Negative role of cAMP-dependent protein kinase A in RANTES-mediated transcription of proinflammatory mediators through Raf.

The chemokine RANTES (regulated on activation normal T cell expressed and secreted) is expressed in several inflammatory diseases of the central nervous system and is a powerful stimulus for astrocyte production of proinflammatory mediators. The mechanism of RANTES-mediated astrocyte activation was investigated. RANTES stimulation decreased both intracellular cyclic AMP (cAMP) levels and cAMP-dependent protein kinase A (PKA) activity in cultures of primary mouse astrocytes. H-89, a potent inhibitor of PKA, mimicked RANTES-mediated chemokine and cytokine transcription. RANTES treatments activated Raf-1 kinase activity, and conversely a dominant negative Raf and a Raf-1 inhibitor blocked RANTES-induced chemokine transcription. Transfection with a constitutively active Raf was sufficient to induce transcription of proinflammatory mediators. The combined data indicate that Raf-1 is required for RANTES-mediated astrocyte activation. Decreases of cAMP and PKA activity contributed to the transcription of proinflammatory mediators by cross-talk with the Raf-1/mitogen-activated protein kinase pathway. The results identify an upstream signaling pathway for amplification of proinflammatory mediators in the central nervous system.