Molecular cloning and characterization of the human p27Kip1 gene promoter

The major transcription initiation site of the p27Kip1 gene is conserved in human and mouse and produces a long 5'-UTR

BACKGROUND

The cyclin-dependent kinase inhibitor p27Kip1 is essential for proper control of cell cycle progression. The levels of p27Kip1 are regulated by several mechanisms including transcriptional and translational controls. In order to delineate the molecular details of these regulatory mechanisms it is important to identify the transcription initiation site within the p27Kip1 gene, thereby defining the promoter region of the gene and the 5'-untranslated region of the p27Kip1 mRNA. Although several previous studies have attempted to map p27Kip1 transcription start sites, the results vary widely for both the mouse and human genes. In addition, even though the mouse and human p27Kip1 gene sequences are very highly conserved, the reported start sites are notably different.

RESULTS

In this report, using a method that identifies capped ends of mRNA molecules together with RNase protection assays, we demonstrate that p27Kip1 transcription is initiated predominantly from a single site which is conserved in the human and mouse genes. Initiation at this site produces a 5'-untranslated region of 472 nucleotides in the human p27Kip1 mRNA and 502 nucleotides in the mouse p27Kip1 mRNA. In addition, several minor transcription start sites were identified for both the mouse and human genes.

CONCLUSIONS

These results demonstrate that the major transcription initiation sites in the mouse and human p27Kip1 genes are conserved and that the 5'-UTR of the p27Kip1 mRNA is much longer than generally believed. It will be important to consider these findings when designing experiments to identify elements that are involved in regulating the cellular levels of p27Kip1.

Androgen-driven prostate epithelial cell proliferation and differentiation in vivo involve the regulation of p27

Androgens control both growth and differentiation of the normal prostate gland. However, the mechanisms by which androgens act upon the cell cycle machinery to regulate these two fundamental processes are largely unknown. The cyclin-dependent kinase (cdk) inhibitor p27 is a negative cell cycle regulator involved in differentiation-associated growth arrest. Here, we investigate the role and regulation of p27 in the testosterone proprionate (TP)-stimulated regeneration of the ventral prostate (VP) of castrated rats. Continuous TP administration to castrated rats triggered epithelial cell proliferation, which peaked at 72 h, and then declined despite further treatment. Castration-induced atrophy of the VP was associated with a significant increase in p27 expression as compared with the VP of intact animals. Twelve hours after the initiation of androgen treatment, total p27 levels as well as its fraction bound to cdk2, its main target, significantly dropped in the VP of castrated rats. Thereafter, concomitantly to the induction of epithelial cell proliferation, the glandular morphology of VP was progressively restored at 48-96 h of TP treatment. During this period of the regenerative process, whereas both proliferating basal and secretory epithelial cells did not express p27, the protein was selectively up-regulated in the nonproliferating secretory epithelial compartment. This up-regulation of p27 expression was coincident with an increase in its association with, and presumably inhibition of, cdk2. At each time point of TP treatment, p27 abundance in the VP was inversely correlated with the level of its proteasome-dependent degradation activity measured in vitro in VP lysates, whereas only slight changes in the amount of p27 transcripts were detected. In addition, the antiandrogen flutamide blocked maximal TP-induced p27 degradation completely. Finally, the expression of skp2, the ubiquitin ligase that targets p27 for degradation, was seen to increase with androgen administration, preceding maximal proliferation and concomitantly to augmented p27 degradation activity. Taken together, our data indicate that androgens mediate both proliferation and differentiation signals in normal prostate epithelial cells in vivo, through regulation of p27.

