Ubiquitin-conjugating enzyme complex Uev1A-Ubc13 promotes breast cancer metastasis through nuclear factor-кB mediated matrix metalloproteinase-1 gene regulation
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UEV1A encodes a ubiquitin-conjugating enzyme variant (Ubc13), which is required for Ubc13-catalyzed Lys63-linked polyubiquitination of target proteins and nuclear factor κB (NF-кB) activation. Previous reports have correlated the level of UEV1A expression with tumorigenesis; however, the detailed molecular events leading to tumors particularly breast cancer and metastasis are unclear. This study is to investigate roles of different UEV1 splicing variants, and its close homolog MMS2, in promoting tumorigenesis and metastasis in breast cancer cells.
We experimentally manipulated the UEV1 and MMS2 levels in MDA-MB-231 breast cancer cells and monitored their effects on cell invasion and migration, as well as tumor formation and metastasis in xenograft mice. The underlying molecular mechanisms leading to metastasis were also examined.
It was found that overexpression of UEV1A alone, but not UEV1C or MMS2, is sufficient to induce cell invasion in vitro and metastasis in vivo. This process is mediated by NF-κB activation and requires functional Ubc13. Our experimental data establish that among NF-κB target genes, UEV1A-regulated matrix metalloproteinase-1 (MMP1) expression plays a critical role in cell invasion and metastasis. Interestingly, experimental depletion of UEV1 in MDA-MB-231 cells reduces MMP1 expression and prevents tumor formation and metastasis in a xenograft mouse model, while overexpression of MMP1 overrides the metastasis effects in UEV1-depleted cells.
These results identify UEV1A as a potential therapeutic target in the treatment of metastasic breast cancers.
KeywordsBreast Cancer Breast Cancer Cell Electrophoretic Mobility Shift Assay Xenograft Mouse Model MMP1 Promoter
Dulbecco's modified Eagle's medium
epidermal growth factor
electrophoretic mobility shift assay
hematoxylin and eosin
inhibitor of NF-κB alpha
nuclear factor of kappa-light polypeptide gene enhancer in B-cells
polymerase chain reaction
small hairpin RNA
small interfering RNA
tumor necrosis factor
tumor necrosis factor receptor-associated factor
ubiquitin conjugating enzyme variant.
UEV1, also known as CROC1 or CIR1, encodes a ubiquitin (Ub)-conjugating enzyme variant (Uev) . It was also identified as a mammalian homolog of yeast MMS2 and as a co-factor of Ubc13 . Indeed, a Uev (Uev1A or Mms2) is absolutely required for Ubc13-mediated Lys(K)63-linked polyubiquitin chain assembly [5, 6, 7, 8, 9]. Despite their shared biochemical activity, Mms2 and Uev1A appear to function differently in mammalian cells: Ubc13-Mms2 is required for DNA-damage responses whereas Ubc13-Uev1A is involved in nuclear factor κB (NF-κB) activation . Previous studies implicate UEV1 as a potential proto-oncogene. UEV1 was initially identified as a transactivator of the c-fos promoter . It is downregulated when HT29-M6 colon cancer cells undergo chemical-induced differentiation, and upregulated when Simian virus 40-transformed human embryonic kidney cells become immortal . Furthermore, UEV1 is variably upregulated in all tumor cell lines examined , and maps to chromosome 20q13.2 , a region where DNA amplification is frequently reported in breast cancers [11, 12, 13, 14] and other tumors , as well as in virus-transformed immortal cells .
Ubc13-Uev1A is involved in NF-κB activation [10, 17, 18] and inhibits stress-induced apoptosis in HepG2 cells . Very recently, it was reported that a small-molecule inhibitor of Ubc13-Uev1A interaction can inhibit proliferation and survival of diffuse large B-cell lymphoma cells . These observations collectively establish a close correlation between UEV1 expression and tumorigenic potential; however, whether UEV1 plays a role in promoting tumorigenesis or progression and how this is accomplished remains to be elucidated.
