Parallel Regulation of von Hippel-Lindau Disease by pVHL-Mediated Degradation of B-Myb and Hypoxia-Inducible Factor α

Abstract

U biquitin-mediated proteolysis by the 26S proteasome plays an important role in the elimination of short-lived proteins (1), including those involved in cell cycle progression, cellular signaling in response to environmental stress or extracellular ligands, morphogenesis, secretion, DNA repair, and organelle biogenesis (2, 3). The pathway consists of two key steps, namely, the covalent attachment of multiple ubiquitin molecules to a target protein and the degradation of the ubiquitinated protein by the 26S proteasome complex. Several components act in concert to attach ubiquitin to a target protein, including a ubiquitin-activating enzyme (E1), a ubiquitin-conjugating enzyme (E2), and a ubiquitin-protein isopeptide ligase (E3). E3 is directly responsible for substrate recognition. On the basis of structural similarity, E3 enzymes are classified into three families: the HECT (homologous to E6-AP COOH terminus) family, the U-box family, and the RING finger-containing protein family.The elongin B and C-Cul2 or Cul5-SOCS box protein (ECS) family belongs to the cullin RING ligase (CRL) superfamily (4). pVHL, the protein product of the von Hippel-Lindau (VHL) tumor suppressor gene, is a member of the ECS family. pVHL forms a complex with elongins B and C, Cul2, and the RING finger protein Rbx1 (5, 6). The CRL2 pVHL complex has ubiquitin ligase activity and targets the hypoxia-inducible factor ␣ (HIF-␣) family of transcription factors (HIF-1 to -3␣) for proteasomal degradation (7). At normal oxygen levels, proline residues in the LXXLAP sequence motif of HIF-␣ proteins are hydroxylated by three prolyl hydroxylases (PHD1 to -3), and an in-depth study revealed that PHD2 is a critical enzyme for the hydroxylation of HIF-1␣ (8, 9). Hydroxylated HIF-␣ is targeted by pVHL for polyubiquitination and proteasomal degradation (10-12). Under conditions of hypoxia (low oxygen level), HIF-␣ is not hydroxylated by PHDs and is therefore not recognized or targeted for degradation by pVHL. The unhydroxylated HIF-␣ dimerizes with constitutively expressed HIF-1␤, also known as an aryl hydrocarbon receptor nuclear translocator (ARNT), and translocates to the nucleus, where it induces the transcription of downstream target genes, including the genes coding for vascular endothelial growth factor A (VEGFA), solute carrier family 2 member 1 (SLC2A1; also known as GLUT1), and platelet-derived growth factor (PDGF) (13). Loss of functional pVHL protein prevents the O 2 -dependent degradation of HIF-␣, resulting in constitutive expression of HIF-dependent genes and VHL disease, which is characterized by a variety of lesions, including hemangioblastomas, renal clear cell carcinomas, pheochromocytomas, pancreatic islet cell tumors, endolymphatic sac tumors, and papillary cystadenomas of the broad ligament (females) and epididymis (males) (13).Studies showing that heterozygous pVHL ϩ/Ϫ mice are phenotypically normal and VHL Ϫ/Ϫ mice die at embryonic day 10.5 (E10.5) to E12.5 (14), together with the existence of many pVHLinteracting proteins (13), a...

Hypoxia-inducible factor-2α stabilizes the von Hippel-Lindau (VHL) disease suppressor, Myb-related protein 2

“…Although it is well known that pVHL targets HIF-2α for proteasomal degradation [13,14], HIF-2α protein levels increased even in pVHL-deficient 786-O cells after treatment with the proteasome inhibitors MG132 and lactacystin, suggesting that HIF-2α might be a substrate of other ubiquitin ligases (Fig 1C and 1D). B-Myb protein levels also increased in 786-O cells after MG132 or lactacystin treatment, suggesting that B-Myb is also regulated by ubiquitin-mediated proteolysis in the absence of pVHL, as suggested previously [11]. Importantly, proteasome inhibition with MG132 treatment nearly completely prevented the downregulation of B-Myb induced by HIF-2α knockdown (Fig 1C).…”

COX-2/PGE2 Axis Regulates HIF2α Activity to Promote Hepatocellular Carcinoma Hypoxic Response and Reduce the Sensitivity of Sorafenib Treatment

“…While p-VHL plays an important role in the recognition and subsequent degradation of HIF-2α, nuclear translocation is another crucial means of executing its transcriptional activity (42,43). Our data first provided the evidence that the COX-2/PGE2 axis regulates HIF-2α levels through a p-VHL-mediated degradation pathway during posttranscriptional processing but not at the transcription level.…”

The Tumor Suppressor Roles of MYBBP1A, a Major Contributor to Metabolism Plasticity and Stemness

“…It has been described that all members of the MYB family, including c-MYB, interact with pVHL [37]. B-MYB is also targeted by pVHL for degradation via the proteasome [19,81,82]. Altogether, these results suggest that the absence of pVHL is needed to achieve an appropriately regulated expression of c-MYB, which is a key factor involved in the MYBBP1A downregulation response.…”