Molecular cloning and characterization of the human p19(INK4d) gene promoter

E2F1-mediated upregulation of p19INK4d determines its periodic expression during cell cycle and regulates cellular proliferation

Background

A central aspect of development and disease is the control of cell proliferation through regulation of the mitotic cycle. Cell cycle progression and directionality requires an appropriate balance of positive and negative regulators whose expression must fluctuate in a coordinated manner. p19INK4d, a member of the INK4 family of CDK inhibitors, has a unique feature that distinguishes it from the remaining INK4 and makes it a likely candidate for contributing to the directionality of the cell cycle. p19INK4d mRNA and protein levels accumulate periodically during the cell cycle under normal conditions, a feature reminiscent of cyclins.

Methodology/Principal Findings

In this paper, we demonstrate that p19INK4d is transcriptionally regulated by E2F1 through two response elements present in the p19INK4d promoter. Ablation of this regulation reduced p19 levels and restricted its expression during the cell cycle, reflecting the contribution of a transcriptional effect of E2F1 on p19 periodicity. The induction of p19INK4d is delayed during the cell cycle compared to that of cyclin E, temporally separating the induction of these proliferative and antiproliferative target genes. Specific inhibition of the E2F1-p19INK4d pathway using triplex-forming oligonucleotides that block E2F1 binding on p19 promoter, stimulated cell proliferation and increased the fraction of cells in S phase.

Conclusions/Significance

The results described here support a model of normal cell cycle progression in which, following phosphorylation of pRb, free E2F induces cyclin E, among other target genes. Once cyclinE/CDK2 takes over as the cell cycle driving kinase activity, the induction of p19 mediated by E2F1 leads to inhibition of the CDK4,6-containing complexes, bringing the G1 phase to an end. This regulatory mechanism constitutes a new negative feedback loop that terminates the G1 phase proliferative signal, contributing to the proper coordination of the cell cycle and provides an additional mechanism to limit E2F activity.

P19INK4D links endomitotic arrest and megakaryocyte maturation and is regulated by AML-1

The molecular mechanisms that regulate megakaryocyte (MK) ploidization are poorly understood. Using MK differentiation from primary human CD34(+) cells, we observed that p19(INK4D) expression was increased both at the mRNA and protein levels during ploidization. p19(INK4D) knockdown led to a moderate increase (31.7% +/- 5%) in the mean ploidy of MKs suggesting a role of p19(INK4D) in the endomitotic arrest. This increase in ploidy was associated with a decrease in the more mature MK population (CD41(high)CD42(high)) at day 9 of culture, which was related to a delay in differentiation. Inversely, p19(INK4D) overexpression in CD34(+) cells resulted in a decrease in mean ploidy level associated with an increase in CD41 and CD42 expression in each ploidy class. Confirming these in vitro results, bone marrow MKs from p19(INK4D) KO mice exhibited an increase in mean ploidy level from 18.7N (+/- 0.58N) to 52.7N (+/- 12.3N). Chromatin immunoprecipitation assays performed in human MKs revealed that AML-1 binds in vivo the p19(INK4D) promoter. Moreover, AML-1 inhibition led to the p19(INK4D) down-regulation in human MKs. These results may explain the molecular link at the transcriptional level between the arrest of endomitosis and the acceleration of MK differentiation.

Induction of p19INK4d in response to ultraviolet light improves DNA repair and confers resistance to apoptosis in neuroblastoma cells

The genetic instability driving tumorigenesis is fueled by DNA damage and by errors made by the DNA replication. Upon DNA damage the cell organizes an integrated response not only by the classical DNA repair mechanisms but also involving mechanisms of replication, transcription, chromatin structure dynamics, cell cycle progression, and apoptosis. In the present study, we investigated the role of p19INK4d in the response driven by neuroblastoma cells against DNA injury caused by UV irradiation. We show that p19INK4d is the only INK4 protein whose expression is induced by UV light in neuroblastoma cells. Furthermore, p19INK4d translocation from cytoplasm to nucleus is observed after UV irradiation. Ectopic expression of p19INK4d clearly reduces the UV-induced apoptosis as well as enhances the cellular ability to repair the damaged DNA. It is clearly shown that DNA repair is the main target of p19INK4d effect and that diminished apoptosis is a downstream event. Importantly, experiments performed with CDK4 mutants suggest that these p19INK4d effects would be independent of its role as a cell cycle checkpoint gene. The results presented herein uncover a new role of p19INK4d as regulator of DNA-damage-induced apoptosis and suggest that it protects cells from undergoing apoptosis by allowing a more efficient DNA repair. We propose that, in addition to its role as cell cycle inhibitor, p19INK4d is involved in maintenance of DNA integrity and, therefore, would contribute to cancer prevention.

