Sp1 is involved in the transcriptional activation of p16(INK4) by p21(Waf1) in HeLa cells

Salvianolic acid B protects against pulmonary fibrosis by attenuating stimulating protein 1-mediated macrophage and alveolar type 2 cell senescence

Idiopathic pulmonary fibrosis (IPF), as the most common idiopathic interstitial pneumonia, is caused by a complex interaction of pathological mechanisms. Interestingly, IPF frequently occurs in the middle-aged and elderly populations but rarely affects young people. Salvianolic acid B (SAB) exerts antioxidant, antiinflammatory, and antifibrotic bioactivities and is considered a promising drug for pulmonary disease treatment. However, the pharmacological effects and mechanisms of SAB on cellular senescence of lung cells and IPF development remain unclear. We used bleomycin (BLM)-induced pulmonary fibrosis mice and different lung cells to investigate the antisenescence impact of SAB and explain its underlying mechanism by network pharmacology and the Human Protein Atlas database. Here, we found that SAB significantly prevented pulmonary fibrosis and cellular senescence in mice, and reversed the senescence trend and typical senescence-associated secretory phenotype (SASP) factors released from lung macrophages and alveolar type II (AT2) epithelial cells, which further reduced lung fibroblasts activation. Additionally, SAB alleviated the epithelial-mesenchymal transition process of AT2 cells induced by transforming growth factor beta. By predicting potential targets of SAB that were then confirmed by chromatin immunoprecipitation-qPCR technology, we determined that SAB directly hampered the binding of transcription factor stimulating protein 1 to the promoters of SASPs (P21 and P16), thus halting lung cell senescence. We demonstrated that SAB reduced BLM-induced AT2 and macrophage senescence, and the subsequent release of SASP factors that activated lung fibroblasts, thereby dual-relieving IPF. This study provides a new scientific foundation and perspective for pulmonary fibrosis therapy.

p21WAF1/Cip1 Regulation by hYSK1 Activates SP-1 Transcription Factor and Increases MMP-2 Expression under Hypoxic Conditions

The hYSK1, a serine/threonine kinase (STK)-25, has been implicated in a variety of cellular functions including cell migration and polarity. We have recently reported that hYSK1 down-regulated the expression and functions of p16^INK4a, a cell cycle regulatory protein, thereby enhancing migration and growth of cancer cells under hypoxic conditions. In this study, we further investigated the mechanisms underlying downregulation of p16^INK4a and anti-migratory function of hYSK1. Our study revealed that p21^WAF1/Cip1 is a novel binding partner of hYSK1. Moreover, the interaction between hYSK1 and p21^WAF1/Cip1 led to the inhibition of SP-1 transcriptional activity, as revealed by a significant down-regulation of SP-1-mediated transactivation of p16^INK4a promoter, and accelerated MMP-2 expression. Conversely, the knock-down of hYSK1 enhanced the p16^INK4a promoter activity and protein expression, and diminished MMP-2 transcription and protein levels in hypoxic conditions as compared to control. Taken together, hYSK1 blocks the p21^WAF1/Cip1 functions by direct interaction and inhibits the p16^INK4a expression and induces MMP-2 expression by its regulations of SP-1 transcriptional activity under the hypoxia conditions.

3' UTR lengthening as a novel mechanism in regulating cellular senescence

Cellular senescence has been viewed as a tumor suppression mechanism and also as a contributor to individual aging. Widespread shortening of 3' untranslated regions (3' UTRs) in messenger RNAs (mRNAs) by alternative polyadenylation (APA) has recently been discovered in cancer cells. However, the role of APA in the process of cellular senescence remains elusive. Here, we found that hundreds of genes in senescent cells tended to use distal poly(A) (pA) sites, leading to a global lengthening of 3' UTRs and reduced gene expression. Genes that harbor longer 3' UTRs in senescent cells were enriched in senescence-related pathways. Rras2, a member of the Ras superfamily that participates in multiple signal transduction pathways, preferred longer 3' UTR usage and exhibited decreased expression in senescent cells. Depletion of Rras2 promoted senescence, while rescue of Rras2 reversed senescence-associated phenotypes. Mechanistically, splicing factor TRA2B bound to a core "AGAA" motif located in the alternative 3' UTR of Rras2, thereby reducing the RRAS2 protein level and causing senescence. Both proximal and distal poly(A) signals showed strong sequence conservation, highlighting the vital role of APA regulation during evolution. Our results revealed APA as a novel mechanism in regulating cellular senescence.

