Beta-Transducin Repeats-Containing Proteins as an Anticancer Target

Simple Summary Beta-transducin repeat-containing proteins (β-TrCPs) are a component of the E3 ubiquitin ligase complex and function in many cellular processes to maintain protein homeostasis. Mounting evidence suggests that β-TrCPs are aberrantly upregulated in cancer tissues and are potential targets for cancer treatment. Although extensive studies have been performed to understand the mode of regulation of their substrates and its biological consequences, little attention has been paid to the mechanisms of the regulation of β-TrCPs themselves. The current review is focused on the modulation of β-TrCPs’ activities and the implications for cancer treatment. Abstract Beta-transducin repeat-containing proteins (β-TrCPs) are E3-ubiquitin-ligase-recognizing substrates and regulate proteasomal degradation. The degradation of β-TrCPs’ substrates is tightly controlled by various external and internal signaling and confers diverse cellular processes, including cell cycle progression, apoptosis, and DNA damage response. In addition, β-TrCPs function to regulate transcriptional activity and stabilize a set of substrates by distinct mechanisms. Despite the association of β-TrCPs with tumorigenesis and tumor progression, studies on the mechanisms of the regulation of β-TrCPs’ activity have been limited. In this review, we studied publications on the regulation of β-TrCPs themselves and analyzed the knowledge gaps to understand and modulate β-TrCPs’ activity in the future.


Introduction
It is now well established that strict control of protein stability is important to maintain normal cellular physiology and prevent various diseases as well.The uncontrolled accumulation of unwanted or damaged proteins in cells or tissues leads to the disruption of homeostasis and, eventually, the development of diseases including cancers [1,2].In addition, targeted protein degradation (TPD) utilizing the ubiquitin-proteasome system (UPS) has been acknowledged recently as a promising strategy for undruggable targets, which include the proteolysis-targeting chimeric molecule (PROTAC), small-molecule PROTAC, chloroalkane-containing PROTAC (HaloPROTAC), in-cell click-formed PROTAC (CLIP-TAC), RNA-PROTAC, and transcription-factor-targeting chimera (TRAFTAC) [1][2][3][4][5][6][7][8][9][10].The UPS is composed of a series of enzymes: E1-ubiquitin-activating, E2-ubiquitin-conjugating, and E3 ubiquitin ligases [11].Poly-ubiquitination at the lysine 48 and/or 11 (K48/K11) Cancers 2023, 15, 4248 2 of 32 residues play the role of a signal for degradation by the 26S proteasome, while monoubiquitination or ubiquitin chains at K6, K27, K33, and K63 are involved in various biological processes such as kinase activation, subcellular localization, DNA replication and repair, lysosomal degradation, and stress responses.More than 700 members (~5% of the human genome) of the E3 ubiquitin ligases have been identified and participate in the control of protein stability in human cells and are suggested as potential drug targets [11][12][13].
Cancers 2023, 15, x FOR PEER REVIEW 2 of 32 E2-ubiquitin-conjugating, and E3 ubiquitin ligases [11].Poly-ubiquitination at the lysine 48 and/or 11 (K48/K11) residues play the role of a signal for degradation by the 26S proteasome, while mono-ubiquitination or ubiquitin chains at K6, K27, K33, and K63 are involved in various biological processes such as kinase activation, subcellular localization, DNA replication and repair, lysosomal degradation, and stress responses.More than 700 members (~5% of the human genome) of the E3 ubiquitin ligases have been identified and participate in the control of protein stability in human cells and are suggested as potential drug targets [11][12][13].

