Defective placental vasculogenesis causes embryonic lethality in VHL-deficient mice

Failure to prolyl hydroxylate hypoxia-inducible factor alpha phenocopies VHL inactivation in vivo

Many functions have been assigned to the von Hippel-Lindau tumor suppressor gene product (pVHL), including targeting the alpha subunits of the heterodimeric transcription factor HIF (hypoxia-inducible factor) for destruction. The binding of pVHL to HIFalpha requires that HIFalpha be hydroxylated on one of two prolyl residues. We introduced HIF1alpha and HIF2alpha variants that cannot be hydroxylated on these sites into the ubiquitously expressed ROSA26 locus along with a Lox-stop-Lox cassette that renders their expression Cre-dependent. Expression of the HIF2alpha variant in the skin and liver induced changes that were highly similar to those seen when pVHL is lost in these organs. Dual expression of the HIF1alpha and HIF2alpha variants in liver, however, more closely phenocopied the changes seen after pVHL inactivation than did the HIF2alpha variant alone. Moreover, gene expression profiling confirmed that the genes regulated by HIF1alpha and HIF2alpha in the liver are overlapping but non-identical. Therefore, the pathological changes caused by pVHL inactivation in skin and liver are due largely to dysregulation of HIF target genes.

Placental but not heart defects are associated with elevated hypoxia-inducible factor alpha levels in mice lacking prolyl hydroxylase domain protein 2

PHD1, PHD2, and PHD3 are prolyl hydroxylase domain proteins that regulate the stability of hypoxia-inducible factor alpha subunits (HIF-alpha). To determine the roles of individual PHDs during mouse development, we disrupted all three Phd genes and found that Phd2(-/-) embryos died between embryonic days 12.5 and 14.5 whereas Phd1(-/-) or Phd3(-/-) mice were apparently normal. In Phd2(-/-) mice, severe placental and heart defects preceded embryonic death. Placental defects included significantly reduced labyrinthine branching morphogenesis, widespread penetration of the labyrinth by spongiotrophoblasts, and abnormal distribution of trophoblast giant cells. The expression of several trophoblast markers was also altered, including an increase in the spongiotrophoblast marker Mash2 and decreases in the labyrinthine markers Tfeb and Gcm1. In the heart, trabeculae were poorly developed, the myocardium was remarkably thinner, and interventricular septum was incompletely formed. Surprisingly, while there were significant global increases in HIF-alpha protein levels in the placenta and the embryo proper, there was no specific HIF-alpha increase in the heart. Taken together, these data indicate that among all three PHD proteins, PHD2 is uniquely essential during mouse embryogenesis.

Effect of gestational nutrition on vascular integrity in the murine placenta

Maternal undernutrition is thought to result in smaller offspring and programme disease in later life. It is not known whether gestational nutrition affects development and functioning of placental vessels. The aim of this study was to ascertain if disturbed angiogenesis and junctional integrity were features of the labyrinthine vessels of placenta taken from transgenic Tie2-GFP mice fed either 18% (control) or 9% (low protein; MLP) casein diet. MLP animals showed a significant decrease in fetal weight at E14.5 and in placental and fetal weight at E18.5, however, maternal weight and litter size remained unaffected. Stereological analyses revealed that the fraction of components of the placenta remained similar in both study groups. There was a significant reduction in labyrinthine blood vessel length but not in luminal diameter in the E18.5 MLP group. In both MLP groups, perturbation of vascular endothelial Cadherin and beta-catenin, regulators of junctional integrity, permeability and quiescence, was observed with higher percentage of vessels showing weak or no junctional immunoreactivity. The reduction in length of the labyrinthine vessels and the downregulation of the adhesion molecules suggest that gestational undernutrition causes vascular dysfunction in the murine placenta. This may play a role in the early life programming of disease risk.

Hypoxia-inducible factors in the kidney

Tissue hypoxia not only occurs under pathological conditions but is also an important microenvironmental factor that is critical for normal embryonic development. Hypoxia-inducible factors HIF-1 and HIF-2 are oxygen-sensitive basic helix-loop-helix transcription factors, which regulate biological processes that facilitate both oxygen delivery and cellular adaptation to oxygen deprivation. HIFs consist of an oxygen-sensitive alpha-subunit, HIF-alpha, and a constitutively expressed beta-subunit, HIF-beta, and regulate the expression of genes that are involved in energy metabolism, angiogenesis, erythropoiesis and iron metabolism, cell proliferation, apoptosis, and other biological processes. Under conditions of normal Po(2), HIF-alpha is hydroxylated and targeted for rapid proteasomal degradation by the von Hippel-Lindau (VHL) E3-ubiquitin ligase. When cells experience hypoxia, HIF-alpha is stabilized and either dimerizes with HIF-beta in the nucleus to form transcriptionally active HIF, executing the canonical hypoxia response, or it physically interacts with unrelated proteins, thereby enabling convergence of HIF oxygen sensing with other signaling pathways. In the normal, fully developed kidney, HIF-1alpha is expressed in most cell types, whereas HIF-2alpha is mainly found in renal interstitial fibroblast-like cells and endothelial cells. This review summarizes some of the most recent advances in the HIF field and discusses their relevance to renal development, normal kidney function and disease.

Severe combined immunodeficiency associated with nephrogenic diabetes insipidus and a deletion in the Xq28 region

We evaluated a baby boy with severe combined immunodeficiency (SCID) and X-linked nephrogenic diabetes insipidus (NDI). This patient had less than 10% CD3+ T cells, almost all of which were positive for CD4 and CD45RO. Genetic studies demonstrated a 34.4 kb deletion at Xq28 which included AVPR2, the gene responsible for NDI; ARHGAP4, a hematopoietic specific gene encoding a GTPase-activating protein; and a highly conserved segment of DNA between ARHGAP4 and ARD1A, a gene involved in the response to hypoxia. Other patients with NDI, but without immunodeficiency, have had deletions that remove all ARHGAP4 except exon 1; however, no other patients have had deletions of the highly conserved intragenic region between ARHGAP4 and ARD1A. X chromosome inactivation studies, done on sorted cells from the mother and grandmother of the patient, carriers of the deletion, demonstrated exclusive use of the non-mutant X chromosome as the active X in CD4 and CD8 T cells. Surprisingly, NK cells, monocytes and neutrophils from these women demonstrated preferential use of the mutant X chromosome as the active X. These results are consistent with an X-linked form of SCID, due to the loss of regulatory elements that control the response to hypoxia in hematopoietic cells.

