NF-κB-Induced R-Loops and Genomic Instability in HTLV-1-Infected and Adult T-Cell Leukemia Cells.

Human T-cell leukemia virus type 1 (HTLV-1) is a human delta retrovirus that causes adult T-cell leukemia/lymphoma (ATL) in 3-5% of the infected population after decades of clinical latency. HTLV-1 Tax is a potent activator of IKK/NF-κB and a clastogen. While NF-κB activities are associated with cell survival and proliferation, constitutive NF-κB activation (NF-κB hyperactivation) by Tax leads to senescence and oncogenesis.

NF-κB-induced R-loop accumulation and DNA damage select for nucleotide excision repair deficiencies in adult T cell leukemia.

Constitutive NF-κB activation (NF-κB) confers survival and proliferation advantages to cancer cells and frequently occurs in T/B cell malignancies including adult T cell leukemia (ATL) caused by human T-cell leukemia virus type 1 (HTLV-1). Counterintuitively, NF-κB by the HTLV-1 transactivator/oncoprotein Tax induces a senescence response, and HTLV-1 infections in culture mostly result in senescence or cell-cycle arrest due to NF-κB How NF-κB induces senescence, and how ATL cells maintain NF-κB and avert senescence, remain unclear. Here we report that NF-κB by Tax increases R-loop accumulation and DNA double-strand breaks, leading to senescence.

RNF8 Dysregulation and Down-regulation During HTLV-1 Infection Promote Genomic Instability in Adult T-Cell Leukemia.

The genomic instability associated with adult T cell leukemia/lymphoma (ATL) is causally linked to Tax, the HTLV-1 viral oncoprotein, but the underlying mechanism is not fully understood. We have previously shown that Tax hijacks and aberrantly activates ring finger protein 8 (RNF8) - a lysine 63 (K63)-specific ubiquitin E3 ligase critical for DNA double-strand break (DSB) repair signaling - to assemble K63-linked polyubiquitin chains (K63-pUbs) in the cytosol. Tax and the cytosolic K63-pUbs, in turn, initiate additional recruitment of linear ubiquitin assembly complex (LUBAC) to produce hybrid K63-M1 pUbs, which trigger a kinase cascade that leads to canonical IKK:NF-κB activation.

OTUB1 non-catalytically stabilizes the E2 ubiquitin-conjugating enzyme UBE2E1 by preventing its autoubiquitination.

OTUB1 is a deubiquitinating enzyme that cleaves Lys-48-linked polyubiquitin chains and also regulates ubiquitin signaling through a unique, noncatalytic mechanism. OTUB1 binds to a subset of E2 ubiquitin-conjugating enzymes and inhibits their activity by trapping the E2∼ubiquitin thioester and preventing ubiquitin transfer. The same set of E2s stimulate the deubiquitinating activity of OTUB1 when the E2 is not charged with ubiquitin.

SUMO-Targeted Ubiquitin Ligases (STUbLs) Reduce the Toxicity and Abnormal Transcriptional Activity Associated With a Mutant, Aggregation-Prone Fragment of Huntingtin.

Cell viability and gene expression profiles are altered in cellular models of neurodegenerative disorders such as Huntington's Disease (HD). Using the yeast model system, we show that the SUMO-targeted ubiquitin ligase (STUbL) Slx5 reduces the toxicity and abnormal transcriptional activity associated with a mutant, aggregation-prone fragment of huntingtin (Htt), the causative agent of HD. We demonstrate that expression of an aggregation-prone Htt construct with 103 glutamine residues (103Q), but not the non-expanded form (25Q), results in severe growth defects in Δ and Δ cells.

SUMO-Targeted Ubiquitin Ligase (STUbL) Slx5 regulates proteolysis of centromeric histone H3 variant Cse4 and prevents its mislocalization to euchromatin.

Centromeric histone H3, CENP-A, is essential for faithful chromosome segregation. Stringent regulation of cellular levels of CENP-A restricts its localization to centromeres. Mislocalization of CENP-A is associated with aneuploidy in yeast, flies and tumorigenesis in human cells; thus, defining pathways that regulate CENP-A levels is critical for understanding how mislocalization of CENP-A contributes to aneuploidy in human cancers.

A SUMO-targeted ubiquitin ligase is involved in the degradation of the nuclear pool of the SUMO E3 ligase Siz1.

The Slx5/Slx8 heterodimer constitutes a SUMO-targeted ubiquitin ligase (STUbL) with an important role in SUMO-targeted degradation and SUMO-dependent signaling. This STUbL relies on SUMO-interacting motifs in Slx5 to aid in substrate targeting and carboxy-terminal RING domains in both Slx5 and Slx8 for substrate ubiquitylation. In budding yeast cells, Slx5 resides in the nucleus, forms distinct foci, and can associate with double-stranded DNA breaks.

The SUMO E3 ligase Siz2 exerts a locus-dependent effect on gene silencing in Saccharomyces cerevisiae.

In the yeast Saccharomyces cerevisiae, the two silent mating-type loci and subtelomeric regions are subjected to a well-characterized form of gene silencing. Establishment of silencing involves the formation of a distinct chromatin state that is refractory to transcription. This structure is established by the action of silent information regulator proteins (Sir2, Sir3, and Sir4) that bind to nucleosomes and initiate the deacetylation of multiple lysine residues in histones H3 and H4.