Michael Ohh

Ubiquitination of hypoxia-inducible factor requires direct binding to the β-domain of the von Hippel-Lindau protein

von Hippel–Lindau (VHL) disease is a hereditary cancer syndrome that is characterized by the development of multiple vascular tumors and is caused by inactivation of the von Hippel–Lindau protein (pVHL). Here we show that pVHL, through its β-domain, binds directly to hypoxia-inducible factor (HIF), thereby targeting HIF for ubiquitination in an α-domain-dependent manner. This is the first function to be ascribed to the pVHL β-domain. Furthermore, we provide the first direct evidence that pVHL has a function analogous to that of an F-box protein, namely, to recruit substrates to a ubiquitination machine. These results strengthen the link between overaccumulation of HIF and development of VHL disease.

The von Hippel-Lindau Tumor Suppressor Protein Is Required for Proper Assembly of an Extracellular Fibronectin Matrix

Fibronectin coimmunoprecipitated with wild-type von Hippel-Lindau protein (pVHL) but not tumor-derived pVHL mutants. Immunofluorescence and biochemical fractionation experiments showed that fibronectin colocalized with a fraction of pVHL associated with the endoplasmic reticulum, and cold competition experiments suggested that complexes between fibronectin and pVHL exist in intact cells. Assembly of an extracellular fibronectin matrix by VHL-/- renal carcinoma cells, as determined by immunofluorescence and ELISA assays, was grossly defective compared with VHL+/+ renal carcinoma cells. Reintroduction of wildtype, but not mutant, pVHL into VHL-/- renal carcinoma cells partially corrected this defect. Finally, extracellular fibronectin matrix assembly by VHL-/- mouse embryos and mouse embryo fibroblasts (MEFs), unlike their VHL+/+ counterparts, was grossly impaired. These data support a direct role of pVHL in fibronectin matrix assembly.

Regulation of Hypoxia-Inducible mRNAs by the von Hippel-Lindau Tumor Suppressor Protein Requires Binding to Complexes Containing Elongins B/C and Cul2

ABSTRACT The von Hippel-Lindau tumor suppressor protein (pVHL) binds to elongins B and C and posttranscriptionally regulates the accumulation of hypoxia-inducible mRNAs under normoxic (21% O2) conditions. Here we report that pVHL binds, via elongin C, to the human homolog of the Caenorhabditis elegans Cul2 protein. Coimmunoprecipitation and chromatographic copurification data suggest that pVHL-Cul2 complexes exist in native cells. pVHL mutants that were unable to bind to complexes containing elongin C and Cul2 were likewise unable to inhibit the accumulation of hypoxia-inducible mRNAs. A model for the regulation of hypoxia-inducible mRNAs by pVHL is presented based on the apparent similarity of elongin C and Cul2 to Skp1 and Cdc53, respectively. These latter proteins form complexes that target specific proteins for ubiquitin-dependent proteolysis.

The updated biology of hypoxia‐inducible factor

Oxygen is essential for eukaryotic life and is inextricably linked to the evolution of multicellular organisms. Proper cellular response to changes in oxygen tension during normal development or pathological processes, such as cardiovascular disease and cancer, is ultimately regulated by the transcription factor, hypoxia‐inducible factor (HIF). Over the past decade, unprecedented molecular insight has been gained into the mammalian oxygen‐sensing pathway involving the canonical oxygen‐dependent prolyl‐hydroxylase domain‐containing enzyme (PHD)–von Hippel‐Lindau tumour suppressor protein (pVHL) axis and its connection to cellular metabolism. Here we review recent notable advances in the field of hypoxia that have shaped a more complex model of HIF regulation and revealed unique roles of HIF in a diverse range of biological processes, including immunity, development and stem cell biology.

