Fabiola Moretti

p53 re-expression inhibits proliferation and restores differentiation of human thyroid anaplastic carcinoma cells

Alterations of the tumor suppressor gene p53 are uncommon in differentiated thyroid neoplasia but are detected at high frequency in anaplastic thyroid carcinoma suggesting that impaired p53 function may contribute to the undifferentiated and highly aggressive phenotype of these tumors. Effects of wild type p53 (wt-p53) re-expression were investigated in a human anaplastic thyroid carcinoma cell line (ARO) expressing a mutated p53. ARO cells were stably transfected with the temperature-sensitive p53 Val135 gene (ts-p53) which exhibits wild type-like activity at 32°C. Exogenous wt-p53 function in ARO-tsp53 clones was assessed by evaluating its transcriptional activity on a CAT reporter vector containing p53 binding sites. At 32°C, a significant reduction in the proliferation rate (≈percnt;50%) was observed, with accumulation of cells in the G0/G1 phase of the cell cycle. This effect was accompanied by induction of the expression of the growth inhibitor p21/Waf1 gene. At 32°C, ARO-tsp53 clones also showed a marked impairment of their tumorigenic potential. Furthermore, transfected clones re-acquired the ability to respond to thyrotropin (TSH) stimulation showing an increased expression of thyroid-specific genes (thyroglobulin, thyroperoxidase and TSH receptor). In conclusion, re-expression of wt-p53 activity in ARO cells, inhibits cell proliferation and restores responsiveness to physiological stimuli.

Epithelial-restricted gene profile of primary cultures from human prostate tumors: a molecular approach to predict clinical behavior of prostate cancer

The histopathologic and molecular heterogeneity of prostate cancer and the limited availability of human tumor tissue make unraveling the mechanisms of prostate carcinogenesis a challenging task. Our goal was to develop an ex vivo model that could be reliably used to define a prognostic signature based on gene expression profiling of cell cultures that maintained the tumor phenotype. To this end, we derived epithelial cultures from tissue explanted from 59 patients undergoing radical prostatectomy or cistoprostatectomy because of prostate benign hyperplasia/prostate cancer or bladder carcinoma. Patient selection criteria were absence of hormonal neoadjuvant treatment before surgery and diagnosis of clinically localized disease. Using this unique experimental material, we analyzed expression of 22,500 transcripts on the Affymetrix Human U133A GeneChip platform (Affymetrix, Inc., High Wycombe, United Kingdom). Cultures from normal/hyperplastic tissues with a prevalent luminal phenotype and from normal prostate epithelial tissue with basal phenotype (PrEC) served as controls. We have established a large number of prostate primary cultures highly enriched in the secretory phenotype. From them, we derived an epithelial-restricted transcriptional signature that (a) differentiated normal from tumor cells and (b) clearly separated cancer-derived lines into two distinct groups, which correlated with indolent or aggressive clinical behavior of the disease. Our findings provide (a) a method to expand human primary prostate carcinoma cells with a luminal phenotype, (b) a powerful experimental model to study primary prostate cancer biology, and (c) a novel means to characterize these tumors from a molecular genetic standpoint for prognostic and/or predictive purposes. (Mol Cancer Res 2006;4(2):79–92)

Signaling through estrogen receptors modulates telomerase activity in human prostate cancer

Sex steroid hormone receptors play a central role in all stages of prostate cancer. Here, we tested whether estrogen receptor (ER) signaling contributes to telomerase activation, an early event in prostate tumorigenesis. Following 17beta-estradiol (E(2)) treatment, both mRNA encoding the catalytic subunit of human telomerase (hTERT) and telomerase activity were promptly induced in human prostate normal epithelial cells, fresh explants from benign prostate hyperplasia, and prostate cancer explants and cell lines. Reporter expression studies and in vivo chromatin immunoprecipitation assays revealed E(2)-dependent hTERT promoter induction and showed that both ERalpha and ERbeta bound this sequence. Crucially, addition of the anti-estrogen 4-hydroxytamoxifen caused a differential recruitment in vivo of ERalpha and ERbeta onto the hTERT promoter and inhibited telomerase activity. Treatment with the aromatase inhibitor letrozole, which prevented testosterone-mediated interaction between ER and the hTERT estrogen response element, resulted in a negative regulation of telomerase activity. Thus, intracellular conversion of androgens to estrogens may contribute to the etiopathogenesis of prostate cancer. Given the present evidence for direct control of hTERT gene expression and telomerase activity in the prostate by the ER, we suggest that this transcriptional regulator represents a possible therapeutic target in prostate cancer.

