Debora Bencivenga

p57(Kip2) and cancer: time for a critical appraisal.

p57Kip2 is a cyclin-dependent kinase inhibitor belonging to the Cip/Kip family, which also includes p21Cip1 and p27Kip1. So far, p57Kip2 is the least-studied Cip/Kip protein, and for a long time its relevance has been related mainly to its unique role in embryogenesis. Moreover, genetic and molecular studies on animal models and patients with Beckwith-Wiedemann syndrome have shown that alterations in CDKN1C (the p57Kip2 encoding gene) have functional relevance in the pathogenesis of this disease. Recently, a number of investigations have identified and characterized heretofore unexpected roles for p57Kip2. The protein appears to be critically involved in initial steps of cell and tissue differentiation, and particularly in neuronal development and erythropoiesis. Intriguingly, p27Kip1, the Cip/Kip member that is most homologous to p57Kip2, is primarily involved in the process of cell cycle exit. p57Kip2 also plays a critical role in controlling cytoskeletal organization and cell migration through its interaction with LIMK-1. Furthermore, p57Kip2 appears to modulate genome expression. Finally, accumulating evidence indicates that p57Kip2 protein is frequently downregulated in different types of human epithelial and nonepithelial cancers as a consequence of genetic and epigenetic events. In summary, the emerging picture is that several aspects of p57Kip2s functions are only poorly clarified. This review represents an appraisal of the data available on the p57Kip2 gene and protein structure, and its role in human physiology and pathology. We particularly focus our attention on p57Kip2 changes in cancers and pharmacological approaches for modulating p57Kip2 levels. Mol Cancer Res; 9(10); 1269–84. ©2011 AACR.

Targeting p27Kip1 protein: its relevance in the therapy of human cancer

Introduction: Cell division cycle progression is achieved by a sequential and stringently concerted activation of a family of serine–threonine kinases, namely the cyclin-dependent kinases (CDKs). p27Kip1 is a pivotal CDK inhibitor and a tight modulator of CDK-dependent phenotypes. Thus, p27Kip1 plays a fundamental role in key cellular processes such as proliferation, differentiation, apoptosis, substrate adhesion and motility. Intriguingly, when p27Kip1 is localized in the nucleus, it acts as an antiproliferative protein, while, in the cytosol, p27Kip1 promotes cytoskeleton remodeling and might positively influence metastatization. Downregulation of p27Kip1 nuclear level or its cytosolic mislocalization are consistently correlated with poor prognosis of numerous types of human epithelial and non-epithelial cancers. Areas covered: This review illustrates the basic structural features of p27Kip1 protein, its metabolism and alterations in human malignancies, along with describing anticancer strategies based on targeting p27Kip1. Expert opinion: Given the role of p27Kip1 in the control of cell proliferation and its decreased level observed in malignancies with poor outcome, drugs able to handle the protein levels and localization might represent an important goal for novel specific and effective anticancer strategies. Although no convincing proofs have been reported, putative negative consequences of p27Kip1 targeting might be also conceivable.

Retinoic acid induces p27Kip1 nuclear accumulation by modulating its phosphorylation.

