Abstract LB084: AXL as a therapeutic target in adenoid cystic carcinoma: preclinical evaluation of AXL targeting antibody-drug conjugate (ADCT-601)

Background: Adenoid cystic carcinoma (ACC) is a rare cancer of salivary gland cancer with a significant unmet clinical need. Conventional systemic therapy has limited efficacy and there are currently no FDA approved drugs for these patients with metastatic disease. Advances in molecular profiling led identification of potential therapeutic targets, and previous studies with proteomic analysis identified AXL as a potential therapeutic target in ACC. AXL, a member of the TAM tyrosine kinase receptor family has been generally associated with poor prognosis, and its overexpression and activation is implicated in conferring resistance to conventional systemic therapies in solid tumors. ADCT-601 is an antibody drug conjugate targeting human AXL carrying novel pyrrolobenzodiazepine dimer cytotoxin as the payload.

Methods: The goal of this study is to determine the in vitro and in vivo anti-tumor activity of ADCT-601 in ACC cell lines and patient-derived xenograft (ACCX) models with varying AXL expression determined by Western blot. ADCT-601 treated cell lines in vitro were analyzed by MTT-based cytotoxic assay and immunoblotting, and anti-tumorigenicity was assessed in two ACCX models.

Results: ADCT-601 induced a potent dose-dependent cytotoxic effect in ACC cell lines, but its cytotoxic activity was attenuated after AXL knockdown. Biochemical data indicated that ADCT-601 treatment led to DNA damage and induction of apoptotic factors accompanied by phosphorylation of gamma-H2AX. In high AXL expressing ACCX6 model, a single dose of either 0.5 or 1.0 mg/kg ADCT-601 significantly suppressed tumor growth resulting 3/5 partial responders (PRs) at 1mg/kg and 2/5 PRs at 0.5 mg/kg, compared to the vehicle or isotype-ADC control treated mice (0/5 PR and 1/5 PR, respectively). ADCT-601 appears to eliminate tumor, as no evidence of recovery in tumor growth was observed during a 60-day period. In low AXL expressing ACCX9 model, ADCT-601 treatment (at either the 0.5 or 1.0 mg/kg dose) delayed tumor growth compared to isotype-ADC control, but did not lead to durable partial responses.

Conclusion: This study demonstrated that ADCT-601 induced a potent and specific in vitro and in-vivo anti-tumor activities in AXL expressing ACC models and suggests further development of ADCT-601 in biomarker driven clinical trials.

Citation Format: Joseph O. Humtsoe, Leilani Jones, Shorook Naara, Francesca Zammarchi, Nicole Spardy Burr, Patrick H. van Berkel, Patrick K. Ha, Hyunseok Kang. AXL as a therapeutic target in adenoid cystic carcinoma: preclinical evaluation of AXL targeting antibody-drug conjugate (ADCT-601) [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr LB084.

Development and Characterization of MYB-NFIB Fusion Expression in Adenoid Cystic Carcinoma

Adenoid cystic carcinoma (ACC) is the second most common cancer type arising from the salivary gland. The frequent occurrence of chromosome t(6;9) translocation leading to the fusion of MYB and NFIB transcription factor genes is considered a genetic hallmark of ACC. This inter-chromosomal rearrangement may encode multiple variants of functional MYB-NFIB fusion in ACC. However, the lack of an ACC model that harbors the t(6;9) translocation has limited studies on defining the potential function and implication of chimeric MYB-NFIB protein in ACC. This report aims to establish a MYB-NFIB fusion protein expressing system in ACC cells for in vitro and in vivo studies. RNA-seq data from MYB-NFIB translocation positive ACC patients' tumors and MYB-NFIB fusion transcript in ACC patient-derived xenografts (ACCX) was analyzed to identify MYB breakpoints and their frequency of occurrence. Based on the MYB breakpoint identified, variants of MYB-NFIB fusion expression system were developed in a MYB-NFIB deficient ACC cell lines. Analysis confirmed MYB-NFIB fusion protein expression in ACC cells and ACCXs. Furthermore, recombinant MYB-NFIB fusion displayed sustained protein stability and impacted transcriptional activities of interferon-associated genes set as compared to a wild type MYB. In vivo tumor formation analysis indicated the capacity of MYB-NFIB fusion cells to grow as implanted tumors, although there were no fusion-mediated growth advantages. This expression system may be useful not only in studies to determine the functional aspects of MYB-NFIB fusion but also in evaluating effective drug response in vitro and in vivo settings.

