Transcriptional regulation of p57kip2 expression during development, differentiation and disease.

p57kip2 is the most complex member of the CIP/KIP family of cyclin-dependent kinase inhibitors and plays a fundamental role in regulating cell cycle and differentiation during mammalian development. Consistently with a key role for p57kip2 in the spatial and temporal control of cell proliferation, its expression is fine-tuned by multiple regulatory mechanisms, resulting in a tissue-, developmental phase- and cell type-specific pattern. Moreover, p57kip2 is an imprinted gene, further supporting the importance of its proper expression dosage. Importantly, misregulation of p57kip2 expression has been associated, more frequently than mutations in its coding region, to human growth disorders, such as Beckwith-Wiedemann and Silver-Russell syndromes, as well as to the onset of several types of cancers. This review will summarize the molecular mechanisms regulating p57kip2 transcription during differentiation and development, their relationship with the imprinting control and their alterations in growth-related diseases and cancer. Particular attention will be given to the role of epigenetic mechanisms, involving DNA methylation, histone modifications, long-range chromatin interactions and non-coding RNAs in modulating and integrating the functions of cis-regulatory elements and trans-acting factors.

A cross-talk between DNA methylation and H3 lysine 9 dimethylation at the KvDMR1 region controls the induction of Cdkn1c in muscle cells.

The cdk inhibitor p57kip2, encoded by the Cdkn1c gene, plays a critical role in mammalian development and in the differentiation of several tissues. Cdkn1c protein levels are carefully regulated via imprinting and other epigenetic mechanisms affecting both the promoter and distant regulatory elements, which restrict its expression to particular developmental phases or specific cell types. Inappropriate activation of these regulatory mechanisms leads to Cdkn1c silencing, causing growth disorders and cancer. We have previously reported that, in skeletal muscle cells, induction of Cdkn1c expression requires the binding of the bHLH myogenic factor MyoD to a long-distance regulatory element within the imprinting control region KvDMR1. Interestingly, MyoD binding to KvDMR1 is prevented in myogenic cell types refractory to the induction of Cdkn1c. In the present work, we took advantage of this model system to investigate the epigenetic determinants of the differential interaction of MyoD with KvDMR1. We show that treatment with the DNA demethylating agent 5-azacytidine restores the binding of MyoD to KvDMR1 in cells unresponsive to Cdkn1c induction. This, in turn, promotes the release of a repressive chromatin loop between KvDMR1 and Cdkn1c promoter and, thus, the upregulation of the gene. Analysis of the chromatin status of Cdkn1c promoter and KvDMR1 in unresponsive compared to responsive cell types showed that their differential responsiveness to the MyoD-dependent induction of the gene does not involve just their methylation status but, rather, the differential H3 lysine 9 dimethylation at KvDMR1. Finally, we report that the same histone modification also marks the KvDMR1 region of human cancer cells in which Cdkn1c is silenced. On the basis of these results, we suggest that the epigenetic status of KvDMR1 represents a critical determinant of the cell type-restricted expression of Cdkn1c and, possibly, of its aberrant silencing in some pathological conditions.

Poly(ADP-ribosyl)ation is required to modulate chromatin changes at c-MYC promoter during emergence from quiescence.

Poly(ADP-ribosyl)ation is a post-translational modification of various proteins and participates in the regulation of chromatin structure and transcription through complex mechanisms not completely understood. We have previously shown that PARP-1, the major family member of poly(ADP-ribose)polymerases, plays an important role in the cell cycle reactivation of resting cells by regulating the expression of Immediate Early Response Genes, such as c-MYC, c-FOS, JUNB and EGR-1. In the present work we have investigated the molecular mechanisms by which the enzyme induces c-MYC transcription upon serum stimulation of quiescent cells. We show that PARP-1 is constitutively associated in vivo to a c-MYC promoter region recognized as biologically relevant for the transcriptional regulation of the gene. Moreover, we report that serum stimulation causes the prompt accumulation of ADP-ribose polymers on the same region and that this modification is required for chromatin decondensation and for the exchange of negative for positive transcriptional regulators. Finally we provide evidence that the inhibition of PARP activity along with serum stimulation impairs c-MYC induction by preventing the proper accumulation of histone H3 phosphoacetylation, a specific chromatin mark for the activation of Immediate Early Response Genes. These findings not only suggest a novel strategy by which PARP-1 regulates the transcriptional activity of promoters but also provide new information about the complex regulation of c-MYC expression, a critical determinant of the transition from quiescence to proliferation.

PARP-1 interaction with VP1 capsid protein regulates polyomavirus early gene expression.

