Tracheal development and the von Hippel-Lindau tumor suppressor homolog in Drosophila

Mechanisms of action of NME metastasis suppressors - a family affair

Metastatic progression is regulated by metastasis promoter and suppressor genes. NME1, the prototypic and first described metastasis suppressor gene, encodes a nucleoside diphosphate kinase (NDPK) involved in nucleotide metabolism; two related family members, NME2 and NME4, are also reported as metastasis suppressors. These proteins physically interact with members of the GTPase dynamin family, which have key functions in membrane fission and fusion reactions necessary for endocytosis and mitochondrial dynamics. Evidence supports a model in which NDPKs provide GTP to dynamins to maintain a high local GTP concentration for optimal dynamin function. NME1 and NME2 are cytosolic enzymes that provide GTP to dynamins at the plasma membrane, which drive endocytosis, suggesting that these NMEs are necessary to attenuate signaling by receptors on the cell surface. Disruption of NDPK activity in NME-deficient tumors may thus drive metastasis by prolonging signaling. NME4 is a mitochondrial enzyme that interacts with the dynamin OPA1 at the mitochondria inner membrane to drive inner membrane fusion and maintain a fused mitochondrial network. This function is consistent with the current view that mitochondrial fusion inhibits the metastatic potential of tumor cells whereas mitochondrial fission promotes metastasis progression. The roles of NME family members in dynamin-mediated endocytosis and mitochondrial dynamics and the intimate link between these processes and metastasis provide a new framework to understand the metastasis suppressor functions of NME proteins.

Modeling Neoplastic Growth in Renal Cell Carcinoma and Polycystic Kidney Disease

Renal cell carcinoma (RCC) and autosomal dominant polycystic kidney disease (ADPKD) share several characteristics, including neoplastic cell growth, kidney cysts, and limited therapeutics. As well, both exhibit impaired vasculature and compensatory VEGF activation of angiogenesis. The PI3K/AKT/mTOR and Ras/Raf/ERK pathways play important roles in regulating cystic and tumor cell proliferation and growth. Both RCC and ADPKD result in hypoxia, where HIF-α signaling is activated in response to oxygen deprivation. Primary cilia and altered cell metabolism may play a role in disease progression. Non-coding RNAs may regulate RCC carcinogenesis and ADPKD through their varied effects. Drosophila exhibits remarkable conservation of the pathways involved in RCC and ADPKD. Here, we review the progress towards understanding disease mechanisms, partially overlapping cellular and molecular dysfunctions in RCC and ADPKD and reflect on the potential for the agile Drosophila genetic model to accelerate discovery science, address unresolved mechanistic aspects of these diseases, and perform rapid pharmacological screens.

Genetically Encoded Tools for Research of Cell Signaling and Metabolism under Brain Hypoxia

Hypoxia is characterized by low oxygen content in the tissues. The central nervous system (CNS) is highly vulnerable to a lack of oxygen. Prolonged hypoxia leads to the death of brain cells, which underlies the development of many pathological conditions. Despite the relevance of the topic, different approaches used to study the molecular mechanisms of hypoxia have many limitations. One promising lead is the use of various genetically encoded tools that allow for the observation of intracellular parameters in living systems. In the first part of this review, we provide the classification of oxygen/hypoxia reporters as well as describe other genetically encoded reporters for various metabolic and redox parameters that could be implemented in hypoxia studies. In the second part, we discuss the advantages and disadvantages of the primary hypoxia model systems and highlight inspiring examples of research in which these experimental settings were combined with genetically encoded reporters.

The conservation and functionality of the oxygen-sensing enzyme Factor Inhibiting HIF (FIH) in non-vertebrates

The asparaginyl hydroxylase, Factor Inhibiting HIF (FIH), is a cellular dioxygenase. Originally identified as oxygen sensor in the cellular response to hypoxia, where FIH acts as a repressor of the hypoxia inducible transcription factor alpha (HIF-α) proteins through asparaginyl hydroxylation, FIH also hydroxylates many proteins that contain ankyrin repeat domains (ARDs). Given FIH's promiscuity and the unclear functional effects of ARD hydroxylation, the biological relevance of HIF-α and ARD hydroxylation remains uncertain. Here, we have employed evolutionary and enzymatic analyses of FIH, and both HIF-α and ARD-containing substrates, in a broad range of metazoa to better understand their conservation and functional importance. Utilising Tribolium castaneum and Acropora millepora, we provide evidence that FIH from both species are able to hydroxylate HIF-α proteins, supporting conservation of this function beyond vertebrates. We further demonstrate that T. castaneum and A. millepora FIH homologs can also hydroxylate specific ARD proteins. Significantly, FIH is also conserved in several species with inefficiently-targeted or absent HIF, supporting the hypothesis of important HIF-independent functions for FIH. Overall, these data show that while oxygen-dependent HIF-α hydroxylation by FIH is highly conserved in many species, HIF-independent roles for FIH have evolved in others.

Nucleoside diphosphate kinases (NDPKs) in animal development

In textbooks of biochemistry, nucleoside diphosphate conversion to a triphosphate by nucleoside diphosphate 'kinases' (NDPKs, also named NME or NM23 proteins) merits a few lines of text. Yet this essential metabolic function, mediated by a multimeric phosphotransferase protein, has effects that lie beyond a simple housekeeping role. NDPKs attracted more attention when NM23-H1 was identified as the first metastasis suppressor gene. In this review, we examine these NDPK enzymes from a developmental perspective because of the tractable phenotypes found in simple animal models that point to common themes. The data suggest that NDPK enzymes control the availability of surface receptors to regulate cell-sensing cues during cell migration. NDPKs regulate different forms of membrane enclosure that engulf dying cells during development. We suggest that NDPK enzymes have been essential for the regulated uptake of objects such as bacteria or micronutrients, and this evolutionarily conserved endocytic function contributes to their activity towards the regulation of metastasis.

