Wild-type p53 regulates its own transcription in a cell-type specific manner

Heterozygous loss of TSC2 alters p53 signaling and human stem cell reprogramming

Tuberous sclerosis complex (TSC) is a pediatric disorder of dysregulated growth and differentiation caused by loss of function mutations in either the TSC1 or TSC2 genes, which regulate mTOR kinase activity. To study aberrations of early development in TSC, we generated induced pluripotent stem cells using dermal fibroblasts obtained from patients with TSC. During validation, we found that stem cells generated from TSC patients had a very high rate of integration of the reprogramming plasmid containing a shRNA against TP53. We also found that loss of one allele of TSC2 in human fibroblasts is sufficient to increase p53 levels and impair stem cell reprogramming. Increased p53 was also observed in TSC2 heterozygous and homozygous mutant human stem cells, suggesting that the interactions between TSC2 and p53 are consistent across cell types and gene dosage. These results support important contributions of TSC2 heterozygous and homozygous mutant cells to the pathogenesis of TSC and the important role of p53 during reprogramming.

Prohepcidin binds to the HAMP promoter and autoregulates its own expression

Hepcidin is the major regulatory peptide hormone of iron metabolism, encoded by the HAMP (hepcidin antimicrobial peptide) gene. Hepcidin is expressed mainly in hepatocytes, but is also found in the blood in both a mature and prohormone form. Although, the function of mature hepcidin and the regulation of the HAMP gene have been extensively studied, the intracellular localization and the fate of prohepcidin remains controversial. In the present study, we propose a novel role for prohepcidin in the regulation of its own transcription. Using indirect immunofluorescence and mCherry tagging, a portion of prohepcidin was detected in the nucleus of hepatocytes. Prohepcidin was found to specifically bind to the STAT3 (signal transducer and activator of transcription 3) site in the promoter of HAMP. Overexpression of prohepcidin in WRL68 cells decreased HAMP promoter activity, whereas decreasing the amount of prohepcidin caused increased promoter activity measured by a luciferase reporter-gene assay. Moreover, overexpression of the known prohepcidin-binding partner, α-1 antitrypsin caused increased HAMP promoter activity, suggesting that only the non-α-1 antitrypsin-bound prohepcidin affects the expression of its own gene. The results of the present study indicate that prohepcidin can bind to and transcriptionally regulate the expression of HAMP, suggesting a novel autoregulatory pathway of hepcidin gene expression in hepatocytes.

A critical role of PUMA in maintenance of genomic integrity of murine spermatogonial stem cell precursors after genotoxic stress

Neonatal gonocytes are precursors of spermatogonial stem cells. Preserving their integrity by elimination of damaged germ cells may be crucial to avoid the transmission of genetic alterations to progeny. Using gamma-irradiation, we investigated by immunohistochemistry, flow cytometry and real-time PCR components of the death machinery in neonatal gonocytes. Their death was correlated with caspase 3 activation but not with AIF translocation into the nucleus. The in vivo contribution of both the extrinsic and the intrinsic pathways was then investigated. We focused on the roles of TRAIL/Death Receptor 5 (DR5) and PUMA. Our results were validated using knockout mice. Whereas DR5 expression was upregulated at the cell surface after radiation, caspase 8 was not activated. However, we detected caspase 9 cleavage associated with cytochrome c release. In mice deficient for PUMA, radiation-induced gonocyte apoptosis was reduced, whereas invalidation of TRAIL had no effect. Overall, our results show that genotoxic stress-induced apoptosis of gonocytes is caspase-dependent and involves almost exclusively the intrinsic pathway. Furthermore, PUMA plays a critical role in the maintenance of genomic integrity of spermatogonial stem cell precursors.

p73 or p53 directly regulates human p53 transcription to maintain cell cycle checkpoints

Whereas the p53 tumor suppressor protein plays a central role in cellular checkpoints that respond to damage or stress to prevent tumorigenesis, the transcriptional control of the p53 gene has remained unclear. We show that chemotherapeutic agents induce p53 transcription and that p73 or p53 transactivates endogenous p53 expression through direct binding to the p53 promoter. Silencing of p53 or p73 by RNA interference significantly suppresses p53 transcription under physiologic conditions or in response to cellular stress. Mutational analysis of the human p53 promoter localized a p53 DNA-binding site, which confers p53- or p73-dependent p53 promoter activation. Importantly, impaired p53-mediated autoregulation of p53 transcription by inducible-interfering RNA results in aberrant cell cycle regulation and suppression of p53-mediated apoptosis. Thus, a positive feedback loop regulates human p53 expression and involves p73 and p53. Disruption of p53 transcription contributes to defective checkpoint control.

