Negative regulation of hypoxia-inducible genes by the von Hippel-Lindau protein

Improved detection of germline mutations in the von Hippel-Lindau disease tumor suppressor gene

von Hippel-Lindau disease (VHL) is an inherited neoplastic disorder characterized by the development of tumors in the eyes, brain, spinal cord, inner ear, adrenal gland, pancreas, kidney, and epididymis. The VHL tumor suppressor gene was identified in 1993. Initial studies reported the detection of germline mutations in the VHL gene in 39-75% of VHL families. We used tests that detect different types of mutations to improve the frequency of detection of germline mutations in VHL families. The methods included quantitative Southern blotting to detect deletions of the entire VHL gene, Southern blotting to detect gene rearrangements, fluorescence in situ hybridization (FISH) to confirm deletions, and complete sequencing of the gene. Here we report that we have detected germline mutations in the VHL gene in 100% (93/93) of VHL families tested. In addition, we describe 13 novel intragenic VHL germline mutations. With the methodology described in this article, it is now possible to identify germline mutations in virtually all families with VHL.

von Hippel-Lindau syndrome: target for anti-vascular endothelial growth factor (VEGF) receptor therapy

von Hippel-Lindau (VHL) syndrome is a familial cancer syndrome caused by germline mutations in the VHL tumor suppressor gene. Mutations in the VHL gene result in the constitutive stabilization of transcription factors hypoxia-inducible factors 1alpha and 2alpha, which bind to specific enhancer elements in the vascular endothelial growth factor (VEGF) gene and stimulate angiogenesis. This increase in angiogenesis under normoxic conditions in key target organs such as the brain, kidney, and eye leads to high morbidity and reduced life expectancy. Drugs designed to block the VEGF signaling pathway may prevent the long-term complications of the disease. To test this hypothesis, a clinical study was initiated to evaluate the effect of the VEGF tyrosine kinase receptor inhibitor SU5416 in patients with VHL syndrome. Preliminary data on SU5416 indicate that it is well tolerated when administered chronically in such patients. However, since little is known about the long-term use of such inhibitors, patients will need careful monitoring. Data obtained from monitoring these patients will provide valuable information for adjuvant treatment trials in cancer patients.

Somatic mRNA turnover mutants implicate tristetraprolin in the interleukin-3 mRNA degradation pathway

Control of mRNA stability is critical for expression of short-lived transcripts from cytokines and proto-oncogenes. Regulation involves an AU-rich element (ARE) in the 3' untranslated region (3'UTR) and cognate trans-acting factors thought to promote either degradation or stabilization of the mRNA. In this study we present a novel approach using somatic cell genetics designed to identify regulators of interleukin-3 (IL-3) mRNA turnover. Mutant cell lines were generated from diploid HT1080 cells transfected with a reporter construct containing green fluorescent protein (GFP) linked to the IL-3 3'UTR. GFP was expressed at low levels due to rapid decay of the mRNA. Following chemical mutagenesis and selection of GFP-overexpressing cells, we could isolate three mutant clones (slowA, slowB, and slowC) with a specific, trans-acting defect in IL-3 mRNA degradation, while the stability of IL-2 and tumor necrosis factor alpha reporter transcripts was not affected. Somatic cell fusion experiments revealed that the mutants are genetically recessive and form two complementation groups. Expression of the tristetraprolin gene in both groups led to reversion of the mutant phenotype, thereby linking this gene to the IL-3 mRNA degradation pathway. The genetic approach described here should allow identification of the defective functions by gene transfer and is also applicable to the study of other mRNA turnover pathways.

Role of chromosome 3p12-p21 tumour suppressor genes in clear cell renal cell carcinoma: analysis of VHL dependent and VHL independent pathways of tumorigenesis

AIMS

Chromosome 3p deletions and loss of heterozygosity (LOH) for 3p markers are features of clear cell renal cell carcinoma but are rare in non-clear cell renal cell carcinoma. The VHL tumour suppressor gene, which maps to 3p25, is a major gatekeeper gene for clear cell renal cell carcinoma and is inactivated in most sporadic cases of this disease. However, it has been suggested that inactivation of other 3p tumour suppressor genes might be crucial for clear cell renal cell carcinoma tumorigenesis, with inactivation (VHL negative) and without inactivation (VHL positive) of the VHL tumour suppressor gene. This study set out to investigate the role of non-VHL tumour suppressor genes in VHL negative and VHL positive clear cell renal cell carcinoma.

METHODS

Eighty two clear cell renal cell carcinomas of known VHL inactivation status were analysed for LOH at polymorphic loci within the candidate crucial regions for chromosome 3p tumour suppressor genes (3p25, LCTSGR1 at 3p21.3, LCTSGR2 at 3p12 and at 3p14.2).

RESULTS

Chromosome 3p12-p21 LOH was frequent both in VHL negative and VHL positive clear cell renal cell carcinoma. However, although the frequency of 3p25 LOH in VHL negative clear cell renal cell carcinoma was similar to that at 3p12-p21, VHL positive tumours demonstrated significantly less LOH at 3p25 than at 3p12-p21. Although there was evidence of LOH for clear cell renal cell carcinoma tumour suppressor genes at 3p21, 3p14.2, and 3p12, both in VHL negative and VHL positive tumours, the major clear cell renal cell carcinoma LOH region mapped to 3p21.3, close to the lung cancer tumour suppressor gene region 1 (LCTSGR1). There was no association between tumour VHL status and tumour grade and stage.

CONCLUSIONS

These findings further indicate that VHL inactivation is not sufficient to initiate clear cell renal cell carcinoma and that loss of a gatekeeper 3p21 tumour suppressor gene is a crucial event for renal cell carcinoma development in both VHL negative and VHL positive clear cell renal cell carcinoma.

Sporadic endocrine tumours and their relationship to the hereditary endocrine neoplasia syndromes

In the last years of the previous century the genes involved in the aetiology of five endocrine tumour syndromes have been identified. The tumour-suppressor gene that is responsible for Von Hippel-Lindau Disease was cloned in 1993; multiple endocrine neoplasia (MEN) types 2A and 2B and familial medullary thyroid carcinoma were found to be caused by activating mutations in the ret proto-oncogene in 1993 and 1994, and most recently the menin-gene, another tumour-suppressor gene, was shown to be associated with MEN-1. As usual, the answer to one question leads to innumerable new questions. And so, now we want to know the extent to which germ-line mutations (de novo, or otherwise previously undetected) in these genes play a role in the occurrence of the various endocrine tumours that are associated with these syndromes in apparently sporadic cases. We also want to know if the nature of the (germ-line) mutation conveys any information about the characteristics (phenotype) of the disease. We want to know the role of somatic mutations in these genes in truly sporadic tumours. And finally we want to know the exact function of the proteins that are encoded by these genes. The paper by Roijers et al. [1] elsewhere in this issue is an example of a small but well-directed step on the way to address some of these questions with respect to the menin-gene. It addresses the problem of patient selection when looking for germ-line mutations in apparently sporadic MEN-1 patients. In this review we want to give a brief summary of the present status with regard to some of the questions mentioned above, in relation to the endocrine tumour syndromes caused by the vhl, ret and menin genes.

