Failure in cytokinesis, the final step in cell division, by generating tetra- and polyploidization promotes chromosomal instability, a hallmark of cancer. Here we show that HIPK2, a kinase involved in cell fate decisions in development and response to stress, controls cytokinesis and prevents tetraploidization through its effects on histone H2B. HIPK2 binds and phosphorylates histone H2B at S14 (H2B-S14(P)), and the two proteins colocalize at the midbody. HIPK2 depletion by targeted gene disruption or RNA interference results in loss of H2B-S14(P) at the midbody, prevention of cell cleavage, and tetra- and polyploidization. In HIPK2 null cells, restoration of wild-type HIPK2 activity or expression of a phosphomimetic H2B-S14D derivative abolishes cytokinesis defects and rescues cell proliferation, showing that H2B-S14(P) is required for a faithful cytokinesis. Overall, our data uncover mechanisms of a critical HIPK2 function in cytokinesis and in the prevention of tetraploidization.

HIPK2 was found to be involved in cell-cycle regulation dependent on p21(Waf-1/Cip-1) and independent of DNA damage.

c‐Abl function is strictly dependent on its subcellular localization. Using an in vitro approach, we identify c‐Abl as a new substrate for p300, CBP (CREB‐binding protein) and PCAF (p300/CBP‐associated factor) histone acetyltransferases. Remarkably, acetylation markedly alters its subcellular localization. Point mutagenesis indicated that Lys 730, located in the second nuclear localization signal, is the main target of p300 activity. It has previously been reported that c‐Abl accumulates in the cytoplasm during myogenic differentiation. Here, we show that c‐Abl protein is acetylated at early stages of myogenic differentiation. Indeed, acetylation on Lys 730 drives c‐Abl accumulation in the cytoplasm and promotes differentiation. Thus, Lys 730 acetylation is a novel post‐translational modification of c‐Abl and a novel mechanism for modulating its subcellular localization that contributes to myogenic differentiation.

The conversion of skeletal myoblasts to terminally differentiated myocytes is negatively controlled by several growth factors and oncoproteins. In this study, we have investigated the molecular mechanisms by which v-Src, a prototypic tyrosine kinase, perturbs myogenesis in primary avian myoblasts and in established murine C2C12 satellite cells. We determined the expression levels of the cell cycle regulators pRb, cyclin D1 and D3 and cyclindependent kinase inhibitors p21 and p27 in v-Srctransformed myoblasts and found that, in contrast to myogenin, they are normally modulated by differentiative cues, implying that v-Src affects myogenesis independent of cell proliferation. We then examined the levels of expression, DNA-binding ability and transcription-activation potentials of myogenic regulatory factors in transformed myoblasts and in myotubes after reactivation of a temperature-sensitive allele of v-Src. Our results reveal two distinct potential modes of repression targeted to myogenic factors. On the one hand, we show that v-Src reversibly inhibits the expression of MyoD and myogenin in C2C12 cells and of myogenin in quail myoblasts. Remarkably, these loci become resistant to activation of the kinase in the postmitotic compartment. On the other hand, we demonstrate that v-Src efficiently inhibits muscle gene expression by repressing the transcriptional activity of myogenic factors without affecting MyoD DNAbinding activity. Indeed, forced expression of MyoD and myogenin allows terminal differentiation of transformed myoblasts. Finally, we found that ectopic expression of the coactivator p300 restores transcription from extrachromosomal muscle-specific promoters.

The homeodomain-interacting protein kinase 2 (HIPK2) protein is a member of a recently identified family of nuclear protein kinases that are well conserved in various organisms. HIPK2 can bind to several homeotic factors and to a series of proteins involved in the regulation of cell survival and proliferation in response to morphogenetic and genotoxic signals. Here we report Hipk2-targeted disruption in mouse; Hipk2(-/-) mice are viable and fertile but significantly smaller than their wild-type littermates. This feature is present at birth and retained throughout the mouse adulthood. Mouse embryo fibroblasts from Hipk2(-/-) mice show a reduced proliferation rate, compared to the wild-type counterparts, with accumulation in the G0/G1 phase of the cell cycle and altered levels of the cell cycle regulators cyclin D and CDK6. Restoration of wild-type HIPK2 expression in Hipk2(-/-) cells rescues the normal phenotype supporting a role for HIPK2 in the regulation of cell proliferation.