Exosome secretion, including the DNA damage-induced p53-dependent secretory pathway, is severely compromised in TSAP6/Steap3-null mice.

TSAP6 (tumor suppressor-activated pathway 6), also known as Steap3, is a direct p53 transcriptional target gene. It regulates protein secretion, for example translationally controlled tumor protein (TCTP), which is implicated in tumor reversion. In keeping with the latter, we show herein that TSAP6 is a glycosylated protein present in the trans-Golgi network, endosomal-vesicular compartment and cytoplasmic membrane. To further investigate the physiological function of TSAP6, we have generated TSAP6-deficient mice. These mice exhibit microcytic anemia with abnormal reticulocyte maturation and deficient transferrin receptor downregulation, a process known to be dependent on exosomal secretion. Moreover, we provide direct evidence that exosome production is severely compromised in TSAP6-null cells. Finally, we show that the DNA damage-induced p53-dependent nonclassical exosomal secretory pathway is abrogated in TSAP6-null cells. Given the fact that exosomes are used as cell-free vaccines against cancer and that they could be involved in the biogenesis and spread of human immunodeficiency virus, it is important to understand their regulation. The results presented here provide the first genetic demonstration that exosome formation is a tightly controlled biological process dependent of TSAP6.

TCTP protects from apoptotic cell death by antagonizing bax function.

Translationally controlled tumor protein (TCTP) is a potential target for cancer therapy. It functions as a growth regulating protein implicated in the TSC1-TSC2 -mTOR pathway or a guanine nucleotide dissociation inhibitor for the elongation factors EF1A and EF1Bbeta. Accumulating evidence indicates that TCTP also functions as an antiapoptotic protein, through a hitherto unknown mechanism. In keeping with this, we show here that loss of tctp expression in mice leads to increased spontaneous apoptosis during embryogenesis and causes lethality between E6.5 and E9.5. To gain further mechanistic insights into this apoptotic function, we solved and refined the crystal structure of human TCTP at 2.0 A resolution. We found a structural similarity between the H2-H3 helices of TCTP and the H5-H6 helices of Bax, which have been previously implicated in regulating the mitochondrial membrane permeability during apoptosis. By site-directed mutagenesis we establish the relevance of the H2-H3 helices in TCTP's antiapoptotic function. Finally, we show that TCTP antagonizes apoptosis by inserting into the mitochondrial membrane and inhibiting Bax dimerization. Together, these data therefore further confirm the antiapoptotic role of TCTP in vivo and provide new mechanistic insights into this key function of TCTP.

Mdm2, but not Mdm4, protects terminally differentiated smooth muscle cells from p53-mediated caspase-3-independent cell death.

p53 is a potent inhibitor of cell growth and an inducer of apoptosis. During embryonic development, Mdm2 and Mdm4 inhibit the growth suppressive activities of p53. However, whether tight surveillance of p53 activity is required in quiescent cells is unknown. To test this, conditional inactivation of mdm2 and mdm4 was carried out in smooth muscle cells (SMCs). Upon SMC-specific inactivation of mdm2, and not of mdm4, mice rapidly became ill and died. Necropsy showed small intestinal dilation, and histological analyses indicated a severe reduction in the number of intestinal SMCs. Increased p53 levels and activity were detected in the remaining SMCs, and the phenotype was completely rescued on a p53-null background. Interestingly, intestinal SMCs are caspase-3-negative and therefore did not undergo caspase-3-dependent apoptotic cell death. Together, Mdm2, but not Mdm4, prevents accumulation of active p53 in quiescent SMCs and thereby the induction of p53-mediated caspase-3-independent cell death.

The ornithine decarboxylase gene is essential for cell survival during early murine development.

Overexpression and inhibitor studies have suggested that the c-Myc target gene for ornithine decarboxylase (ODC), the enzyme which converts ornithine to putrescine, plays an important role in diverse biological processes, including cell growth, differentiation, transformation, and apoptosis. To explore the physiological function of ODC in mammalian development, we generated mice harboring a disrupted ODC gene. ODC-heterozygous mice were viable, normal, and fertile. Although zygotic ODC is expressed throughout the embryo prior to implantation, loss of ODC did not block normal development to the blastocyst stage. Embryonic day E3.5 ODC-deficient embryos were capable of uterine implantation and induced maternal decidualization yet failed to develop substantially thereafter. Surprisingly, analysis of ODC-deficient blastocysts suggests that loss of ODC does not affect cell growth per se but rather is required for survival of the pluripotent cells of the inner cell mass. Therefore, ODC plays an essential role in murine development, and proper homeostasis of polyamine pools appears to be required for cell survival prior to gastrulation.