BTRC (the gene encoding β-TrCP1
) is located at human chromosome 10q24.3[21], which contains a gene that is mutated in prostate tumorigenesis [22] and frequently deleted in medulloblastomas [23,24].FBXW11 (the gene encoding β-TrCP2) is located at human chromosome 5q35.1,composed of at least 14 exons and encoding three isoforms transcribed by alternative splicing [25].The roles of F-box proteins and their involvement in cancer have been reviewed recently [16,26].However, the regulation of β-TrCP itself has not been well studied yet, whereas the regulation by it has been extensively in- Online Software [19,20].ATM, ataxia telangiectasia mutated.
BTRC (the gene encoding β-TrCP1) is located at human chromosome 10q24.3[21], which contains a gene that is mutated in prostate tumorigenesis [22] and frequently deleted in medulloblastomas [23,24].FBXW11 (the gene encoding β-TrCP2) is located at human chromosome 5q35.1,composed of at least 14 exons and encoding three isoforms transcribed by alternative splicing [25].The roles of F-box proteins and their involvement in cancer have been reviewed recently [16,26].However, the regulation of β-TrCP itself has not been well studied yet, whereas the regulation by it has been extensively investigated [27].In this review, we analyzed the literature on the role of β-TrCP, especially in cancer and its potential as a therapeutic target for cancer treatment.
The prior phosphorylation-independent degradation of β-TrCP substrates, such as SMA and MAD family member 3 (SMAD3), has also been reported.SMAD3 has no DSGΨXS motif [49], and its phosphorylation may not be necessary for the transforminggrowth-factor-β (TGFβ)-induced degradation [49,50].More interestingly, it has been reported that β-TrCP1 can bind to the nonphosphorylated DDG motif (DDGφXD) in CDC25A and CDC25B, leading to the ubiquitination and degradation of CDC25A and CDC25B [51].
The HIV protein Vpu is a unique β-TrCP-binding molecule, which acts as an adaptor molecule to recruit β-TrCP1 to other cellular proteins to modulate their stability.The proteasomal degradation of cluster of differentiation 4 (CD4) is mediated by recruitment of β-TrCP1 through binding to Vpu, which contains the DSGΨXS motif [52].Phosphoprotein Vpu acts as an adapter protein for ubiquitin-mediated degradation of CD4, which is the major cellular receptor for HIV-1.On the other hand, Vpu interferes with the degradation of IκBα, leading to the downregulation of NF-κB activity [53].The ubiquitous casein kinase II constitutively phosphorylates DSGΨXS motifs in Vpu [54], and this p-Vpu has a dominant negative effect on β-TrCP1 function [53].Unlike the initial demonstrations, Vpu is also degraded by β-TrCP-mediated ubiquitinylation [55].
NF-κB1 p105 is controlled by two proteolytic pathways: complete (degradation) and limited (processing to p50) [58].Since NF-κB1 p105 also functions as an IκB, the degradation of NF-κB1 p105 results in the release of p50, v-rel avian reticuloendotheliosis viral oncogene homolog (c-REL), and v-rel avian reticuloendotheliosis viral oncogene homolog A (RELA) in the cytoplasm, and they translocate into the nucleus to modulate their target gene transcription [59,60].NF-κB1 p105 contains a conserved motif that is similar to the DS(P)GΨXS(P) motif in the C-terminal PEST domain, and the prior phosphorylation of two serine residues by IKKα/β serves as the signal for the degradation of p105 [45,61].
In some cases, the ubiquitination of substrates results in their lysosomal degradation.Membrane receptors, such as interferon α receptor 1 (IFNAR1), are phosphorylated and subsequently bind to β-TrCP2.However, ubiquitinylated INFAR1 is degraded in the lysosome after endocytosis [62].Endocytosis and the degradation of the growth hormone receptor (GHR) are also regulated by β-TrCP2 through GHR binding to β-TrCP2 via its ubiquitin-dependent endocytosis (UbE) motif [63].Interestingly, β-TrCP2 binding to GHR is independent of a prior phosphorylation.
The biological consequence of β-TrCP-mediated degradation and processing depends on the cellular context.The degradation of many substrates promotes tumorigenesis or tumor progression, while the destabilization of other substrates functions to suppress tumors (Figure 2).The detailed list of the substrates, kinases responsible for prior phosphorylation, biological consequences, and β-TrCP paralogues are summarized in Supplementary Table S1.
sis viral oncogene homolog (c-REL), and v-rel avian reticuloendotheliosis viral oncogene homolog A (RELA) in the cytoplasm, and they translocate into the nucleus to modulate their target gene transcription [59,60].NF-κB1 p105 contains a conserved motif that is similar to the DS(P)GΨXS(P) motif in the C-terminal PEST domain, and the prior phosphorylation of two serine residues by IKKα/β serves as the signal for the degradation of p105 [45,61].
In some cases, the ubiquitination of substrates results in their lysosomal degradation.Membrane receptors, such as interferon α receptor 1 (IFNAR1), are phosphorylated and subsequently bind to β-TrCP2.However, ubiquitinylated INFAR1 is degraded in the lysosome after endocytosis [62].Endocytosis and the degradation of the growth hormone receptor (GHR) are also regulated by β-TrCP2 through GHR binding to β-TrCP2 via its ubiquitin-dependent endocytosis (UbE) motif [63].Interestingly, β-TrCP2 binding to GHR is independent of a prior phosphorylation.
Similarly, HIV1 Vpu has also been reported to sequester β-TrCP1 in the cytoplasm, leading to the accumulation of its substrates including β-catenin, IκBα, and ATF4 [65].However, the functional consequences of the inhibition of β-TrCP1 by Vpu is not fully understood.

Limited Processing by β-TrCP
A distinct β-TrCP-recognition motif is found in substrates that are processed limitedly rather than complete destruction (Table 1) [68].In addition, a glycine-rich region (GRR) in the substrates plays the role of a STOP signal for the 26S proteasome's digestion [69][70][71].This limited processing by β-TrCP1 is also dependent on the prior phosphorylation of substrates [68] and subsequent neddylation [72].
Both NF-κB1 p105 and NF-κB2 p100 are limitedly processed by β-TrCP [68,73].The limited processing of NF-κB precursors is differentially regulated by two upstream kinases, IKKβ and IKKα, respectively.Subsequently, the limited processing of NF-κB1 p105 has been reported to be independent of SCF β-TrCP1 , whereas its degradation is mediated by SCF β-TrCP1 , which is stimulated by IKKβ [74].The processing of NF-κB2 p100 is primed by IKKα, which is activated by NF-κB-inducing kinase (NIK) [75].NIK not only phosphorylates its downstream kinase IKKα, but also plays the role of a docking molecule to tether IKKα to p100.
The ubiquitous transcription factor specific protein 1 (SP1) is also regulated by proteolytic processing.Under normal conditions, SP1 is constitutively repressed by N-terminal SUMOylation [76].Upon mitotic stimulation, SP1 is activated by proteolytic processing, which is mediated by a canonical β-TrCP-binding motif in a cyclin-A/cyclin-dependentkinase-2 (CDK2)-mediated phosphorylation-dependent manner [77].However, it remains to be determined which paralogue contributes to SP1 processing.The contribution of β-TrCP1 in the transcription of genes has been reported.β-TrCP1 binds to and co-localizes with the p300 transcriptional coactivator to β-catenin target gene promoters [80].However, unlike other β-TrCP1 binding proteins, p300 is not degraded by β-TrCP1-dependent proteolysis under normal growth conditions.In addition, β-TrCP1 also activates SMAD3-mediated transcription cooperatively with p300 [80].The detailed molecular mechanism of the β-TrCP1-mediated transcriptional regulation remains to be determined.

