Re-expression of DNA methylation-silenced CD44 gene in a resistant NB4 cell line: rescue of CD44-dependent cell death by cAMP.

In the acute promyelocytic leukemia cell line, NB4, activation of the CD44 receptor triggers apoptosis. This pathway does not operate in the retinoid-maturation-resistant NB4-LR1 subclone. In this work, we show that the CD44 gene is silenced in these cells. The molecular defect involves DNA methylation of cytosine phosphate guanine (CpG) island and underacetylation of histone H3 at CD44 promoter. The methylating inhibitor 5-aza-CdR and cyclic AMP (cAMP) reverse the CD44 gene silencing. Contrary to 5-aza-CdR, cAMP does not induce DNA demethylation or histone modification at the CD44 promoter, whereas an H3pS10/AcK14 dual modification is observed on a global level. cAMP also induces the expression of c-Jun transcription factor and its recruitment at the CD44 promoter. Chromatin immunoprecipitation assays further show the association of brahma (Brm), a subunit of SWI/SNF chromatin-remodelling complex involved in the crosstalk between transcription and RNA polymerase II (RNA Pol II) processing, as well as the binding of phosphorylated RNA Pol II to the proximal promoter region of CD44. Finally, our study reveals that cAMP re-establishes the CD44-mediated cell death signalling. We propose that one of the actions of cAMP in restoring normal cell phenotype of leukaemia cells may consist in a broad trans-reactivation of silenced genes, despite marked hypermethylation of their promoters, as illustrated here with CD44 re-expression.