Cycloastragenol suppresses M1 and promotes M2 polarization in LPS-stimulated BV-2 cells and ischemic stroke mice

There are two distinct phenotypes of activated microglia, pro-inflammatory M1 and anti-inflammatory M2. Accumulating evidence indicates that shifting the microglial polarization from M1 to M2 is a potential strategy for the treatment of neuroinflammation-associated brain diseases, including ischemic stroke. Cycloastragenol (CAG) is a hydrolysis product of astragaloside IV, the major active component of Astragalus radix. We have previously demonstrated that CAG has anti-inflammatory effect in a mouse model of ischemic stroke. This study investigated the effect of CAG on the phenotype polarization of microglia in lipopolysaccharide (LPS)-stimulated BV-2 mouse microglial cells and ischemic stroke mice. In LPS-treated BV-2 cells, we found that CAG significantly reduced the expression of M1 markers, including pro-inflammatory cytokines and enzymes. In contrast, CAG promoted the expression of M2 markers, including anti-inflammatory cytokines and growth factor. In addition, CAG inhibited the activation of nuclear factor-κB (NF-κB) and enhanced the activation of nuclear factor E2-related factor 2 (Nrf2) and the expression of its downstream heme oxygenase-1 (HO-1). Furthermore, CAG also inhibited levels of M1 markers, promoted those of M2 markers, and enhanced Nrf2 activation and HO-1 expression in ischemic mouse brain. Importantly, the effect of CAG on M2 markers, but not M1 markers, was reversed by Nrf2 siRNA in LPS-stimulated BV-2 cells. Together, our results suggested that CAG promoted microglial M2 and suppressed M1 polarization through activating Nrf2 and inhibiting NF-κB, respectively, in LPS-stimulated BV-2 cells and ischemic mouse brain. CAG is a promising candidate for the treatment of neuroinflammation-related diseases, including ischemic stroke.