Mutations at multiple CDK phosphorylation consensus sites on Cdt2 increase the affinity of CRL4Cdt2 for PCNA and its ubiquitination activity in S phase

LOC101929709 promotes gastric cancer progression by aiding LIN28B to stabilize c-MYC mRNA

BACKGROUND

LIN28B plays a critical role in the Warburg effect. However, its underlying mechanism remains elusive. Recently, it has been reported that LIN28B could collaborate with IGF2BP3, which can bind to m6A-modified c-MYC transcripts. Therefore, this study investigated if LIN28B recognises methylated c-MYC mRNA to promote the Warburg effect in gastric cancer.

METHODS

Effects of LIN28B on gastric cancer were confirmed in vitro and in vivo. On the basis of bioinformatics analysis, the association between LIN28B and c-MYC mRNA was shown using RNA immunoprecipitation (RIP) and luciferase reporter assays. The role of m6A was identified by RNA pull-down assays. We further performed RIP-seq to search for long non-coding RNAs (lncRNAs) participating in the LIN28B binding process. Chromatin immunoprecipitation was used to show the impact of c-MYC on transcription of LIN28B and lncRNAs.

RESULTS

LIN28B was identified to stabilize c-MYC mRNA by recognizing m6A. Furthermore, the interaction between c-MYC mRNA and LIN28B is speculated to be supported by LOC101929709, which binds to both LIN28B and IGF2BP3. Functional experiments revealed that LOC101929709 promotes the proliferation, migration and glycolysis of gastric cancer. Mechanistically, LOC101929709 enriched in the cytoplasm helps LIN28B stabilize c-MYC mRNA. Moreover, c-MYC promoted the transcription of both LOC101929709 and LIN28B. Additionally, LOC101929709 also activated the PI3K/AKT pathway.

CONCLUSIONS

The c-MYC/LOC101929709/LIN28B axis promotes aerobic glycolysis and tumour progression. Thus, LOC101929709 can be a novel potential target for gastric cancer treatment.

Transcriptome Analysis of Sugarcane Young Leaves and Protoplasts after Enzymatic Digestion

Sugarcane somatic cell hybridization can break through the barrier of genetic incompatibility between distantly related species in traditional breeding. However, the molecular mechanisms of sugarcane protoplast regeneration and the conditions for protoplast preparation remain largely unknown. In this study, young sugarcane (ROC22) leaves were enzymatically digested, and the viability of protoplasts reached more than 90% after enzymatic digestion (Enzymatic combination: 2% cellulase + 0.5% pectinase + 0.1% dissociative enzyme + 0.3% hemicellulase, pH = 5.8). Transcriptome sequencing was performed on young sugarcane leaves and protoplasts after enzymatic digestion to analyze the differences in gene expression in somatic cells before and after enzymatic digestion. A total of 117,411 unigenes and 43,460 differentially expressed genes were obtained, of which 21,123 were up-regulated and 22,337 down-regulated. The GO terms for the 43,460 differentially expressed genes (DEGs) were classified into three main categories: biological process, cellular component and molecular function, which related to developmental process, growth, cell proliferation, transcription regulator activity, signal transducer activity, antioxidant activity, oxidative stress, kinase activity, cell cycle, cell differentiation, plant hormone signal transduction, and so on. After enzymatic digestion of young sugarcane leaves, the expressions of GAUT, CESA, PSK, CyclinB, CyclinA, CyclinD3 and cdc2 genes associated with plant regeneration were significantly down-regulated to 65%, 47%, 2%, 18.60%, 21.32%, 52% and 45% of young leaves, respectively. After enzymatic digestion, Aux/IAA expression was up-regulated compared with young leaves, and Aux/IAA expression was 3.53 times higher than that of young leaves. Compared with young leaves, these key genes were significantly changed after enzymatic digestion. These results indicate that the process of somatic enzymatic digestion process may affect the regeneration of heterozygous cells to a certain extent.

CRL4Cdt2 Ubiquitin Ligase, A Genome Caretaker Controlled by Cdt2 Binding to PCNA and DNA

The ubiquitin ligase CRL4Cdt2 plays a vital role in preserving genomic integrity by regulating essential proteins during S phase and after DNA damage. Deregulation of CRL4Cdt2 during the cell cycle can cause DNA re-replication, which correlates with malignant transformation and tumor growth. CRL4Cdt2 regulates a broad spectrum of cell cycle substrates for ubiquitination and proteolysis, including Cdc10-dependent transcript 1 or Chromatin licensing and DNA replication factor 1 (Cdt1), histone H4K20 mono-methyltransferase (Set8) and cyclin-dependent kinase inhibitor 1 (p21), which regulate DNA replication. However, the mechanism it operates via its substrate receptor, Cdc10-dependent transcript 2 (Cdt2), is not fully understood. This review describes the essential features of the N-terminal and C-terminal parts of Cdt2 that regulate CRL4 ubiquitination activity, including the substrate recognition domain, intrinsically disordered region (IDR), phosphorylation sites, the PCNA-interacting protein-box (PIP) box motif and the DNA binding domain. Drugs targeting these specific domains of Cdt2 could have potential for the treatment of cancer.

