p27Kip1 represses the Pitx2-mediated expression of p21Cip1 and regulates DNA replication during cell cycle progression

Environmentally relevant concentrations of antimony pose potential risks to human health: An evaluation on human umbilical vein endothelial cells

Antimony (Sb) ore exploitation and the use of Sb-containing drugs pose known health risks. This study investigated the toxicity of environmentally relevant concentrations of Sb (0.12-12 mg L-1) on human umbilical vein endothelial cells (HUVECs). The 50 % lethal concentration (LC50) of Sb to HUVECs was 11.4 mg L-1. Exposing to high level of Sb induced cell cycle arrest by altering the expression of cell cycle regulators, inhibiting the transitions of G0/G1 to S and S to G2/M. At 1.2 mg L-1 Sb, CKD6 and p21 expressions in HUVECs changed to 0.75 and 1.32 folds that of no-Sb control, respectively (p < 0.01). At 12 mg L-1 Sb, CDK2, CKD6, and p27 expressions decreased by 1.54, 4.41, and 1.54 folds (p < 0.001), while p21 expression increased by 3.03 folds (p < 0.001) as compared to control. Sb also led to cell apoptosis, evidenced by Annexin V-FITC/PI staining and changes in the expressions of Bax (1.21-1.30 folds, p < 0.01) and Bcl-2 (0.65-0.83 folds). Oxidative damage was a pivotal factor driving cell apoptosis, probably through down-regulating antioxidant genes (CAT, GPX1, and GSTP1) and up-regulating stress response genes (HO-1, SOD1, and TrxR1). The elevated H2O2 generated in mitochondria likely contributed to cell apoptosis due to the imbalance in H2O2 metabolism. These findings suggest that environmentally relevant concentrations of Sb can exert cytotoxicity to HUVECs, which should be of potential concern for human cardiovascular disease.

Phosphorylation of PED/PEA-15 at Ser116 and phosphorylation of p27 at Thr187 indicates a poor prognosis in hepatocellular carcinoma

Hepatocellular carcinoma (HCC) constitutes a deadly cancer with a high rate of recurrence and metastasis. Phosphoprotein enriched in diabetes/phosphoprotein enriched in astrocytes-15 (PED/PEA-15) is a protein involved in the metabolism of glucose that regulates numerous cellular processes, including cell division, apoptosis and migration in numerous types of cancer. However, PED/PEA-15 may act as a tumor-promotor or a tumor-suppressor depending on its phosphorylation status. In the present study, the association between the phosphorylation of PED/PEA-15 at Ser116 [PED/PEA-15(S116)], the phosphorylation of P27 at Thr187 [P-p27(T187)] and the clinicopathological features and prognosis of patients with HCC was assessed. The levels of PED/PEA-15(S116) and P-p27(T187) were determined using immunohistochemistry and western blotting analysis in resected liver tumor tissues and adjacent non-cancerous tissues obtained from 60 patients with HCC as well as normal liver tissues from 12 patients with benign lesions. The association between the expression levels of these two markers and the clinicopathological features of patients with HCC was explored. Using the Kaplan-Meier method, the prognostic value of PED/PEA-15(S116) and P-p27(T187) expression levels was determined. The results demonstrated that the levels of PED/PEA-15(S116) and P-p27(T187) proteins were remarkably higher in the HCC group compared with those in the adjacent and normal tissue groups (both P<0.05). In addition, a moderate positive correlation was observed between the levels of PED/PEA-15(S116) and P-p27(T187) (r=0.434; P<0.05). The levels of these two proteins were associated with the Edmondson grade, Tumor-Node-Metastasis (TNM) stage, vascular invasion and tumor multiplicity (all P<0.05). Furthermore, the Kaplan-Meier analysis results demonstrated that patients with HCC that presented with positive expression of PED/PEA-15(S116) and P-p27(T187) exhibited a dismal prognosis compared with that in patients with negative expression regarding the overall survival (OS), as well as disease-free survival (both P<0.05). Multivariate Cox analysis revealed that the TNM stage (P<0.05), vascular invasion (P<0.05), PED/PEA-15(S116) levels (P<0.001) and P-p27(T187) levels (P<0.05) were independent prognostic factors for OS in patients with HCC. In conclusion the results of the present study demonstrated that PED/PEA-15(S116) and P-p27(T187) levels were upregulated in HCC tissues compared with those in the adjacent and normal tissues; PED/PEA-15(S116) and P-p27(T187) expression may serve as an indicator of a poor prognosis in patients with HCC, suggesting that these proteins may be prospective therapeutic targets for HCC.

