Arnim Pause

Inhibition of transcription elongation by the VHL tumor suppressor protein

Germline mutations in the von Hippel-Lindau tumor suppressor gene (VHL) predispose individuals to a variety of tumors, including renal carcinoma, hemangioblastoma of the central nervous system, and pheochromocytoma. Here, a cellular transcription factor, Elongin (SIII), is identified as a functional target of the VHL protein. Elongin (SIII) is a heterotrimer consisting of a transcriptionally active subunit (A) and two regulatory subunits (B and C) that activate transcription elongation by RNA polymerase II. The VHL protein was shown to bind tightly and specifically to the Elongin B and C subunits and to inhibit Elongin (SIII) transcriptional activity in vitro. These findings reveal a potentially important transcriptional regulatory network in which the VHL protein may play a key role.

Transcription-Dependent Nuclear-Cytoplasmic Trafficking Is Required for the Function of the von Hippel-Lindau Tumor Suppressor Protein

ABSTRACT 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.

The RNA-Binding Protein HuR Promotes Cell Migration and Cell Invasion by Stabilizing the β-actin mRNA in a U-Rich-Element-Dependent Manner

ABSTRACT A high expression level of the β-actin protein is required for important biological mechanisms, such as maintaining cell shape, growth, and motility. Although the elevated cellular level of the β-actin protein is directly linked to the long half-life of its mRNA, the molecular mechanisms responsible for this effect are unknown. Here we show that the RNA-binding protein HuR stabilizes the β-actin mRNA by associating with a uridine-rich element within its 3′ untranslated region. Using RNA interference to knock down the expression of HuR in HeLa cells, we demonstrate that HuR plays an important role in the stabilization but not in the nuclear/cytoplasmic distribution of the β-actin mRNA. HuR depletion in HeLa cells alters key β-actin-based cytoskeleton functions, such as cell adhesion, migration, and invasion, and these defects correlate with a loss of the actin stress fiber network. Together our data establish that the posttranscriptional event involving HuR-mediated β-actin mRNA stabilization could be a part of the regulatory mechanisms responsible for maintaining cell integrity, which is a prerequisite for avoiding transformation and tumor formation.

Mammalian mediator subunit mMED8 is an Elongin BC-interacting protein that can assemble with Cul2 and Rbx1 to reconstitute a ubiquitin ligase.

The heterodimeric Elongin BC complex has been shown to interact in vitro and in cells with a conserved BC-box motif found in an increasing number of proteins including RNA polymerase II elongation factor Elongin A, suppressor of cytokine signaling (SOCS)-box proteins, and the von Hippel–Lindau tumor suppressor protein. Recently, the Elongin BC complex was found to function as an adaptor that links these BC-box proteins to a module composed of Cullin family members Cul2 or Cul5 and RING-H2 finger protein Rbx1 to reconstitute a family of E3 ubiquitin ligases that activate ubiquitylation by the E2 ubiquitin-conjugating enzyme Ubc5. As part of our effort to understand the functions of Elongin BC-based ubiquitin ligases, we exploited a modified yeast two-hybrid screen to identify a mammalian BC-box protein similar in sequence to Saccharomyces cerevisiae Mediator subunit Med8p. In this report we demonstrate (i) that mammalian MED8 is a subunit of the mammalian Mediator complex and (ii) that MED8 can assemble with Elongins B and C, Cul2, and Rbx1 to reconstitute a ubiquitin ligase. Taken together, our findings are consistent with the model that MED8 could function to recruit ubiquitin ligase activity directly to the RNA polymerase II transcriptional machinery.

INTERACTION OF VON HIPPEL-LINDAU TUMOR SUPPRESSOR GENE PRODUCT WITH ELONGIN

Publisher Summary This chapter discusses the interaction of von Hippel-Lindau (VHL) tumor suppressor gene product with elongin. Germline mutations in the VHL tumor suppressor gene predispose affected individuals to a variety of tumors, including clear cell renal carcinoma, hemangioblastoma of the central nervous system, and pheochromocytoma. VHL gene inactivation is also associated with the majority (>75%) of sporadic renal cell carcinomas. The B and C regulatory subunits of cellular transcription factor elongin (SIII) have been identified as targets of the VHL protein. Elongin, a heterotrimeric complex of ll0-kDa (A), 18-kDa (B), and 15-kDa (C) subunits, activates transcription elongation by RNA polymerase II by suppressing transient pausing by polymerase at many sites within transcription units. The chapter describes the expression and purification of recombinant human VHL protein and recombinant elongin A, B, and C proteins, as well as methods for measuring their activity in the oligo(dC)-tailed template transcription assay and the adenovirus major late (AdML) transcription runoff assay.