By Max Bingham, PhD
Topical Fluoxetine Proposed for Diabetic Foot Wound Healing
Fluoxetine applied to nonhealing skin wounds may be able to improve healing, according to Nguyen et al. (p. ), potentially setting the drug up as a topical treatment for diabetic foot ulcers. The experiments they report initially focus on cellular experiments and an in vitro scratch assay, which is used to model cell migration. In this case, the authors focus on keratinocyte migration, finding that the application of serotonin resulted in enhanced migration in a dose-dependent manner; the highest dose resulted in ∼55% coverage of the scratch wound area, while untreated controls managed ∼42%. The addition of fluoxetine, however, resulted in coverage of 67% at the highest dose. Investigations into the involved pathways suggested that fluoxetine increases keratinocyte migration via serotonin signaling and specifically via 5-HT receptor 2A. They also found that in mice with diabetes, wounds treated with topical fluoxetine showed decreased wound area and less exudate after 10 days compared with controls. Re-epithelialization in control mice reached ∼40%, while mice treated with fluoxetine managed ∼66%. Using an immunohistochemistry approach, they found evidence of increased angiogenesis and wound bed vascularity in wounds treated with fluoxetine. There was also a reduction in immune cells, indicative of reduced inflammation in comparison to that seen in controls. Further analyses with flow cytometry and RT-PCR revealed more insights into pathways and mechanisms. Finally, the authors report that systemic levels of fluoxetine did not increase following topical application of the drug and that there were no overt psychological effects following administration via this route. In their article, Nguyen et al. conclude the following: “Since fluoxetine has undergone extensive toxicology profiling and safety evaluation, and postmarketing adverse event reporting, the path to translation to clinic for this novel indication could be shortened, and a therapeutic need filled rapidly. We believe that this topical therapeutic represents a safe alternative for the challenging clinical problem of chronic, nonhealing wounds in patients with diabetes.”
Representative wound images from day 0 to day 10 after excisional biopsy in db/db mice treated with topical PEG vehicle (control), topical 0.2% fluoxetine (FLX), or topical serotonin (5-HT). Scale bar = 1 mm. Average percent re-epithelialization per treatment group was quantified at day 10.
Representative wound images from day 0 to day 10 after excisional biopsy in db/db mice treated with topical PEG vehicle (control), topical 0.2% fluoxetine (FLX), or topical serotonin (5-HT). Scale bar = 1 mm. Average percent re-epithelialization per treatment group was quantified at day 10.
Modified Urocortin 2 for the Treatment of Type 2 Diabetes, Obesity?
A modified version of the neuropeptide urocortin 2 (UCN2) appears to have therapeutic potential in obesity, insulin resistance, and type 2 diabetes, according to Borg et al. (p. ). Injections of the compound into obese mice resulted in substantially reduced food intake and rapid weight loss as well as improved glucose tolerance and uptake into skeletal muscle. Based on the results, the authors suggest that the modified compound might be acting as an insulin sensitizer in skeletal muscle and thus might have therapeutic potential for type 2 diabetes. As they point out, exercise and dietary interventions can have potent effects toward the same outcomes and yet adherence is poor, which, they suggest, leads to a need for pharmacological interventions to achieve the same outcomes. The study initially focuses on mice fed a high-fat diet for 20 weeks who then received daily injections of a control vehicle or the test compound for 14 days. There was substantial weight loss in the compound group whether or not the mice were also exposed to exercise (wheel running). There was also a substantial (84%) drop in food intake immediately that was followed by an overall reduction of 28% at the end of the treatment period. Further investigations pointed toward weight loss in the compound-treated group being due to reductions in fat mass (as opposed to lean mass). Moving to ex vivo experiments, the authors then assessed insulin sensitivity in leg muscles after exposure to the compound or vehicle. They found increased insulin-stimulated Akt phosphorylation following compound treatment compared with control as well as increased glucose transport. They also found that the compound acutely promoted GLUT4 translocation, which suggests there were direct effects on glucose clearance and uptake into skeletal muscle. In their article, Borg et al. conclude: “Our results fill a therapeutic void by providing new evidence for a treatment for type 2 diabetes that acts on skeletal muscle to enhance insulin sensitivity and glucose transport.”