Vascular defects and liver damage by the acute inactivation of the VHL gene during mouse embryogenesis

Inactivation of the von Hippel-Lindau (VHL) tumor suppressor gene leads to the development of central nervous system hemangioblastomas, pheochromocytomas and renal cell carcinomas. The biological role of the VHL gene during development is poorly understood because of early lethality of VHL-null embryos. To overcome early embryo lethality observed in the conventional knockout mouse, we introduced a tamoxifen-inducible Cre (CreER(TM)) transgene for the stage specific inactivation of the VHL gene. Acute tamoxifen-induced inactivation of the VHL gene at E10.5 resulted in embryonic lethality between E14.5 and E15.0 with extensive hemorrhage and necrosis, while littermate controls showed normal development. Examination of the VHL-inactivated embryos between E10.5 and E14.5 revealed dilated blood vessels, hemorrhage and necrotizing liver damage. Concomitant with severe hemorrhage and abnormal vasculature at E15.0, blood circulation in the yolk sac was impaired in the VHL-inactivated embryos, which may be the cause of embryo death. Placental development looked normal before embryo death (E14.5); however, at E16.5 following embryo death, we observed reduced growth of the placental labyrinthine layer. Inactivation of the VHL gene resulted in hypoxia-inducible factor (HIF)-1alpha stabilization and induction of its target genes, VEGF and CAIX, in mouse embryonic fibroblasts (MEFs). In addition, we observed lactate overproduction and acidification of culture media by the inactivation of the VHL gene. Thus, by using a novel conditional VHL knockout mouse model, we could show that the VHL gene plays an important role in the developing vasculature and liver during embryogenesis through regulation of HIF-1alpha and its target genes. This mouse model will be useful for the screening of anti-HIF or anti-VEGF drugs in vivo. Additionally, this acute VHL inactivation system may provide a useful tool for the in vivo study of genes that cause early embryonic lethality.

pVHL function is essential for endothelial extracellular matrix deposition

The tumor suppressor von Hippel-Lindau protein (pVHL) is critical for cellular molecular oxygen sensing, acting to target degradation of the hypoxia-inducible factor alpha transcription factor subunits under normoxic conditions. We have found that independent of its function in regulating hypoxic response, the VHL gene plays a critical role in embryonic endothelium development through regulation of vascular extracellular matrix assembly. We created mice lacking the VHL gene in endothelial cells; these conditional null mice died at the same stage as homozygous VHL-null mice, with similar vascular developmental defects. These included defective vasculogenesis in the placental labyrinth, a collapsed endocardium, and impaired vessel network patterning. The defects in embryonic vascularization were correlated with a diminished vascular fibronectin deposition in vivo and defective endothelial extracellular fibronectin assembly in vitro. We found that the impaired migration and adhesion of VHL-null endothelial cells can be partially rescued by the addition of back exogenous fibronectin, which indicates that pVHL regulation of fibronectin deposition plays an important functional role in vascular patterning and maintenance of vascular integrity.

Hypoxia and HIF-1 in Chondrogenesis

In endochondral bone development chondrocytes undergo proliferation, hypertrophic differentiation, mineralization of the surrounding matrix, death, blood vessel invasion, and finally replacement of cartilage with bone. The chondrocytic growth plate is a unique mesenchymal tissue, as it is avascular but it requires blood vessel invasion in order to be replaced by bone. We have recently provided evidence that the growth plate is hypoxic during fetal development. Adaptation to hypoxia is a critical event in numerous pathological settings, such as tumor progression and survival of tissues in which blood flow has been suddenly interrupted. One of the hallmarks of the response to hypoxia is activation of the transcription factor HIF-1alpha. The von Hippel-Lindau (VHL) tumor suppressor protein is a component of a ubiquitin ligase promoting proteolysis of HIF-1alpha. By using a genetic approach, we have demonstrated that VHL and HIF-1alpha are critical regulators of endochondral bone development.

A novel mutation in the von Hippel-Lindau tumor suppressor gene identified in a Japanese family with pheochromocytoma and hepatic hemangioma

Von Hippel-Lindau (VHL) syndrome is a neoplastic syndrome caused by a mutation in the VHL gene. There is a discrepancy between the phenotypes of human VHL syndrome and VHL gene-disrupted mouse models. A heterozygous VHL gene-disrupted model (vhl +/-) developed hepatic vascular lesions; in contrast, hepatic hemangioma is a rare manifestation of human VHL syndrome. We identified a novel mutation (P154S) in the VHL gene in a Japanese family with pheochromocytoma. One of the members demonstrated hepatic hemangiomas, suggesting that there may be a relationship between the mutation of the VHL gene and hepatic vascular lesions, even in humans.

Oxygen, epigenetics and stem cell fate

Coordinated interactions between the embryo/fetus and its environment are critical for proper development. In addition to acting as metabolic substrates for cellular homeostasis, basic physiological factors, such as oxygen tension, have a profound influence on developmental outcomes. Since the mammalian embryo resides in a physiologically hypoxic environment during gestation, understanding its responses to oxygen deprivation on a cellular level is critical. In this review, we analyze interactions between the hypoxia-inducible factor family of transcriptional regulators and epigenetic mechanisms governing chromatin structure. The ability of hypoxia-inducible factors to interact physically with histone deacetylase (HDAC) enzymes and modulate nuclear HDAC activities places them in the pivotal position of integrating physiological and epigenetic effectors. Multiple embryonic and extra-embryonic stem cell populations in mice and humans rely on this interaction – an important determinant of stem cell fate. Dissection of the pathways involved will provide novel insights into the metabolic as well as molecular determinants of the stem cell niches that allow self-renewal of progenitors in an undifferentiated state.

Genes critical to vasculogenesis as defined by systematic analysis of vascular defects in knockout mice

To identify genes important to the process of vasculogenesis, we evaluated embryonic vascular anomalies from 100 mouse knockout studies using a novel meta-analysis approach. By applying this method, termed approach for ranking of embryonic vascular anomalies (AREVA), rank scores were calculated for each knockout based on the occurrence of vascular defects during periods of vasculogenesis in specific embryonic regions. As a result, 12 genes (fibronectin, VEGFR-1/Flt-1, VEGFR-2/Flk-1, alpha 5 integrin, Tek/Tie2, VE-cadherin, VEGFA, connexin 45, ShcA, cytochrome P450 reductase, CD148/DEP-1, and EphrinB2) were determined to play critical roles in vasculogenesis. Functional categorization of these genes revealed the fundamental importance of VEGF signaling since 10 of the 12 genes (fibronectin, VEGFR-1/Flt-1, VEGFR-2/Flk-1, alpha 5 integrin, VE-cadherin, VEGFA, ShcA, cytochrome P450 reductase, CD148/DEP-1, and EphrinB2) relate to this pathway. Furthermore, the findings highlight a potential network for regulating VEGF signaling involving integration of fibronectin, EphrinB2, Tie2, and connexin 45 signaling pathways via the ShcA/Ras/Raf/Mek/Erk cascade. In addition to retrospective application of AREVA as done herein, AREVA can be used prospectively to determine the relevancy to vasculogenesis of newly inactivated genes.