hYSK1 promotes cancer cell proliferation and migration through negative regulation of p16INK4a under hypoxic conditions

The alteration of expression of p16^INK4a, a well-known cyclin-dependent kinase inhibitor involved in cell cycle control, in tumors is unclear, especially under hypoxic conditions. To evaluate p16^INK4a regulation, we performed a protein microarray analysis. Among 1,800 proteins in the array, we identified hYSK1 as a novel protein that interacts with the tumor suppressor p16^INK4a. hYSK1, a member of the Ste20 family of serine/threonine protein kinases, promotes cell migration and tumorigenesis and is activated by oxidative stress. However, the molecular mechanisms underlying the oncogenic potential of hYSK1 remain elusive. Here, we report that hYSK1 interacts with p16^INK4a under hypoxic conditions in tumors, where it negatively regulates p16^INK4a, enhancing cancer cell migration. Hypoxic stimulation of hYSK1 reduces p16^INK4a accumulation through p16 promoter regulation to interact with unphosporylated SP-1 and increases matrix metalloproteinase-2 (MMP-2) expression by activating the MMP-2 promoter associated with cell migration and proliferation.Conversely, knocking down hYSK1 expression activated p16^INK4a expression and suppressed MMP-2 expression. Thus, hYSK1 is necessary as a trigger for inactivating p16^INK4a and activating MMP-2 during tumor migration, suggesting that hYSK1 is a specific negative regulator of the tumor suppressor p16^INK4a and may represent a novel molecular target for reactivation of tumor suppressor genes in humans.

Cell cycle regulation by long non-coding RNAs

The mammalian cell cycle is precisely controlled by cyclin-dependent kinases (CDKs) and related pathways such as the RB and p53 pathways. Recent research on long non-coding RNAs (lncRNAs) indicates that many lncRNAs are involved in the regulation of critical cell cycle regulators such as the cyclins, CDKs, CDK inhibitors, pRB, and p53. These lncRNAs act as epigenetic regulators, transcription factor regulators, post-transcription regulators, and protein scaffolds. These cell cycle-regulated lncRNAs mainly control cellular levels of cell cycle regulators via various mechanisms, and may provide diversity and reliability to the general cell cycle. Interestingly, several lncRNAs are induced by DNA damage and participate in cell cycle arrest or induction of apoptosis as DNA damage responses. Therefore, deregulations of these cell cycle regulatory lncRNAs may be involved in tumorigenesis, and they are novel candidate molecular targets for cancer therapy and diagnosis.

Regulatory mechanisms of tumor suppressor P16(INK4A) and their relevance to cancer

P16(INK4A) (also known as P16 and MTS1), a protein consisting exclusively of four ankyrin repeats, is recognized as a tumor suppressor mainly because of the prevalence of genetic inactivation of the p16(INK4A) (or CDKN2A) gene in virtually all types of human cancers. However, it has also been shown that an elevated level of expression (upregulation) of P16 is involved in cellular senescence, aging, and cancer progression, indicating that the regulation of P16 is critical for its function. Here, we discuss the regulatory mechanisms of P16 function at the DNA level, the transcription level, and the posttranscriptional level, as well as their implications for the structure-function relationship of P16 and for human cancers.