p27 as a Transcriptional Regulator: New Roles in Development and Cancer

p27 binds and inhibits cyclin-CDK to arrest the cell cycle. p27 also regulates other processes including cell migration and development independent of its cyclin-dependent kinase (CDK) inhibitory action. p27 is an atypical tumor suppressor-deletion or mutational inactivation of the gene encoding p27, CDKN1B, is rare in human cancers. p27 is rarely fully lost in cancers because it can play both tumor suppressive and oncogenic roles. Until recently, the paradigm was that oncogenic deregulation results from either loss of growth restraint due to excess p27 proteolysis or from an oncogenic gain of function through PI3K-mediated C-terminal p27 phosphorylation, which disrupts the cytoskeleton to increase cell motility and metastasis. In cancers, C-terminal phosphorylation alters p27 protein-protein interactions and shifts p27 from CDK inhibitor to oncogene. Recent data indicate p27 regulates transcription and acts as a transcriptional coregulator of cJun. C-terminal p27 phosphorylation increases p27-cJun recruitment to and action on target genes to drive oncogenic pathways and repress differentiation programs. This review focuses on noncanonical, CDK-independent functions of p27 in migration, invasion, development, and gene expression, with emphasis on how transcriptional regulation by p27 illuminates its actions in cancer. A better understanding of how p27-associated transcriptional complexes are regulated might identify new therapeutic targets at the interface between differentiation and growth control.

Role of p27Kip1 as a transcriptional regulator

The protein p27Kip1 is a member of the Cip/Kip family of cyclin-dependent kinase (Cdk) inhibitors. It interacts with both the catalytic and the regulatory subunit (cyclin) and introduces a region into the catalytic cleave of the Cdk inducing its inactivation. Its inhibitory capacity can be modulated by specific tyrosine phosphorylations. p27Kip1 also behaves as a transcriptional regulator. It associates with specific chromatin domains through different transcription factors. ChIP on chip, ChIP-seq and expression microarray analysis allowed the identification of the transcriptional programs regulated by p27Kip1. Thus, important cellular functions as cell division cycle, respiration, RNA processing, translation and cell adhesion, are under p27Kip1 regulation. Moreover, genes involved in pathologies as cancer and neurodegeneration are also regulated by p27Kip1, suggesting its implication in these pathologies. The carboxyl moiety of p27Kip1 can associate with different proteins, including transcriptional regulators. In contrast, its NH2-terminal region specifically interacts with cyclin-Cdk complexes. The general mechanistic model of how p27Kip1 regulates transcription is that it associates by its COOH region to the transcriptional regulators on the chromatin and by the NH2-domain to cyclin-Cdk complexes. After Cdk activation it would phosphorylate the specific targets on the chromatin leading to gene expression. This model has been demonstrated to apply in the transcriptional regulation of p130/E2F4 repressed genes involved in cell cycle progression. We summarize in this review our current knowledge on the role of p27Kip1 in the regulation of transcription, on the transcriptional programs under its regulation and on its relevance in pathologies as cancer and neurodegeneration.

p27Kip1 regulates alpha-synuclein expression

Alpha-synuclein (α-SYN) is the main component of anomalous protein aggregates (Lewy bodies) that play a crucial role in several neurodegenerative diseases (synucleinopathies) like Parkinson’s disease and multiple system atrophy. However, the mechanisms involved in its transcriptional regulation are poorly understood. We investigated here the role of the cyclin-dependent kinase (Cdk) inhibitor and transcriptional regulator p27^Kip1 (p27) in the regulation of α-SYN expression. We observed that selective deletion of p27 by CRISPR/Cas9 technology in neural cells resulted in increased levels of α-SYN. Knock-down of the member of the same family p21^Cip1 (p21) also led to increased α-SYN levels, indicating that p27 and p21 collaborate in the repression of α-SYN transcription. We demonstrated that this repression is mediated by the transcription factor E2F4 and the member of the retinoblastoma protein family p130 and that it is dependent of Cdk activity. Chromatin immunoprecipitation analysis revealed specific binding sites for p27, p21 and E2F4 in the proximal α-SYN gene promoter. Finally, luciferase assays revealed a direct action of p27, p21 and E2F4 in α-SYN gene expression. Our findings reveal for the first time a negative regulatory mechanism of α-SYN expression, suggesting a putative role for cell cycle regulators in the etiology of synucleinopathies.

