Leveraging the Role of the Metastatic Associated Protein Anterior Gradient Homologue 2 in Unfolded Protein Degradation: A Novel Therapeutic Biomarker for Cancer

Abstract

Effective diagnostic, prognostic and therapeutic biomarkers can help in tracking disease progress, predict patients' survival, and considerably affect the drive for successful clinical management. The present review aims to determine how the metastatic-linked protein anterior gradient homologue 2 (AGR2) operates to affect cancer progression, and to identify associated potential diagnostic, prognostic and therapeutic biomarkers, particularly in central nervous system (CNS) tumors. Studies that show a high expression level of AGR2, and associate the protein expression with the resilience to chemotherapeutic treatments or with poor cancer survival, are reported. The primary protein structures of the seven variants of AGR2, including their functional domains, are summarized. Based on experiments in various biological models, this review shows an orchestra of multiple molecules that regulate AGR2 expression, including a feedback loop with p53. The AGR2-associated molecular functions and pathways including genomic integrity, proliferation, apoptosis, angiogenesis, adhesion, migration, stemness, and inflammation, are detailed. In addition, the mechanisms that can enable the rampant oncogenic effects of AGR2 are clarified. The different strategies used to therapeutically target AGR2-positive cancer cells are evaluated in light of the current evidence. Moreover, novel associated pathways and clinically relevant deregulated genes in AGR2 high CNS tumors are identified using a meta-analysis approach.

Keywords: adhesion; angiogenesis; anterior gradient homologue 2; apoptosis; genomic integrity; inflammation; metastasis; migration; proliferation; stemness; unfolded protein response.

Figure 1

Illustrations of the AGR2 gene and its corresponding protein structure, as well as the detected splice variants. A-H represent AGR2 splice variants. Functional domains for AGR2 protein are shown as: (I) 1-20aa CHPS, directs the AGR2 protein to the ER and contains block of hydrophobic amino acid residues; (II) 20-21aa Ala-Lys, is a leader sequence; (III) 21-45aa, is important for the extracellular secretion; (IV) 60-64aa, is required for dimerization; (V) CXXS motif, interacts with intermediates in redox reactions during folding and retrograde transport in the secretory pathway; (VI) A peptide binding lobe that is important for protein–protein interactions; and (VII) KETL, works as an ER retention signal for targeting proteins into the secretory pathway and to traffic AGR2 protein to different compartments in the cell.

Figure 2

The expression of AGR2 is regulated by multiple proteins. IRE1α, modulated ATF6α, HIF-1α, RAD9A, phosphorylated-AKT, FOXA1/2 have all been shown to contribute to the upregulation of AGR2. In contrast, the upregulation of SMAD4, miR-342-3p, miR-217, miR-1291, and TMED2 have been shown to contribute to the downregulation of AGR2. While p53, and FoxM1 appear to be involved in a feedback regulatory loop. The numbers in red indicate references.

Figure 3

The overexpression of intracellular AGR2 and the presence of extracellular AGR2 (eAGR2) impact several pathways. AGR2 overexpression has been shown to effect pathways related to genomic integrity, apoptosis, angiogenesis, adhesion, migration, and stemness. Several proteins have been shown to be upregulated or downregulated in the presence of AGR2. In addition, several proteins have been shown to directly interact with AGR2 including DAG1, GRp78, RuvB-like 2, Muc1/2, LYPD3, and PAUF (also known as ZG16B). In addition, eAGR2 particularly affects the expression of molecules related to adhesion and stemness and is able to selectively promote monocyte attraction.

Figure 4

The mechanism of action for AGR2 in the UPR machinery. AGR2 has several roles in the UPR. (a) Dimeric AGR2 directly interact with GRp78 or TMED2 in the ER-lumen. (b) The AGR2 overexpression affects the expression of phosphorylated eIF2α. (c) The expression levels of AGR2 affects the ERAD process through the regulation of EDEM1. In cancer cells, AGR2 is likely to play a master role in preventing the degradation of mutated or misfolded proteins, thus allowing errors to be unaccounted for.

Figure 5

(a) Using HG 133 Plus 2.0 microarrays and a probe set (209173_at) interrogating mostly the coding region between exon 4 to exon 8, a significant higher expression is revealed for meningiomas compared to normal brain tissue and GBM. Normal brain and GBM files were derived from GEO submission GSE33331 and meningiomas from GSE16581. Dots indicate expression values of individual samples. Files were analyzed using Transcriptome Analysis Software (Thermo Fisher Scientific). Stars mark p-values < 0.05. (b) The merged network is based on the top three networks that were most significantly related to the microarray expression profiles of 245 DEGs that were retrieved from the meta-analysis of microarray expression data on meningiomas and were observed in at least three different studies. The Ingenuity Pathway Analysis (IPA) (Qiagen, Hilden, Germany) and its curated molecular knowledge base were employed to generate the merged network. Network molecules are related skeletal and muscular system development and function, cancer, endocrine system disorders, embryonic development, nervous system development and function, cellular function and maintenance, and tissue development (Table 2). Upregulated network molecules include, ABCC9, ADAMTS5, AGR2, ALDOA, ARHGAP20, ATP1A2, CDKN2A, CFH, COL18A1, CPA4, CTSV, DMD, DSC2, DSG2, EPS8L2, ERO1A, GAP43, GDF15, HACD1, ID4, KCNJ8, KIF23, LAMA1, LUM, MYO1B, NEFL, NID1, OLFM4, PPARGC1A, PPFIBP1, PSAT1, RAPGEF5, RELN, RNASE1, TMEM108, VTN, WNT6, and ZIC1. Downregulated network molecules include, ADAM22, B4GALT1, BCAR3, CD200, CDH5, CLIC5, CYP1B1, DOK5, EBF1, FBXO32, FRY, GALNT12, GDAP1, GSTM5, IGFBP5, KALRN, KCNMA1, LRRN1, MEG3, NT5E, NTN1, PRRX1, RASGRF2, RBP4, RUNX1T1, SLC39A14, SLC44A1, SORL1, TRIM2, and ZNF521. Molecules retrieved from the IPA molecular base include, actin, alpha catenin, collagen type I, collagen type IV, collagen(s), Ctbp, DNA-methyltransferase, ERK1/2, F Actin, fibrin, growth hormone, Hdac, hedgehog, hemoglobin, histone deacetylase, Histone h4, Hsp27, integrin, integrin alpha V beta 3, Jnk, laminin (complex), laminin1, LDL, Mek, N-cor, P glycoprotein, Pdgf (complex), PEPCK, PI3K (complex), Pkg, Rap1, Sod, STAT5a/b, troponin t, trypsin, vitaminD3-VDR-RXR, and Wnt.

Figure 6

The most clinically relevant differentially expressed cancer-related genes in meningioma tumors that have high AGR2 expression.