Abstract
The accumulation of glucose exerts various cytotoxic effects on endothelial and other
vascular cells, and thereby contributes to the development of microvascular complications
in diabetes. Since tissues, in which vascular complications typically occur, do not
take up glucose in an insulin regulated manner, it is an important question to know
whether other mechanisms exist in these cells to restrict the uptake and the accumulation
of glucose. To study this question, we used microvascular endothelial cells isolated
from rat heart endothelial cells (RHEC). In RHEC, the non-insulin regulated glucose
transporter (Glut-1) was detected as a broad protein band of 50 - 65 kD. In contrast,
the Glut-1 from rat brain, which was taken as reference, had a molecular weight of
45 kD. After treatment with endoglycosidase F, both proteins formed a band of approximately
40 kD on SDS-PAGE, demonstrating a more extensive glycosylation of Glut-1 in RHEC
as compared to brain. Incubation of the cells in high glucose (22 mM, up to 10 days)
did not down-regulate either Glut-1 protein or mRNA. In contrast to high glucose,
deprivation of the cells from glucose led to an increase in Glut-1 mRNA and protein
which is partly non-glycosylated. In cells from hearts of streptozotocin-diabetic
rats (DRHEC), Glut-1 protein, but not Glut-1 mRNA, was reduced by about 40%. Additionally,
a significant amount of glycosyl residues was resistant to the enzymatic treatment
with N-endoglycosidase F. Both changes in Glut-1 were also observed when the cells
were cultivated in low glucose (5.5 mM) for several passages indicating a long lasting,
hardly reversible modification of Glut-1 by diabetes. These data indicate that Glut-1
is not down-regulated in RHEC by high glucose, and that this important mechanism to
protect the endothelium against an intracellular accumulation of glucose is missing
in RHEC. As a consequence, increases in blood glucose may lead to a glucose overload
with the described deleterious effects on the structure and function of endothelium.
Key words
Diabetes Mellitus - Glucose Transporter - Heart Endothelial Cells