Modified UCN2 increases GLUT4 translocation and glucose transport into skeletal muscle. L6-GLUT4-Myc myoblasts were stimulated with insulin, compound A (Comp A), or clenbuterol, and GLUT4 translocation to the cell membrane was assessed.
Modified UCN2 increases GLUT4 translocation and glucose transport into skeletal muscle. L6-GLUT4-Myc myoblasts were stimulated with insulin, compound A (Comp A), or clenbuterol, and GLUT4 translocation to the cell membrane was assessed.
Genetic Regulation of Insulin Sensitivity in Adipose Tissue Uncovered
The genetic basis of insulin resistance (IR) is explored by Sharma et al. (p. ), who reveal a complex picture of differential gene expression correlating with measures of insulin across two completely separate cohorts of different ethnicities. On top of that, they also uncover a role for the enoyl-CoA hydratase domain-containing 3 gene in regulating IR, successfully demonstrating that its knockdown does affect glucose uptake in adipocytes, at least in vitro. Using a combination of gene expression, genotyping, and insulin sensitivity data, the authors initially focus on a cohort of ∼250 individuals of African American ancestry with no diabetes. The individuals did, however, have a broad range of insulin sensitivity. They found that roughly 5,000 transcripts from adipose tissue either positively or negatively correlated with insulin sensitivity (as measured by the Matsuda index). Even after adjusting for BMI, 83% still had a significant correlation with insulin sensitivity, which together had some potential involvement in over 200 canonical pathways. To then validate the findings, they turned to a cohort of individuals with European ancestry and ran the same analysis again. They found that just under 4,000 transcripts were significantly correlated with the measure of insulin sensitivity in both cohorts and that 97.5% had concordant direction of effect. Out of all the transcripts that the cohorts shared, the top ranked was the enoyl-CoA hydratase domain-containing 3 gene. The authors go on to detail how it might affect insulin sensitivity and specifically show that silencing the gene in adipocytes resulted in significant reductions in insulin-stimulated glucose uptake, suggesting a role for the gene. Author Swapan K. Das commented: “Our study suggests a role for a novel molecular mechanism in IR, but more importantly it provides evidence for a complex genetic regulatory mechanism for IR, which involves hundreds of genes. Future studies will be required to fully define gene regulatory networks and driver genes that are causally linked to IR and to identify common and ethnically predominant mechanisms of insulin sensitivity.”
AMPK Regulates Muscle Glucose Uptake After Exercise (but Not During)
An analysis by Kjøbsted et al. (p. ) suggests that AMPK and its downstream target TBC1D1 are involved in regulating muscle glucose uptake but only in the period after exercise and not during exercise. Previous studies on the role of AMPK in exercise and glucose uptake have resulted in inconsistencies, and according to the authors, this might be related to the way AMPK/glucose readings were captured in those studies. Specifically, studies using AMPK knockout mice have only ever assessed glucose uptake during and after exercise or only after exercise but never during and after exercise separately. That meant the effect of the AMPK knockout had not been assessed completely. Using two different AMPK knockout mouse models and a TBC1D1-deficient model, the authors found that glucose uptake during exercise was not compromised in the AMPK knockout mice, suggesting no role for it while exercise is undertaken. However, immediately following exercise, glucose uptake reversed much faster toward resting levels in the knockout mice compared with wild-type controls. They also found that muscle glucose uptake after contraction experiments was positively linked to phosphorylation of TBC1D1, the downstream target of AMPK, and that skeletal muscle of mice deficient in TBC1D1 had impaired glucose uptake after contraction. The authors go on to discuss some of the implications of their observations, ultimately concluding that they may well reconcile some of the inconsistencies seen in previous studies and that they would redefine the role of AMPK activation during and after exercise. In their article, Kjøbsted et al. state the following: “Based on these findings, we propose a model by which activation of AMPK in skeletal muscle during exercise acts to maintain a high glucose transport capacity in recovery from exercise. We propose that AMPK in this way secures a faster normalization of myocellular energy and fuel status after contractile activity.”