Newly Identified Members of FGFR1 Splice Variants Engage in Cross-talk with AXL/AKT Axis in Salivary Adenoid Cystic Carcinoma

Adenoid cystic carcinoma (ACC) is the second most common malignancy of the salivary gland. Although characterized as an indolent tumor, ACC often leads to incurable metastatic disease. Patients with ACC respond poorly to currently available therapeutic drugs and factors contributing to the limited response remain unknown. Determining the role of molecular alterations frequently occurring in ACC may clarify ACC tumorigenesis and advance the development of effective treatment strategies. Applying Splice Expression Variant Analysis and outlier statistics on RNA sequencing of primary ACC tumors and matched normal salivary gland tissues, we identified multiple alternative splicing events (ASE) of genes specific to ACC. In ACC cells and patient-derived xenografts, FGFR1 was a uniquely expressed ASE. Detailed PCR analysis identified three novel, truncated, intracellular domain-lacking FGFR1 variants (FGFR1v). Cloning and expression analysis suggest that the three FGFR1v are cell surface proteins, that expression of FGFR1v augmented pAKT activity, and that cells became more resistant to pharmacologic FGFR1 inhibitor. FGFR1v-induced AKT activation was associated with AXL function, and inhibition of AXL activity in FGFR1v knockdown cells led to enhanced cytotoxicity in ACC. Moreover, cell killing effect was increased by dual inhibition of AXL and FGFR1 in ACC cells. This study demonstrates that these previously undescribed FGFR1v cooperate with AXL and desensitize cells to FGFR1 inhibitor, which supports further investigation into combined FGFR1 and AXL inhibition as an effective ACC therapy.This study identifies several FGFR1 variants that function through the AXL/AKT signaling pathway independent of FGF/FGFR1, desensitizing cells to FGFR1 inhibitor suggestive of a potential resistance mechanism in ACC. SIGNIFICANCE: This study identifies several FGFR1 variants that function through the AXL/AKT signaling pathway independent of FGF/FGFR1, desensitizing cells to FGFR1 inhibitor, suggestive of a potential resistance mechanism in ACC.

Abstract A27: Differential protein-interactome of MYB-NF1B fusion in adenoid cystic carcinoma

Chromosomal translocations occur frequently in adenoid cystic carcinomas (ACC). In salivary gland ACC, the commonly identified rearrangement occurs at chromosome t(6:9), resulting in the fusion of the transcription factor genes, MYB and NF1B. The gene fusion encoding a chimeric MYB-NF1B is a unique genetic feature, regarded as a hallmark in ACC carcinogenesis. Although MYB-NF1B may be involved in the deregulation of MYB expression and its downstream gene targets, the precise role of MYB-NF1B in the development and maintenance of ACC remains to be clearly defined. Here, we engineered an inducible system to express either MYB or MYB-NF1B in ACC-01 cells, which express low levels of endogenous MYB but not MYB-NF1B, to study the molecular and mechanistic role of MYB-NF1B. Immunoprecipitation and mass spectrometry analysis revealed the presence of previously unreported MYB- and MYB-NF1B-interacting partners, including vimentin and nuclear myosin (MYO1C). These interactions were positively validated in several patient-derived xenograft models of ACC. Interestingly, MYB and MYB-NF1B interaction with vimentin and MYO1C appeared to be differentially regulated, and altering vimentin expression reduces MYB levels, while maintaining relatively steady levels of MYB-NF1B. MYO1C is involved in transcriptional regulation, and altered MYB-NF1B binding activity with nuclear myosin may modulate downstream gene targets. While detailed further study is needed, these findings suggest a potential distinct function of MYB-NF1B that may play an important role in ACC pathogenesis.

Citation Format: Joseph O. Humtsoe, Hyun-Su Kim, Margaret Soucheray, Danielle Swaney, Patrick K. Ha. Differential protein-interactome of MYB-NF1B fusion in adenoid cystic carcinoma [abstract]. In: Proceedings of the AACR-AHNS Head and Neck Cancer Conference: Optimizing Survival and Quality of Life through Basic, Clinical, and Translational Research; 2019 Apr 29-30; Austin, TX. Philadelphia (PA): AACR; Clin Cancer Res 2020;26(12_Suppl_2):Abstract nr A27.

