Proteasome-dependent degradation of transcription factor activating enhancer-binding protein 4 (TFAP4) controls mitotic division.

TFAP4, a basic helix-loop-helix transcription factor that regulates the expression of a multitude of genes involved in the regulation of cellular proliferation, stemness, and epithelial-mesenchymal transition, is up-regulated in colorectal cancer and a number of other human malignancies. We have found that, during the G2 phase of the cell division cycle, TFAP4 is targeted for proteasome-dependent degradation by the SCF(ßTrCP) ubiquitin ligase. This event requires phosphorylation of TFAP4 on a conserved degron. Expression of a stable TFAP4 mutant unable to interact with ßTrCP results in a number of mitotic defects, including chromosome missegregation and multipolar spindles, which eventually lead to the activation of the DNA damage response. Our findings reveal that ßTrCP-dependent degradation of TFAP4 is required for the fidelity of mitotic division.

Degradation of Tiam1 by casein kinase 1 and the SCFßTrCP ubiquitin ligase controls the duration of mTOR-S6K signaling.

Tiam1 (T-cell lymphoma invasion and metastasis 1) is a guanine nucleotide exchange factor that specifically controls the activity of the small GTPase Rac, a key regulator of cell adhesion, proliferation, and survival. Here, we report that in response to mitogens, Tiam1 is degraded by the ubiquitin-proteasome system via the SCF(ßTrCP) ubiquitin ligase. Mitogenic stimulation triggers the binding of Tiam1 to the F-box protein ßTrCP via its degron sequence and subsequent Tiam1 ubiquitylation and proteasomal degradation. The proteolysis of Tiam1 is prevented by ßTrCP silencing, inhibition of CK1 and MEK, or mutation of the Tiam1 degron site. Expression of a stable Tiam1 mutant that is unable to interact with ßTrCP results in sustained activation of the mTOR/S6K signaling and increased apoptotic cell death. We propose that the SCF(ßTrCP)-mediated degradation of Tiam1 controls the duration of the mTOR-S6K signaling pathway in response to mitogenic stimuli.

Differential dysregulation of ß-TrCP1 and -2 by HIV-1 Vpu leads to inhibition of canonical and non-canonical NF-κB pathways in infected cells.

The HIV-1 Vpu protein is expressed late in the virus lifecycle to promote infectious virus production and avoid innate and adaptive immunity. This includes the inhibition of the NF-κB pathway which, when activated, leads to the induction of inflammatory responses and the promotion of antiviral immunity. Here we demonstrate that Vpu can inhibit both canonical and non-canonical NF-κB pathways, through the direct inhibition of the F-box protein ß-TrCP, the substrate recognition portion of the Skp1-Cul1-F-box (SCF)ß-TrCP ubiquitin ligase complex. There are two paralogues of ß-TrCP (ß-TrCP1/BTRC and ß-TrCP2/FBXW11), encoded on different chromosomes, which appear to be functionally redundant. Vpu, however, is one of the few ß-TrCP substrates to differentiate between the two paralogues. We have found that patient-derived alleles of Vpu, unlike those from lab-adapted viruses, trigger the degradation of ß-TrCP1 while co-opting its paralogue ß-TrCP2 for the degradation of cellular targets of Vpu, such as CD4. The potency of this dual inhibition correlates with stabilization of the classical IκBα and the phosphorylated precursors of the mature DNA-binding subunits of canonical and non-canonical NF-κB pathways, p105/NFκB1 and p100/NFκB2, in HIV-1 infected CD4+ T cells. Both precursors act as alternative IκBs in their own right, thus reinforcing NF-κB inhibition at steady state and upon activation with either selective canonical or non-canonical NF-κB stimuli. These data reveal the complex regulation of NF-κB late in the viral replication cycle, with consequences for both the pathogenesis of HIV/AIDS and the use of NF-κB-modulating drugs in HIV cure strategies. IMPORTANCE The NF-κB pathway regulates host responses to infection and is a common target of viral antagonism. The HIV-1 Vpu protein inhibits NF-κB signaling late in the virus lifecycle, by binding and inhibiting ß-TrCP, the substrate recognition portion of the ubiquitin ligase responsible for inducing IκB degradation. Here we demonstrate that Vpu simultaneously inhibits and exploits the two different paralogues of ß-TrCP by triggering the degradation of ß-TrCP1 and co-opting ß-TrCP2 for the destruction of its cellular targets. In so doing, it has a potent inhibitory effect on both the canonical and non-canonical NF-κB pathways. This effect has been underestimated in previous mechanistic studies due to the use of Vpu proteins from lab-adapted viruses. Our findings reveal previously unappreciated differences in the ß-TrCP paralogues, revealing functional insights into the regulation of these proteins. This study also raises important implications for the role of NF-κB inhibition in the immunopathogenesis of HIV/AIDS and the way that this may impact on HIV latency reversal strategies based on the activation of the non-canonical NF-κB pathway.

TRIM67 Suppresses TNFalpha-Triggered NF-kB Activation by Competitively Binding Beta-TrCP to IkBa.