RB18A enhances expression of mutant p53 protein in human cells

RB18A (TRAP220/DRIP205) is a cofactor of transcription. We herein demonstrated that RB18A downregulated p53 and upregulated MDM2 promoters. These RB18A regulations, not modified by p53wt expression, were inhibited by mutant p53 (p53mut) expression, which directly interacts with RB18A D5 domain. In addition, RB18A via its D4 domain, also interacts directly and specifically with MDM2 protein inhibiting p53mut degradation. Altogether, these mechanisms contribute to maintain a high level of p53mut expression in tumor proliferating cells. Therefore, RB18A plays a central role to control p53wt and p53mut protein content and functions in cells through a loop of regulation, which involves MDM2.

Choline kinase inhibition induces the increase in ceramides resulting in a highly specific and selective cytotoxic antitumoral strategy as a potential mechanism of action

Choline kinase (ChoK, E.C. 2.7.1.32) is involved in the synthesis of phosphatidylcholine (PC), and has been found to be increased in human tumors and tumor-derived cell lines. Furthermore, ChoK inhibitors have been reported to show a potent and selective antitumoral activity both in vitro and in vivo. Here, we provide the basis for a rational understanding of the antitumoral activity of ChoK inhibitors. In normal cells, blockage of de novo phosphorylcholine (PCho) synthesis by inhibition of ChoK promotes the dephosphorylation of pRb, resulting in a reversible cell cycle arrest at G0/G1 phase. In contrast, ChoK inhibition in tumor cells renders cells unable to arrest in G0/G1 as manifested by a lack of pRb dephosphorylation. Furthermore, tumor cells specifically suffer a drastic wobble in the metabolism of main membrane lipids PC and sphingomyelin (SM). This lipid disruption results in the enlargement of the intracellular levels of ceramides. As a consequence, normal cells remain unaffected, but tumor cells are promoted to apoptosis. Thus, we provide in this study the rationale for the potential clinical use of ChoK inhibitors.

E5 protein of human papillomavirus type 16 protects human foreskin keratinocytes from UV B-irradiation-induced apoptosis

The human papillomavirus type 16 (HPV16) E5 protein associates with the epidermal growth factor receptor (EGFR) and enhances the activation of the EGFR after stimulation by EGF in human keratinocytes. Phosphatidylinositol 3-kinase (PI3K) and ERK1/2 mitogen-activated protein kinase (ERK1/2 MAPK), two signal molecules downstream of the EGFR, have been recognized as participants in two survival signal pathways in response to stress. The fact that E5 can enhance EGFR activation suggests that E5 might act as a survival factor. To test this hypothesis, the apoptotic response of UV B-irradiated primary keratinocytes infected with either control retrovirus, LXSN, or HPV16 2E5-expressing recombinant retrovirus was quantitated. Under the same conditions, LXSN-infected cells showed extensive apoptosis, while E5-expressing cells demonstrated a significant reduction in UV B-irradiation-induced apoptosis. The E5-mediated protection against apoptosis was blocked by wortmannin and PD98059, specific inhibitors of the PI3K and ERK1/2 MAPK pathways, respectively, suggesting that the PI3K and ERK1/2 MAPK pathways are involved in this process. Western blot analysis showed that Akt (also named protein kinase B), which is a downstream effector of PI3K, and ERK1/2 MAPK were activated by EGF. When cells were stimulated by EGF and irradiated by UV B, the levels of phospho-Akt and phospho-ERK1/2 activated by EGF in E5-expressing cells were about twofold greater than those in LXSN-infected cells. Two other UV-activated stress pathways, p38 and JNK, were activated to the same level during UV B irradiation in both LXSN-infected cells and E5-expressing cells, indicating that E5 protein did not affect these two pathways. After UV B irradiation, p53 was activated in both LXSN-infected cells and E5-expressing cells, and cell cycle analysis showed that nearly all cells in both cell populations were growth arrested. These data suggest that unlike HPV16 E6, which blocks apoptosis by inactivation of p53, HPV16 E5 protects cells from apoptosis by enhancing the PI3K-Akt and ERK1/2 MAPK signal pathways.

