β-catenin/CBP activation of mTORC1 signaling promotes partial epithelial-mesenchymal states in head and neck cancer

Head and neck cancers, which include oral squamous cell carcinoma (OSCC) as a major subsite, exhibit cellular plasticity that includes features of an epithelial-mesenchymal transition (EMT), referred to as partial-EMT (p-EMT). To identify molecular mechanisms contributing to OSCC plasticity, we performed a multiphase analysis of single cell RNA sequencing (scRNAseq) data from human OSCC. This included a multiresolution characterization of cancer cell subgroups to identify pathways and cell states that are heterogeneously represented, followed by casual inference analysis to elucidate activating and inhibitory relationships between these pathways and cell states. This approach revealed signaling networks associated with hierarchical cell state transitions, which notably included an association between β-catenin-driven CREB-binding protein (CBP) activity and mTORC1 signaling. This network was associated with subpopulations of cancer cells that were enriched for markers of the p-EMT state and poor patient survival. Functional analyses revealed that β-catenin/CBP induced mTORC1 activity in part through the transcriptional regulation of a raptor-interacting protein, chaperonin containing TCP1 subunit 5 (CCT5). Inhibition of β-catenin-CBP activity through the use of the orally active small molecule, E7386, reduced the expression of CCT5 and mTORC1 activity in vitro, and inhibited p-EMT-associated markers and tumor development in a murine model of OSCC. Our study highlights the use of multiresolution network analyses of scRNAseq data to identify targetable signals for therapeutic benefit, thus defining an underappreciated association between β-catenin/CBP and mTORC1 signaling in head and neck cancer plasticity.

Abstract 6049: Exogenous serine promotes cancer stem cells in oral squamous cell carcinoma

Cancer development and progression is associated with metabolic reprogramming of tumor cells required to meet their proliferative, bioenergetic and survival challenges. Some metabolites, including a non-essential amino acid, serine, have been shown to play a role in tumorigenesis by promoting aggressive cell states, such as cancer stem cells (CSCs). Previous studies from our laboratory showed that the nuclear branch of the Wnt/β-catenin signaling pathway, the β-catenin/CBP/MLL1 axis, enhanced CSC states during oral squamous cell carcinoma (OSCC) evolution through H3K4 trimethylation (H3K4me3) and epigenetic remodeling of the chromatin landscape. Given that metabolism has also been shown to modulate cell plasticity via epigenomic modifications, we have aimed to decode the relationship between serine synthesis and cell identity in OSCC.

We have shown that under serine starvation conditions, OSCC cells upregulate steady-state mRNA and protein levels of serine synthesis pathway enzymes. The latter is associated with an increase in the by-product, alpha-ketoglutarate (αKG), a co-substrate for nuclear αKG-dependent dioxygenases, which demethylate histone marker H3K27me3 and de-repress differentiation-associated genes. Furthermore, increased production of αKG is associated with downregulation of the β-catenin/CBP/MLl1 complex and CSC-associated H3K4me3.

In this study, we used human OSCC cell lines CAL27 and HSC3, derived from a primary tongue tumor and a metastatic site, respectively. Each cell line was cultured in either complete medium or serine starvation medium. Serine starvation conditions led to a downregulation of H3K27me3 along with H3K4me3, promoting a switch from cancer stem cell to differentiation-enhancing chromatin landscape. The observed changes in cell plasticity were further investigated through tumorsphere formation, a surrogate assay for cancer stem cells. We found that OSCC CAL27 and HSC3 cells readily formed tumorspheres replete with stem cell markers BMI1, KRT14 and SOX2, under non-adhesive conditions when grown in complete media, supporting our findings that utilization of exogenous serine enables OSCC cells to maintain stemness. Our findings suggest that a switch from exogenous serine uptake to the endogenous serine biosynthesis promotes OSCC cell differentiation concomitant with the loss of CSC identity.

Citation Format: Stacy Ann Jankowski, Nina C. Hardy, Maria A. Kukuruzinska. Exogenous serine promotes cancer stem cells in oral squamous cell carcinoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 6049.

Inhibition of LSD1 attenuates oral cancer development and promotes therapeutic efficacy of immune checkpoint blockade and Yap/Taz inhibition

Lysine-specific demethylase 1 (LSD1) is a histone demethylase that contributes to the etiology of oral squamous cell carcinoma (OSCC) in part by promoting cancer stem cell phenotypes. The molecular signals regulated by LSD1, or acting with LSD1, are poorly understood, particularly in the development of OSSC. In this study, we show that conditional deletion of the Lsd1 gene or pharmacological inhibition of LSD1 in the tongue epithelium leads to reduced development of OSCC following exposure to the tobacco carcinogen 4NQO. LSD1 inhibition attenuated proliferation and clonogenic survival and showed an additive effect when combined with the YAP inhibitor Verteporfin. Interestingly, LSD1 inhibition upregulated the expression of PD-L1, leading to immune checkpoint inhibitor therapy responses. Implications: Collectively, our studies reveal a critical role for LSD1 in OSCC development and identification of tumor growth targeting strategies that can be combined with LSD1 inhibition for improved therapeutic application.

Abstract 2004: Understanding the role of α1,2-fucosylation in head and neck cancer

Head and neck cancer is the seventh most common cancer in the world, claiming ~500,000 deaths yearly. Low expression of FUT2, an α1,2-fucosyltransferase that modifies glycoproteins and glycolipids, is associated with lower overall survival in head and neck squamous cell carcinoma (HNSCC). We previously showed that inhibition of nuclear β-catenin/CBP led to an increase in antennary fucosylation of the epidermal growth factor receptor (EGFR) and reduced tumor growth in a mouse orthotopic tumor xenograft model of HNSCC (Chandler KB et al., 2020). One possible explanation for this finding could be attenuation of HNSCC cellular proliferation and survival via restoration of fucose modification-mediated regulation of cell signaling and adhesion, leading to reinforcement of epithelial cell characteristics. To explore this hypothesis, FUT2 was overexpressed in the metastatic HNSCC cell line HSC-3. To compare the rate of proliferation between empty vector (HSC-3-EV) and FUT2-overexpressing cells (HSC-3-FT2), cells were plated in triplicate and photographed every 24 hours using an EVOS Imaging System. Independently, indolent CAL27 cells with high endogenous FUT2 expression and HSC-3-FT2 cells were subject to flow cytometry analyses to probe for fucosylated epitopes on the cell surface. To identify proteins bearing fucosylated determinants, CAL27 and HSC-3 cell lysates were subjected to proteolysis and analyzed via liquid chromatography-tandem mass spectrometry (LC-MS/MS). MS/MS data were searched with PEAKS software (Bioinformatics Solutions, Inc.), and with Byonic (Protein Metrics). Forced expression of FUT2 in HSC-3 cells (HSC-3-FT2) led to a decrease in proliferation compared to HSC-3-EV cells. Flow cytometry analyses revealed the presence of α1,2-fucosylated glycan epitopes on the cell surface of CAL27 cells, but not on parental HSC-3 or HSC-3-EV cells. To probe the role of FUT2 in HNSCC, we sought to identify fucosylated proteins in HNSCC cell lines by applying proteomic and glycoproteomic analyses of CAL27 and HSC-3 cell lysates to identify glycoproteins with differential display of fucosylated epitopes. Among the 1379 proteins in CAL27 cells and 1340 proteins in parental HSC-3 cells (1% FDR cutoff), with 1137 proteins common to both cell lines, we detected at least one glycopeptide for 55 proteins. Adhesion molecules and cell surface receptors were over-represented in the set and will serve as the focus of future inquiry into the mechanism of FUT2-mediated survival in HNSCC. In conclusion, FUT2 overexpression decreases cell proliferation in HSC-3 tongue squamous carcinoma cells. Low endogenous levels of α1,2-fucosylated epitopes were detected on HSC-3 cells with demonstrated metastatic potential, while higher levels were detected on non-metastatic CAL27 cells that express FUT2. We plan to further investigate the role of FUT2 in HNSCC, by exploring the impact of α1,2-fucose modification on signaling and adhesion molecules.

Citation Format: Evan Ales, Bach-Cuc Nguyen, Winston H. Elliott, Maria A. Kukuruzinska, Catherine E. Costello, Robert Sackstein, Kevin Brown Chandler. Understanding the role of α1,2-fucosylation in head and neck cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 2004.

Gene expression alterations in salivary gland epithelia of Sjögren's syndrome patients are associated with clinical and histopathological manifestations

Sjögren's syndrome (SS) is a complex autoimmune disease associated with lymphocytic infiltration and secretory dysfunction of salivary and lacrimal glands. Although the etiology of SS remains unclear, evidence suggests that epithelial damage of the glands elicits immune and fibrotic responses in SS. To define molecular changes underlying epithelial tissue damage in SS, we laser capture microdissected (LCM) labial salivary gland epithelia from 8 SS and 8 non-SS controls for analysis by RNA sequencing (RNAseq). Computational interrogation of gene expression signatures revealed that, in addition to a division of SS and non-SS samples, there was a potential intermediate state overlapping clustering of SS and non-SS samples. Differential expression analysis uncovered signaling events likely associated with distinct SS pathogenesis. Notable signals included the enrichment of IFN-γ and JAK/STAT-regulated genes, and the induction of genes encoding secreted factors, such as LTF, BMP3, and MMP7, implicated in immune responses, matrix remodeling and tissue destruction. Identification of gene expression signatures of salivary epithelia associated with mixed clinical and histopathological characteristics suggests that SS pathology may be defined by distinct molecular subtypes. We conclude that gene expression changes arising in the damaged salivary epithelia may offer novel insights into the signals contributing to SS development and progression.

