The protein p57Kip2 (hereafter called p57) was independently identified by two groups in 1995. It belongs to the family of cyclin-dependent kinase inhibitor (CKI) CIP/KIP along with p21Cip1 (hereafter called p21) and p27Kip1 (hereafter called p27). The role of this CKI family is to inhibit cell cycle progression by binding to cyclin D-CDK4/CDK6 and cyclin E/cyclin A-CDK2 complexes (Lee et al. 1995; Matsuoka et al. 1995). Studies from knockout (KO) mice in late 1990s revealed that CIP/KIP members exert only partially overlapping functions. In particular, while mice lacking p21 or p27 are viable, p57 KO mice are characterized by perinatal lethality and severe developmental defects. Moreover, the knock-in of p27 in p57 KO mice only partially overcomes the phenotype (Yan et al. 1997; Susaki et al. 2009).
While p21 and p27 have a wide tissue distribution, p57 expression is strictly regulated. The highest levels and widespread tissue localization of p57 have been observed during embryogenesis and differentiation. In contrast, few adult tissues, such as the muscle, brain, heart, lung, kidney, and placenta, maintain high levels of p57 expression. In adults, p57 is involved in the differentiation of several cell types such as myoblasts, hematopoietic stem cells, podocytes, placental cells, keratinocytes, pancreatic cells, hepatocytes, T lymphocytes, and spermatozoa (Pateras et al. 2009). More recently, it is emerging that p57 contribute to the control of cellular response to stress being involved in the induction of apoptosis or senescence (Rossi and Antonangeli 2015).
p57 Structure and Functions
p57 is important during embryonal development and is found in derivatives of all three germ layers. p57 is important also for the differentiation of some adult tissues where it not only regulates cell cycle exit but is able to directly interact with specific transcription factors. Indeed, in the differentiation of the nervous system, p57 interacts with the transcription factors Neuro D and Mash (Joseph et al. 2009) and, in myogenesis, binds and stabilizes the master gene of muscle differentiation MyoD (Reynaud et al. 2000). In the hematopoietic system, p57 is important for keeping hematopoietic stem cells in an undifferentiated, quiescent state, and its expression declines when the stem cells start to differentiate (Scandura et al. 2004).
In addition, p57 participates in the cellular response to stress as it has been demonstrated that through its QT domain, it interacts with and inhibits the kinase activity of c-Jun NH2-terminal kinase/stress-activated protein kinase (JNK/SAPK) (Chang et al. 2003). Through this interaction, p57 can participate in the regulation of many cellular responses to stress including cell death. p57 exerts a proapoptotic role also translocating in the mitochondria where it induces BAX activation and loss of mitochondrial membrane potential (Kavanagh et al. 2012). Some authors reported that the interaction with LIMK-1, by an indirect mechanism that involves actin filaments stabilization, is also responsible for apoptosis induction (Yokoo et al. 2003). Recently, it has been demonstrated that p57 participates also in the DNA damage response (Jia et al. 2015) being activated after genotoxic stimuli to induce cell cycle arrest and allow the DNA damage repair.
Transcriptional and Posttranscriptional Regulation of p57
The spatial and temporal expression of p57 is achieved through a complex epigenetic regulation. First of all, its gene, cdkn1c, is located in an imprinted domain, in which only the maternal allele is expressed and the paternal allele is silenced. The silencing of the paternal allele is achieved through the activity of a differentially methylated region, the KvDMR1. This region, 150 kbp far from p57 gene, is able to establish repressive chromatin markers on the imprinted genes of the locus through the formation of higher-order repressive chromatin and through the action of a long noncoding RNA, kcnq1ot1 (Shin et al. 2008). On the active maternal allele, both proximal and distal elements contribute to the regulation of p57 expression. In particular, the promoter region is bound by the TGFβ effector GATA2 (Billing et al. 2016), by Smad1 and Atf2 after DNA damage (Jia et al. 2015), and by Sp1 and EGR1 during muscle differentiation. In addition it has also been described that a p57 enhancer is bound by the Notch effector Hes1 in myoblast (Zalc et al. 2014).
On the maternal allele, the occurrence of a repressive chromatin loop between the KvDMR1 and p57 promoter has been demonstrated. This loop is mediated by the insulator protein CTCF. Epigenetic modifications, such as DNA methylation and histone methylation or acetylation occurring both at p57 promoter and at the KvDMR1, influence the accessibility of the gene to the transcriptional machinery and so p57 gene expression (Busanello et al. 2012).
Regarding the posttranscriptional regulation of p57, its mRNA is a direct target of several miRs among which the family of miR 221/222, which has been frequently observed upregulated in cancer cells.