All-trans-retinoic acid (ATRA), the most biologically active metabolite of vitamin A, controls cell proliferation, apoptosis, and differentiation depending on the cellular context. These activities point to ATRA as a candidate for cancer therapy. A pivotal effect of the molecule is the modulation of p27Kip1, a cyclin-dependent kinase (CDK) inhibitor (CDKI). Here, we investigate the mechanisms by which ATRA regulates p27Kip1 level in LAN-5, a neuroblastoma cell line. When added to the cells, ATRA causes a rapid nuclear increase of p27Kip1, which clearly precedes growth arrest. The early buildup is not due to impairment of the CDKI degradation, in contrast to previous observations. Particularly, we did not detect the down-regulation of Skp2 and Cks1, two proteins involved in the nuclear ubiquitin-dependent p27Kip1 removal. Moreover, the morphogen does not impair the CDKI nuclear export and does not cause CDK2 relocalization. The characterization of CDKI isoforms by two-dimensional PAGE/immunoblotting showed that ATRA induces an early nuclear up-regulation of monophosphorylated p27Kip1. Immunologic studies established that this isoform corresponds to p27Kip1 phosphorylated on S10. The buildup of phospho(S10)p27Kip1 precedes the CDKI accumulation and increases its half-life. Finally, ATRA-treated nuclear LAN-5 extracts showed an enhanced capability of phosphorylating p27Kip1 on S10, thus explaining the nuclear up-regulation of the isoform. In conclusion, our data suggest a novel mechanism of ATRA antiproliferative activity, in which the morphogen rapidly up-regulates a nuclear kinase activity that phosphorylates p27Kip1 on S10. In turn, this event causes the stabilization of p27Kip1 and its accumulation in the nuclear compartment.

Histone deacetylase inhibitors upregulate p57Kip2 level by enhancing its expression through Sp1 transcription factor

Histone deacetylase inhibitors (HDACIs) represent a new class of targeted anticancer agents. Here, we evaluate the effects of butyrate (BuA) and other HDACIs on p57(Kip2), a cyclin-dependent kinase inhibitor (cki). We observed that inhibitors of class I/II histone deacetylases (HDACs), but not of class III HDACs, induce a remarkable accumulation of p57(Kip2) in several cells. The cki upregulation is associated with an increased gene expression that was not prevented by cycloheximide, indicating that HDACIs affect directly p57(Kip2) transcription. The characterization of p57(Kip2) promoter indicates that the first 165 bp are mostly involved in the BuA effects. Chromatin immunoprecipitation studies demonstrated that the BuA treatment causes the recruitment of Sp1 transcription factor. The Sp1 importance was confirmed by the reduction of BuA effects by mithramycin A (an Sp1 antagonist) and, most stringently, by Sp1 downregulation due to Sp1 siRNA. Moreover, both the treatments reduce the p57(Kip2) transcription in untreated cells, suggesting that Sp1 is required for the constitutive cki expression. Studies employing plasmids containing parts of the 165 bp of p57(Kip2) promoter indicate that the promoter region between -87 and -113 bp, which includes two putative Sp1 consensus sequences, plays a critical role in the response to HDACIs. Since this p57(Kip2) promoter region also embraces the consensus sequence for the transcriptional repressor chicken ovalbumin upstream promoter transcription factor-interacting protein 2 (CTIP2), we evaluated whether this factor is involved into the BuA effect. When CTIP2 was downregulated by a specific siRNA, we observed the enhancement of BuA activity on p57(Kip2) expression suggesting that CTIP2 might also be involved in HDACIs effects.

The Tyrosine Kinase Inhibitor Dasatinib Induces a Marked Adipogenic Differentiation of Human Multipotent Mesenchymal Stromal Cells

Background The introduction of specific BCR-ABL inhibitors in chronic myelogenous leukemia therapy has entirely mutated the prognosis of this hematologic cancer from being a fatal disorder to becoming a chronic disease. Due to the probable long lasting treatment with tyrosine-kinase inhibitors (TKIs), the knowledge of their effects on normal cells is of pivotal importance. Design and Methods We investigated the effects of dasatinib treatment on human bone marrow-derived mesenchymal stromal cells (MSCs). Results Our findings demonstrate, for the first time, that dasatinib induces MSCs adipocytic differentiation. Particularly, when the TKI is added to the medium inducing osteogenic differentiation, a high MSCs percentage acquires adipocytic morphology and overexpresses adipocytic specific genes, including PPARγ, CEBPα, LPL and SREBP1c. Dasatinib also inhibits the activity of alkaline phosphatase, an osteogenic marker, and remarkably reduces matrix mineralization. The increase of PPARγ is also confirmed at protein level. The component of osteogenic medium required for dasatinib-induced adipogenesis is dexamethasone. Intriguingly, the increase of adipocytic markers is also observed in MSCs treated with dasatinib alone. The TKI effect is phenotype-specific, since fibroblasts do not undergo adipocytic differentiation or PPARγ increase. Conclusions Our data demonstrate that dasatinib treatment affects bone marrow MSCs commitment and suggest that TKIs therapy might modify normal phenotypes with potential significant negative consequences.