Computational and experimental characterization of dVHL establish a Drosophila model of VHL syndrome

The von Hippel-Lindau (VHL) cancer syndrome is associated with mutations in the VHL gene. The pVHL protein is involved in response to changes in oxygen availability as part of an E3-ligase that targets the Hypoxia-Inducible Factor for degradation. pVHL has a molten globule configuration with marginal thermodynamic stability. The cancer-associated mutations further destabilize it. The Drosophila homolog, dVHL, has relatively low sequence similarity to pVHL, and is also involved in regulating HIF1-α. Using in silico, in vitro and in vivo approaches we demonstrate high similarity between the structure and function of dVHL and pVHL. These proteins have a similar fold, secondary and tertiary structures, as well as thermodynamic stability. Key functional residues in dVHL are evolutionary conserved. This structural homology underlies functional similarity of both proteins, evident by their ability to bind their reciprocal partner proteins, and by the observation that transgenic pVHL can fully maintain normal dVHL-HIF1-α downstream pathways in flies. This novel transgenic Drosophila model is thus useful for studying the VHL syndrome, and for testing drug candidates to treat it.

Interaction between Nm23 and the tumor suppressor VHL

Among the anti-tumor genes (tumor suppressors and metastasis suppressors), the von-Hippel Lindau gene and the Nm23 family of genes are among the more intriguing ones. Both are small (long and short forms of VHL are 30 and 19 kD, respectively, and Nm23 is ~17 kD), and both possess diverse molecular and cellular functions. Despite extensive studies, the entire spectra of functions and the molecular function-phenotype correlation of these two proteins have not been completely elucidated. In this report, we present data showing these two proteins interact physically. We also summarize and confirm the previous studies that demonstrated the endocytic function of these two genes and further show that the endocytic function of VHL is mediated through the activity of Nm23. These functional and molecular interactions are evolutionarily conserved from Drosophila to human.

The archipelago ubiquitin ligase subunit acts in target tissue to restrict tracheal terminal cell branching and hypoxic-induced gene expression

The Drosophila melanogaster gene archipelago (ago) encodes the F-box/WD-repeat protein substrate specificity factor for an SCF (Skp/Cullin/F-box)-type polyubiquitin ligase that inhibits tumor-like growth by targeting proteins for degradation by the proteasome. The Ago protein is expressed widely in the fly embryo and larva and promotes degradation of pro-proliferative proteins in mitotically active cells. However the requirement for Ago in post-mitotic developmental processes remains largely unexplored. Here we show that Ago is an antagonist of the physiologic response to low oxygen (hypoxia). Reducing Ago activity in larval muscle cells elicits enhanced branching of nearby tracheal terminal cells in normoxia. This tracheogenic phenotype shows a genetic dependence on sima, which encodes the HIF-1α subunit of the hypoxia-inducible transcription factor dHIF and its target the FGF ligand branchless (bnl), and is enhanced by depletion of the Drosophila Von Hippel Lindau (dVHL) factor, which is a subunit of an oxygen-dependent ubiquitin ligase that degrades Sima/HIF-1α protein in metazoan cells. Genetic reduction of ago results in constitutive expression of some hypoxia-inducible genes in normoxia, increases the sensitivity of others to mild hypoxic stimulus, and enhances the ability of adult flies to recover from hypoxic stupor. As a molecular correlate to these genetic data, we find that Ago physically associates with Sima and restricts Sima levels in vivo. Collectively, these findings identify Ago as a required element of a circuit that suppresses the tracheogenic activity of larval muscle cells by antagonizing the Sima-mediated transcriptional response to hypoxia.

Deletion of von Hippel-Lindau protein converts renin-producing cells into erythropoietin-producing cells

States of low perfusion pressure of the kidney associate with hyperplasia or expansion of renin-producing cells, but it is unknown whether hypoxia-triggered genes contribute to these changes. Here, we stabilized hypoxia-inducible transcription factors (HIFs) in mice by conditionally deleting their negative regulator, Vhl, using the Cre/loxP system with renin-1d promoter-driven Cre expression. Vhl (−/−(REN)) mice were viable and had normal BP. Deletion of Vhl resulted in constitutive accumulation of HIF-2α in afferent arterioles and glomerular cells and HIF-1α in collecting duct cells of the adult kidney. The preglomerular vascular tree developed normally, but far fewer renin-expressing cells were present, with more than 70% of glomeruli not containing renin cells at the typical juxtaglomerular position. Moreover, these mice had an attenuated expansion of renin-producing cells in response to a low-salt diet combined with an ACE inhibitor. However, renin-producing cells of Vhl (−/−(REN)) mice expressed the erythropoietin gene, and they were markedly polycythemic. Taken together, these results suggest that hypoxia-inducible genes, regulated by VHL, are essential for normal development and physiologic adaptation of renin-producing cells. In addition, deletion of Vhl shifts the phenotype of juxtaglomerular cells from a renin- to erythropoietin-secreting cell type, presumably in response to HIF-2 accumulation.

