FBW7 regulates DNA interstrand cross-link repair by modulating FAAP20 degradation

Poly(ADP-ribosyl)ation of TIMELESS limits DNA replication stress and promotes stalled fork protection

Poly(ADP-ribosyl)ation (PARylation), catalyzed mainly by poly(ADP-ribose) polymerase (PARP)1, is a key posttranslational modification involved in DNA replication and repair. Here, we report that TIMELESS (TIM), an essential scaffold of the replisome, is PARylated, which is linked to its proteolysis. TIM PARylation requires recognition of auto-modified PARP1 via two poly(ADP-ribose)-binding motifs, which primes TIM for proteasome-dependent degradation. Cells expressing the PARylation-refractory TIM mutant or under PARP inhibition accumulate TIM at DNA replication forks, causing replication stress and hyper-resection of stalled forks. Mechanistically, aberrant engagement of TIM with the replicative helicase impedes RAD51 loading and protection of reversed forks. Accordingly, defective TIM degradation hypersensitizes BRCA2-deficient cells to replication damage. Our study defines TIM as a substrate of PARP1 and elucidates how the control of replisome remodeling by PARylation is linked to stalled fork protection. Therefore, we propose a mechanism of PARP inhibition that impinges on the DNA replication fork instability caused by defective TIM turnover.

FBXW7 and the Hallmarks of Cancer: Underlying Mechanisms and Prospective Strategies

FBXW7, a member of the F-box protein family within the ubiquitin–proteasome system, performs an indispensable role in orchestrating cellular processes through ubiquitination and degradation of its substrates, such as c-MYC, mTOR, MCL-1, Notch, and cyclin E. Mainly functioning as a tumor suppressor, inactivation of FBXW7 induces the aberrations of its downstream pathway, resulting in the occurrence of diseases especially tumorigenesis. Here, we decipher the relationship between FBXW7 and the hallmarks of cancer and discuss the underlying mechanisms. Considering the interplay of cancer hallmarks, we propose several prospective strategies for circumventing the deficits of therapeutic resistance and complete cure of cancer patients.

Tumor Suppressor FBXW7 and Its Regulation of DNA Damage Response and Repair

The proper DNA damage response (DDR) and repair are the central molecular mechanisms for the maintenance of cellular homeostasis and genomic integrity. The abnormality in this process is frequently observed in human cancers, and is an important contributing factor to cancer development. FBXW7 is an F-box protein serving as the substrate recognition component of SCF (SKP1-CUL1-F-box protein) E3 ubiquitin ligase. By selectively targeting many oncoproteins for proteasome-mediated degradation, FBXW7 acts as a typical tumor suppressor. Recent studies have demonstrated that FBXW7 also plays critical roles in the process of DDR and repair. In this review, we first briefly introduce the processes of protein ubiquitylation by SCF^FBXW7 and DDR/repair, then provide an overview of the molecular characteristics of FBXW7. We next discuss how FBXW7 regulates the process of DDR and repair, and its translational implication. Finally, we propose few future perspectives to further elucidate the role of FBXW7 in regulation of a variety of biological processes and tumorigenesis, and to design a number of approaches for FBXW7 reactivation in a subset of human cancers for potential anticancer therapy.

Glycogen synthase kinase 3β inhibition synergizes with PARP inhibitors through the induction of homologous recombination deficiency in colorectal cancer

Monotherapy with poly ADP-ribose polymerase (PARP) inhibitors results in a limited objective response rate (≤60% in most cases) in patients with homologous recombination repair (HRR)-deficient cancer, which suggests a high rate of resistance in this subset of patients to PARP inhibitors (PARPi). To overcome resistance to PARPi and to broaden their clinical use, we performed high-throughput screening of 99 anticancer drugs in combination with PARPi to identify potential therapeutic combinations. Here, we found that GSK3 inhibitors (GSK3i) exhibited a strong synergistic effect with PARPi in a panel of colorectal cancer (CRC) cell lines with diverse genetic backgrounds. The combination of GSK3β and PARP inhibition causes replication stress and DNA double-strand breaks, resulting in increased anaphase bridges and abnormal spindles. Mechanistically, inhibition or genetic depletion of GSK3β was found to impair the HRR of DNA and reduce the mRNA and protein level of BRCA1. Finally, we demonstrated that inhibition or depletion of GSK3β could enhance the in vivo sensitivity to simmiparib without toxicity. Our results provide a mechanistic understanding of the combination of PARP and GSK3 inhibition, and support the clinical development of this combination therapy for CRC patients.