Comparative sequence analysis of the VHL tumor suppressor gene

Comparative genome analysis may provide novel insights into gene evolution and function. To investigate the von Hippel-Lindau (VHL) disease tumor suppressor gene, we sequenced the VHL gene in seven primate species. Comparative analysis was performed for human, primate, and rodent VHL genes and for a putative Caenorhabditis elegans VHL homologue identified by database analysis. The VHL gene has two translation initiation sites (at codons 1 and 54); however, the relative importance of the full-length translation product (pVHL30) and that translated from the second internal translation initiation site (pVHL19) is unclear. The N-terminal sequence of pVHL30 contains eight copies of a GXEEX acidic repeat motif in human and higher primates, but only three copies were present in the marmoset, and only one copy was present in rodent VHL genes. Evolutionary analysis suggested that the N-terminal repetitive sequence in pVHL30 was of less functional importance than those regions present in both pVHL30 and pVHL19. The VHL gene product is reported to form complexes with various proteins including elongin B, elongin C, VBP-1, fibronectin, Spl, CUL2, and HIF-1. Although most of the regions in pVHL that had been implicated in binding specific proteins demonstrated evolutionary conservation, the carboxy-terminal putative VBP-1 binding site was less well conserved, suggesting that VBP-1 binding may have less functional significance. Although an amino acid substitution (K171T) close to the pVHL elongin binding region was found in baboon, analysis of the structure of human pVHL suggested that this substitution would not interfere with pVHL/elongin C interaction. In general, there was a good correlation between the pVHL domains that demonstrated most evolutionary conservation and those that were most frequently mutated in tumors. Analysis of human/C. elegans conservation and human germline and somatic mutation patterns identified a highly conserved mutation cluster region between codons 74 and 90. However, this region is likely to be important for the structural integrity of pVHL rather than representing an additional protein binding domain.

Ran-mediated nuclear export of the von Hippel-Lindau tumor suppressor protein occurs independently of its assembly with cullin-2

Inactivating mutations of the von Hippel-Lindau (VHL) tumor suppressor gene cause the VHL cancer syndrome and sporadic renal clear cell carcinoma. VHL engages in a nucleocytoplasmic shuttle, which is required for its function. Here, we pursue our investigation to identify mechanisms by which VHL-green fluorescent protein (VHL-GFP) is exported from the nucleus. We show that nuclear export of VHL-GFP in living cells requires ongoing RNA polymerase II activity, and is mediated by mechanisms that are temperature-sensitive and energy-dependent. In vitro nuclear export of VHL-GFP is inhibited by nuclear pore-specific lectins, requires ATP hydrolysis and polyadenylated mRNAs, and occurs with kinetics that are similar to those of proteins containing a nuclear export signal. Biochemical fractionation has revealed that nuclear export of VHL-GFP occurs by way of a Ran-dependent pathway. Size exclusion column chromatography and deletion mutant analysis suggest that VHL-GFP does not require assembly with one of its associated proteins, cullin-2, to engage in nuclear export. These results demonstrate that nuclear export of VHL-GFP is Ran-mediated and ATP hydrolysis-dependent. They also suggest that sequences outside the elongin C binding box may function as a nuclear export domain, potentially providing a novel role for this region of VHL frequently mutated in renal cell carcinoma.

von Hippel-Lindau disease

An autosomal dominant tumor predisposition syndrome, von Hippel-Lindau disease (VHL) is characterized by the presence of benign and malignant tumors. Hallmark lesions include retinal angiomas, hemangioblastomas of the cerebellum and spinal cord, and renal cell carcinomas. Affected persons may also have angiomatous or cystic lesions of the kidneys, pancreas, and epididymis, as well as adrenal pheochromocytomas. In this article, we discuss the clinical features and diagnostic criteria for this clinically underdiagnosed condition. An update on recent findings regarding the molecular genetics of VHL is provided, including a discussion of the evolving understanding of genotype-phenotype correlations. Understanding the molecular and functional aspects of this condition will lead to the development of strategies for the management and treatment of inherited and sporadic VHL-associated tumors.

Vascular endothelial growth factor expression is independent of hypoxia in human malignant glioma spheroids and tumours

We recently showed that severe hypoxia was not universally present adjacent to necrosis in human glioma xenografts and spheroids established from the M059K, M006, M006X, M006XLo and M010b cell lines. Using these glioma models, we wished to test whether oxygen serves as a regulator of cellular VEGF expression in situ. In situ hybridization (ISH) and immunohistochemistry (IHC) were used to detect vascular endothelial growth factor (VEGF) mRNA and protein expression in sections of glioma xenografts and spheroids in which hypoxic regions and regions with well-oxygenated necrosis were identified on contiguous sections by use of the hypoxia-specific marker, 3H-misonidazole. Independent validation of the presence of radiobiologically hypoxic cells in M006 xenografts was undertaken using the comet assay. Northern blotting analyses of monolayer cells demonstrated significant up-regulation of VEGF mRNA in the M006X line at oxygen concentrations of 6% and below. ISH analysis of VEGF mRNA showed unexpectedly strong staining for VEGF mRNA across the entire viable rim of M006X and M006XLo glioma spheroids. Similarly, in virtually all xenograft tumours of the M059K, M006 and M010b lines, VEGF ISH showed similar staining across all regions of healthy cells up to the border of necrosis. Only in one M006X tumour was there a suggestion of increased VEGF expression in cells adjacent to necrosis. IHC for VEGF showed good concordance with the ISH results. IHC analysis of the VEGF receptor flt-1 showed strong tumour cell staining in M006XLo glioma cells. In human glioma spheroids and xenograft tumours, regions of severe hypoxia do not correspond to areas of up-regulated VEGF expression; in fact, VEGF expression is quite uniform. Furthermore, this and our previous study demonstrate that levels of VEGF expression vary among sublines (M006, M006X and M006XLo) derived from a single human glioma specimen.