The transcription factor NF-κB plays an important role in modulation of inflammatory pathways, which are associated with inflammatory diseases, neurodegeneration, apoptosis, immune responses, and cancer. Increasing evidence indicates that TRIM proteins are crucial role in the regulation of NF-κB signaling pathways. In this study, we identified TRIM67 as a negative regulator of TNFα-triggered NF-κB activation. Ectopic expression of TRIM67 significantly represses TNFα-induced NF-κB activation and the expression of pro-inflammatory cytokines TNFα and IL-6. In contrast, Trim67 depletion promotes TNFα-induced expression of TNFα, IL-6, and Mcp-1 in primary mouse embryonic fibroblasts. Mechanistically, we found that TRIM67 competitively binding ß-transducin repeat-containing protein (ß-TrCP) to IκBα results inhibition of ß-TrCP-mediated degradation of IκBα, which finally caused inhibition of TNFα-triggered NF-κB activation. In summary, our findings revealed that TRIM67 function as a novel negative regulator of NF-κB signaling pathway, implying TRIM67 might exert an important role in regulation of inflammation disease and pathogen infection caused inflammation.

miR-224 targets BTRC and promotes cell migration and invasion in colorectal cancer.

Our study aims to investigate the impact of miR-224 on cell migration and invasion in colorectal cancer (CRC) as well as its molecular mechanisms. The results showed that miR-224 was significantly upregulated in CRC compared to normal tissues via the TCGA database. Overexpression of miR-224 promoted CRC cell migration and invasion, while inhibition of miR-224 demonstrated the opposite result via transwell assays. In addition, we found that BTRC was a target gene of miR-224 through the miRecords database and dual-luciferase assay, while western blot together with RT-qPCR showed that inhibition of miR-224 led to elevated BTRC expression in protein level but not in mRNA level, and also decreased the expression of ß-catenin. In reference to the Human Protein Atlas, BTRC protein expression was higher in normal tissues than in CRC tissues. In conclusion, miR-224 regulates its target BTRC protein expression and its related Wnt/ß-catenin pathway. Its impact on cell migration and invasion in CRC cells suggested that miR-224 could be a prospective therapeutic target for early-stage non-metastatic CRC.

Ultraviolet B irradiation induced Nrf2 degradation occurs via activation of TRPV1 channels in human dermal fibroblasts.

Ultraviolet (UV) irradiation causes cellular oxidative stress. Under redox imbalance, Keap1-dependent Nrf2 degradation is minimal. In this study, we examined the role of Ca2+ in Nrf2 homeostasis after UVB irradiation using human dermal fibroblasts. UVB irradiation stimulates 12-lipoxygenase and the product 12-hydroxyeicosatetraenoic acid then activates TRPV1 increasing the cell's cytosolic Ca2+ concentration. UVB irradiation induced reactive oxygen species generation and apoptosis are inhibited in the absence of Ca2+ or in the presence of either a 12-lipoxygenase inhibitor or a TRPV1 inhibitor during and after UVB irradiation. Thus, the Ca2+ increase via TRPV1 is a critical factor in UVB irradiation induced oxidative stress. UVB irradiation induces a Ca2+ dependent Nrf2 degradation and thus activation of TRPV1 with 12-hydroxyeicosatetraenoic acid also decreasing Nrf2 levels. UVB irradiation induced Nrf2 degradation is inhibited by co-treatment of cells with W-7, cyclosporin A, SB-216763 or MG-132, which are inhibitors of calmodulin, calcineurin, GSK3ß and the proteasome, respectively. Furthermore, UVB irradiation in parallel induces GSK3ß dephosphorylation in a Ca2+ dependent manner. Co-immunoprecipitation showed that UVB irradiation induces an increase in Nrf2 phosphorylation, an increase in the binding of ß-TrCP and Nrf2, and an increase in Nrf2 ubiquitination; these effects are all Ca2+ dependent. These findings suggest that UVB irradiation induced GSK3ß activation in a Ca2+ dependent manner, which then stimulates the phosphorylation and ubiquitination of Nrf2 via ß-TrCP. Indeed, silencing of ß-TrCP was found to inhibit UVB irradiation-induced oxidative stress, Nrf2 degradation and apoptosis, while it had no effect on the Ca2+ increase. Taken together, our results suggest that a Ca2+ influx via TRPV1 is responsible for UVB irradiation-induced Nrf2 degradation and that modulation of the Ca2+-calmodulin-calcineurin-GSK3ß-Nrf2-ß-TrCP-Cullin-1 pathway may explain Ca2+ dependent Nrf2 degradation.

The Tumor Suppressor MIG6 Controls Mitotic Progression and the G2/M DNA Damage Checkpoint by Stabilizing the WEE1 Kinase.

MIG6 is an important tumor suppressor that binds to and negatively regulates epidermal growth factor receptor (EGFR). Here, we report an EGFR-independent function for MIG6 as an integral component of the cell cycle machinery. We found that depletion of MIG6 causes accelerated entry into and delayed exit from mitosis. This is due to premature and prolonged activation of CDK1, a key regulator of mitotic progression at the G2/M and meta- and anaphase transitions. Furthermore, MIG6 is required for inhibition of CDK1 upon DNA damage and subsequent G2/M cell cycle arrest. Mechanistically, we found that MIG6 depletion results in reduced phosphorylation of CDK1 on the inhibitory WEE1-targeted tyrosine-15 residue. MIG6 interacts with WEE1 and promotes its stability by interfering with the recruitment of the ßTrCP-SCF E3 ubiquitin ligase and consequent proteasomal degradation of WEE1. Our findings uncover a critical role of MIG6 in cell cycle progression that is likely to contribute to its potent tumor-suppressive properties.