Downregulation of retinal GLUT1 in diabetes by ubiquitinylation.
OBJECTIVE
To investigate the effect of chronic hyperglycemia on the levels of the glucose transporter GLUT1 in retina and its ubiquitinylation.


METHODS
Two diabetic animal models (Goto Kakizaki rats and alloxan-induced diabetic rabbits) and retinal endothelial cells in culture were used. GLUT1 content was determined by western blotting. Glut1 mRNA was determined by RT-PCR and northern blotting. Ubiquitin conjugates were evaluated by western blot analysis. In vitro ubiquitin conjugation activity was evaluated in supernatants using radiolabeled ubiquitin. Evidence for GLUT1 ubiquitinylation was further investigated by transfecting HEK293 cells with a hemagglutinin (HA)-tagged ubiquitin cDNA followed by immunoprecipitation of the cell lysates.


RESULTS
Chronic hyperglycemia resulted in a significant decrease on the amount of GLUT1 protein without significant changes on the GLUT1 mRNA in the retinas of diabetic GK rats and alloxan treated rabbits, and in high glucose treated retinal endothelial cells, compared to controls. The content of high molecular weight ubiquitin conjugates was higher both in the membrane fractions of diabetic retinas and in endothelial cells incubated with high glucose concentrations. GLUT1 immunoprecipitated from diabetic retinas crossreacted with antibodies directed against ubiquitin suggesting that GLUT1 is posttranslationally modified by monoubiquitinylation. Cells transfected with HA-tagged ubiquitin revealed crossreactivity with anti-GLUT1 antibodies on the HA immunoprecipitates.


CONCLUSIONS
The data indicate that retinal GLUT1 abundance decreases in experimental diabetes and with exposure of retinal endothelial cells to elevated glucose concentrations. Results further suggest that decreased abundance of GLUT1 may be associated with its increased degradation by a ubiquitin dependent mechanism. Ubiquitinylation of GLUT1 may be the mechanism targeting GLUT1 for degradation in diabetes.
Fernandes
Rosa
R
Biomedical Institute for Research in Light and Image, Center of Ophthalmology, University of Coimbra, Coimbra, Portugal.
Carvalho
Ana Luisa
AL
Kumagai
Arno
A
Seica
Raquel
R
Hosoya
Ken-ichi
K
Terasaki
Tetsuya
T
Murta
Joaquim
J
Pereira
Paulo
P
Faro
Carlos
C
D000818:Animals; D001786:Blood Glucose / Q000032:analysis; D001813:Blood-Retinal Barrier; D015152:Blotting, Northern; D015153:Blotting, Western; D002460:Cell Line; D002908:Chronic Disease; D003921:Diabetes Mellitus, Experimental / Q000378:metabolism*; D003930:Diabetic Retinopathy / Q000378:metabolism*; D015536:Down-Regulation; D004730:Endothelium, Vascular / Q000378:metabolism*; D051272:Glucose Transporter Type 1; D006943:Hyperglycemia / Q000378:metabolism; D009004:Monosaccharide Transport Proteins / Q000235:genetics / Q000378:metabolism*; D012333:RNA, Messenger / Q000378:metabolism; D011817:Rabbits; D051381:Rats; D017208:Rats, Wistar; D012171:Retinal Vessels / Q000378:metabolism*; D020133:Reverse Transcriptase Polymerase Chain Reaction; D014162:Transfection; D025801:Ubiquitin / Q000235:genetics / Q000378:metabolism*
D001786:Blood Glucose; D051272:Glucose Transporter Type 1; D009004:Monosaccharide Transport Proteins; D012333:RNA, Messenger; C495679:Slc2a1 protein, rat; D025801:Ubiquitin