A point mutation in a silencer module reduces the promoter activity for the human mercaptopyruvate sulfurtransferase

A promoter region of human mercaptopyruvate sulfurtransferase (MST) [EC 2.8.1.2] is G+C-rich and TATA-less, showing features of a house-keeping gene. In the core promoter, a GC box (-284:GGGGCGTGGC:-275) and an initiator (-219:TTATATG:-225) are found. A cap site hunting analysis for human liver cDNA revealed four possible transcriptional start sites, nucleotides -223, -159, -35 and -25. Point mutagenesis and deletion studies suggest that a module of the silencer element is -394:GCTG:-391. A replacement of -391G to C lost the silencer function; on the other hand, a replacement of -394G to T or C, -393C to T or -392T to G markedly reduced the promoter activity.

Trichostatin A activates p18INK4c gene: differential activation and cooperation with p19INK4d gene

We have reported that histone deacetylase (HDAC) inhibitors activate a member of the INK4 family, the p19INK4d gene, causing G1 phase arrest. We report here that HDAC inhibitor, Trichostatin A, activates another member of the INK4 family, the p18INK4c gene, through its promoter in Jurkat cells. Interestingly, the activation patterns of the p18INK4c gene were different from those of p19INK4d. Furthermore, mouse embryo fibroblasts lacking p18Ink4c or p18Ink4c/p19Ink4d were resistant to the growth inhibitory effects of TSA as compared to their wild-type counterpart. Our findings suggest that p18INK4c is involved in TSA-mediated cell growth inhibition and cooperates with p19INK4d.

Histone deacetylase inhibitors activate INK4d gene through Sp1 site in its promoter

Histone deacetylase (HDAC) inhibitors are known to arrest human tumor cells at the G1 phase of the cell cycle and activate the cyclin-dependent kinase inhibitor, p21(WAF1/Cip1). However, several studies have suggested the existence of a p21(WAF1/Cip1)-independent molecular pathway. We report here that HDAC inhibitors activate a member of the INK4 family, the INK4d gene, causing G1 phase arrest, in the human T cell leukemia cell line, Jurkat. One of the major Trichostatin A (TSA)-responsive elements is a specific Sp1 binding site in the INK4d promoter. Electrophoretic mobility-shift assay revealed that Sp1 and Sp3 can specifically interact with this Sp1 binding site. Furthermore, using chromatin immunoprecipitation assay, we demonstrated that HDAC2 was present in the INK4d proximal promoter region in the absence, but not the presence, of TSA. Taken together, these results suggest that treatment with TSA transcriptionally activates INK4d by releasing HDAC2 from the histone-DNA complex at the INK4d promoter. Using a p21(WAF1/Cip1)-deleted human colorectal carcinoma cell line, HCT116 p21 (-/-), we show that upregulation of p19(INK4d) by TSA is associated with inhibition of cell proliferation. Moreover, mouse embryo fibroblasts lacking Ink4d were resistant to the growth inhibitory effects of TSA as compared to their wild-type counterpart. Our findings suggest that p19(INK4d) in addition to p21(WAF1/Cip1) is an important molecular target of HDAC inhibitors inducing growth arrest.

p15(INK4b) in HDAC inhibitor-induced growth arrest

Histone deacetylase (HDAC) inhibitors arrest human tumor cells at the G1 phase of the cell cycle and activate the cyclin-dependent kinase inhibitor, p21(WAF1/Cip1). However, several studies have suggested the existence of a p21(WAF1/Cip1)-independent molecular pathway. We report here that HDAC inhibitors, trichostatin A (TSA) and sodium butyrate, activate the p15(INK4b) gene, a member of the INK4 gene family, through its promoter in HaCaT cells. Furthermore, we show that up-regulation of p15(INK4b) by TSA is associated with cell growth inhibition of HCT116 p21 (-/-) cells. Our findings suggest that p15(INK4b) is one of the important molecular targets of HDAC inhibitors.