A team led by Max Planck Institute researchers this week published a report linking a single microRNA — miR-802 — to obesity-related insulin resistance and type II diabetes. The findings suggest that the miRNA and its targets may offer new opportunities for treating the diseases.
According to the paper, which appeared in Nature, the investigators generated miRNA expression profiles from RNA isolated from liver tissue from two mouse models of obesity and insulin resistance: high-fat diet-fed mice compared to normal chow diet-fed mice, and mice homozygous for the diabetes mutation of the leptin receptor, or Leprdb/db versus wild-type controls.
The screen confirmed the increased expression of miRNAs previously associated with obesity-induced insulin resistance, including miR-103, miR-107, miR-143, and miR-335, the team wrote. In addition, they found an “even more pronounced” over-expression of miR-802 in the liver of the mouse models compared to control animals.
Specifically, qRT-PCR analyses revealed a 5.5-fold up-regulation of miR-802 expression in the livers of the high-fat diet mice and a 30-fold increase in the miRNA's expression in the livers of the Leprdb/db mice.
“Our analysis revealed that murine miR-802 expression is highly enriched in the liver of lean mice,” the team noted. “Moreover, we compared miR-802 expression in primary hepatocytes versus non-hepatocytes isolated from control mice, revealing that miR-802 expression was tenfold higher in hepatocytes versus non-hepatocytes, indicating that liver parenchymal cells represent the main source of miR-802 expression in this tissue.”
The scientists then examined miR-802 expression in human livers, and found that it was significantly increased in overweight individuals, and that hepatic miR-802 expression “significantly correlated” with the body mass index of these people.
“Taken together, hepatic expression of miR-802 is increased in dietary and genetic mouse models of obesity, as well as in overweight human subjects,” they wrote in Nature.
Hypothesizing that increased miR-802 expression may contribute to insulin resistance, the investigators over-expressed the miRNA in a murine hepatoma cell line, and found that it diminished insulin's ability to phosphorylate Akt, a central signaling node of insulin action. Additionally, miR-802 over-expression was found to enhance basal and forskolin-induced expression of glucose 6 phosphatase, indicating an impairment of glucose metabolism.
Aiming to bear these findings out in vivo, the researchers generated mice with the capacity for doxycycline-induced over-expression of miR-802. They determined that transgenic over-expression of miR-802 does not interfere with transcription and processing of small RNAs in a non-specific manner, confirming that transgenic over-expression of the miRNA essentially mimicked its expression observed in obese mice.
“Although we detected no differences in blood glucose concentrations, transgenic mice developed glucose intolerance and insulin resistance upon miR-802over-expression,” they wrote. “Furthermore, homeostatic model assessment of insulin resistance indices of mice over-expressing miR-802 were significantly increased.” Taken together, these data show that miR-802 over-expression impairs glucose homeostasis in mice.
To determine miR-802's functional effect on insulin resistance, the team synthesized locked nucleic acids against the miRNA and administered them to mice receiving a high-fat diet and normal controls. While the treatment had no effect on body weight, body fat content, or circulating serum leptin concentrations, the obesity-associated rise in serum insulin concentrations was diminished in the obese mice.
Meanwhile, an improvement in glucose tolerance was observed with the inhibition of miR-802, as was an improvement in insulin sensitivity and a greater ability of insulin to inhibit glucose 6 phosphatase.
In all, “the findings show that reduction of miR-802 expression in diet-induced obese mice results in improvement of insulin sensitivity predominantly by increasing insulin action in the liver,” according to the Nature paper.
A bioinformatics analysis revealed a number of murine genes targeted by miR-802, including one encoding hepatocyte nuclear factor 1 beta, or Hnf1b, which has been causally linked to predisposition for type II diabetes. The researchers created shRNAs against this gene and found that its suppression in the liver triggered glucose intolerance and insulin signaling impairment, and promoted hepatic gluconeogenesis.
When the investigators examined the over-expression of Hnf1bin Leprdb/db, they observed an improvement of insulin sensitivity, “supporting the model that miR-802-mediated reduction of hepatic Hnf1b expression contributes to the metabolic impairment observed upon development of obesity,” they wrote.
Overall, the findings demonstrate an “important role” for obesity-induced over-expression of miR-802 in the development of obesity-associated insulin resistance, the researchers concluded.
"The observation that over-expression of miR-802 causes insulin resistance and impairs glucose tolerance, whereas reducing miR-802 expression in obese mice improves these metabolic parameters, clearly indicates a functional role for increased miR-802expression in the development of obesity-associated insulin resistance,” they wrote. “Importantly, increased miR-802 expression is not restricted to murine obesity models, but is also detected in obese humans, thus characterizing miR-802 and its target genes as potential new targets for the treatment of obesity-associated insulin resistance and type II diabetes.”