Scientists have identified two miRNAs that appear to play a role in regulating insulin sensitivity. Studies in obese mouse models silencing miR-103 and miR-107 in adipocytes was shown to improve glucose homeostasis and upregulate caveolin-1, which occurs alongside stabilization of the insulin receptor, enhanced insulin signaling, decreased adipocyte size, and enhanced insulin-stimulated glucose uptake.
The researchers were led by Markus Stoffel, Ph.D., at Switzerland’s ETH Zurich. Results, published in Nature, are described in a paper titled “MicroRNAs 103 and 107 regulate insulin sensitivity.”
To identify miRNAs that may be deregulated in obesity and insulin resistance, the researchers carried out miRNA microarray analysis on the livers of two types of obese mice: leptin-deficient ob/ob mice and diet-induced obese (DIO) mice.
Compared with miRNAs in wild-type mice, the miR-103/miR-107 family was among the five most-upregulated miRNAs in the livers of both obese models; an increase in the expression of these miRNAs has also been previously reported in a diabetic rat model, the team notes.
A twofold to threefold upregulation of miR-103 and miR-107 was observed in the livers of both mouse models, and these miRNAs were also found to be upregulated in liver biopsies from patients with alcoholoic liver disease, non-alcoholic fatty liver disease, and non-alcoholic steatohepatitis, conditions often associate with diabetes. Furthermore, there was a positive correlation between the subjects’ homeostatic model assessment (HOMA) index and miR-103/107 expression levels, indicating a link between these miRNAs with insulin resistance.
The researchers then investigated the effects of over-expression of miR-107 using adenoviral-based gene delivery. Injection of wild-type mice with miR-107 caused a rise in blood-glucose levels and also in insulin levels. The miRNA also impaired glucose tolerance after an intraperitoneal glucose injection and decreased insulin sensitivity relative to that in control mice. Hepatic overexpression of miR-107 resulted in increased glucose production during an intraperitoneal pyruvate-tolerance test.
Conversely, while silencing both miRNA-103 and miRNA-107 in the livers and fat tissues of wild-type mice fed a normal chow diet had no effect on blood glucose levels, it did lower plasma glucose levels in both the obese mouse models. Glucose- and insulin-tolerance tests showed that there was improved glucose tolerance and insulin sensitivity in ob/ob and DIO mice in which miR-103 and miR-107 were silenced. In addition, liver glycogen content was increased and plasma insulin levels were decreased.
Interestingly, silencing miR-103 and miR-107 specifically in the liver (but not in fat tissues or muscle) of the ob/ob mouse model had no effect on blood glucose levels, plasma insulin levels, glucose-tolerance, or insulin-tolerance.
Expression of miR-103 is about eightfold higher in adipose tissue than in liver and muscle, the authors note, and when the two miRNAs were silenced specifically in the adipose tissue of ob/ob mice, the animals showed a slight reduction in bodyweight, and both mouse models had reduced levels of total fat due to decreases in both subcutaneous and visceral adipose tissues. This was associated with an increased number of small adipocytes and a decreased number of large adipocytes, equating to about a 10–20% increase in the overall number of fat cells.
Previous studies have shown that smaller adipocytes are associated with increased insulin sensitivity in human and rodent models, and further analyses on primary adipocytes taken from ob/ob mice showed that silencing the miRNAs led to increases in basal and insulin-stimulated glucose uptake.
The researchers carried out gene expression analyses to try and figure out the mechanism by which miR-103 and miR-107 might regulate insulin sensitivity. The gene encoding caveolin-1 (Cav1), which is a key component of caveolae (lipid- and cholesterol-enriched vascular invaginations at the plasma membrane) and a mediator of insulin signaling, was among those that were downregulated after overexpression of miR-107 in the liver and upregulated after silencing. Furthermore, miR-103 silencing in the fat resulted in an approximately 3.5-fold upregulation of Cav1 mRNA levels.
Subsequent studies confirmed that in the fat and liver of ob/ob mice, silencing of miR-103/107 resulted in increased Cav1 levels, whereas no expression of the protein was detected in skeletal muscle. In addition, the expression of insulin receptor β-subunit (IRβ) in adipocytes was increased and insulin-stimulated levels of phosphorylated Akt1 and IRβ (pAkt1 and pIRβ) were augmented in the fat and livers of the miR-103/107-silenced animals.
Conversely, wild-type mice treated to overexpress miR-107 in their inguinal fat pad showed a reduction in Cav1 expression and decreased IRβ and pAkt1 levels. Overexpression of miR-107 in the livers of wild-type mice similarly led to diminished Cav1 and pAKt1 levels. These findings point to one potential mechanism in which miRNA-103 and miR-107 target Cav1, resulting in diminished numbers of insulin receptors in caveolae-enriched plasma membranes and reduced downstream insulin signaling.
The overall results thus suggest that miR-103 and miR-107 are negative regulators of insulin sensitivity, the authors conclude. “Their increased hepatic expression in rodents and humans with insulin resistance and hepatic steatosis indicates that they might contribute to the etiology of diabetes. Our finding that silencing miR-103/107 in obese animals improves glucose homeostasis implicates these miRNAs as novel therapeutic targets for the treatment of diabetes.”