p57Kip2 is a downstream effector of BCR-ABL kinase inhibitors in chronic myelogenous leukemia cells

Chronic myelogenous leukemia (CML) is characterized by the expression of BCR-ABL tyrosine kinase, which results in increased cell proliferation and inhibition of apoptosis. In this study, we show that BCR-ABL-positive CML cell lines treated with imatinib (STI571) undergo G₁ cell cycle arrest associated with the accumulation of p57(Kip)², a cyclin-dependent kinase inhibitor (CKI). Interestingly, p57(Kip)² increase precedes the reported STI571-dependent upregulation of p27(Kip)¹. A number of complementary approaches allow the demonstration that p57(Kip)² buildup is due to the transcriptional activation of CDKN1C, the p57(Kip)²-encoding gene, while neither p57(Kip)² half-life elongation nor its cell relocalization were observed. We also identified a heretofore undescribed pattern of p57(Kip)² phosphorylated isoforms which, however, did not change in response to STI571 cell treatment. The imatinib-dependent p57(Kip)² upregulation occurs only in STI571-responsive cells, while the CKI accumulation was not evidenced in an imatinib-resistant clone. Nilotinib and dasatinib (second-generation BCR-ABL inhibitors), at concentrations comparable to those used in therapy, increase the CKI but do not affect p27(Kip)¹ level. Finally, CD34(+) cells from CML patients display a clear imatinib-dependent p57(Kip)² upregulation, which was not observed in CD34(+) cells from control subjects. In conclusion, our study points to p57(Kip)² as a novel and precocious effector of BCR-ABL targeting drugs.

Congenital erythrocytosis associated with gain-of-function HIF2A gene mutations and erythropoietin levels in the normal range

Hypoxia-inducible factor 2α (HIF-2α) plays a pivotal role in the balancing of oxygen requirements throughout the body. The protein is a transcription factor that modulates the expression of a wide array of genes and, in turn, controls several key processes including energy metabolism, erythropoiesis and angiogenesis. We describe here the identification of two cases of familial erythrocytosis associated with heterozygous HIF2A missense mutations, namely Ile533Val and Gly537Arg. Ile533Val is a novel mutation and represents the genetic HIF2A change nearest to Pro-531, the primary hydroxyl acceptor residue, so far identified. The Gly537Arg missense mutation has already been described in familial erythrocytosis. However, our patient is the only described case of a de novo HIF2A mutation associated with the development of congenital polycythemia. Functional in vivo studies, based on exogenous expression of hybrid HIF-2α transcription factors, indicated that these genetic alterations lead to the stabilization of HIF-2α protein. All the identified polycythemic subjects with HIF2A mutations show serum erythropoietin in the normal range, independently of the hematocrit values and phlebotomy frequency. The erythroid precursors obtained from the peripheral blood of patients showed an altered phenotype, including an increased rate of growth and a modified expression of some HIF-2α target genes. These results suggest the novel proposal that polycythemia observed in subjects with HIF2A mutations might also be due to primary changes in hematopoietic cells and not only secondary to increased erythropoietin levels.

Resveratrol mimics insulin activity in the adipogenic commitment of human bone marrow mesenchymal stromal cells.