Induction of p27(KIP1) as a mechanism underlying NS398-induced growth inhibition in human lung cancer cells

Increased expression of cyclooxygenase-2 (COX-2) causes enhanced production of prostaglandins, which are emerging as important mediators of growth stimulation of cancer cells. Overexpression of COX-2 has been found in human non-small cell lung cancer tissues and cell lines. In vitro and in vivo studies showed that nonselective cyclooxygenase inhibitors (like aspirin and indomethacin) may suppress growth of lung cancer cells and may prevent lung tumorigenesis induced by the tobacco-specific carcinogens. However, the molecular mechanisms that mediated the anticancer action of these inhibitors are not well defined. In this study, we examined the effect of a specific COX-2 inhibitor, N-(2-cyclohexyloxy-4-nitrophenyl)methanesulfonamide (NS398), on high COX-2-expressing A549 lung cancer cells. Our results indicated that NS398 inhibited prostaglandin E(2) synthesis and induced G(1) growth arrest in these cells. NS398 specifically up-regulated cyclin-dependent kinase inhibitor p27(KIP1), whereas the expressions of G(1)-acting cyclins and cyclin-dependent kinases were not changed. Additionally, NS398 effectively suppressed cyclin E-associated kinase activity in A549 cells. The molecular mechanism responsible for the induction of p27(KIP1) by NS398 was characterized. We found that NS398 did not induce p27(KIP1) through transcriptional activation because this drug could not stimulate the p27(KIP1) promoter. Metabolic labeling experiments showed that the synthesis rate of p27(KIP1) protein was not altered by NS398. Conversely, pulse-chase assays demonstrated that degradation of p27(KIP1) protein was obviously reduced in NS398-treated cells. We conclude that NS398 enhances p27(KIP1) expression via post-translational regulation, and our results provide a new mechanism by which specific COX-2 inhibitors suppress proliferation of cancer cells.

Regulation of G(1) cyclin-dependent kinases in the mammalian cell cycle

Cyclin-dependent kinases are the key regulators of cell-cycle transitions. In mammalian cells, Cdk2, Cdk4, Cdk6 and associated cyclins control the G(1) to S phase transition. Because proper regulation of this transition is critical for an organism's survival, these protein kinases are exquisitely regulated at different mechanistic levels and in response to a large variety of intrinsic and extrinsic signals.

A U-rich element in the 5' untranslated region is necessary for the translation of p27 mRNA

Increased translation of p27 mRNA correlates with withdrawal of cells from the cell cycle. This raised the possibility that antimitogenic signals might mediate their effects on p27 expression by altering complexes that formed on p27 mRNA, regulating its translation. In this report, we identify a U-rich sequence in the 5' untranslated region (5'UTR) of p27 mRNA that is necessary for efficient translation in proliferating and nonproliferating cells. We show that a number of factors bind to the 5'UTR in vitro in a manner dependent on the U-rich element, and their availability in the cytosol is controlled in a growth- and cell cycle-dependent fashion. One of these factors is HuR, a protein previously implicated in mRNA stability, transport, and translation. Another is hnRNP C1 and C2, proteins implicated in mRNA processing and the translation of a specific subset of mRNAs expressed in differentiated cells. In lovastatin-treated MDA468 cells, the mobility of the associated hnRNP C1 and C2 proteins changed, and this correlated with increased p27 expression. Together, these data suggest that the U-rich dependent RNP complex on the 5'UTR may regulate the translation of p27 mRNA and may be a target of antimitogenic signals.

Expression and role of p27(kip1) in neuronal differentiation of embryonal carcinoma cells

We examined the expression and the regulation of p21(waf1) and p27(kip1) cdk inhibitors in P19 mouse embryonal carcinoma (EC) cells following treatment with all-trans retinoic acid (ATRA) to induce neuronal differentiation. The levels of p27 mRNA and protein increased within 24 h of treatment with ATRA, reaching a plateau 4-5 days later prior to neurite formation. In contrast, levels of p21 expression remained low until after neurites were extensively formed. Induction of muscle differentiation from P19 cells by treatment with dimethyl sulfoxide caused only transient increases in p27 levels. In a mutant P19 cell line, RAC65, treatment with ATRA induced neither p27 accumulation nor neuronal differentiation, but p21 mRNA expression increased markedly. In contrast, treatment of RAC65 cells with 9-cis retinoic acid induced both p27 expression and neuronal differentiation. Correlation between p27 expression and neuronal differentiation was also observed in NT2/D1 human EC cells. Luciferase reporter assays showed that p27 promoter activity increased in ATRA-treated cells, consistent with the elevation of p27 mRNA levels. Arrest of neuronal differentiation of P19 cells by okadaic acid resulted in inhibition of p27 expression, whereas p21 mRNA expression was greatly enhanced. Conversely, inhibition of p27 expression by antisense p27 oligonucleotides resulted in blockade of neuronal differentiation. Taken together, these results strongly suggest that the expression of p27 is indispensable for neuronal differentiation of EC cells.