Poly(ADP-ribose)polymerases are involved in fundamental cellular events as well as they seem to be associated to some viral infection process. In this work, the poly(ADP-ribose)polymerase-1 (PARP-1) role in the polyomavirus life cycle has been investigated. Early viral transcription was reduced by competitive inhibitors of PARPs in Swiss 3T3 cells and almost abolished in PARP-1 knockout fibroblasts and in wild-type fibroblasts when PARP-1 was silenced by RNA interference. In vivo chromatin immunoprecipitation assays showed that poly(ADP-ribosyl)ation (poly(ADP-ribose)) facilitates the release of the capsid protein viral protein 1 (VP1) from the chromatin of infecting virions. In vitro experiments demonstrated that VP1 stimulates the enzymatic activity of PARP-1 and binds non-covalently both protein-free and PARP-1-bound poly(ADP-ribose). Our studies suggest that PARP-1 promotes the complete VP1 displacement from viral DNA favouring the viral early transcription.

p57Kip2 is induced by MyoD through a p73-dependent pathway.

The cyclin-dependent-kinase inhibitors p21 and p57 are highly expressed in skeletal muscle where they redundantly control cell cycle arrest during differentiation. We have previously shown that p57 is a target of the myogenic factor MyoD in cells lacking p21. Here we show that MyoD induces p57 at the transcriptional level through a mechanism different from that involved in p21 regulation, since it is E-box-independent and requires new synthesized protein(s). We have identified p73 family members as the factors that mediate the activation of p57 through a 165bp promoter region. The levels of p73 alpha, beta and delta isoforms increase during muscle differentiation both in MyoD-expressing fibroblasts and in spontaneously differentiating C2 myoblasts. Moreover, the expression of a p73 dominant negative mutant interferes with the induction of p57. Finally, each of the isoforms up-regulated by MyoD, even when over-expressed alone, is capable of inducing p57 in p21-lacking fibroblasts. In contrast, the same p73 isoforms, either induced by MyoD or exogenously over-expressed, are unable to activate the expression of p57 in p21-expressing fibroblasts. Our finding that a transfected p57 promoter-reporter construct, unlike the endogenous gene, is responsive to both MyoD and p73 even in these cells, suggests that a cis-acting mechanism, probably involving a repressive chromatin structure, prevents the induction of p57 in p21-expressing fibroblasts.

MyoD induces the expression of p57Kip2 in cells lacking p21Cip1/Waf1: overlapping and distinct functions of the two cdk inhibitors.

The myogenic factor MyoD induces the expression of the cdk inhibitor p21 to promote cell cycle withdrawal in differentiating myoblasts. Although the cdk inhibitor p57 is also highly expressed in skeletal muscle and is thought to redundantly control myogenesis, little is known about its regulation, that has been suggested to be independent of MyoD. Here we show, for the first time, that MyoD is capable to induce the expression of p57. Intriguingly, this ability is restricted to cells lacking p21, suggesting that the two cdk inhibitors may be expressed in different muscle cell lineages. We also suggest that the functions of p21 and p57 in myoblast cells are only in part redundant. In fact, while the two cdk inhibitors play a similar role in cells undergoing G1 arrest during MyoD-induced differentiation, p57 does not replace p21 in cells escaping G1 arrest and undergoing MyoD-induced apoptosis. This difference can be ascribed both to a different subcellular localization and to a differential ability of the two cdk inhibitors to interact with cell cycle regulators.

MyoD induces apoptosis in the absence of RB function through a p21(WAF1)-dependent re-localization of cyclin/cdk complexes to the nucleus.

During differentiation of skeletal myoblasts, MyoD promotes growth arrest through the induction of the cdk inhibitor p21 and the accumulation of hypophosphorylated RB protein. Myoblasts lacking RB function fail to accomplish full differentiation and undergo apoptosis. Here we show that exogenous MyoD induces apoptosis in several cell backgrounds sharing RB inactivation. This process is associated with increased levels of cell cycle-driving proteins and aberrant cell cycle progression. The inability of MyoD to induce apoptosis in a p21-null background, highlights a requirement of p21 in RB-regulated apoptosis during myogenesis. This pro-apoptotic function of p21 cannot be exerted by simple p21 over-expression, but requires the co-operation of MyoD. We also suggest that the essential aspect of p21 activity involved in such a process is related to its ability to induce the nuclear accumulation and aberrant activity of cyclin/cdk complexes. These results establish a novel link between MyoD, p21 and RB during myogenesis, providing new insights into the antagonism between muscle differentiation and loss of RB function.