Encyclopedia of Signaling Molecules

2018 Edition
| Editors: Sangdun Choi

CHFR

Reference work entry
DOI: https://doi.org/10.1007/978-3-319-67199-4_101581

Synonyms

Historical Background

Chfr (Checkpoint with FHA and RING domains) is a ubiquitously expressed E3 ubiquitin ligase that is involved in a mitotic cell cycle checkpoint. It was identified from an expressed sequence tag (EST) database search for proteins containing forkhead-associated (FHA) motifs (Scolnick and Halazonetis 2000). Protein BLAST comparison revealed that Chfr is conserved across species, from unicellular eukaryotic yeast cell, Schizosaccharomyces pombe and Saccharomyces cerevisiae to mice. Chfr orthologues in yeast, Dma1 and its paralog, Dma2, have been shown to be involved in regulating various cell division cycle processes such as spindle assembly, spindle positioning, and cytokinesis. More significantly, Dma1 and Dma2 were also demonstrated to regulate mitotic exit in fission yeast and budding yeast via SIN (Septation Initiation Network) and MEN (Mitotic Exit Network) respectively (analogous to Hippo-kinase signaling pathway in Drosophila and mammals) (Hergovich and Hemmings 2012).

The role of Chfr in mammalian mitosis was established by the observation of differences in mitotic indices (number of condensed chromosomes) between normal primary cells and tumor cell lines expressing nonfunctional Chfr during exposure to mitotic stresses. Scholnick and Thomas noted that tumor cell lines without functional Chfr exhibited high mitotic indices upon treatment with microtubule depolymerizing agent, nocodazole. Conversely, normal wild-type cells treated similarly exhibited a lower number of condensed chromosomes. Interestingly, ectopic expression of wild-type Chfr in U20S and DLD1 cells (CHFR-null cell lines) lowered the low mitotic index upon treatment with nocodazole. These findings suggest that the low mitotic index in normal cells treated with microtubule poison is regulated in a Chfr-dependent manner. The authors further demonstrated that the low mitotic index observed in cells treated with mitotic stresses is due to the delay in S-M phase transition.

A study from the Fang group further elucidated the molecular mechanism of Chfr in regulating mitotic progression. Researchers showed that Chfr is a ubiquitin ligase as evident from the accumulation of high molecular weight ubiquitin conjugates in cells ectopically expressing Chfr. To investigate the function of Chfr in regulating cell cycle progression into mitosis, the authors employed an elegant assay using Xenopus cell-free extracts to monitor mitotic entry. Addition of a nondestructible form of cyclin B to interphase Xenopus cell extracts activated mitotic kinase, Cdc2, activity and promoted mitotic entry. Intriguingly, the addition of Chfr to the extracts delayed mitotic entry as a result of reduced activity of Cdc2. In contrast, mitotic entry delay was absent with addition of the inactive form of Chfr, indicating that the ubiquitin ligase activity of Chfr is essential for delaying mitotic entry.

Structural Properties of Chfr

CHFR encodes for a 73 kDa protein, which consists of three distinctive domains; an N-terminal FHA domain, a central RING (Really Interesting New Gene) finger domain, and a cysteine-rich C-terminal domain embedded with Poly-ADP-ribose binding zinc finger (PBZ) motif (Fig. 1). The FHA domain is a phosphoprotein-binding module that is present in many prokaryotic and eukaryotic proteins. This domain is indispensible for Chfr to function as a mitotic checkpoint or NF-ķB signaling inhibitor. Crystallographic analysis revealed that Chfr is capable of interacting with tungstate (a phosphate analog), suggesting that Chfr binds to phosphopeptides through the FHA domain. However, the Chfr FHA-interacting partners remain elusive.
CHFR, Fig. 1

Structure of the Chfr. Chfr consists of three domains; a FHA domain (16–122 amino acids), a central RING finger domain (303–346 amino acids), and a cysteine-rich C-terminal domain (476–644 amino acids), embedded with Poly-ADP-ribose binding zinc finger (PBZ) motif (620–644 amino acids)

A RING finger domain is located at the position of the 303–346 amino acid of Chfr. This domain is often found on many E3 ubiquitin ligases involved in a wide range of cellular processes such as cell signaling, cell cycle progression and cell cycle checkpoints, and DNA repair. The RING domain of E3 ubiquitin ligase facilitates the transfer of ubiquitin from E2-ubiquitin to the substrate. Previous studies demonstrated that the RING domain is essential for ubiquitin ligase activity. Consistent with this, a deletion of RING domain or mutation of Isoleucine-306 to alanine abolishes the E3 ubiquitin ligase activity of Chfr.

