Albumin enhances superoxide production in cultured microglia

Microglial activation and disruption of blood-brain barrier (BBB) are known to occur and contribute to neuronal damages in cerebral ischemic conditions and in some neurodegenerative diseases. To investigate whether a serum factor leaked out from circulation enhances microglial activation, we examined the effect of normal rat serum on superoxide (O2-) production by cultured microglia. Microglia cultured from neonatal rat brains were studied on their O2- production induced by the addition of phorbol myristate acetate by a method of acetyl-cytochrome c reduction. The O2- production was significantly increased by the addition of 0.01% rat serum, and the maximal enhancement was observed at about 0.1% rat serum. After the serum was fractionated using a molecular sieve membrane, we observed the enhancing effect only in a greater molecular weight fraction (>50 kDa). Furthermore, three kinds of bovine serum albumin (BSA) with different purity, and human serum and plasma albumins, also enhanced O2- production to a similar extent to that by rat serum. However, other proteins tested showed no significant effect. The enhancement of O2- production by BSA was observed dose-dependently, and the effect of 50 microg/ml of purified BSA was equivalent to that of 0.1% rat serum, suggesting that albumin itself enhances O2- production by microglia. These results imply that albumin leaked out through impaired BBB may activate microglia and that the potentiation of O2- production by albumin results in the pathogenesis of neuronal damage in cerebral ischemia and some neurodegenerative diseases.