Serum amyloid a differentially activates microglia and astrocytes via the PI3K pathway.

Microglia and astrocytes in the brain play an important role in the development and progression of Alzheimer's disease (AD). Serum amyloid A (SAA) is a major acute-phase protein produced locally in the brain and colocalizes with senile plaques in AD patients. We investigated whether SAA plays a role in the development of AD. The viability of cultured primary microglia and astrocytes was measured by MTT; cell cycle and apoptosis analysis was also conducted. Cultured microglia and astrocytes were stimulated with 1 μM SAA for different periods of time (2, 4, 6, 12 h) or treated with 1 μM SAA with or without 15 min pretreatment of MAPK or PI3K inhibitors. Total RNA was extracted for qPCR analysis. SAA induced morphological changes of primary microglia but not astrocytes. Interestingly, SAA increased the viability of microglia by inhibiting their apoptosis and reduced the viability of astrocytes by inducing G1 cell cycle arresting. SAA treatment increased the mRNA levels of IL-6, TNF-α, IL12p40, IL23p19, and IL-10, with higher potency in microglia than in astrocytes. However, SAA induced more iNOS mRNA in astrocytes than in microglia. SAA induced these cytokines and iNOS expression by activating the PI3K pathway in both glial cells, but selectively activated the JNK pathway in microglia and the NF-κB pathway in astrocytes. These results suggest that SAA can stimulate a different reactive phenotype in microglia and astrocytes, and SAA regulates cell viability differently in these two glial cells in part through the PI3K pathway.