Stabilization of Oncogene Products by β-TrCP
In contrast to the proteasomal degradation of target proteins by β-TrCPs, they can stabilize proteins through the distinct ubiquitinylation of their target proteins.The ubiquitinylation of MYC by β-TrCPs results in the stabilization of MYC rather than degradation.
The formation of heterotypic polyubiquitin chains on MYC by β-TrCPs antagonizes the FBXW7-mediated degradation of MYC [81].The stability of MYC is controlled by the SCF FBXW7 complex in the GSK3β-dependent phosphorylation of MYC [82,83].However, the phosphorylation of MYC by Polo-like kinase 1 (PLK1) triggers the β-TrCP-dependent ubiquitinylation of MYC and blocks its proteasomal degradation.Consistent with this, the targeted degradation of β-TrCP by small-molecule mTORC1/P70S6K inhibitors reduces MYC protein levels in TNBC cells.
A recent study demonstrated that β-TrCP upregulates the hypoxia-inducible factor 1α (HIF-1α) protein level and its transcriptional activity by competing with its binding to heat shock protein 70 (HSP70)/the carboxy terminus of HSP70-interacting protein (CHIP), antagonizing CHIP E3 ligase activity in prostate cancer [84].Direct binding of β-TrCP to HSP70 disrupts both HSP70-HIF-1α and HSP70-CHIP interaction.This modulation of other E3 ligases by β-TrCP is characteristic since it is not dependent on β-TrCP E3-ligase-mediated proteasomal degradation.