Germline CBL mutations cause developmental abnormalities and predispose to juvenile myelomonocytic leukemia

CBL encodes a member of the Cbl family of proteins, which functions as an E3 ubiquitin ligase. We describe a dominant developmental disorder resulting from germline missense CBL mutations, which is characterized by impaired growth, developmental delay, cryptorchidism and a predisposition to juvenile myelomonocytic leukemia (JMML). Some individuals experienced spontaneous regression of their JMML but developed vasculitis later in life. Importantly, JMML specimens from affected children show loss of the normal CBL allele through acquired isodisomy. Consistent with these genetic data, the common p.371Y>H altered Cbl protein induces cytokine-independent growth and constitutive phosphorylation of ERK, AKT and S6 only in hematopoietic cells in which normal Cbl expression is reduced by RNA interference. We conclude that germline CBL mutations have developmental, tumorigenic and functional consequences that resemble disorders that are caused by hyperactive Ras/Raf/MEK/ERK signaling and include neurofibromatosis type 1, Noonan syndrome, Costello syndrome, cardiofaciocutaneous syndrome and Legius syndrome.

VHL Promotes E2 Box-Dependent E-Cadherin Transcription by HIF-Mediated Regulation of SIP1 and Snail

ABSTRACT The product of the von Hippel-Lindau gene (VHL) acts as the substrate-recognition component of an E3 ubiquitin ligase complex that ubiquitylates the catalytic α subunit of hypoxia-inducible factor (HIF) for oxygen-dependent destruction. Although emerging evidence supports the notion that deregulated accumulation of HIF upon the loss of VHL is crucial for the development of clear-cell renal cell carcinoma (CC-RCC), the molecular events downstream of HIF governing renal oncogenesis remain unclear. Here, we show that the expression of a homophilic adhesion molecule, E-cadherin, a major constituent of epithelial cell junctions whose loss is associated with the progression of epithelial cancers, is significantly down-regulated in primary CC-RCC and CC-RCC cell lines devoid of VHL. Reintroduction of wild-type VHL in CC-RCC (VHL−/−) cells markedly reduced the expression of E2 box-dependent E-cadherin-specific transcriptional repressors Snail and SIP1 and concomitantly restored E-cadherin expression. RNA interference-mediated knockdown of HIFα in CC-RCC (VHL−/−) cells likewise increased E-cadherin expression, while functional hypoxia or expression of VHL mutants incapable of promoting HIFα degradation attenuated E-cadherin expression, correlating with the disengagement of RNA polymerase II from the endogenous E-cadherin promoter/gene. These findings reveal a critical HIF-dependent molecular pathway connecting VHL, an established “gatekeeper” of the renal epithelium, with a major epithelial tumor suppressor, E-cadherin.

Differential Dependence of Hypoxia-inducible Factors 1α and 2α on mTORC1 and mTORC2

Constitutive expression of hypoxia-inducible factor (HIF) has been implicated in several proliferative disorders. Constitutive expression of HIF1α and HIF2α has been linked to a number of human cancers, especially renal cell carcinoma (RCC), in which HIF2α expression is the more important contributor. Expression of HIF1α is dependent on the mammalian target of rapamycin (mTOR) and is sensitive to rapamycin. In contrast, there have been no reports linking HIF2α expression with mTOR. mTOR exists in two complexes, mTORC1 and mTORC2, which are differentially sensitive to rapamycin. We report here that although there are clear differences in the sensitivity of HIF1α and HIF2α to rapamycin, both HIF1α and HIF2α expression is dependent on mTOR. HIF1α expression was dependent on both Raptor (a constituent of mTORC1) and Rictor (a constitutive of mTORC2). In contrast, HIF2α was dependent only on the mTORC2 constituent Rictor. These data indicate that although HIF1α is dependent on both mTORC1 and mTORC2, HIF2α is dependent only on mTORC2. We also examined the dependence of HIFα expression on the mTORC2 substrate Akt, which exists as three different isoforms, Akt1, Akt2, and Akt3. Interestingly, the expression of HIF2α was dependent on Akt2, whereas that of HIF1α was dependent on Akt3. Because HIF2α is apparently more critical in RCC, this study underscores the importance of targeting mTORC2 and perhaps Akt2 signaling in RCC and other proliferative disorders in which HIF2α has been implicated.