Specification of the patterning of a ductal tree during branching morphogenesis of the submandibular gland

The development of ductal structures during branching morphogenesis relies on signals that specify ductal progenitors to set up a pattern for the ductal network. Here, we identify cellular asymmetries defined by the F-actin cytoskeleton and the cell adhesion protein ZO-1 as the earliest determinants of duct specification in the embryonic submandibular gland (SMG). Apical polarity protein aPKCζ is then recruited to the sites of asymmetry in a ZO-1-dependent manner and collaborates with ROCK signaling to set up apical-basal polarity of ductal progenitors and further define the path of duct specification. Moreover, the motor protein myosin IIB, a mediator of mechanical force transmission along actin filaments, becomes localized to vertices linking the apical domains of multiple ductal epithelial cells during the formation of ductal lumens and drives duct maturation. These studies identify cytoskeletal, junctional and polarity proteins as the early determinants of duct specification and the patterning of a ductal tree during branching morphogenesis of the SMG.

Abstract 5940: Inhibition of β-catenin/CBP signaling alters EGFR fucosylation status in head and neck squamous cell carcinoma

Head and neck squamous cell carcinoma (HNSCC) is the sixth most common malignancy in the world with oral squamous cell carcinomas (OSCC) accounting for the majority of HNSCC cases. A major driver of OSCC is the epidermal growth factor receptor (EGFR), a receptor tyrosine kinase (RTK) with 12 N-glycosylation sites. Fucosylated N-linked glycans on EGFR are associated with survival e.g., they suppress receptor dimerization and signaling. High levels of fucosylated glycan epitopes have been observed in OSCC, where invasive regions lose expression of linkage-specific fucosylated epitopes, suggesting that fucosylated glycans are involved in the suppression of cell growth and invasion. Previously, it was shown that inhibition of the interaction between nuclear β-catenin and CREB-binding protein (CBP) in human OSCC cells and in mouse tumor xenografts with a small molecule inhibitor ICG-001, interfered with OSCC proliferation and aggressive features in cellular, zebrafish, and murine models. E7386, a novel β-catenin/CBP modulator displays activity profile that closely overlaps with that of ICG-001 and exhibits ~50 - 100-fold lower EC50 values. Treatment with ICG-001 and E7386, increased expression of two glycosyltransferases, FUT2 and FUT3 coincident with decreased EGFR abundance that was accompanied by higher fucosylation of EGFR and upregulated expression of E-cadherin and junctional β-catenin. Further, genomic analyses showed a positive correlation between the ICG-001 and E7386 treatments and EGFR inhibition, suggesting that higher expression of antennary fucosyltransferase genes suppresses EGFR signaling. We now show using nLC-MS/MS analyses that EGFR from metastatic HSC-3 cells had low levels of fucosylated N-glycans, while EGFR from indolent CAL27 cells displayed higher levels of fucosylation at multiple EGFR N-linked glycosylation sites, and that these changes were statistically significant. In-depth characterization of multiply-fucosylated N-glycans via tandem mass spectrometry of EGFR glycopeptides revealed new insights into the identity of fucosylated glycan epitopes. Collectively, these results suggest that the β-catenin/CBP axis promotes EGFR signaling through downregulation of fucosyltransferase expression and activity. We conclude that inhibition of β-catenin/CBP signaling with a novel small molecule E7386 may serve as a therapeutic approach to downregulate EGFR pro-tumorigenic activity in HNSCC patients.

Citation Format: Kevin B. Chandler, Khalid Alamoud, Bac-Cuc Nguyen, Vanessa L. Stahl, Takashi Owa, Kenichi Nomoto, Stefano Monti, Maria A. Kukuruzinska, Catherine E. Costello. Inhibition of β-catenin/CBP signaling alters EGFR fucosylation status in head and neck squamous cell carcinoma [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 5940.

Abstract 2442: Identification of a novel role for the β-catenin/CBP signaling in epigenetic regulation of the N-glycosylation gene, DPAGT1, in head and neck cancer

Aberrant activation of the DPAGT1 gene, encoding an essential enzyme in the metabolic pathway of protein N-glycosylation, has been shown to be associated with head and neck squamous cell carcinoma (HNSCC). We have shown that DPAGT1 inhibits intercellular adhesion and functions in a positive feedback loop with Wnt/β-catenin signaling, and that the nuclear β-catenin/CBP signaling underlies the progression of HNSCC to advanced disease. However, the tumor promoting effects of DPAGT1 and its molecular links to the nuclear β-catenin/CBP axis are not well defined. We carried out genomic and functional analyses of DPAGT1 perturbation in indolent (CAL27) and metastatic (HSC-3) HNSCC cells, and in orthotopic HSC-3-derived xenografts in mice. We further generated and annotated DPAGT1 inhibition signature in HSC-3 cells and interrogated it in TCGA HNSCC. We then examined the effects of inhibition of β-catenin-CBP interaction with E7386 on DPAGT1 expression using ChIP-seq and computational approaches. E7386, a novel β-catenin/CBP modulator displays activity profile that closely overlaps with that of ICG-001, but exhibits ~50 - 100-fold lower EC50 values. Ectopic expression of DPAGT1 in indolent CAL27 cells induced epithelial-to-mesenchymal transition (EMT) which coincided with increased abundance of active β-catenin. Partial knockdown of DPAGT1 with siRNA in metastatic HSC-3 cells inhibited EMT, diminished cell migration and enhanced intercellular adhesion. Inhibition of the DPAGT1 enzyme activity using tunicamycin interfered with orthotopic tongue tumor growth and metastasis. DPAGT1 knockdown in HSC-3 cells defined DPAGT1-activated gene signature as enriched in pro-tumorigenic signaling pathways, including stem cell-like genes. Integrative analysis of the DPAGT1-activated genes in TCGA validated the association of DPAGT1 activity with the EMT transcription factors, ZEB1, Twist1/2 and Slug. ChIP-seq analyses without and with the E7386 treatment revealed reduced occupancy of H3K4me3 at two DPAGT1 transcription start sites following the E7386 treatment. In conclusion, our studies align aberrant activation of DPAGT1 with the induction of EMT and stem cell associated genes and suggest a novel role of β-catenin/CBP/MLL1 in the epigenetic regulation DPAGT1 and protein N-glycosylation in HNSCC.

Citation Format: Khalid A. Alamoud, Vinay Kartha, Huamei Yang, Andrew Tilston-Lunel, Anthony Federico, Manish Bais, Takachi Owa, Kenichi Nomoto, Xalarabos Varelas, Stefano Monti, Maria A. Kukuruzinska. Identification of a novel role for the β-catenin/CBP signaling in epigenetic regulation of the N-glycosylation gene, DPAGT1, in head and neck cancer [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 2442.

Abstract 2453: Inhibition of β-catenin/CBP signaling with E7386 targets epigenetic changes associated with cancer stem cells in head and neck cancer

Head and neck squamous cell carcinoma (HNSCC) is a pernicious malignancy that arises from populations of cancer stem cells (CSCs). We and others have shown that the Wnt/β-catenin signaling pathway drives CSC gene expression mediated, in part, by epigenetic alterations directed by interactions between nuclear β-catenin and the cAMP-responsive element binding (CREB)-binding protein (CBP). In HNSCC, the β-catenin/CBP complex recruits the histone methyltransferase, MLL1, to drive trimethylation of H3K4me3 to induce an open chromatin structure and expression of CSC genes. Further, β-catenin/CBP signaling is highly correlated with the activity of the paralogous transcriptional regulators YAP and TAZ (YAP/TAZ), which are pro-tumorigenic factors in HNSCC. We reported that a small molecule inhibitor of the β-catenin-CBP interaction, ICG-001, blocks oncogenic phenotypes in cellular, zebrafish, and murine models of HNSCC, concomitant with the reduction of CSC traits. Recently, a novel β-catenin/CBP modulator, E7386, has been shown to be effective against a number of neoplasms in preclinical studies. Here, we compared anti-cancer properties of E7386 with ICG-001 to define its molecular mechanisms and validate the β-catenin/CBP axis as a bona fide therapeutic target in HNSCC.

Anti-HNSCC activity of E7386 was evaluated in four human HNSCC cell lines using genomic, molecular, biochemical and functional approaches, including global ChIP-seq for H3K4me3. The set of transcripts significantly down-regulated by E7386 in HNSCC cells was projected onto a TCGA RNA-seq data (n=318) using ASSIGN, where samples were scored based on the coordinated expression of the gene signature which, in turn, reflected the level of E7386 inhibition per sample. The E7386 inhibition score was then tested for its association with survival by stratifying TCGA patients (n=318) into high- and low-score groups. Results showed that E7386 had highly overlapping activity signatures with ICG-001 (R = 0.997) with ~50 - 100-fold lower EC50. Similar to ICG-001, treatment with E7386 blocked association between β-catenin and CBP with a concomitant reduction in CBP and MLL1 abundance and global H3K4 trimethylation. E7386 repressed an oncogenic gene expression signature regulated by YAP1/TAZ and impeded HNSCC cell proliferation, promoting E-cadherin adhesion and junctional localization of β-catenin. Importantly, E7386 inhibition-associated transcriptional signatures tracked with tumor grade and poor human HNSCC patient survival. In conclusion, inhibiting β-catenin/CBP activity with E7386 represents a novel approach aimed at targeting epigenetically driven changes in the chromatin structure in HNSCC.