Abstract 64: Abnormal splicing events in adenoid cystic carcinoma

Salivary gland adenoid cystic carcinoma (ACC) is a relatively rare and understudied malignancy. While this disease can often be controlled locally by surgery and radiation therapy, distant metastases occur in nearly 50% of cases. Interestingly, these metastases can remain asymptomatic for several years, and there are currently no factors that are known to contribute to their dormancy or aggressiveness. Furthermore, there are no targeted chemotherapeutic agents available to treat any stage of this disease.

To identify aberrations that contribute to ACC tumorigenesis, we performed whole genome methylation profiling, RNA sequencing, and exome sequencing on 18 primary ACC tumors and matched normal salivary gland tissue. A novel analysis methodology of Splice Expression Variability Analysis (SEVA) coupled with outlier statistical analyses of mRNA splice junctions revealed the presence of several abnormal splicing events (ASEs) in the studied tumors. Interestingly, the ASEs were significantly overrepresented in tumors as compared to matched normal salivary gland tissue. In vitro analyses found that many of the ASEs are also expressed in the only validated ACC cell line, MDA-ACC01, and multiple patient-derived xenograft (PDX) models of ACC. Overall, these studies have identified an unanticipated aberration in ACC and developed a model in which to study ASEs. Future work will be aimed at determining the functional impact that ASEs have on tumor initiation and growth.

Citation Format: Brandon Leonard, Joseph O. Humtsoe, Hyun-Su Kim, Shizhang Ling, Luigi Marchionni, Elana J. Fertig, Michael Considine, Patrick Ha. Abnormal splicing events in adenoid cystic carcinoma [abstract]. In: Proceedings of the AACR-AHNS Head and Neck Cancer Conference: Optimizing Survival and Quality of Life through Basic, Clinical, and Translational Research; April 23-25, 2017; San Diego, CA. Philadelphia (PA): AACR; Clin Cancer Res 2017;23(23_Suppl):Abstract nr 64.

ErbB3 upregulation by the HNSCC 3D microenvironment modulates cell survival and growth

Head and neck squamous carcinomas (HNSCC) present as dense epithelioid three-dimensional (3D) tumor nests that can mediate signals via the human epidermal growth factor receptor (ErbB) tyrosine kinase family to promote intratumoral survival and growth. We examined the role of the tumor microenvironment on ErbB receptor family expression and found that the status of intercellular organization altered the receptor profile. We showed that HNSCC cells forced into tumor island-like 3D aggregates strongly upregulated ErbB3 at the level of transcription. Not only was the elevated ErbB3 responsive to HRG-β1-induced enhanced signaling mechanism, but also analysis by siRNA-knockdown and kinase inhibitor strategies revealed that the ErbB3/AKT signaling pathway was sufficient to enhance tumor cell survival and growth potential. Elevated ErbB3 expression in the high-density 3D culture system was strongly associated with hypoxia-induced HIF-1α. Hypoxia-regulated ErbB3 expression was mediated by the HIF-1α-binding consensus sequence in the ErbB3 proximal promoter. The findings show that the local 3D tumor microenvironment can trigger reprograming and switching of ErbB family members and thereby influence ErbB3-driven tumor growth.

Transcriptional profiling identifies upregulated genes following induction of epithelial-mesenchymal transition in squamous carcinoma cells

During the progression of head and neck squamous cell carcinoma (HNSCC), the induction of an epithelial-mesenchymal transition (EMT) program may play a critical role in the dissemination of cells from the primary tumor to distant metastatic foci. The process of EMT involves the activation of several important genes and pathways to help maintain survival and growth and evolve into highly invasive and metastatic variants. In this study, expression microarray analysis identified a set of 145 upregulated genes in EMT-like HNSCC cells. Some of the strongly upregulated transcripts include genes that are reportedly involved in invasion and metastasis, such as DOCK10, LOX, ROBO1 and SRGN. Importantly, the Tbx3 gene, a member of the T-box transcription factor, was strongly upregulated in SCC cells displaying an EMT-like phenotype compared to cells with an epitheloid, non-EMT behavior. Tbx3 was also found to be strongly upregulated at the protein and gene expression level in an experimental model of snail-induced EMT cells. In addition, siRNA-induced Tbx3 depletion modestly suppressed cell invasion while enhancing Tbx3-mediated resistance to anoikis. Our findings provide evidence that Tbx3 overexpression promotes SCC cell survival displaying an EMT phenotype. This set of newly identified genes that are modulated during EMT-like conversion may be important diagnostic biomarkers during the process of HNSCC progression.