Association of β-TrCP with Cancer
Mounting evidence supports that β-TrCP is oncogenic [15].The mutation or overexpression of β-TrCPs has been associated with the tumorigenesis of various cancers such as skin, gastric, prostate, and colon cancers [22,[85][86][87][88].It has been suggested that the overexpression of β-TrCP1 induces β-catenin accumulation and the activation of the downstream targets of β-catenin such as cyclin D1, glutamine synthetase, and chemotaxin 2, leading to tumorigenesis in these cancers [89].For example, an increase in β-TrCP1 expression has been associated with colorectal cancer, which leads to the activation of β-catenin and the NF-κB pathway [87].High levels of BTRC mRNA and β-TrCP1 have been found in tumor samples from patients with colorectal cancer compared to normal tissues.In addition, high β-TrCP1 levels are significantly linked to decreased apoptosis in tumor cells.In addition, the upregulation of BTRC mRNA and the concordant accumulation of β-TrCP1 in the cytoplasm and nucleus are found in clinical samples of patients with hepatoblastoma and hepatoblastoma cell lines [90].
Somatic BTRC mutations (5.3%), such as A99V, H342Y, H425Y, C206Y, and G260E, have been identified in gastric cancer samples [91].Tumor tissues with these mutations demonstrate moderate to strong cytoplasmic accumulation of β-catenin.However, the functional consequence of these mutations remains to be determined.A 9 bp insertion or deletion (9N ins/del) polymorphism (rs16405) in the 3 -UTR of the BTRC gene has been negatively associated with hepatocellular carcinoma (HCC) risk in a Chinese population [92].Among the rs16405 genotypes, the 9N ins/del and 9N del/del are associated with a reduced HCC risk compared to 9N ins/ins.In addition, the mRNA levels of BTRC with 9N ins/del or 9N del/del were reduced in HCC tumor tissues compared to 9N ins/ins.The 9N del disrupts the binding of miR-920, a negative regulator for β-TrCP1, on the 3 -UTR of the BTRC gene, leading to the upregulation of BTRC mRNA expression [92].On the contrary, the 9N ins/del of the BTRC gene had no association in epithelial ovarian cancer in a Chinese population [93].Furthermore, the cancer-related copy number variation (CNV) of the BTRC gene has been associated with CRC prognosis in 518 Chinese patients (amplification vs. wildtype, hazards ratio = 0.42, 95% confidence interval: 0.19, 0.97, p = 0.05; amplification + deletion vs. wildtype, hazards ratio = 0.39, 95% confidence interval: 0.17, 0.88, p = 0.023) [94].
Due to the β-TrCP1/2 control cell-cycle-dependent activity of CDK1 by regulating its upstream effectors including CDC25 [42,43], WEE1 [95], and F-box only protein 5 (FBXO5) (also known as early mitotic inhibitor 1 (EMI1)) [96,97], the dysregulation of β-TrCP1/2 may contribute to the development of tumors.The increased expression of β-TrCP1 has been reported to confer the constitutive activation of NF-κB in chemoresistant pancreatic cancer cells [98].The targeting of β-TrCP1 by siRNA downregulates NF-κB activity and etoposide resistance in pancreatic cancer cell lines.In addition, IL-1R antagonist treatment partially inhibits β-TrCP1 expression in a chemoresistant pancreatic cancer cell line, PancTu-1.The transient expression of β-TrCP1 induces IL-1β secretion in an NF-κB-dependent manner by degrading IκBα.Consistent with this, a considerable expression of β-TrCP1 is detected in clinical samples of pancreatic ductal adenocarcinoma [98].The overexpression of β-TrCP1 promotes cell proliferation by the activation of TNF-dependent NF-κB in diffuse large B cell lymphoma cells [99].
The potential role of β-TrCP1 in mammary gland tumorigenesis has been reported [100].Mammary-gland-specific hypoplasia has been found in β-TrCP1 −/− female mice.In addition, mammary-gland-specific expression of β-TrCP1 under the control of the mouse mammary tumor virus (MMTV) long terminal repeat promoter induces the proliferation of mammary epithelia and an increased NF-κB DNA binding activity.About 40% of these mice develop tumors such as mammary, ovarian, and uterine carcinomas.On the other hand, the lymphoid-organ-specific expression of β-TrCP1 by the CD4 promoter displays no effects on these organs.
The low-level expression of β-TrCPs has been reported in glioma tissue [21] and associated with the poor survival of patients with glioma [101].A subsequent study demonstrated that the overexpression of β-TrCP reduces migration, invasion, and proliferation in glioma cell lines [102].
The loss of β-TrCP1 is also found in several lung cancer cell lines and subsets of lung cancer specimens [103].In such cases, the stable expression of β-TrCP1, potentially through the downregulation of CDC25A, leads to the negative regulation of cell motility, cell growth in soft agar, and tumor growth in xenografts.
A high level of expression of β-TrCP2 has been reported in human cancer cell lines and primary breast tumors [104].On the contrary, the downregulation of β-TrCP2 has been reported in clinical chondrosarcoma samples [105].In addition, the recovery of β-TrCP2 suppresses chondrosarcoma cell growth and induces apoptosis.A high level of expression of β-TrCP2 has also been reported in patients with lymphocytic leukemia [106].The overexpression of β-TrCP2 in lymphocytic leukemia cells promotes cell proliferation in vitro and tumor formation in vivo through the stimulation of cell cycle progression.
The tumor suppressive function of β-TrCPs is occasionally impaired by the stabilization of their substrates through decreased phosphorylation and/or binding capability.The oncogenic activation of β-catenin is achieved by the decreased phosphorylation of the degradation motif (degron) in cancer cells [107].Interestingly, the induction of β-TrCPs has been reported in cells expressing an oncogenic β-catenin mutant, which leads to the activation of the NF-κB transcription factor [88].In addition, the epigenetic regulation of BTRC and AXIN2 by promoter hypermethylation and histone deacetylation has been associated with nuclear β-catenin accumulation in NCSLC cell lines and patient samples [108].The stabilization of the prolactin receptor (PRLR) is also correlated with enhanced expression of it in breast cancer.The reduced phosphorylation of PRLR in phospho-degron results in inefficient recruitment of β-TrCP and the accumulation of PRLR in breast cancer cells and tissues [109].Targeting PRLR has been reported to exert anticancer effects on breast cancer cells both in vitro and in xenograft models [110].In human medulloblastomas and neuroblastomas, the RE1-silencing transcription factor (REST) plays the role of an oncogene and evades β-TrCP1-mediated degradation by C-terminal truncations [18].The inactivation of kinases for prior phosphorylation is another mechanism of evasion of β-TrCP-mediated degradation in cancer.The inactivation of GSK3β, the kinase for CDC25A priming, has been associated with CDC25A overproduction in human tumor tissues [111].The stabilization of PRLR is also mediated by the human epidermal growth factor receptor 2 (HER2)-/RAS-signaling-induced inhibitory phosphorylation of GSK3β in breast cancer cells, and elevated PRLR levels are correlated with GSK3β inactivation in breast cancer specimens [112].
Taken together, β-TrCP1/2 may function either as an oncogene or tumor suppressor in a cellular-context-dependent manner.A detailed understanding of the complex regulation of β-TrCP1/2 in the differential cellular context will provide new insights into tumorigenesis or tumor suppression and an alternative strategy to develop novel targeted therapeutics.
Post-translational modification of β-TrCPs remains largely unknown [27].As shown in Figure 1, many putative phosphorylation residues are found in β-TrCPs.However, the corresponding kinases are waiting to be discovered.Previously, mTORC2, but not mTORC1, has been reported to inhibit β-TrCP degradation in TNBC cells [27].The pharmacological inhibition of mTORC2 by a small-molecule inhibitor, WYE-354, induces the reduction of β-TrCP levels in a dose-dependent manner.On the contrary, rapamycin does not.In addition, treatment by a PI3K/mTOR inhibitor, PI-103, reduces the serine/threonine phosphorylation of β-TrCP and reduces its protein levels.The PI-103-induced degradation of β-TrCP is dependent on proteasomal activity since MG132 treatment abolishes its degradation in the presence of PI-103.In addition, the knockdown of β-TrCP1 by siRNA markedly reduces the proliferation of TNBC cells in vitro [27].Taken together, the phosphorylation of β-TrCP may contribute to its stability in cancer cells.