Citation Format: Huamei Yang, Vinay Kartha, Khalid A. Alamoud, Anthony Federico, Andrew Tilston-Lunel, Bach-Cuc Nguyen, Takashi Owa, Kenichi Nomoto, Xaralabos Varelas, Stefano Monti, Maria A. Kukuruzinska. Inhibition of β-catenin/CBP signaling with E7386 targets epigenetic changes associated with cancer stem cells in head and neck cancer [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 2453.

β-Catenin/CBP inhibition alters epidermal growth factor receptor fucosylation status in oral squamous cell carcinoma

Epidermal growth factor receptor (EGFR) is a major driver of head and neck cancer, a devastating malignancy with a major sub-site in the oral cavity manifesting as oral squamous cell carcinoma (OSCC). EGFR is a glycoprotein receptor tyrosine kinase (RTK) whose activity is upregulated in >80% OSCC. Current anti-EGFR therapy relies on the use of cetuximab, a monoclonal antibody against EGFR, although it has had only a limited response in patients. Here, we uncover a novel mechanism regulating EGFR activity, identifying a role of the nuclear branch of the Wnt/β-catenin signaling pathway, the β-catenin/CBP axis, in control of post-translational modification of N-glycans on the EGFR. Genomic and structural analyses reveal that β-catenin/CBP signaling represses fucosylation on the antennae of N-linked glycans on EGFR. By employing nUPLC-MS/MS, we determined that malignant human OSCC cells harbor EGFR with a paucity of N-glycan antennary fucosylation, while indolent cells display higher levels of fucosylation at sites N420 and N579. Additionally, treatment with either ICG-001 or E7386, which are both small molecule inhibitors of β-catenin/CBP signaling, leads to increased transcriptional expression of fucosyltransferases FUT2 and FUT3, with a concomitant increase in EGFR N-glycan antennary fucosylation. In order to discover which fucosylated glycan epitopes are involved in the observed effect, we performed in-depth characterization of multiply-fucosylated N-glycans via tandem mass spectrometry analysis of the EGFR tryptic glycopeptides. Data are available via ProteomeXchange with identifier PXD017060. We propose that β-catenin/CBP signaling promotes EGFR oncogenic activity in OSCC by inhibiting its N-glycan antennary fucosylation through transcriptional repression of FUT2 and FUT3.

Abstract 2633: Deciphering the role of protein glycosylation in oral cancer: insights into tumor biology and treatment

Head and neck cancer is a debilitating malignancy, with the majority of cases arising in the oral cavity as oral squamous cell carcinoma (OSCC). A major driver of OSCC is the epidermal growth factor receptor (EGFR), whose activity is aberrantly upregulated in >80% of tumors. EGFR is highly modified with N-linked glycans; fucosylation of N-glycans interferes with EGFR dimerization and activation. Thus, post-transcriptional changes may govern EGFR activity. In OSCC, EGFR signaling converges on Wnt/β-catenin activity, known to play pivotal roles in the pathobiology of this malignancy through the interaction of nuclear β-catenin with the histone acetyltransferase CREB-binding protein (CBP). We have shown that a small molecule inhibitor of β-catenin-CBP interaction, ICG-001, interferes with OSCC proliferation and aggressive features in cellular, zebrafish and murine models. Also, OSCC-cell line derived mouse tumor xenografts exhibit reduced EGFR abundance, and genomic analyses show a positive correlation between ICG-001 and EGFR inhibition. Given that modification of EGFR with N-glycans impacts its cell-surface localization and signaling, we hypothesized that ICG-001 affects EGFR N-glycosylation. To determine the effect of inhibition of β-catenin/CBP activity on cellular N-glycosylation programs, N-glycans from CAL27 and HSC3 cells treated with ICG-001 or vehicle control were released, permethylated, and analyzed via MALDI-TOF MS. Next, EGFR glycopeptides from CAL27 and HSC3 cells treated with ICG-001 or vehicle control, were analyzed with an Orbitrap Fusion™ Lumos™ Tribrid™ mass spectrometer (Thermo Scientific) using EThcD. In CAL27 cells, we observed higher levels of high mannose (less processed) N-glycans and complex fucosylated N-glycans, whereas in HSC3 cells we observed complex, afucosylated N-glycans. After ICG-001 treatment, HSC3 cells displayed higher levels of fucosylated N-glycans, suggesting that ICG-001, via inhibition of β-catenin/CBP signaling, promotes a more indolent-like glycan profile. Similarly, EGFR from CAL27 cells had highly fucosylated N-glycans, while EGFR from HSC3 cells displayed N-glycans with a paucity of fucose. Treatment of HSC3 cells with ICG-001 led to higher fucosylation, potentially inhibiting EGFR signaling. Parallel analyses of gene expression signatures in response to ICG-001 treatment in HSC-3 cells showed increased transcriptional expression of fucosyltransferases, FUT2 and FUT3. Our studies suggest that the β-catenin/CBP axis promotes EGFR signaling through downregulation of FUT2 and FUT3 expression and activity. Thus, inhibition of β-catenin/CBP signaling with ICG-001 may serve as a therapeutic approach to downregulate EGFR pro-tumorigenic activity in OSCC. Supported by NIH grants P41 GM104603 (CEC), F32 CA196157 (KBC), and by the Evans Center for Interdisciplinary Biomedical Research ARC #9950000118 (MAK).

Citation Format: Kevin B. Chandler, Vanessa L. Stahl, Khalid Alamoud, Bach-Cuc Nguyen, Vinay Kartha, Khikmet Sadykov, Stefano Monti, Maria A. Kukuruzinska, Catherine E. Costello. Deciphering the role of protein glycosylation in oral cancer: insights into tumor biology and treatment [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 2633.

Functional and genomic analyses reveal therapeutic potential of targeting β-catenin/CBP activity in head and neck cancer

BACKGROUND

Head and neck squamous cell carcinoma (HNSCC) is an aggressive malignancy characterized by tumor heterogeneity, locoregional metastases, and resistance to existing treatments. Although a number of genomic and molecular alterations associated with HNSCC have been identified, they have had limited impact on the clinical management of this disease. To date, few targeted therapies are available for HNSCC, and only a small fraction of patients have benefited from these treatments. A frequent feature of HNSCC is the inappropriate activation of β-catenin that has been implicated in cell survival and in the maintenance and expansion of stem cell-like populations, thought to be the underlying cause of tumor recurrence and resistance to treatment. However, the therapeutic value of targeting β-catenin activity in HNSCC has not been explored.

METHODS

We utilized a combination of computational and experimental profiling approaches to examine the effects of blocking the interaction between β-catenin and cAMP-responsive element binding (CREB)-binding protein (CBP) using the small molecule inhibitor ICG-001. We generated and annotated in vitro treatment gene expression signatures of HNSCC cells, derived from human oral squamous cell carcinomas (OSCCs), using microarrays. We validated the anti-tumorigenic activity of ICG-001 in vivo using SCC-derived tumor xenografts in murine models, as well as embryonic zebrafish-based screens of sorted stem cell-like subpopulations. Additionally, ICG-001-inhibition signatures were overlaid with RNA-sequencing data from The Cancer Genome Atlas (TCGA) for human OSCCs to evaluate its association with tumor progression and prognosis.

RESULTS

ICG-001 inhibited HNSCC cell proliferation and tumor growth in cellular and murine models, respectively, while promoting intercellular adhesion and loss of invasive phenotypes. Furthermore, ICG-001 preferentially targeted the ability of subpopulations of stem-like cells to establish metastatic tumors in zebrafish. Significantly, interrogation of the ICG-001 inhibition-associated gene expression signature in the TCGA OSCC human cohort indicated that the targeted β-catenin/CBP transcriptional activity tracked with tumor status, advanced tumor grade, and poor overall patient survival.

CONCLUSIONS

Collectively, our results identify β-catenin/CBP interaction as a novel target for anti-HNSCC therapy and provide evidence that derivatives of ICG-001 with enhanced inhibitory activity may serve as an effective strategy to interfere with aggressive features of HNSCC.

