Toll‐like receptor 4‐dependent innate immune responses are mediated by intracrine corticosteroids and activation of glycogen synthase kinase‐3β in astrocytes

Reactive astrocytes are important pathophysiologically and synthesize neurosteroids. We observed that LPS increased immunoreactive TLR4 and key steroidogenic enzymes in cortical astrocytes of rats and investigated whether corticosteroids are produced and mediate astrocytic TLR4-dependent innate immune responses. We found that LPS increased steroidogenic acute regulatory protein (StAR) and StAR-dependent aldosterone production in purified astrocytes. Both increases were blocked by the TLR4 antagonist TAK242. LPS also increased 11β-hydroxysteroid dehydrogenase type 1 (11β-HSD1) and corticosterone production, and both were prevented by TAK242 and by siRNAs against 11β-HSD1, StAR, or aldosterone synthase (CYP11B2). Knockdown of 11β-HSD1, StAR, or CYP11B2 or blocking either mineralocorticoid receptors (MR) or glucocorticoid receptors (GR) prevented dephosphorylation of p-Ser⁹GSK-3β, activation of NF-κB, and the GSK-3β-dependent increases of C3, IL-1β, and TNF-α caused by LPS. Exogenous aldosterone mimicked the MR- and GSK-3β-dependent pro-inflammatory effects of LPS in astrocytes, but corticosterone did not. Supernatants from astrocytes treated with LPS reduced MAP2 and viability of cultured neurons except when astrocytic StAR or MR was inhibited. In adrenalectomized rats, intracerebroventricular injection of LPS increased astrocytic TLR4, StAR, CYP11B2, and 11β-HSD1, NF-κB, C3 and IL-1β, decreased astrocytic p-Ser⁹GSK-3β in the cortex and was neurotoxic, except when spironolactone was co-injected, consistent with the in vitro results. LPS also activated NF-κB in some NeuN⁺ and CD11b⁺ cells in the cortex, and these effects were prevented by spironolactone. We conclude that intracrine aldosterone may be involved in the TLR4-dependent innate immune responses of astrocytes and can trigger paracrine effects by activating astrocytic MR/GSK-3β/NF-κB signaling.