Both β-TrCP paralogs are functionally redundant due to their lack of selectivity in substrate recognition.In addition, β-TrCP paralogs reciprocally regulate each other.AMPK is activated and phosphorylates β-TrCP1 for subsequent SCFβ-TrCP2-mediated ubiquitination and degradation in glucose deprivation or serum starvation, and SCFβ-TrCP1 promotes β-TrCP2 ubiquitination and degradation mediated by some unknown kinase(s) [116].In addition, β-TrCP2 inhibits autophagy and senescence and promotes cell proliferation and migration, whereas β-TrCP1 suppresses cell growth in TNBC cells.In addition, β-TrCP2, not β-TrCP1, governs the activity of mTORC1, a central regulator of autophagy and growth, by preferentially degrading the DEP domain-containing mTOR-interacting protein (DEP-TOR) and regulated in development and DNA damage response (1REDD1).DEPTOR and REDD1 are two well-known substrates of SCF β-TrCP and inhibitors of mTORC1.Thus, βTrCP2 acts as a dominant paralog with oncogenic properties in the regulation of cell autophagy and growth [116].Interestingly, it has been reported that the β-TrCP-mediated degradation of HER2 in HER2+ breast cancer cells was abrogated by DEPTOR through its interaction with HER2 [165].DNA damage also activates β-TrCP1 via phosphorylation at S158 by ATM [115].ATM-mediated phosphorylation protects β-TrCP1 from β-TrCP2mediated degradation.Phosphorylated β-TrCP1 enhances the proteasomal degradation of β-TrCP2 and mouse double minute 2 (MDM2), leading to G2/M cell cycle arrest to promote DNA repair in response to DNA damage.Degradation of MDM2 is mediated by the inhibition of the polyubiquitination at K63 of MDM2 by β-TrCP2, which is directly ubiquitinated at K48 by β-TrCP1 and subsequently undergoes proteasomal degradation [115].β-TrCP1 regulates MDM2 negatively by abrogating the K63-linked ubiquitination of MDM2 by β-TrCP2 and promoting the polyubiquitination of MDM2 at K48.Of note, the polyubiquitination of TNF receptor-associated factor 6 (TRAF6) at K63 is reduced by β-TrCP to inhibit lipopolysaccharide (LPS)-induced IKK activation [166].These results imply that β-TrCP paralogs play the role of a differential cellular process by the reciprocal regulation of each other upon various extracellular and intracellular signals.Since β-TrCP1 and β-TrCP2 form either a homodimer or a heterodimer with differential potency in promoting substrate degradation [28], further studies are needed to understand the fine regulation of cellular processes by β-TrCP paralogs.
The overexpression of the tumor suppressor RAS-associated domain-containing protein 1A (RASSF1A) shows an antiproliferative effect and decreasing cyclin D1 levels, potentially by restricting cells in the retinoblastoma (RB) cell cycle checkpoint to prevent them entering the S-phase [167].In addition, RASSF1A inhibits SCF β-TrCP activity to allow the G-to-S transition through the upregulation of the levels of FBXO5/EMI, which blocks anaphase-promoting complex (APC) activity [130].The underlying mechanism of RASSF1A-mediated inhibition of SCF β-TrCP activity remains elusive.
RASSF1C, an isoform of tumor suppressor RASSF1, has been reported to inhibit β-catenin degradation through direct interaction with β-TrCP1 [131].The interaction between RASSF1C and β-TrCP1 is mediated by the SSGYXS motif in the N-terminus of RASSF1C, which is absent in RASSF1A.Although the SSGYXS motif is reminiscent of the phospho-degron motif that is recognized by β-TrCP1, RASSF1C binding to β-TrCP1 is not mediated by WD40 repeats in β-TrCP1.The binding of RASSF1C to β-TrCP1 may inhibit the interaction of β-catenin with β-TrCP1, leading to a change in the β-catenin subcellular localization from the nucleus to the cytoplasm.Interestingly, the silencing of RASSF1A in cells expressing both RASSF1A and RASSF1C is enough to induce β-catenin accumulation.RASSF1C is suggested to be a pseudosubstrate or negative modulator of β-TrCP1 to block the degradation of β-TrCP1 substrates [131].
The inhibition of Janus Kinase 2 (JAK2) either by small-molecule inhibitor AG490 or by knockdown with shRNA results in an increase in β-TrCP and GSK3α/β at both the mRNA and protein level in both human leukemia Jurkat cells and human erythroleukemia HEL cells [168].JAK2-blockade-induced β-TrCP activation leads to the degradation of IκB and the nuclear translocation of NF-κB.However, the exact molecular mechanism of JAK2-reguated β-TrCP activity remains to be determined.
Proto-oncogene SRC (SRC) is a nonreceptor tyrosine kinase that inhibits the Hippo pathway from enhancing tafazzin (TAZ) decay mediated by β-TrCP.TAZ is a transcription coactivator, shuttling from the cytoplasm to the nucleus.Hippo pathway kinase large tumor suppressor homolog 1/2 (LATS1/2) reduces TAZ nuclear localization and minimizes TAZ cytoplasmic levels by the E3 ligase β-TrCP [169].The polyomavirus-middle-T-antigen (PyMT)-mediated SRC activation inhibits TAZ degradation via β-TrCP, leading to the expression of the CTGF and ANKRD1 genes, which are nuclear targets of TAZ and the YES-associated protein (YAP).The inhibition of β-TrCP by SRC is also observed with IκB [138].However, the mechanism involved in the attenuation of β-TrCP E3 ubiquitin ligase activity by SRC remains to be determined.The stability of TAZ in chondrocytes is negatively regulated by tumor protein 53 (TP53) by the physical interaction between TP53 and TAZ, promoting TAZ degradation by β-TrCP [170].A β-TrCP substrate, TIAM1, also contributes to TAZ degradation by enhancing β-TrCP-TAZ interactions to inhibit the invasion of intestinal epithelial cells [171].
Centromere protein W (CENP-W) and heterogeneous nuclear ribonucleoprotein U (hnRNP U) interact with the region of F box and the WD40 domain of β-TrCP1, respectively [122].The interaction complex leads to a stable shuttling complex, resulting in the accumulation of β-TrCP1 in the nucleus and promoting cell migration.It has been proposed that CENP-W may enhance the oncogenic potential of β-TrCP1 by promoting its nuclear accumulation [122].
Ubiquitin-specific peptidase (24USP24) belongs to the superfamily of deubiquitinases (DUBs), which have been correlated with cancer progression.Elevated USP24 in malignant cancer cells and M2 macrophages promotes metastasis by positively regulating IL-6 expression through stabilizing p300 and β-TrCP, leading to increases of histone-3 acetylation and NF-κB and decreases in DNA methyltransferase 1 (DNMT1) and IκB levels [142].However, the underlying mechanism of the USP24-mediated stabilization of β-TrCP remains to be elucidated.
Previously, it has been demonstrated that extracellular stresses, such as ultraviolet (UV) radiation, hydrogen peroxide (H 2 O 2 ), and tumor necrosis factor α (TNFα), upregulate β-TrCP1 via the elevation of its mRNA level in 293T cells [114].The stabilization of β-TrCP1 by H 2 O 2 is achieved by the oxidative modification of cysteine 308 residues in β-TrCP1 [175].Since C308 is required for maximal binding between β-TrCP1s, the oxidation of cysteine thiols results in the diminished degradation of IκBα in lipopolysaccharide-stimulated cells in response to H 2 O 2 exposure, leading to the anti-inflammatory effects of H 2 O 2 in immune cells such as neutrophils.In addition, c-JUN N-terminal kinase (JNK) and p38 have also been reported to upregulate β-TrCP1 through the stabilization of BTRC mRNA [114].Constitutively active mutant upstream kinases of JNK, such as JNK kinase 2 (JNKK2) or MAPK kinase 6 (MKK6), also induce BTRC mRNA.On the contrary, MEK1 or IKKβ does not induce β-TrCP1 accumulation.Again, the effector molecules for JNK/p38, which mediate BTRC mRNA stabilization, are largely unknown.
Mammalian miRNAs regulate various genes' expression through binding on the 3 -UTR of target mRNAs, leading to mRNA degradation.A series of miRNAs has been reported to downregulate either BTRC or FBXW11 mRNA (Table 2).For example, BTRC and FBXW11 contain a highly conserved miR-10a [150] and miR-182 [153] binding site, respectively, within their 3 -UTRs.miR-10a and miR-182 directly bind to the 3 -UTR of BTRC, and FBXW11 degrades their mRNAs in human aortic endothelial cells [150] and in pancreatic cancer cells [153], respectively.The overexpression of miR-182 has been reported to promote pancreatic cancer cell proliferation and migration in a β-TrCP2dependent manner [153].
Long non-coding RNAs (lncRNAs) have a crucial role in the signaling cascade of tumorigenesis and chemoresistance.LncRNAs also contribute to the modulation of β-TrCP activity (Table 2).LncRNA SLC7A11-AS1 expression is elevated in gemcitabine-resistant pancreatic ductal adenocarcinoma (PDAC) cells [161].SLC7A11-AS1 interacts with the F-box motif of β-TrCP1, preventing NRF2 ubiquitination and subsequent proteasomal degradation in the nucleus.Stabilized NRF2 reduces intracellular ROS for the maintenance of PDAC stemness and chemoresistance [161].
Circular RNA has been reported to regulate β-TrCP activity.CircHIPK3 functions as a scaffold for ELAV-like protein 1 (ELAVL1) and β-TrCP1 to enhance β-TrCP1-mediated ubiquitination and degradation of ELAVL1, leading to a decrease in the p21 level and cardiac senescence with a concordant increase in telomere length [145].Studies suggest that circHIPK3 has a dual role in tumorigenesis and tumor progression [176][177][178].Further studies are needed to delineate the circHIPK3-mediated regulation of β-TrCP1 activity over a diverse set of its substrates and their functional consequences in tumor development.