Abstract 2516: Inhibition of Β-catenin/CBP signaling in oral cancer alters EGFR N-glycosylation and abundance

Head and neck cancer is a debilitating malignancy, with the majority of cases arising in the oral cavity as oral squamous cell carcinoma (OSCC). A major driver of OSCC is the epidermal growth factor receptor (EGFR), whose activity is aberrantly upregulated in >90% of tumors. EGFR is highly modified with N-linked glycans; fucosylation of N-linked glycans interferes with EGFR dimerization and activation. Thus, post-transcriptional changes may govern EGFR activity. In OSCC, EGFR signaling converges on Wnt/ Β-catenin activity, known to play pivotal roles in the pathobiology of this malignancy through the interaction of nuclear Β-catenin with the histone acetyltransferase CREB-binding protein (CBP). We have shown that a small-molecule inhibitor of Β-catenin-CBP interaction, ICG-001, interferes with OSCC proliferation and aggressive features in cellular, zebrafish and murine models. Also, OSCC-cell line derived mouse tumor xenografts exhibit reduced EGFR abundance, and genomic analyses show a positive correlation between ICG-001 and EGFR inhibition. Given that modification of EGFR with N-glycans impacts its cell-surface localization and signaling, we hypothesized that ICG-001 affected EGFR N-glycosylation. We immunoprecipitated EGFR from indolent CAL27 and metastatic HSC-3 cells after treatment with ICG-001 or vehicle control and determined the effect of inhibition of Β-catenin/CBP activity on its N-glycosylation status. We subjected immunoprecipitated EGFR to proteolysis, performed glycopeptide enrichment via hydrophilic interaction liquid chromatography (HILIC), analyzed glycopeptides with an Agilent 6550 Quadrupole Time-of-Flight (Q-TOF) MS using collision-induced dissociation, and compared site-specific glycoform patterns for the two cell types +/- ICG-001. At specific N-glycosylation sites, EGFR from indolent CAL27 cells had highly fucosylated N-glycans, while EGFR from metastatic HSC-3 cells displayed N-linked glycans with a paucity of fucose. Treatment of HSC-3 cells with ICG-001 revealed higher fucosylation at sites N151, N420, suggesting that ICG-001 promoted modification with terminal fucose, potentially inhibiting EGFR signaling. Parallel analyses of gene expression signatures in response to ICG-001 treatment in HSC-3 cells showed increased transcriptional expression of fucosyltransferases, FUT2 and FUT3 that fucosylate residues on the outer arms of N-linked glycans. Our studies suggest that the Β-catenin/CBP axis promotes EGFR signaling by inhibiting its fucosylation through downregulation of FUT2 and FUT3 expression and activity. Thus, inhibition of Β-catenin/CBP signaling with ICG-001 may serve as a therapeutic approach to downregulate EGFR protumorigenic activity in OSCC.

Supported by NIH grants P41 GM104603 (CEC), F32 CA196157 (KBC), and by the Evans Center for Interdisciplinary Biomedical Research ARC #9950000118 (MAK).

Citation Format: Kevin B. Chandler, Khalid Alamoud, Vinay K. Kartha, Khikmet Sadykov, Stefano Monti, Maria A. Kukuruzinska, Catherine E. Costello. Inhibition of Β-catenin/CBP signaling in oral cancer alters EGFR N-glycosylation and abundance [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 2516.

Emerging Insights into Wnt/β-catenin Signaling in Head and Neck Cancer

Head and neck cancer presents primarily as head and neck squamous cell carcinoma (HNSCC), a debilitating malignancy fraught with high morbidity, poor survival rates, and limited treatment options. Mounting evidence indicates that the Wnt/β-catenin signaling pathway plays important roles in the pathobiology of HNSCC. Wnt/β-catenin signaling affects multiple cellular processes that endow cancer cells with the ability to maintain and expand immature stem-like phenotypes, proliferate, extend survival, and acquire aggressive characteristics by adopting mesenchymal traits. A central component of canonical Wnt signaling is β-catenin, which balances its role as a structural component of E-cadherin junctions with its function as a transcriptional coactivator of numerous target genes. Recent genomic characterization of head and neck cancer revealed that while β-catenin is not frequently mutated in HNSCC, its activity is unchecked by more common mutations in genes encoding upstream regulators of β-catenin, NOTCH1, FAT1, and AJUBA. Wnt/β-catenin signaling affects a wide range epigenetic and transcriptional activities, mediated by the interaction of β-catenin with different transcription factors and transcriptional coactivators and corepressors. Furthermore, Wnt/β-catenin functions in a network with many signaling and metabolic pathways that modulate its activity. In addition to its effects on tumor epithelia, β-catenin activity regulates the tumor microenvironment by regulating extracellular matrix remodeling, fibrotic processes, and immune response. These multifunctional oncogenic effects of β-catenin make it an attractive bona fide target for HNSCC therapy.

The Integrated Role of Wnt/β-Catenin, N-Glycosylation, and E-Cadherin-Mediated Adhesion in Network Dynamics

The cellular network composed of the evolutionarily conserved metabolic pathways of protein N-glycosylation, Wnt/β-catenin signaling pathway, and E-cadherin-mediated cell-cell adhesion plays pivotal roles in determining the balance between cell proliferation and intercellular adhesion during development and in maintaining homeostasis in differentiated tissues. These pathways share a highly conserved regulatory molecule, β-catenin, which functions as both a structural component of E-cadherin junctions and as a co-transcriptional activator of the Wnt/β-catenin signaling pathway, whose target is the N-glycosylation-regulating gene, DPAGT1. Whereas these pathways have been studied independently, little is known about the dynamics of their interaction. Here we present the first numerical model of this network in MDCK cells. Since the network comprises a large number of molecules with varying cell context and time-dependent levels of expression, it can give rise to a wide range of plausible cellular states that are difficult to track. Using known kinetic parameters for individual reactions in the component pathways, we have developed a theoretical framework and gained new insights into cellular regulation of the network. Specifically, we developed a mathematical model to quantify the fold-change in concentration of any molecule included in the mathematical representation of the network in response to a simulated activation of the Wnt/ β-catenin pathway with Wnt3a under different conditions. We quantified the importance of protein N-glycosylation and synthesis of the DPAGT1 encoded enzyme, GPT, in determining the abundance of cytoplasmic β-catenin. We confirmed the role of axin in β-catenin degradation. Finally, our data suggest that cell-cell adhesion is insensitive to E-cadherin recycling in the cell. We validate the model by inhibiting β-catenin-mediated activation of DPAGT1 expression and predicting changes in cytoplasmic β-catenin concentration and stability of E-cadherin junctions in response to DPAGT1 inhibition. We show the impact of pathway dysregulation through measurements of cell migration in scratch-wound assays. Collectively, our results highlight the importance of numerical analyses of cellular networks dynamics to gain insights into physiological processes and potential design of therapeutic strategies to prevent epithelial cell invasion in cancer.

Head and neck cancer: from research to therapy and cure

Cumulative findings from many research groups have identified new signaling mechanisms associated with head and neck cancers. We summarize these findings, including discussion of aberrant NOTCH, PI3K, STAT3, immune recognition, oxidative pathway, and regulation of cell cycle and cell death. The genomic landscape of head and neck cancers has been shown to differ depending on human papillomavirus (HPV) status. We discuss studies examining the integration of HPV into genomic regions, as well as the epigenetic alterations that occur in response to HPV infection, and how these may help reveal new biomarker and treatment predictors. The characterization of premalignant lesions is also highlighted, as is evidence indicating that the surgical removal of these lesions is associated with better clinical outcomes. Current surgical methods are also discussed, including several less aggressive approaches such as minimal invasive robotic surgery. While much remains to be done in the fight against head and neck cancer, continued integration of basic research with new treatment options will likely lead to more effective therapeutic strategies directed against this disease.

Protein N-glycosylation in oral cancer: dysregulated cellular networks among DPAGT1, E-cadherin adhesion and canonical Wnt signaling

N-Linked glycosylation (N-glycosylation) of proteins has long been associated with oncogenesis, but not until recently have the molecular mechanisms underlying this relationship begun to be unraveled. Here, we review studies describing how dysregulation of the N-glycosylation-regulating gene, DPAGT1, drives oral cancer. DPAGT1 encodes the first and rate-limiting enzyme in the assembly of the lipid-linked oligosaccharide precursor in the endoplasmic reticulum and thus mediates N-glycosylation of many cancer-related proteins. DPAGT1 controls N-glycosylation of E-cadherin, the major epithelial cell-cell adhesion receptor and a tumor suppressor, thereby affecting intercellular adhesion and cytoskeletal dynamics. DPAGT1 also regulates and is regulated by Wnt/β-catenin signaling, impacting the balance between proliferation and adhesion in homeostatic tissues. Thus, aberrant induction of DPAGT1 promotes a positive feedback network with Wnt/β-catenin that represses E-cadherin-based adhesion and drives tumorigenic phenotypes. Further, modification of receptor tyrosine kinases (RTKs) with N-glycans is known to control their surface presentation via the galectin lattice, and thus increased DPAGT1 expression likely contributes to abnormal activation of RTKs in oral cancer. Collectively, these studies suggest that dysregulation of the DPAGT1/Wnt/E-cadherin network underlies the etiology and pathogenesis of oral cancer.

N-glycosylation induces the CTHRC1 protein and drives oral cancer cell migration

Oral squamous cell carcinoma (OSCC) is one of the most pernicious malignancies, but the mechanisms underlying its development and progression are poorly understood. One of the key pathways implicated in OSCC is the canonical Wnt/β-catenin signaling pathway. Previously, we reported that canonical Wnt signaling functions in a positive feedback loop with the DPAGT1 gene, a principal regulator of the metabolic pathway of protein N-glycosylation, to hyperglycosylate E-cadherin and reduce intercellular adhesion. Here, we show that in OSCC, DPAGT1 and canonical Wnt signaling converge to up-regulate CTHRC1 (collagen triple helix repeat containing 1), an N-glycoprotein implicated in tumor invasion and metastasis. We found that in human OSCC specimens, amplification of the levels of CTHRC1 was associated with its hyperglycosylation. Partial inhibition of DPAGT1 expression in OSCC CAL27 cells reduced CTHRC1 abundance by increasing protein turnover, indicating that N-glycosylation stabilizes CTHRC1. Additionally, canonical Wnt signaling promoted β-catenin/T-cell factor transcriptional activity at the CTHRC1 promoter to further elevate CTHRC1 levels. We demonstrate that DPAGT1 promotes cell migration and drives the localization of CTHRC1 to cells at the leading edge of a wound front coincident with drastic changes in cell morphology. We propose that in OSCC, dysregulation of canonical Wnt signaling and DPAGT1-dependent N-glycosylation induces CTHRC1, thereby driving OSCC cell migration and tumor spread.