Jak3 selectively regulates Bax and Bcl-2 expression to promote T-cell development.

Jak3-deficient mice display vastly reduced numbers of lymphoid cells. Thymocytes and peripheral T cells from Jak3-deficient mice have a high apoptotic index, suggesting that Jak3 provides survival signals. Here we report that Jak3 regulates T lymphopoiesis at least in part through its selective regulation of Bax and Bcl-2. Jak3-deficient thymocytes express elevated levels of Bax and reduced levels of Bcl-2 relative to those in wild-type littermates. Notably, up-regulation of Bax in Jak3-deficient T cells is physiologically relevant, as Jak3 Bax double-null mice have marked increases in thymocyte and peripheral T-cell numbers. Rescue of T lymphopoiesis by Bax loss was selective, as mice deficient in Jak3 plus p53 or in Jak3 plus Fas remained lymphopenic. However, Bax loss failed to restore proper ratios of peripheral CD4/CD8 T cells, which are abnormally high in Jak3-null mice. Transplantation into Jak3-deficient mice of Jak3-null bone marrow transduced with a Bcl-2-expressing retrovirus also improved peripheral T-cell numbers and restored the ratio of peripheral CD4/CD8 T cells to wild-type levels. The data support the concepts that Jak kinases regulate cell survival through their selective and cell context-dependent regulation of pro- and antiapoptotic Bcl-2 family proteins and that Bax and Bcl-2 play distinct roles in T-cell development.

Apoptosis of Sertoli cells after conditional ablation of murine double minute 2 (Mdm2) gene is p53-dependent and results in male sterility.

Beside its well-documented role in carcinogenesis, the function of p53 family has been more recently revealed in development and female reproduction, but it is still poorly documented in male reproduction. We specifically tested this possibility by ablating Mdm2, an E3 ligase that regulates p53 protein stability and transactivation function, specifically in Sertoli cells (SCs) using the AMH-Cre line and created the new SC-Mdm2(-/-) line. Heterozygous SC-Mdm2(-/+) adult males were fertile, but SC-Mdm2(-/-) males were infertile and exhibited: a shorter ano-genital distance, an extra duct along the vas deferens that presents a uterus-like morphology, degenerated testes with no organized seminiferous tubules and a complete loss of differentiated germ cells. In adults, testosterone levels as well as StAR, P450c17 (Cyp17a1) and P450scc (Cyp11a1) mRNA levels decreased significantly, and both plasma LH and FSH levels increased. A detailed investigation of testicular development indicated that the phenotype arose during fetal life, with SC-Mdm2(-/-) testes being much smaller at birth. Interestingly, Leydig cells remained present until adulthood and fetal germ cells abnormally initiated meiosis. Inactivation of Mdm2 in SCs triggered p53 activation and apoptosis as early as 15.5 days post conception with significant increase in apoptotic SCs. Importantly, testis development occurred normally in SC-Mdm2(-/-) lacking p53 mice (SC-Mdm2(-/-)p53(-/-)) and accordingly, these mice were fertile indicating that the aforementioned phenotypes are entirely p53-dependent. These data not only highlight the importance of keeping p53 in check for proper testicular development and male fertility but also certify the critical role of SCs in the maintenance of meiotic repression.

A role for Xenopus Gli-type zinc finger proteins in the early embryonic patterning of mesoderm and neuroectoderm.

Gli-type zinc finger proteins play important regulatory roles in vertebrate and invertebrate embryogenesis. In Xenopus, the Gli-type proteins XGli-3 and XGli-4 are first expressed in earliest stages of mesoderm and neural development. Transient transfection assays reveal that XGli-3 and XGli-4 can function as transcription repressors. Counteracting the Gli-protein repressor activity by ectopic expression of a fusion protein that contains the Gli-zinc finger cluster connected to the E1A activator domain in Xenopus embryos results in specific morphological alterations in the developing somites and in the central nervous system. Altered expression characteristics for a broad set of molecular markers highlighting specific aspects of mesodermal and neural differentiation demonstrate an important role for Gli-type zinc finger proteins in the early mesodermal and neural patterning of Xenopus embryos.

