Synergistic effect of retinoic acid and cytokines on the regulation of glial fibrillary acidic protein expression.

Glial fibrillary acidic protein (GFAP) is the main astroglial marker during astrogliogenesis, but it is also expressed in other cell types, including neural stem cells and old neurons. Activation of the JAK/STAT pathway by the IL-6 family of cytokines is the canonical pathway regulating GFAP expression, whereas retinoic acid is thought to be the only inducer of GFAP to operate independently of this pathway. Here, we show that retinoic acid receptor α not only links retinoic acid signaling to the canonical cytokine-stimulated pathway leading to GFAP expression but that it also plays a key role in the synergistic actions of retinoic acid and cytokines on this pathway. Cytokines both potentiate retinoic acid receptor α expression and enhance its binding to DNA and to the Stat3-p300/CBP-Smad transcriptional complex, the cornerstone of the canonical pathway. PI3K is upstream to all the key events leading to the expression of GFAP. Our results give new insights about the role of retinoic acid signaling in GFAP expression.