Activation of Melanocortin 4 Receptors Reduces the Inflammatory Response and Prevents Apoptosis Induced by Lipopolysaccharide and Interferon-γ in Astrocytes

Abstract

α-MSH exerts an immunomodulatory action in the brain and may play a neuroprotective role acting through melanocortin 4 receptors (MC4Rs). In the present study, we show that MC4Rs are constitutively expressed in astrocytes as determined by immunocytochemistry, RT-PCR, and Western blot analysis. α-MSH (5 μm) reduced the nitric oxide production and the expression of inducible nitric oxide synthase (iNOS) induced by bacterial lipopolysaccharide (LPS, 1 μg/ml) plus interferon-γ (IFN-γ, 50 ng/ml) in cultured astrocytes after 24 h. α-MSH also attenuated the stimulatory effect of LPS/IFN-γ on prostaglandin E2 release and cyclooxygenase-2 (COX-2) expression. Treatment with HS024, a selective MC4R antagonist, blocked the antiinflammatory effects of α-MSH, suggesting a MC4R-mediated mechanism in the action of this melanocortin. In astrocytes, LPS/IFN-γ treatment reduced cell viability, increased the number of terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling-positive cells and activated caspase-3. α-MSH prevented these apoptotic events, and this cytoprotective effect was abolished by HS024. LPS/IFN-γ decreased Bcl-2, whereas it increased Bax protein expression in astrocytes, thus increasing the Bax/Bcl-2 ratio. α-MSH produced a shift in Bax/Bcl-2 ratio toward astrocyte survival because it increased Bcl-2 expression and also prevented the effect of LPS/IFN-γ on Bax and Bcl-2 expression. In summary, these findings suggest that α-MSH, through MC4R activation, attenuates LPS/IFN-γ-induced inflammation by decreasing iNOS and COX-2 expression and prevents LPS/IFN-γ-induced apoptosis of astrocytes by modulating the expression of proteins of the Bcl-2 family.