Differential dependence of HIF1α and HIF2α on mTORC1 and mTORC2

Constitutive expression of hypoxia-inducible factor (HIF) has been implicated in several proliferative disorders. Constitutive expression of HIF1α and HIF2α has been linked to a number of human cancers, especially renal cell carcinoma (RCC), in which HIF2α expression is the more important contributor. Expression of HIF1α is dependent on the mammalian target of rapamycin (mTOR) and is sensitive to rapamycin. In contrast, there have been no reports linking HIF2α expression with mTOR. mTOR exists in two complexes, mTORC1 and mTORC2, which are differentially sensitive to rapamycin. We report here that although there are clear differences in the sensitivity of HIF1α and HIF2α to rapamycin, both HIF1α and HIF2α expression is dependent on mTOR. HIF1α expression was dependent on both Raptor (a constituent of mTORC1) and Rictor (a constitutive of mTORC2). In contrast, HIF2α was dependent only on the mTORC2 constituent Rictor. These data indicate that although HIF1α is dependent on both mTORC1 and mTORC2, HIF2α is dependent only on mTORC2. We also examined the dependence of HIFα expression on the mTORC2 substrate Akt, which exists as three different isoforms, Akt1, Akt2, and Akt3. Interestingly, the expression of HIF2α was dependent on Akt2, whereas that of HIF1α was dependent on Akt3. Because HIF2α is apparently more critical in RCC, this study underscores the importance of targeting mTORC2 and perhaps Akt2 signaling in RCC and other proliferative disorders in which HIF2α has been implicated.

Multiple splice variants of the human HIF-3α locus are targets of the VHL E3 ubiquitin ligase complex

Functional inactivation of the von Hippel-Lindau (VHL) tumor suppressor protein is the cause of familial VHL disease and sporadic kidney cancer. The VHL gene product (pVHL) is a component of an E3 ubiquitin ligase complex that targets the hypoxia-inducible factor (HIF) 1 and 2 α subunits for polyubiquitylation. This process is dependent on the hydroxylation of conserved proline residues on the α subunits of HIF-1/2 in the presence of oxygen. In our effort to identify orphan HIF-like proteins in the data base that are potential targets of the pVHL complex, we report multiple splice variants of the human HIF-3α locus as follows: hHIF-3α1, hHIF-3α2 (also referred to as hIPAS; human inhibitory PAS domain protein), hHIF-3α3, hHIF-3α4, hHIF-3α5, and hHIF-3α6. We demonstrate that the common oxygen-dependent degradation domain of hHIF-3α1–3 splice variants is targeted for ubiquitylation by the pVHL complex in vitro and in vivo. This activity is enhanced in the presence of prolyl hydroxylase and is dependent on a proline residue at position 490. Furthermore, the ubiquitin conjugation occurs on lysine residues at position 465 and 568 within the oxygen-dependent degradation domain. These results demonstrate additional targets of the pVHL complex and suggest a growing complexity in the regulation of hypoxia-inducible genes by the HIF family of transcription factors.

pVHL modification by NEDD8 is required for fibronectin matrix assembly and suppression of tumor development

ABSTRACT Functional inactivation of the von Hippel-Lindau (VHL) tumor suppressor gene is the cause of the familial VHL disease and most sporadic renal clear-cell carcinomas (RCC). pVHL has been shown to play a role in the destruction of hypoxia-inducible factor α (HIF-α) subunits via ubiquitin-mediated proteolysis and in the regulation of fibronectin matrix assembly. Although most disease-causing pVHL mutations hinder the regulation of the HIF pathway, every disease-causing pVHL mutant tested to date has failed to promote the assembly of the fibronectin matrix, underscoring its potential importance in VHL disease. Here, we report that a ubiquitin-like molecule called NEDD8 covalently modifies pVHL. A nonneddylateable pVHL mutant, while retaining its ability to ubiquitylate HIF, failed to bind to and promote the assembly of the fibronectin matrix. Expression of the neddylation-defective pVHL in RCC cells, while restoring the regulation of HIF, failed to promote the differentiated morphology in a three-dimensional growth assay and was insufficient to suppress the formation of tumors in SCID mice. These results suggest that NEDD8 modification of pVHL plays an important role in fibronectin matrix assembly and that in the absence of such regulation, an intact HIF pathway is insufficient to prevent VHL-associated tumorigenesis.