ATG5 attenuates inflammatory signaling in mouse embryonic stem cells to control differentiation

Attenuated inflammatory response is a property of embryonic stem cells (ESCs). However, the underlying mechanisms are unclear. Moreover, whether the attenuated inflammatory status is involved in ESC differentiation is also unknown. Here, we found that autophagy-related protein ATG5 is essential for both attenuated inflammatory response and differentiation of mouse ESCs and that attenuation of inflammatory signaling is required for mouse ESC differentiation. Mechanistically, ATG5 recruits FBXW7 to promote ubiquitination and proteasome-mediated degradation of β-TrCP1, resulting in the inhibition of nuclear factor κB (NF-κB) signaling and inflammatory response. Moreover, differentiation defects observed in ATG5-depleted mouse ESCs are due to β-TrCP1 accumulation and hyperactivation of NF-κB signaling, as loss of β-TrCP1 and inhibition of NF-κB signaling rescued the differentiation defects. Therefore, this study reveals a previously uncharacterized mechanism maintaining the attenuated inflammatory response in mouse ESCs and further expands the understanding of the biological roles of ATG5.

ATG5 nonautophagically regulates inflammation and differentiation in mouse embryonic stem cells

Embryonic stem cells (ESCs), with abilities of infinite proliferation (self-renewal) and to differentiate into distinct cell types (pluripotency), show attenuated inflammatory response against cytokines or pathogens, which is recognized as a unique characteristic of ESCs compared with somatic cells. However, the underlying molecular mechanisms remain unclear, and whether the attenuated inflammatory state is involved in ESC differentiation is completely unknown. Our recent study demonstrated that macroautophagy/autophagy-related protein ATG5 inhibits the inflammatory response of mouse ESCs (MmESCs) by promoting the degradation of BTRC/β-TrCP1 and further the downregulation of NFKB/NF-κB signaling. In addition, maintenance of an attenuated inflammation status in MmESCs is required for their differentiation. In conclusion, ATG5 is a key regulator for the regulation of inflammatory response and differentiation of MmESCs.

Insights into the role of HIV-1 Vpu in modulation of NF-ĸB signaling pathways

HIV-1 inhibits the activation of nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) to prevent the induction of a proinflammatory state but also activates the NF-κB pathway to promote viral transcription. Thus, optimal regulation of this pathway is important for the viral life cycle. In recent work, Pickering et al. (3) demonstrate that HIV-1 viral protein U has contrasting effects on the two distinct paralogs of β-transducin repeat-containing protein (β-TrCP1 and β-TrCP2) and that this interaction has important implications for the regulation of both the canonical and non-canonical NF-κB pathways. Additionally, the authors identified the viral requirements for the dysregulation of β-TrCP. In this commentary, we discuss how these findings further our understanding of how the NF-κB pathway functions during viral infection.

MK2 promotes Tfcp2l1 degradation via β-TrCP ubiquitin ligase to regulate mouse embryonic stem cell self-renewal

Tfcp2l1 can maintain mouse embryonic stem cell (mESC) self-renewal. However, it remains unknown how Tfcp2l1 protein stability is regulated. Here, we demonstrate that β-transducin repeat-containing protein (β-TrCP) targets Tfcp2l1 for ubiquitination and degradation in a mitogen-activated protein kinase (MAPK)-activated protein kinase 2 (MK2)-dependent manner. Specifically, β-TrCP1 and β-TrCP2 recognize and ubiquitylate Tfcp2l1 through the canonical β-TrCP-binding motif DSGDNS, in which the serine residues have been phosphorylated by MK2. Point mutation of serine-to-alanine residues reduces β-TrCP-mediated ubiquitylation and enhances the ability of Tfcp2l1 to promote mESC self-renewal while repressing the speciation of the endoderm, mesoderm, and trophectoderm. Similarly, inhibition of MK2 reduces the association of Tfcp2l1 with β-TrCP1 and increases the self-renewal-promoting effects of Tfcp2l1, whereas overexpression of MK2 or β-TrCP genes decreases Tfcp2l1 protein levels and induces mESC differentiation. Collectively, our study reveals a posttranslational modification of Tfcp2l1 that will expand our understanding of the regulatory network of stem cell pluripotency.