NF-кB is a sequence-specific transcription factor known to be involved in innate immunity, anti-apoptosis and inflammation [21, 22, 23], and its uncontrolled activation is associated with several types of cancers including breast cancer [24, 25]. It regulates a panel of genes that collectively play pro-survival and anti-apoptotic roles [26, 27]. It also controls the expression of genes linked with invasion, angiogenesis, and metastasis of cancer, including the matrix metalloproteinase (MMP) family [28, 29] and chemokine (C-X-C motif) ligand (CXCL) family genes [30, 31]. Activation of NF-κB is a tightly regulated event. In normal cells, NF-κB becomes activated only after the appropriate stimulation, and then it upregulates the transcription of its target genes . NF-κB is activated by many divergent stimuli, including proinflammatory cytokines such as TNF-α, IL-1b, epidermal growth factor (EGF), T- and B-cell mitogens and bacterial lipopolysaccharides (LPS) . Previous studies reported that the Uev1A-Ubc13 heterodimer is involved in TNF receptor-associated factor 6 (TRAF6) [17, 33] and TRAF2-mediated  activation of NF-κB, in which Ubc13-Uev1A probably serves as a unique Ubc/E2 along with TRAF proteins to polyubiquitinate NF-κB essential modulator/inhibitor of κB proteinkinase (NEMO/IKKγ) [18, 35] and/or Rieske iron-sulfur polypeptide 1(RIP1)  to activate IκB kinase (IKK). Activated IKK leads to the phosphorylation and degradation of IκBα, resulting in the release of NF-κB RelA (p65) subunits to translocate into the nucleus .
In this study we demonstrate that in MDA-MB-231 breast cancer cells, the UEV1A transcript level is moderately elevated compared to normal breast cells. Overexpression of UEV1A alone in MDA-MB-231 cells is sufficient to activate NF-кB, which in turn upregulates the MMP1 expression to enhance breast cancer cell metastasis. More importantly, experimental depletion of Uev1 in MDA-MB-231 cells reduces MMP1 expression and reduces their ability to grow tumors and metastasize in a xenograft mouse model. These observations provide the experimental and theoretical cornerstone for therapeutic targeting of Uev1A in the treatment of metastatic breast cancers.
Human breast cancer cell lines MDA-MB-231, MCF7, MDA-MB-468, MDA-MB-361, MDA-MB-453, MDA-MB-436 and SK-BRIII were obtained from the American Type Culture Collection (ATCC, Manassan, VA, USA). The cells were cultured in Dulbecco’s minimum essential medium (DMEM) (Invitrogen, Burlington, ON, Canada) supplemented with 10% fetal bovine serum, 100 units/ml penicillin, and 100 μg/ml streptomycin (Invitrogen) in a 5% CO2 atmosphere at 37°C. The MCF10A immortalized human mammary epithelial cells were obtained from ATCC and cultured in DMEM/F12 medium supplemented with 10% horse serum, 100 units/ml penicillin, 100 μg/ml streptomycin (Invitrogen), 10 μg/ml insulin (Sigma, St. Louis, MO, USA), 100 ng/ml choleratoxin (Sigma), 0.5 mg/ml hydrocortisone (Sigma), and 20 ng/ml EGF (Peprotech). MDA-MB-231-TR stable cell lines were created by transfecting MDA-MB-231 cell lines with pLenti6-TR-lentivirus (Invitrogen) and selecting with 10 μg/ml blasticidin (Invitrogen).
Plasmids and pLentivirus vector preparation
Human MMS2, UEV1A, and UEV1C open reading frames (ORFs) were PCR-amplified as KpnI-XhoI fragments and cloned into the pcDNA4.0/TO/HA(+) plasmid vector. The 1.9-kb human MMP1 promoter sequence [GenBank: AJ002550.1] was PCR-amplified as a KpnI-HindIII fragment and then cloned into the same sites of pGL4.2 (Invitrogen). The NF-кB target site was subsequently mutated by site-directed mutagenesis using a quick-exchange method (Stratagene, La Jolla, CA, USA). The sense primer for creating the NF-кB binding site mutation is 5′-AAAGG CAGAA GGGAA CCTCA AGAGG TTTTG AAGAG CCACC GTAAA GTGAG-3′ (mutated sequence italicized). The mutated Ubc13-binding site (F38E) in Uev1A was designed based on a previous study with Mms2-F13E . The modified sequence for UEV1 small hairpin RNA (shRNA) delivered by lentiviral particles was from Santa Cruz Biotechnology, Inc (Dallas, Texas, USA). The lentiviral particle infection of breast cancer cells was performed following instructions of the supplier. The MMP1 and MMP9 small interfering RNAs (siRNAs) were purchased from Genepharma Co Ltd (Shanghai, China). The sequence for MMP1 siRNA is 5′- GCGUGUGACAGUAAGCUAATT-3′ and that for MMP9 siRNA is 5′-CGCUCAUGUACCCUAUGUATT-3′.
