these conditions (72) .…”