Lipid phosphate phosphatase-3 regulates tumor growth via β-catenin and CYCLIN-D1 signaling

Background

The acquisition of proliferative and invasive phenotypes is considered a hallmark of neoplastic transformation; however, the underlying mechanisms are less well known. Lipid phosphate phosphatase-3 (LPP3) not only catalyzes the dephosphorylation of the bioactive lipid sphingosine-1-phosphate (S1P) to generate sphingosine but also may regulate embryonic development and angiogenesis via the Wnt pathway. The goal of this study was to determine the role of LPP3 in tumor cells.

Results

We observed increased expression of LPP3 in glioblastoma primary tumors and in U87 and U118 glioblastoma cell lines. We demonstrate that LPP3-knockdown inhibited both U87 and U118 glioblastoma cell proliferation in culture and tumor growth in xenograft assays. Biochemical experiments provided evidence that LPP3-knockdown reduced β-catenin, CYCLIN-D1, and CD133 expression, with a concomitant increase in phosphorylated β-catenin. In a converse experiment, the forced expression of LPP3 in human colon tumor (SW480) cells potentiated tumor growth via increased β-catenin stability and CYCLIN-D1 synthesis. In contrast, elevated expression of LPP3 had no tumorigenic effects on primary cells.

Conclusions

These results demonstrate for the first time an unexpected role of LPP3 in regulating glioblastoma progression by amplifying β-catenin and CYCLIN-D1 activities.

Differential epidermal growth factor receptor signaling regulates anchorage-independent growth by modulation of the PI3K/AKT pathway

Tumor cells are capable of surviving loss of nutrients and anchorage in hostile microenvironments. Under these conditions adapting to specific signaling pathways may shift the balance between growth and cellular dormancy. Here we report a mechanism by which EGFR differentially modulates the PI3K/AKT pathway in cellular stress conditions. When carcinoma cells were cultured as multicellular aggregates (MCA), cyclin D1 was induced through a serum-dependent EGFR activating pathway, triggering cell proliferation. The expression of cyclin D1 required both EGFR-mediated ERK and AKT activation. In serum-starved MCAs, EGFR activation was associated with active ERK1/2 but not AKT and failed to induce cyclin D1. Analysis revealed that, under serum-starved conditions, EGFR-Y1086 residue was poorly autophosphorylated and this correlated with failure to phosphorylate Gab1. Accordingly, the EGFR activation failed to induce EGFR/PI3K complex formation or AKT activation, preventing cyclin D1 induction. Furthermore, we show that in serum-starved MCA, expression of constitutively active AKT re-established cyclin D1 expression and induced proliferation in an EGFR-dependent manner. Thus, modulation of the PI3K/AKT pathway by context-dependent EGFR signaling may regulate tumor cell growth and dormancy.

Detection and quantification of poly-ADP-ribosylated cellular proteins of spleen and liver tissues of mice in vivo by slot and Western blot immunoprobing using polyclonal antibody against mouse ADP-ribose polymer

Poly-ADP-ribosylation (PAR) of cellular proteins has been shown to have decisive roles in diverse cellular functions including carcinogenesis. There are indications that metabolic level of poly-ADP-ribosylated cellular proteins might indicate carcinogenesis and, therefore, could be potentially used in cancer screening program. Keeping in mind the limitations of currently available assays of cellular PAR, a new assay is being reported that measures the metabolic level of poly-ADP-ribosylated cellular proteins. The ELISA based slot and Western blot immunoassay used polyclonal antibody against natural, heterogeneous ADP-ribose polymers. It could be successfully employed to qualitatively and quantitatively assay metabolic levels of poly-ADP-ribosylated proteins of spleen and liver tissues of normal mice or mice exposed to dimethylnitrosamine for up to 8 weeks; potentially PAR of cellular proteins could be assayed in any tissue or biopsy. Implications of the results in cancer screening program have been discussed.