Polymorphisms in promoter sequences of MDM2, p53, and p16 genes in normal Japanese individuals

Research has been conducted to identify sequence polymorphisms of gene promoter regions in patients and control subjects, including normal individuals, and to determine the influence of these polymorphisms on transcriptional regulation in cells that express wild-type or mutant p53. In this study we isolated genomic DNA from whole blood of healthy Japanese individuals and sequenced the promoter regions of the MDM2, p53, and p16(INK4a) genes. We identified polymorphisms comprising 3 nucleotide substitutions at exon 1 and intron 1 regions of the MDM2 gene and 1 nucleotide insertion at a poly(C) nucleotide position in the p53 gene. The Japanese individuals also exhibited p16(INK4a) polymorphisms at several positions, including position -191. Reporter gene analysis by using luciferase revealed that the polymorphisms of MDM2, p53, and p16(INK4a) differentially altered luciferase activities in several cell lines, including the Colo320DM, U251, and T98G cell lines expressing mutant p53. Our results indicate that the promoter sequences of these genes differ among normal Japanese individuals and that polymorphisms can alter gene transcription activity.

Transforming growth factor-alfa gene (TGFA), human tooth agenesis, and evidence of segmental uniparental isodisomy

We have previously reported an association between variants in the transforming growth factor-alfa gene (TGFA) and human tooth agenesis. To demonstrate in greater detail that TGFA contributes to tooth agenesis, we investigated additional markers in the gene. Cheek swab samples were obtained for DNA analysis from 116 patient/parent trios. Probands had at least one developmentally missing tooth, excluding third molars. Genotyping was performed using TaqMan assays. Linkage disequilibrium analysis and test of the transmission distortion of the marker alleles were performed. We confirmed that TGFA variants and haplotypes are associated with tooth agenesis. Moreover, it appears that preferential premolar agenesis is associated with TGFA, and patients with a family history of tooth agenesis would have an associated haplotype. Finally, we excluded that a TGFA microdeletion could cause sporadic agenesis in a case of upper lateral incisors and lower second premolars and suggest this case may be consequence of a segmental uniparental isodisomy.

Transcriptional upregulation and unmethylation of the promoter region of p16 in invasive basal cell carcinoma cells and partial co-localization with the gamma 2 chain of laminin-332

Basal cell carcinoma cells show low proliferation rates at the invasive front and a concordant upregulation of the cdk-inhibitor p16, limiting proliferative capacity. Little is known about the mechanisms of p16 regulation in normal and malignant cells apart from that many transcription factors such as Ets1, Ets2, SP1, SP3, JunB and the polycomb protein Bmi1 have the potential to induce or repress p16 expression. Therefore, the aim of this study was to determine how p16 is regulated in basal cell carcinoma with special focus on its upregulation in invasive cells. By analysing various microdissected areas of basal cell carcinoma using real-time quantitative PCR we observed upregulation of p16 mRNA in invasive tumour cells compared to centrally localized tumour cells. The methylation status of the p16 promoter, analysed by methylation-specific PCR, also showed diminished methylation in tumour cells at the invasive front, supporting the hypothesis that promoter methylation can affect the transcriptional activation of p16 in vivo. There was only sporadic co-localization of Ets, or ERK1/2 phosphorylation with p16 upregulation at the invasive front, suggesting that these factors were not directly involved in the regulation of p16. Furthermore, the gamma 2 chain of laminin-332 has been reported to be increased at the invasive front compared to the central areas of many tumours. Interestingly, in basal cell carcinoma we observed partial co-localization between p16 and the gamma 2 chain of laminin-332 in tumour cells towards areas of ulceration and in the majority of clearly infiltrative tumour cells but not in p16 positive tumour cells with a more pushing invasive growth pattern. These data suggest that concurrent p16 upregulation and decreased proliferation are more general phenomena in different types of invasive growth patterns in basal cell carcinomas and that these only partially overlap with the gamma 2 chain of laminin-332 associated invasion patterns.

Sp1 is essential for p16 expression in human diploid fibroblasts during senescence

BACKGROUND

p16(INK4a) tumor suppressor protein has been widely proposed to mediate entrance of the cells into the senescent stage. Promoter of p16(INK4a) gene contains at least five putative GC boxes, named GC-I to V, respectively. Our previous data showed that a potential Sp1 binding site, within the promoter region from -466 to -451, acts as a positive transcription regulatory element. These results led us to examine how Sp1 and/or Sp3 act on these GC boxes during aging in cultured human diploid fibroblasts.