RNA preparation and real-time RT-PCR (qRT-PCR)
Total RNA was prepared from cultured breast cancer cells by using TRIzol reagent (Invitrogen). First-strand cDNA was synthesized from total RNA with SuperScript (Invitrogen) according to manufacturer’s instructions. The human breast cancer cDNAs TissueScan™ cancer qPCR Arrays (#BCRT102) were purchased from Origene (Beijing, China). The clinical information is shown on the website  and Additional file 1. qRT-PCR analysis was performed on the iQ5 cycler (Bio-Rad, Hercules, CA, USA). The specific primer sets were as follows: GADPH, 5′- GAAGGTGAAGGTCGGAGTC-3′ and 5′- GAAGATGGTGATGGGATTTC-3′; UEV1, 5′- TCTCCACAGCAATCCTATGAGGTTGA-3′ and 5′- CCAACAGTCGGAAATTGCGAGGG-3′; UEV1A, 5′- GAGAGGTTCAAGCGTCTTACCTGAA-3′ and 5′-ACTGTGCCATCTCCTACTCCTTTCT -3′; UEV1C, 5′-GCAGCCACCACGGGCTCG-3′ and 5′- CAATTATCATCCCTGTCCATCTTGT-3′; MMS2, 5′- CGCTTGTTGGAAGAACTTGA-3′ and 5′- CGGAGGAGCTTCTGGGTAT-3′; MMP1 5′- AAATGCAGGAATTCTTTGGG-3′ and 5′-ATGGTCCACATCTGCTCTTG-3′; MMP9 5′-CATCGTCATCCAGTTTGGTG-3′ and 5′- TCGAAGATGAAGGGGAAGTG-3′. The relative expression levels were calculated using the comparative cycle threshold (CT) method (2-ΔCT) on the Bio-Rad iQ5 (Bio-Rad).
Luciferase reporter assay
Cells were seeded in 24-well plates at a density of 1 × 105. After 24 hr, the cells were transfected using X-tremeGENE HP DNA Transfection Reagent (Roche, Indianapolis, IN, USA). Briefly, luciferase reporter gene constructs (400 ng), pcDNA-Uevs plasmids (400 ng) and the pRL-SV40 Renilla luciferase construct (5 ng) (for normalization) were co-transfected into the wells. Cell extracts were prepared 48 hr after transfection and the luciferase activity was measured using the Dual-Luciferase reporter assay system (Promega, Madison, WI, USA).
Western blot analysis
Cells were grown to log phase and lysed in Dulbecco’s PBS (150 mM NaCl, 10 mM Na2HPO4 and 10 mM NaH2PO4, pH 7.4) with 1% SDS and the protease inhibitor cocktail for mammalian cells (Sigma-Aldrich). Total protein concentration was determined by the Bradford method using a commercial reagent from Bio-Rad. Cell extracts or purified proteins were electrophoresed in 10% or 15% SDS-polyacrylamide gel electrophoresis (PAGE) gels, transferred to polyvinyl difluoride (PVDF) membrane, and incubated with specific primary antibodies. Monoclonal antibodies (mAbs) LN2B (anti-Uev1) and 4E11 (anti-Ubc13) were from the laboratory stock [10, 39]. Primary antibodies against HA (sc-7392), MMP1 (sc-30069), NF-κB (sc-372), Lamin B (sc-166729), β-tublin (sc-6216), and secondary goat anti-mouse antibody IgG-horseradish peroxidase (HRP) (sc-2005) and goat anti-rabbit IgG-HRP (sc-2004) antibody were from Santa Cruz. The P-IκBα antibody (#2859S) was from Cell Signaling Technology (Whitby, ON, Canada), while an anti-MMP9 antibody (ab38898) was from Abcam (Toronto, ON, Canada) and β-actin antibody (BM0627) was from Boster (Wunah, China).