Murine lipid phosphate phosphohydrolase-3 acts as a cell-associated integrin ligand

Lipid phosphate phosphohydrolase-3 (LPP3) is a cell surface protein that exhibits ectoenzyme activity. Previously, we identified human LPP3 in a functional assay of angiogenesis and showed that the Arg-Gly-Asp (RGD) motif in the proposed second extracellular domain interacts with a subset of integrins to mediate cell-cell adhesion. In contrast to the RGD domain of human LPP3, murine Lpp3 contains a variant sequence, Arg-Gly-Glu (RGE). Whether the RGE motif of murine Lpp3 mediates cell-cell interaction has not been studied. In this report, we test the hypothesis that the cell adhesion function of the LPP3 protein is conserved across mouse and human. A glutathione S-transferase (GST) fusion protein of the proposed second extracellular loop of the murine Lpp3 sequence (GST-mLpp3-RGE) promoted attachment of cells in a long-term cell adhesion assay. GST-mLpp3-RGE interacted with alpha(5)beta(1) and alpha(v)beta(3) integrins in a solid-phase ELISA, while a mutant control, GST-hLPP3-RAD, did not. Long-term adhesion of endothelial cells to GST-mLpp3-RGE induced phosphorylation of FAK, SHC, and CAS, whereas adhesion to GST-hLPP3-RAD failed to do so. Upon long-term adhesion both the GST-hLPP3-RGD and GST-mLpp3-RGE substrates bound to the alpha(5)beta(1) integrin of FRT-alpha(5)(+) cells, an interaction that was inhibited by an anti-alpha(5) integrin antibody. In addition, a cell aggregation assay showed that the intact mLpp3-RGE protein interacts with alpha(5)beta(1) and alpha(v)beta(3) integrins expressed by adjacent cells, an interaction that can be blocked by GRGDSP peptides and anti-LPP3-RGD antibodies. These data, together with the known importance of integrins in angiogenesis, provide a mechanism for the function of LPP3 in cell-cell interactions in both human and mouse.

Anti-lipid phosphate phosphohydrolase-3 (LPP3) antibody inhibits bFGF- and VEGF-induced capillary morphogenesis of endothelial cells

Background

Angiogenesis, or the remodeling of existing vasculature serves as a lifeline to nourish developing embryos and starved tissues, and to accelerate wound healing, diabetic retinopathy, and tumor progression. Recent studies indicate that angiogenesis requires growth factor activity as well as cell adhesion events mediated by α₅β₁ and α_vβ₃ integrins. We previously demonstrated that human lipid phosphate phosphohydrolase-3 (LPP3) acts as a cell-associated ligand for α₅β₁ and α_vβ₃ integrins. Here, we test the hypothesis that an anti-LPP3 antibody can inhibit basic fibroblast growth factor (bFGF)-and vascular endothelial growth factor (VEGF)-induced capillary morphogenesis of endothelial cells (ECs).

Results

We report that bFGF and VEGF up-regulate LPP3 protein expression in ECs. Immunoprecipitation analyses show that LPP3 is a cell surface protein and undergoes N-glycosylation. Fluorescent activated cell sorting (FACS) data suggest that anti-LPP3-RGD detects native neoepitope on the surface of activated ECs. Moreover, we demonstrate LPP3 protein expression in tumor endothelium alongside VEGF. The embedding of ECs into three-dimensional type I collagen in the presence of bFGF and VEGF induce capillary formation. Importantly, we show that the addition of an anti-LPP3 antibody specifically and significantly blocks bFGF- and VEGF-induced capillary morphogenesis of ECs.

Conclusion

These data suggest that activated ECs as well as tumor endothelium express LPP3 protein. In an in vitro assay, the anti-LPP3-RGD specifically blocks bFGF and VEGF induced capillary morphogenesis of ECs. Our results, therefore, suggest a role for LPP3 in angiogenesis.