METHODOLOGY/PRINCIPAL FINDINGS

Mutagenesis studies revealed that GC-I, II and IV, especially GC-II, are essential for p16(INK4a) gene expression in senescent cells. Electrophoretic mobility shift assays (EMSA) and ChIP assays demonstrated that both Sp1 and Sp3 bind to these elements and the binding activity is enhanced in senescent cells. Ectopic overexpression of Sp1, but not Sp3, induced the transcription of p16(INK4a). Both Sp1 RNAi and Mithramycin, a DNA intercalating agent that interferes with Sp1 and Sp3 binding activities, reduced p16(INK4a) gene expression. In addition, the enhanced binding of Sp1 to p16(INK4a) promoter during cellular senescence appeared to be the result of increased Sp1 binding affinity, not an alteration in Sp1 protein level.

CONCLUSIONS/SIGNIFICANCE

All these results suggest that GC- II is the key site for Sp1 binding and increase of Sp1 binding activity rather than protein levels contributes to the induction of p16(INK4a) expression during cell aging.

Induction of p16INK4A mediated by beta-catenin in a TCF4-independent manner: implications for alterations in p16INK4A and pRb expression during trans-differentiation of endometrial carcinoma cells

Excessive beta-catenin is considered to contribute to tumor progression by inducing transcription of cell cycle-related genes such as cyclin D1 and c-myc. In contrast, our recent studies demonstrated that beta-catenin could inhibit cell proliferation through activation of p14(ARF)/p53/p21(WAF1) pathway during trans-differentiation toward morular phenotype of endometrial carcinoma (Em Ca) cells. Here, we focused on associations with alterations in p16(INK4A) and pRb expression during this process. In clinical cases, p16(INK4A) immunoreactivity was found to frequently overlap with nuclear beta-catenin accumulation in small-sized morules and surrounding glandular carcinomas (Sur-Ca), demonstrating a significant positive correlation (r = 0.447, p < 0.0001) overall, while the immunoreactions showed stepwise decrease in enlarged morules, despite persistent accumulation of beta-catenin and p21(WAF1) in nuclei. Immunoreactivity for both total pRb and its phosphorylated form was apparently decreased in all morules as compared to Sur-Ca lesions, with a significantly positive correlation. In cell lines, transcriptional activation of p16(INK) (4A) promoter by active form beta-catenin, as well as p21(WAF1), occurred through the region from -385 to -280 bp relative to the translation start site, in a TCF4-independent manner. Moreover, cell proliferation was accompanied with phosphorylation of pRb and increased p16(INK4A) expression, while its inhibition by serum starvation caused decreased expression of total pRb but not p16(INK4A), resulting in high relative amounts of the latter. These findings indicate that induction of p16(INK4A) mediated by nuclear beta-catenin and p21(WAF1), along with loss of pRb expression, may be important for initial steps during trans-differentiation of Em Ca cells. In addition, its down-regulation is associated with progression of lesions.

p21Waf1/Cip1 plays a critical role in modulating senescence through changes of DNA methylation

It has been reported that genomic DNA methylation decreases gradually during cell culture and an organism's aging. However, less is known about the methylation changes of age-related specific genes in aging. p21(Waf1/Cip1) and p16(INK4a) are cyclin-dependent kinase (Cdk) inhibitors that are critical for the replicative senescence of normal cells. In this study, we show that p21(Waf1/Cip1) and p16(INK4a) have different methylation patterns during the aging process of normal human 2BS and WI-38 fibroblasts. p21(Waf1/Cip1) promoter is gradually methylated up into middle-aged fibroblasts but not with senescent fibroblasts, whereas p16(INK4a) is always unmethylated in the aging process. Correspondently, the protein levels of DNA methyltransferase 1 (DNMT1) and DNMT3a increase from young to middle-aged fibroblasts but decrease in the senescent fibroblasts, while DNMT3b decreases stably from young to senescent fibroblasts. p21(Waf1/Cip1) promoter methylation directly represses its expression and blocks the radiation-induced DNA damage-signaling pathway by p53 in middle-aged fibroblasts. More importantly, demethylation by 5-aza-CdR or DNMT1 RNA interference (RNAi) resulted in an increased p21(Waf1/Cip1) level and premature senescence of middle-aged fibroblasts demonstrated by cell growth arrest and high beta-Galactosidase expression. Our results suggest that p21(Waf1/Cip1) but not p16(INK4a) is involved in the DNA methylation mediated aging process. p21(Waf1/Cip1) promoter methylation may be a critical biological barrier to postpone the aging process.