Bone marrow mesenchymal stromal cells (BM-MSCs) are multipotent cells capable of differentiating toward osteoblatic and adipocytic phenotypes. BM-MSCs play several key roles including bone remodeling, establishment of hematopoietic niche and immune tolerance induction. Here, we investigated the effect of resveratrol (RSV), a therapeutically promising natural polyphenol, on the commitment of human BM-MSCs primary cultures. Cell differentiation was evaluated by means of morphological analysis, specific staining and expression of osteogenic and adipocytic master genes (Runx-2, PPARγ). To maintain BM-MSC multipotency, all experiments were performed on cells at very early passages. At any concentration RSV, added to standard medium, did not affect the phenotype of confluent BM-MSCs, while, when added to osteogenic or adipogenic medium, 1 μM RSV enhances the differentiation toward osteoblasts or adipocytes, respectively. Conversely, the addition of higher RSV concentration (25 μM) to both differentiation media resulted exclusively in BM-MSCs adipogenesis. Surprisingly, the analysis of RSV molecular effects demonstrated that the compound completely substitutes insulin, a key component of adipogenic medium. We also observed that RSV treatment is associated to enhanced phosphorylation of CREB, a critical effector of insulin adipogenic activity. Finally, our observations contribute to the mechanistic elucidation of the well-known RSV positive effect on insulin sensitivity and type 2 diabetes mellitus.

p27(Kip1) and human cancers: A reappraisal of a still enigmatic protein.

p27Kip1 is a cell cycle regulator firstly identified as a cyclin-dependent kinase inhibitor. For a long time, its function has been associated to cell cycle progression inhibition at G1/S boundary in response to antiproliferative stimuli. The picture resulted complicated by the discovery that p27Kip1 is an intrinsically unstructured protein, with numerous CDK-dependent and -independent functions and involvement in many cellular processes, such as cytoskeleton dynamics and cell motility control, apoptosis and autophagy activation. Depending on the cell context, these activities might turn to be oncogenic and stimulate cancer progression and metastatization. Nevertheless, p27Kip1 role in cancer biology suppression was underscored by myriad data reporting its down-regulation and/or cytoplasmic relocalization in different tumors, while usually no genetic alterations were found in human cancers, making the protein a non-canonical oncosuppressor. Recently, mostly due to advances in genomic analyses, CDKN1B, p27Kip1 encoding gene, has been found mutated in several cancers, thus leading to a profound reappraisal of CDKN1B role in tumorigenesis. This review summarizes the main p27Kip1 features, with major emphasis to its role in cancer biology and to the importance of CDKN1B mutations in tumor development.

p27Kip1 serine 10 phosphorylation determines its metabolism and interaction with cyclin-dependent kinases

p27Kip1 is a critical modulator of cell proliferation by controlling assembly, localization and activity of cyclin-dependent kinase (CDK). p27Kip1 also plays important roles in malignant transformation, modulating cell movement and interaction with the extracellular matrix. A critical p27Kip1 feature is the lack of a stable tertiary structure that enhances its “adaptability” to different interactors and explains the heterogeneity of its function. The absence of a well-defined folding underlines the importance of p27Kip1 post-translational modifications that might highly impact the protein functions. Here, we characterize the metabolism and CDK interaction of phosphoserine10-p27Kip1 (pS10- p27Kip1), the major phosphoisoform of p27Kip1. By an experimental strategy based on specific immunoprecipitation and bidimensional electrophoresis, we established that pS10-p27Kip1 is mainly bound to cyclin E/CDK2 rather than to cyclin A/CDK2. pS10- p27Kip1 is more stable than non-modified p27Kip1, since it is not (or scarcely) phosphorylated on T187, the post-translational modification required for p27Kip1 removal in the nucleus. pS10-p27Kip1 does not bind CDK1. The lack of this interaction might represent a mechanism for facilitating CDK1 activation and allowing mitosis completion. In conclusion, we suggest that nuclear p27Kip1 follows 2 almost independent pathways operating at different rates. One pathway involves threonine-187 and tyrosine phosphorylations and drives the protein toward its Skp2-dependent removal. The other involves serine-10 phosphorylation and results in the elongation of p27Kip1 half-life and specific CDK interactions. Thus, pS10-p27Kip1, due to its stability, might be thought as a major responsible for the p27Kip1-dependent arrest of cells in G1/G0 phase.