It is well established that patients with type 2 DM often have a co-occurrence of insulin resistance, hyperinsulinemia, hyperglycemia, dyslipidemia, and/or obesity [26–28]. In the present study, we showed that dietary administration of LC-MUFAs for 8 weeks attenuated hyperinsulinemia and hyperlipidemia and decreased fat pad weight in diabetic KK-Ay mice.
Insulin resistance has been assigned a central place in the metabolic disturbances associated with obesity and type 2 DM, and compensatory hyperinsulinemia stemming from peripheral insulin resistance is a well-recognized metabolic disturbance in type 2 DM [29–31]. Furthermore, it has been demonstrated that mice transfected with extra copies of the insulin gene produce elevated insulin levels, and they show insulin resistance, hyperglycemia, and hypertriglyceridemia, suggesting that insulin itself is an important contributor to insulin resistance . Besides hyperinsulinemia, obesity-derived hyperlipidemia is one of the suggested possible mechanisms for type 2 DM. Serum FFAs, which are elevated in most obese subjects, induce metabolic insulin resistance and inhibit insulin signaling through different mechanisms [33, 34]. The LC-MUFA diet decreased plasma insulin and FFA concentrations that were markedly elevated in spontaneous type 2 diabetic KK-Ay mice, suggesting that treatment of diabetic mice with LC-MUFAs may favorably impact the risk factors for type 2 DM. On the other hand, non-fasting levels of plasma triglycerides and glucose were not significantly different between the control and LC-MUFA diet group. It is therefore suggested that the antidiabetic effect of LC-MUFAs in KK-Ay mice was mild. In addition, although there were no significant differences in hepatic triglycerides and total cholesterol, LC-MUFA intake increased liver weight. It is not yet clear whether this is due to the changes in the fatty acid composition of the liver with LC-MUFA administration, and further studies are needed to address whether LC-MUFAs have any impact on liver health and fatty liver in particular.
Because type 2 DM is closely associated with obesity, ~80–90% of people diagnosed with type 2 DM are also diagnosed as being overweight or obese, and several studies of human subjects have shown that the risk of developing DM is reduced by weight loss [35, 36]. Research suggests that adipose tissue not only serves as a storage site for fat but also functions as an endocrine organ [37, 38]. Adipose tissue grows by two mechanisms: hyperplasia and hypertrophy, the latter occurring prior to hyperplasia to meet the need for additional fat storage capacity as obesity progresses . In fact, obesity is characterized by adipocyte hypertrophy followed by increased angiogenesis, immune cell infiltration, extracellular matrix overproduction, and, consequently, by increased production of proinflammatory adipocytokines and FFAs, which are potentially involved in the pathogenesis of insulin resistance. In the present study, whereas there were no differences in body weight gain between the LC-MUFA group and control group, dietary treatment of KK-Ay mice with LC-MUFAs reduced fat accumulation and caused decreased adipocyte size, which may in turn have improved lipid metabolism and attenuated compensatory hyperinsulinemia. The LC-MUFA diet increased plasma and organ levels of C20:1 and C22:1 isomers, suggesting a negative correlation between LC-MUFA intake and risk factors for type 2 DM in KK-Ay mice. Plasma, liver and WAT levels of linoleic acid (C18:2 n-6), a major fatty acid in soybean oil used in the control diet, decreased significantly in mice fed the LC-MUFA diet, however, suggesting a minor role for linoleic acid in the improvement of type 2 DM with LC-MUFA treatment. Worthy of note, the predominant LC-MUFA isomers in the current study is C22:1 n-11, an isomer of erucic acid (C22:1 n-9) which is an important fatty acid of rapeseed oil, and LC-MFUA diet increased plasma and organ levels of C22:1 n-11 significantly. Given that there is continuing controversy as to the health effects of erucic acid [40–42], further study is required to examine the physiologic similarities and differences between the C22:1 isomers. Furthermore, LC-MUFA diet increased palmitoleic acid (C16:1 n-7) levels in plasma and liver significantly, and in adipose tissues non-significantly (p < 0.15). Although it has been proposed some beneficial effects of palmitoleic aicd in cell culture and animal models [43, 44], human plasma palmitoleic acid content has been reported to be a consistent and independent predictor in incident diabetes [45, 46]. It is therefore suggested that no all MUFAs are uniformly beneficial, and we could not exclude the possibility that a rise in palmitoleic acid with LC-MUFA diet could have some potential negative implications in a diabetic animal model.
To explore the potential positive impact of reducing adipocyte size and lowering plasma FFAs, we evaluated the expression of Pparg, Lpl, Fatp, and Cd36 mRNA in adipose tissue. Pparg, which is expressed predominantly in adipose tissue, plays crucial roles in regulating adipocyte differentiation, fatty acid metabolism, and insulin signal transduction [47, 48]. Overexpression of Pparg in mature 3T3-L1 adipocytes results in a decrease in both cell size and intracellular triglyceride content, and Pparg activation results in a marked improvement of insulin and glucose parameters resulting from an improvement of whole-body insulin sensitivity in type 2 diabetic patients [49–51]. The Pparg target genes Lpl, Fatp, and Cd36 are important control elements in FFA homeostasis [52, 53]. FFAs liberated from circulating lipoproteins through hydrolysis by LPL bind to albumin, and cell surface receptors such as Fatp and Cd36 facilitate rapid uptake and coordinate the import of FFAs. Our data demonstrate that Pparg mRNA level increased in adipose tissue of KK-Ay mice fed LC-MUFAs, which was paralleled by increases in Lpl, Fatp, and Cd36 mRNAs, suggesting that reduction in adipocyte size and an increase in FFA uptake in adipose tissue were partly attributed to upregulation of Pparg and its target genes, Lpl, Fatp, and Cd36. Furthermore, concomitant with increase in Pparg gene expression, plasma leptin levels were decreased with the LC-MUFA diet. Leptin, a 16 KDa protein hormone that plays a key role in regulating adipose tissue mass and energy balance, is secreted primarily by adipocytes . Circulating leptin levels are highly correlated with body fat stores, and high plasma leptin levels are observed in obese subjects as a result of increased production in enlarged fat cells . Studies have demonstrated that leptin is located at Pparg downstream, and Pparg activation inhibits leptin gene transcription . It is therefore suggested that LC-MUFAs decreased plasma leptin levels may possibly through activating Pparg in adipose. Furthermore, it has been demonstrated that adipose inflammation plays key roles in the vascular complications of obesity, insulin resistance, as well as type 2 DM, and hypertrophic adipocytes within adipose tissue directly augment systemic inflammation . The present study shows that LC-MUFA feeding downregulated the expression of inflammatory marker Saa3 in adipose tissue with concomitant decreases in adipocyte size. These results suggest possible mechanisms for the beneficial effects of a LC-MUFA-rich diet.
Insulin resistance in subjects with type 2 DM and obesity is connected with an imbalance between the availability and the oxidation of lipids . Cpt1a and Cs are associated with fatty acid utilization and oxidation capacity [59, 60], and the increase in Cpt1a and Cs expression observed in the current study was possibly related to the elevated uptake of circulating FFAs by adipose tissue. Studies have shown that inhibition of Cpt1a increases lipid deposition and exacerbates insulin resistance when animals are placed on a high-fat diet, whereas overexpression of Cpt1a protects myotubes against lipid-induced insulin resistance [61–63]. The present study suggests that a diet supplemented with LC-MUFAs may promote fatty acid oxidation by upregulating Cpt1a and Cs expression, which is possibly associated with a diminution of risk factors for type 2 DM.