A cysteine-rich region spans across 476–644 amino acids of the Chfr C-terminal end and has been reported to interact with various substrates of Chfr such as Aurora A, Kif22, HDAC1, and TOPK (T-lymphokine-activated killer cell-originated protein kinase) (Shinde et al. 2013). This cysteine-rich region also interacts with Mad2 to enhance Mad2-Cdc20 binding.

The 620–644 amino acids of Chfr consist of a PBZ motif that is embedded within the cysteine-rich domain (Oberoi et al. 2010). It has been shown that the PBZ motif of CHFR interacts with PARylated proteins that are involved in cell cycle checkpoint and DNA damage response. Loss of the PBZ motif of CHFR has been revealed to abrogate antephase checkpoint (Ahel et al. 2008) and recruitment of Chfr to the DNA lesion (Liu et al. 2013).

CHFR Regulation

CHFR is ubiquitously expressed in normal human tissues. CHFR is frequently silenced in many types of cancers through hypermethylation of CpG islands in its promoter region. Moreover, CHFR knockout mice display higher incidences of tumor formation, suggesting that CHFR possibly acts as a tumor suppressor. It is therefore not surprising that CHFR is under tight regulation in the cell. To date, several studies have shown that CHFR is regulated at the transcriptional, posttranscriptional, and posttranslational levels.

For instance, expression of CHFR is regulated in a cell division cycle-dependent manner (Chaturvedi et al. 2002). Ectopic expression of CHFR in cells released from G1/S phase synchronization showed that the CHFR expression was relatively high in G1/S phase and significantly reduced in cells at G2/M phase transition. The decrease in CHFR expression level at G2/M phase transition is likely to help facilitate mitotic entry. CHFR expression peaks when cells encounter mitotic stresses such as exposure to microtubule poison, nocodazole.

Posttranscriptional regulation of CHFR has also been documented. In a global screen of microRNA involved in cell proliferation, miR-26b was found to target CHFR. However, the underlying mechanism of miRNA-dependent CHFR regulation remains unclear.

Two posttranslational modification pathways that regulate the stability of Chfr have been identified (Fig. 2). Similar to most of the RING E3 ubiquitin ligase, Chfr level is controlled by autoubiquitylation via its RING domain, which leads to proteasomal degradation. Lack of ubiquitin ligase activity, resulting from the expression of CHFR with RING domain deletion or from a Chfr-I306A mutant, abolishes the cell cycle-dependent degradation of Chfr. In addition, E3 ubiquitin ligase-competent mutant Chfr-K2A (with mutation of putative autoubiquitylation target lysine residues of Chfr to alanine-K384A and K393A) greatly reduces the ubiquitination of Chfr and leads to stabilization of the protein. The autoubiquitination of Chfr can be alleviated by USP7/HAUSP (ubiquitin-specific-processing protease 7/Herpesvirus-associated-ubiquitin specific protease)-dependent deubiquitination, which also stabilizes Chfr [reviewed in (Sanbhnani and Yeong 2012)]. These data suggest that Chfr level during cell cycle is regulated via a balance between autoubiquitylation and deubiquitylation that determines the degradation of Chfr.
CHFR, Fig. 2

Regulation of Chfr. Chfr degradation is facilitated by Chfr autoubiquitylation and Chfr SUMOylation