NME genes in epithelial morphogenesis

The NME family of genes encodes highly conserved multifunctional proteins that have been shown to participate in nucleic acid metabolism, energy homeostasis, cell signaling, and cancer progression. Some family members, particularly isoforms 1 and 2, have attracted extensive interests because of their potential anti-metastasis activity. Unfortunately, there have been few consensus mechanistic explanations for this critical function because of the numerous molecular functions ascribed to these proteins, including nucleoside diphosphate kinase, protein kinase, nuclease, transcription factor, growth factor, among others. In addition, different studies showed contradictory prognostic correlations between NME expression levels and tumor progression in clinical samples. Thus, analyses using pliable in vivo systems have become critical for unraveling at least some aspects of the complex functions of this family of genes. Recent works using the Drosophila genetic system have suggested a role for NME in regulating epithelial cell motility and morphogenesis, which has also been demonstrated in mammalian epithelial cell culture. This function is mediated by promoting internalization of growth factor receptors in motile epithelial cells, and the adherens junction components such as E-cadherin and β-catenin in epithelia that form the tissue linings. Interestingly, NME genes in epithelial cells appear to function in a defined range of expression levels. Either down-regulation or over-expression can perturb epithelial integrity, resulting in different aspects of epithelial abnormality. Such biphasic functions provide a plausible explanation for the documented anti-metastatic activity and the suspected oncogenic function. This review summarizes these recent findings and discusses their implications.

Complex cellular functions of the von Hippel-Lindau tumor suppressor gene: insights from model organisms

The von Hippel-Lindau tumor suppressor gene (VHL) has attracted intensive interest not only because its mutations predispose carriers to devastating tumors, but also because it is involved in oxygen sensing under physiological conditions. VHL loss-of-function mutations result in organ-specific tumors, such as hemangioblastoma of the central nervous system and renal cell carcinoma, both untreatable with conventional chemotherapies. The VHL protein is best known as an E3 ubiquitin ligase that targets hypoxia-inducible factor-α (HIF-α), but many diverse, non-canonical cellular functions have also been assigned to VHL, mainly based on studies in cell culture systems. As such, although the HIF-dependent role of VHL is critical, the full spectrum of pathophysiological functions of VHL is still unresolved. Such understanding requires careful cross-referencing with physiologically relevant experimental models. Studies in model systems, such as Caenorhabditis elegans, Drosophila, zebrafish and mouse have provided critical in vivo confirmation of the VHL-HIF pathway, and verification of potentially important cellular functions including microtubule stabilization and epithelial morphogenesis. More recently, animal models have also suggested systemic roles of VHL in hematopoiesis, metabolic homeostasis and inflammation. In this review, the studies performed in model organisms will be summarized and placed in context with existing clinical and in vitro data.

Drosophila von Hippel-Lindau tumor suppressor gene function in epithelial tubule morphogenesis

Mutations in the human von Hippel-Lindau (VHL) gene are the cause of VHL disease that displays multiple benign and malignant tumors. The VHL gene has been shown to regulate angiogenic potential and glycolic metabolism via its E3 ubiquitin ligase function against the alpha subunit of hypoxia-inducible factor (HIF-alpha). However, many HIF-independent functions of VHL have been identified. Recent evidence also indicates that the canonical function cannot fully explain the VHL mutant cell phenotypes, although it is still unclear how many of these noncanonical functions relate to the pathophysiological processes because of a lack of tractable genetic systems. Here, we report the first genomic mutant phenotype of Drosophila melanogaster VHL (dVHL) in the epithelial tubule network, the trachea, and show that dVHL regulates branch migration and lumen formation via its endocytic function. The endocytic function regulates the surface level of the chemotactic signaling receptor Breathless and promotes clearing of the lumen matrix during maturation of the tracheal tubes. Importantly, the regulatory function in tubular morphogenesis is conserved in the mammalian system, as conditional knockout of Vhl in mouse kidney also resulted in similar cell motility and lumen phenotypes.

Drosophila VHL tumor-suppressor gene regulates epithelial morphogenesis by promoting microtubule and aPKC stability

Mutations in the human von Hippel-Lindau (VHL) genes are the cause of VHL disease, which displays multiple benign and malignant tumors. The VHL gene has been shown to regulate angiogenic potential and glycolic metabolism via its E3 ubiquitin ligase function against the alpha subunit of hypoxia-inducible factor (HIF). However, many other HIF-independent functions of VHL have been identified and recent evidence indicates that the canonical function cannot fully explain the VHL mutant cell phenotypes. Many of these functions have not been verified in genetically tractable systems. Using an established follicular epithelial model in Drosophila, we show that the Drosophila VHL gene is involved in epithelial morphogenesis via stabilizing microtubule bundles and aPKC. Microtubule defects in VHL mutants lead to mislocalization of aPKC and subsequent loss of epithelial integrity. Destabilizing microtubules in ex vivo culture of wild-type egg chambers can also result in aPKC mislocalization and epithelial defects. Importantly, paclitaxel-induced stabilization of microtubules can rescue the aPKC localization phenotype in Drosophila VHL mutant follicle cells. The results establish a developmental function of the VHL gene that is relevant to its tumor-suppressor activity.