ChIP-Seq analysis identifies p27(Kip1)-target genes involved in cell adhesion and cell signalling in mouse embryonic fibroblasts

The protein p27^Kip1 (p27), a member of the Cip-Kip family of cyclin-dependent kinase inhibitors, is involved in tumorigenesis and a correlation between reduced levels of this protein in human tumours and a worse prognosis has been established. Recent reports revealed that p27 also behaves as a transcriptional regulator. Thus, it has been postulated that the development of tumours with low amounts of p27 could be propitiated by deregulation of transcriptional programs under the control of p27. However, these programs still remain mostly unknown. The aim of this study has been to define the transcriptional programs regulated by p27 by first identifying the p27-binding sites (p27-BSs) on the whole chromatin of quiescent mouse embryonic fibroblasts. The chromatin regions associated to p27 have been annotated to the most proximal genes and it has been considered that the expression of these genes could by regulated by p27. The identification of the chromatin p27-BSs has been performed by Chromatin Immunoprecipitation Sequencing (ChIP-seq). Results revealed that p27 associated with 1839 sites that were annotated to 1417 different genes being 852 of them protein coding genes. Interestingly, most of the p27-BSs were in distal intergenic regions and introns whereas, in contrast, its association with promoter regions was very low. Gene ontology analysis of the protein coding genes revealed a number of relevant transcriptional programs regulated by p27 as cell adhesion, intracellular signalling and neuron differentiation among others. We validated the interaction of p27 with different chromatin regions by ChIP followed by qPCR and demonstrated that the expressions of several genes belonging to these programs are actually regulated by p27. Finally, cell adhesion assays revealed that the adhesion of p27^-/- cells to the plates was much higher that controls, revealing a role of p27 in the regulation of a transcriptional program involved in cell adhesion.

Human bone marrow mesenchymal stem cells suppress the proliferation of hepatic stellate cells by inhibiting the ubiquitination of p27

Bone marrow mesenchymal stem cells (BMSCs) have considerable therapeutic potential for the treatment of end-stage liver disease. Previous studies have demonstrated that BMSCs secrete growth factors and cytokines that inactivate hepatic stellate cells (HSCs), which inhibited the progression of hepatic fibrosis. The aim of this study was to determine the mechanism by which BMSCs suppress the function of HSCs in fibrosis. Our results showed that co-culture of BMSCs and HSCs induced cell cycle arrest at the G10/G1 phase and cell apoptosis of HSCs, which finally inhibited the cell proliferation of HSCs. Consistent with the cell cycle arrest, co-culture of BMSCs and HSCs increased the abundance of the cell cycle protein p27. Mechanistically, we further uncovered that following the co-culture with BMSCs, the expression level of the E3 ligase S-phase kinase-associated protein 2 (SKP2) that is responsible for the ubiquitination of p27 was decreased, which attenuated the ubiquitination of p27 and increased the stability of p27 in HSCs. Collectively, our results indicated the potential involvement of the SKP2-p27 axis for the inhibitory effect of BSMCs on the cell proliferation of HSCs.

Clinical performance validation of PITX2 DNA methylation as prognostic biomarker in patients with head and neck squamous cell carcinoma

BACKGROUND

Despite advances in combined modality therapy, outcomes in head and neck squamous cell cancer (HNSCC) remain dismal with five-year overall survival rates of less than 50%. Prognostic biomarkers are urgently needed to identify patients with a high risk of death after initial curative treatment. Methylation status of the paired-like homeodomain transcription factor 2 (PITX2) has recently emerged as a powerful prognostic biomarker in various cancers. In the present study, the clinical performance of PITX2 methylation was validated in a HNSCC cohort by means of an independent analytical platform (Infinium HumanMethylation450 BeadChip, Illumina, Inc.).

METHODS

A total of 528 HNSCC patients from The Cancer Genome Atlas (TCGA) were included in the study. Death was defined as primary endpoint. PITX2 methylation was correlated with overall survival and clinicopathological parameters.

RESULTS

PITX2 methylation was significantly associated with sex, tumor site, p16 status, and grade. In univariate Cox proportional hazards analysis, PITX2 hypermethylation analyzed as continuous and dichotomized variable was significantly associated with prolonged overall survival of HNSCC patients (continuous: hazard ratio (HR) = 0.19 [95%CI: 0.04-0.88], p = 0.034; dichotomized: HR = 0.52 [95%CI: 0.33-0.84], p = 0.007). In multivariate Cox analysis including established clinicopathological parameters, PITX2 promoter methylation was confirmed as prognostic factor (HR = 0.28 [95%CI: 0.09-0.84], p = 0.023).

CONCLUSION

Using an independent analytical platform, PITX2 methylation was validated as a prognostic biomarker in HNSCC patients, identifying patients that potentially benefit from intensified surveillance and/or administration of adjuvant/neodjuvant treatment, i.e. immunotherapy.