β-TrCP1-variant 4, a novel splice variant of β-TrCP1, is a negative regulator of β-TrCP1-variant 1 in β-catenin degradation

β-transducin repeats-containing protein-1 (β-TrCP1) serves as the substrate recognition subunit for SCFβ-TrCP E3 ubiquitin ligases, which specifically ubiquitinate phosphorylated substrates. Three variants of β-TrCP1 are known and act as homodimer or heterodimer complexes. Here, we identified a novel full-sequenced variant, β-TrCP1-variant 4, which harbours exon II instead of exon III of variant 1, with no change in the open reading frame. The expression of β-TrCP1-variant 4 is lower than that of variant 1 or 2 in ovarian cancer cell lines, whereas it is abundantly expressed in normal and cancerous ovarian tissues. Moreover, β-TrCP1-variant 2 was aberrantly expressed more than variant 1 in ovarian cancer tissues whereas variant 1 was expressed more in normal tissues. Similar to variants 1 and 2, β-TrCP1-variant 4 directly interacts with β-catenin, one of the substrates of SCFβ-TrCP E3 ubiquitin ligase and down-regulates the transcriptional activity and protein expression of β-catenin with a significantly weaker effect than that by variants 1 and 2. However, the co-expression of β-TrCP1-variant 4 with variant 1 in same proportion has no effect, whereas other combinations effectively down-regulate the activity of β-catenin, indicating that the heterodimer of variants 1 and 4 has no function. Thus, β-TrCP1-variant 4 could play a critical role in SCFβ-TrCP E3 ligase-mediated ubiquitination by acting as a negative regulator of β-TrCP1-variant 1.

CENP-W inhibits CDC25A degradation by destabilizing the SCFβ-TrCP-1 complex at G2/M

Skp, Cullin, F-box (SCF)β-TrCP-1 ubiquitin ligases play a central role in cell cycle regulation and tumorigenesis via proteolytic cleavage of many essential cell cycle regulators. In this study, we propose that centromere protein (CENP)-W, a newly identified kinetochore component, is a novel negative regulator of the SCFβ-TrCP-1 complex. CENP-W interacts with Cullin (CUL)-1 and β-Transducin repeat-containing protein (β-TrCP)-1 through highly overlapped binding sites with S-phase kinase-associated protein (SKP)-1. CENP-W is incorporated into the SCFβ-TrCP-1 complex to promote complex disassembly. Unlike other known regulators that increase SCFβ-TrCP-1 ubiquitin ligase activity by promoting complex reassociation, CENP-W-mediated complex disorganization induced β-TrCP1 degradation and consequently decreased its activity. The association between CENP-W and the SCFβ-TrCP-1 complex was prominent during the G2/M transition in the nucleus. Especially, CENP-W knockdown decreased the cell division cycle-25A protein level, leading to a delay in mitotic progression. We propose that CENP-W participates in cell cycle regulation by modulating SCFβ-TrCP-1 ubiquitin ligase activity.-Cheon, Y., Lee, S. CENP-W inhibits CDC25A degradation by destabilizing the SCFβ-TrCP-1 complex at G2/M.

Transcriptional Regulation of CRD-BP by c-myc: Implications for c-myc Functions

The coding region determinant binding protein, CRD-BP, is a multifunctional RNA binding protein involved in different processes such as mRNA turnover, translation control, and localization. It is mostly expressed in fetal and neonatal tissues, where it regulates many transcripts essential for normal embryonic development. CRD-BP is scarce or absent in normal adult tissues but reactivated and/or overexpressed in various neoplastic and preneoplastic tumors and in most cell lines. Its expression has been associated with the most aggressive form of some cancers. CRD-BP is an important regulator of different genes including a variety of oncogenes or proto-oncogenes (c-myc, β-TrCP1, GLI1, etc.). Regulation of CRD-BP expression is critical for proper control of its targets as its overexpression may play an important role in abnormal cell proliferation, suppression of apoptosis, invasion, and metastasis. Molecular bases of the regulatory mechanisms governing CRD-BP expression are still not completely elucidated. In this article, we have identified c-myc as a novel transcriptional regulator of CRD-BP. We show that c-myc binds to CRD-BP promoter and induces its transcription. This induction of CRD-BP expression contributes to the role of c-myc in the regulation of translation, increase in cell size, and acceleration of cell cycle progression via a mechanism involving upregulation of β-TrCP1 levels and activities and accelerated degradation of PDCD4.