A recent study by the Seol group revealed that SUMOylation plays an important role in regulating Chfr abundance during cell division. SUMOylation is a posttranslational modification where a SUMO (small-ubiquitin-like modifier) is conjugated to a target protein. The lysine-663 residue of the Chfr C-terminal end can be conjugated with SUMO-1 by SUMO E2-enzyme, Ubc9 (Ubiquitin conjugating enzyme 9). Mutation of Chfr lysine-663R to arginine, Chfr -K663R (SUMOylation defective Chfr), prevents the degradation of Chfr, indicating that SUMOylation plays a role in maintaining the stability of Chfr (Kwon et al. 2013). Intriguingly, SUMOylation promotes Chfr ubiquitination and proteasomal protein degradation independent of its ubiquitin ligase activity. This suggests that Chfr SUMOylation together with an unidentified E3 ubiquitin ligase is responsible for the ubiquitination and subsequent proteasomal degradation of Chfr (Bae et al. 2013). The SUMO-1 moiety on Chfr can be removed by deSUMOylation enzyme, SENP2. Taken together, Chfr stability is tightly regulated by the balance of ubiquitination-deubiquitination in a SUMOylation-dependent and independent manner. The fine-tuned expression of CHFR is essential for cell cycle progression and tumor suppression.

Another layer of Chfr regulation is via its trafficking in the cell. Fluorescence microscopy examination of ectopically expressed GFP-tagged CHFR revealed that its intracellular distribution was regulated in a cell cycle-dependent manner. During interphase, GFP-Chfr accumulated mostly in large and small nuclear foci that were excluded from the nucleolus, with low levels distributed in nucleoplasm. As cells progressed to prophase, GFP-Chfr was dispersed from the foci and redistributed to the nucleoplasm. Interestingly, GFP-Chfr was excluded from the region surrounding the chromosomes. The large GFP-Chfr foci were colocalized with Premyelocytic Leukemia Protein (PML) bodies. Deletion of the N-terminal FHA-domain disrupted the association of Chfr with PML bodies and conferred a dominant-negative effect on mitotic entry delay. Consistent with this, PML −/− mouse embryonic fibroblasts (MEFs) were insensitive to mitotic stresses, suggesting that PML bodies were essential for CHFR function as an antephase checkpoint [reviewed in (Sanbhnani and Yeong 2012)]. However, contradictory observations were reported by other groups using immunofluorescence staining that specifically recognize endogenous Chfr, showing it localizes to the cytoplasm and centrosomes during interphase and on the mitotic spindles during mitosis (Burgess et al. 2008; Privette et al. 2008). This discrepancy can be explained by the fact that over-expression of GFP-CHFR in HeLa cells might not truly reflect endogenous Chfr localization. In agreement with this, Burgess and colleagues demonstrated that YFP-Chfr localized to the nucleus and PML bodies when over-expressed (Burgess et al. 2008).

Chfr in Antephase Checkpoint

The term “Antephase” is defined as a time frame between late G2 to early prophase when chromosome condensation becomes visible. During early mid-twentieth century, Carlson observed an interesting phenomenon where exposure of X-ray irradiation to early-mid prophase grasshopper neuroblast contributed to the cell reversion from prophase to earlier phase. Intriguingly, the reversion was transient as cells were able to re-enter and progress through mitosis. A similar phenomenon was also observed in early prophase avian and mammalian cells exposed to X-ray irradiation.

Further studies in early prophase human and rat kangaroo cells also revealed that cells were able to decondense their chromosome and return to earlier phase when encountering insults such as microtubule poisons, low temperature, and DNA damage. Consistent to Carlson’s observation, cells were also able to resume progression into mitosis. These observations suggest that a cell cycle checkpoint might exist in antephase to delay mitotic entry [reviewed in (Chin and Yeong 2010)]. Matsusaka and Pines subsequently termed this checkpoint as antephase checkpoint (Matsusaka and Pines 2004).

Interestingly, Scolnick and Halazonetis discovered an E3 ubiquitin ligase that is a key protein to delay cell cycle progression into mitosis when exposed to colcemid (Scolnick and Halazonetis 2000). The E3 ligase turned out to be Chfr, now found to be often downregulated in a wide range of cancers. Further studies by Kang and co-workers showed that Plx1 (a Polo-like kinase belonging to the Plk family which generally regulates mitotic entry, such as by preventing activation of Cdc25 phosphatase that is required for Cdk1 activation) is targeted for proteasomal degradation by Chfr in Xenopus laevis extracts. This results in delayed mitotic progression.