Modulation of β-TrCP Activity by Protein-Protein Interactions
Protein-protein interactions function as regulators of β-TrCP activity or substrate binding (Table 3).Competitive inhibition of the interaction between β-TrCPs and their substrates controls β-TrCPs' function.For example, 14-3-3ζ (also known as YWHAZ) competitively binds to β-catenin to dissociate it from β-TrCP binding [179].Since 14-3-3ζ has been reported to be elevated in many human cancers including NSCLC, the 14-3-3ζmediated upregulation of β-catenin by increasing its stability could be a mechanistic basis for lung cancer malignancy [179].Interestingly, transcription factors are involved in the regulation of β-TrCP activity through physical interactions.Activating enhancer-binding protein 2-β (AP2-β) suppresses the proliferation of cervical cancer cells [180].AP2-β binds to β-TrCP and enhances its activity toward β-catenin.A negative correlation between AP2-β and the β-catenin protein is found in clinical cervical cancer tissues.A xenograft study further demonstrated that AP2-β reduces cervical tumor growth by inhibiting the expression of WNT target genes.
Estrogen receptor α (ERα) is a nuclear hormone receptor that is specifically activated by 17β-estradiol (E2) [188].The treatment by E2 of HA22T HCC cells results in enhanced binding of ERα to β-catenin, triggers the binding of β-catenin and β-TrCP, and promotes the degradation of β-catenin, leading to the inhibition of the migration and invasion of HA22T cells [181].In addition, transcription factor AP2-β also activates the CK1/GSK3βmediated phosphorylation-dependent proteasomal degradation of β-catenin by binding to β-catenin and β-TrCP, leading to the suppression of cervical cancer cell proliferation [180].
RASSF5 is a tumor suppressor and direct RAS effector [189].RAS can stimulate SCF β-TrCP1 via RASSF5.Activated RASSF5 directly forms a complex with β-TrCP1 and enhances β-catenin degradation [182].Interestingly, RASSF5 does not affect IκB stability.The mechanism of this differential regulation of β-TrCP1-mediated degradation by RASSF5 remains to be determined.
Tribbles homolog 2 (TRIB2), a substrate of β-TrCP1 [190], reciprocally inhibits β-TrCP1 activity by protein-protein interaction to stabilize YAP in liver cancer cells [183].In addition, TRIB2 contributes to the negative regulation of WNT/β-catenin/TCF4 signaling specifically in liver cancer cells by physically binding to β-TrCP, COP1, and SMAD ubiquitination regulatory factor 1 (SMURF1) [191].In addition, SMURF1 increases the protein stability of β-TrCP by reducing the autoubiquitination of β-TrCP in liver cancer cells [139].Mechanistically, TRIB2 enhances nuclear co-accumulation of β-TrCP E3 ligases and β-catenin, promoting the destabilization of β-catenin and TCF4 in liver cancer.Taken together, the TRIB2-β-TrCP interaction may contribute to the tight control of β-TrCP activity in a spaciotemporal and/or tissue-specific manner in normal physiological conditions, and the dysregulation of this protein-protein interaction functions in tumorigenesis.Interestingly, TRIB3 has also been reported to modulate β-TrCP activity through direct binding to TAZ.Taken together, WNT signaling is finely regulated by β-TrCP at multiple levels.
Ubiquitin-domain-containing protein 1 (UBTD1) interacts with the E2-ubiquitinconjugating enzymes of the ubiquitin proteasome system [193,194].UBTD1 interacts with the YAP degradation complex and enhances β-TrCP-dependent YAP degradation [187].The mechano-transducer C-X-C chemokine receptor type 4 (CXCR4) downregulates UBTD1 and stabilizes YAP in HCC cells in response to the extracellular matrix's stiffness [187].The stability and activation of YAP1 are also regulated by apurinic/apyrimidinic endonuclease 1 (APE1), which binds to β-TrCP, possibly competing with the YAP1-β-TrCP interaction in response to acidic bile salt exposure in esophageal adenocarcinoma cells [195].Interestingly, extracellular matrix stiffness induces the degradation of mammalian STE20-like protein kinase 2 (MST2), a Hippo kinase, by β-TrCP, in human breast epithelial cells [196].Enhanced MST2 degradation in human breast epithelial cells is also induced by the hyperactivation of integrins via intergrin-linked kinase (ILK) [196].It remains to be determined whether there is a differential regulation of the Hippo pathway via β-TrCP in normal and cancerous cells in response to extracellular matrix stiffness or not.MEK1 also interacts with YAP to promote its stability independent of MST/LATS/Hippo and ERK in liver cancer cells [197].Importantly, MEK1-YAP interaction promotes tumorigenesis in liver cancer cells.