Bcl-2/Bax protein ratio predicts 5-fluorouracil sensitivity independently of p53 status

p53 tumour-suppressor gene is involved in cell growth control, arrest and apoptosis. Nevertheless cell cycle arrest and apoptosis induction can be observed in p53-defective cells after exposure to DNA-damaging agents such as 5-fluorouracil (5-FU) suggesting the importance of alternative pathways via p53-independent mechanisms. In order to establish relationship between p53 status, cell cycle arrest, Bcl-2/Bax regulation and 5-FU sensitivity, we examined p53 mRNA and protein expression and p53 protein functionality in wild-type (wt) and mutant (mt) p53 cell lines. p53 mRNA and p53 protein expression were determined before and after exposure to equitoxic 5-FU concentration in six human carcinoma cell lines differing in p53 status and displaying marked differences in 5-FU sensitivity, with IC(50)values ranging from 0.2-22.6 mM. 5-FU induced a rise in p53 mRNA expression in mt p53 cell lines and in human papilloma virus positive wt p53 cell line, whereas significant decrease in p53 mRNA expression was found in wt p53 cell line. Whatever p53 status, 5-FU altered p53 transcriptional and translational regulation leading to up-regulation of p53 protein. In relation with p53 functionality, but independently of p53 mutational status, after exposure to 5-FU equitoxic concentration, all cell lines were able to arrest in G1. No relationship was evidenced between G1 accumulation ability and 5-FU sensitivity. Moreover, after 5-FU exposure, Bax and Bcl-2 proteins regulation was under p53 protein control and a statistically significant relationship (r = 0.880, P = 0.0097) was observed between Bcl-2/Bax ratio and 5-FU sensitivity. In conclusion, whatever p53 status, Bcl-2 or Bax induction and Bcl-2/Bax protein ratio were correlated to 5-FU sensitivity.

Additive effect between NF-kappaB subunits and p53 protein for transcriptional activation of human p53 promoter

The tumor suppressor p53 plays a pivotal role in the cellular response to DNA damage as it controls DNA repair, cell cycle arrest and apoptosis. We studied the autoregulation of human p53 gene transcription in colon cancer cell lines. Wild-type p53 has been shown to autoregulate its own transcription either positively or negatively and probably in a cell-type-specific manner. Indeed, a p53 binding site has been described in the human and murine p53 promoters, but a direct binding of wild-type p53 protein to this site has never been reported. In this study, we demonstrated a transactivation of human p53 promoter by wild-type p53 in human colon cancer cells. We identified in the human p53 promoter a novel potential p53-responsive element that binds wild-type p53. Moreover, wild-type p53 protein transactivated a reporter plasmid containing a luciferase gene driven by a minimal promoter harboring this p53 binding site. Finally, as the p53 promoter contains an NF-kappaB binding site, we demonstrated an additive effect when NF-kappaB subunits and p53 protein combined to transactivate the human p53 promoter.

Inhibition of tumor growth and metastasis of human melanoma by intracellular anti-ATF-1 single chain Fv fragment

Activating transcription factor-1 (ATF-1) and cAMP-responsive element (CRE)-binding protein (CREB) have been implicated in cAMP and Ca2+-induced transcriptional activation. The expression of the transcription factors CREB and ATF-1 is upregulated in metastatic melanoma cells. However, how overexpression of ATF-1/CREB contributes to the acquisition of the metastatic phenotype remains unclear. Here, the effect of disrupting ATF-1 activity was investigated using intracellular expression of an inhibitory anti-ATF-1 single chain antibody fragment (ScFv). Intracellular expression of ScFv anti-ATF-1 in MeWo melanoma cells caused significant reduction in CRE-dependent promoter activation. In addition, expression of ScFv anti-ATF-1 in melanoma cells suppressed their tumorigenicity and metastatic potential in nude mice. ScFv anti-ATF-1 rendered the melanoma cells susceptible to thapsigargin-induced apoptosis in vitro and caused massive apoptosis in tumors transplanted subcutaneously into nude mice, suggesting that ATF-1 and its associated proteins act as survival factor for human melanoma cells. This is the first report to demonstrate the potential of ScFv anti-ATF-1 as an inhibitor of tumor growth and metastasis of solid tumor in vivo. Oncogene (2000).