We immunoprecipitated 1 mg of protein samples in a total volume of 1 ml with 2 μg of antibody and 20 μl of Protein-A beads (for rabbit polyclonal antibodies) or Protein-G beads (for mouse monoclonal antibodies). The samples were rotated at 4°C overnight. The beads were washed 4 times with 1 ml of cold NP40 lysis buffer containing protease inhibitors. The beads were then boiled for 10 minutes in the presence of 25 μl 2 × sample buffer and the released proteins fractionated by SDS-PAGE in 12% or 15% gels. Proteins were detected by immunoblotting as described above.
Cell invasion and migration assays
In vitro invasion assays were conducted using Transwells (Costar, Cambridge, MA, USA) with 8-μm-pore-size polycarbonate membrane filters in 24-well culture plates. The upper surface of the filter was coated with Matrigel (Becton Dickinson, Bedford, MA, USA) in a volume of 12.5 μl per filter. The Matrigel was dried and reconstituted at 37°C into a solid gel on the filter surface. The lower surface of the filter was coated in fibronectin (20 μg/ml), vitronectin (10 μg/ml), collagen IV (50 μg/ml), or 10% (BSA)-DMEM as chemoattractants. After starving in BSA-free DMEM overnight, 2 × 104 cells were seeded in the upper chamber. The cells were allowed to invade for 48 hr. Cells that invaded the lower surface of the filter were counted in five random fields under a light-microscope at high magnification. Experiments were conducted at least in triplicate. The in vitro cell migration ability was detected by wound healing assay or transwell assay without Matrigel coating. The wound healing assay was performed by seeding 2 × 105 cells onto 96-well plates. Confluent monolayers were wounded using a pipette tip. After 48 hr the cell migration distance was measured under a light-microscope at high magnification in at least three random fields. The transwell assay without Matrigel was conducted using Transwells with 8-μm-pore-size polycarbonate membrane filters in 24-well culture plates. The lower surface of the filter was coated with 10% BSA-DMEM as chemoattractants. After starving in a BSA-free DMEM medium overnight, 5 × 104 cells were seeded in the upper chamber. Cells were allowed to invade for 6 to 36 hr and those migrated to the lower surface of the filter were counted in five random fields under a light-microscope at high magnification. Experiments were conducted at least in triplicate.
Metastasis assay in a xenograft mouse model
The mice were sacrificed 6 weeks after cell injection. For experimental metastasis assays by intravenous (i.v.) injection, 2 × 105 breast cancer cells were injected into the tail veins of the 4- to 5-week-old female BALB/c nude mice. Endpoint assays were conducted 5 weeks after injection. Metastatic lung nodules 0.5 mm in diameter were counted. Analysis of variance (ANOVA) was used for statistical analyses. To ensure representative sampling of lung tumor nodules, four sections were made per lung at various depths along the coronal plane of the lung. The nodules per lung (four sections) were counted under a light-microscope.
Formalin-fixed lungs were paraffin-embedded, and tissue sections derived from tumor nodules or other tissues were stained with H&E to evaluate the morphology and invasiveness of breast cancer cells. Metastatic tumor nodules were counted throughout the entire lung section at all three depths under a light-microscope. Anti-Uev1A (LN1), anti-MMP1 (sc-30069) and NF-κB p65 (sc-372) primary antibodies from Santa Cruz were used for immunohistochemistry (IHC). TissueFocus™ breast cancer tissue microarrays (CT565863) for IHC were obtained from Origene (Beijing, China). Microscopic images were captured by a SPOT digital camera mounted in a light-microscope.
Preparation of nuclear fraction
HeLa cells were treated with 40 ng/ml TNF-α for 2 hr. Cells were washed, scraped with PBS, and centrifuged at 3,000 rpm at 4°C. The pellet was suspended in 10 mM Tris (pH 8.0) with 1.5 mM MgCl2, 1 mM dithiothreitol, and 0.1% NP-40, and incubated on ice for 15 minutes. Nuclei were separated from cytosol by centrifugation at 12,000 rpm at 4°C for 15 minutes. The cytosolic supernatants were removed and the precipitated pellets were suspended in 10 mM Tris (pH 8.0) containing 100 mM NaCl and stored on ice for 30 minutes. After agitation for 30 minutes at 4°C, the lysate was centrifuged at 12,000 rpm for 15 minutes at 4°C, and the supernatant was collected.
