FIG. 4.

Inhibition of glycosphingolipid metabolism increases insulin sensitivity in retinal neurons. R28 cells were pretreated with glucosylceramide synthase inhibitors NB-DGJ (100 μmol/l) or PPMP (1 μmol/l) for 24 h before treatments with high glucose (HG, 30 mmol/l) or glucosamine (Gln, 5 mmol/l) for an additional 24 h. Cells were then serum starved for 2 h under the same conditions and treated with insulin (10 nmol/l) for 15 min. Western blots were then performed, and the phosphorylation state of p70 S6K, a kinase whose activity is diminished in the diabetic retina, was assessed. A representative blot of three separate experiments. The differences in basal activity of these drugs may reflect different pharmacokinetic and pharmacodynamic characteristics of these drugs.