Expression of angiogenic growth factors in paragangliomas

OBJECTIVE/HYPOTHESIS

To determine if angiogenic growth factors including vascular endothelial growth factor (VEGF) and platelet-derived endothelial cell growth factor (PD-ECGF) are expressed in human paragangliomas.

STUDY DESIGN

A histopathologic and molecular examination of paraganglioma specimens obtained from surgical cases or retrieved from the Pathology Department of the Massachusetts Eye and Ear Infirmary.

METHODS

Fresh tumor or archival, paraffin-embedded paraganglioma specimens were analyzed by immunohistochemistry, Western blotting, and ELISA.

RESULTS

Positive immunohistochemical staining for VEGF was observed in five of nine surgical specimens and in six of eight archival specimens (11/17, or 65%). PD-ECGF immunoreactivity was detected in four of five surgical specimens and six of eight archival specimens (10/13, or 77%). The presence of PD-ECGF was confirmed by Western blot assay and ELISA confirmed the presence of VEGF in tumor extract.

CONCLUSIONS

Both VEGF and PD-ECGF are expressed in paragangliomas and may contribute to the extreme vascularity of these tumors. Key Words. Vascular endothelial growth factor, platelet-derived, endothelial cell growth factor, hypoxia, tumor vasculature.

An important von Hippel-Lindau tumor suppressor domain mediates Sp1-binding and self-association

VHL is the causative gene for both von Hippel-Lindau (VHL) disease and sporadic clear-cell renal cancer. We showed earlier that VHL downregulates vascular endothelial growth factor transcription by directly binding and inhibiting the transcriptional activator Sp1. We have now mapped the VHL Sp1-binding domain to amino acids 96-122. The 96-122 domain is disproportionately affected by substitution mutations, which interfere with the VHL-Sp1 interaction. Deletion of the 96-122 domain prevents VHL effects on Sp1 DNA binding and on VHL target gene expression, indicating the domain contributes importantly to VHL tumor suppressor activity. Nevertheless, prevention of the VHL-Sp1 interaction only partially abrogates VHL's transcriptional repressor activity, supporting the existence of VHL transcriptional effectors in addition to Sp1. VHL also directly interacts with the Sp1 zinc fingers and self-associates via the 96-122 domain, which furthermore suggest the domain may bind other metalloproteins and contribute to VHL dominant-negative effects.

Two distinct phenotypes caused by two different missense mutations in the same codon of the VHL gene

We have identified a family segregating von Hippel-Lindau (VHL) disease with a previously unreported T547A mutation in exon 1 of the VHL gene that causes a Tyr112 to Asn missense alteration in the protein. The mutation was identified by nucleotide sequencing and confirmed by restriction enzyme digestion. The mutation cosegregated with the disease in all five tested affected individuals from the extended family. The family consists of more than 100 at-risk individuals over seven generations. To date, we have identified 13 affected individuals of whom seven have had renal cell carcinoma and one has had a pheochromocytoma. No other case of a neuroendocrine tumor of the pancreas or adrenal gland (pheochromocytoma) was found or recognized retrospectively. Other manifestations in this family include retinal angioma and hemangioblastoma of the central nervous system. We also found the T547A mutation in three asymptomatic members of the family, ages 12, 19, and 20. Another mutation, T547C, which causes Tyr112 to His, has been seen at the same position and has been associated with VHL type 2A (pheochromocytoma, but no renal cell carcinoma) in two families with a total of 22 affected individuals [Chen F, Slife L, Kishida T, Mulvihill J, Tisherman SE, Zbar B, 1996: J Med Genet 33:716-717]. Thus, different amino acid changes at the same position can cause very distinct clinical phenotypes. It will be interesting to elucidate the functional differences that underlie the different phenotypes.

Okadaic acid stimulates the expression of vascular endothelial growth factor gene

Vascular endothelial growth factor (VEGF) is a specific mitogen for vascular endothelial cells and has been implicated in tumor angiogenesis. Okadaic acid, an inhibitor of protein phosphatases 1 and 2A, is a non-12-O-tetradecanoylphorbol-13-acetate (TPA)-type tumor promoter in two-stage carcinogenesis experiments in mouse skin. To elucidate the role of VEGF in the angiogenesis of these experimental tumors, the effect of okadaic acid on VEGF gene expression was examined. In NIH 3T3, Rat1, HeLa, and A431 cells, VEGF mRNA was upregulated by 5- to 10-fold after incubation with okadaic acid. Furthermore, the amount of VEGF protein in the culture medium was significantly increased after stimulation with okadaic acid. Interestingly, okadaic acid-induced upregulation of VEGF mRNA was not suppressed by protein kinase C (PKC) inhibitor or by tumor necrosis factor alpha blocking antibody, although TPA-induced VEGF upregulation was strongly suppressed by PKC inhibitor. Our results indicate that okadaic acid is a new and potent inducer of VEGF, suggesting the involvement of VEGF as an angiogenic factor during multistep carcinogenesis in vivo.

Identification of the von Hippel-lindau tumor-suppressor protein as part of an active E3 ubiquitin ligase complex

Mutations of von Hippel-Lindau disease (VHL) tumor-suppressor gene product (pVHL) are found in patients with dominant inherited VHL syndrome and in the vast majority of sporadic clear cell renal carcinomas. The function of the pVHL protein has not been clarified. pVHL has been shown to form a complex with elongin B and elongin C (VBC) and with cullin (CUL)-2. In light of the structural analogy of VBC-CUL-2 to SKP1-CUL-1-F-box ubiquitin ligases, the ubiquitin ligase activity of VBC-CUL-2 was examined in this study. We show that VBC-CUL-2 exhibits ubiquitin ligase activity, and we identified UbcH5a, b, and c, but not CDC34, as the ubiquitin-conjugating enzymes of the VBC-CUL-2 ubiquitin ligase. The protein Rbx1/ROC1 enhances ligase activity of VBC-CUL-2 as it does in the SKP1-CUL-1-F-box protein ligase complex. We also found that pVHL associates with two proteins, p100 and p220, which migrate at a similar molecular weight as two major bands in the ubiquitination assay. Furthermore, naturally occurring pVHL missense mutations, including mutants capable of forming a complex with elongin B-elongin C-CUL-2, fail to associate with p100 and p220 and cannot exhibit the E3 ligase activity. These results suggest that pVHL might be the substrate recognition subunit of the VBC-CUL-2 E3 ligase. This is also, to our knowledge, the first example of a human tumor-suppressor protein being directly involved in the ubiquitin conjugation system which leads to the targeted degradation of substrate proteins.