Dose-dependent modulation of HIF-1alpha/sima controls the rate of cell migration and invasion in Drosophila ovary border cells

The role of the hypoxic response during metastasis was analysed in migrating border cells of the Drosophila ovary. Acute exposure to 1% O(2) delayed or blocked border cell migration (BCM), whereas prolonged exposure resulted in the first documented accelerated BCM phenotype. Similarly, manipulating the expression levels of sima, the Drosophila hypoxia-inducible factor (HIF)-1alpha ortholog, revealed that Sima can either block or restore BCM in a dose-dependent manner. In contrast, over-expression of Vhl (Drosophila von Hippel-Lindau) generated a range of phenotypes, including blocked, delayed and accelerated BCM, whereas over-expression of hph (Drosophila HIF prolyl hydroxylase) only accelerated BCM. Mosaic clone analysis of sima or tango (HIF-1beta ortholog) mutants revealed that cells lacking Hif-1 transcriptional activity were preferentially detected in the leading cell position of the cluster, resulting in either a delay or acceleration of BCM. Moreover, in sima mutant cell clones, there was reduced expression of nuclear slow border cells (Slbo) and basolateral DE-cadherin, proteins essential for proper BCM. These results show that Sima levels define the rate of BCM in part through regulation of Slbo and DE-cadherin, and suggest that dynamic regulation of Hif-1 activity is necessary to maintain invasive potential of migrating epithelial cells.

Developmental function of Nm23/awd: a mediator of endocytosis

The metastasis suppressor gene Nm23 is highly conserved from yeast to human, implicating a critical developmental function. Studies in cultured mammalian cells have identified several potential functions, but many have not been directly verified in vivo. Here, we summarize the studies on the Drosophila homolog of the Nm23 gene, named a bnormal w ing d iscs (awd), which shares 78% amino acid identity with the human Nm23-H1 and H2 isoforms. These studies confirmed that awd gene encodes a nucleoside diphosphate kinase, and provided strong evidence of a role for awd in regulating cell differentiation and motility via regulation of growth factor receptor signaling. The latter function is mainly mediated by control of endocytosis. This review provides a historical account of the discovery and subsequent analyses of the awd gene. We will also discuss the possible molecular function of the Awd protein that underlies the endocytic function.

Central role of the oxygen-dependent degradation domain of Drosophila HIFalpha/Sima in oxygen-dependent nuclear export

The Drosophila HIFalpha homologue, Sima, is localized mainly in the cytoplasm in normoxia and accumulates in the nucleus upon hypoxic exposure. We have characterized the mechanism governing Sima oxygen-dependent subcellular localization and found that Sima shuttles continuously between the nucleus and the cytoplasm. We have previously shown that nuclear import depends on an atypical bipartite nuclear localization signal mapping next to the C-terminus of the protein. We show here that nuclear export is mediated in part by a CRM1-dependent nuclear export signal localized in the oxygen-dependent degradation domain (ODDD). CRM1-dependent nuclear export requires both oxygen-dependent hydroxylation of a specific prolyl residue (Pro850) in the ODDD, and the activity of the von Hippel Lindau tumor suppressor factor. At high oxygen tension rapid nuclear export of Sima occurs, whereas in hypoxia, Sima nuclear export is largely inhibited. HIFalpha/Sima nucleo-cytoplasmic localization is the result of a dynamic equilibrium between nuclear import and nuclear export, and nuclear export is modulated by oxygen tension.

Regulation of Drosophila embryonic tracheogenesis by dVHL and hypoxia

The tracheal system of Drosophila melanogaster is an interconnected network of gas-filled epithelial tubes that develops during embryogenesis and functions as the main gas-exchange organ in the larva. Larval tracheal cells respond to hypoxia by activating a program of branching and growth driven by HIF-1alpha/sima-dependent expression of the breathless (btl) FGF receptor. By contrast, the ability of the developing embryonic tracheal system to respond to hypoxia and integrate hard-wired branching programs with sima-driven tracheal remodeling is not well understood. Here we show that embryonic tracheal cells utilize the conserved ubiquitin ligase dVHL to control the HIF-1 alpha/sima hypoxia response pathway, and identify two distinct phases of tracheal development with differing hypoxia sensitivities and outcomes: a relatively hypoxia-resistant 'early' phase during which sima activity conflicts with normal branching and stunts migration, and a relatively hypoxia-sensitive 'late' phase during which the tracheal system uses the dVHL/sima/btl pathway to drive increased branching and growth. Mutations in the archipelago (ago) gene, which antagonizes btl transcription, re-sensitize early embryos to hypoxia, indicating that their relative resistance can be reversed by elevating activity of the btl promoter. These findings reveal a second type of tracheal hypoxic response in which Sima activation conflicts with developmental tracheogenesis, and identify the dVHL and ago ubiquitin ligases as key determinants of hypoxia sensitivity in tracheal cells. The identification of an early stage of tracheal development that is vulnerable to hypoxia is an important addition to models of the invertebrate hypoxic response.

Endothelial function of von Hippel-Lindau tumor suppressor gene: control of fibroblast growth factor receptor signaling

von Hippel-Lindau (VHL) disease results from germline and somatic mutations in the VHL tumor suppressor gene and is characterized by highly vascularized tumors. VHL mutations lead to stabilization of hypoxia-inducible factor (HIF), which up-regulates proangiogenic factors such as vascular endothelial growth factor (VEGF). This pathway is therefore believed to underlie the hypervascular phenotypes of the VHL tumors. However, recent studies have identified novel VHL functions that are independent of the HIF-VEGF pathway. In addition, a potential role of VHL in the tumor microenvironment, which carries heterozygous VHL mutations in VHL patients, has been overlooked. Here, we report a novel HIF-independent VHL function in the endothelium. VHL knockdown in primary human microvascular endothelial cells caused defective turnover of surface fibroblast growth factor (FGF) receptor, increased extracellular signal-regulated kinase signaling, and ETS1 activation, leading to increased cell motility in response to FGF and three-dimensional cord formation in vitro. HIF-alpha knockdown in VHL loss-of-function endothelial cells does not impede their elevated in vitro angiogenic activity. Importantly, the elevated angiogenic response to FGF is recapitulated in Vhl-heterozygous mice. Thus, partial loss of function of VHL in endothelium may be a contributing factor in tumor angiogenesis through a HIF-VEGF-independent mechanism.

