Phenotypic polarization of activated astrocytes: The critical role of lipocalin-2 in the classical inflammatory activation of astrocytes

Astrocytes provide structural and functional support for neurons, as well as display neurotoxic or neuroprotective phenotypes depending upon the presence of an immune or inflammatory microenvironment. This study was undertaken to characterize multiple phenotypes of activated astrocytes and to investigate the regulatory mechanisms involved. We report that activated astrocytes in culture exhibit two functional phenotypes with respect to pro- or anti-inflammatory gene expression, glial fibrillary acidic protein expression, and neurotoxic or neuroprotective activities. The two distinct functional phenotypes of astrocytes were also demonstrated in a mouse neuroinflammation model, which showed pro- or anti-inflammatory gene expression in astrocytes following challenge with classical or alternative activation stimuli; similar results were obtained in the absence of microglia. Subsequent studies involving recombinant lipocalin-2 (LCN2) protein treatment or Lcn2-deficient mice indicated that the pro- or antiinflammatory functionally polarized phenotypes of astrocytes and their intracellular signaling pathway were critically regulated by LCN2 under in vitro and in vivo conditions. Astrocyte-derived LCN2 promoted classical proinflammatory activation of astrocytes but inhibited IL-4-STAT6 signaling, a canonical pathway involved in alternative anti-inflammatory activation. Our results suggest that the secreted protein LCN2 is an autocrine modulator of the functional polarization of astrocytes in the presence of immune or inflammatory stimuli and that LCN2 could be targeted therapeutically to dampen proinflammatory astrocytic activation and related pathologies in the CNS.