Our results demonstrate that palmitoleic acid positively affects hyperglycemia, hypertriglyceridemia, and insulin resistance in spontaneously diabetic KK-Ay mice. Risk factors for type 2 diabetes mellitus include insulin resistance, hyperglycemia, dyslipidemia, and metabolic syndrome, with insulin resistance being the key underlying metabolic perturbation. Insulin resistance is characterized by reduced responsiveness of target tissues (liver, skeletal muscle, adipocytes) to normal circulating levels of insulin, followed by a progressive decline in insulin secretion from the pancreas . Free fatty acids (saturated fatty acids, in particular) promote insulin resistance and reduce glucose utilization in skeletal muscle [16, 17]. However, there are indications that the monounsaturated palmitoleic acid improves glycemic control and increases glucose transport into skeletal muscle cells. This effect is mediated, at least in part, by upregulation of the activities of the glucose transporters GLUT1 and GLUT4 [18, 19]. Hyperinsulinemic-euglycemic clamp studies have shown that palmitoleic acid, in the form of a triglyceride, potentiates the insulin-signaling pathway in mice . In the present study, administration of palmitoleic acid, but not palmitic acid, improved insulin resistance. It is likely, therefore, that the effects of palmitoleic acid on hyperglycemia and hypertriglyceridemia can be attributed to improved insulin sensitivity.
To investigate the possible mechanisms behind the beneficial effect of palmitoleic acid on insulin sensitivity, we extended our study to examine adipocytokine gene expression. Numerous studies have shown the close relationship between insulin resistance-linked type 2 diabetes and adipocytokines that are mainly derived from adipose tissue [20, 21]. Adiponectin, a key adipocytokine, has been shown to increase insulin sensitivity at least in part through stimulating β-oxidation in skeleton muscle and decreasing hepatic glucose output . However, adiponectin mRNA expression levels did not change by palmitoleic acid administration in the present study, suggesting a minor effect of palmitoleic acid on adiponectin on a gene expression level. On the other hand, repeated administration of palmitoleic acid down-regulated mRNA expressions of TNFα and resistin, the adipocytokines that have been demonstrated to contribute to insulin resistance [23, 24]. It is thereby suggested that the beneficial effect of palmitoleic acid on improvement of insulin resistance may be partially owed to suppressing proinflammatory gene expression. In addition, our results also show that pancreas weight increased by repeated administration of palmitoleic acid. In type 2 diabetes, the pancreas fails to produce enough insulin, and evidence suggests that loss of beta cells contributes to this impairment [25, 26]. Preventing beta-cell apoptosis and promoting its proliferation, therefore, may represent an important mechanism for improving the diabetic condition. In vitro studies have shown that palmitoleic acid prevented the beta cells from high glucose- and palmitic acid-induced impairment of beta-cell proliferation possibly via induction of Bcl-2 . Nevertheless, how palmitoleic acid improves beta-cell function and in turn ameliorates insulin resistance in diabetic model has not been entirely elucidated.
Our present data show that palmitoleic acid suppresses lipid accumulation in the liver. To investigate its mechanism, we assessed liver mRNA levels of genes involved in lipogenesis. Analysis of gene expression with RT-PCR indicated that palmitoleic acid markedly down-regulated mRNA expressions of lipogenic genes such as SREBP-1, FAS and SCD-1. SREBP-1 and its target genes FAS as well as SCD-1 are involved in adipogenesis, and SREBP-1 play a central role in regulating fatty acid metabolism . Suppressing effect of palmitoleic acid on hepatic lipid accumulation is thereby possibly due to inhibiting de novo lipogenesis. Cao et al.  demonstrated that fatty acid binding protein-deficient mice have dramatically elevated levels of circulating palmitoleic acid compared to wild-type mice, and that high levels of palmitoleic acid down-regulate genes involved in de novo lipogenesis within the liver. It has been reported that hepatic lipid accumulation closely correlates with obesity, insulin resistance, and type 2 diabetes mellitus . In the liver, insulin suppresses hepatic glucose output by inhibiting gluconeogenesis and stimulating glycogen synthesis . In contrast, hepatic steatosis seems to stimulate gluconeogenesis by activating protein kinase C epsilon type and c-Jun N-terminal kinase 1 . These activated proteins subsequently interfere with tyrosine phosphorylation of insulin receptor substance 1 and 2 and impair the ability of insulin to activate glycogen synthase. We therefore infer that palmitoleic acid improves insulin resistance in diabetic mice, at least in part by decreasing hepatic lipid accumulation.
A growing body of evidence indicates that excess body weight is linked to type 2 diabetes [32, 33]. Administration of palmitoleic acid to KK-Ay mice reduced body weight gain, which may in turn have improved glucose and lipid metabolism. Interestingly, palmitoleic acid treatment resulted in a lower food intake compared with the control group, and studies have shown favorable effects of low calorie intake on obesity-induced metabolic disorders possibly by improving insulin sensitivity [34, 35]. Some unsaturated fatty acids, such as the n-3 polyunsaturated eicosapentaenoic acid and docosahexaenoic acid, as well as some short-chain saturated fatty acids, have been shown to regulate secretion of hunger hormones (e.g., the adipose tissue-derived hormone leptin) and gastrointestinal peptides (e.g., glucagon-like peptide-1, cholecystokinin) [36–39]. On the other hand, levels of hunger hormones correlate with energy intake and glucose metabolism [40–42]. The mechanisms by which palmitoleic acid affects food intake, however, remain unresolved.