Estrogen Receptor-α and Endothelial Nitric Oxide Synthase Nuclear Complex Regulates Transcription of Human Telomerase

We report that in endothelial cells, the angiogenic effect of 17&bgr;-estradiol (E2) is inhibited by the estrogen receptor (ER) antagonist ICI or the NO synthase (NOS) inhibitor 7-nitroindazole via downregulation of hTERT, the telomerase catalytic subunit, suggesting that E2 and NO are involved in controlling hTERT transcription. Quantitative Real-Time PCR and chromatin immunoprecipitations in E2-treated human umbilical vein endothelial cells, showed recruitment of ERs on the hTERT promoter and concomitant enrichment in histone 3 methylation at Lysine 79, a modification associated with transcription-competent chromatin. Confocal microscopy and re-chromatin immunoprecipitations revealed that on E2 induction, endothelial (e)NOS rapidly localized into the nucleus and associated with ER&agr; on the hTERT promoter. Transfections of a constitutively active eNOS mutant (S1177D) strongly induced the hTERT promoter, indicating a direct role of the protein in hTERT transcriptional regulation. Mutation of the estrogen response element in the promoter abolished response to both ERs and active eNOS, demonstrating that the estrogen response element integrity is required for hTERT regulation by these factors. To investigate this novel regulation in a reduced NO environment, pulmonary endothelial cells were isolated from eNOS−/− mice and grown with/without E2. In wild-type cells, E2 significantly increased telomerase activity. In eNOS−/− cells, basal telomerase activity was rescued by exogenous eNOS or an NO donor, whereas responsiveness to E2 demanded the active protein. In conclusion, we document the novel findings of a combinatorial eNOS/ER&agr; complex at the hTERT estrogen response element site and that active eNOS and ligand-activated ERs cooperate in regulating hTERT expression in the endothelium.

Telomerase Mediates Vascular Endothelial Growth Factor-dependent Responsiveness in a Rat Model of Hind Limb Ischemia

Telomere dysfunction contributes to reduced cell viability, altered differentiation, and impaired regenerative/proliferative responses. Recent advances indicate that telomerase activity confers a pro-angiogenic phenotype to endothelial cells and their precursors. We have investigated whether telomerase contributes to tissue regeneration following hind limb ischemia and vascular endothelial growth factor 165 (VEGF165) treatment. VEGF delivery induced angiogenesis and increased expression of the telomerase reverse transcriptase (TERT) and telomerase activity in skeletal muscles and satellite and endothelial cells. Adenovirus-mediated transfer of wild type TERT but not of a dominant negative mutant, TERTdn, significantly induced capillary but not arteriole formation. However, when co-delivered with VEGF, TERTdn abrogated VEGF-dependent angiogenesis, arteriogenesis, and blood flow increase. This effect was paralleled by in vitro evidence that telomerase inhibition by 3′-azido-3′-deoxythymidine in VEGF-treated endothelial cells strongly reduced capillary density and promoted apoptosis in the absence of serum. Similar results were obtained with adenovirus-mediated expression of TERTdn and AKTdn, both reducing endogenous TERT activity and angiogenesis on Matrigel. Mechanistically, neo-angiogenesis in our system involved: (i) VEGF-dependent activation of telomerase through the nitric oxide pathway and (ii) telomerase-dependent activation of endothelial cell differentiation and protection from apoptosis. Furthermore, detection of TERT in activated satellite cells identified them as VEGF targets during muscle regeneration. Because TERT behaves as an angiogenic factor and a downstream effector of VEGF signaling, telomerase activity appears required for VEGF-dependent remodeling of ischemic tissue at the capillaries and arterioles level.

MDM4 (MDMX) localizes at the mitochondria and facilitates the p53-mediated intrinsic-apoptotic pathway

MDM4 is a key regulator of p53, whose biological activities depend on both transcriptional activity and transcription‐independent mitochondrial functions. MDM4 binds to p53 and blocks its transcriptional activity; however, the main cytoplasmic localization of MDM4 might also imply a regulation of p53‐mitochondrial function. Here, we show that MDM4 stably localizes at the mitochondria, in which it (i) binds BCL2, (ii) facilitates mitochondrial localization of p53 phosphorylated at Ser46 (p53Ser46P) and (iii) promotes binding between p53Ser46P and BCL2, release of cytochrome C and apoptosis. In agreement with these observations, MDM4 reduction by RNA interference increases resistance to DNA‐damage‐induced apoptosis in a p53‐dependent manner and independently of transcription. Consistent with these findings, a significant downregulation of MDM4 expression associates with cisplatin resistance in human ovarian cancers, and MDM4 modulation affects cisplatin sensitivity of ovarian cancer cells. These data define a new localization and function of MDM4 that, by acting as a docking site for p53Ser46P to BCL2, facilitates the p53‐mediated intrinsic‐apoptotic pathway. Overall, our results point to MDM4 as a double‐faced regulator of p53.

