ABSTRACT
Circular RNAs are a class of noncoding RNAs with covalently linked 5' and 3' ends that arise from backsplicing events. The absence of a 5' cap and a 3' poly(A) tail makes circular RNAs relatively more stable than their linear counterparts. They are evolutionary conserved and tissue-specific, and some show disease-specific expression patterns. Although their biological functions remain largely unknown, circular RNAs have been shown to play regulatory roles by acting as microRNA sponges, regulators of RNA-binding proteins, alternative splicing, and parental gene expression, and they could even encode proteins. Over the past few decades, circular RNAs have attracted wide attention in oncology owing to their implications in various tumors. Many circular RNAs have been characterized as key players in gastrointestinal cancers and influence cancer growth, progression, metastasis, and therapeutic resistance. Accumulating evidence reveals that their unique characteristics, coupled with their critical roles in tumorigenesis, make circular RNAs promising non-invasive clinical biomarkers for gastrointestinal cancers. In the present review, we summarized the biological roles of the emerging circular RNAs and their potential as biomarkers and therapeutic targets, which may help better understand their clinical significance in the management of gastrointestinal cancers.
Subject(s)
Biomarkers, Tumor , Gastrointestinal Neoplasms , RNA, Circular , RNA , Humans , Gastrointestinal Neoplasms/genetics , Gastrointestinal Neoplasms/therapy , Gastrointestinal Neoplasms/pathology , RNA, Circular/genetics , RNA, Circular/physiology , RNA, Circular/metabolism , Biomarkers, Tumor/genetics , Biomarkers, Tumor/metabolism , RNA/genetics , Molecular Targeted Therapy , MicroRNAs/genetics , MicroRNAs/metabolism , Gene Expression Regulation, Neoplastic , RNA-Binding Proteins/genetics , RNA-Binding Proteins/metabolism , Alternative Splicing/genetics , Disease ProgressionABSTRACT
BACKGROUND: Spalt-like protein 4 (SALL4) is a stemness-related transcription factor whose abnormal re-expression contributes to cancer initiation and progression. However, the role of SALL4 in cancer angiogenesis remains unknown. METHODS: Analyses of clinical specimens via TCGA datasets were performed to determine the expression level and clinical significance of SALL4 in STAD (Stomach Adenocarcinoma). SALL4 knockdown, knockout, and overexpression were achieved by siRNA, CRISPR/Cas9, and plasmid transfection. The effects of conditioned medium (CM) from SALL4 knockdown or overexpression of gastric cancer cells on endothelial cell proliferation, migration, and tube formation were investigated by CCK-8 assay, transwell migration assay, and tube formation assay. The regulation of VEGF gene expression by SALL4 was studied by qRT-PCR, western blot, chromatin immunoprecipitation (ChIP) assay, and electrophoretic mobility shift assay (EMSA). Engineered exosomes from 293T cells loaded with si-SALL4-B and thalidomide were produced to test their therapeutic effect on gastric cancer progression. RESULTS: SALL4 expression was increased in STAD and positively correlated with tumor progression and poor prognosis. SALL4-B knockdown or knockout decreased while over-expression increased the promotion of human umbilical vein endothelial cells (HUVEC) cell proliferation, migration, and tube formation by gastric cancer cell-derived CM. Further investigation revealed a widespread association of SALL4 with angiogenic gene transcription through the TCGA datasets. Additionally, SALL4-B knockdown reduced, while over-expression enhanced the expression levels of VEGF-A, B, and C genes. The results of ChIP and EMSA assays indicated that SALL4 could directly bind to the promoters of VEGF-A, B, and C genes and activate their transcription, which may be associated with increased histone H3-K79 and H3-K4 modifications in their promoter regions. Furthermore, si-SALL4-B and thalidomide-loaded exosomes could be efficiently uptaken by gastric cancer cells and significantly reduced SALL4-B and Vascular Endothelial Growth Factor (VEGF) expression levels in gastric cancer cells, thus inhibiting the pro-angiogenic role of their derived CM. CONCLUSION: These findings suggest that SALL4 plays an important role in angiogenesis by transcriptionally regulating VEGF expression. Co-delivery of the functional siRNA and anticancer drug via exosomes represents a useful approach to inhibiting cancer angiogenesis by targeting SALL4/VEGF pathway.
ABSTRACT
SALL4 (split-like protein 4) is a member of the mammalian homologs of the Drosophila homoeotic gene spalt (sal) and acts as a zinc finger transcription factor to govern the self-renewal and pluripotency of embryonic stem cells. SALL4 expression gradually decreases during development and is even absent in most adult tissues. However, increasing evidence suggests that SALL4 expression is restored in human cancers and its aberrant expression is associated with the progression of many hematopoietic malignancies and solid tumors. The potent roles of SALL4 in regulating cancer cell proliferation, apoptosis, metastasis, and drug resistance have been reported. SALL4 plays a dual role in epigenetic modulation by acting as either an activator or a repressor of its target genes. Furthermore, SALL4 interacts with other partners to control the expression of many downstream genes and the activation of various key signaling transduction pathways. SALL4 is considered as a promising diagnostic and prognostic biomarker and therapeutic target for cancer. In this review, we highlighted the major advances in the roles and mechanisms of SALL4 in cancer and the therapeutic strategies for targeting SALL4 to treat cancer.