von Hippel-Lindau protein induces hypoxia-regulated arrest of tyrosine hydroxylase transcript elongation in pheochromocytoma cells

Rat pheochromocytoma (PC12) cells were stably transfected with either wild type or mutated human von Hippel-Lindau tumor suppressor protein (hpVHL). These proteins have opposing effects on regulating expression of the gene encoding tyrosine hydroxylase (TH), the rate-limiting enzyme in catecholamine synthesis. Whereas wild type hpVHL represses levels of TH mRNA and protein 5-fold, a truncated pVHL mutant, pVHL(1-115), induces accumulation of TH mRNA and protein 3-fold. hpVHL-induced inhibition of TH gene expression does not involve either a decrease in TH mRNA stability or repression of TH promoter activity. However, repression results from inhibition of RNA elongation at a downstream region of the TH gene. This elongation pause is accompanied by hpVHL sequestration in the nuclear extracts of elongins B and C, regulatory components of the transcription elongation heterotrimer SIII (elongin A/B/C). Hypoxia, a physiological stimulus for TH gene expression, alleviates the elongation block. A truncated pVHL mutant, pVHL(1-115), stimulates TH gene expression by increasing the efficiency of TH transcript elongation. This is the first report showing pVHL-dependent regulation of specific transcript elongation in vivo, as well as dominant negative activity of pVHL mutants in pheochromocytoma cells.

The von Hippel-Lindau tumor suppressor gene inhibits hepatocyte growth factor/scatter factor-induced invasion and branching morphogenesis in renal carcinoma cells

Loss of function in the von Hippel-Lindau (VHL) tumor suppressor gene occurs in familial and most sporadic renal cell carcinomas (RCCs). VHL has been linked to the regulation of cell cycle cessation (G(0)) and to control of expression of various mRNAs such as for vascular endothelial growth factor. RCC cells express the Met receptor tyrosine kinase, and Met mediates invasion and branching morphogenesis in many cell types in response to hepatocyte growth factor/scatter factor (HGF/SF). We examined the HGF/SF responsiveness of RCC cells containing endogenous mutated (mut) forms of the VHL protein (VHL-negative RCC) with that of isogenic cells expressing exogenous wild-type (wt) VHL (VHL-positive RCC). We found that VHL-negative 786-0 and UOK-101 RCC cells were highly invasive through growth factor-reduced (GFR) Matrigel-coated filters and exhibited an extensive branching morphogenesis phenotype in response to HGF/SF in the three-dimensional (3D) GFR Matrigel cultures. In contrast, the phenotypes of A498 VHL-negative RCC cells were weaker, and isogenic RCC cells ectopically expressing wt VHL did not respond at all. We found that all VHL-negative RCC cells expressed reduced levels of tissue inhibitor of metalloproteinase 2 (TIMP-2) relative to the wt VHL-positive cells, implicating VHL in the regulation of this molecule. However, consistent with the more invasive phenotype of the 786-0 and UOK-101 VHL-negative RCC cells, the levels of TIMP-1 and TIMP-2 were reduced and levels of the matrix metalloproteinases 2 and 9 were elevated compared to the noninvasive VHL-positive RCC cells. Moreover, recombinant TIMPs completely blocked HGF/SF-mediated branching morphogenesis, while neutralizing antibodies to the TIMPs stimulated HGF/SF-mediated invasion in vitro. Thus, the loss of the VHL tumor suppressor gene is central to changes that control tissue invasiveness, and a more invasive phenotype requires additional genetic changes seen in some but not all RCC lines. These studies also demonstrate a synergy between the loss of VHL function and Met signaling.

Role of vascular endothelial growth factor in the regulation of angiogenesis

Compelling evidence indicates that vascular endothelial growth factor (VEGF) is a fundamental regulator of normal and abnormal angiogenesis. The loss of a single VEGF allele results in defective vascularization and early embryonic lethality. VEGF plays also a critical role in kidney development, and its inactivation during early postnatal life results in the suppression of glomerular development and kidney failure. Recent evidence indicates that VEGF is also essential for angiogenesis in the female reproductive tract and for morphogenesis of the epiphyseal growth plate and endochondral bone formation. Substantial experimental evidence also implicates VEGF in pathological angiogenesis. Anti-VEGF monoclonal antibodies or other VEGF inhibitors block the growth of several human tumor cell lines in nude mice. Furthermore, the concentrations of VEGF are elevated in the aqueous and vitreous humors of patients with proliferative retinopathies such as the diabetic retinopathy. In addition, VEGF-induced angiogenesis results in a therapeutic benefit in several animal models of myocardial or limb ischemia. Currently, both therapeutic angiogenesis using recombinant VEGF or VEGF gene transfer and inhibition of VEGF-mediated pathological angiogenesis are being pursued clinically.

Studying interactions of four proteins in the yeast two-hybrid system: structural resemblance of the pVHL/elongin BC/hCUL-2 complex with the ubiquitin ligase complex SKP1/cullin/F-box protein

The yeast two-hybrid system is a powerful technique that detects interactions between two proteins and has been useful in identifying new binding partners. However, the system fails to detect protein-protein interactions that require the presence of additional components of a multisubunit complex. Here we demonstrate that the vector YIpDCE1 can be used to express elongins B and C in yeast, and that these proteins form a stable complex that interacts with the von Hippel-Lindau tumor-suppressor gene product (pVHL). Only when pVHL and elongins B and C (VBC) are present does an interaction with the cullin family member, hCUL-2, occur, forming the heterotetrameric pVHL/elongin BC/hCUL-2 complex. This system was then used to map the binding region of hCUL-2 for the VBC complex. The first amino-terminal 108 aa of hCUL-2 are necessary for interaction with the VBC complex. The elongin BC dimer acts as a bridge between pVHL and hCUL-2 because pVHL and hCUL-2 can form distinct complexes with elongins B and C. These results reveal a striking structural resemblance of pVHL/elongin BC/hCUL-2 complex with the E3-like ubiquitin ligase complex SKP1/Cullin/F-box protein with respect to protein composition and sites of interactions. Thus, it seems possible that pVHL/elongin BC/hCUL-2 complex will possess ubiquitin ligase activity targeting specific proteins for degradation by the proteasome.

