Abstract
Diphlorethohydroxycarmalol (DPHC), a type of phlorotannin isolated from the marine alga Ishige okamurae, reportedly alleviates impaired glucose tolerance. However, the molecular mechanisms of DPHC regulatory activity and by which it exerts potential beneficial effects on glucose transport into skeletal myotubes to control glucose homeostasis remain largely unexplored. The aim of this study was to evaluate the effect of DPHC on cytosolic Ca2+ levels and its correlation with blood glucose transport in skeletal myotubes in vitro and in vivo. Cytosolic Ca2+ levels upon DPHC treatment were evaluated in skeletal myotubes and zebrafish larvae by Ca2+ imaging using Fluo-4. We investigated the effect of DPHC on the blood glucose level and glucose transport pathway in a hyperglycemic zebrafish. DPHC was shown to control blood glucose levels by accelerating glucose transport; this effect was associated with elevated cytosolic Ca2+ levels in skeletal myotubes. Moreover, the increased cytosolic Ca2+ level caused by DPHC can facilitate the Glut4/AMPK pathways of the skeletal muscle in activating glucose metabolism, thereby regulating muscle contraction through the regulation of expression of troponin I/C, CaMKII, and ATP. Our findings provide insights into the mechanism of DPHC activity in skeletal myotubes, suggesting that increased cytosolic Ca2+ levels caused by DPHC can promote glucose transport into skeletal myotubes to modulate blood glucose levels, thus indicating the potential use of DPHC in the prevention of diabetes.
Highlights
Glucose is stored as glycogen in the skeletal muscle, a major site of glucose uptake that is critical to whole-body glucose metabolism in humans
We present data supporting an association between the cytosolic Ca2+ release by DPHC in skeletal myotubes and its antidiabetes activity [20] that can improve impaired glucose uptake in diabetes to enable the maintenance of glucose homeostasis
Our results showed that DPHC can stimulate glucose transporter 4 (Glut4) translocation and phosphorylation of AMPK, which was dependent on cytosolic Ca2+ in skeletal myotubes, while there was no effect on Akt activation
Summary
Glucose is stored as glycogen in the skeletal muscle, a major site of glucose uptake that is critical to whole-body glucose metabolism in humans. Some of the signaling mechanisms that mediate an increase in the expression of glucose transporter 4 (Glut4) in response to exercise have been identified to be driven by a rise in cytosolic Ca2+ levels due to increased release of Ca2+ from the sarcoplasmic reticulum (SR) [3, 4]. These studies showed that an increase in glucose uptake during muscle contractions does not require membrane depolarization but only a release of Ca2+ to the cytoplasm [5]. Cytosolic Ca2+ in the skeletal muscle regulates several signaling pathways and metabolic events related to contraction and relaxation [6]
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