Modulation of β-TrCP Activity by Viral Proteins
Interestingly, viral oncoproteins target β-TrCP to suppress immune reaction or tumor suppression (Table 4).The A49 protein of poxvirus inhibits β-TrCP-dependent IκBα degradation by molecular mimicry [198].It contains a motif conserved in IκBα and phosphorylated by IKKβ and subsequently binds to β-TrCP to prevent IκBα ubiquitination and degradation.As a result, the activity of NF-κB is reduced and immune evasion is promoted.The adenoviral E1A protein upregulates β-TrCP1 by unknown mechanisms to induce β-TrCP1-dependent degradation of the REST tumor suppressor, leading to viral transformation [199].
The ORF61 of simian varicella virus and the varicella-zoster virus also inhibit the NF-κB pathway by binding to β-TrCP [204].The ORF2 of the hepatitis E virus binds to β-TrCP to inhibit IκBα degradation, leading to the suppression of host immune reaction [205].

Modulation of β-TrCP Activity by Subcellular Localization
The regulation of the subcellular localization of β-TrCPs is another mechanism to control the β-TrCP-dependent degradation of their target proteins.For example, in glioblas-toma cell lines and patient-derived tumor neurospheres, the mislocalization of β-TrCP1 in the nucleus has been reported to uncouple PH domain leucine-rich repeat-containing protein phosphatase 1 (PHLPP1)/the AKT negative feedback loop [209].Consistent with this, the restoration of β-TrCP1 in the cytoplasm rescues the regulation of PHLPP1 stability by AKT.The nuclear localization of MEK also contributes to the enhanced nuclear localization of β-TrCP  5) or a dominant negative β-TrCP ∆F mutant inhibits the growth and survival of human breast cancer cells and augments the cytotoxic effects of anticancer drugs including doxorubicin, tamoxifen, and paclitaxel [211].In addition, the stable expression of β-TrCP1 ∆F in murine myeloma cells has been reported to reduce myeloma cell growth and survival in mice independent of the host immune status [212].The administration of an IκB-ubiquitin ligase inhibitor, pyrrolidine dithiocarbamate (PDTC) [213], to wildtype β-TrCP1 myeloma tumor-bearing mice reduces tumor burden in the bone.The transgenic expression of β-TrCP2 ∆F in mouse skin results in a decrease in UVB-induced edema, hyperplasia, and inflammatory response and an increase in UVB-induced apoptosis [214].These results suggest that targeting β-TrCP activity provides a therapeutic opportunity, at least in specific types of cancer.
As mentioned earlier, the phosphorylation of β-TrCP may contribute to its stability in TNBC cells [27].A small-molecule kinase inhibitor, PI-103, targeting PI3K/mTOR, reduces the levels of β-TrCP in a series of TNBC cells and inhibits cell viability.The β-TrCP target proteins including cyclin E and MYC are downregulated by PI-103 treatment in a dosedependent manner.In addition, siRNA-based knockdown of β-TrCP1 markedly reduces the proliferation of TNBC cells.
In prostate cancer cells, the knockdown of β-TrCP1/2 results in a reduction of cancer cell growth both in vitro and in vivo [215].The depletion of β-TrCP1/2 induces aryl hydrocarbon receptor (AhR) expression in a ligand-independent manner.How does β-TrCP1/2 depletion induce AhR and its consequences remain to be determined.