Contrary to the observations in X. laevis, ectopic expression of Chfr failed to decrease Plk1 levels in both human cancer cells lines and rat kangaroo kidney cells, PtK1 [reviewed in (Chin and Yeong 2010)]. The Cho group recently proposed that Plk1 degradation is regulated in a cell cycle-dependent manner (Kim et al. 2011). Indeed, the authors observed that overexpression of auto-ubiquitylation-defective Chfr-K2A and Chfr-K5A mutants in HeLa cells significantly reduced the level of Plk1 as compared to its wild-type counterpart. However, the reduction in Plk1 level was only specific to G2-phase and not S-phase synchronized cells. The notion that Chfr promotes degradation of Plk1 only at G2-phase was corroborated by an in vivo ubiquitination assay showing elevated Plk1 ubiquitination in CHFR-K2A expressing cells. These results indicate that Chfr promotes delay in mitotic entry partly through Plk1 degradation.

In addition to Plk1, Aurora A is also a key target of Chfr. Aurora A is a serine/threonine kinase that is implicated in mitosis and often overexpressed in many human tumors. Moreover, Aurora A is crucial in regulating many key mitotic events including mitotic entry. During G2-phase, Aurora A phosphorylates and activates Plk1. The activated Plk1 mediates the localization of Aurora A onto the centrosomes during late G2-phase. This centrosome localization of Aurora A promotes the recruitment of cyclin B1 to the centrosomes. Besides promoting localization of cyclin B1 onto the centrosome, Aurora A is also involved in activating cyclin B1-Cdk1 activity. The cyclin B1-Cdk1 is activated at the centrosome by Cdc25c phosphatase which requires Aurora A-dependent phosphorylation for activation.

A study from Yu and co-workers revealed that Aurora A physically interacted with Chfr and was ubiquitinated by Chfr in vitro and in vivo (Yu et al. 2005). Notably, Aurora A expression was higher in Chfr-deficient MEFs as compared to wild type. MG132 (a proteasome inhibitor) treatment of wild-type MEFs stabilized Aurora A level. These data indicate that Aurora A is a bona fide substrate of Chfr. Thus, downregulation of Plk1 and Aurora A in a Chfr-dependent manner might be responsible for mitotic entry delay of antephase checkpoint.

In addition to Plk1 and Aurora A that both drive cell progression into mitosis, Kif22 (a chromokinesin) was also reported to be a Chfr substrate (Maddika et al. 2009). Kif22 is a motor protein associated with DNA and microtubules and is involved in spindle assembly and chromosome segregation during mitosis and meiosis. Chfr binds to both N and C-terminal of Kif22 via its cysteine-rich domain. Binding of Chfr to Kif22 promotes the polyubiquitination of Kif22 and targets it for proteasomal degradation. Overexpression of Kif22 in human mammary epithelial cells (HMEC) resulted in defects in mitotic spindles, kinetochore arrangement, and aberrant centrosomes number, which resemble abnormalities observed in CHFR-shRNA knockdown cells. A partial knockdown of Kif22 in CHFR-shRNA knockdown cells rescued the mitotic spindle defects, suggesting that negative regulation of Kif22 by Chfr is critical in maintaining genomic stability. Besides Kif22, Chfr was also reported to bind to α-tubulin and Translationally Controlled Tumor Protein (TCTP) that localizes on microtubules. The interaction between Chfr with α-tubulin, Kif22, and TCTP might be important to act as a microtubule stress sensor of antephase checkpoint.

Chfr in DNA Damage Response

In addition to the antephase checkpoint, CHFR has also been implicated in DNA damage response. Chfr has been shown to work synergistically with RNF8 (RING Finger Protein 8) to regulate ATM activation during DNA damage to maintain genomic stability (Wu et al. 2011).

Surprisingly, Chfr is one of the earliest E3 ubiquitin ligase to be recruited to the DNA damage site. The mechanism that regulates this process is mediated by the interaction between Poly(ADP-Ribose) (PAR) and the PBZ motif of CHFR at the DNA lesion. Upon DNA damage, Poly(ADP-ribose) polymerase-1 (PARP-1) polymerizes PAR on proteins at the DNA damage site and initiates DNA repair. Concurrently, PARP-1 is also PARylated through auto-PARylation to promote chromatin relaxation. The PAR mediates DNA repair at the DNA lesion by recruiting proteins involved in DNA repair. However, hyperactivity of PARP-1 is detrimental to cells due to the fact that depletion of the intracellular pool of nicotinamide adenine dinucleotide (NAD+) will cause impairment in ATP production and ultimately, genomic instability. To prevent continuous activation of PARP-1 during DNA damage response, cells degrade chromatin-associated PARP-1 after initiation of DNA repair machinery. Chfr has been shown to recognize PAR via its PBZ motif. Binding of Chfr to PARylated PARP-1 polyubiquitinates PARP-1 for proteasome degradation (Liu et al. 2013).

