Hypoxia-inducible factor-1alpha (HIF1alpha) is a central regulator of the cellular response to hypoxia. Prolyl-hydroxylation of HIF1alpha by PHD enzymes is prerequisite for HIF1alpha degradation. Here, we demonstrate that the abundance of PHD1 and PHD3 are regulated via their targeting for proteasome-dependent degradation by the E3 ubiquitin ligases Siah1a/2, under hypoxia conditions. Siah2 null fibroblasts exhibit prolonged PHD3 half-life, resulting in lower levels of HIF1alpha expression during hypoxia. Significantly, hypoxia-induced HIF1alpha expression was completely inhibited in Siah1a/2 null cells, yet could be rescued upon inhibition of PHD3 by RNAi. Siah2 targeting of PHD3 for degradation increases upon exposure to even mild hypoxic conditions, which coincides with increased Siah2 transcription. Siah2 null mice subjected to hypoxia displayed an impaired hyperpneic respiratory response and reduced levels of hemoglobin. Thus, the control of PHD1/3 by Siah1a/2 constitutes another level of complexity in the regulation of HIF1alpha during hypoxia.

Defects in the structure or function of the primary cilium, an antennae-like structure whose functional integrity has been linked to the suppression of uncontrolled kidney epithelial cell proliferation, are a common feature of genetic disorders characterized by kidney cysts. However, the mechanisms by which primary cilia are maintained remain poorly defined. von Hippel-Lindau (VHL) disease is characterized by the development of premalignant renal cysts and arises because of functional inactivation of the VHL tumour suppressor gene product, pVHL. Here, we show that pVHL and glycogen synthase kinase (GSK)3beta are key components of an interlinked signalling pathway that maintains the primary cilium. Although inactivation of either pVHL or GSK3beta alone did not affect cilia maintenance, their combined inactivation leads to loss of cilia. In VHL patients, GSK3beta is subjected to inhibitory phosphorylation in renal cysts, but not in early VHL mutant lesions, and these cysts exhibit reduced frequencies of primary cilia. We propose that pVHL and GSK3beta function together in a ciliary-maintenance signalling network, disruption of which enhances the vulnerability of cells to lose their cilia, thereby promoting cyst formation.

The von Hippel-Lindau (VHL) tumor suppressor gene is mutated as an early event in almost all cases of clear cell renal cell carcinoma (ccRCC), the most frequent form of kidney cancer. In this review we discuss recent advances in understanding how dysregulation of the many hypoxia-inducible factor α-dependent and -independent functions of the VHL tumor suppressor protein (pVHL) can contribute to tumor initiation and progression. Recent evidence showing extensive inter- and intratumoral genetic diversity has given rise to the idea that ccRCC should actually be considered as a series of molecularly related, yet distinct, diseases defined by the pattern of combinatorial genetic alterations present within the cells of the tumor. We highlight the range of genetic and epigenetic alterations that recur in ccRCC and discuss the mechanisms through which these events appear to function cooperatively with a loss of pVHL function in tumorigenesis.

In patients with von Hippel-Lindau (VHL) disease, renal cysts and clear cell renal cell carcinoma (ccRCC) arise from renal tubular epithelial cells containing biallelic inactivation of the VHL tumour suppressor gene. However, it is presumed that formation of renal cysts and their conversion to ccRCC involve additional genetic changes at other loci. Here, we show that cystic lesions in the kidneys of patients with VHL disease also demonstrate activation of the phosphatidylinositol-3-kinase (PI3K) pathway. Strikingly, combined conditional inactivation of Vhlh and the Pten tumour suppressor gene, which normally antagonises PI3K signalling, in the mouse kidney, elicits cyst formation after short latency, whereas inactivation of either tumour suppressor gene alone failed to produce such a phenotype. Interestingly, cells lining these cysts frequently lack a primary cilium, a microtubule-based cellular antenna important for suppression of uncontrolled kidney epithelial cell proliferation and cyst formation. Our results support a model in which the PTEN tumour suppressor protein cooperates with pVHL to suppress cyst development in the kidney.

