PRMT4 is a novel coactivator of c-Myb-dependent transcription in haematopoietic cell lines.

Protein arginine methyltransferase 4 (PRMT4)-dependent methylation of arginine residues in histones and other chromatin-associated proteins plays an important role in the regulation of gene expression. However, the exact mechanism of how PRMT4 activates transcription remains elusive. Here, we identify the chromatin remodeller Mi2α as a novel interaction partner of PRMT4. PRMT4 binds Mi2α and its close relative Mi2ß, but not the other components of the repressive Mi2-containing NuRD complex. In the search for the biological role of this interaction, we find that PRMT4 and Mi2α/ß interact with the transcription factor c-Myb and cooperatively coactivate c-Myb target gene expression in haematopoietic cell lines. This coactivation requires the methyltransferase and ATPase activity of PRMT4 and Mi2, respectively. Chromatin immunoprecipitation analysis shows that c-Myb target genes are direct transcriptional targets of PRMT4 and Mi2. Knockdown of PRMT4 or Mi2α/ß in haematopoietic cells of the erythroid lineage results in diminished transcriptional induction of c-Myb target genes, attenuated cell growth and survival, and deregulated differentiation resembling the effects caused by c-Myb depletion. These findings reveal an important and so far unknown connection between PRMT4 and the chromatin remodeller Mi2 in c-Myb signalling.

E-box-independent regulation of transcription and differentiation by MYC.

MYC proto-oncogene is a key player in cell homeostasis that is commonly deregulated in human carcinogenesis(1). MYC can either activate or repress target genes by forming a complex with MAX (ref. 2). MYC also exerts MAX-independent functions that are not yet fully characterized(3). Cells possess an intrinsic pathway that can abrogate MYC-MAX dimerization and E-box interaction, by inducing phosphorylation of MYC in a PAK2-dependent manner at three residues located in its helix-loop-helix domain(4). Here we show that these carboxy-terminal phosphorylation events switch MYC from an oncogenic to a tumour-suppressive function. In undifferentiated cells, MYC-MAX is targeted to the promoters of retinoic-acid-responsive genes by its direct interaction with the retinoic acid receptor-α (RARα). MYC-MAX cooperates with RARα to repress genes required for differentiation, in an E-box-independent manner. Conversely, on C-terminal phosphorylation of MYC during differentiation, the complex switches from a repressive to an activating function, by releasing MAX and recruiting transcriptional co-activators. Phospho-MYC synergizes with retinoic acid to eliminate circulating leukaemic cells and to decrease the level of tumour invasion. Our results identify an E-box-independent mechanism for transcriptional regulation by MYC that unveils previously unknown functions for MYC in differentiation. These may be exploited to develop alternative targeted therapies.

Epigenetics and senescence: learning from the INK4-ARF locus.

Cellular senescence is the biological consequence of aging. However, the same mechanisms that provoke senescence during aging have been proven to act in tumor suppression and thus to occur in premalignant cells. All the diverse aspects of the senescent phenotype, as are observed for many other cell fates, arise from alterations of the chromatin architecture. Relatively little is known overall about the changes in chromatin structure, and which regulatory networks are implicated in these. Major insight into the epigenetic contributions to senescence has been gained by studying the regulation of the INK4-ARF locus. Activation of the tumor suppressors encoded by this locus leads to an irreversible cell cycle exit. Importantly, epigenetic alterations at this locus have been associated with the onset of cancer. Here we discuss the recent findings that link epigenetics to the senescence pathway.