Signal Transduction Pathways of Chfr

Although targets of Chfr were reported, the underlying mechanism of how Chfr activates antephase checkpoint remains poorly understood. Matsusaka and Pines proposed that Chfr delays mitotic entry via p38 mitogen-activated protein kinase (MAPK) signaling cascade (Matsusaka and Pines 2004). Chfr presumably activates the p38 signaling pathway by activating TAK1 kinase (a MAPK kinase kinase) through K63 ubiquitylation (Fig. 3).
CHFR, Fig. 3

Overview of possible pathways involving Chfr. Upstream factors triggering the antephase checkpoint and the targets of Chfr implicated in delaying mitotic entry

TAK1 kinase can be activated by TRAF6 ubiquitin ligase in response to interferon. In a yeast two-hybrid screen for Chfr binding partners, T6BP (a TRAF6 Binding protein) has been isolated. Chfr, together with TBP6, activates TAK1 and ultimately contributes to delay in mitotic entry. TAK1 kinase has been shown to negatively regulate the activity of Cdc25C through phosphorylation at serine-216, which promotes the binding of 14–3-3 protein and abrogates the ability of Cdc25C to dephosphorylate and alleviate inhibitory phosphorylation on Cdk1 at threonine-14 and tyrosine-15. This leads to retention of cyclin B1-Cdk1 in the cytoplasm and delay in mitotic entry. This model is supported by the finding that overexpression of cyclin B1 with a mutated nuclear export sequence can bypass the antephase checkpoint, suggesting Chfr delays mitotic entry via inhibition of cyclin B1 nuclear localization. Once cyclin B1 enters the nucleus and activates Cdk1, cells are committed to mitosis and reversion to earlier phase is not an option.

Data from other reports indicate that there is an alternative pathway involving Akt in the antephase checkpoint. TOPK has been proposed to be a novel Chfr substrate that plays a role in regulating antephase checkpoint (Shinde et al. 2013). The authors showed that Chfr is capable of ubiquitinating TOPK and eventually causing proteasomal degradation of TOPK. Similar to overexpression of CHFR, shRNA knockdown of TOPK significantly decreased the mitotic index upon nocodazole treatment in HeLa cells. In relation to the antephase checkpoint, TOPK has been shown to promote activation of the Akt pathway via phosphorylation of PTEN at serine-380, thereby inhibiting its activity. This in turn activates the Akt signaling cascade to facilitate G2/M transition. Hence, upon activation of antephase checkpoint, Chfr might target TOPK for proteasomal degradation, which negatively regulates the Akt pathway to halt mitotic entry. However, the mechanistic link between Akt and Chfr remains elusive.

More recently, a report from Perdereau and co-workers showed that Chfr is a novel effector in control of insulin-induced cell proliferation (Perdereau et al. 2015). In a yeast two-hybrid screen using the N-terminal domain of Grb14 as bait, a protein with high amino acid homology with human Chfr was isolated. Grb14 is an inhibitor of insulin receptor signaling that negatively regulates insulin-induced cell division. Immuno-precipitation using lysates from COS cells with ectopic expression of Chfr and Grb14 further validated the interaction between these proteins. Intriguingly, the Grb14-Chfr complex formation was detected in the absence of insulin though it increased with time upon insulin stimulation. Treatment of cells with phosphoinositide-3-kinase (PI3K) inhibitor, LY294002, abolished the complex formation, indicating that PI3K pathway might play a role in mediating this interaction. Furthermore, the complex formation is dependent on the phosphorylation of Chfr at threonine-39 by Akt, given that a Chfr-T39A mutant failed to interact with Grb14. Conversely, the phospho-mimetic mutant of Chfr (Chfr-T39D) was bound to Grb14 even without insulin stimulation. Thus, the phosphorylation on Chfr increases the affinity to Grb14 and ultimately potentiates the inhibitory effect on insulin-induced cell division by Grb14.

