Glucose‐Induced Map Kinase Activation And Aldose Reductase Expression In Primary Schwann Cell Culture

Abstract

Diabetic hyperglycaemia creates biochemical alterations in nerve that lead to Wallerian degeneration, resulting in the modification of the Schwann cell phenotype. This suggests that the peripheral neuroglia could play a crucial role in diabetes‐induced peripheral neuropathy. Hyperglycaemia‐induced cellular stresses result in up‐regulation of aldose reductase (AR) protein expression. This study aimed to determine whether activation of mitogen‐activated protein kinases (MAPK) in Schwann cells in vitro might represent an early step in the transduction of hyperglycaemia to diabetes‐induced increased AR protein expression. We observed MAPK activation (Western blots for JNK and p38) in response to raised glucose and MAPK responses were expressed as the ratio of phosphorylated to total form. Neonatal rat Schwann cell cultures were treated with glucose (50, 200mM) for up to 24 hours. Mannitol (45, 195mM) was used as an osmotic control. Glucose for 1 hour (200mM) caused transient activation of both p38 (1.9 fold) and JNK (p < 0.01 for p56 and p46) versus controls (5.6mM glucose). Similar results were seen for JNK in response to mannitol. The mannitol‐induced p38 activation was sustained from 1 to 8 hours of treatment (p < 0.01 versus controls and p < 0.05 versus glucose treatment). Similarly, we observed a significant increase of AR content (p < 0.01 and p < 0.05 in response to glucose and mannitol, respectively). However, the maximum response occurred at 24 hours of treatment. Immunocytochemistry studies showed that activated JNK was located both in cytoplasm and nucleus, while accumulation of AR was mainly restricted to the cytoplasm surrounding the nucleus. These results suggest that JNK and/or p38 could transduce increased AR expression in response to high glucose, at least under in vitro conditions.