[Molecular biology of the clear cell renal cell carcinoma: principles for a selective treatment]

Renal cell carcinoma (RCC) and its most frequent subtype, the clear cell hystology type, has shown resistance to chemotherapy and radiotherapy treatment when disease was already spread in patients. Recently, a huge advance in the molecular biology of this tumor has been performed. This fact allowed a deeper and better knowledge of the disease and the development of new drugs that work against the growth factors involved in tumor origin. In this review article it is summarized the molecular milestones that are involved in the development of clear cell renal cell carcinomas.

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.

Analysis of the hypoxia-sensing pathway in Drosophila melanogaster

The mechanism by which hypoxia induces gene transcription involves the inhibition of HIF-1alpha (hypoxia-inducible factor-1 alpha subunit) PHD (prolyl hydroxylase) activity, which prevents the VHL (von Hippel-Lindau)-dependent targeting of HIF-1alpha to the ubiquitin/proteasome pathway. HIF-1alpha thus accumulates and promotes gene transcription. In the present study, first we provide direct biochemical evidence for the presence of a conserved hypoxic signalling pathway in Drosophila melanogaster. An assay for 2-oxoglutarate-dependent dioxygenases was developed using Drosophila embryonic and larval homogenates as a source of enzyme. Drosophila PHD has a low substrate specificity and hydroxylates key proline residues in the ODD (oxygen-dependent degradation) domains of human HIF-1alpha and Similar, the Drosophila homologue of HIF-1alpha. The enzyme promotes human and Drosophila [(35)S]VHL binding to GST (glutathione S-transferase)-ODD-domain fusion protein. Hydroxylation is enhanced by proteasomal inhibitors and was ascertained using an anti-hydroxyproline antibody. Secondly, by using transgenic flies expressing a fusion protein that combined an ODD domain and the green fluorescent protein (ODD-GFP), we analysed the hypoxic cascade in different embryonic and larval tissues. Hypoxic accumulation of the reporter protein was observed in the whole tracheal tree, but not in the ectoderm. Hypoxic stabilization of ODD-GFP in the ectoderm was restored by inducing VHL expression in these cells. These results show that Drosophila tissues exhibit different sensitivities to hypoxia.

Endocytic function of von Hippel-Lindau tumor suppressor protein regulates surface localization of fibroblast growth factor receptor 1 and cell motility

The tumor suppressor VHL (von Hippel-Lindau protein) serves as a negative regulator of hypoxia-inducible factor-alpha subunits. However, accumulated evidence indicates that VHL may play additional roles in other cellular functions. We report here a novel hypoxia-inducible factor-independent function of VHL in cell motility control via regulation of fibroblast growth factor receptor 1 (FGFR1) endocytosis. In VHL null tumor cells or VHL knock-down cells, FGFR1 internalization is defective, leading to surface accumulation and abnormal activation of FGFR1. The enhanced FGFR1 activity directly correlates with increased cell migration. VHL disease mutants, in two of the mutation hot spots favoring development of renal cell carcinoma, failed to rescue the above phenotype. Interestingly, surface accumulation of the chemotactic receptor appears to be selective in VHL mutant cells, since other surface proteins such as epidermal growth factor receptor, platelet-derived growth factor receptor, IGFR1, and c-Met are not affected. We demonstrate that 1) FGFR1 endocytosis is defective in the VHL mutant and is rescued by reexpression of wild-type VHL, 2) VHL is recruited to FGFR1-containing, but not EGFR-containing, endosomal vesicles, 3) VHL exhibits a functional relationship with Rab5a and dynamin 2 in FGFR1 internalization, and 4) the endocytic function of VHL is mediated through the metastasis suppressor Nm23, a protein known to regulate dynamin-dependent endocytosis.

The Hsp60C gene in the 25F cytogenetic region in Drosophila melanogaster is essential for tracheal development and fertility

Earlier studies have shown that of the four genes (Hsp60A, Hsp60B, Hsp60C, Hsp60D genes) predicted to encode the conserved Hsp60 family chaperones in Drosophila melanogaster, the Hsp60A gene (at the 10A polytene region) is expressed in all cell types of the organism and is essential from early embryonic stages, while the Hsp60B gene (at 21D region) is expressed only in testis, being essential for sperm individualization. In the present study, we characterized the Hsp60C gene (at 25F region), which shows high sequence homology with the other three Hsp60 genes of D. melanogaster. In situ hybridization of Hsp60C-specific riboprobe shows that expression of this gene begins in late embryonic stages (stage 14 onwards), particularly in the developing tracheal system and salivary glands; during larval and adult stages, it is widely expressed in many cell types but much more strongly in tracheae and in developing and differentiating germ cells. A P-insertion mutant (Hsp60C(1)) allele with the P transposon inserted at -251 position of the Hsp60C gene promoter was generated. This early larval recessive lethal mutation significantly reduces levels of Hsp60C transcripts in developing tracheae and this is associated with a variety of defects in the tracheal system, including lack of liquid clearance. About 10% of the homozygotes survive as weak, shortlived and completely sterile adults. Testes of the surviving mutant males are significantly smaller, with fewer spermatocytes, most of which do not develop beyond the round spermatid stage. In situ and Northern hybridizations show significantly reduced levels of the Hsp60C transcripts in Hsp60C(1) homozygous adult males. The absence of early meiotic stages in the Hsp60C(1) homozygous testes contrasts with the effect of testis-specific Hsp60B (21D) gene, whose mutation affects individualization of sperm bundles later in spermiogenesis. In view of the specific effects in tracheal development and in early stages of spermatogenesis, it is likely that, besides its functions as a chaperone, Hsp60C may have signalling functions and may also be involved in cation transport across the developing tracheal epithelial cells.