Activation of the human p27(Kip1) promoter by IFNalpha 2b

p27(Kip1) is one of the key regulatory proteins in cell cycle through inhibition of pRB phosphorylation by suppression of the activity of several cyclin/Cdk complexes. The expression of p27(Kip1) has been shown to be controlled by a posttranslational mechanism, although vitamin D(3) and neuronal differentiation can also induce its mRNA. Recently, the p27(Kip1) promoter was isolated and sequenced from a human leukocyte genomic library. In this report, we demonstrate that IFNalpha 2b, activates the human p27(Kip1) promoter-driven luciferase reporter gene in transient expression assays in H82 cells. This induction might involve two IRF 1-like binding sites present in the p27(Kip1) promoter. To our knowledge this is the first report on the direct activation of the human p27(Kip1) promoter by IFNalpha 2b.

Sp1 and NF-Y synergistically mediate the effect of vitamin D(3) in the p27(Kip1) gene promoter that lacks vitamin D response elements

Vitamin D(3) promotes myeloid leukemic cell lines to differentiate terminally into monocytes/macrophages. It has been reported that overexpression of the cdk inhibitor p27(Kip1) results in the differentiation of the myelomonocytic U937 cell line and that this gene is the target of vitamin D(3). To identify the sequences required for the positive regulation of p27(Kip1) transcription by vitamin D(3), a 3.6-kilobase 5'-flanking region of the human p27(Kip1) gene was examined by transiently transfecting luciferase reporter constructs into U937 cells. The transcriptional activity of this construct was activated by vitamin D(3). Deletion and mutational analysis revealed that both a GGGCGG sequence (-545/-539) and a CCAAT sequence (-525/-520) were necessary to induce p27(Kip1) gene expression. Importantly, the region containing both of these elements conferred positive responsiveness to vitamin D(3) to a heterologous promoter. Gel shift assays showed that Sp1 binds to the GGGCGG sequence and that NF-Y binds to the CCAAT sequence. Consistent with the roles of these transcription factors, treatment with vitamin D(3) stimulated the DNA binding activities of these factors to each element and induced the change of one NF-Y subunit. We conclude that vitamin D(3) stimulates transcription of the p27(Kip1) gene by a novel mechanism involving Sp1 and NF-Y, but not the vitamin D receptor, during the early stages of U937 cell differentiation.

The ubiquitous transcription factor NF-Y positively regulates the transcription of human p27Kip1 through a CCAAT box located in the 5-upstream region of the p27Kip1 gene

Abnormally low levels of the cyclin-dependent kinase inhibitor p27Kip1 are found frequently in human carcinomas, and these levels correlate directly with both histological aggressiveness and patient mortality. p27Kip1 is haplo-insufficient for tumor suppression. Thus, p27Kip1 may be a useful molecule for the development of cancer therapies. To know the possible mechanisms underlying transcriptional control, we previously cloned the promoter region of human p27Kip1 gene. We report here the characterization of the 5'-regulatory region of the human p27Kip1 gene. Promoter analysis using 5'-deletion mutants revealed that a 39-bp region between -549 and -511 was required for maximal promoter activity. Point mutation analysis revealed that a CCAAT box within this region was essential for promoter activity. Gel shift assays and cotransfection experiments using a dominant negative form of the NF-Y transcription factor showed that NF-Y directly regulates p27Kip1 transcription through this CCAAT box. This finding might provide a clue to approach the mechanism of tumorigenesis.