The von Hippel-Lindau tumor suppressor protein is a component of an E3 ubiquitin-protein ligase activity

pVHL, the product of the VHL tumor suppressor gene, plays an important role in the regulation of cell growth and differentiation of human kidney cells, and inactivation of the VHL gene is the most frequent genetic event in human kidney cancer. The biochemical function of pVHL is unknown. Here we report that pVHL exists in vivo in a complex that displays ubiquitination-promoting activity in conjunction with the universally required components E1, E2, and ubiquitin. pVHL-associated ubiquitination activity requires, at a minimum, pVHL to bind elongin C and Cul-2, relatives of core components of SCF (Skp1-Cdc53/Cul-1-F-box protein) E3 ligase complexes. Notably, certain tumor-derived mutants of pVHL demonstrate loss of associated ubiquitination promoting activity. These results identify pVHL as a component of a potential SCF-like E3 ubiquitin-protein ligase complex and suggest a direct link between pVHL tumor suppressor and the process of ubiquitination.

The von Hippel-Lindau tumour suppressor protein: new perspectives

von Hippel-Lindau (VHL) disease is a hereditary cancer syndrome caused by germline mutations of the VHL tumour suppressor gene. The VHL gene product, pVHL, forms multiprotein complexes that contain elongin B, elongin C and Cul-2, and negatively regulates hypoxia-inducible mRNAs. pVHL is suspected to play a role in ubiquitination given the similarity of elongin C and Cul-2 with Skp1 and Cdc53, respectively. pVHL can also interact with fibronectin and is required for the assembly of a fibronectin matrix. Finally, pVHL, at least indirectly, plays a role in the ability of cells to exit the cell cycle. Thus, pVHL is a tumour suppressor protein that regulates angiogenesis, extracellular matrix formation and the cell cycle.

Processed pseudogene from the von Hippel-Lindau disease gene is located on human chromosome 1

The von Hippel-Lindau (VHL) disease gene is a tumor suppressor located at 3p25-26. While amplifying intron 1 of this gene, a smaller-than-expected product was found. This fragment was sequenced and was approximately 78% similar in sequence to the VHL gene and completely lacked sequence from the intron. No stop codons were found in the sequenced region. Using this DNA fragment as a probe for Northern blot hybridization analysis, no evidence was found for expression of a unique RNA. Because of the lack of intron 1 sequence and the likely lack of expression, the new sequence is most probably a part of a VHL processed pseudogene. The putative pseudogene was mapped to human chromosome band 1q12 using the polymerase chain reaction with template DNA from human/rodent somatic cell hybrids, a radiation hybrid panel, and a set of primers that were chosen to be maximally divergent from the genuine VHL gene. The human/rodent somatic cell hybrid DNAs were then used on Southern blots to determine which human bands are from the pseudogene and which are from the functional gene. This knowledge is valuable in interpreting Southern blot evidence of VHL gene abnormalities.

The tumour suppressor protein VHL targets hypoxia-inducible factors for oxygen-dependent proteolysis

Hypoxia-inducible factor-1 (HIF-1) has a key role in cellular responses to hypoxia, including the regulation of genes involved in energy metabolism, angiogenesis and apoptosis. The alpha subunits of HIF are rapidly degraded by the proteasome under normal conditions, but are stabilized by hypoxia. Cobaltous ions or iron chelators mimic hypoxia, indicating that the stimuli may interact through effects on a ferroprotein oxygen sensor. Here we demonstrate a critical role for the von Hippel-Lindau (VHL) tumour suppressor gene product pVHL in HIF-1 regulation. In VHL-defective cells, HIF alpha-subunits are constitutively stabilized and HIF-1 is activated. Re-expression of pVHL restored oxygen-dependent instability. pVHL and HIF alpha-subunits co-immunoprecipitate, and pVHL is present in the hypoxic HIF-1 DNA-binding complex. In cells exposed to iron chelation or cobaltous ions, HIF-1 is dissociated from pVHL. These findings indicate that the interaction between HIF-1 and pVHL is iron dependent, and that it is necessary for the oxygen-dependent degradation of HIF alpha-subunits. Thus, constitutive HIF-1 activation may underlie the angiogenic phenotype of VHL-associated tumours. The pVHL/HIF-1 interaction provides a new focus for understanding cellular oxygen sensing.

Conjugation of the ubiquitin-like protein NEDD8 to cullin-2 is linked to von Hippel-Lindau tumor suppressor function

The von Hippel-Lindau tumor suppressor protein pVHL assembles with cullin-2 (hCUL-2) and elongin B/C forming a protein complex, CBCVHL, that resembles SKP1-CDC53-F-box protein ubiquitin ligases. Here, we show that hCUL-2 is modified by the conserved ubiquitin-like protein NEDD8 and that NEDD8-hCUL-2 conjugates are part of CBCVHL complexes in vivo. Remarkably, the formation of these conjugates is stimulated by the pVHL tumor suppressor. A tumorigenic pVHL variant, however, is essentially deficient in this activity. Thus, ligation of NEDD8 to hCUL-2 is linked to pVHL activity and may be important for pVHL tumor suppressor function.

The elongin B ubiquitin homology domain. Identification of Elongin B sequences important for interaction with Elongin C

Mammalian Elongin B is a 118-amino acid protein composed of an 84-amino acid amino-terminal ubiquitin-like domain and a 34-amino acid carboxyl-terminal tail. Elongin B is found in cells as a subunit of the heterodimeric Elongin BC complex, which was originally identified as a positive regulator of RNA polymerase II elongation factor Elongin A and subsequently as a component of the multiprotein von Hippel-Lindau tumor suppressor and suppressor of cytokine signaling complexes. As part of our effort to understand how the Elongin BC complex regulates the activity of Elongin A, we are characterizing Elongin B functional domains. In this report, we show that the Elongin B ubiquitin-like domain is necessary and sufficient for interaction with Elongin C and for positive regulation of Elongin A transcriptional activity. In addition, by site-directed mutagenesis of the Elongin B ubiquitin-like domain, we identify a short Elongin B region that is important for its interaction with Elongin C. Finally, we observe that both the ubiquitin-like domain and carboxyl-terminal tail are conserved in Drosophila melanogaster and Caenorhabditis elegans Elongin B homologs that efficiently substitute for mammalian Elongin B in reconstitution of the transcriptionally active Elongin ABC complex, suggesting that the carboxyl-terminal tail performs an additional function not detected in our assays.