Scientific and clinical opportunities for modeling blood disorders with embryonic stem cells

Our considerable wealth of data concerning hematologic processes has come despite difficulties working with stem and progenitor cells in vitro and their propensity to differentiate. Key methodologies that have sought to overcome such limitations include transgenic/knock-out animals and in vitro studies using murine embryonic stem cells, because both permit investigation of the formation of hematopoietic tissue from nonhematopoietic precursors. Although there have been many successful studies in model animals for understanding hematopoietic-cell development, differences between lower vertebrates and humans have left gaps in our understanding. Clearly, human-specific strategies to study the onset of hematopoiesis, particularly the earliest events leading to the specification of both normal and abnormal hematopoietic tissue, could bring an investigational renaissance. The recent availability of human embryonic stem (hES) cells suggests that such a system is now at hand. This review highlights the potential of hES cells to model human hematologic processes in vitro with an emphasis on disease targets.

Hypoxia inducible factor 1 alpha regulates T cell receptor signal transduction

Low oxygen pressures exist in many solid tissues, including primary and secondary lymphoid organs. One key element in cellular adaptation to hypoxia is induced expression of hypoxia inducible factor (Hif) 1alpha. Here, we have examined the effect of Hif-1alpha, isolated from the myriad other effects of hypoxia, on T cell receptor (TCR) signaling in thymocytes. Because pVHL (von Hippel-Lindau protein) directs the proteolysis of Hif-1alpha under "normoxic" conditions, we achieved constitutive stabilization of Hif-1alpha through thymic deletion of Vhlh and reversed Hif-1alpha stabilization with double deletion of Vhlh and Hif-1alpha. We found that constitutive activity of Hif-1alpha resulted in diminished Ca(2+) response upon TCR crosslinking despite equivalent activation of phospholipase C(gamma1), normal intracellular Ca(2+) stores, and normal entry of Ca(2+) across the plasma membrane. Altered Ca(2+) response was instead due to accelerated removal of Ca(2+) from the cytoplasm into intracellular compartments, which occurred in association with Hif-1alpha-dependent overexpression of the calcium pump SERCA2 (sarcoplasmic/endoplasmic reticulum calcium ATPase 2). These data suggest a unique mechanism for control of TCR signaling through Hif-1alpha, which may be operative at the physiologic oxygen tensions seen in solid lymphoid organs.

Lung cancer susceptibility in Fhit-deficient mice is increased by Vhl haploinsufficiency

The FHIT gene plays important roles in cancer development, including lung cancers, in which the Fhit protein is frequently lost. To determine if Fhit-deficient mice exhibit increased susceptibility to carcinogen-induced lung cancer, mice were treated with the pulmonary carcinogen 4-methylnitrosamino-1-3-pyridyl-1-butanone. Wild-type and Fhit-deficient animals did not exhibit significantly different frequencies of lung lesions, but Fhit-/- mice showed significantly increased average tumor volume (1.62 mm3) and multiplicity in tumor-bearing mice, compared with wild-type mice (0.70 mm3). Tumors of Fhit-/- mice were all carcinomas, whereas Fhit+/+ mice did not develop carcinomas. To determine if Fhit absence, in combination with deficiency of an additional 3p tumor suppressor, would affect the frequency of tumor induction, we examined the spontaneous and dimethylnitrosamine-induced tumor phenotype of Fhit-/-Vhl+/- mice. Whereas no spontaneous lung tumors were observed in Fhit-/- or Vhl+/- mice, 44% of Fhit-/-Vhl+/- mice developed adenocarcinomas by 2 years of age. Dimethylnitrosamine (6 mg/kg body weight) induced lung tumors (adenomas and carcinomas) in 100% of Fhit-/-Vhl+/- mice and adenomas in 40% of Fhit-/- mice by 20 months of age. Thus, double deficiency in murine homologues of 3p suppressor genes, including haploinsufficiency of Vhl, predisposes to spontaneous and induced lung cancers, showing that Fhit-deficient mice will be useful, in combination with other 3p tumor suppressors, in recapitulating a pattern of lung cancer development similar to the human pattern; such double- or triple-deficient mice will be excellent lung cancer prevention and therapy models.

O2 enhancement of human trophoblast differentiation and hCYP19 (aromatase) gene expression are mediated by proteasomal degradation of USF1 and USF2

When cultured in 20% O(2), human cytotrophoblasts fuse to form the syncytiotrophoblast with marked induction of hCYP19 (aromatase) gene expression. When cultured in 2% O(2), cytotrophoblast fusion and induced hCYP19 expression are prevented. These effects of hypoxia are mediated by increased expression of mammalian achaete/scute homologue-2 (Mash-2), which increases levels of upstream stimulatory factors 1 and 2 (USF1/2) and their binding as heterodimers to E-boxes surrounding the hCYP19 promoter. In studies to define mechanisms for O(2) regulation of syncytiotrophoblast differentiation, we found that hypoxia and overexpression of Mash-2 markedly increased cyclin B1 levels in cultured trophoblasts and the proportion of cells at the G(2)/M transition. Unlike USF proteins, USF1/2 mRNA levels are unaffected by O(2) tension. To determine whether increased O(2) might enhance proteasomal degradation of USF1/2, human trophoblasts were cultured in 2% or 20% O(2) with or without proteasome inhibitors. In cells cultured in 20% O(2), proteasome inhibitors increased USF1/2 protein levels and blocked spontaneous induction of hCYP19 expression, cell fusion, and differentiation. Like hypoxia, inhibitory effects of proteasome inhibitors on hCYP19 expression were mediated by increased binding of USF1/2 to the E-boxes. In human trophoblast cells cultured in 20% O(2), increased polyubiquitylation of USF1/2 proteins was observed. Thus, early in gestation when the placenta is relatively hypoxic, increased USF1/2 may block trophoblast differentiation and hCYP19 gene expression. In the second trimester, increased O(2) tension promotes proteasomal degradation of USF1/2, resulting in syncytiotrophoblast differentiation and induction of hCYP19 expression.

Role of elongin-binding domain of von hippel lindau gene product on HuR-mediated VPF/VEGF mRNA stability in renal cell carcinoma

Vascular endothelial growth factor (VEGF), also known as vascular permeability factor (VPF), is a key mediator of angiogenesis for both physiological and pathological conditions. It is well established that the hypoxic induction of VPF/VEGF is in large part an increase in the stability of its mRNA. A Hu family ubiquitously expressed RNA-binding protein HuR has recently been shown to be important for VPF/VEGF mRNA stabilization. In renal cancer cells, the inactivation of the tumor suppressor protein von Hippel Lindau (VHL) leads to an increase in VPF/VEGF expression. VHL not only inhibits the transcription of VPF/VEGF but also plays a significant role in decreasing its mRNA stability. Here we delineate a possible mechanism by which VHL can control the function of HuR in order to regulate the stability of VPF/VEGF mRNA. The experiments presented here suggest that the association of the elongin-binding domain of VHL with a specific RNA-binding domain of HuR (RRM1) is important for the destabilizing function of VHL on VPF/VEGF mRNA.

