FOXO transcription factors cooperate with δEF1 to activate growth suppressive genes in B lymphocytes

Forkhead transcription factors regulate many aspects of lymphocyte development and function. The FOXO subgroup of Forkhead factors opposes proliferation and survival, and FOXO inactivation is an important outcome of Ag receptor signaling. FOXO activity at target promoters is modulated by other transcription factors in a manner dependent on cell type and external stimulus. We have investigated the mechanisms by which FOXO proteins activate the promoters of two target genes in murine B lymphocytes, Ccng2 (encoding cyclin G,) and Rbl2 (p130), each of which has been implicated in cell cycle arrest. FOXO proteins bound directly to both promoters in vitro and in vivo, augmented transcriptional activity in reporter assays, and increased expression of the endogenous genes. Each of the promoter sequences has consensus binding sites for the delta EF1 transcription factor, previously shown to either repress or activate different promoters. delta EF1 bound to the Ccng2 and Rbl2 promoters in vitro and in vivo and increased reporter activity as well as endogenous mRNA levels for these genes. Strikingly, delta EF1 synergized with FOXO proteins to strongly activate transcription from both promoters. Coexpression of delta EF1 enhanced FOXO-induced cell cycle arrest in B lymphoma cells. These findings establish a novel mechanism of FOXO function at target promoters: cooperation with delta EF1.