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Kapinos et al. show that nuclear pore complex permeability and cargo release functionalities are concomitantly regulated by karyopherin occupancy and turnover in a systematic continuum. This highlights increasingly important roles for the soluble nucleocytoplasmic transport machinery that depart from established views of the nuclear pore complex selectivity mechanism., Nucleocytoplasmic transport is sustained by karyopherins (Kaps) and a Ran guanosine triphosphate (RanGTP) gradient that imports nuclear localization signal (NLS)–specific cargoes (NLS-cargoes) into the nucleus. However, how nuclear pore complex (NPC) barrier selectivity, Kap traffic, and NLS-cargo release are systematically linked and simultaneously regulated remains incoherent. In this study, we show that Kapα facilitates Kapβ1 turnover and occupancy at the NPC in a RanGTP-dependent manner that is directly coupled to NLS-cargo release and NPC barrier function. This is underpinned by the binding affinity of Kapβ1 to phenylalanine–glycine nucleoporins (FG Nups), which is comparable with RanGTP·Kapβ1, but stronger for Kapα·Kapβ1. On this basis, RanGTP is ineffective at releasing standalone Kapβ1 from NPCs. Depleting Kapα·Kapβ1 by RanGTP further abrogates NPC barrier function, whereas adding back Kapβ1 rescues it while Kapβ1 turnover softens it. Therefore, the FG Nups are necessary but insufficient for NPC barrier function. We conclude that Kaps constitute integral constituents of the NPC whose barrier, transport, and cargo release functionalities establish a continuum under a mechanism of Kap-centric control.

Cholesterol metabolism produces essential membrane components as well as metabolites with a variety of biological functions. In the tumour microenvironment, cell-intrinsic and cell-extrinsic cues reprogram cholesterol metabolism and consequently promote tumourigenesis. Cholesterol-derived metabolites play complex roles in supporting cancer progression and suppressing immune responses. Preclinical and clinical studies have shown that manipulating cholesterol metabolism inhibits tumour growth, reshapes the immunological landscape and reinvigorates anti-tumour immunity. Here, we review cholesterol metabolism in cancer cells, its role in cancer progression and the mechanisms through which cholesterol metabolites affect immune cells in the tumour microenvironment. We also discuss therapeutic strategies aimed at interfering with cholesterol metabolism, and how the combination of such approaches with existing anti-cancer therapies can have synergistic effects, thus offering new therapeutic opportunities.

Nuclear pore complexes (NPCs) mediate molecular transport between the nucleus and cytoplasm in eukaryotic cells. Tethered within each NPC lie numerous intrinsically disordered proteins known as FG nucleoporins (FG Nups) that are central to this process. Over two decades of investigation has converged on a view that a barrier mechanism consisting of FG Nups rejects non-specific macromolecules while promoting the speed and selectivity of karyopherin (Kaps) receptors (and their cargoes). Yet, the number of NPCs in the cell is exceedingly small compared to the number of Kaps, so that in fact there is a high likelihood the pores are always populated by Kaps. Here, we contemplate a view where Kaps actively participate in regulating the selectivity and speed of transport through NPCs. This so-called “Kap-centric” control of the NPC accounts for Kaps as essential barrier reinforcements that play a prerequisite role in facilitating fast transport kinetics. Importantly, Kap-centric control reconciles both mechanistic and kinetic requirements of the NPC, and in so doing potentially resolves incoherent aspects of FG-centric models. On this basis, we surmise that Kaps prime the NPC for nucleocytoplasmic transport by fine-tuning the NPC microenvironment according to the functional needs of the cell.

The Rac1/JNK cascade plays important roles in DNA damage-induced apoptosis. However, how this cascade is activated upon DNA damage remains to be fully understood. We show here that, in untreated cells, Tiam1, a Rac1-specific guanine nucleotide exchange factor, is phosphorylated by casein kinase 1 (CK1) at its C terminus, leading to Skp, Cullin, F-box-containing(β-TrCP) recognition, ubiquitination, and proteasome-mediated degradation. Upon DNA-damaging anticancer drug treatment, CK1/β-TrCP-mediated Tiam1 degradation is abolished, and the accumulated Tiam1 contributes to downstream activation of Rac1/JNK. Consistently, tumor cells overexpressing Tiam1 are hypersensitive to DNA-damaging drug treatment. In xenograft mice, Tiam1-high cells are more susceptible to doxorubicin treatment. Thus, our results uncover that inhibition of proteasome-mediated Tiam1 degradation is an upstream event leading to Rac1/JNK activation and cell apoptosis in response to DNA-damaging drug treatment.

Wnt/β-catenin signalling is widely implicated in embryogenesis, tissue homeostasis and tumorigenesis. The key event in Wnt signalling activation is β-catenin accumulation, which is controlled by both its production and degradation. However, much more emphasis has been placed on the understanding of its degradation. Here, we show that the synthesis of β-catenin protein, which requires a group of serine/arginine-rich splicing factors (SRSF), also contributes to its tumorigenic activity. Overexpression of SRSF1 and SRSF9 promote β-catenin accumulation via the recruitment of β-catenin mRNA and by enhancing its translation in an mTOR-dependent manner. We further demonstrate that, like SRSF1, SRSF9 is also an oncogene, and is frequently overexpressed in multiple types of human tumours. Finally, our results suggest that promoting degradation and blocking production of β-catenin synergistically reduce β-catenin levels under pathological conditions and that a combinational therapy could be a promising approach for the treatment of cancer patients.

The beta-catenin-lymphoid enhancer factor (LEF) protein complex is the key mediator of canonical Wnt signaling and initiates target gene transcription upon ligand stimulation. In addition to beta-catenin and LEF themselves, many other proteins have been identified as necessary cofactors. Here we report that the evolutionally conserved splicing factor and transcriptional co-regulator, SKIP/SNW/NcoA62, forms a ternary complex with LEF1 and HDAC1 and mediates the repression of target genes. Loss-of-function studies showed that SKIP is obligatory for Wnt signaling-induced target gene transactivation, suggesting an important role of SKIP in the canonical Wnt signaling. Consistent with its involvement in beta-catenin signaling, the C-terminally truncated forms of SKIP are able to stabilize beta-catenin and enhance Wnt signaling. In Xenopus embryos, both overexpression and knockdown of Skip lead to reduced neural crest induction, consistent with down-regulated Wnt signaling in both cases. Our results indicate that SKIP is a novel component of the beta-catenin transcriptional complex.

Sodium butyrate (NaBu) has been shown to induce erythroid cell differentiation. In this study, we investigated the mechanisms involved in NaBu-induced activation of erythroid-specific 5-aminolevulinate synthase gene (ALAS2). We showed that NaBu upregulated ALAS2 gene transcription in different cell lineages. By using site-directed mutagenesis of putative responsive elements at ALAS2 promoter and reporter gene analysis, we identified that the Sp1 binding sites within the ALAS2 promoter were responsive to NaBu stimulation. Results from the chromatin immunoprecipitation (ChIP) assays indicated that upon the NaBu stimulation, binding of Sp1 protein to ALAS2 promoter increased significantly, with concurrent increases in acetylation level of histone H3 and dimethylation level of H3-Lysine4 at ALAS2 promoter. Also, our data suggested that HDAC1 may be a target enzyme of NaBu action.