Electrophoretic mobility shift assay (EMSA)
The secquence of biotin-labelled sense NF-κB probe for EMSA is 5′-GAACCTCAGAGAACCCCGAAGAGCC-3′. The cold probe is the same NF-κB sequence without biotin label. The sequence of biotin-labelled mutated sense NF-κB probe is 5′-GAA CCTCA AGAGGTTTTG AAGAGCC-3′ (mutated sequence underlined): 1 ng of the probe was incubated together with 10 to 20 μg of cell extracts or 5 to 10 μg of nuclear extracts for 30 minutes at 25°C in a final volume of 20 μl. The binding reaction was subsequently separated on a 5.5-7% poly-acrylamide gel in 1x Tris-Borate-EDTA (TBE) buffer (90 mM Tris, 90 mM boric acid).
The statistical significance of differential findings between the experimental and control groups was determined by Student’s t-test as implemented by Microsoft Excel 2010 (*P <0.05, **P <0.01 and ***P <0.001).
Alternative UEV1transcript levels in breast cancer cell lines and samples
Western blot analysis of endogenous Uev1s using a Uev1-specific monoclonal antibody LN2B  could only detect Uev1C in several breast cancer cell lines including MDA-MB-231 and MCF7, and MCF10A, an immortalized normal mammary epithelial cell line (Figure 1B). qRT-PCT analysis revealed that the relative transcript level of UEV1C is approximately 100-fold higher than that of UEV1A in the above cell lines (Figure 1C), consistent with observations in other cell lines including HeLa and U2OS (data not shown).
We next examined relative transcript levels of UEV1A and UEV1C in breast cancer lines using MCF10A as a reference. Interestingly, the UEV1A transcript level is elevated in all breast cancer cell lines examined (Figure 1D), with no significant upregulation of UEV1C (Figure 1E) or MMS2 (Additional file 2: Figure S1A) in these lines. It has been previously reported that the UEV1A level may be elevated when normal cells undergo immortalization . To further assess relative UEV1A expression in normal versus breast tumor tissues, we measured the UEV1A transcript level in five normal human breast samples and 43 breast cancer samples from TissueScan microarrays. Compared with the five normal human breast samples, 33/43 or 77% of breast cancer samples display UEV1A expression above the highest UEV1A level in normal samples, or at least 1.7-fold higher than the average level among the five normal samples (Additional file 2: Figure S1B), suggesting that Uev1A may play a role in promoting breast tumorigenesis.
Overexpression of UEV1A promotes breast cancer cell invasion in vitroand metastasis in a xenograft model
The cell growth and cell cycle progression of stable MDA-MB-231 transfectants were first measured, with no obvious alterations among each group (Additional file 2: Figure S3). The effects of ecotopic genes on breast cancer cell migration and invasion were then measured. The wound-healing experimental data show that the overexpression of UEV1A doubles the mobility compared with the same cells without ecotopic UEV1A expression or vector-transfected MDA-MB-231-TR cells with Dox treatment, while overexpression of UEV1C or MMS2 does not affect cell mobility compared with uninduced cells (Figure 2B, C and Additional file 2: Figure S4). In a transwell assay, the invasiveness of UEV1A transfectants after induction was approximately 2.3-fold higher than the control, UEV1C or MMS2 transfectants, whereas there was no significant difference among control, UEV1C and MMS2 transfectants regardless of Dox treatment (Figure 2D and E). These results suggest that UEV1A regulates breast cancer cell migration and invasion in vitro.