Coordinate regulation of N-glycosylation gene DPAGT1, canonical Wnt signaling and E-cadherin adhesion

The metabolic pathway of protein N-glycosylation influences intercellular adhesion by affecting the composition and cytoskeletal association of E-cadherin protein complexes, or adherens junctions (AJs). In sparse cells, E-cadherin is modified extensively with complex N-glycans and forms nascent AJs, while in dense cultures, hypoglycosylated E-cadherin drives the assembly of mature AJs with increased levels of γ- and α-catenins. N-glycosylation of E-cadherin is controlled by the DPAGT1 gene, a key regulator of the N-glycosylation pathway. DPAGT1 is a target of the canonical Wnt signaling pathway, with both β- and γ-catenins binding to Tcf at its promoter. We now report that DPAGT1 senses cell density through canonical Wnt signaling. In dense cells, depletion of β-catenin from the DPAGT1 promoter correlated with downregulation of its cellular abundance, while loss of nuclear γ-catenin reflected its greater recruitment to AJs. DPAGT1 itself affected canonical Wnt signaling, with forced changes in its expression resulting in corresponding changes in transcriptionally active β-catenin and canonical Wnt activity. Remarkably, a 2.4-fold increase in the DPAGT1 mRNA level resulted in increased N-glycosylation and reduced membrane localization of E-cadherin, coincident with dramatic changes in cell morphology. Lastly, we present evidence that N-glycosylation status of E-cadherin controls its antagonism of canonical Wnt signaling. Transfection of hypoglycosylated E-cadherin mutant, V13, but not fully N-glycosylated E-cadherin, into sparse cells inhibited canonical Wnt activity by depleting nuclear β- and γ-catenins. Collectively, our studies show that cells coordinate DPAGT1 expression and protein N-glycosylation with canonical Wnt signaling and E-cadherin adhesion via positive and negative feedback mechanisms.

N-glycosylation gene DPAGT1 is a target of the Wnt/beta-catenin signaling pathway

Protein N-glycosylation and the Wnt/β-catenin signaling pathways play critical roles in development and cancer. Although N-glycosylation has been shown to influence Wnt signaling through its effects on Wnt ligands, it is unclear whether the Wnt/β-catenin pathway impacts protein N-glycosylation. In this study, we show that promoters of the first N-glycosylation gene, DPAGT1, from Chinese hamster ovary (CHO), Madin-Darby canine kidney (MDCK), and human epidermoid carcinoma (A253) cells contain the T-cell factor/lymphoid enhancer-binding factor (TCF/LEF) consensus sequence. Treatment of cells with a Wnt activator, lithium chloride, up-regulated DPAGT1 transcript levels that correlated with an increase in the β-catenin abundance. Furthermore, exposure of cells to a Wnt receptor ligand, Wnt3a, resulted in an increase in the DPAGT1 transcript levels that was abrogated by the Wnt inhibitor, Dickkopf-1. DNA mobility shift assays revealed specific protein complexes at the DPAGT1 TCF/LEF binding region that were competed off with antibodies to either Tcf3/4 or β-catenin. Chromatin immunoprecipitation analysis confirmed the presence of β-catenin at the DPAGT1 promoter in vivo. In addition, the DPAGT1 TCF/LEF sequence drove the expression of the luciferase reporter gene. Furthermore, up-regulation of DPAGT1 transcripts by Wnt3a led to altered N-glycosylation of E-cadherin. Interestingly, the DPAGT1 TCF/LEF sequence also interacted with γ-catenin, a close homologue of β-catenin, although not in a lithium chloride-dependent manner. Our results provide the first evidence that the Wnt/β-catenin signaling pathway regulates the metabolic pathway of protein N-glycosylation by targeting DPAGT1 expression. Moreover, they suggest the existence of another regulatory mechanism involving the interaction of Tcf with γ-catenin at the DPAGT1 promoter.

Hypoglycosylated E-cadherin promotes the assembly of tight junctions through the recruitment of PP2A to adherens junctions

Epithelial cell-cell adhesion is controlled by multiprotein complexes that include E-cadherin-mediated adherens junctions (AJs) and ZO-1-containing tight junctions (TJs). Previously, we reported that reduction of E-cadherin N-glycosylation in normal and cancer cells promoted stabilization of AJs through changes in the composition and cytoskeletal association of E-cadherin scaffolds. Here, we show that enhanced interaction of hypoglycosylated E-cadherin-containing AJs with protein phosphatase 2A (PP2A) represents a mechanism for promoting TJ assembly. In MDCK cells, attenuation of cellular N-glycosylation with siRNA to DPAGT1, the first gene in the N-glycosylation pathway, reduced N-glycosylation of surface E-cadherin and resulted in increased recruitment of stabilizing proteins gamma-catenin, alpha-catenin, vinculin and PP2A to AJs. Greater association of PP2A with AJs correlated with diminished binding of PP2A to ZO-1 and claudin-1 and with increased pools of serine-phosphorylated ZO-1 and claudin-1. More ZO-1 was found in complexes with occludin and claudin-1, and this corresponded to enhanced transepithelial resistance (TER), indicating physiological assembly of TJs. Similar maturation of AJs and TJs was detected after transfection of MDCK cells with the hypoglycosylated E-cadherin variant, V13. Our data indicate that E-cadherin N-glycans coordinate the maturity of AJs with the assembly of TJs by affecting the association of PP2A with these junctional complexes.

N-glycosylation status of E-cadherin controls cytoskeletal dynamics through the organization of distinct β-catenin- and γ-catenin-containing AJs

N-glycosylation of E-cadherin has been shown to inhibit cell-cell adhesion. Specifically, our recent studies have provided evidence that the reduction of E-cadherin N-glycosylation promoted the recruitment of stabilizing components, vinculin and serine/threonine protein phosphatase 2A (PP2A), to adherens junctions (AJs) and enhanced the association of AJs with the actin cytoskeleton. Here, we examined the details of how N-glycosylation of E-cadherin affected the molecular organization of AJs and their cytoskeletal interactions. Using the hypoglycosylated E-cadherin variant, V13, we show that V13/β-catenin complexes preferentially interacted with PP2A and with the microtubule motor protein dynein. This correlated with dephosphorylation of the microtubule-associated protein tau, suggesting that increased association of PP2A with V13-containing AJs promoted their tethering to microtubules. On the other hand V13/γ-catenin complexes associated more with vinculin, suggesting that they mediated the interaction of AJs with the actin cytoskeleton. N-glycosylation driven changes in the molecular organization of AJs were physiologically significant because transfection of V13 into A253 cancer cells, lacking both mature AJs and tight junctions (TJs), promoted the formation of stable AJs and enhanced the function of TJs to a greater extent than wild-type E-cadherin. These studies provide the first mechanistic insights into how N-glycosylation of E-cadherin drives changes in AJ composition through the assembly of distinct β-catenin- and γ-catenin-containing scaffolds that impact the interaction with different cytoskeletal components.

Overexpression of DPAGT1 leads to aberrant N-glycosylation of E-cadherin and cellular discohesion in oral cancer

Cancer cells are frequently characterized by aberrant increases in protein N-glycosylation and by disruption of E-cadherin-mediated adherens junctions. The relationship between altered N-glycosylation and loss of E-cadherin adhesion in cancer, however, remains unclear. Previously, we reported that complex N-glycans on the extracellular domains of E-cadherin inhibited the formation of mature adherens junctions. Here, we examined whether dysregulated N-glycosylation was one of the underlying causes for cellular discohesion in oral cancer. We show that dense cultures of human salivary epidermoid carcinoma A253 cells exhibited elevated expression of DPAGT1, the gene that initiates protein N-glycosylation. Overexpression of DPAGT1 correlated with the production of E-cadherin-bearing complex N-glycans in nascent adherens junctions. Partial inhibition of DPAGT1 with small interfering RNA reduced the complex N-glycans of E-cadherin and increased the abundance of alpha-catenin and stabilizing proteins in adherens junctions. This was associated with the assembly of functional tight junctions. The inverse relationship between DPAGT1 expression and intercellular adhesion was a feature of oral squamous cell carcinoma. Oral squamous cell carcinomas displayed overexpression of DPAGT1 that correlated with diminished localization of E-cadherin and alpha-catenin at the sites of adherens junctions. Our studies show for the first time that DPAGT1 is an upstream regulator of E-cadherin N-glycosylation status and adherens junction composition and suggest that dysregulation of DPAGT1 causes disturbances in intercellular adhesion in oral cancer.