Acetylation modulates the Fanconi anemia pathway by protecting FAAP20 from ubiquitin-mediated proteasomal degradation

Fanconi anemia (FA) is a chromosome instability syndrome of children caused by inherited mutations in one of FA genes, which together constitute a DNA interstrand cross-link (ICL) repair, or the FA pathway. Monoubiquitination of Fanconi anemia group D2 protein (FANCD2) by the multisubunit ubiquitin E3 ligase, the FA core complex, is an obligate step in activation of the FA pathway, and its activity needs to be tightly regulated. FAAP20 is a key structural component of the FA core complex, and regulated proteolysis of FAAP20 mediated by prolyl cis-trans isomerization and phosphorylation at a consensus phosphodegron motif is essential for preserving the integrity of the FA core complex, and thus FANCD2 monoubiquitination. However, how ubiquitin-dependent FAAP20 degradation is modulated to fine-tune FA pathway activation remains largely un-known. Here, we present evidence that FAAP20 is acetylated by the acetyltransferase p300/CBP on lysine 152, the key residue that when polyubiquitinated results in the degradation of FAAP20. Acetylation or mutation of the lysine residue stabilizes FAAP20 by preventing its ubiquitination, thereby protecting it from proteasome-dependent FAAP20 degradation. Consequently, disruption of the FAAP20 acetylation pathway impairs FANCD2 activation. Together, our study reveals a competition mechanism between ubiquitination and acetylation of a common lysine residue that controls FAAP20 stability and highlights a complex balancing between different posttranslational modifications as a way to refine the FA pathway signaling required for DNA ICL repair and genome stability.

Fanconi anemia and the underlying causes of genomic instability

Fanconi anemia (FA) is a rare genetic disorder, characterized by birth defects, progressive bone marrow failure, and a predisposition to cancer. This devastating disease is caused by germline mutations in any one of the 22 known FA genes, where the gene products are primarily responsible for the resolution of DNA interstrand cross-links (ICLs), a type of DNA damage generally formed by cytotoxic chemotherapeutic agents. However, the identity of endogenous mutagens that generate DNA ICLs remains largely elusive. In addition, whether DNA ICLs are indeed the primary cause behind FA phenotypes is still a matter of debate. Recent genetic studies suggest that naturally occurring reactive aldehydes are a primary source of DNA damage in hematopoietic stem cells, implicating that they could play a role in genome instability and FA. Emerging lines of evidence indicate that the FA pathway constitutes a general surveillance mechanism for the genome by protecting against a variety of DNA replication stresses. Therefore, understanding the DNA repair signaling that is regulated by the FA pathway, and the types of DNA lesions underlying the FA pathophysiology is crucial for the treatment of FA and FA-associated cancers. Here, we review recent advances in our understanding of the relationship between reactive aldehydes, bone marrow dysfunction, and FA biology in the context of signaling pathways triggered during FA-mediated DNA repair and maintenance of the genomic integrity. Environ. Mol. Mutagen. 2020. © 2020 Wiley Periodicals, Inc.

Recent insight into the role of FBXW7 as a tumor suppressor

FBXW7 (also known as Fbw7, Sel10, hCDC4, or hAgo) is a tumor suppressor and the most frequently mutated member of the F-box protein family in human cancers. FBXW7 functions as the substrate recognition component of an SCF-type E3 ubiquitin ligase. It specifically controls the proteasome-mediated degradation of many oncoproteins such as c-MYC, NOTCH, KLF5, cyclin E, c-JUN, and MCL1. In this review, we summarize the molecular and biological features of FBXW7 and its substrates as well as the impact of mutations of FBXW7 on cancer development. We also address the clinical potential of anticancer therapy targeting FBXW7.

Conditional degradation of SDE2 by the Arg/N-End rule pathway regulates stress response at replication forks

Multiple pathways counteract DNA replication stress to prevent genomic instability and tumorigenesis. The recently identified human SDE2 is a genome surveillance protein regulated by PCNA, a DNA clamp and processivity factor at replication forks. Here, we show that SDE2 cleavage after its ubiquitin-like domain generates Lys-SDE2^Ct, the C-terminal SDE2 fragment bearing an N-terminal Lys residue. Lys-SDE2^Ct constitutes a short-lived physiological substrate of the Arg/N-end rule proteolytic pathway, in which UBR1 and UBR2 ubiquitin ligases mediate the degradation. The Arg/N-end rule and VCP/p97^UFD1-NPL4 segregase cooperate to promote phosphorylation-dependent, chromatin-associated Lys-SDE2^Ct degradation upon UVC damage. Conversely, cells expressing the degradation-refractory K78V mutant, Val-SDE2^Ct, fail to induce RPA phosphorylation and single-stranded DNA formation, leading to defects in PCNA-dependent DNA damage bypass and stalled fork recovery. Together, our study elucidates a previously unappreciated axis connecting the Arg/N-end rule and the p97-mediated proteolysis with the replication stress response, working together to preserve replication fork integrity.