Dial 9-1-1 for p53: mechanisms of p53 activation by cellular stress

The tumor suppressor protein, p53, is part of the cell's emergency team that is called upon following cellular insult. How do cells sense DNA damage and other cellular stresses and what signal transduction pathways are used to alert p53? How is the resulting nuclear accumulation of p53 accomplished and what determines the outcome of p53 induction? Many posttranslational modifications of p53, such as phosphorylation, dephosphorylation, acetylation and ribosylation, have been shown to occur following cellular stress. Some of these modifications may activate the p53 protein, interfere with MDM2 binding and/or dictate cellular localization of p53. This review will focus on recent findings about how the p53 response may be activated following cellular stress.

Thymidylate synthase protein and p53 mRNA form an in vivo ribonucleoprotein complex

A thymidylate synthase (TS)-ribonucleoprotein (RNP) complex composed of TS protein and the mRNA of the tumor suppressor gene p53 was isolated from cultured human colon cancer cells. RNA gel shift assays confirmed a specific interaction between TS protein and the protein-coding region of p53 mRNA, and in vitro translation studies demonstrated that this interaction resulted in the specific repression of p53 mRNA translation. To demonstrate the potential biological role of the TS protein-p53 mRNA interaction, Western immunoblot analysis revealed nearly undetectable levels of p53 protein in TS-overexpressing human colon cancer H630-R10 and rat hepatoma H35(F/F) cell lines compared to the levels in their respective parent H630 and H35 cell lines. Polysome analysis revealed that the p53 mRNA was associated with higher-molecular-weight polysomes in H35 cells compared to H35(F/F) cells. While the level of p53 mRNA expression was identical in parent and TS-overexpressing cell lines, the level of p53 RNA bound to TS in the form of RNP complexes was significantly higher in TS-overexpressing cells. The effect of TS on p53 expression was also investigated with human colon cancer RKO cells by use of a tetracycline-inducible system. Treatment of RKO cells with a tetracycline derivative, doxycycline, resulted in 15-fold-induced expression of TS protein and nearly complete suppression of p53 protein expression. However, p53 mRNA levels were identical in transfected RKO cells in the absence and presence of doxycycline. Taken together, these findings suggest that TS regulates the expression of p53 at the translational level. This study identifies a novel pathway for regulating p53 gene expression and expands current understanding of the potential role of TS as a regulator of cellular gene expression.

Loss of AP-2 results in downregulation of c-KIT and enhancement of melanoma tumorigenicity and metastasis

Expression of the tyrosine kinase receptor, c-KIT, progressively decreases during local tumor growth and invasion of human melanomas. We have previously shown that enforced c-KIT expression in highly metastatic cells inhibited tumor growth and metastasis in nude mice. Furthermore, the ligand for c-KIT, SCF, induces apoptosis in human melanoma cells expressing c-KIT under both in vitro and in vivo conditions. Here we show that loss of c-KIT expression in highly metastatic cells correlates with loss of expression of the transcription factor AP-2. The c-KIT promoter contains three binding sites for AP-2 and EMSA gels demonstrated that AP-2 protein binds directly to the c-KIT promoter. Transfection of wild-type AP-2 into c-KIT-negative A375SM melanoma cells activated a c-KIT promoter-driven luciferase reporter gene, while expression of a dominant-negative AP-2B in c-KIT-positive Mel-501 cells inhibited its activation. Endogenous c-KIT mRNA and expression of proteins were upregulated in AP-2-transfected cells, but not in control cells. In addition, re-expression of AP-2 in A375SM cells suppressed their tumorigenicity and metastatic potential in nude mice. These results indicate that the expression of c-KIT is highly regulated by AP-2 and that enforced AP-2 expression suppresses tumorigenicity and metastatic potential of human melanoma cells, possibly through c-KIT transactivation and SCF-induced apoptosis. Therefore, loss of AP-2 expression might be a crucial event in the development of malignant melanoma.