Hypoxia-inducible factor as a physiological regulator

Hypoxia-inducible factor (HIF) is a transcription complex which responds to changes in oxygen, providing cells with a master regulator that coordinates changes in gene transcription. HIF operates in all mammalian cell types and is ancient in evolutionary terms, being conserved in C. elegans and D. melanogaster. This review summarizes recent insights into the molecular events that link reduced oxygenation to HIF activation and emerging insights into the extensive role of HIF in a broad range of physiological processes.

Hereditary hormone excess: genes, molecular pathways, and syndromes

Hereditary origin of a tumor helps toward early discovery of its mutated gene; for example, it supports the compilation of a DNA panel from index cases to identify that gene by finding mutations in it. The gene for a hereditary tumor may contribute also to common tumors. For some syndromes, such as hereditary paraganglioma, several genes can cause a similar syndrome. For other syndromes, such as multiple endocrine neoplasia 2, one gene supports variants of a syndrome. Onset usually begins earlier and in more locations with hereditary than sporadic tumors. Mono- or oligoclonal ("clonal") tumor usually implies a postnatal delay, albeit less delay than for sporadic tumor, to onset and potential for cancer. Hormone excess from a polyclonal tissue shows onset at birth and no benefit from subtotal ablation of the secreting organ. Genes can cause neoplasms through stepwise loss of function, gain of function, or combinations of these. Polyclonal hormonal excess reflects abnormal gene dosage or effect, such as activation or haploinsufficiency. Polyclonal hyperplasia can cause the main endpoint of clinical expression in some syndromes or can be a precursor to clonal progression in others. Gene discovery is usually the first step toward clarifying the molecule and pathway mutated in a syndrome. Most mutated pathways in hormone excess states are only partly understood. The bases for tissue specificity of hormone excess syndromes are usually uncertain. In a few syndromes, tissue selectivity arises from mutation in the open reading frame of a regulatory gene (CASR, TSHR) with selective expression driven by its promoter. Polyclonal excess of a hormone is usually from a defect in the sensor system for an extracellular ligand (e.g., calcium, glucose, TSH). The final connections of any of these polyclonal or clonal pathways to hormone secretion have not been identified. In many cases, monoclonal proliferation causes hormone excess, probably as a secondary consequence of accumulation of cells with coincidental hormone-secretory ability.

Hypoxia and HIF-1α in chondrogenesis

In endochondral bone development chondrocytes undergo well-ordered and controlled phases of proliferation, hypertrophic differentiation, mineralization of the surrounding matrix, death, blood vessel invasion, and finally replacement of cartilage with bone. The chondrocytic growth plate is a unique mesenchymal tissue, as it is avascular but it requires blood vessel invasion, i.e. the angiogenic switch, in order to be replaced by bone. We have recently provided evidence that the growth plate is hypoxic during fetal development. Adaptation to hypoxia is a critical event in numerous pathological settings, such as tumor progression and survival of tissues in which blood flow has been suddenly interrupted. One of the hallmarks of the response to hypoxia is activation of the transcription factor HIF-1alpha. The von Hippel Lindau tumor suppressor protein VHL is a component of a ubiquitin ligase promoting proteolysis of HIF-1alpha. By using a genetic approach, we have demonstrated the essential role of the hypoxia/VHL/HIF-1alpha pathway in endochondral bone development. Hypoxia-dependent up regulation of HIF-1alpha transcriptional activity is critical for survival of hypoxic chondrocyte, and it shapes up the fetal growth plate by inhibiting chondrocyte proliferation, increasing matrix accumulation and probably modulating cell size. The findings overall highlight the usefulness of studying fetal growth plate development as a model to address issues such as adaptation of normal tissues to hypoxia, survival of hypoxic cells, and regulation of the angiogenic switch. They also demonstrate a crucial role of hypoxia and HIF-1alpha in development and differentiation.

Decreased growth of Vhl-/- fibrosarcomas is associated with elevated levels of cyclin kinase inhibitors p21 and p27

Inactivating mutations within the von Hippel-Lindau (VHL) tumor suppressor gene predispose patients to develop a variety of highly vascularized tumors. pVHL targets alpha subunits of the heterodimeric transcription factor hypoxia-inducible factor (HIF), a critical regulator of energy metabolism, angiogenesis, hematopoiesis, and oxygen (O(2)) delivery, for ubiquitin-mediated degradation in an O(2)-dependent manner. To investigate the role of Vhl in cellular proliferation and tumorigenesis, we utilized mouse embryonic fibroblasts (MEFs), a common tool for analyzing cell cycle regulation, and generated Vhl(-)(/)(-) MEF-derived fibrosarcomas. Surprisingly, growth of both Vhl(-)(/)(-) MEFs and fibrosarcomas was impaired, although tumor vascularity was increased. Decreased proliferation of Vhl(-)(/)(-) MEFs was correlated with an overexpression of cyclin kinase inhibitors (CKIs) p21 and p27. The transcription of p21 and p27 is inhibited by c-Myc; therefore, the induction of CKIs was attributed to the ability of HIF to antagonize c-Myc activity. Indeed, p21 mRNA levels were elevated under normoxia in Vhl(-)(/)(-) MEFs, while c-Myc transcriptional activity was markedly reduced. Gene silencing of HIF-1alpha by small interfering RNA reduced p21 and p27 protein and mRNA levels in Vhl(-)(/)(-) MEFs. The induction of p21 and p27, mediated by constitutive activation of the HIF pathway, provides a mechanism for the decreased proliferation rates of Vhl(-)(/)(-) MEFs and fibrosarcomas. These results demonstrate that a loss of pVHL can induce growth arrest in certain cells types, which suggests that additional genetic mutations are necessary for VHL-associated tumorigenesis.

Mutation of SENP1/SuPr-2 reveals an essential role for desumoylation in mouse development

The covalent modification of proteins by the small ubiquitin-like protein SUMO has been implicated in the regulation of numerous biological processes, including nucleocytoplasmic transport, genomic stability, and gene transcription. Sumoylation occurs by a multienzyme process similar to ubiquitination and, in Saccharomyces cerevisiae, is reversed by desumoylating enzymes encoded by the Ulp1 and Smt4/Ulp2 genes. The physiological importance of desumoylation has been revealed by mutations in either gene, which lead to nonoverlapping defects in cell cycle transition and meiosis. Several mammalian Ulp homologues have been identified, but, to date, nothing is known of the phenotypic effects of their loss of function. Here, we describe a random retroviral insertional mutation of one homolog, mouse SENP1/SuPr-2. The mutation causes increased steady-state levels of the sumoylated forms of a number of proteins and results in placental abnormalities incompatible with embryonic development. Our findings provide the first insight into the critical importance of regulating sumoylation in mammals.

Development of structures and transport functions in the mouse placenta

The placenta is essential for sustaining the growth of the fetus during gestation, and defects in its function result in fetal growth restriction or, if more severe, fetal death. Several molecular pathways have been identified that are essential for development of the placenta, and mouse mutants offer new insights into the cell biology of placental development and physiology of nutrient transport.

