Labeling and imaging of GLUT4 in live L6 cells with quantum dots

SIRT1 interacts with metabolic transcriptional factors in the pancreas of insulin-resistant and calorie-restricted rats

Sirtuin 1 (SIRT1) is one member of the silent information regulator 2 (Sir2)-like family of proteins involved in glucose homeostasis in mammals. It has been reported that SIRT1 modulates endocrine signaling of glucose and fat homeostasis by regulating transcription factors such as forkhead transcription factor 3a (FOXO3a), glucose transporter 4 (GLUT4), peroxisome proliferator-activated receptor gamma (PPARγ) and PPARγ coactivator (PGC-1α). However, it is still not clear how SIRT1 is involved in the development of insulin resistance. To determine the location and expression of SIRT1 and its target proteins in rats and analyze the interactions and functions of these proteins in insulin resistance. Forty-eight male Sprague-Dawley rats were randomly divided into four regimen groups: normal control (NC), calorie restriction (CR), high-fat (HFa), and high-fructose (HFr). Animals were fed for 12 weeks and blood samples collected from tail veins at weeks 2, 4, 6, 8 and 12 after fasting for 16 h. Baseline metabolic parameters such as fasting blood sugar, insulin, cholesterol and triglycerides were analyzed. A glucose tolerance test was carried out at the end of the study. Visceral fat, consisting of epididymis and perirenal fat, was isolated and weighed. The pancreas from each animal was also immediately removed. Immunohistochemical staining was performed to detect the locations of SIRT1, FOXO3a, GLUT4, PPARγ and PGC-1α in the β-cell of the rat pancreas. Expression in the pancreas was analyzed by western blotting. Blood biochemical analysis indicated that the HFa and HFr groups were insulin-resistant. Immunohistochemical staining showed that GLUT4 was a nuclear protein. SIRT1, FOXO3a, PPARγ and PGC-1α were present in both the nucleus and the cytoplasm of β-cells of pancreatic islets. The expression of SIRT1, GLUT4 and PGC-1α increased significantly in response to CR, but decreased in the HFr and HFa groups. FOXO3a was similar in the CR and the NC groups, whereas it declined in the HFa and HFr groups. PPARγ was elevated in the HFa group, but dropped in the CR and HFr groups. These data suggest that SIRT1 and its regulators are involved in the development of insulin resistance.

Observing GLUT4 translocation in live L6 cells using quantum dots

The glucose transporter 4 (GLUT4) plays a key role in maintaining whole body glucose homeostasis. Tracking GLUT4 in space and time can provide new insights for understanding the mechanisms of insulin-regulated GLUT4 translocation. Organic dyes and fluorescent proteins were used in previous studies for investigating the traffic of GLUT4 in skeletal muscle cells and adipocytes. Because of their relative weak fluorescent signal against strong cellular autofluorescence background and their fast photobleaching rate, most studies only focused on particular segments of GLUT4 traffic. In this study, we have developed a new method for observing the translocation of GLUT4 targeted with photostable and bright quantum dots (QDs) in live L6 cells. QDs were targeted to GLUT4myc specifically and internalized with GLUT4myc through receptor-mediated endocytosis. Compared with traditional fluorescence dyes and fluorescent proteins, QDs with high brightness and extremely photostability are suitable for long-term single particle tracking, so individual GLUT4-QD complex can be easily detected and tracked for long periods of time. This newly described method will be a powerful tool for observing the translocation of GLUT4 in live L6 cells.