The Effect of Different High Fat Diets on Plasma Concentration and Hepatic Expression of Retinol‐Binding Protein 4 in Rats

Abstract

Retinol‐binding protein 4 (RBP4) has been associated with insulin resistance and hepatic steatosis. Production of hepatic insulin resistance by high‐fat diets depends on their fatty acid composition. Here, the effect of three high‐fat diets on plasma concentration and hepatic expression of RBP4 (Rbp4) was compared with respect to insulin resistance, dyslipidemia, and hepatic steatosis.Weight‐matched groups (N=10/group) of male Sprague‐Dawley rats received one of four diets for eight weeks: low‐fat control (LF, 15% kcal as fat), or a high‐fat (HF, 55% kcal as fat) safflower oil (polyunsaturated), olive oil (cis‐monounsaturated, MUFA) or a trans‐fat (trans‐MUFA) diet. Blood samples were collected as a time course study at wks 3, 6, and 9 (N=16), and after an eight‐week feeding in the fasted or refed (fat‐free, high carbohydrate meal) state (N=40). Liver samples were collected at wk 9 in the fasted or refed state to compare plasma concentration and hepatic expression of RBP4 (Rbp4) and its response to insulin. Plasma RBP4 concentrations were measured by enzyme‐linked immunosorbent assay (ELISA). Hepatic expression of Rbp4 and other genes (e.g. Srebf, Fasn) was measured by real‐time PCR.Body weights and percent body fat of all HF groups were similar at wk 9. Plasma RBP4 concentrations were higher in the trans‐fat vs. the LF group at wk 3, 6, and 9 (p< 0.05). The olive oil and the trans‐fat group had elevated plasma RBP4 at wk 9 vs. the LF group (p<0.05). Liver expression of Rbp4 under fasted or re‐fed conditions did not differ among the groups, a finding consistent with a non‐hepatic (e.g. adipose tissue) source of RBP4. The olive oil group had higher fasting plasma insulin levels (p<0.05), higher HOMA‐IR values (p<0.005), higher liver lipid concentration (p < 0.05) vs. other groups. Expression of hepatic genes in the lipogenic pathway was positively correlated with plasma triglyceride levels in both the olive oil and trans‐fat groups (p< 0.05), and the olive oil group had greater stimulation of lipogenic gene expression by refeeding than did other groups (p<0.05). The HF safflower group did not show differences from the LF group at wk 9.The HF olive oil (cis‐MUFA) diet was associated with elevated plasma RBP4, hyperinsulinemia, hepatic steatosis and insulin resistance, as well as an enhanced hepatic expression of lipogenic genes. The HF trans‐fat (trans‐MUFA) diet was also associated with elevated plasma RBP4 and increased lipogenic gene expression, and both MUFA groups showed this expression to be correlated with plasma triglyceride levels. These findings show that HF diet‐induced increases in RBP4 are dependent on the type of fat and suggest that non‐hepatic RBP4 may promote hepatic expression of lipogenic genes.