Inactivation of the arylhydrocarbon receptor nuclear translocator (Arnt) suppresses von Hippel-Lindau disease-associated vascular tumors in mice

Patients with germ line mutations in the VHL tumor suppressor gene are predisposed to the development of highly vascularized tumors within multiple tissues. Loss of pVHL results in constitutive activation of the transcription factors HIF-1 and HIF-2, whose relative contributions to the pathogenesis of the VHL phenotype have yet to be defined. In order to examine the role of HIF in von Hippel-Lindau (VHL)-associated vascular tumorigenesis, we utilized Cre-loxP-mediated recombination to inactivate hypoxia-inducible factor-1alpha (Hif-1alpha) and arylhydrocarbon receptor nuclear translocator (Arnt) genes in a VHL mouse model of cavernous liver hemangiomas and polycythemia. Deletion of Hif-1alpha did not affect the development of vascular tumors and polycythemia, nor did it suppress the increased expression of vascular endothelial growth factor (Vegf) and erythropoietin (Epo). In contrast, phosphoglycerokinase (Pgk) expression was substantially decreased, providing evidence for target gene-dependent functional redundancy between different Hif transcription factors. Inactivation of Arnt completely suppressed the development of hemangiomas, polycythemia, and Hif-induced gene expression. Here, we demonstrate genetically that the development of VHL-associated vascular tumors in the liver depends on functional ARNT. Furthermore, we provide evidence that individual HIF transcription factors may play distinct roles in the development of specific VHL disease manifestations.

The VHL tumor suppressor in development and disease: functional studies in mice by conditional gene targeting

The von Hippel-Lindau tumor suppressor pVHL plays a critical role in the pathogenesis of familial and sporadic clear cell carcinomas of the kidney and hemangioblastomas of the retina and central nervous system. pVHL targets the oxygen sensitive alpha subunit of hypoxia-inducible factor (HIF) for proteasomal degradation, thus providing a direct link between tumorigenesis and molecular pathways critical for cellular adaptation to hypoxia. Cell type specific gene targeting of VHL in mice has demonstrated that proper pVHL mediated HIF proteolysis is fundamentally important for survival, proliferation and differentiation of many cell types and furthermore, that inactivation of pVHL may, unexpectedly, inhibit tumor growth under certain conditions. Mouse knock out studies have provided novel mechanistic insights into VHL associated tumorigenesis and established a central role for HIF in the development of the VHL phenotype.

The adverse effects of maternal smoking on the human placenta: A review

Studies of placental pathologies associated with maternal cigarette smoking have led to many interesting observations. For example, maternal smoking impairs human placental development by changing the balance between cytotrophoblast (CTB) proliferation and differentiation. It is likely that chronic exposure to tobacco constituents in early pregnancy can affect placental development directly or indirectly by reducing blood flow, which creates a pathologically hypoxic environment. To understand this process at a molecular level, tissue samples from non-smoking and smoking mothers were studied to determine whether active and/or passive cigarette smoke exposure affects CTB expression of molecules that govern cellular responses to oxygen tension: the von Hippel-Lindau tumor suppressor protein (pVHL), hypoxia-inducible transcription factors (HIFs) and vascular endothelial growth factor-A (VEGF). The results show that maternal smoking dysregulates CTB expression of all three types of molecules. In addition, cell columns and proliferating cells were reduced while there was a corresponding increase in cell islands. All three phenomena were most obvious in the placentas of heavy smokers. Interestingly, a subset of the aforementioned effects can be detected in samples obtained from women who were passively exposed to cigarette smoke during pregnancy. These observations suggest that tobacco constituents exert direct effects on CTB proliferation and differentiation and help to explain the mechanisms by which smoking negatively effects human pregnancy outcome.

The hypoxia-inducible factor and tumor progression along the angiogenic pathway

The hypoxia-inducible factor (HIF) is a transcription factor that plays a key role in the response of cells to oxygen levels. HIF is a heterodimer of alpha- and beta-subunits where the alpha-subunit is translated constitutively but has a very short half-life under normal oxygen concentrations. Negative regulation of the half-life and activity of the alpha-subunit is dependent on its posttranslational hydroxylation by hydroxylases that are dependent on oxygen for activity. Thus under low oxygen (hypoxic) conditions the hydroxylases are inactive and the alpha-subunit is stable and able to interact with the beta-subunit to bind and induce transcription of target genes. Hypoxic conditions are encountered in development and in disease states such as cancer. Tumors that have outstripped their blood supply become hypoxic and express high levels of HIF. HIF in turn targets genes that induce survival, glycolysis, and angiogenesis, a form of neovascularization, which ensures the tumor with a continued supply of oxygen and nutrients for further growth.

Role of VHL gene mutation in human cancer

Germline inactivation of the von Hippel-Lindau (VHL) tumor suppressor gene causes the von Hippel-Lindau hereditary cancer syndrome, and somatic mutations of this gene have been linked to the development of sporadic hemangioblastomas and clear-cell renal carcinomas. The VHL tumor suppressor protein (pVHL), through its oxygen-dependent polyubiquitylation of hypoxia-inducible factor (HIF), plays a central role in the mammalian oxygen-sensing pathway. This interaction between pVHL and HIF is governed by post-translational prolyl hydroxylation of HIF in the presence of oxygen by a conserved family of Egl-nine (EGLN) enzymes. In the absence of pVHL, HIF becomes stabilized and is free to induce the expression of its target genes, many of which are important in regulating angiogenesis, cell growth, or cell survival. Moreover, preliminary data indicate that HIF plays a critical role in pVHL-defective tumor formation, raising the possibility that drugs directed against HIF or its downstream targets (such as vascular endothelial growth factor) might one day play a role in the treatment of hemangioblastoma and renal cell carcinoma. On the other hand, clear genotype-phenotype correlations are emerging in VHL disease and can be rationalized if pVHL has functions separate from its control of HIF.

The von Hippel-Lindau tumor suppressor protein: roles in cancer and oxygen sensing

Biallelic inactivation of the von Hippel-Lindau (VHL) tumor suppressor gene is a common event in hereditary (von Hippel- Lindau disease) and sporadic hemangioblastomas and clear-cell renal carcinomas. Germ-line VHL mutations are also linked to some hereditary pheochromocytoma families. The VHL gene product, pVHL, interacts with a number of cellular proteins and is implicated in the control of angiogenesis, extracellular matrix formation, cell metabolism, and mitogenesis. The best understood function of pVHL relates to its role as the substrate recognition unit of an E3 ligase that targets the heterodimeric transcription factor HIF (hypoxia-inducible factor) for destruction in the presence of oxygen. Down-regulation of HIF appears to be both necessary and sufficient for renal tumor suppression by pVHL, and HIF is strongly suspected of contributing to hemangioblastoma development as well. Recent work suggests that pVHL's role in pheochromocytoma is not related to HIF but rather to the ability of pVHL to regulate neuronal apoptosis, which is mediated by c-Jun, when growth factors such as NGF become limiting. Loss of pVHL leads to up-regulation of JunB, which antagonizes c-Jun and blunts apoptosis.