Unlike other Chfr targets, Grb14 itself is not a Chfr ubiquitin ligase substrate. However, Chfr ubiquitin ligase-inactive mutant (C303A and C331A) failed to exert Grb14 inhibitory effect on insulin-induced GVBD (germinal vesicle breakdown, a marker for oocyte maturation). These data suggest that Grb14-Chfr complex formation requires prerequisite phosphorylation of Chfr by Akt, but the E3 ubiquitin ligase activity is not essential. Co-injection of Chfr and Grb14 into Xenopus oocytes prevented the oocytes maturation. Of interest, both substrates of Chfr that are involved in entry into mitosis, Plx1 and Eg2 (Xenopus Aurora A), were transiently decreased after 1 h of insulin stimulation and the phosphorylated form of Eg2 failed to accumulate even after 18 h. This indicates that binding of Grb14-Chfr complex promotes ubiquitin ligase activity of Chfr and in turn down regulates Polo-like kinase and Aurora A kinase to delay mitotic entry.

Another recent report suggested that an antephase-dependent delay into mitosis is mediated by the stabilization of Chfr via interaction of Chfr-PARP-1 (Brodie et al. 2015; Kashima et al. 2012). It has been shown that Chfr interacts with PARP-1 through its PBZ binding domain. A study from the Brandes group demonstrated that the interaction between PARP-1 and Chfr is limited to G2/M phase and increased drastically upon exposure to the microtubule-stabilizing agent, docetaxel. In contrast, disruption of Chfr-PARP-1 interaction using the Chfr-(C623A, C629A) mutant (mutations in PBZ domain that disrupt PARP-1 interaction) triggers the auto-ubiquitylation of Chfr and eventually causes proteasomal degradation of Chfr. Furthermore, treatment of cells with small molecule inhibitor, A3, that disrupts Chfr-PARP-1 complex exhibited a decrease in Chfr level (Brodie et al. 2015). These data suggest that formation of Chfr-PARP-1 complex is crucial in stabilizing Chfr when cells encounter mitotic stresses. However, the cell cycle-dependent interaction between Chfr-PARP-1 was not established.

As alluded to above, Chfr localizes on spindle during mitosis. A yeast two-hybrid screen using Chfr as bait isolated TCTP as a novel interacting partner. TCTP is a highly expressed protein that is involved in diverse cellular processes such as microtubule stabilization, apoptosis, and Na/K-ATPase inhibition and is able to function as guanine nucleotide exchange factor (GEF). Further analysis using immuno-precipitation verified the Chfr-TCTP interaction and identified β-tubulin as a third member of this protein complex. Notably, the Chfr-TCTP complex was stable throughout the cell cycle, but was disrupted upon nocodazole challenge. Hence, Burgess and co-workers proposed that Chfr could be the sensor for microtubule perturbation. When cells encounter microtubule stresses, the Chfr-TCTP-β-tubulin complex dissociated and freed Chfr from the spindle to activate antephase checkpoint [reviewed in (Sanbhnani and Yeong 2012)]. Nonetheless, the detailed mechanism of how Chfr detects the defect in microtubules remains poorly understood.

Implication of CHFR in Cancers

Several lines of evidence suggest that CHFR acts as a tumor suppressor gene. In their initial study, Scolnick and Halazonetis (Scolnick and Halazonetis 2000) discovered that CHFR was either absent or downregulated in several human cancer cell lines examined. CHFR messenger-RNA (mRNA) was not detected in human colorectal cancer (DLD1 and HCT116) and neuroblastoma (IMR-5) cell lines. Interestingly, Chfr level was lowered in osteosarcoma cell line, U20S. The decrease in expression level of CHFR in U20S cell line was due to a mutation at valine-580 to methionine and contributed to the destabilization of CHFR mRNA pool (Scolnick and Halazonetis 2000).