How do endothelial cells orientate?

In sprouting angiogenesis, endothelial cells must orientate in the tissue environment in order to effectively invade tissues and form vascular patterns according to the local needs. Here, we review recent data indicating that sprouting angiogenesis is a guided process resembling axonal guidance and insect trachea formation. Angiogenesis requires functional specialization of endothelial cells within the sprout. Cells situated at the tip of the sprouts sense and navigate the environment using long filopodia, whereas cells in the sprout stalks proliferate and form a vascular lumen. Migration of the tip cells depends on a graded distribution of VEGF-A and activation of VEGFR2 located on the tip-cell filopodia. Proliferation in the stalk is concomitantly regulated by the local VEGF-A levels. Thus, the shape of the VEGF-A gradient controls the balance between tip cell migration and stalk cell proliferation, which in turn determines the initial vascular pattern. An imbalance between the two processes may explain why abnormal vascular patterns develop in pathological angiogenesis.

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.

The von Hippel-Lindau tumor suppressor gene and kidney cancer

The von Hippel-Lindau tumor suppressor gene (VHL), which resides on chromosome 3p25, is mutated or silenced in >50% of sporadic clear cell renal cell carcinomas. Germ-line VHL mutations give rise to VHL disease, which is characterized by an increased risk of blood vessel tumors (hemangioblastomas) and renal cell carcinomas. In this setting, VHL inactivation gives rise to premalignant renal cysts. Additional genetic alterations are presumably required for conversion of these cysts to renal cell carcinomas. Restoration of VHL function in VHL-/- renal cell carcinomas is sufficient to inhibit tumorigenesis in vivo. On the basis of these and other data, VHL appears to be a critical gatekeeper with respect to the development of renal cell carcinoma. The VHL gene product, pVHL, is the substrate recognition module of an E3 ubiquitin ligase that targets the hypoxia-inducible factor (HIF) for destruction in the presence of oxygen. Hypoxic cells, or cells lacking pVHL, accumulate high levels of HIF, which activates the transcription of a variety of genes, including vascular endothelial growth factor, platelet-derived growth factor B, and transforming growth factor alpha. We have demonstrated that inhibition of HIF is necessary and sufficient for tumor suppression by pVHL in renal cell carcinoma nude mouse xenograft assays. This provides a rationale for treating VHL-/- renal cell carcinoma with inhibitors of HIF or its downstream targets. Genotype-phenotype correlations in VHL disease suggest, however, that pVHL has targets in addition to HIF. Elucidating these targets should provide a more complete picture of how pVHL suppresses tumor growth.

The novel plant homeodomain protein rhinoceros antagonizes Ras signaling in the Drosophila eye

The sequential specification of cell fates in the Drosophila eye requires repeated activation of the epidermal growth factor receptor (EGFR)/Ras/MAP kinase (MAPK) pathway. Equally important are the multiple layers of inhibitory regulation that prevent excessive or inappropriate signaling. Here we describe the molecular and genetic analysis of a previously uncharacterized gene, rhinoceros (rno), that we propose functions to restrict EGFR signaling in the eye. Loss of rno results in the overproduction of photoreceptors, cone cells, and pigment cells and a corresponding reduction in programmed cell death, all phenotypes characteristic of hyperactivated EGFR signaling. Genetic interactions between rno and multiple EGFR pathway components support this hypothesis. rno encodes a novel but evolutionarily conserved nuclear protein with a PHD zinc-finger domain, a motif commonly found in chromatin-remodeling factors. Future analyses of rno will help to elucidate the regulatory strategies that modulate EGFR signaling in the fly eye.

Branching Morphogenesis of theDrosophilaTracheal System

Many organs including the mammalian lung and vascular system consist of branched tubular networks that transport essential gases or fluids, but the genetic programs that control the development of these complex three-dimensional structures are not well understood. The Drosophila melanogaster tracheal (respiratory) system is a network of interconnected epithelial tubes that transports oxygen and other gases in the body and provides a paradigm of branching morphogenesis. It develops by sequential sprouting of primary, secondary, and terminal branches from an epithelial sac of approximately 80 cells in each body segment of the embryo. Mapping of the cell movements and shape changes during the sprouting process has revealed that distinct mechanisms of epithelial migration and tube formation are used at each stage of branching. Genetic dissection of the process has identified a general program in which a fibroblast growth factor (FGF) and fibroblast growth factor receptor (FGFR) are used repeatedly to control branch budding and outgrowth. At each stage of branching, the mechanisms controlling FGF expression and the downstream signal transduction pathway change, altering the pattern and structure of the branches that form. During terminal branching, FGF expression is regulated by hypoxia, ensuring that tracheal structure matches cellular oxygen need. A branch diversification program operates in parallel to the general budding program: Regional signals locally modify the general program, conferring specific structural features and other properties on individual branches, such as their substrate outgrowth preferences, differences in tube size and shape, and the ability to fuse to other branches to interconnect the network.

