ICAM-1-Coupled Signaling Pathways in Astrocytes Converge to Cyclic AMP Response Element-Binding Protein Phosphorylation and TNF-α Secretion

In the CNS, astrocytes play a key role in immunological and inflammatory responses through ICAM-1 expression, cytokine secretion (including TNF-α), and regulation of blood-brain barrier permeability. Because ICAM-1 transduces intracellular signals in lymphocytes and endothelial cells, we investigated in the present study ICAM-1-coupled signaling pathways in astrocytes. Using rat astrocytes in culture, we report that ICAM-1 binding by specific Abs induces TNF-α secretion together with phosphorylation of the transcription factor cAMP response element-binding protein. We show that ICAM-1 binding induces cAMP accumulation and activation of the mitogen-activated protein kinase extracellular signal-regulated kinase. Both pathways are responsible for cAMP response element-binding protein phosphorylation and TNF-α secretion. Moreover, these responses are partially dependent protein kinase C, which acts indirectly, as a common activator of cAMP/protein kinase A and extracellular signal-regulated kinase pathways. These results constitute the first evidence of ICAM-1 coupling to intracellular signaling pathways in glial cells and demonstrate the convergence of these pathways onto transcription factor regulation and TNF-α secretion. They strongly suggest that ICAM-1-dependent cellular adhesion to astrocytes could contribute to the inflammatory processes observed during leukocyte infiltration in the CNS.