Small Molecule Compounds That Modulate β-TrCP Activity
Small-molecule compounds have also been found to inhibit β-TrCPs' function either directly or indirectly (Table 6).An example of a β-TrCP-specific inhibitor has been developed from a ubiquitin-based engineered inhibitor for β-TrCP2 using the phage display technique [218].Interestingly, the ubiquitin-based inhibitors competitively bind on the SKP1-F-box interface to block CUL1 binding to the same site, resulting in the inhibition of ligase activity.Further engineering of inhibitors results in developing a highly specific inhibitor for β-TrCP2, while it binds very weakly to β-TrCP1 and does not bind to other E3 ligases tested.Further clinical development of this inhibitor remains to be disclosed.For example, erioflorin interferes with the interaction between β-TrCP1 and programmed cell death protein 4 (PDCD4) and stabilizes PDCD4 protein levels with a concomitant alteration of the cell cycle progression and suppression of the cell proliferation of various cancer cell lines [219].

Echinomycin HIF1
• inhibits the expression of β-TrCPs, leading to reduced lung adenocarcinoma and lymphoma tumor growth by blocking the expression of the MYC and HIF-1α proteins [231] Erioflorin β-TrCP1 • inhibits the interaction between β-TrCP1 and PDCD4 • alters cell cycle progression and suppresses the cell proliferation of various cancer cell lines [219] Euphorbiasteroid AMPK • increases β-TrCP with a concomitant reduction of p-GSK3β S9 and exerts anticancer activity in NSCLC cells [232] Fisetin SIRTs • upregulates β-TrCPs by an unknown mechanism, leading to the degradation of β-catenin in human melanoma cells [233] Gallic acid - • induces the expression of β-TrCPs, by an unknown mechanism, leading to a reduction in prostate cancer growth by reducing the REST protein's stability [236] MLN4924 (pevonedistat) NEDD8 • inhibits β-TrCPs' ubiquitinylation and degradation via blocking neddylation, leading to mitochondrial fusion by inducing MFN1 accumulation, resulting in anticancer effects [237] • suppresses the growth of liver cancer cell through IκBα degradation via the accumulation of β-TrCPs  A recent study reported that a small molecule enhances the β-TrCP-β-catenin interaction [247].As these kinds of drugs target naturally occurring protein-protein interactions, the molecular glue strategy may provide an alternative method to interfere with hard-totarget proteins [247].However, the clinical implications of these small molecules remain to be determined.
Targeting cancer cells based on the Warburg effect, metabolic shifting to aerobic glycolysis, by natural-product-based energy restriction-mimetic agents (ERMAs), is a new potential cancer therapy strategy [248][249][250].Thiazolidinediones (TZDs) have been developed as selective ligands for peroxisome-proliferator-activated receptor gamma (PPARγ) and have been identified as a novel class of ERMAs [251].TZDs activate the β-TrCP1mediated proteolysis of its target proteins such as β-catenin, cyclin D1, and SP1 via an increase in the β-TrCP1 expression level [245,246,252].When glucose is deprived, TZDs activate silent information regulator 1 (SIRT1), AMP-activated protein kinase (AMPK), and ER stress [251].In addition, TZDs upregulate β-TrCP1 through protein stabilization in an SIRT1-dependnet manner.The energy restriction by TZDs induces apoptosis via β-TrCP1mediated proteasomal degradation and transcriptional repression in cancer cells [251].Since SIRT1 is an NAD + -dependent deacetylase, the potential involvement of acetylation in the regulation of β-TrCP1 stability remains to be elucidated.
Hydroquinone (HQ) induces the demethylation of the Forkhead box protein P3 (FOXP3) gene, resulting in FOXP3 gene expression in U937 cells [155].As a result, FOXP3 induces the expression of miR-183, leading to a reduction in β-TrCP1 mRNA stability.The downregulation of β-TrCP1 results in the upregulation of its target SP1 expression in U937 cells.SP1-induced ADAM17 and LYB contributes to the proliferation and clonogenicity of U937 cells.
As mentioned earlier, JAK2 negatively regulates β-TrCP activity in both human leukemia Jurkat cells and human erythroleukemia HEL cells [168].The knockdown of β-TrCP1 by shRNA results in reversing the downregulation of β-catenin in the presence of the JAK2 inhibitor, AG490.However, the role of β-TrCP1 in leukemia remains elusive.

Conclusions
β-TrCP1/2 function as either oncogenes or tumor suppressors in a cellular-contextdependent manner.Compared to the long lists of β-TrCP1/2 substrates and their functions, upstream effectors that regulate the expression of mRNAs and the protein stability, function, and localization of β-TrCP1/2 have not been well established yet.Since evidence suggests that β-TrCP1/2 are potential targets to treat certain types of cancers, further studies on the transcriptional and post-translational modulation on β-TrCP1/2 warrant the development of new therapeutic entities to overcome malignant diseases.

7 .
[210].The sequestration of β-TrCP in the nucleus results in the stabilization of YAP in KRAS mutant cancer cells.In addition, the inhibition of mutant KRAS triggers MEK nuclear transportation, leading to KRAS-targeted drug resistance [210].Targeting β-TrCP in Cancer 7.1.β-TrCP as a Target for Cancer Treatment Since β-TrCPs upregulates the NF-κB activities that are important for cancer cells' survival, targeting β-TrCPs has been suggested as a potential effective means to treat cancer [17].Early evidence that β-TrCPs is a potential target to treat cancer has been demonstrated in several human breast cancer cells.The inhibition of β-TrCPs by either siRNA (Table

Table 1 .
β-TrCP substrates that are modified by the proteasome-mediated limited processing.

Table 5 .
The effects of β-TrCP silencing in cancer.