Targeting Mdmx to treat breast cancers with wild-type p53.

The function of the tumor suppressor p53 is universally compromised in cancers. It is the most frequently mutated gene in human cancers (reviewed). In cases where p53 is not mutated, alternative regulatory pathways inactivate its tumor suppressive functions. This is primarily achieved through elevation in the expression of the key inhibitors of p53: Mdm2 or Mdmx (also called Mdm4) (reviewed). In breast cancer (BrCa), the frequency of p53 mutations varies markedly between the different subtypes, with basal-like BrCas bearing a high frequency of p53 mutations, whereas luminal BrCas generally express wild-type (wt) p53. Here we show that Mdmx is unexpectedly highly expressed in normal breast epithelial cells and its expression is further elevated in most luminal BrCas, whereas p53 expression is generally low, consistent with wt p53 status. Inducible knockdown (KD) of Mdmx in luminal BrCa MCF-7 cells impedes the growth of these cells in culture, in a p53-dependent manner. Importantly, KD of Mdmx in orthotopic xenograft transplants resulted in growth inhibition associated with prolonged survival, both in a preventative model and also in a treatment model. Growth impediment in response to Mdmx KD was associated with cellular senescence. The growth inhibitory capacity of Mdmx KD was recapitulated in an additional luminal BrCa cell line MPE600, which expresses wt p53. Further, the growth inhibitory capacity of Mdmx KD was also demonstrated in the wt p53 basal-like cell line SKBR7 line. These results identify Mdmx growth dependency in wt p53 expressing BrCas, across a range of subtypes. Based on our findings, we propose that Mdmx targeting is an attractive strategy for treating BrCas harboring wt p53.

Functional analysis of ZNF85 KRAB zinc finger protein, a member of the highly homologous ZNF91 family.

We previously identified the ZNF85 (HPF4) KRAB zinc finger gene, a member of the human ZNF91 family. Here, we show that the ZNF85 gene is highly expressed in normal adult testis, in seminomas, and in the NT2/D1 teratocarcinoma cell line. Immunocytochemical localization of a panel of beta-Gal/ZNF85 fusion proteins revealed that ZNF85 contains at least one nuclear localization signal located in the spacer region connecting the KRAB domain with the zinc finger repeats. Bacterially expressed ZNF85 zinc finger domain bound strongly and exclusively to DNA in vitro in a zinc-dependent manner. The KRAB(A) domain of the ZNF85 protein and of several other members of the ZNF91 family exhibited repressing activity when tested in Gal4 fusion protein assays. The repression was significantly enhanced by the addition of the KRAB (B) domain, whereas further addition of other conserved regions had no effect. The ZNF85 KRAB(A) and (B) domains in vitro bound several nuclear proteins that might constitute critical cofactors for repression.

Enhanced expression in seminoma of human zinc finger genes located on chromosome 19.

Six Krüppel-type zinc finger (ZF) genes were cloned from a seminoma cDNA library. One, ZFS-1, showed high sequence homology to the ZNF91 KRAB (Krüppel-associated box) ZF gene family and also the same chromosomal assignment. Interestingly, Northern blot analyses using ZFS-1 and ZNF91 revealed that multiple ZF genes on chromosome 19 were predominantly expressed in seminomas. In addition, the testis and the seminoma showed specific expression of 2.3 kb transcript. Our results suggest that ZF genes on chromosome 19 may be implicated in the development and/or growth of seminomas.

MDM2 restrains estrogen-mediated AKT activation by promoting TBK1-dependent HPIP degradation.

