C-terminal p73 Isoforms Repress Transcriptional Activity of the Human Telomerase Reverse Transcriptase (hTERT) Promoter

Abstract

Activation of telomerase is linked to tumorigenesis and has been observed in a variety of human tumors. Previous reports demonstrated that p53 represses human telomerase reverse transcriptase (hTERT), a key component for telomerase activity. The p73 protein displays a tumor suppressor activity similar to p53. In the present study, we examined the effect of transactivation competent p73 isoforms on hTERT expression in p53-negative human H1299 cells. Overexpression of C-terminal p73 isoforms (␣, ␤, ␥, ␦) resulted in a clear down-regulation of hTERT promoter activity. The strongest inhibitory effect, comparable with p53, was observed for p73␤. Moreover, suppression of hTERT expression was also mediated by endogenous p73 after activation of E2F1 in H1299ER-E2F1 cells. Mutations in the Sp1 transcription factor-binding sites of the proximal core promoter region significantly abolished p73-induced repression, suggesting that the effect is mediated by Sp1. Finally, we demonstrate that p73 directly interacts with Sp1, suggesting that formation of a p73-Sp1 complex is the underlying mechanism for p73-triggered inhibition of hTERT expression. Our findings provide additional evidence that p73 mimics p53 in many aspects in cells lacking functional p53, thereby contributing to tumor surveillance.Telomerase is the enzyme required for the addition of telomeric repeats to the ends of linear chromosomes. It consists of an RNA subunit, a catalytic subunit (hTERT, 2 human telomerase reverse transcriptase) and telomeraseassociated proteins. The RNA component of human telomerase provides the template for telomere repeat synthesis (1). Whereas the RNA subunit of telomerase is expressed in most cells, expression of hTERT is observed at high levels in most malignant tumors and cancer cell lines but not in normal tissues (2) and was found to be closely associated with telomerase activity (3). Moreover, ectopic expression of hTERT has been shown to facilitate immortalization of human cells and to be required for the transformation of human primary cells by H-Ras and SV40 large T antigen oncoproteins (4). These findings led to the conclusion that hTERT expression is a rate-limiting step in telomerase activity and a hallmark of cancer. Although telomerase enzymatic activity can be regulated at multiple levels, several studies demonstrated that transcriptional regulation of hTERT is the major mechanism of telomerase regulation. Deletion analyses in reporter assays showed that the 200-bp proximal region of the hTERT promoter (core promoter) is responsible for most of the transcriptional activity (5). Elucidation of the mechanisms governing hTERT expression revealed two general principles for hTERT up-regulation: (i) the activation of the hTERT promoter through oncoproteins or viral integration and (ii) the derepression of the hTERT promoter through the loss of tumor suppressors. It has been shown that oncogenes such as c-myc and the human papillomavirus E6 protein activate hTERT transcription (6). Furthermore, abrogation of p53 function by th...

Regulation of p57KIP2 during Muscle Differentiation: Role of Egr1, Sp1 and DNA Hypomethylation

“…15 However, we did not find any canonical p73/p53 binding site in the minimal p57 promoter. Since it has been previously reported that Sp1 and p53 family members can physically and functionally interact in regulating transcription, [23][24][25] we asked whether p73 could participate in the Sp1-containing complex that binds to p57 promoter. As shown in Fig.…”

The jury is in: p73 is a tumor suppressor after all: Figure 1.

“…TAp73 is a known transcriptional inducer of ⌬Np73 (Grob et al 2001), but in TAp73-deficient mice a striking increase in ⌬Np73 RNA levels was observed. TAp73 has been reported to repress the expression of several genes by interacting with other transcription factors such as Sp1 in the promoters and regulatory regions of these genes (Salimath et al 2000;Uramoto et al 2004;Innocente and Lee 2005;Racek et al 2005). Many transcriptional repressors have binding sites in the ⌬Np73 promoter, including Sp1 (J. Rosenbluth and J. Pietenpol, unpubl.).…”

Molecular Mechanism of p73-Mediated Regulation of Hepatitis B Virus Core Promoter/Enhancer II: Implications for Hepatocarcinogenesis

“…27 Koutsodontis et al found that the 101 Cterminal amino acids of p53, which include the oligomerization and regulatory domains of the protein, are required for the physical interactions with Sp1. 28 Sp1 has two binding sites in the HBV cp and one in the upstream EII (Fig.…”