As an increased ability of cancer cells to migrate and invade in vitro is a faithful indicator of cell metastasis, to further confirm the correlation between UEV1A expression and breast cancer metastasis, we assessed the effects of UEV1A on metastasis using an in vivo xenograft mouse model. Stably-transfected MDA-MB-231-TR cells were injected into the lateral flanks of 4- to 5-week-old BALB/c female nude mice and Dox (625 mg/kg) was added in feed as soon as the cells were injected. Tumor growth and metastasis were then monitored. All mice (10/10) injected with the UEV1A-expressing cells had massively enlarged lymph nodes containing invasive breast cancer cells (Figure 2F). In contrast, there were no tumor metastasis foci in lymph nodes of mice injected with vector control or UEV1C-expressing cells, although some lymph nodes were enlarged (data not shown). Furthermore, overexpression of UEV1A but not UEV1C accelerated tumor growth compared to vector-transfected cells (Figure 2G). In the tail-vein injection groups, compared with UEV1C or vector control, overexpression of UEV1A significantly promoted lung metastasis colony fomation (Figure 2H, and I). These observations collectively demonstrate that elevated expression of UEV1A alone is sufficient to promote tumor growth and metastasis.
Depletion of Uev1 prevents breast cancer cell invasion in vitroand metastasis in nude mice
Overexpression of UEV1Aactivates NF-κB in MDA-MB-231 cells in a Ubc13-dependent manner
It has been previously reported that Uev1A is a cofactor of Ubc13 in the NF-κB signaling pathway [10, 17]. To ask whether the above Uev1A function is indeed dependent on Ubc13, we created a Uev1A-F38E mutation as the corresponding Mms2-F12E mutation (Figure 1A) absolutely abolishes its interaction with Ubc13 and its ability to promote Ubc13-mediated K63 polyubiquitination . Indeed we confirmed that the Uev1A-F38E substitution does not affect its expression (Additional file 2: Figure S2F) but abolishes its interaction with Ubc13 in vivo (Figure 4C). As expected, overexpression of UEV1A-F38E failed to activate NF-κB, as judged by a lack of IκBα phosphorylation and p65 nucleaar translocation (Figure 4D).
MMP1 and MMP9 are tightly regulated by UEV1
MMP1 is a downstream effector for Uev1A-induced metastasis
UevA-Ubc13 control MMP1expression by regulating NF-κB
It has been reported previously that human cells contain two UEV genes, UEV1 and MMS2, which share >90% amino acid sequence-identity in their core domains . Although both Uev1A and Mms2 proteins serve as cofactors for Ubc13-mediated K63-linked polyubiquitination, their biological functions are apparently distinct and only the Uev1A-Ubc13 complex is involved in NF-κB signaling . It has been puzzling us that Uev1A and Mms2 have different molecular weights and migrate differently; however, a monoclonal antibody capable of recognizing both purified Uev1A and Mms2 only detects a single band in western blot analysis, and siRNA depletion of either Mms2 or Uev1 only partially reduced the intensity of this band. A careful examination in this study reveals that in addition to the previously reported two UEV1 splicing variants UEV1A and UEV1B, cultured human cells contain a novel UEV1 splicing variant, UEV1C, lacking the N-terminal 30 amino acid unique region of Uev1A. The resulting 147-residue protein would co-migrate with Mms2 during electrophoresis. It turns out that the UEV1C transcript is much more abundant than UEV1A, and a Uev1-specific monoclonal antibody can detect cellular Uev1C but not Uev1A, unless the latter is experimentally overexpressed.
The current study investigates roles of UEV1A, UEV1C and MMS2 in tumorigenesis using a breast cancer model. With comparable levels of ectopic expression, it was found that only UEV1A, but not UEV1C or MMS2, is able to promote cell migration and invasion. Similarly, overexpression of UEV1A, but not UEV1C promotes tumor growth and metastasis in a xenograft mouse model. The above results are highly reliable, as the target gene expression is under tight regulation of a Tet-on promoter, and the phenotypes were only observed under Dox-induced conditions. In a reverse experiment, depletion of Uev1 in cultured breast cancer cells significantly reduces cell migration and invasion, as well as tumor growth and metastasis in a dose-dependent manner, indicating that the cellular Uev1 (presumbly Uev1A) level plays a critical role in breast tumorigenesis and metastasis.