Pathological studies using patient’s samples revealed that the CHFR is often downregulated due to epigenetic silencing. Hypermethylation of CHFR at its promoter CpG, presumably by DNA methyltransferaseses Dnmt1 and Dnmt3b, lead to silencing of the gene. The association between CHFR promoter hypermethylation and tumors has been observed in multiple cancers such as primary lung cancers, nonsmall cell lung carcinoma (NSCLC), gastric cancers, and colon cancers [reviewed in (Sanbhnani and Yeong 2012)]. These data suggest there is a correlation between downregulation of CHFR and occurrence of tumor incidences.

The role of CHFR downregulation in tumorigenesis was further supported by experiments conducted using CHFR knockout mice. Yu and colleagues (Yu et al. 2005) demonstrated that CHFR was dispensable for mouse embryogenesis as evident from complete embryo development in Chfr −/− mice. Strikingly, CHFR-null mice developed invasive lymphomas in multiple organs. Furthermore, dorsal skin treatment of dimethylbenz(a)anthracene (a chemical carcinogen) exacerbated the skin tumor incidence in Chfr −/− mice. Moreover, aneuploidy, defects in chromosome segregation and cytokinesis, had been observed in Chfr −/− MEFs, suggesting that Chfr is important in maintaining genomic stability (Yu et al. 2005). Zheng and co-workers further demonstrated that spontaneous tumor formation in CHFR-null mice could be exacerbated by simultaneous knockout of a gene involved in DNA mismatch repair, Mlh1. Chfr −/− Mlh1 −/− MEFs karyotype analysis showed significant aneuploidy or polyploidy. Interestingly, homozygous or heterozygous deletion of Mlh1 together with wild-type CHFR (Chfr +/+ Mlh1 +/− or Chfr +/+ Mlh1 −/− ) displayed near to normal karyotypes suggesting that Chfr plays a role in maintaining genomic stability. These data support the notion that Chfr possibly functions as a tumor suppressor gene in mice.

Some hints as to the mechanistic link between CHFR downregulation and tumorigenesis were revealed by the study from the Seol group [reviewed in (Sanbhnani and Yeong 2012)]. The authors identified histone deacetylase1 (HDAC1) as a Chfr-interacting partner. HDAC1, an enzyme that facilitates histones deacetylation and transcriptional repression, appears to be negatively regulated by Chfr through its polyubiquitination activity. This is consistent with the finding that an overexpression of CHFR led to HDAC1 degradation and hence increased expression level of Cdk inhibitor p21 (CIP/WAF1), metastasis suppressor KAI1, and E-cadherin.

Summary

Extensive studies during past decade have shed light on the likely role of Chfr as a checkpoint component in monitoring a wide range of stresses that could perturb faithful cell cycle progression. In particular, Chfr has been implicated in the DNA damage responses as well as the antephase checkpoint, both of which are critical in preventing genomic instability. Consistent with its roles in these pathways, it is not surprising that epigenetic silencing of CHFR is associated with multiple human cancers. Moreover, CHFR downregulation is often associated with poor prognosis and sensitivity to taxane-based chemotherapy drugs. In spite of the seemingly important functions of Chfr, presently, the molecular pathways related to activation of Chfr and how Chfr targets effectors of the checkpoints have not been fully elucidated. There are, however, hints that its ubiquitin ligase activity is needed to target key regulators in the DNA damage and mitotic entry pathways. For instance, PARP-1, Plk1, and Aurora A are important regulators of the two checkpoint pathways and are thought to be substrates of Chfr. Other studies indicate that Chfr might act through the p38 MAPK pathway to activate the antephase checkpoint. Moreover, Chfr has been reported to localize to the microtubules. However, a coherent picture of how these different pathways interact with one another in relation to Chfr activation and function is lacking. Further studies are therefore needed to better understand the molecular pathways involving Chfr. Such studies would provide invaluable information that might support the use of CHFR as a cancer biomarker, as well as for the development of treatment strategies to combat the occurrence of CHFR-dependent chemotherapy drug resistance.

Notes

Acknowledgments

YFM is funded by the Singapore Ministry of Education Tier 2 grant (R183-000-328-112).

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Copyright information

© Springer International Publishing AG 2018

Authors and Affiliations

  1. 1.Department of Biochemistry, Yong Loo Lin School of MedicineNational University of SingaporeSingaporeSingapore