Restoration of p53 tumor suppressor function through inhibition of its interaction and/or enzymatic activity of its E3 ligase, MDM2, is a promising therapeutic approach to treat cancer. However, because the MDM2 targetome extends beyond p53, MDM2 inhibition may also cause unwanted activation of oncogenic pathways. Accordingly, we identified the microtubule-associated HPIP, a positive regulator of oncogenic AKT signaling, as a novel MDM2 substrate. MDM2-dependent HPIP degradation occurs in breast cancer cells on its phosphorylation by the estrogen-activated kinase TBK1. Importantly, decreasing Mdm2 gene dosage in mouse mammary epithelial cells potentiates estrogen-dependent AKT activation owing to HPIP stabilization. In addition, we identified HPIP as a novel p53 transcriptional target, and pharmacological inhibition of MDM2 causes p53-dependent increase in HPIP transcription and also prevents HPIP degradation by turning off TBK1 activity. Our data indicate that p53 reactivation through MDM2 inhibition may result in ectopic AKT oncogenic activity by maintaining HPIP protein levels.

Identification of a ZEB2-MITF-ZEB1 transcriptional network that controls melanogenesis and melanoma progression.

Deregulation of signaling pathways that control differentiation, expansion and migration of neural crest-derived melanoblasts during normal development contributes also to melanoma progression and metastasis. Although several epithelial-to-mesenchymal (EMT) transcription factors, such as zinc finger E-box binding protein 1 (ZEB1) and ZEB2, have been implicated in neural crest cell biology, little is known about their role in melanocyte homeostasis and melanoma. Here we show that mice lacking Zeb2 in the melanocyte lineage exhibit a melanoblast migration defect and, unexpectedly, a severe melanocyte differentiation defect. Loss of Zeb2 in the melanocyte lineage results in a downregulation of the Microphthalmia-associated transcription factor (Mitf) and melanocyte differentiation markers concomitant with an upregulation of Zeb1. We identify a transcriptional signaling network in which the EMT transcription factor ZEB2 regulates MITF levels to control melanocyte differentiation. Moreover, our data are also relevant for human melanomagenesis as loss of ZEB2 expression is associated with reduced patient survival.

p53 promotes VEGF expression and angiogenesis in the absence of an intact p21-Rb pathway.

There is growing evidence that the p53 tumour suppressor downregulates vascular endothelial growth factor (VEGF) expression, although the underlying mechanisms remain unclear and controversial. Here we provide insights from in vitro experiments and in vivo xenotransplantation assays that highlight a dual role for p53 in regulating VEGF during hypoxia. Unexpectedly, and for the first time, we demonstrate that p53 rapidly induces VEGF transcription upon hypoxia exposure by binding, in an HIF-1α-dependent manner, to a highly conserved and functional p53-binding site within the VEGF promoter. However, during sustained hypoxia, p53 indirectly downregulates VEGF expression via the retinoblastoma (Rb) pathway in a p21-dependent manner, which is distinct from its role in cell-cycle regulation. Our findings have important implications for cancer therapy, especially for tumours that harbour wild-type TP53 and a dysfunctional Rb pathway.

RNF4 is required for DNA double-strand break repair in vivo.

Unrepaired DNA double-strand breaks (DSBs) cause genetic instability that leads to malignant transformation or cell death. Cells respond to DSBs with the ordered recruitment of signaling and repair proteins to the sites of DNA lesions. Coordinated protein SUMOylation and ubiquitylation have crucial roles in regulating the dynamic assembly of protein complexes at these sites. However, how SUMOylation influences protein ubiquitylation at DSBs is poorly understood. We show herein that Rnf4, an E3 ubiquitin ligase that targets SUMO-modified proteins, accumulates in DSB repair foci and is required for both homologous recombination (HR) and non-homologous end joining repair. To establish a link between Rnf4 and the DNA damage response (DDR) in vivo, we generated an Rnf4 allelic series in mice. We show that Rnf4-deficiency causes persistent ionizing radiation-induced DNA damage and signaling, and that Rnf4-deficient cells and mice exhibit increased sensitivity to genotoxic stress. Mechanistically, we show that Rnf4 targets SUMOylated MDC1 and SUMOylated BRCA1, and is required for the loading of Rad51, an enzyme required for HR repair, onto sites of DNA damage. Similarly to inactivating mutations in other key regulators of HR repair, Rnf4 deficiency leads to age-dependent impairment in spermatogenesis. These findings identify Rnf4 as a critical component of the DDR in vivo and support the possibility that Rnf4 controls protein localization at DNA damage sites by integrating SUMOylation and ubiquitylation events.

Suppression of Myc oncogenic activity by nucleostemin haploinsufficiency.