The WD40 domain of FBXW7 is a poly(ADP-ribose)-binding domain that mediates the early DNA damage response

FBXW7, a classic tumor suppressor, is a substrate recognition subunit of the Skp1-cullin-F-box (SCF) ubiquitin ligase that targets oncoproteins for ubiquitination and degradation. We recently found that FBXW7 is recruited to DNA damage sites to facilitate nonhomologous end-joining (NHEJ). The detailed underlying molecular mechanism, however, remains elusive. Here we report that the WD40 domain of FBXW7, which is responsible for substrate binding and frequently mutated in human cancers, binds to poly(ADP-ribose) (PAR) immediately following DNA damage and mediates rapid recruitment of FBXW7 to DNA damage sites, whereas ATM-mediated FBXW7 phosphorylation promotes its retention at DNA damage sites. Cancer-associated arginine mutations in the WD40 domain (R465H, R479Q and R505C) abolish both FBXW7 interaction with PAR and recruitment to DNA damage sites, causing inhibition of XRCC4 polyubiquitination and NHEJ. Furthermore, inhibition or silencing of poly(ADP-ribose) polymerase 1 (PARP1) inhibits PAR-mediated recruitment of FBXW7 to the DNA damage sites. Taken together, our study demonstrates that the WD40 domain of FBXW7 is a novel PAR-binding motif that facilitates early recruitment of FBXW7 to DNA damage sites for subsequent NHEJ repair. Abrogation of this ability seen in cancer-derived FBXW7 mutations provides a molecular mechanism for defective DNA repair, eventually leading to genome instability.

Proteolytic control of genome integrity at the replication fork

Faithful duplication of the genome is critical for the survival of an organism and prevention of malignant transformation. Accurate replication of a large amount of genetic information in a timely manner is one of the most challenging cellular processes and is often perturbed by intrinsic and extrinsic barriers to DNA replication fork progression, a phenomenon referred to as DNA replication stress. Elevated DNA replication stress is a primary source of genomic instability and one of the key hallmarks of cancer. Therefore, targeting DNA replication stress is an emerging concept for cancer therapy. The replication machinery associated with PCNA and other regulatory factors coordinates the synthesis and repair of DNA strands at the replication fork. The dynamic interaction of replication protein complexes with DNA is essential for sensing and responding to various signaling events relevant to DNA replication and damage. Thus, the disruption of the spatiotemporal regulation of protein homeostasis at the replication fork impairs genome integrity, which often involves the deregulation of ubiquitin-mediated proteolytic signaling. Notably, emerging evidence has highlighted the role of the AAA+ATPase VCP/p97 in extracting ubiquitinated protein substrates from the chromatin and facilitating the turnover of genome surveillance factors during DNA replication and repair. Here, we review recent advances in our understanding of chromatin-associated degradation pathways at the replication fork and the implication of these findings for cancer therapy.

FBW7 Regulates the Autophagy Signal in Mesangial Cells Induced by High Glucose

Aims

Abnormal regulation of autophagy participates in the development of diabetic nephropathy. mTOR is the most common negative regulator of the autophagy signaling pathway. FBW7 constitutes the SCF (Skp1-Cullin1-F-box protein) recognition subunit of E3 ubiquitin ligase, and mTOR is a substrate of FBW7 that can be modified by ubiquitination and be degraded via proteasomes. In this study, we explored the relationship between FBW7 and autophagy and examined the effects of FBW7 on the occurrence of diabetic nephropathy in vitro.

Materials and Methods

We cultured mesangial cells induced by high glucose in vitro and used rapamycin as a specific mTOR inhibitor, performed FBW7 gene overexpression, and detected the expression of autophagy signal and inflammatory factors by WB, ELISA, RT-PCR, and immunofluorescence.

Results

High glucose can downregulate the expression of FBW7 and activate mTOR signal, which leads to diminished autophagy in renal mesangial cells, as well as renal inflammatory cytokines and fibrotic factors. RAPA, as a specifically inhibitor of mTOR, can decrease inflammatory cytokines and fibrotic factors by inhibiting mTOR. Moreover, FBW7 gene overexpression can increase autophagy by inhibiting mTOR signal; at the same time, the inflammatory cytokines and fibrotic factors were decreased in mesangial cells.

