Diabetologisk disputation
Disputation & avhandling - INSULIN CONTROL OF GLUCOSE TRANSPORT IN CAVEOLAE MICRODOMAINS OF THE PLASMA MEMBRANEOktober 1997
Johanna Gustavsson
Linköping University Medical Dissertation No. 530 INSULIN CONTROL OF GLUCOSE TRANSPORT IN CAVEOLAE MICRODOMAINS OF THE PLASMA MEMBRANE
Akademisk avhandling försvaras fredagen den 10 oktober 1997 kl 09.oo i Linköping. Fakultetsopponent: Doc Harriet Wallberg-Henriksson, Sektionen för klinisk fysiologi, Karolinska Sjukhuset, StockholmABSTRACT
Caveolae are invaginated, dynamic microdomains in the plasma membrane and believed to be involved in receptor-mediated uptake of small molecules (potocytosis) and in signal transduction. A phopsphatidylinisotol glycan, a precursor of potential insulin second messangers, has been found to be enriched in the caveolae-fraction of adipocyte plasma membranes (Parpal et al., 1995, J Cell Biol 131:125-131). We now demonstrate that the insulin receptor is localized to caveolae micordomains. This was investigated in 1) 3T3-L1 adipocyte plasma membranes by a morphological method (double immunofluoroscence labeling and confocal microscopy) and in 2) caveolae isolated in biochemical, detergent-free method. The insulin receptor was enriched in caveolae and, in response to insulin, phosphorylated on tyrosine which indicated that the insulin receptor was active. Insulin stimulated the translocation of glucose transporter proteins >from intracellular stores to the plasma membrane which leads to an increased glucose uptake. Long-chain 1,2-diacylglycerol, one of the two potential second messangers for insulin, has been found to stimulate glucose uptake in rat adipocytes (Strålfors, 1988, Nature, 335:554-556). Here, we report that long-chain 1,2-diacylglycerols are taken up by different cell types in amounts sufficient to have biological effects, equilally well in the absence of taurodeoxycolate. We also report that a rapid translocation of GLUT4 to the plasma membrane was followed by a slower transition of GLUT4 into caveolae. Accumulation of GLU4 in caveolae coincided with the insulin-stimulated increase in glucose uptake. This offers a mechanistic explanation for the observed discrepancy between the appearance of GLUT4 in the plasma membrane and the delayed increase in glucose uptake.
Non-hydrolyzable GTP-analogs stimulate the translocation of GLUT 4 and increase glucose uptake in permeabilized cells. The small GTP-binding protein Rab4 is suggested to be involved in these processes since Ra4 has been localized to GLUT4-containing vesicles and is redistributed in response to insulin. We found that Rab4 is inriched in caveolae and that the amount of Rab4 increased in caveolae, in the same extent as GLUT4 did, in response to insulin.
Caveolae are charachterized by high levels of sphingolipids and cholesterol. Depletion of cholesterol, which disrupts the integrity of caveolae, abolished insulin-stimulated glucose uptake reversibly. Insulin´s control of protein phosphorylation was abolised while beta-adrenergic signaling was unaffected.
The results suggest that caveolae are crucial for insulin-sginaling in adipocytes and a disruption of these structures may have consequence for the development of insulin resistance and diabetes mellitus.
Dpt of Biomedicine and Surgery, Division of Cell Biology Faculty of Helath Sciences,
S-581 85 Linköping, Sweden
ISBN 91-7871-794-9
NyhetsINFO 971022
Johanna.Gustavsson@MCB.LiU.SE