Tamoxifen induces p21WAF1 and p27KIP1 expression in estrogen receptor-negative lung cancer cells

Tamoxifen (Tam), besides its action as an anti-estrogen, also inhibits cell proliferation of estrogen receptor (ER)-negative cancer cells by an unknown mechanism. In this study, we used ER-negative lung cancer cells to clarify such ER-independent inhibitory effect of Tam. We found that Tam induced G1 growth arrest in these cells. However, our results indicated that the expression of G1 cyclins (including D1, 2, 3 and E) was not regulated by Tam in these lung cancer cells. Additionally, the protein levels of G1 acting cyclin-dependent kinases (CDKs), CDK2, 4 and 6, was unaltered in Tam-treated lung cancer cells with the exception of CDK2 expression in H322 cells which was attenuated by Tam in a cell line-specific manner. We next examined the effect of Tam on the expression of cyclin-dependent kinase inhibitors (CDKIs) and our results demonstrated that the expression of p21WAF1 and p27KIP1, but not p57KIP2, was strongly activated by Tam in these cells. The amounts of p21WAF1 and p27KIP1 co-immunoprecipitated with cyclin E were obviously increased after Tam treatment and reduced activity of cyclin E-associated kinases and accumulation of hypo-phosphorylated retinoblastoma (Rb) protein were clearly detected in Tam-incubated cells. No consentaneous induction of CDKIs was found when ER-negative lung cancer cells were incubated with cytotoxic drugs, cisplatin and etoposide, this indicates that enhancement of CDKI expression is not a non-specific effect of Tam. We also found that Tam may up-regulate p21WAF1 expression via transcription activation. Considered together, these results suggest that Tam-induced growth inhibition in ER-negative lung cancer cells is associated with induction of p21WAF1 and p27KIP1.

Two short sequences have positive effects on the human p27Kip1 gene transcription

The cyclin-dependent kinase (Cdk) inhibitor p27Kip1 plays an important role in the progression from G1 to S phase in the cell cycle. To study the activities of its promoter and other regulatory elements, we have cloned and characterized the 5'-flanking region of the human p27Kip1 gene. This region, about 3kb in length, is GC-rich and shares homology with that of the mouse p27Kip1 gene. Transcription start points (tsp) determined by the oligo-capping method are mapped in two regions, the cluster I (-479 to -403) and cluster II (-280 to -273). The cluster I was the primary functional site in transcription initiation. The luciferase activities of serial deletion mutants indicated that two short sequences (-581 to -557 and -556 to -526) had positive effects on transcription. The gel shift assay showed that factors in HeLa nuclear extract bound to these sequences. Sp1 was the major binding factor to the sequence of -556 to -526, wheres yet unidentified positive factors bound to the sequence of -581 to -557.

Differential expression of the p27Kip1 mRNA in IFN-sensitive and resistant cell lines

IFNs arrest the growth of a small cell lung cancer (SCLC) cell line NCI-H82 in the G1 phase but not the growth of the derived cell line NCI-H82R. Progression through the G1 phase is controlled by positive and negative regulatory genes. Oncoviral genes can override this control. In this study, we compared the effects of human IFN alpha 2b on the mRNA levels of the Cdk inhibitor p27Kip1 in NCI-H82, NCI-H82R and HPV 16 E7-transfected NCI-H82 cell lines. Induction of the 2-5 Oligoadenylate synthetase (2-5 OAS) gene was used as a marker of IFN-dependent signal transduction The expression of p27Kip1 mRNA increased at 48 and 72 hr after IFN alpha 2b addition in sensitive cells. In contrast, p27Kip1 mRNA had only slight variations in both the resistant and E7-transfected cells. Interestingly, the E7-transfected NCI-H82 cells became resistant to the IFN alpha 2b anti-proliferative effect. Our results suggest that p27Kip1 could be a key mediator of the IFN alpha 2b-induced growth arrest and that HPV 16 E7 might affect p27Kip1 inducibility, originating IFN alpha 2b-resistant cells.