Vhlh gene deletion induces Hif-1-mediated cell death in thymocytes

The von Hippel-Lindau gene product (pVHL) targets the alpha subunit of basic helix-loop-helix transcription factor hypoxia-inducible factor (HIF) for proteasomal degradation. Inactivation of pVhl in the mouse germ line results in embryonic lethality, indicating that tight control of Hif-mediated adaptive responses to hypoxia is required for normal development and tissue function. In order to investigate the role of pVhl in T-cell development, we generated mice with thymocyte-specific inactivation of Vhlh resulting in constitutive transcriptional activity of Hif-1, as well as mice with thymocyte-specific repression of Hif-1 in a wild-type and Vhlh-deficient background. Thymi from Vhlh-deficient mice were small due to a severe reduction in the total number of CD4/CD8-double-positive thymocytes which was associated with increased apoptosis in vivo and in vitro. Increased apoptosis was a result of enhanced caspase 8 activity, while Bcl-2 and Bcl-XL transgene expression had little effect on this phenotype. Inactivation of Hif-1 in Vhlh-deficient thymocytes restored thymic cellularity as well as thymocyte viability in vitro. Our data suggest that tight regulation of Hif-1 via pVhl is required for normal thymocyte development and viability and that an increase in Hif-1 transcriptional activity enhances caspase 8-mediated apoptosis in thymocytes.

Trophoblast differentiation during embryo implantation and formation of the maternal-fetal interface

Trophoblasts, the specialized cells of the placenta, play a major role in implantation and formation of the maternal-fetal interface. Through an unusual differentiation process examined in this review, these fetal cells acquire properties of leukocytes and endothelial cells that enable many of their specialized functions. In recent years a great deal has been learned about the regulatory mechanisms, from transcriptional networks to oxygen tension, which control trophoblast differentiation. The challenge is to turn this information into clinically useful tests for monitoring placental function and, hence, pregnancy outcome.

In vitroandIn vivoModels Analyzing von Hippel-Lindau Disease-Specific Mutations

Mutations in the von Hippel-Lindau (VHL) tumor suppressor gene cause tissue-specific tumors, with a striking genotype-phenotype correlation. Loss of VHL expression predisposes to hemangioblastoma and clear cell renal cell carcinoma, whereas specific point mutations predispose to pheochromocytoma, polycythemia, or combinations of hemangioblastoma, renal cell carcinoma, and/or pheochromocytoma. The VHL protein (pVHL) has been implicated in many cellular activities including the hypoxia response, cell cycle arrest, apoptosis, and extracellular matrix remodeling. We have expressed missense pVHL mutations in Vhl(-/-) murine embryonic stem cells to test genotype-phenotype correlations in euploid cells. We first examined the ability of mutant pVHL to direct degradation of the hypoxia inducible factor (HIF) subunits HIF1alpha and HIF2alpha. All mutant pVHL proteins restored proper hypoxic regulation of HIF1alpha, although one VHL mutation (VHL(R167Q)) displayed impaired binding to Elongin C. This mutation also failed to restore HIF2alpha regulation. In separate assays, these embryonic stem cells were used to generate teratomas in immunocompromised mice, allowing independent assessment of the effects of specific VHL mutations on tumor growth. Surprisingly, teratomas expressing the VHL(Y112H) mutant protein displayed a growth disadvantage, despite restoring HIFalpha regulation. Finally, we observed increased microvessel density in teratomas derived from Vhl(-/-) as well as VHL(Y112H), VHL(R167Q), and VHL(R200W) embryonic stem cells. Together, these observations support the hypothesis that pVHL plays multiple roles in the cell, and that these activities can be separated via discrete VHL point mutations. The ability to dissect specific VHL functions with missense mutations in a euploid model offers a novel opportunity to elucidate the activities of VHL as a tumor suppressor.

Activating Met mutations produce unique tumor profiles in mice with selective duplication of the mutant allele

Tyrosine kinase-activating mutations in Met have been observed in hereditary papillary renal carcinomas as well as in other cancers. These mutations have been examined in several in vitro systems, where they cause constitutive Met activation, focus formation, and cell motility, and are tumorigenic in xenografts. To study the influence of these mutations on tumorigenesis in vivo, we generated mice with targeted mutations in the murine met locus. The following five mouse lines with mutant Met were created: WT, D1226N, Y1228C, M1248T, and M1248T/L1193V. We observed that mice harboring D1226N, Y1228C(,) and M1248T/L1193V mutations developed a high frequency of sarcomas and some lymphomas, whereas the M1248T mice developed carcinomas and lymphomas. Of considerable interest, we observed trisomy of chromosome 6 and duplication of the mutant met allele in a majority of the tumors, similar to what has been reported in patients with hereditary renal papillary carcinomas. These results demonstrate that activating Met mutations and met amplification play key roles in promoting tumorigenesis in vivo. Moreover, our findings show that different mutations in the Met kinase domain can influence the types of cancers that develop.

Deletion of Vhlh in chondrocytes reduces cell proliferation and increases matrix deposition during growth plate development

The von Hippel Lindau tumor suppressor protein (pVHL) is a component of a ubiquitin ligase that promotes proteolysis of the transcription factor hypoxia-inducible-factor 1alpha (HIF1alpha), the key molecule in the hypoxic response. We have used conditional inactivation of murine VHL (Vhlh) in all cartilaginous elements to investigate its role in endochondral bone development. Mice lacking Vhlh in cartilage are viable, but grow slower than control littermates and develop a severe dwarfism. Morphologically, Vhlh null growth plates display a significantly reduced chondrocyte proliferation rate, increased extracellular matrix, and presence of atypical large cells within the resting zone. Furthermore, stabilization of the transcription factor HIF1alpha leads to increased expression levels of HIF1alpha target genes in Vhlh null growth plates. Lastly, newborns lacking both Vhlh and Hif1a genes in growth plate chondrocytes display essentially the same phenotype as Hif1a null single mutant mice suggesting that the Vhlh null phenotype could result, at least in part, from increased activity of accumulated HIF1alpha. This is the first study reporting the novel and intriguing findings that pVHL has a crucial role in endochondral bone development and is necessary for normal chondrocyte proliferation in vivo.