Regulation of the INK4a/ARF locus by histone deacetylase inhibitors

Despite the importance of the INK4a/ARF locus in tumor suppression, its modulation by histone deacetylase inhibitors (HDACis) remains to be characterized. Here, we have shown that the levels of p16INK4a are decreased in human and murine fibroblasts upon exposure to relatively high concentrations of trichostatin A and sodium butyrate. Interestingly, the levels of p19ARF are strongly upregulated in murine cells even at low concentrations of HDACis. Using ARF-deficient cells, we have demonstrated that p19ARF plays an active role in HDACi-triggered cytostasis and the contribution of p19ARF to this arrest is of higher magnitude than that of the well established HDACi target p21Waf1/Cip. Moreover, chemically induced fibrosarcomas in ARF-null mice are more resistant to the therapeutic effect of HDACis than similar tumors in wild type or p21Waf1/Cip-null mice. Together, our results have established the tumor suppressor ARF as a relevant target for HDACi chemotherapy.

The histone deacetylase inhibitor butyrate downregulates cyclin B1 gene expression via a p21/WAF-1-dependent mechanism in human colon cancer cells

Histone deacetylase (HDAC) inhibitors are showing promise as treatment for a variety of human cancers, but their precise mechanism of action has not been elucidated. We examined the effects of the HDAC inhibitor butyrate on colon cancer cells, focusing on its effect on the cell cycle promoter cyclin B(1). In HT-29 cells, sodium butyrate-mediated growth inhibition is associated with a marked decrease in cyclin B(1) mRNA levels. The decrease in cyclin B(1) occurred in a delayed fashion (at 24 h), is completely blocked by concomitant treatment with protein synthesis inhibitors, and appears to be dependent on changes in transcription. Cyclin B(1) repression is linked to the differentiation process in colon cancer cells, not merely with growth arrest. The mechanism of cyclin B(1) repression by butyrate requires prolonged histone hyperacetylation and is at least partly dependent on p21 expression. In fact, p21/WAF-1 appears to directly repress a minimal cyclin B(1) promoter (-90 bp), a process that can be mediated by the amino-terminal portion of the p21 protein. These findings highlight key molecular mechanisms by which HDAC inhibitors mediate their beneficial effects on human cancer cells.

Myocyte enhancer factor 2 activates promoter sequences of the human AbetaH-J-J locus, encoding aspartyl-beta-hydroxylase, junctin, and junctate

Alternative splicing of the locus AbetaH-J-J generates three functionally distinct proteins: an enzyme, AbetaH (aspartyl-beta-hydroxylase), a structural protein of the sarcoplasmic reticulum membrane (junctin), and an integral membrane calcium binding protein (junctate). Junctin and junctate are two important proteins involved in calcium regulation in eukaryotic cells. To understand the regulation of these two proteins, we identified and functionally characterized one of the two promoter sequences of the AbetaH-J-J locus. We demonstrate that the P2 promoter of the AbetaH-J-J locus contains (i) a minimal sequence localized within a region -159 bp from the transcription initiation site, which is sufficient to activate transcription of both mRNAs; (ii) sequences which bind known transcriptional factors such as those belonging to the myocyte enhancer factor 2 (MEF-2), MEF-3, and NF-kappaB protein families; and (iii) sequences bound by unknown proteins. The functional characterization of the minimal promoter in C2C12 cells and in the rat soleus muscle in vivo model indicates the existence of cis elements having positive and negative effects on transcription. In addition, our data demonstrate that in striated muscle cells the calcium-dependent transcription factor MEF-2 is crucial for the transcription activity directed by the P2 promoter. The transcription directed by the AbetaH-J-J P2 promoter is induced by high expression of MEF-2, further stimulated by calcineurin and Ca2+/calmodulin-dependent protein kinase I, and inhibited by histone deacetylase 4.