Diverse roles of E-cadherin in the morphogenesis of the submandibular gland: insights into the formation of acinar and ductal structures

The formation of acinar and ductal structures during epithelial tissue branching morphogenesis is not well understood. We report that in the mouse submandibular gland (SMG), acinar and ductal cell fates are determined early in embryonic morphogenesis with E-cadherin playing pivotal roles in development. We identified two morphologically distinct cell populations at the single bud stage, destined for different functions. The outer layer of columnar cells with organized E-cadherin junctions expressed the neonatal acinar marker B1 by E13.5, demonstrating their acinar fate. The interior cells initially lacked distinct E-cadherin junctions, but with morphogenesis formed cytokeratin 7 (K7) -positive ductal structures with organized E-cadherin junctions and F-actin filaments. Inhibition of E-cadherin function with either siRNA or function blocking antibody caused extensive apoptosis of ductal cells and aberrantly dilated lumens, providing the first evidence that E-cadherin regulates ductal lumen formation during branching morphogenesis of the salivary gland.

Gamma-secretase activation of notch signaling regulates the balance of proximal and distal fates in progenitor cells of the developing lung

Little is known about the mechanisms by which the lung epithelial progenitors are initially patterned and how proximal-distal boundaries are established and maintained when the lung primordium forms and starts to branch. Here we identified a number of Notch pathway components in respiratory progenitors of the early lung, and we investigated the role of Notch in lung pattern formation. By preventing gamma-secretase cleavage of Notch receptors, we have disrupted global Notch signaling in the foregut and in the lung during the initial stages of murine lung morphogenesis. We demonstrate that Notch signaling is not necessary for lung bud initiation; however, Notch is required to maintain a balance of proximal-distal cell fates at these early stages. Disruption of Notch signaling dramatically expands the population of distal progenitors, altering morphogenetic boundaries and preventing formation of proximal structures. Our data suggest a novel mechanism in which Notch and fibroblast growth factor signaling interact to control the proximal-distal pattern of forming airways in the mammalian lung.

N-glycosylation affects the molecular organization and stability of E-cadherin junctions

Epithelial cell-cell adhesion is mediated by E-cadherin, an intercellular N-glycoprotein adhesion receptor that functions in the assembly of multiprotein complexes anchored to the actin cytoskeleton named adherens junctions (AJs). E-cadherin ectodomains 4 and 5 contain three potential N-glycan addition sites, although their significance in AJ stability is unclear. Here we show that sparse cells lacking stable AJs produced E-cadherin that was extensively modified with complex N-glycans. In contrast, dense cultures with more stable AJs had scarcely N-glycosylated E-cadherin modified with high mannose/hybrid and limited complex N-glycans. This suggested that variations in AJ stability were accompanied by quantitative and qualitative changes in E-cadherin N-glycosylation. To further examine the role of N-glycans in AJ function, we generated E-cadherin N-glycosylation variants lacking selected N-glycan addition sites. Characterization of these variants in CHO cells, lacking endogenous E-cadherin, revealed that site 1 on ectodomain 4 was modified with a prominent complex N-glycan, site 2 on ectodomain 5 did not have a substantial oligosaccharide, and site 3 on ectodomain 5 was decorated with a high mannose/hybrid N-glycan. Removal of complex N-glycan from ectodomain 4 led to a dramatically increased interaction of E-cadherin-catenin complexes with vinculin and the actin cytoskeleton. The latter effect was further enhanced by the deletion of the high mannose/hybrid N-glycan from site 3. In MDCK cells, which produce E-cadherin, a variant lacking both complex and high mannose/hybrid N-glycans functioned like a dominant positive displaying increased interaction with gamma-catenin and vinculin compared with the endogenous E-cadherin. Collectively, our studies show that N-glycans, and complex oligosaccharides in particular, destabilize AJs by affecting their molecular organization.

A hypomorphic allele of the first N-glycosylation gene, ALG7, causes mitochondrial defects in yeast

The modification of proteins at asparagine residues with oligosaccharides (N-glycans) plays critical roles in diverse cell functions. N-glycans originate from a common lipid-linked oligosaccharide (LLO) precursor whose synthesis is initiated by the Dol-P-dependent GlcNAc-1-P transferase (GPT) encoded by an essential ALG7 gene. To identify cellular processes affected by ALG7 and N-glycosylation, we replaced the genomic copy of ALG7 with its hypomorphic allele in two genetically distinct haploid yeast cells. We show that ALG7 knockdown gave rise to an unexpected phenotype of mitochondrial dysfunction. The alg7 mutants did not grow on glycerol and DNA arrays revealed the absence of mitochondrial genes' expression. Accordingly, the alg7 mutants displayed no detectable mtDNA and respiratory activity. Both mutants exhibited diminished abundance of LLO and under-glycosylation of carboxypeptidase Y (CPY). Moreover, another N-glycosylation mutant with a LLO defect, alg6, was respiratory deficient. Collectively, our studies provide evidence that the dysregulation of N-glycosylation in haploid yeast cells leads to mitochondrial dysfunction.

Inhibition of DNA replication by fish oil-treated cytoplasm is counteracted by fish oil-treated nuclear extract

We have recently noted that cells treated with fish oil and n-3-fatty acids show slower DNA replication rates than cells treated with a control emulsion or corn oil only. However, it is not clearly understood how such an effect is induced. Fish oil and its metabolites are known to have several modulating effects on signal transduction pathways. Alternatively, they may influence DNA replication by interacting directly with nuclear components. To investigate this problem in greater detail, we have studied the kinetics of DNA synthesis in a cell-free system derived from HeLa cells. Nuclei and cytosolic extract were isolated from cells synchronized in early S phase after treatment with control emulsion, corn oil, or fish oil, respectively. The nuclei were reconstituted with cytosolic extract and a reaction mixture containing bromodeoxyuridine (BrdU) triphosphate to label newly synthesized DNA. The rate of DNA synthesis was measured by bivariate DNA/BrdU analysis and flow cytometry. We show that fish oil-treated cytosol inhibits the elongation of newly synthesized DNA by ~80% in control nuclei. However, nuclei treated with fish oil escape this inhibitory effect. We also show that addition of nuclear extract from fish oil-treated cells reverses the inhibitory effect seen in the reconstitution system of control nuclei and fish oil-treated cytosol. These results indicate that polyunsaturated fatty acids can modulate DNA synthesis through cytosolic as well as soluble nuclear factors.

Regulation of cadherin junctions during mouse submandibular gland development

Submandibular gland (SMG) development involves branching morphogenesis of the salivary epithelium into the surrounding mesenchyme, accompanied by proliferation and differentiation of immature salivary cells along acinar and ductal cell lineages. During development, salivary cell sorting and cell-cell adhesion are likely to be directed by cadherin adhesion receptors. We show that two classic cadherins, N- and E-cadherin, participate in SMG development. Early in embryonic morphogenesis, both cadherins displayed diffuse staining with regionalized localization to cell-cell borders. At this stage, significant pools of N- and E-cadherins were Triton-soluble, suggesting that fractions of these molecules were not localized to stable junctional complexes associated with the actin cytoskeleton. With cytodifferentiation, cadherins became progressively Triton-insoluble, and this correlated with their organization at cell-cell interfaces. In the cytodifferentiated SMG, N-cadherin was absent, whereas E-cadherin remained at cell-cell interfaces. Early in morphogenesis, beta-catenin was also primarily Triton-soluble, and its association with the actin cytoskeleton and localization to the adherens junctions increased with cytodifferentiation. Greater recruitment of cadherins and beta-catenin to cell-cell borders was paralleled by changes in membrane association of two Rho GTPases, Cdc42 and RhoA. N-cadherin was detected only at early stages of postnatal development, whereas E-cadherin and beta-catenin became progressively Triton-insoluble during differentiation. Our results indicate that N-cadherin functions transiently in SMG development. On the other hand, E-cadherin and beta-catenin appear to play different roles during tissue organization and cytodifferentiation. In early morphogenesis, E-cadherin and beta-catenin are likely to participate in SMG remodeling, whereas during cytodifferentiation, they form stable cell-cell contacts, and may collaborate with Rho GTPases in the establishment and maintenance of salivary cell polarity.

A comprehensive analysis of gene expression profiles in a yeast N-glycosylation mutant

Although protein N-glycosylation is critical to many cell functions, its downstream targets remain largely unknown. In all eukaryotes, N-glycosylation utilizes the lipid-linked oligosaccharide (LLO) precursor, whose synthesis is initiated by the ALG7 gene. To elucidate the key signaling and metabolic events affected by N-glycosylation, we performed genomewide expression profiling of yeast cells carrying a hypomorphic allele of ALG7. DNA microarrays showed that of more than 97% of known or predicted yeast genes, 29 displayed increased expression while 23 were repressed in alg7 mutants. Changes in transcript abundance were observed for a and alpha mating-type genes, for genes functioning in several mitogen-activated protein kinase (MAPK) cascades, as well as in phosphate, amino acid, carbohydrate, mitochondrial and ATP metabolism. Therefore, DNA microarrays have revealed direct and indirect targets, including internal feedback loops, through which N-glycosylation affects signaling and metabolic activities and is functionally linked with cellular regulatory circuitry.