Identification of an Aberrantly Spliced Form of HDMX in Human Tumors: A New Mechanism for HDM2 Stabilization

The HDMX protein is closely related to HDM2 with which it shares different structural domains, particularly the p53 binding domain and the ring finger domain, where the two HDM proteins interact. Several oncogenic forms derived from splicing of HDM2 have been described in cancer. This work aimed at investigating whether analogous forms of HDMX exist in human tumors. Here, we report the characterization of an aberrantly spliced form of HDMX, HDMX211, isolated from the thyroid tumor cell line, ARO. HDMX211 binds and stabilizes the HDM2 protein. Although it lacks the p53 binding domain, HDMX211 also stabilizes p53 by counteracting its degradation by HDM2. However, the resulting p53 is transcriptionally inactive and increasingly associated to its inhibitor HDM2. Expression of HDMX211 strongly enhances the colony-forming ability of human cells in the presence or absence of wild-type p53. Conversely, depletion of HDMX211 by small interfering RNA significantly reduces the growth of ARO cells and increases their sensitivity to chemotherapy. Screening of lung cancer biopsies shows the presence of HDMX211 in samples that overexpress HDM2 protein, supporting a pathologic role for this new protein. This is the first evidence of a variant form of HDMX that has oncogenic potential independently of p53. HDMX211 reveals a new mechanism for overexpression of the oncoprotein HDM2. Most interestingly, it outlines a possible molecular explanation for a yet unclarified tumor phenotype, characterized by simultaneous overexpression of HDM2 and wild-type p53.

Analysis of human MDM4 variants in papillary thyroid carcinomas reveals new potential markers of cancer properties

A wild-type (wt) p53 gene characterizes thyroid tumors, except for the rare anaplastic histotype. Because p53 inactivation is a prerequisite for tumor development, alterations of p53 regulators represent an alternative way to impair p53 function. Indeed, murine double minute 2 (MDM2), the main p53 negative regulator, is overexpressed in many tumor histotypes including those of the thyroid. A new p53 regulator, MDM4 (a.k.a. MDMX or HDMX) an analog of MDM2, represents a new oncogene although its impact on tumor properties remains largely unexplored. We estimated levels of MDM2, MDM4, and its variants, MDM4-S (originally HDMX-S) and MDM4-211 (originally HDMX211), in a group of 57 papillary thyroid carcinomas (PTC), characterized by wt tumor protein 53, in comparison to matched contra-lateral lobe normal tissue. Further, we evaluated the association between expression levels of these genes and the histopathological features of tumors. Quantitative real-time polymerase chain reaction revealed a highly significant downregulation of MDM4 mRNA in tumor tissue compared to control tissue (P < 0.0001), a finding confirmed by western blot on a subset of 20 tissue pairs. Moreover, the tumor-to-normal ratio of MDM4 levels for each individual was significantly lower in late tumor stages, suggesting a specific downregulation of MDM4 expression with tumor progression. In comparison, MDM2 messenger RNA (mRNA) and protein levels were frequently upregulated with no correlation with MDM4 levels. Lastly, we frequently detected overexpression of MDM4-S mRNA and presence of the aberrant form, MDM4-211 in this tumor group. These findings indicate that MDM4 alterations are a frequent event in PTC. It is worthy to note that the significant downregulation of full-length MDM4 in PTC reveals a novel status of this factor in human cancer that counsels careful evaluation of its role in human tumorigenesis and of its potential as therapeutic target.

Mitochondrial MDM4 (MDMX): an unpredicted role in the p53-mediated intrinsic apoptotic pathway.

P53 is a crucial regulator of cell response to DNA damage. MDM4 and MDM2 are the two main negative regulators of p53 activity. Upon DNA damage, their constraint is released and p53 becomes activated and exerts its safeguard function by arresting cell growth or by killing excessively damaged cells. Under these conditions, increasing data suggest that MDM4 and MDM2 play novel roles. In this respect, we recently published that MDM4 exerts a positive activity towards p53 mitochondrial apoptosis. We observed that a fraction of MDM4 stably localizes at the mitochondria where upon lethal stress conditions, promotes the mitochondrial localization of p53 phosphorylated at Ser46 (p53Ser46P) and facilitates its binding to BCL2, cytochrome C release and apoptosis. Most importantly, we observed a correlation of MDM4 expression with cisplatin-resistance in a group of human ovarian cancers suggesting that MDM4 proapoptotic activity may have in vivo relevance. Here, we discuss about these and some new findings and compare them with previous data trying to settle some apparent contradictions. In addition, this review discusses the potential relevance of our data to the field of human cancer.

MDM4 (MDMX) overexpression enhances stabilization of stress-induced p53 and promotes apoptosis.

Rescue of embryonic lethality in MDM4-/- mice through concomitant loss of p53 has revealed a functional partnership between the two proteins. Biochemical studies have suggested that MDM4 may act as a negative regulator of p53 levels and activity. On the other hand, MDM4 overexpression has been reported to stabilize p53 levels and to counteract MDM2-degradative activity. We have investigated the functional role of MDM4 overexpression on cell behavior. In both established and primary cells cultured under stress conditions, overexpression of MDM4 significantly increased p53-dependent cell death, in correlation with enhanced induction of the endogenous p53 protein levels. This phenomenon was associated with induced p53 transcriptional activity and increased levels of the proapoptotic protein, Bax. Further, p53 stabilization was accompanied by decreased association of the protein to its negative regulator, MDM2. These findings reveal a novel role for MDM4 by demonstrating that in non-tumor cells under stress conditions it may act as a positive regulator of p53 activity, mainly by controlling p53 levels. They also indicate a major distinction between the biological consequences of MDM4 and MDM2 overexpression.