To understand the molecular mechanism by which Uev1A promotes tumorigenesis, we demonstrated that overexpression of UEV1A, but not UEV1C or MMS2, is able to promote IκBα phorsphorylation and NF-κB translocation into the nucleus, and that this effect absolutely relies on its physical interaction with Ubc13. It is conceivable that as previously reported, the Ubc13-Uev1A heteromider serves as an E2 to assemble K63-linked poly-Ub chains along with cognate really interesting new gene (RING)-finger E3s like TRAF2 and/or TRAF6 [17, 34], which recruit K63 polyUb-binding proteins like NEMO  and TAB2/3 [47, 48] to phorsphorylate and subsequently degrade IκBα, leading to NF-κB activation.
NF-κB activation promotes the transcription of many downstream genes in the signaling cascade . However, the moderate level of NF-κB activation by UEV1A overexpression does not appear to induce all NF-κB targeting genes. To understand how overexpression of UEV1A leads to tumorigenesis and particularly metastasis in breast cancer cells, we surveyed NF-κB and metastasis-related genes and focused on two candidate genes, MMP1 and MMP9, both of which are highly induced upon UEV1A overexpression. Experimental results as presented in this report indicate that both genes are tightly regulatd by cellular Uev1 levels; however, depletion of MMP9 was not as effective as that of MMP1 on cell migration and invasion. As ecotopic expression of MMP1 to restore the wild-type level in Uev1-depleted cells also restored wild-type level of invasiveness, it is plausible to conclude that MMP1 is the critical downstream effector of UEV1A-induced breast cancer metastasis, although this study does not rule out the contributions of MMP9 and possibly other genes. The signal transduction cascade of Uev1A → NF-κB → MMP1 → metastasis is further confirmed by showing that Uev1A-induced MMP1 expression is dependent on both Ubc13 and the predicted NF-κB binding site located in the MMP1 promoter. Although this report only presents data from one human breast cancer line, we have obtained comparable results with a different breast cancer cell line MCF7 (data not shown), indicating that the tumorigenic and metastatic effects of Uev1A is a general phenomenon in breast cancers.
While the experimental evidence as shown in this report clearly indicates that UEV1A can function as a proto-oncogene, the clinical relevance of this finding awaits future investigation. Nevertheless, our limited TissueScan microarray data indicate a low (less than 2-fold) variation in UEV1A transcript levels among five normal human breast samples, compared with an increase of up to 20-fold in some breast cancer samples. As NF-κB activation is commonly observed in breast cancers [24, 25, 49] and UEV1 upregulation is also frequently observed in breast cancer samples [12, 13, 14] and in cultured tumor cell lines , and is found to be correlated to tumorigenic indicators [2, 3], it is conceivable that a certain percentage of breast cancer samples with NF-κB activation is due to elevated UEV1A expression.
This study demonstrated that the N-terminal region of Uev1A is the molecular determinant of its cellular function(s) in the NF-κB signaling pathway. Although the exact cellular function of Uev1C remains a mystery, our previous studies  have shown that truncated Uev1A missing the N-terminal 30 amino acids behaves like Mms2 in terms of subcellular localization and promotion of K63-linked di-Ub versus poly-Ub chains in vitro, suggesting that Uev1C may play an Mms2-related role.
Given the importance of Uev1 in signaling and tumorigenesis, small-molecule inhibitors against Uev1 have been isolated [20, 50] based on their interference with the Ubc13-Uev1A interaction, and one appears to be able to inhibit proliferation and survival of diffuse large B-cell lymphoma cells. It is unclear whether these inhibitors also interfere with the Ubc13-Mms2 interaction, as critical residues responaible for the heterdimer formation are conserved between Uev1 and Mms2 . Furthermore, this study provides evidence that a desired inhibitor should target the N-terminal region of Uev1A instead of the Ubc13-Uev1 interface. Hence, this report provides an experimental and theoretical cornerstone for future diagnosis and therapy by targeting Uev1A for the cure of breast cancer.
Among three Uev gene products, only Uev1A promotes breast tumor metastasis, which is primarily through activating the NF-κB target gene MMP1. Hence, UEV1A is considered a proto-oncogene and a therapeutic target for breast cancers.
The authors wish to thank the Department of Laboratory Animal Science of Peking University Health Science Center for animal care assistance, Michelle Hanna for proofreading the manuscript and other laboratory members for discussion. This work was supported by a Capital Normal University 211 Special fund 10531182313 and the Canadian Breast Cancer Foundation research grant C7022 to WX.
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