Oxygen-regulated expression of TGF-beta 3, a growth factor involved in trophoblast differentiation

The transforming growth factor-beta 3 (TGF-beta 3) is involved in oxygen-dependent differentiation processes during placental development and pregnancy disorders. However, the importance of oxygen partial pressure for the regulation of TGF-beta 3 expression is presently unclear. We and others presented preliminary evidence that the hypoxia-inducible factor-1 (HIF-1) confers TGF-beta 3 transcription but it was unknown whether this occurred directly or indirectly. To analyze how HIF-1 regulates TGF-beta 3 gene transcription, we cloned and sequenced the mouse TGF-beta 3 promoter region. Multiple putative HIF-1 binding sites (HBSs) were identified, many of which co-localized with two G+C rich CpG islands 5' to the TGF-beta 3 transcription start site. A 6.8 kb fragment of the TGF-beta 3 promoter induced reporter gene expression under hypoxic conditions or when treated with an iron chelator known to stabilize and activate the HIF-1 alpha subunit. Deletion of a 2.4 kb fragment upstream of the distal CpG island abolished inducibility of reporter gene expression. Two HBSs (HBS1 and HBS6) that bound the HIF-1 protein could be identified within this 2.4 kb fragment. These results suggest that TGF-beta 3 gene expression is directly regulated by HIF-1.

Determinants of Placental Vascularity

PROBLEM

Vascular growth during implantation and placentation is critical for successful gestation and it is thought that vascular insufficiencies during placentation contribute to a number of obstetrical complications. However, relatively little is known regarding the regulation of angiogenesis in the placenta.

METHOD OF STUDY

We review literature concerning the potential significance of inadequate placental vascularity as a contributor to the obstetrical complications of spontaneous abortion, fetal growth restriction and preeclampsia. Gene expression assays were used to compare fluctuations of placenta growth factor (PlGF) and PlGF receptor expression in normal and preeclamptic trophoblast in vitro.

RESULTS

Studies have shown that common obstetrical complications manifest altered placental vascularity. Both intrinsic defects (gene knockouts) and extrinsic factors (O(2) tension, cytokines, etc) may be responsible for the defects. Some of these factors have been shown to influence trophoblast vascular endothelial growth factor (VEGF)/PlGF expression suggesting this particular family of angiogenic proteins play an important role in placental angiogenesis.

CONCLUSION

Placental vascularization reflects a complex interaction of regulatory factors. Understanding the regulation of vascular growth in the placenta will provide much needed insight into placenta-related vascular insufficiencies.

Von Hippel-Lindau tumor suppressor protein and hypoxia-inducible factor in kidney cancer

The development of hereditary von Hippel-Lindau (VHL) disease and the majority of sporadic kidney cancers are due to the functional inactivation of the VHL gene. The product of the VHL gene, pVHL, in association with elongins B and C, cullin 2, and Rbx1 form an E3 ubiquitin-ligase complex VEC that targets the alpha subunits of hypoxia-inducible factor (HIF) for ubiquitination. Ubiquitin-tagged HIF-alpha proteins are subsequently degraded by the common 26S proteasome. pVHL functions as the substrate-docking interface that specifically recognizes prolyl-hydroxylated HIF-alpha. This hydroxylation occurs only in the presence of oxygen or normoxia. Thus, under hypoxia, HIF-alpha subunits are no longer subjected to degradation and are thereby able to dimerize with the common and constitutively stable beta subunits. The heterodimeric HIFs upregulate a myriad of hypoxia-inducible genes, triggering our physiologic response to hypoxia. Inappropriate accumulations of HIF-alpha in VHL disease are believed to contribute to the pathogenesis via the upregulation of several of these HIF target genes. Our current molecular understanding of the roles of HIF and pVHL in the development of VHL-associated clear-cell renal cell carcinoma (CC-RCC) is the focus of this review.

Molecular Cloning and Characterization of the von Hippel-Lindau-Like Protein

von Hippel-Lindau (VHL) tumor suppressor protein—inactivated in VHL disease and sporadic kidney cancer—is a component of an E3 ubiquitin ligase complex that selectively ubiquitinates the α subunit of the hypoxia-inducible factor (HIF) transcription factor for subsequent destruction by the 26S proteasome. Here, we report the identification and characterization of the first VHL homologue, VHL-like protein (VLP), located on chromosome 1q21.2. A 676-bp partial cDNA encoding a 139-amino acid protein that is 78% similar to VHL was isolated by reverse transcription-PCR from human brain cerebellum and several cancer cell lines. The expression of VLP transcript is most abundant in the placenta. Like VHL, VLP contains a β domain capable of binding HIFα. However, unlike VHL, it does not contain a recognizable α domain, which is required for nucleating the multiprotein E3 ubiquitin ligase complex. The increased expression of VLP in the presence of VHL attenuated the ubiquitination of HIFα and led to the accumulation of downstream HIF target genes. These results taken together indicate that VLP functions as a dominant-negative VHL to serve as a protector of HIFα.

Somatic mutations inVHL germline deletion kindred correlate with mild phenotype

Generally, von Hippel-Lindau (VHL) disease is caused by a germline mutation of the VHL gene (chromosome 3p), and tumorigenesis is initiated from a "second-hit" deletion. A subset of VHL patients have a germline deletion of the VHL gene, and the molecular events leading to tumorigenesis are not fully understood. To determine the molecular pathogenesis of tumor formation in this setting, we analyzed five central nervous system hemangioblastomas from three patients of a single VHL germline deletion kindred, all displaying mild clinical phenotype. Rather than loss of heterozygosity (the "second hit" in VHL germline mutation patients), all tumors from this kindred showed "second-hit" point mutations on the wild-type allele. Moreover, in two patients who each had two hemangioblastomas resected each tumor contained a unique mutation. The specific germline deletion and the overall genetic makeup of the patient did not predict these random "second-hit" point mutations. These results suggest that in patients with germline deletion of a tumor suppressor gene there is a unique genetic mechanism underlying tumorigenesis. This unique genetic mechanism correlates with and may help to understand the mild clinical phenotype seen in these patients.

HIFs, hypoxia, and vascular development

Cellular oxygen (O2) concentrations are tightly regulated to maintain ATP levels required for metabolic reactions in the human body. Responses to changes in O2 concentrations are primarily regulated by the transcription factor hypoxia inducible factor (HIF). HIF activates transcription of genes that increase systemic O2 delivery or provide cellular metabolic adaptation under conditions of hypoxia. HIF activity is essential for embryogenesis and various processes in postnatal life, and therefore, HIF levels need to be precisely controlled. Abnormal HIF expression is related to numerous diseases of the vascular system, including heart disease, cancer, and chronic obstructive pulmonary disease.

Identification of Jade1, a gene encoding a PHD zinc finger protein, in a gene trap mutagenesis screen for genes involved in anteroposterior axis development

In a gene trap screen for genes expressed in the primitive streak and tail bud during mouse embryogenesis, we isolated a mutation in Jade1, a gene encoding a PHD zinc finger protein previously shown to interact with the tumor suppressor pVHL. Expressed sequence tag analysis indicates that Jade1 is subject to posttranscriptional regulation, resulting in multiple transcripts and at least two protein isoforms. The fusion Jade1-beta-galactosidase reporter produced by the gene trap allele exhibits a regulated expression during embryogenesis and localizes to the nucleus and/or cytoplasm of different cell types. In addition to the primitive streak and tail bud, beta-galactosidase activity was found in other embryonic regions where pluripotent or tissue-specific progenitors are known to reside, including the early gastrulation epiblast and the ventricular zone of the cerebral cortex. Prominent reporter expression was also seen in the extraembryonic tissues as well as other differentiated cell types in the embryo, in particular the developing musculature. We show that the gene trap mutation produces a null allele. However, homozygotes for the gene trap integration are viable and fertile. Database searches identified a family of Jade proteins conserved through vertebrates. This raises the possibility that the absence of phenotype is due to a functional compensation by other family members.

