K63-ubiquitylation of VHL by SOCS1 mediates DNA double-strand break repair

DNA repair is essential for maintaining genomic stability, and defects in this process significantly increase the risk of cancer. Clear-cell renal cell carcinoma (CCRCC) caused by inactivation of the von Hippel-Lindau (VHL) tumor suppressor gene is characterized by high genomic instability. However, the molecular mechanism underlying the association between the loss of VHL and genomic instability remains unclear. Here, we show that suppressor of cytokine signaling 1 (SOCS1) promotes nuclear redistribution and K63-ubiquitylation of VHL in response to DNA double-strand breaks (DSBs). Loss of VHL or VHL mutations that compromise its K63-ubiquitylation attenuates the DNA-damage response (DDR), resulting in decreased homologous recombination repair and persistence of DSBs. These results identify VHL as a component of the DDR network, inactivation of which contributes to the genomic instability associated with CCRCC.

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.

Loss of JAK2 regulation via a heterodimeric VHL-SOCS1 E3 ubiquitin ligase underlies Chuvash polycythemia

Chuvash polycythemia is a rare congenital form of polycythemia caused by homozygous R200W and H191D mutations in the VHL (von Hippel-Lindau) gene, whose gene product is the principal negative regulator of hypoxia-inducible factor. However, the molecular mechanisms underlying some of the hallmark abnormalities of Chuvash polycythemia, such as hypersensitivity to erythropoietin, are unclear. Here we show that VHL directly binds suppressor of cytokine signaling 1 (SOCS1) to form a heterodimeric E3 ligase that targets phosphorylated JAK2 (pJAK2) for ubiquitin-mediated destruction. In contrast, Chuvash polycythemia-associated VHL mutants have altered affinity for SOCS1 and do not engage with and degrade pJAK2. Systemic administration of a highly selective JAK2 inhibitor, TG101209, reversed the disease phenotype in Vhl(R200W/R200W) knock-in mice, an experimental model that recapitulates human Chuvash polycythemia. These results show that VHL is a SOCS1-cooperative negative regulator of JAK2 and provide biochemical and preclinical support for JAK2-targeted therapy in individuals with Chuvash polycythemia.

Abstract 3123: Differential effects of polyoma virus middle tumor antigen mutants upon gap junctional, intercellular communication

Gap junctions are channels that connect the cytoplasm of adjacent cells. Gap junctional, intercellular communication (GJIC) is blocked in cells transformed by oncogenes such as the middle Tumor antigen of polyoma virus (mT), an oncoprotein which associates with and is tyrosine-phosphorylated by cSrc family members. Specific phosphotyrosines provide docking sites for the phosphotyrosine binding domain of Shc (mT-tyr250) and the SH2 domain of the phosphatidylinositol 3-kinase (mT-tyr315), resulting in the activation of their downstream signaling cascades, Ras/Raf/Erk and PI-3 kinase/Akt, respectively.

To examine the effect of these mT-initiated pathways upon gap junctional communication and neoplasia, GJIC was assessed in mT-mutant-expressing, rat liver epithelial T51B cells which normally have extensive GJIC, using a novel technique of in situ electroporation we developed. The results show that although low levels of wt-mT are sufficient to interrupt gap junctional communication, GJIC suppression still requires an intact tyr250 site, that is activation of the Ras pathway. In sharp contrast, activation of the PI-3 kinase pathway is not required for GJIC suppression. It is remarkable that T51B cells expressing a mutant deficient in binding of Shc, hence activation of the Ras pathway, are still morphologically transformed and do grow into tumors in syngeneic rats, albeit with an altered morphology. These results indicate that GJIC suppression requires Ras/Erk but not PI-3 kinase/Akt signalling, and is independent of neoplastic conversion in rat liver epithelial cells.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 3123. doi:10.1158/1538-7445.AM2011-3123