VHL: oxygen sensing and vasculogenesis

Tumor derived vasculogenesis in von Hippel-Lindau disease-associated tumors

von Hippel-Lindau disease (VHL) patients develop highly vascular tumors, including central nervous system hemangioblastomas. It has been hypothesized that the vascular nature of these tumors is the product of reactive angiogenesis. However, recent data indicate that VHL-associated hemangioblastoma neoplastic cells originate from embryologically-arrested hemangioblasts capable of blood and endothelial cell differentiation. To determine the origin of tumor vasculature in VHL-associated hemangioblastomas, we analyzed the vascular elements in tumors from VHL patients. We demonstrate that isolated vascular structures and blood vessels within VHL-associated hemangioblastomas are a result of tumor-derived vasculogenesis. Further, similar to hemangioblastomas, we demonstrate that other VHL-associated lesions possess vascular tissue of tumor origin and that tumor-derived endothelial cells emerge within implanted VHL deficient UMRC6 RCC murine xenografts. These findings further establish the embryologic, developmentally arrested, hemangioblast as the tumor cell of origin for VHL-associated hemangioblastomas and indicate that it is also the progenitor cell for other VHL-associated tumors.

EAF2 loss enhances angiogenic effects of Von Hippel-Lindau heterozygosity on the murine liver and prostate

Von Hippel-Lindau (VHL) disease results from the inactivation of the VHL gene and is characterized by highly vascular tumors. A consequence of VHL loss is the stabilization of hypoxia-inducible factor (HIF) alpha subunits and increased expression of HIF target genes, which include pro-angiogenic growth factors such as vascular endothelial growth factor (VEGF). In mice, homozygous deletion of VHL is embryonic lethal due to vascular abnormalities in the placenta; and, VHL^+/− mice develop proliferative vascular lesions in several major organs, most prominently the liver. Loss of ELL-associated factor (EAF2) in murine models has also been shown to induce increased vascular density in the liver as well as the prostate. Previously, EAF2 was determined to be a binding partner of VHL and loss of EAF2 induced a reduction in pVHL levels and an increase in hypoxia induced factor 1α (HIF1α) levels in vitro. Here we characterized the cooperative effects of VHL- and EAF2-deficiency on angiogenesis in the liver and prostate of male mice. VHL deficiency consistently increased the incidence of hepatic vascular lesions across three mouse strains. These vascular lesions where characterized by an increase in microvessel density, and staining intensity of VHL target proteins HIF1α and VEGF. EAF2^−/−VHL^+/− mice had increased incidence of proliferative hepatic vascular lesions (4/4) compared to VHL^+/− (10/18) and EAF2^−/− (0/5) mice. Prostates of EAF2^−/−VHL^+/− mice also displayed an increase in microvessel density, as well as stromal inflammation and prostatic intraepithelial neoplasia. These results suggest that cooperation of VHL and EAF2 may be critical for angiogenic regulation of the liver and prostate, and concurrent loss of these two tumor suppressors may result in a pro-angiogenic phenotype.

VHL and PTEN loss coordinate to promote mouse liver vascular lesions

Von Hippel-Lindau (VHL) inactivation develops a tumor syndrome characterized by highly vascularized tumors as a result of hypoxia inducible factors (HIF) stabilization. The most common manifestation is the development of hemangioblastomas typically located in the central nervous system and other organs including the liver. PTEN (Phosphatase and tension homologue deleted on chromosome 10) inactivation also upregulates HIF-1alpha and may take part in promoting vascular lesions in tumors. The coordinate effect of loss of these tumor suppressors on HIF levels, and the subsequent effect on vascular lesion formation would elucidate the potential for mechanisms to modify HIF dosage supplementally and impact tumor phenotype. We therefore employed models of somatic conditional inactivation of Vhl, Pten, or both tumor suppressor genes in individual cells of the liver by Cre-loxP recombination to study the cooperativity of these two tumor suppressors in preventing tumor formation. Nine months after tumor suppressor inactivation, Vhl conditional deletion (Vhl (loxP/loxP)) mice showed no abnormalities, Pten conditional deletion (Pten (loxP/loxP)) mice developed liver steatosis and focal nodular expansion of hepatocytes containing lipid droplet and fat. Vhl and Pten conditional deletion (Vhl (loxP/loxP);Pten (loxP/loxP)) mice, however, developed multiple cavernous liver lesions reminiscent of hemangioblastoma. Liver hemangioblastomas in VHL disease may, therefore, require secondary mutation in addition to VHL loss of heterozygosity which is permissive for vascular lesion development or augments levels of HIF-1alpha.

VHL type 2B mutations retain VBC complex form and function

BACKGROUND

von Hippel-Lindau disease is characterized by a spectrum of hypervascular tumors, including renal cell carcinoma, hemangioblastoma, and pheochromocytoma, which occur with VHL genotype-specific differences in penetrance. VHL loss causes a failure to regulate the hypoxia inducible factors (HIF-1alpha and HIF-2alpha), resulting in accumulation of both factors to high levels. Although HIF dysregulation is critical to VHL disease-associated renal tumorigenesis, increasing evidence points toward gradations of HIF dysregulation contributing to the degree of predisposition to renal cell carcinoma and other manifestations of the disease.

METHODOLOGY/PRINCIPAL FINDINGS

This investigation examined the ability of disease-specific VHL missense mutations to support the assembly of the VBC complex and to promote the ubiquitylation of HIF. Our interaction analysis supported previous observations that VHL Type 2B mutations disrupt the interaction between pVHL and Elongin C but maintain partial regulation of HIF. We additionally demonstrated that Type 2B mutant pVHL forms a remnant VBC complex containing the active members ROC1 and Cullin-2 which retains the ability to ubiquitylate HIF-1alpha.

CONCLUSIONS

Our results suggest that subtypes of VHL mutations support an intermediate level of HIF regulation via a remnant VBC complex. These findings provide a mechanism for the graded HIF dysregulation and genetic predisposition for cancer development in VHL disease.