Loss of alpha3beta1 integrin function results in an altered differentiation program in the mouse submandibular gland

Mammalian submandibular gland (SMG) development leads to the establishment of highly organized secretory acinar and nonsecretory ductal epithelial cells. The ability of maturing salivary epithelial cells to attain their differentiated state has been shown to depend, in part, on interactions between extracellular matrix (ECM) proteins and their integrin receptors. In a search for key regulators of salivary cell lineage, we have studied alpha3beta1 integrin, a receptor for the basement membrane protein laminin, by characterizing embryonic day 18 (E18) SMGs isolated from mice carrying a targeted mutation in the alpha3 integrin gene. Transmission electron microscopy studies showed that the mutant SMGs exhibited an aberrant differentiation phenotype with defects in the apical-basal polarity axis and in the basement membrane. Based on immunohistochemistry and Western blot analyses, the alpha3beta1-deficient SMGs had altered expression and/or localization of several ECM and adhesive molecules, including laminin beta1, fibronectin, alpha5 integrin, and E-cadherin. These changes correlated with alterations in the activation state of Ras-extracellular signal-regulated kinase (ERK), as well as the expression and/or localization of Cdc42 and RhoA, two Rho GTPases that regulate the organization of the actin cytoskeleton. We conclude that alpha3beta1 is required for normal salivary cell differentiation and that its absence affects multiple components of adhesive complexes and their associated signalling pathways.

Differential expression of proliferative, cytoskeletal, and adhesive proteins during postnatal development of the hamster submandibular gland

Although the submandibular gland (SMG) plays important exocrine and endocrine roles, little is known about the molecular details underlying its development. Previously, we reported that in the postnatally developing hamster SMG, GPT, the protein product of the first N-glycosylation gene, ALG7, was an in vivo marker for salivary cell proliferation. Here we investigated the proliferative, cytoskeletal, and adhesive changes during SMG postnatal development. The cellular localization and abundance of GPT, filamentous actin, and beta1 integrin receptor were examined using confocal microscopy and immunoblotting. In neonatal glands, high GPT levels marked extensive cell proliferation throughout the tissue. The apical regions of immature salivary cells displayed intense actin staining, while most of the beta1 integrin was diffusely distributed throughout the tissue. As development proceeded, discrete regions of the gland expressed attenuated levels of GPT, an increased organization of actin to the cell cortex, and beta1 integrin to the basal lamina. In the adult SMG, differentiated salivary cells displayed low levels of GPT and actin. While the abundance of beta1 integrin remained unchanged throughout development, in the adult, it was found exclusively in regions where cells contact the basal lamina. These data indicate that SMG development entails regionalized cell proliferation and polarization, and that these processes are temporally and spatially coordinated with the establishment of stable cell-substratum interactions.

ALG gene expression and cell cycle progression

The evolutionarily conserved ALG genes function in the dolichol pathway in the synthesis of the lipid-linked oligosaccharide precursor for protein N-glycosylation. Increasing evidence suggests a role for these genes in the cell cycle. In Saccharomyces cerevisiae, coordinate regulation of the ALG genes makes up the primary genomic response to growth stimulation; several features of the ALG genes' expression resemble mammalian early growth response genes. However, only the first gene in the pathway, ALG7, is downregulated in response to an antimitogenic signal that leads to cell cycle arrest and differentiation, suggesting that selective inhibition of the first gene may be sufficient to regulate the dolichol pathway for the withdrawal from the cell cycle. The availability of mutants in the early essential ALG genes has established functional relationships between these genes' expression and G1/S transition, budding, progression through G2 and withdrawal from the cell cycle. Analysis of the regulation of ALG7 has provided insights into how this gene's expression is controlled at the molecular level. Recent studies have also begun to reveal how ALG7 expression is linked to cell cycle arrest in response to antimitogenic cues and have identified G1 cyclins as some of its downstream targets. Since the functions of the ALG genes appear to be as conserved among eukaryotes as the cell cycle machinery, it is likely that these genes play a similar role in mammalian cell proliferation and differentiation.

Confocal imaging of gene expression during hamster submandibular gland biogenesis

The data presented here provide evidence that the abundance of the ALG7 protein product, GPT, correlates with high proliferative activity during the postnatal development of the hamster SMG development, and that it becomes downregulated with differentiation. Based on our previous studies with yeast, changes in the level of ALG7 expression may be necessary for the events directing salivary cell polarization, migration, differentiation, and apoptosis at distinct developmental stages.

Molecular dissection of the genetic targets of ALG7 in the serpentine receptor-mediated signal transduction pathway in yeast

These initial studies show that deregulated expression of ALG7 affects diverse cellular functions crucial to development, including proliferation, differentiation, and morphogenesis. Furthermore, the data suggest multiple genetic targets for ALG7 and provide the basis for future dissection of these developmentally relevant pathways.

Characterization of multiple transcripts of the hamster dolichol-P-dependent N-acetylglucosamine-1-P transferase suggests functionally complex expression

The evolutionarily conserved dolichol-P-dependent N-acetylglucosamine-1-P transferase gene, ALG7, functions by initiating the dolichol pathway of protein N-glycosylation. In yeast, ALG7 has a complex expression pattern and plays a critical role in diverse cellular functions, including proliferation and morphological response. In Chinese hamster ovary cells (CHO), ALG7 gives rise to three mRNAs of 1.5, 1.9 and 2.2 kb. We report results of RNA blotting assays, ribonuclease protection, PCR-amplification and sequencing of the CHO ALG7 transcripts 5' and 3' ends which suggest that the 1.5 and 1.9 kb transcripts are produced as a consequence of initiation at 2 distinct start sites, 350-379 bp apart. The transcriptional start site for the 1.5 kb mRNA is positioned between the first two in frame ATGs, while that of the 1.9 kb species is located upstream of these two in-frame ATGs. In order to test the translational competence of the 1.5 and 1.9 kb mRNAs, we constructed DNA templates specifying these transcripts and used them for in vitro transcription/translation. Our data show that the 1.9 kb mRNA served in the synthesis of 36 and 24 kDa species, as well as a low-abundance 32 kDa protein. The 1.5 kb transcript gave rise to a translation product of 32 kDa. The latter is synthesized in CHO cells and hamster submandibular glands. These results suggest the possibility that the 1.5 and 1.9 kb transcripts give rise to related protein isoforms with different lengths of their NH2-terminal regions.

Protein N-glycosylation: molecular genetics and functional significance

Protein N-glycosylation is a metabolic process that has been highly conserved in evolution. In all eukaryotes, N-glycosylation is obligatory for viability. It functions by modifying appropriate asparagine residues of proteins with oligosaccharide structures, thus influencing their properties and bioactivities. N-glycoprotein biosynthesis involves a multitude of enzymes, glycosyltransferases, and glycosidases, encoded by distinct genes. The majority of these enzymes are transmembrane proteins that function in the endoplasmic reticulum and Golgi apparatus in an ordered and well-orchestrated manner. The complexity of N-glycosylation is augmented by the fact that different asparagine residues within the same polypeptide may be modified with different oligosaccharide structures, and various proteins are distinguished from one another by the characteristics of their carbohydrate moieties. Furthermore, biological consequences of derivatization of proteins with N-glycans range from subtle to significant. In the past, all these features of N-glycosylation have posed a formidable challenge to an elucidation of the physiological role for this modification. Recent advances in molecular genetics, combined with the availability of diverse in vivo experimental systems ranging from yeast to transgenic mice, have expedited the identification, isolation, and characterization of N-glycosylation genes. As a result, rather unexpected information regarding relationships between N-glycosylation and other cellular functions--including secretion, cytoskeletal organization, proliferation, and apoptosis--has emerged. Concurrently, increased understanding of molecular details of N-glycosylation has facilitated the alignment between N-glycosylation deficiencies and human diseases, and has highlighted the possibility of using N-glycan expression on cells as potential determinants of disease and its progression. Recent studies suggest correlations between N-glycosylation capacities of cells and drug sensitivities, as well as susceptibility to infection. Therefore, knowledge of the regulatory features of N-glycosylation may prove useful in the design of novel therapeutics. While facing the demanding task of defining properties, functions, and regulation of the numerous, as yet uncharacterized, N-glycosylation genes, glycobiologists of the 21st century offer exciting possibilities for new approaches to disease diagnosis, prevention, and cure.

Deregulation of the first N-glycosylation gene, ALG7, perturbs the expression of G1 cyclins and cell cycle arrest in Saccharomyces cerevisiae

The evolutionarily conserved ALG7 gene encodes the dolichol-P-dependent N-acetylglucosamine-1-P transferase (GPT) and functions by initiating the dolichol pathway of protein N-glycosylation. In Saccharomyces cerevisiae, ALG7 has been shown to play a role in cell proliferation. The yeast alpha-factor-induced cell cycle arrest in G1 occurs, in part, by downregulation of CLN1 and CLN2. The function of ALG7 in G1 arrest was examined in alg7 mutants containing diminished GPT activity. In wild type, CLN1 and CLN2 mRNAs were rapidly downregulated, while in alg7 mutants, these transcripts were only transiently repressed before becoming greatly augmented. Analyses of DNA contents and budding indices showed that alg7 mutants resumed cycling when wild type cells remained arrested. Thus, deregulation of ALG7 interferes with cell cycle arrest by preventing a sustained downregulation of CLN1 and CLN2 mRNAs. These results provide a molecular insight into the role of ALG7, and protein N-glycosylation in general, in proliferation.