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

Mutations of von Hippel-Lindau tumor-suppressor gene and congenital polycythemia

The von Hippel-Lindau (pVHL) protein plays an important role in hypoxia sensing. It binds to the hydroxylated hypoxia-inducible factor 1 alpha (HIF-1 alpha) and serves as a recognition component of an E3-ubiquitin ligase complex. In hypoxia or secondary to a mutated VHL gene, the nondegraded HIF-1 alpha forms a heterodimer with HIF-beta and leads to increased transcription of hypoxia-inducible genes, including erythropoietin (EPO). The autosomal dominant cancer-predisposition von Hippel-Lindau (VHL) syndrome is due to inheritance of a single mutated allele of VHL. In contrast, we recently showed that homozygous germline 598C-->T VHL mutation leads to Chuvash polycythemia (CP). We subsequently found VHL mutations in three unrelated individuals unaffected with CP, one of whom was compound heterozygous for the 598C-->T mutation and another VHL mutation. We now report seven additional polycythemic patients with VHL mutations in both alleles. Two Danish siblings and another American boy were homozygous for the VHL 598C-->T mutation. Three unrelated white Americans were compound heterozygotes for 598C-->T and another VHL mutation, 562C-->G in two and 574C-->T in the third. Additionally, a Croatian boy was homozygous for a 571C-->G VHL mutation, the first example of homozygous VHL germline mutation causing polycythemia, other than the VHL 598C-->T mutation. We have not observed VHL syndrome-associated tumors in polycythemic subjects or their heterozygous relatives; however, this will need to be evaluated by longitudinal studies. Over all, we found that up to half of the consecutive patients with apparent congenital polycythemia and increased serum Epo we have examined have mutations of both VHL alleles. Those findings, along with reports of CP, underscore that VHL mutations are the most frequent cause of congenital polycythemia and define a new class of polycythemic disorder, polycythemias due to augmented hypoxia sensing.

Understanding the molecular responses to hypoxia using Drosophila as a genetic model

We have previously discovered that Drosophila melanogaster could recover from extended periods of anoxia (0% oxygen) with no apparent consequential injuries. We have since employed forward and reverse genetic approaches to decipher the molecular basis for anoxia tolerance. In so doing, we have identified several independent mutant lines that demonstrated increased sensitivity to anoxia. Characterization of one of these mutants resulted in the identification of a dADAR gene that plays a role in the sensitivity to low levels of oxygen. We have also used microarrays to study all known Drosophila genes, the expression of which may be altered by hyoxia. Microarrays experiments have generated a large body of information that is being currently analyzed. We believe that these undertakings will provide insight into the genetic mechanisms of hypoxia tolerance and ischemic injuries.

Heparan sulfate fine structure and specificity of proteoglycan functions

Heparan sulfate chains have markedly heterogeneous structures in which distinct patterns of sulfation determine the binding specificity for ligand proteins. These "fine structures" of heparan sulfate are mainly produced by the regulated introduction of sulfate groups at the N-, 2-O-, 6-O-, and 3-O-positions of the sugar chain. Recent biochemical, histochemical, and genetic studies have demonstrated that different fine structures mediate distinct molecular recognition events to regulate a variety of cellular functions. In this review, we focus on the molecular basis of growth factor control by the sulfation status of heparan sulfate.

The TRC8 hereditary kidney cancer gene suppresses growth and functions with VHL in a common pathway

VHL is part of an SCF related E3-ubiquitin ligase complex with 'gatekeeper' function in renal carcinoma. However, no mutations have been identified in VHL interacting proteins in wild type VHL tumors. We previously reported that the TRC8 gene was interrupted by a t(3;8) translocation in a family with hereditary renal and non-medullary thyroid cancer. TRC8 encodes a multi-membrane spanning protein containing a RING-H2 finger with in vitro ubiquitin ligase activity. We isolated the Drosophila homologue, DTrc8, and studied its function by genetic manipulations and a yeast 2-hybrid screen. Human and Drosophila TRC8 proteins localize to the endoplasmic reticulum. Loss of either DTrc8 or DVhl resulted in an identical ventral midline defect. Direct interaction between DTrc8 and DVhl was confirmed by GST-pulldown and co-immunoprecipitation experiments. CSN-5/JAB1 is a component of the COP9 signalosome, recently shown to regulate SCF function. We found that DTrc8 physically interacts with CSN-5 and that human JAB1 localization is dependent on VHL mutant status. Lastly, overexpression of DTrc8 inhibited growth consistent with its presumed role as a tumor suppressor gene. Thus, VHL, TRC8, and JAB1 appear to be linked both physically and functionally and all three may participate in the development of kidney cancer.

