Anti-inflammatory effects of progesterone in human microglia via TLR4/NLRP3 pathway modulation: Relevance to drug-resistant epilepsy

Background: Progesterone (P4) is used as an antiseizure medication (ASM) to treat catamenial epilepsy, refractory to first-line drugs. P4 and other neurosteroids (NSs) are important regulators of multiple nervous system functions, including neuronal excitability and synaptic plasticity. In addition to their antiseizure properties, P4 and other NSs are also anti-inflammatory agents. Neuroinflammation is an important pathophysiological mechanism of epilepsy refractory to ASMs. Accordingly, we evaluated the ability of P4 to modulate neuroinflammation, using human microglia activated by lipopolysaccharide (LPS). Methods: Human microglia (HMC3) were stimulated for 3 h with LPS in the absence or presence of various concentrations of P4. Thereafter, levels of (i) toll-like receptor 4 (TLR4), (ii) the NLRP3 inflammasome, and (iii) pro-inflammatory cytokines were quantitated by real-time PCR and Western blot analyses. Phagocytic activity was also assessed using a phagocytosis assay employing fluorescent beads. Results: P4 treatment significantly reduced the microglial inflammatory state induced by LPS, which was mediated by upregulation of the TLR4- and NLRP3-axes. The protective effects of P4 were mediated by inhibition of Nuclear Factor kappa-light-chain-enhancer of activated B cells (NFκB) phosphorylation and reduced activation of Mitogen-Activated Protein Kinases (MAPK). The effects of P4 included a significant reduction in mRNA levels of the main pro-inflammatory cytokines and a reduction in phagocytic activity of HMC3. Conclusions: P4 is endowed with significant anti-inflammatory properties, which may be involved in the beneficial effects reported for drug-resistant catamenial epilepsy. Further research is required to clarify P4 post-receptor mechanisms of action and to explore the roles of other P4-derived NSs.