Chemosensitivity of human malignant glioma: modulation by p53 gene transfer

Loss of wild-type p53 activity is one of the most common molecular abnormalities in human cancers including malignant gliomas. The p53 status is also thought to modulate sensitivity to irradiation and chemotherapy. Here, we studied the effect of a p53 gene transfer on the chemosensitivity of three human glioma cell lines with different endogenous p53 status (LN-229, wild-type; LN-18, mutant; LN-308, deleted), using the murine temperature-sensitive p53 val135 mutant. Expression of mutant p53 enhanced proliferation of LN-308 cells but reduced proliferation in the other cell lines. Expression of wild-type p53 caused reversible growth arrest of all cell lines but failed to induce apoptosis. Growth arrest induced by wild-type p53 was associated with strong induction of p21 expression. Strong induction of BAX expression and loss of BCL-2 expression, which are associated with p53-dependent apoptosis rather than growth arrest, were not observed. Wild-type p53 failed to sensitize glioma cells to cytotoxic drugs including BCNU, cytarabine, doxorubicin, teniposide and vincristine. The combined effects of wild-type p53 gene transfer and drug treatment were less than additive rather than synergistic, suggesting that the intracellular cascades activated by p53 and chemotherapy are redundant. Unexpectedly, forced expression of mutant p53 modulated drug sensitivity in that it enhanced the toxicity of some drugs but attenuated the effects of others. These effects may represent a dominant negative effect of mutant p53 in LN-229 cells which have wild-type p53 activity but must be considered a gain of function-type effect in the other two cell lines which have no wild-type p53 activity. Importantly, no clear-cut pattern emerged among the three cell lines studied. We conclude that somatic gene therapy based on the reintroduction of p53 will limit the proliferation of human malignant glioma cells but is unlikely to induce clinically relevant sensitization to chemotherapy in these tumors.

Loss of AP-2 results in up-regulation of MCAM/MUC18 and an increase in tumor growth and metastasis of human melanoma cells

MCAM/MUC18 is a cell-surface glycoprotein of 113 kDa, originally identified as a melanoma antigen, whose expression is associated with tumor progression and the development of metastatic potential. We have previously shown that enforced expression of MCAM/MUC18 in primary cutaneous melanoma led to increased tumor growth and metastatic potential in nude mice. The mechanism for up-regulation of MCAM/MUC18 during melanoma progression is unknown. Here we show that up-regulation of MCAM/MUC18 expression in highly metastatic cells correlates with loss of expression of the transcription factor AP-2. The MCAM/MUC18 promoter contains four binding sites for AP-2, and electrophoretic mobility shift assay gels demonstrated that the AP-2 protein bound directly to the MCAM/MUC18 promoter. Transfection of AP-2 into highly metastatic A375SM melanoma cells (AP-2-negative and MCAM/MUC18-positive) inhibited MCAM/MUC18 promoter-driven chloramphenicol acetyltransferase reporter gene in a dose-dependent manner. MCAM/MUC18 mRNA and protein expression were down-regulated in AP-2-transfected but not in control cells. In addition, re-expression of AP-2 in A375SM cells inhibited their tumorigenicity and metastatic potential in nude mice. These results indicate that the expression of MCAM/MUC18 is regulated by AP-2 and that enforced AP-2 expression suppresses tumorigenicity and metastatic potential of human melanoma cells, possibly by down-regulating MCAM/MUC18 gene expression. Since AP-2 also regulates other genes that are involved in the progression of human melanoma such as c-KIT, E-cadherin, MMP-2, and p21(WAF-1), we propose that loss of AP-2 is a crucial event in the development of malignant melanoma.

Autoregulation of eukaryotic transcription factors

The structures of several promoters regulating the expression of eukaryotic transcription factors have in recent years been examined. In many cases there is good evidence for autoregulation, in which a given factor binds to its own promoter and either activates or represses transcription. Autoregulation occurs in all eukaryotes and is an important component in controlling expression of basal, cell cycle specific, inducible response and cell type-specific factors. The basal factors are autoregulatory, being strictly necessary for their own expression, and as such must be epigenetically inherited. Autoregulation of stimulus response factors typically serves to amplify cellular signals transiently and also to attenuate the response whether or not a given inducer remains. Cell cycle-specific transcription factors are positively and negatively autoregulatory, but this frequently depends on interlocking circuits among family members. Autoregulation of cell type-specific factors results in a form of cellular memory that can contribute, or define, a determined state. Autoregulation of transcription factors provides a simple circuitry, useful in many cellular circumstances, that does not require the involvement of additional factors, which, in turn, would need to be subject to another hierarchy of regulation. Autoregulation additionally can provide a direct means to sense and control the cellular conce]ntration of a given factor. However, autoregulatory loops are often dependent on cellular pathways that create the circumstances under which autoregulation occurs.