Comprehensive mutational analysis of the VHL gene in sporadic renal cell carcinoma: relationship to clinicopathological parameters

To delineate more precisely the somatic von Hippel-Lindau disease (VHL) gene alteration as well as to elucidate its etiologic role in renal tumorigenesis, we examined a total of 240 sporadic renal cell carcinomas (RCCs) for somatic VHL gene alterations by DNA-SSCP followed by sequencing, methylation-specific PCR assay, microsatellite LOH study, and Southern blot analysis. Intragenic mutation of the VHL gene was found exclusively in clear-cell or variant-type RCCs at a frequency of 51% (104/202). Hypermethylation of the VHL promoter region was detected in an additional 11 clear-cell RCCs. Microsatellite analysis demonstrated that LOH of the VHL locus was found in 140/155 (90%) informative clear-cell RCCs. The VHL gene therefore seems to be inactivated in a two-hit manner by intragenic mutation or hypermethylation plus allelic loss in clear-cell RCC. Genomic rearrangement of the VHL gene detected by Southern analysis was not found (0/216 cases); this is in contrast to germ lines in which Southern aberrations consisted of 7-19% of the mutations. Clinicopathologic data demonstrated that VHL mutation/LOH did not vary according to tumor progression in clear-cell RCC, including tumor diameter, stage, grading, distant metastasis, and lymph node metastasis. Interestingly, VHL mutation was significantly less frequent in RCCs occurring in younger (< or = 55 years) than that in older (> or = 56 years) patients. These data suggested that the inactivation of the VHL tumor-suppressor gene is a specific genetic change in clear-cell RCC, and that it may occur at an early or first step in the clear-cell tumorigenic pathway rather than as a late event.

Hypoxia: the tumor's gateway to progression along the angiogenic pathway

Decreased aerobic (hypoxic) conditions in tumors induce the release of cytokines that promote vascularization and thereby enhance tumor growth and metastasis. Recent major advances have provided insight into the role hypoxia plays in cancer biology. The domain structure of the hypoxia-inducible factor 1alpha (HIF-1alpha) has been elucidated, as has the mechanism by which stabilization of HIF-1alpha leads to initiation of the transcription of target genes involved in growth of blood vessels.

A conserved family of prolyl-4-hydroxylases that modify HIF

Mammalian cells respond to changes in oxygen availability through a conserved pathway that is regulated by the hypoxia-inducible factor (HIF). The alpha subunit of HIF is targeted for degradation under normoxic conditions by a ubiquitin-ligase complex that recognizes a hydroxylated proline residue in HIF. We identified a conserved family of HIF prolyl hydoxylase (HPH) enzymes that appear to be responsible for this posttranslational modification. In cultured mammalian cells, inappropriate accumulation of HIF caused by forced expression of the HIF-1alpha subunit under normoxic conditions was attenuated by coexpression of HPH. Suppression of HPH in cultured Drosophila melanogaster cells by RNA interference resulted in elevated expression of a hypoxia-inducible gene (LDH, encoding lactate dehydrogenase) under normoxic conditions. These findings indicate that HPH is an essential component of the pathway through which cells sense oxygen.

Hypoxia: the tumor's gateway to progression along the angiogenic pathway

Decreased aerobic (hypoxic) conditions in tumors induce the release of cytokines that promote vascularization and thereby enhance tumor growth and metastasis. Recent major advances have provided insight into the role hypoxia plays in cancer biology. The domain structure of the hypoxia-inducible factor 1alpha (HIF-1alpha) has been elucidated, as has the mechanism by which stabilization of HIF-1alpha leads to initiation of the transcription of target genes involved in growth of blood vessels.

The von Hippel-Lindau tumor suppressor gene

Germline mutations of the von Hippel-Lindau tumor suppressor gene (VHL) in humans causes a hereditary cancer syndrome characterized by the development of retinal and central nervous system hemangioblastomas. Other tumors associated with von Hippel-Lindau disease include clear cell renal carcinomas and pheochromocytomas. Tumor development in this setting is due to functional loss of the remaining wild-type VHL allele. Biallelic VHL inactivation is also common in nonhereditary hemangioblastomas and clear cell renal carcinomas, in keeping with Knudson's 2-Hit Model of carcinogenesis. The VHL gene product, pVHL, is a component of an E3 ubiquitin ligase that targets the alpha subunits of the HIF (hypoxia-inducible factor) transcription factor for destruction in the presence of oxygen. Consequently, tumor cells lacking pVHL overproduce the products of HIF target genes such as vascular endothelial growth factor and transforming growth factor alpha. pVHL has been implicated in a variety of processes that are central to carcinogenesis including cell-cycle control, differentiation, extracellular matrix formation and turnover, and angiogenesis.

The von Hippel-Lindau tumor suppressor protein

The von Hippel-Lindau tumor suppressor protein (pVHL) has been shown to bind directly to the alpha subunits of the heterodimeric transcription factor HIF (hypoxia inducible factor). pVHL directs the polyubiquitination and, hence, destruction of HIF in the presence of oxygen. Loss of pVHL function leads to deregulation of HIF target genes, which play critical roles in angiogenesis.

Sequence and functional properties of Ets genes in the model organism Drosophila

Detailed molecular and genetic studies, coupled with the recent sequencing of the fly genome, have identified eight Ets-related genes in the model organism Drosophila. All show homology to genes in vertebrate species. Functional analyses of some of the Drosophila ets genes have revealed their essential roles in developmental processes such as metamorphosis, oogenesis, neurogenesis, myogenesis, and eye development. Such studies have yielded important insights into our understanding of the genetic control of hormonally-regulated gene expression, programmed cell death, and signal transduction during cell fate determination and differentiation. The developmental roles of E74 (ELF1), pointed (Ets 1), yan (TEL), and D-elg (GABPalpha) will be reviewed in this article. The context of their participation in signal transduction and gene regulation will also be discussed. The information should be of significant value to the study of related processes in higher organisms due to the growing evidence for the cross species conservation of developmental mechanisms.