The importance of the tumor microenvironment in targeted anticancer therapies has been well recognized. Various protein factors participate in the cross-talk between tumor cells and non-malignant cells. Anterior gradient-2 (AGR2) is overexpressed in diverse human adenocarcinomas and it exists in both intracellular and extracellular spaces. Although intracellular AGR2 has been intensively investigated, the function of secreted AGR2, especially its exact mechanism of action is still poorly understood. Here we report that the secreted AGR2 promotes the angiogenesis and the invasion of vascular endothelial cells and fibroblasts by enhancing the activities of vascular endothelial growth factor (VEGF) and fibroblast growth factor 2 (FGF2). Further study indicated that AGR2 directly binds to these extracellular signaling molecules, and enhances their homodimerization. The extracellular AGR2 activity can be blocked to reduce angiogenesis and inhibit tumor growth in vitro and in vivo by a monoclonal antibody targeting the AGR2 self-dimerization region, and combined treatment with bevacizumab produced maximum inhibition effect. In conclusion, our investigation reveals a mechanism that directly links the secreted AGR2 with extracellular signaling networks, and we propose that the secreted AGR2 is a blockable molecular target, which acts as a chaperon-like enhancer to VEGF and FGF2.

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Anterior gradient 2 (AGR2) is a promising anti-tumor target associated with estrogen receptor expression and metastatic progression of breast cancer. Insulin-like growth factor-1 (IGF-1) is another potent factor that stimulates breast cancer progression and mediates anti-estrogen drug resistance. However, the precise mechanism and connections between these two factors in breast cancer drug resistance have not been fully elucidated. Here, for the first time, we decipher that IGF-1 remarkably induces AGR2 in the MCF7 cell line, through an estrogen response element (ERE) between −802 and −808 bp and a leucine zipper transcription factor-binding site located between −972 and −982 bp on the AGR2 promoter. We also found that the ERK1/2 and AKT pathways mediate estrogen receptor-α at the upstream of ERE and that the JNK pathway activates the leucine zipper site through the c-Jun/c-Fos complex. Additionally, our data suggest that knockdown of AGR2 reduces IGF-1-induced cell proliferation, migration and cell cycle progression. Therefore, we report that AGR2 is a key modulator involved in IGF-1-induced breast cancer development. We propose that the identification of the mechanism linking the IGF-1/insulin signal and AGR2 promoter activation is important, because it provides insights into the development of anti-breast cancer drugs.

Sal-like protein 2 (Sall2), a homeotic transcription factor, is a putative tumor suppressor. We have previously shown that Sall2 activates the transcription of tumor suppressor gene p21 and suppresses tumorigenesis through cell cycle inhibition and induction of apoptosis. To investigate additional Sall2-regulated downstream genes, we analyzed the differences in mRNA expression profiles with and without exogenously expressed Sall2. We identified 1616 Sall2-responsive genes through gene expression arrays. Promoter-reporter assays of p16INK4A and several other tumor-related genes indicated that the Sall2 regulation of these promoters was not significantly different between the two major forms of Sall2 with alternative exon 1 or exon 1A. Additional analysis showed that Sall2-induced p16 promoter activation was Sall2 dose-dependent. Deletion and site-directed mutagenesis of the p16 promoter identified a consensus Sall2 binding site (GGGTGGG) proximal to the p16 transcription start site and was critical for p16 promoter activation. Finally, to confirm the significance of Sall2-activated p16 expression in cell cycle regulation, we co-transfected the SKOV3 cells with a Sall2 expression construct and a p16 minigene and also co-transfected the ES-2 cells with a Sall2 expression construct and the siRNA against p16 for flow cytometry analysis. Our results showed that Sall2 enhanced the p16 minigene blocking of cell cycle progression and p16 knockdown with siRNA abolished most of the Sall2 inhibition of cell cycle progression. These findings indicate that Sall2 targets multiple cell cycle regulators, including p16, through their promoters, adding knowledge to the understanding of Sall2 and p16 gene regulation, and how Sall2 deregulation may promote cancer formation.

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