Id4 Expression Induces Apoptosis in Astrocytic Cultures and Is Down-regulated by Activation of the cAMP-Dependent Signal Transduction Pathway

Abstract

The Id family of helix–loop–helix transcription factors has been implicated in the regulation of cellular differentiation in several different lineages. We have explored the potential regulatory role of the cyclic AMP-dependent signaling pathway on Id gene expression in astroglial primary cultures. We found that primary cultures of mouse forebrain astrocytes constitutively expressed the four known members of the Id gene family, Id1, Id2, Id3, and Id4. During culture in presence of serum for 4 weeks, the expression of Id4 was up-regulated. In these same cultures, treatment with dibutyryl-cyclic AMP, a cyclic AMP analogue known to promote astrocyte differentiation, dramatically and selectively decreased Id4 gene expression. This effect was detectable after short-term treatment and was maintained during long-term treatment. Forskolin and pentoxifylline, two other agents known to elevate intracellular cyclic AMP through different mechanisms, also potently decreased Id4 gene expression. Furthermore, overexpression of Id4 in an astrocyte-derived cell line induced cells to round up and die by apoptosis. These results indicate that the cyclic AMP pathway acts as an inhibitor of Id4 gene expression in astrocytes, identify a new function for Id4, and suggest that Id4 is strategically positioned in the chain of molecular events regulating astrocyte differentiation and apoptosis.