Expression of von Hippel-Lindau protein (VHL-P) in nontumorous and adenomatous human pituitaries

We studied the presence of von Hippel-Lindau protein (VHL-P) in 7 nontumorous pituitaries and 68 pituitary adenomas by immunocytochemistry using a polyclonal antibody which detects both normal and mutated forms. In nontumorous pituitary VHL-P was variable expressed in the cytoplasm of most adenohypophysial cells. In addition, weak diffuse staining was noted in the posterior lobe. Among the 53 VHL-P immunopositive adenomas (78%), 32 showed only cytoplasmic, 7 only nuclear, and 14 both cytoplasmic and nuclear immunoreactivity. In densely (8 cases) and sparsely (7 cases) granulated somatotroph adenomas nuclear and weak cytoplasmic immunoreactivity were common; all 4 sparsely granulated lactotroph adenomas had only moderate cytoplasmic immunostaining; all 7 functioning corticotroph adenomas presented intense cytoplasmic immunoreactivity and 4/7 showing nuclear immunostaining as well; all 3 silent subtype 1 adenomas were negative; 7 3/4 silent subtype 2 and 4/7 silent subtype 3 adenomas exhibited only cytoplasmic immunoreactivity; thyrotroph adenomas showed variable patterns from negative in 1/7, to only cytoplasmic in 2/7, nuclear alone in 2/7, or both in 2/7. Only weak, focal cytoplasmic immunoreactivity was noted in gonadotroph adenomas of female (2/4) and male (1/5) type. Lastly, all 4 null cell and 8 oncocytic adenomas showed moderate to intense cytoplasmic immunoreactivity. It can be concluded that the majority of pituitary adenomas express the VHL-P with variable distributions and intensity in different tumor types. The frequent localization of VHL-P in the nuclei of somatotroph adenomas, the least vascularized tumor type, suggests a possible inhibitory role of VHL-P in pituitary angiogenesis.

Advances in the molecular basis of renal neoplasia

The past 2 years have provided exciting progress in elucidating the molecular basis of renal cancer. Work on the von Hippel-Lindau tumor suppressor, pVHL, in clear-cell renal cancer is already suggesting new potential therapies, and should have important implications in the pathogenesis of renal cystic disease and tumor angiogenesis. In addition, study of the Wilms' tumor suppressor, WT1, is revealing much about the pathogenesis of Wilms' tumor, urogenital development, and glomerular podocyte biology. c-met, the gene encoding the hepatocyte growth factor receptor, has recently been identified as a causative gene for hereditary papillary renal cancer. This review will highlight these and other new molecular advances in the renal cancer field.

Rbx1, a component of the VHL tumor suppressor complex and SCF ubiquitin ligase

The von Hippel-Lindau (VHL) tumor suppressor gene is mutated in most human kidney cancers. The VHL protein is part of a complex that includes Elongin B, Elongin C, and Cullin-2, proteins associated with transcriptional elongation and ubiquitination. Here it is shown that the endogenous VHL complex in rat liver also includes Rbx1, an evolutionarily conserved protein that contains a RING-H2 fingerlike motif and that interacts with Cullins. The yeast homolog of Rbx1 is a subunit and potent activator of the Cdc53-containing SCFCdc4 ubiquitin ligase required for ubiquitination of the cyclin-dependent kinase inhibitor Sic1 and for the G1 to S cell cycle transition. These findings provide a further link between VHL and the cellular ubiquitination machinery.

Structure of the VHL-ElonginC-ElonginB complex: implications for VHL tumor suppressor function

Mutation of the VHL tumor suppressor is associated with the inherited von Hippel-Lindau (VHL) cancer syndrome and the majority of kidney cancers. VHL binds the ElonginC-ElonginB complex and regulates levels of hypoxia-inducible proteins. The structure of the ternary complex at 2.7 angstrom resolution shows two interfaces, one between VHL and ElonginC and another between ElonginC and ElonginB. Tumorigenic mutations frequently occur in a 35-residue domain of VHL responsible for ElonginC binding. A mutational patch on a separate domain of VHL indicates a second macromolecular binding site. The structure extends the similarities to the SCF (Skp1-Cul1-F-box protein) complex that targets proteins for degradation, supporting the hypothesis that VHL may function in an analogous pathway.

Hypoxic regulation of vascular endothelial growth factor mRNA stability requires the cooperation of multiple RNA elements

Vascular endothelial growth factor (VEGF) is a key regulator of developmental, physiological, and tumor angiogenesis. Upregulation of VEGF expression by hypoxia appears to be a critical step in the neovascularization of solid cancers. The VEGF mRNA is intrinsically labile, but in response to hypoxia the mRNA is stabilized. We have systematically analyzed the regions in the VEGF mRNA that are responsible for its lability under normoxic conditions and for stabilization in response to hypoxia. We find that the VEGF mRNA not only contains destabilizing elements in its 3' untranslated region (3'UTR), but also contains destabilizing elements in the 5'UTR and coding region. Each region can independently promote mRNA degradation, and together they act additively to effect rapid degradation under normoxic conditions. Stabilization of the mRNA in response to hypoxia is completely dependent on the cooperation of elements in each of the 5'UTR, coding region, and 3'UTR. Combinations of any of two of these three regions were completely ineffective in responding to hypoxia, whereas combining all three regions allowed recapitulation of the hypoxic stabilization seen with the endogenous VEGF mRNA. We conclude that multiple regions in the VEGF mRNA cooperate both to ensure the rapid degradation of the mRNA under normoxic conditions and to allow stabilization of the mRNA in response to hypoxia. Our findings highlight the complexity of VEGF gene expression and also reveal a mechanism of gene regulation that could become the target for strategies of therapeutic intervention.

Targeting cancer therapy

Cellular responses to hypoxia include modulation of respiration rate and up-regulation of genes which encode for angiogenesis factors. We tested whether human malignant glioma cells vary in their response to hypoxic stress over the range of oxygen concentrations which exist in tumours. In five cell lines tested, decreased oxygen availability resulted in decreased rates of oxygen utilization, however substantial differences in the magnitude of the response were observed. Northern blot analysis was used to study induction of vascular endothelial growth factor mRNA in response to hypoxia. In two cell lines, modest hypoxia increased vascular endothelial growth factor mRNA levels compared with those of aerobic controls. In two additional cell lines, vascular endothelial growth factor mRNA was constituitively expressed under aerobic conditions and was not further increased by hypoxia. These findings demonstrate that differences in the response to hypoxia exist among human malignant glioma cell lines and suggest that therapies designed to exploit tumour hypoxia may have varying effects in tumours with different hypoxic stress responses. © 1999 Cancer Research Campaign

Alternate choice of initiation codon produces a biologically active product of the von Hippel Lindau gene with tumor suppressor activity

The VHL tumor suppressor gene has previously been reported to encode a protein of 213 amino acid residues. Here we report the identification of a second major VHL gene product with an apparent molecular weight of 18 kD, pVHL18, which appears to arise from alternate translation initiation at a second AUG codon (codon 54) within the VHL open reading frame. In vitro and in vivo studies indicate that the internal codon in the VHL mRNA is necessary and sufficient for production of pVHL18. pVHL18 can bind to elongin B, elongin C, and Hs-CUL2. When reintroduced into renal carcinoma cells that lack a wild-type VHL allele, pVHL18 suppresses basal levels of VEGF expression, restores hypoxia-inducibility of VEGF expression, and inhibits tumor formation in nude mice. These data strongly support the existence of two distinct VHL gene products in VHL tumor suppression.