Conclusions

FBW7 was decreased in renal mesangial cells induced by high glucose, and FBW7 gene overexpression can increase autophagy by inhibiting mTOR signaling and ameliorate inflammation and fibrosis.

Glycogen synthase kinase-3 beta inhibitors as novel cancer treatments and modulators of antitumor immune responses

As a kinase at the crossroads of numerous metabolic and cell growth signaling pathways, glycogen synthase kinase-3 beta (GSK-3β) is a highly desirable therapeutic target in cancer. Despite its involvement in pathways associated with the pathogenesis of several malignancies, no selective GSK-3β inhibitor has been approved for the treatment of cancer. The regulatory role of GSK-3β in apoptosis, cell cycle, DNA repair, tumor growth, invasion, and metastasis reflects the therapeutic relevance of this target and provides the rationale for drug combinations. Emerging data on GSK-3β as a mediator of anticancer immune response also highlight the potential clinical applications of novel selective GSK-3β inhibitors that are entering clinical studies. This manuscript reviews the preclinical and early clinical results with GSK-3β inhibitors and delineates the developmental therapeutics landscape for this potentially important target in cancer therapy.

Prolyl isomerization of FAAP20 catalyzed by PIN1 regulates the Fanconi anemia pathway

The Fanconi Anemia (FA) pathway is a multi-step DNA repair process at stalled replication forks in response to DNA interstrand cross-links (ICLs). Pathological mutation of key FA genes leads to the inherited disorder FA, characterized by progressive bone marrow failure and cancer predisposition. The study of FA is of great importance not only to children suffering from FA but also as a model to study cancer pathogenesis in light of genome instability among the general population. FANCD2 monoubiquitination by the FA core complex is an essential gateway that connects upstream DNA damage signaling to enzymatic steps of repair. FAAP20 is a key component of the FA core complex, and regulated proteolysis of FAAP20 mediated by the ubiquitin E3 ligase SCF^FBW7 is critical for maintaining the integrity of the FA complex and FA pathway signaling. However, upstream regulatory mechanisms that govern this signaling remain unclear. Here, we show that PIN1, a phosphorylation-specific prolyl isomerase, regulates the integrity of the FA core complex, thus FA pathway activation. We demonstrate that PIN1 catalyzes cis-trans isomerization of the FAAP20 pSer48-Pro49 motif and promotes FAAP20 stability. Mechanistically, PIN1-induced conformational change of FAAP20 enhances its interaction with the PP2A phosphatase to counteract SCF^FBW7-dependent proteolytic signaling at the phosphorylated degron motif. Accordingly, PIN1 deficiency impairs FANCD2 activation and the DNA ICL repair process. Together, our study establishes PIN1-dependent prolyl isomerization as a new regulator of the FA pathway and genomic integrity.

Fanconi anemia (FA) is a devastating disease of children that leads to birth defects, bone marrow failure, and a variety of cancers early in their lives. Germ-line mutations in FA genes disrupt the DNA repair process, namely the FA pathway, resulting in genome instability and clinical features of FA patients. Thus, understanding the molecular mechanisms by which the FA pathway is regulated is critical for alleviating the burden of children suffering from FA and related cancer. A critical step in this pathway is the monoubiquitination of FANCD2 by a multi-subunit ubiquitin E3 ligase called the FA core complex, and the FAAP20 subunit is required for its functional integrity. Here, we show that proline-directed structural change of FAAP20 catalyzed by the PIN1 prolyl cis-trans isomerase is essential for the FAAP20 stability by counteracting phosphorylation-dependent proteolytic signaling of FAAP20 and thus promotes FANCD2 activation and DNA repair. Our findings reveal how PIN1-mediated phosphorylation signaling cascade and proteolysis preserves genomic integrity and how its deregulation is associated the pathogenesis of FA. Our knowledge on a new regulatory mechanism governing FA pathway activation may lead to the development of a new target for FA and FA-related malignancy.

Fanconi Anemia Signaling and Cancer

The extremely high cancer incidence associated with patients suffering from a rare human genetic disease, Fanconi anemia (FA), demonstrates the importance of FA genes. Over the course of human tumor development, FA genes perform critical tumor-suppression roles. In doing so, FA provides researchers with a unique genetic model system to study cancer etiology. Here, we review how aberrant function of the 22 FA genes and their signaling network contributes to malignancy. From this perspective, we will also discuss how the knowledge discovered from FA research serves basic and translational cancer research.