25-Hydroxycholesterol and 27-hydroxycholesterol induce neuroinflammation by activating microglia

Abstract Interleukin-1β (IL-1β) levels and side-chain oxygenated cholesterol molecules (oxysterols) are elevated in the brains of patients with Alzheimer’s disease (AD), and high cholesterol levels increase the risk of AD. However, roles of high cholesterol and side-chain oxysterols in IL-1β expression remain to be determined. Therefore, we investigated whether side-chain oxysterols such as 24s-hydroxycholesterol (24sOHChol), 25-hydroxycholsterol (25OHChol), and 27-hydroxycholesterol (27OHChol) along with cholesterol are involved in IL-1β expression. Treatment of microglial HMC3 cells with 25OHChol and 27OHChol induced IL-1β expression at both the transcript and protein levels. 25OHChol and 27OHChol also upregulated the surface expression of MHC class II (MHC II), a marker of activated microglia. In contrast, cholesterol and 24sOHChol did not increase IL-1β transcript levels or MHC II surface expression. 25OHChol and 27OHChol more potently increased IL-1β transcript levels than oligomeric amyloid beta. Polymyxin B impaired IL-1β expression induced by lipopolysaccharides, but not by 25OHChol and 27OHChol. Both oxysterols enhanced the phosphorylation of Akt, ERK, and Src, and inhibition of the kinase pathways by LY294002 (a PI3K inhibitor), U0126 (a MEK inhibitor), and PP2 (an Src kinase inhibitor) impaired the expression of IL-1β and MHC II. We also investigated microglial activation under condition of hypercholesterolemia which enhances oxysterol concentration. Immunohistochemistry revealed increased microglial expression of IL-1β and MHC II, as determined by their co-immunoreactivity with Iba-1, in apolipoprotein E-deficient mice. These results indicate that 25OHChol and 27OHChol activate the microglia to secrete IL-1β and the activation requires multiple signaling pathways. Because IL-1β is a key cytokine that drives inflammation in the brain, we suggest that elevated levels of 25OHChol and 27OHChol function as immunosterols, inducing neuroinflammation in patients with AD, and thereby are involved in the development of neurodegenerative diseases.