Anti-angiogenesis therapy and strategies for integrating it with adjuvant therapy

Tumor angiogenesis is critical for the growth of primary cancers above 1-2 mm in diameter. A major vascular growth factor is VEGF, and approaches to inhibit VEGF have shown encouraging results in pre-clinical studies. The mechanisms involved in switching on angiogenesis involve activation of oncogenes and upregulation of the hypoxia-sensing pathway. These provide novel targets for therapy. Many anti-angiogenic drugs are in clinical trial currently and there are problems in assessing these types of drugs if they only cause disease stabilisation. It will be important to develop methods to assess inhibition of vascular growth in vivo. New generations of anti-angiogenesis drugs such as endostatin of angiostatin, which are more potent, may cause tumor regression, but this has not yet been studied in patients. These approaches for advanced disease should be more successful when applied early in an adjuvant situation. This will also require careful monitoring of long-term toxicity.

Renal cell carcinoma: management of advanced disease

PURPOSE

We provide a current review of the management of advanced renal cell carcinoma.

MATERIALS AND METHODS

A comprehensive literature review of peer reviewed articles which address the current management of metastatic renal cell carcinoma was performed.

RESULTS

Renal cell carcinoma is the seventh leading cause of cancer, accounting for 3% of malignancies in men. The incidence of renal cell carcinoma has increased significantly by 38% from 1974 through 1990 at least in part related to earlier diagnosis with the common use of new radiological techniques. Cytotoxic chemotherapy remains poor as a treatment alternative. Interferon-alpha produces responses in 15 to 20% of patients but clinical usefulness as monotherapy has been surpassed by interleukin-2 (IL-2). IL-2 is the first immunotherapy to produce durable remissions resulting in approval by the Food and Drug Administration. Although high dose bolus IL-2 schedules have the longest followup, IL-2 administered on other schedules may have enhanced efficacy. Randomized trials are attempting to delineate the appropriate role for various doses and schedules.

CONCLUSIONS

Advanced renal cell carcinoma, once a disease relegated to the incurable, during the last decade has evolved into a malignancy that may be associated with cure. The first evidence of this potential is the clear and unequivocal demonstration that IL-2 produces durable complete remissions. Building upon this immunotherapeutic approach the future treatment of renal cell carcinoma will incorporate new immunological technology, including gene, dendritic cell, vaccine and antibody therapy.

Transcription-dependent nuclear-cytoplasmic trafficking is required for the function of the von Hippel-Lindau tumor suppressor protein

Mutation of the von Hippel-Lindau tumor suppressor gene (vhl) causes the von Hippel-Lindau cancer syndrome as well as sporadic renal clear cell carcinoma. To pursue our study of the intracellular localization of VHL protein in relation to its function, we fused VHL to the green fluorescent protein (GFP) to produce the VHL-GFP fusion protein. Like VHL, VHL-GFP binds to elongins B and C and Cullin-2 and regulates target gene product levels, including levels of vascular endothelial growth factor and glucose transporter 1. VHL-GFP localizes predominantly to the cytoplasm, with some detectable nuclear signal. Inhibition of transcription by actinomycin D or 5,6-dichlorobenzimidazole riboside (DRB) causes VHL to be redistributed to the nucleus. A cellular fusion assay was used to demonstrate that inhibition of transcription induces a decrease in the nuclear export rate of VHL. The dependence of transcription for trafficking is lost with a deletion of exon 2, a region with a mutation causing a splice defect in the VHL gene in sporadic renal clear cell carcinoma. Addition of a strong nuclear export signal (NES) derived from the Rev protein results in complete nuclear exclusion and abrogates the redistribution of VHL-GFP-NES into the nucleus upon inhibition of transcription. Leptomycin B, which inhibits NES-mediated nuclear export, reverts the distribution of VHL-GFP-NES to that of VHL-GFP and restores sensitivity to actinomycin D and DRB. Uncoupling of VHL-GFP trafficking to transcription either by an exon 2 deletion or fusion to NES abolishes VHL function. We suggest that VHL function requires not only nuclear or cytoplasmic localization, but also exon 2-mediated transcription-dependent trafficking between these two cellular compartments.

Protective function of von Hippel-Lindau protein against impaired protein processing in renal carcinoma cells

The absence of functional von Hippel-Lindau (VHL) tumor suppressor gene leads to the development of neoplasias characteristic of VHL disease, including renal cell carcinoma (RCC). Here, we compared the sensitivity of RCC cells lacking VHL gene function with that of RCC cells expressing the wild-type VHL gene (wtVHL) after exposure to various stresses. While the response to most treatments was not affected by the VHL gene status, glucose deprivation was found to be much more cytotoxic for RCC cells lacking VHL gene function than for wtVHL-expressing cells. The heightened sensitivity of VHL-deficient cells was not attributed to dissimilar energy requirements or to differences in glucose uptake, but more likely reflects a lesser ability of VHL-deficient cells to handle abnormally processed proteins arising from impaired glycosylation. In support of this hypothesis, other treatments which act through different mechanisms to interfere with protein processing (i.e., tunicamycin, brefeldin A, and azetidine) were also found to be much more toxic for VHL-deficient cells. Furthermore, ubiquitination of cellular proteins was elevated in VHL-deficient cells, particularly after glucose deprivation, supporting a role for the VHL gene in ubiquitin-mediated proteolysis. Accordingly, the rate of elimination of abnormal proteins was lower in cells lacking a functional VHL gene than in wtVHL-expressing cells. Thus, pVHL appears to participate in the elimination of misprocessed proteins, such as those arising in the cell due to the unavailability of glucose or to other stresses.