The RepID-CRL4 ubiquitin ligase complex regulates metaphase to anaphase transition via BUB3 degradation

The spindle assembly checkpoint (SAC) prevents premature chromosome segregation by inactivating the anaphase promoting complex/cyclosome (APC/C) until all chromosomes are properly attached to mitotic spindles. Here we identify a role for Cullin–RING ubiquitin ligase complex 4 (CRL4), known for modulating DNA replication, as a crucial mitotic regulator that triggers the termination of the SAC and enables chromosome segregation. CRL4 is recruited to chromatin by the replication origin binding protein RepID/DCAF14/PHIP. During mitosis, CRL4 dissociates from RepID and replaces it with RB Binding Protein 7 (RBBP7), which ubiquitinates the SAC mediator BUB3 to enable mitotic exit. During interphase, BUB3 is protected from CRL4-mediated degradation by associating with promyelocytic leukemia (PML) nuclear bodies, ensuring its availability upon mitotic onset. Deficiencies in RepID, CRL4 or RBBP7 delay mitotic exit, increase genomic instability and enhance sensitivity to paclitaxel, a microtubule stabilizer and anti-tumor drug.

The spindle assembly checkpoint (SAC) safeguards chromosome segregation by regulating the anaphase promoting complex/cyclosome (APC/C), allowing chromosomes to correctly attach to mitotic spindles. Here the authors reveal a role for Cullin–RING ubiquitin ligase complex 4 (CRL4) in regulating metaphase to anaphase transition via BUB3 degradation.

Accurate delineation of cell cycle phase transitions in living cells with PIP-FUCCI

Cell cycle phase transitions are tightly orchestrated to ensure efficient cell cycle progression and genome stability. Interrogating these transitions is important for understanding both normal and pathological cell proliferation. By quantifying the dynamics of the popular FUCCI reporters relative to the transitions into and out of S phase, we found that their dynamics are substantially and variably offset from true S phase boundaries. To enhance detection of phase transitions, we generated a new reporter whose oscillations are directly coupled to DNA replication and combined it with the FUCCI APC/C reporter to create "PIP-FUCCI". The PIP degron fusion protein precisely marks the G1/S and S/G2 transitions; shows a rapid decrease in signal in response to large doses of DNA damage only during G1; and distinguishes cell type-specific and DNA damage source-dependent arrest phenotypes. We provide guidance to investigators in selecting appropriate fluorescent cell cycle reporters and new analysis strategies for delineating cell cycle transitions.

A DNA-binding domain in the C-terminal region of Cdt2 enhances the DNA synthesis-coupled CRL4Cdt2 ubiquitin ligase activity for Cdt1

The Cullin-RING ubiquitin ligase CRL4Cdt2 maintains genome integrity by mediating the cell cycle- and DNA damage-dependent degradation of proteins such as Cdt1, p21 and Set8. Human Cdt2 has two regions, a conserved N-terminal seven WD40 repeat region and a less conserved C-terminal region. Here, we showed that the N-terminal region is sufficient for complex formation with CRL4, but the C-terminal region is required for the full ubiquitin ligase activity. UV irradiation-induced polyubiquitination and degradation of Cdt1 were impaired in Cdt2 (N-terminus only)-expressing cells. Deletion and mutation analysis identified a domain in the C-terminal region that increased ubiquitination activity and displayed DNA-binding activity. The identified domain mediated binding to double-stranded DNA and showed higher affinity binding to single-stranded DNA. As the ligase activity of CRL4Cdt2 depends on proliferating cell nuclear antigen (PCNA) loading onto DNA, the present results suggest that the DNA-binding domain facilitates the CRL4Cdt2-mediated recognition and ubiquitination of substrates bound to PCNA on chromatin.

Direct binding of Cdt2 to PCNA is important for targeting the CRL4Cdt2 E3 ligase activity to Cdt1

The C-terminal end of Cdt2 contains a PIP box for binding to PCNA to promote CRL4^Cdt2 function, creating a new paradigm where the substrate receptor and substrates bind to a common multivalent docking platform for ubiquitination.

The CRL4^Cdt2 ubiquitin ligase complex is an essential regulator of cell-cycle progression and genome stability, ubiquitinating substrates such as p21, Set8, and Cdt1, via a display of substrate degrons on proliferating cell nuclear antigens (PCNAs). Here, we examine the hierarchy of the ligase and substrate recruitment kinetics onto PCNA at sites of DNA replication. We demonstrate that the C-terminal end of Cdt2 bears a PCNA interaction protein motif (PIP box, Cdt2^PIP), which is necessary and sufficient for the binding of Cdt2 to PCNA. Cdt2^PIP binds PCNA directly with high affinity, two orders of magnitude tighter than the PIP box of Cdt1. X-ray crystallographic structures of PCNA bound to Cdt2^PIP and Cdt1^PIP show that the peptides occupy all three binding sites of the trimeric PCNA ring. Mutating Cdt2^PIP weakens the interaction with PCNA, rendering CRL4^Cdt2 less effective in Cdt1 ubiquitination and leading to defects in Cdt1 degradation. The molecular mechanism we present suggests a new paradigm for bringing substrates to the CRL4-type ligase, where the substrate receptor and substrates bind to a common multivalent docking platform to enable subsequent ubiquitination.