A Streptococcal Collagen-like Protein Interacts with the α2β1 Integrin and Induces Intracellular Signaling

The streptococcal collagen-like proteins Scl1 and Scl2 are prokaryotic members of a large protein family with domains containing the repeating amino acid sequence (Gly-Xaa-Yaa)(n) that form a collagen-like triple-helical structure. Here, we test the hypothesis that Scl variant might interact with mammalian collagen-binding integrins. We show that the recombinant Scl protein p176 promotes adhesion and spreading of human lung fibroblast cells through an alpha2beta1 integrin-mediated interaction as shown in cell adhesion inhibition assays using anti-alpha2beta1 and anti-beta1 integrins monoclonal antibodies. Accordingly, C2C12 cells stably expressing alpha2beta1 integrin as the only collagen-binding integrin show productive cell adhesion activities on p176 that can be blocked by an anti-alpha2beta1 integrin antibody. In addition, p176 promotes tyrosine phosphorylation of p125(FAK) of C2C12 cells expressing alpha2beta1 integrin, whereas parental cells do not. Furthermore, C2C12 adhesion of human lung fibroblast cells to p176 induces phosphorylation of p125FAK, p130CAS, and p68Paxillin proteins. In a domain swapping experiment, we show that integrin binds to the collagenous domain of the Scl protein. Moreover, the recombinant inserted domain of the alpha2 integrin interacts with p176 with a relatively high affinity (K(D) = 17 nm). Attempts to identify the integrin sites in p176 suggest that more than one site may be involved. These studies, for the first time, suggest that the collagen-like proteins of prokaryotes retained not only structural but also functional characteristics of their eukaryotic counterparts.

Analysis of VEGF-responsive genes involved in the activation of endothelial cells

BACKGROUND

Identification of the genes and pathways associated with the activation of endothelial cells (ECs) could help uncover the role of ECs in wound healing, vascular permeability, blood brain barrier function, angiogenesis, diabetic retinopathy, atherosclerosis, psoriasis, and growth of solid tumors.

DESIGN

Herein, we embedded ECs in 3D type I collagen gel, left unstimulated or stimulated with VEGF165, and subjected to suppression subtractive hybridization followed by differential display (SSHDD). Gene fragments obtained from SSHDD were subjected to DNA sequence analysis. Database search with nucleotide sequence were performed using the BLAST algorithm and expression of candidate genes determined by northern blot analysis.

RESULTS

A total of approximately 32 cDNA fragments, including known regulators of angiogenesis, and a set of genes that were not reported to be associated with activation of ECs and angiogenesis previously were identified. We confirmed the mRNA expression of KDR, alpha2 integrin, Stanniocalcin, including a set of 11 candidate genes. Western immunoblotting results indicated that KDR, alpha2 integrin, MMP-1, MMP-2, and VE-cadherin genes were indeed active genes.

CONCLUSION

We have identified a set of 11 VEGF-responsive endothelial cell candidate genes. Their expression in endothelial cell is confirmed by northern blot analyses. This preliminary report forms as a foundation for functional studies to be performed to reveal their roles in EC activation and pathophysiological events associated with the vasculature including tumor growth.

Regulation of cell-cell interactions by phosphatidic acid phosphatase 2b/VCIP

We identified vascular endothelial growth factor and type I collagen inducible protein (VCIP), also known as phosphatidic acid phosphatase 2b (PAP2b), in a functional assay of angiogenesis. VCIP/PAP2b exhibits an Arg-Gly-Asp (RGD) cell adhesion sequence. Immunoprecipitation and fluorescence-activated cell sorting analyses demonstrated that VCIP-RGD is exposed to the outside of the cell surface. Retroviral transduction of VCIP induced cell aggregation/cell- cell interactions, modestly increased p120 catenin expression and promoted activation of the Fak, Akt and GSK3beta protein kinases. Furthermore, expression of recombinant VCIP promoted adhesion, spreading and tyrosine phosphorylation of Fak, Shc, Cas and paxillin in endothelial cells. GST-VCIP-RGD, but not GST-VCIP-RGE, specifically interacted with a subset of integrins, and these interactions were effectively blocked by anti-alpha(v)beta(3) and anti-alpha(5)beta(1) integrin antibodies, and by PAP2b/VCIP-derived peptides. Interestingly, PAP2b/VCIP is expressed in close proximity to vascular endothelial growth factor, von Willebrand factor and alpha(v)beta(3) integrin in tumor vasculatures. These findings demonstrate an unexpected function of PAP2b/VCIP, and represent an important step towards understanding the molecular mechanisms by which PAP2b/VCIP-induced cell-cell interactions regulate specific intracellular signaling pathways.