Mechanisms of tumor angiogenesis and therapeutic implications: angiogenesis inhibitors

Angiogenesis is the development of new blood vessels from the existing vascular bed. In normal conditions this tightly regulated process occurs only during embryonic development, the female reproductive cycle and wound repair. In contrast, in pathological conditions such as malignant growth, atherosclerosis and diabetic retinopathy, angiogenesis becomes persistent due to an imbalance in the interplay between the positive and negative regulatory signals controlling the process. Thus, the control of tumor neovascularization may lead to new therapeutic approaches. Indeed, several anti-angiogenic drugs are currently undergoing preclinical characterization and/or clinical investigation. Recent achievement has clarified the mechanisms of action leading to pathological angiogenesis and has highlighted the role of hypoxia, growth factors, growth factor-receptors, enzymes and cell adhesion molecules involved in the process. This knowledge has permitted the design of receptor antagonists, adhesion molecule blockers and new targeted vascular approaches including gene therapy.

aHIF: a natural antisense transcript overexpressed in human renal cancer and during hypoxia

BACKGROUND

Nonpapillary renal carcinoma is the predominant form of human kidney cancer and represents a distinct disease entity, morphologically and molecularly, from papillary renal carcinoma. We have discovered a natural antisense transcript that is complementary to the 3' untranslated region of hypoxia inducible factor alpha (HIF1alpha) messenger RNA (mRNA) and is strikingly overexpressed specifically in nonpapillary kidney cancer. HIF1alpha encodes a protein that is known to have two important functions: 1) to act as a transcription factor for hypoxia inducible genes and 2) to stabilize p53 protein during hypoxia. Because of the importance of HIF1alpha, we have characterized this natural antisense transcript, which we have named "aHIF."

METHODS

Differential display, reverse transcription-polymerase chain reaction, ribonuclease protection, and DNA-sequencing methods were used in our analysis.

RESULTS AND CONCLUSIONS

We show the following: 1) aHIF is a natural antisense transcript derived from HIF1alpha gene sequences encoding the 3' untranslated region of HIF1alpha mRNA; 2) aHIF is specifically overexpressed in all nonpapillary clear-cell renal carcinomas examined, but not in the papillary renal carcinomas examined; 3) aHIF is overexpressed in an established nonpapillary renal carcinoma cell line under both normoxic (i.e., normal aerobic) and hypoxic conditions; and 4) although aHIF is not further induced by hypoxia in nonpapillary disease, it can be induced in lymphocytes where there is a concomitant decrease in HIF1alpha mRNA. To our knowledge, this is the first case of overexpression of a natural antisense transcript exclusively associated with a specific human malignant disease.

Recombinant adenovirus expressing Von Hippel-Lindau-mediated cell cycle arrest is associated with the induction of cyclin-dependent kinase inhibitor p27Kip1

A recombinant adenovirus containing the Von Hippel-Lindau (VHL) cDNA was constructed (AdVHL) and used to investigate the function of this tumor suppressor gene. Exposure of renal and breast cancer cell lines to AdVHL resulted in high levels of VHL mRNA and protein. AdVHL infection resulted in G1 cell cycle arrest and growth inhibition of renal and breast cancer cell lines. AdVHL-mediated cell cycle arrest was associated with induction of the cyclin-dependent kinase inhibitor (CDKI) p27Kip1 and inhibition of CDK2 and cyclinB1-dependent cdc2 activities. Nuclear run-on analyses and actinomycin D inhibition studies indicate that the induction of p27Kip1 RNA by VHL is mediated at least in part through an increase in p27Kip1 mRNA synthesis. Furthermore, [35S]methionine pulse-chase studies indicate that the increase in p27Kip expression is also regulated through posttranscriptional control mechanisms. These studies support a novel concept that the tumor suppressor gene VHL controls cell cycle progression by regulation of p27Kip1 at both the mRNA and protein levels.

The Elongin BC complex interacts with the conserved SOCS-box motif present in members of the SOCS, ras, WD-40 repeat, and ankyrin repeat families

The Elongin BC complex was identified initially as a positive regulator of RNA polymerase II (Pol II) elongation factor Elongin A and subsequently as a component of the multiprotein von Hippel-Lindau (VHL) tumor suppressor complex, in which it participates in both tumor suppression and negative regulation of hypoxia-inducible genes. Elongin B is a ubiquitin-like protein, and Elongin C is a Skp1-like protein that binds to a BC-box motif that is present in both Elongin A and VHL and is distinct from the conserved F-box motif recognized by Skp1. In this report, we demonstrate that the Elongin BC complex also binds to a functional BC box present in the SOCS box, a sequence motif identified recently in the suppressor of cytokine signaling-1 (SOCS-1) protein, as well as in a collection of additional proteins belonging to the SOCS, ras, WD-40 repeat, SPRY domain, and ankyrin repeat families. In addition, we present evidence (1) that the Elongin BC complex is a component of a multiprotein SOCS-1 complex that attenuates Jak/STAT signaling by binding to Jak2 and inhibiting Jak2 kinase, and (2) that by interacting with the SOCS box, the Elongin BC complex can increase expression of the SOCS-1 protein by inhibiting its degradation. These results suggest that Elongin BC is a multifunctional regulatory complex capable of controlling multiple pathways in the cell through interaction with a short degenerate sequence motif found in many different proteins.

Coexpression of Erythropoietin and Vascular Endothelial Growth Factor in Nervous System Tumors Associated With von Hippel-Lindau Tumor Suppressor Gene Loss of Function

Hemangioblastomas are highly vascular tumors of the central nervous system that overexpress the hypoxia-inducible gene, vascular endothelial growth factor (VEGF), as a consequence of mutational inactivation of the von Hippel-Lindau tumor suppressor gene (VHL). Previous reports showed that hemangioblastomas can also express erythropoietin (Epo), which is also hypoxia-inducible. However, Epo expression in hemangioblastomas was observed only in individual cases, and the analyses were mainly based on indirect determination of erythropoiesis-stimulating activity. Therefore, we analyzed a series of 11 hemangioblastomas for Epo, VEGF, and VHL expression by Northern blot analysis and compared the results with normal brain and glioblastomas. Surprisingly, we observed Epo mRNA expression in all hemangioblastoma specimens analyzed, but in none of four glioblastomas. In contrast, VEGF mRNA was expressed in all hemangioblastomas and all glioblastomas. In situ hybridization revealed neoplastic stromal cells as Epo- and VEGF-producing cells in hemangioblastomas. These results suggest that in the nonhypoxic microenvironment of hemangioblastoma, Epo, similar to VEGF, might be negatively regulated by the VHL gene product.

© 1998 by The American Society of Hematology.