Dysregulated
sphingolipid metabolism causes neuronal cell death and is associated with
insulin resistance and diseases. Thus, we hypothesized that diabetes-induced changes in
retinal sphingolipid metabolism may contribute to neuronal pathologies in
diabetic retinopathy. ESI-MS/MS was used to measure
ceramide content and
ceramide metabolites in whole retinas after 2, 4, and 8 weeks of
streptozotocin-induced diabetes. After 4 and 8 weeks of diabetes, a approximately 30% decrease in total
ceramide content was observed, concomitant with a significant approximately 30% increase in
glucosylceramide levels in fed diabetic rats compared with their age-matched controls. Acute
insulin therapy as well as a short-term lowering of
glucose via fasting did not affect the increase in
glucosylceramide composition. To assess the putative
biological consequences of the increase in
glucosylceramide composition, R28 retinal neurons were treated with
glucosylceramide synthase inhibitors. Inhibiting
glycosphingolipid metabolism increased
insulin sensitivity in retinal neurons.
Glycosphingolipid inhibitors augmented
insulin-stimulated p70 S6kinase activity in the presence of inhibitory concentrations of high
glucose or
glucosamine. Inhibition of
glycosphingolipid synthesis also suppressed
glucosamine- and interleukin-1beta-induced death. Consistent with these inhibitor studies, pharmacological accumulation of
glycosphingolipids increased activation of the endoplasmic reticulum stress response, a putative modulator of
insulin resistance and neuronal apoptosis. It is speculated that an increase in
glucosylceramide, and possibly higher-order
glycosphingolipids, could contribute to the pathogenesis of
diabetic retinopathy by contributing to local
insulin resistance, resulting in neuronal cell death. Thus, dysfunctional
glycosphingolipid metabolism may contribute to metabolic stress in diabetes, and therapeutic strategies to restore normal
sphingolipid metabolism may be a viable approach for treatment of
diabetic retinopathy.