The dual role of mRNA half-lives in the expression of the yeast ALG7 gene

The yeast ALG7 gene functions by initiating the synthesis of the dolichol-linked oligosaccharide precursor and plays an important role in the control of protein N-glycosylation. The levels of ALG7 multiple transcripts are modulated by the physiological status of the cell and environmental cues, and deregulation of their abundance is deleterious to several cellular functions. Since ALG7 mRNAs are unstable, we investigated the role of these transcripts' half-lives in determining their steady-state levels. Using a temperature-sensitive RNA polymerase II mutant, we demonstrate that increased stability was the primary determinant of higher ALG7 mRNA abundance in response to glucose limitation or treatment with tunicamycin. In contrast, at the G1/G0 transition point, changes in the decay rates were inversely related to ALG7 transcript accumulation: the decreased abundance of ALG7 mRNAs following exit from the mitotic cycle was associated with lengthening of the decay rates, while their increased accumulation after growth stimulation correlated with decreased stability. This suggests that, depending on the circumstance, mRNA half-lives can either directly determine the level of ALG7 transcript accumulation or oppose regulatory changes at other control levels.

Proliferation-dependent differential regulation of the dolichol pathway genes in Saccharomyces cerevisiae

The dolichol pathway serves in the synthesis of the dolichol-linked oligosaccharide precursor for protein N-glycosylation. Recently, we reported that mRNAs of genes that function at the early steps in the dolichol pathway in yeast, ALG7, ALG1 and ALG2, were co-ordinately induced following growth stimulation of G0-arrested cells in a manner similar to that of the transcripts of the early growth response genes (Kukuruzinska, M.A. and Lennon, K. Glycobiology, 4, 437-443, 1994). To determine whether the entire dolichol pathway was co-ordinately regulated with growth, we examined the expression of genes functioning late in the pathway, including two genes encoding oligosaccharyltransferase subunits, at two critical control points in the G1 phase of cell cycle: G0/G1 and START. We show that early in G1, at the G0/G1 transition point, the late ALG genes and the two oligosaccharyltransferase-encoding genes examined were regulated co-ordinately with the early ALG genes: they were downregulated upon exit from the mitotic cell cycle into G0, and they were induced following growth stimulation in the absence of de novo protein synthesis. All the dolichol pathway genes produced transcripts with short half-lives that were rapidly stabilized in the presence of cycloheximide. In contrast, cell division arrest late in G1, at START, was accompanied by a selective downregulation of only the first dolichol pathway gene, ALG7, and not of the genes functioning later in the pathway. These results indicate that, depending on their position in G1, cells either co-ordinately or differentially regulate the dolichol pathway genes.

Expression of the first N-glycosylation gene in the dolichol pathway, ALG7, is regulated at two major control points in the G1 phase of the Saccharomyces cerevisiae cell cycle

The Saccharomyces cerevisiae ALG7 gene, which functions by initiating the dolichol pathway of protein N-glycosylation, displays properties of an early growth-response gene. To initiate studies of the involvement of ALG7 in cellular proliferation, we have now more precisely analyzed ALG7 expression in the G1 phase of cell cycle. We show that the rapid rate of ALG7 mRNA accumulation following growth stimulation was attenuated soon thereafter and that ALG7 growth induction occurred irrespective of alpha-factor. ALG7 growth induction was observed in mutants conditionally defective for reentry into the cell cycle from the stationary phase, indicating that the induction occurred prior to the performance of START. In addition, the steady-state levels of ALG7 mRNAs declined four-fold in response to START-I cell division arrest brought about by alpha-factor treatment later in G1. Importantly, deregulated expression of ALG7 resulted in an aberrant alpha-factor response. Our data not only indicate that ALG7 expression is regulated at two critical control points in G1 that determine the proliferative potential of cells, but also provide a link between ALG7 and START.

Diminished activity of the first N-glycosylation enzyme, dolichol-P-dependent N-acetylglucosamine-1-P transferase (GPT), gives rise to mutant phenotypes in yeast

The enzyme which initiates the dolichol pathway of protein N-glycosylation, dolichol-P-dependent N-acetylglucosamine-1-P transferase (GPT), is encoded by the ALG7 gene. Essential for viability, ALG7 has been evolutionarily conserved and shown to be involved in a variety of functions. ALG7 is an early growth-response gene in yeast, and downregulation of ALG7 expression results in diminished N-glycosylation and secretion of Xenopus oocyte proteins. We have now investigated the consequences of diminished GPT activity in yeast using mutant ALG7 genes with deletions in the 3' untranslated region (3' UTR). We show that a 2.5- to 4-fold reduction in GPT activity gave rise to distinct phenotypes, whose severity was inversely related to the level of GPT activity. These phenotypes included hypersensitivity to tunicamycin, enlarged cell size, extensive aggregation, lack of a typical stationary (G0) arrest, and defective spore germination. We conclude that yeast cells are sensitive to GPT dosage, and that attenuation of GPT activity interferes with various functions in the yeast life cycle.

Developmental regulation and tissue-specific expression of hamster dolichol-P-dependent N-acetylglucosamine-1-P transferase (GPT)

The first enzyme in the dolichol pathway of protein N-glycosylation, dolichol-P-dependent N-acetylglucosamine-1-phosphate transferase, GPT, has been implicated in the development of a wide variety of eukaryotes. GPT is encoded by ALG7, an early growth-response gene, whose expression has been shown to affect the extent of N-glycosylation and secretion of proteins. To initiate the molecular characterization of ALG7 involvement in mammalian growth and differentiation, we have used the postnatally developing hamster submandibular gland (SMG) as an experimental paradigm. In this study we report that the ALG7 gene was differentially expressed during postnatal development and in terminally differentiated adult tissues. Throughout development, GPT activity paralleled ALG7 mRNA levels, suggesting that the amount of functional enzyme was determined by modulation of transcript abundance.

Growth-related coordinate regulation of the early N-glycosylation genes in yeast

The Saccharomyces cerevisiae ALG7, ALG1 and ALG2 genes, whose products function early in the dolichol pathway of protein N-glycosylation, are essential for cell viability, and perturbation in their expression causes G1-specific cell cycle arrest. Here, we show that expression of the ALG7, ALG1 and ALG2 genes is coordinately regulated at the G0/G1 transition point in the yeast life cycle. Carbon starvation, which induces cells to exit from the G1 stage of the mitotic cycle into G0, resulted in a time-dependent decrease in the levels of the early ALG genes' mRNAs. Accordingly, addition of glucose, which stimulates the G0-arrested cells to resume proliferation, resulted in a rapid induction of their mRNAs. Cycloheximide alone also induced the early ALG transcripts, albeit to a much lower extent than glucose. Simultaneous addition of glucose and cycloheximide caused superinduction of these mRNAs, indicating that more than one control level was involved in their activation. Consistent with this, rapid degradation of ALG7, ALG1 and ALG2 mRNAs was completely abolished in the presence of cycloheximide. These data suggest that in yeast, the early N-glycosylation genes are regulated in a manner similar to that of the early growth-response genes.

Antisense RNA to the first N-glycosylation gene, ALG7, inhibits protein N-glycosylation and secretion by Xenopus oocytes

N-Glycosylation has been shown to affect the rate of glycoprotein transport through the secretory pathway. In order to identify the critical components in the N-glycosylation pathway that directly influence protein secretion, we have studied the effects of downregulation of the first gene in the dolichol pathway, ALG7, on the synthesis, glycosylation and secretion of native and heterologous proteins by Xenopus laevis oocytes. Our strategy involved the use of ALG7 antisense RNA (asRNA) to lower the effective abundance of the ALG7 protein in oocytes. The results showed that there was an inverse dose-response relationship between ALG7 asRNA and the amount of glycosylated and secreted proteins. These effects were also observed for heterologously expressed rat parotid amylase. Since ALG7 asRNA did not inhibit overall protein synthesis, we conclude that downregulation of ALG7 expression directly lowered protein export.

Biosynthesis of asparagine-linked oligosaccharides in Saccharomyces cerevisiae: the alg2 mutation

In the yeast Saccharomyces cerevisiae, the alg2 mutation causes temperature-sensitive growth and abnormal accumulation of the lipid-linked oligosaccharide Man2GlcNAc2-PP-Dol (Jackson et al., Arch. Biochem. Biophys., 272, 203-209, 1989; Huffaker and Robbins, Proc. Natl. Acad. Sci. USA, 80, 7466-7470, 1983). A gene having the function and genomic location of ALG2 was cloned from libraries based on the multicopy plasmid YEp24 and on the centromere plasmid YCp50. Alg2 mutants transformed with plasmids containing ALG2 regained the capacity to grow and to synthesize lipid-linked oligosaccharides normally at the previously non-permissive temperature. ALG2 was essential for viability in haploid and diploid yeast. The ALG2 gene was transcribed into a single mRNA of 1.7 kb in size. The stability of ALG2 mRNA, assessed after thermal inactivation of RNA polymerase II in an rpb1-1 mutant (Herrick et al., Mol. Cell. Biol., 10, 2269-2284, 1990) was very low, with a t1/2 of < 5 min. The ALG2 transcript accumulation was growth dependent, and it was at least an order of magnitude lower in stationary phase cells compared to exponentially growing cells. The putative translation product of ALG2 contained a potential dolichol recognition domain similar to that found in all three glycosyltransferases of the lipid-linked pathway that have been sequenced.(ABSTRACT TRUNCATED AT 250 WORDS)