The von Hippel-Lindau Gene, Kidney Cancer, and Oxygen Sensing

ABSTRACT. Recent studies of a relatively rare hereditary cancer syndrome, von Hippel-Lindau (VHL) disease, have shed new light on the molecular pathogenesis of kidney cancer and, perhaps more important, on how mammalian cells sense and respond to changes in oxygen availability. This knowledge is already translating into new therapeutic targets for kidney cancer as well as for multiple conditions, such as myocardial infarction and stroke, in which ischemia plays a pathogenic role. This review summarizes the current knowledge of the molecular pathogenesis of von Hippel-Lindau disease and the role of the VHL gene product (pVHL) in kidney cancer and the mammalian oxygen sensing pathway. E-mail: william_kaelin@dfci.harvard.edu

USF1 and USF2 mediate inhibition of human trophoblast differentiation and CYP19 gene expression by Mash-2 and hypoxia

In the human placental syncytiotrophoblast, C(19) steroids are converted to estrogens by aromatase P450, product of the CYP19 gene. When human cytotrophoblasts, which lack the capacity to express aromatase, are cultured in 20% O(2), they spontaneously fuse to form a multinuclear syncytiotrophoblast and CYP19 expression is markedly induced. On the other hand, when cytotrophoblasts are cultured in 2% O(2), syncytiotrophoblast differentiation and induction of CYP19 expression are prevented. We previously observed that expression of the transcription factor Mash-2 (mammalian achaete/scute homologue 2), which is elevated in human cytotrophoblasts and maintained at elevated levels by hypoxia, declines with syncytiotrophoblast differentiation. Overexpression of Mash-2 prevents syncytiotrophoblast differentiation and induction of CYP19 expression. In the present study, we observed that unexpectedly immunoreactive Mash-2 protein was localized predominantly to the cytoplasm of human cytotrophoblasts. Elevated cytoplasmic levels of Mash-2 were maintained when trophoblasts were cultured in 2% O(2) and declined to undetectable levels upon culture in 20% O(2). Previously, we found that Mash-2 inhibited CYP19 promoter activity through sequences within a 350-bp region upstream and within placenta-specific exon I.1 containing three E boxes (E1 at -325 bp, 5'-CACTTG-3'; E2 at -58 bp, 5'-CACATG-3'; and E3 at +26 bp, 5'-CACGTG-3'). In this study, we found that trophoblast nuclear protein binding to these E boxes declined with syncytiotrophoblast differentiation in 20% O(2) and was induced by hypoxia; however, Mash-2 did not appear to bind to any of these E boxes. On the other hand, the basic helix-loop-helix leucine zipper transcription factors upstream stimulatory factors 1 and 2 (USF1 and USF2) did bind to E2 and E3 but not E1. Nuclear levels of USF1 and USF2 and DNA-binding activity declined with syncytiotrophoblast differentiation and were maintained at elevated levels by hypoxia and overexpression of Mash-2, whereas USF1 mRNA levels were unaffected. Finally, USF1 overexpression in cultured human trophoblasts markedly inhibited endogenous CYP19 expression, differentiation of cultured human trophoblast cells, and CYP19 promoter activity. These findings suggest that increased protein levels and DNA binding of USF1 and USF2 mediate the inhibitory effects of hypoxia and of Mash-2 on CYP19 gene expression in human placenta.

A randomized trial of bevacizumab, an anti-vascular endothelial growth factor antibody, for metastatic renal cancer

BACKGROUND

Mutations in the tumor-suppressor gene VHL cause oversecretion of vascular endothelial growth factor by clear-cell renal carcinomas. We conducted a clinical trial to evaluate bevacizumab, a neutralizing antibody against vascular endothelial growth factor, in patients with metastatic renal-cell carcinoma.

METHODS

A randomized, double-blind, phase 2 trial was conducted comparing placebo with bevacizumab at doses of 3 and 10 mg per kilogram of body weight, given every two weeks; the time to progression of disease and the response rate were primary end points. Crossover from placebo to antibody treatment was allowed, and survival was a secondary end point.

RESULTS

Minimal toxic effects were seen, with hypertension and asymptomatic proteinuria predominating. The trial was stopped after the interim analysis met the criteria for early stopping. With 116 patients randomly assigned to treatment groups (40 to placebo, 37 to low-dose antibody, and 39 to high-dose antibody), there was a significant prolongation of the time to progression of disease in the high-dose--antibody group as compared with the placebo group (hazard ratio, 2.55; P<0.001). There was a small difference, of borderline significance, between the time to progression of disease in the low-dose--antibody group and that in the placebo group (hazard ratio, 1.26; P=0.053). The probability of being progression-free for patients given high-dose antibody, low-dose--antibody, and placebo was 64 percent, 39 percent, and 20 percent, respectively, at four months and 30 percent, 14 percent, and 5 percent at eight months. At the last analysis, there were no significant differences in overall survival between groups (P>0.20 for all comparisons).

CONCLUSIONS

Bevacizumab can significantly prolong the time to progression of disease in patients with metastatic renal-cell cancer.

von Hippel-Lindau disease: recent advances and therapeutic perspectives

von Hippel-Lindau disease is a hereditary cancer syndrome predisposing carriers to the development of a panel of highly vascularized tumors such as central nervous system and retinal hemangioblastomas, endolymphatic sac tumors, clear-cell renal cell carcinomas, pheochromocytomas and pancreatic neuroendocrine tumors. The disease is the foremost cause of inherited renal cell carcinomas, which is induced by germline mutations of the VHL tumor-suppressor gene also inactivated in most sporadic renal cell carcinomas. VHL appears to be a pivotal gene in the oxygen-sensing pathway, mainly involved in targeting the hypoxia-inducible factors for ubiquitination. This discovery is opening the way for the development of new specific drugs inhibiting hypoxia-inducible factors and/or their downstream targets, possibly representing an attractive treatment not only for von Hippel-Lindau disease but also for sporadic renal cell carcinomas and others cancers.

Molecular mechanisms of renal development

The biology of renal development has become increasingly complex because technical advances in genetics and cell biology have been used to study this aspect of embryogenesis. The molecular biology and genetics of renal development may seem inconsequential and frustrating to the practicing clinician, but insight into fundamental mechanisms of renal development are necessary to understand clinical breakthroughs that will occur in the future. As a basis for appreciating these concepts, specific paradigms of renal development are illustrated and the investigative strategies used to develop them are summarized in this article.