Nucleostemin (NS), a nucleolar GTPase, is highly expressed in stem/progenitor cells and in most cancer cells. However, little is known about the regulation of its expression. Here, we identify the NS gene as a novel direct transcriptional target of the c-Myc oncoprotein. We show that Myc overexpression enhances NS transcription in cultured cells and in pre-neoplastic B cells from Eµ-myc transgenic mice. Consistent with NS being downstream of Myc, NS expression parallels that of Myc in a large panel of human cancer cell lines. Using chromatin immunoprecipitation we show that c-Myc binds to a well-conserved E-box in the NS promoter. Critically, we show NS haploinsufficiency profoundly delays Myc-induced cancer formation in vivo. NS+/-Eµ-myc transgenic mice have much slower rates of B-cell lymphoma development, with life spans twice that of their wild-type littermates. Moreover, we demonstrate that NS is essential for the proliferation of Myc-overexpressing cells in cultured cells and in vivo: impaired lymphoma development was associated with a drastic decrease of c-Myc-induced proliferation of pre-tumoural B cells. Finally, we provide evidence that in cell culture NS controls cell proliferation independently of p53 and that NS haploinsufficiency significantly delays lymphomagenesis in p53-deficient mice. Together these data indicate that NS functions downstream of Myc as a rate-limiting regulator of cell proliferation and transformation, independently from its putative role within the p53 pathway. Targeting NS is therefore expected to compromise early tumour development irrespectively of the p53 status.

Mdm2 controls CREB-dependent transactivation and initiation of adipocyte differentiation.

The role of the E3 ubiquitin ligase murine double minute 2 (Mdm2) in regulating the stability of the p53 tumor suppressor is well documented. By contrast, relatively little is known about p53-independent activities of Mdm2 and the role of Mdm2 in cellular differentiation. Here we report a novel role for Mdm2 in the initiation of adipocyte differentiation that is independent of its ability to regulate p53. We show that Mdm2 is required for cAMP-mediated induction of CCAAT/enhancer-binding protein δ (C/EBPδ) expression by facilitating recruitment of the cAMP regulatory element-binding protein (CREB) coactivator, CREB-regulated transcription coactivator (Crtc2)/TORC2, to the c/ebpδ promoter. Our findings reveal an unexpected role for Mdm2 in the regulation of CREB-dependent transactivation during the initiation of adipogenesis. As Mdm2 is able to promote adipogenesis in the myoblast cell line C2C12, it is conceivable that Mdm2 acts as a switch in cell fate determination.

Mdm2-mediated ubiquitylation: p53 and beyond.

The really interesting genes (RING)-finger-containing oncoprotein, Mdm2, is a promising drug target for cancer therapy. A key Mdm2 function is to promote ubiquitylation and proteasomal-dependent degradation of the tumor suppressor protein p53. Recent reports provide novel important insights into Mdm2-mediated regulation of p53 and how the physical and functional interactions between these two proteins are regulated. Moreover, a p53-independent role of Mdm2 has recently been confirmed by genetic data. These advances and their potential implications for the development of new cancer therapeutic strategies form the focus of this review.

Monoallelic but not biallelic loss of Dicer1 promotes tumorigenesis in vivo.

Human tumors are characterized by widespread reduction in microRNA (miRNA) expression, although it is unclear how such changes come about and whether they have an etiological role in the disease. Importantly, miRNA knockdown has been shown to enhance the tumorigenic potential of human lung adenocarcinoma cells. A defect in miRNA processing is one possible mechanism for global downregulation. To explore this possibility in more detail in vivo, we have manipulated Dicer1 gene dosage in a mouse model of retinoblastoma. We show that although monoallelic loss of Dicer1 does not affect normal retinal development, it dramatically accelerates tumor formation on a retinoblastoma-sensitized background. Importantly, these tumors retain one wild-type Dicer1 allele and exhibit only a partial decrease in miRNA processing. Accordingly, in silico analysis of human cancer genome data reveals frequent hemizygous, but not homozygous, deletions of DICER1. Strikingly, complete loss of Dicer1 function in mice did not accelerate retinoblastoma formation. miRNA profiling of these tumors identified members of the let-7 and miR-34 families as candidate tumor suppressors in retinoblastoma. We conclude that Dicer1 functions as a haploinsufficient tumor suppressor. This finding has implications for cancer etiology and cancer therapy.