Finally, the protein stability is regulated by ubiquitynilation performed by the SCF complex in G1 and early M phase. In S phase, the activity of the kinase SKP2 leads to p57 phosphorylation and subsequent degradation by the proteasome (Pateras et al. 2009).
p57 in Disease
Alteration in p57 expression or abundance has been associated to several pathological conditions spanning from cancer to diabetes, highlighting the importance of this protein in physiology and pathology.
During cancer progression, p57 function is often lost to allow abnormal proliferation. Moreover, downregulation of p57 is associated with poor prognosis and cancer aggressiveness. Several different mechanisms of p57 downregulation have been described in human tumors. The most important seems to be a hypermethylation of cdkn1c promoter that has been described in many different tumors, such as gastric, colorectal, hepatocellular, and hematopoietic cancers. Moreover, an increase in repressive histone markers in cdkn1c promoter has been found in breast cancer, rhabdoid tumor, and gastric cancer. Indeed, drugs that reduce DNA methylation such as 5′ azacytidine or enhance histone acetylation such as inhibitors of histone deacetylases are able to restore cdkn1c expression in cancer cells. Another mechanism of p57 silencing that involves epigenetic modifications at the level of the KvDMR1 has been described in liver, breast, cervical, gastric, and bladder carcinomas. Loss of heterozygosity of p57 locus due to chromosomal deletion has been described in Wilm’s tumors. As reported before, deregulation of miR 221/222 has also been correlated with decreased p57 in different cancer cells such as hepatocellular carcinoma (Pateras et al. 2009).
Overgrowth and Growth Restriction Diseases
A lack or an excess of p57 are associated with overgrowth disorders (Beckwith-Wiedemann syndrome) or with growth restriction diseases, (Silver-Russell syndrome, IMAGe), respectively.
Beckwith-Wiedemann syndrome (BWS). Around 60% of BWS patients present epigenetic defects that lead to loss of CDKN1C expression. BWS is characterized by a complex phenotype that includes malformation in several organs and a 1000-fold increased risk to develop embryonal tumors such as Wilm’s tumor, hepatoblastoma, and rhabdomyosarcoma (Eggermann et al. 2014). The lack of p57 seems to contribute to both developmental defects and cancer susceptibility.
Silver-Russell syndrome (SRS). SRS is characterized by severe intrauterine and postnatal growth retardation (Eggermann et al. 2014). Even if the genetic basis of SRS is heterogeneous, an excess of p57 has been documented in several cases and in mouse models.
IMAGe. The IMAGe syndrome is a very rare disease with only 25 patients reported. It is a growth retardation syndrome characterized by delayed skeletal development and adrenal insufficiency. In IMAGe syndrome, the excess of p57 is due to an increased stability of the protein caused by point mutations in its carboxy-terminus domain (Dias and Maher 2012).
p57 expression has been found increased in acute and end-stage heart failure. Moreover, a microsatellite repeat in the promoter region of p57 that leads to a decreased expression of p57 is associated with atherosclerosis and myocardial infarction (Pateras et al. 2009).
Hyperinsulinism of Infancy (HI) and Diabetes
HI is a rare genetic disorder with a prevalence of 1:50,000 characterized by mild or severe hypoglycemia that can lead to irreversible neurological damage.
It has been demonstrated that while normal βcells express p57 during different stages of development and in adult tissues, this expression is lost in focal HI (Kassem et al. 2000). Moreover, the downregulation of p57 by shRNA in adult βcells promotes their replication supporting a key role of p57 loss in the massive replication of βcells that occurs in focal HI (Avrahami et al. 2014).
Regarding the association with diabetes, even if there is not any functional study, there are some indirect associations. For example, differences in the methylation levels of the KvDMR1 have been associated with type 2 diabetes risk (Travers et al. 2013) as well as gene variants located in a region that contains putative long distance regulative elements for p57 (Yasuda et al. 2008). More studies are necessary to elucidate if these differences reflect a real dysregulation of p57 cellular levels.
p57 is a cell cycle inhibitor belonging to the CIP/KIP family. It is involved in the regulation of G1-S transition by interacting with the complex cyclin-CDK2 and 4. p57 plays a crucial role during embryonal development and in adults for the differentiation programs of many tissues such as muscle. Recent findings highlight other roles of p57 distinct from the cell cycle control as the induction of apoptosis or the control of cytoskeletal dynamics. p57 expression pattern is strictly spatial and temporal regulated, and this regulation is achieved through complex epigenetic mechanisms including imprinting, DNA and histone methylation, three-dimensional chromatin loop, and long noncoding RNA. Dysregulation of p57 expression has been found in many cancer types as well as in several human disorders such as BWS, SRS, and HI. The involvement of p57 in many human pathologies highlights the importance of this protein and in particular of its correct time-tuned expression.
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