Mutational inactivation of VHL is the earliest genetic event in the majority of clear cell renal cell carcinomas (ccRCC), leading to accumulation of the HIF-1α and HIF-2α transcription factors. While correlative studies of human ccRCC and functional studies using human ccRCC cell lines have implicated HIF-1α as an inhibitor and HIF-2α as a promoter of aggressive tumour behaviours, their roles in tumour onset have not been functionally addressed. Herein we show using an autochthonous ccRCC model that Hif1a is essential for tumour formation whereas Hif2a deletion has only minor effects on tumour initiation and growth. Both HIF-1α and HIF-2α are required for the clear cell phenotype. Transcriptomic and proteomic analyses reveal that HIF-1α regulates glycolysis while HIF-2α regulates genes associated with lipoprotein metabolism, ribosome biogenesis and E2F and MYC transcriptional activities. HIF-2α-deficient tumours are characterised by increased antigen presentation, interferon signalling and CD8 + T cell infiltration and activation. Single copy loss of HIF1A or high levels of HIF2A mRNA expression correlate with altered immune microenvironments in human ccRCC. These studies reveal an oncogenic role of HIF-1α in ccRCC initiation and suggest that alterations in the balance of HIF-1α and HIF-2α activities can affect different aspects of ccRCC biology and disease aggressiveness.

Error-free mitosis depends on fidelity-monitoring checkpoint systems that ensure correct temporal and spatial coordination of chromosome segregation by the microtubule spindle apparatus. Defects in these checkpoint systems can lead to genomic instability, an important aspect of tumorigenesis. Here we show that the von Hippel-Lindau (VHL) tumour suppressor protein, pVHL, which is inactivated in hereditary and sporadic forms of renal cell carcinoma, localizes to the mitotic spindle in mammalian cells and its functional inactivation provokes spindle misorientation, spindle checkpoint weakening and chromosomal instability. Spindle misorientation is linked to unstable astral microtubules and is supressed by the restoration of wild-type pVHL in pVHL-deficient cells, but not in naturally-occurring VHL disease mutants that are defective in microtubule stabilization. Impaired spindle checkpoint function and chromosomal instability are the result of reduced Mad2 (mitotic arrest deficient 2) levels actuated by pVHL-inactivation and are rescued by re-expression of either Mad2 or pVHL in VHL-defective cells. An association between VHL inactivation, reduced Mad2 levels and increased aneuploidy was also found in human renal cancer, implying that the newly identified functions of pVHL in promoting proper spindle orientation and chromosomal stability probably contribute to tumour suppression.

Clear cell renal cell carcinomas (ccRCC) frequently exhibit inactivation of the VHL tumour suppressor gene and often harbour multiple copy number alterations in genes that regulate cell cycle progression. We show here that modelling these genetic alterations by combined renal epithelium-specific deletion of Vhl, Trp53 and Rb1 in mice caused ccRCC. These tumours arose from proximal tubule epithelial cells and shared molecular markers and mRNA expression profiles with human ccRCC. Exome sequencing revealed that mouse and human ccRCCs exhibit recurrent mutations in genes associated with the primary cilium, uncovering a mutational convergence on this organelle and implicating a subset of ccRCCs as genetic ciliopathies. Different mouse tumours responded differently to standard therapies for advanced human ccRCC, mimicking the range of clinical behaviours in the human disease. Inhibition of HIF-α transcription factors with Acriflavine as third line therapy had therapeutic effects in some tumours, providing pre-clinical evidence for further investigation of HIF-α inhibition as a ccRCC treatment. This autochthonous mouse ccRCC model represents a tool to investigate the biology of ccRCC and to identify new treatment strategies.

The Drosophila SINA (seven in absentia) protein and its mammalian orthologs (Siah, seven in absentia homolog) are RING domain proteins that function in E3 ubiquitin ligase complexes and facilitate ubiquitination and degradation of a wide range of cellular proteins, including ␤-catenin. Despite these diverse targets, the means by which SINA͞Siah recognize substrates or binding proteins has remained unknown. Here we identify a peptide motif (RPVAxVxPxxR) that mediates the interaction of Siah protein with a range of protein partners. Sequence alignment and mutagenesis scanning revealed residues that are important to this interaction. This consensus sequence correctly predicted a high-affinity interaction with a peptide from the cytoskeletal protein plectin-1 (residues 95-117). The unusually high-affinity binding obtained with a 23-residue peptide (K Dapp ‫؍‬ 29 nM with SINA) suggests that it may serve as a useful dominant negative reagent for SINA͞Siah proteins.