Downregulation of GJA1-Derived circ_753 and circ_755 Differentially Modulates miR-182-5p Expression and Breast Cancer-Associated Phenotypes.

CircRNAs are key regulators of gene expression. The functional roles of exonic versus exon-intron circRNAs in early breast cancer remain unclear. Connexin-43 (GJA1) generates exonic circRNA (ecircRNA) hsa_circ_0077755 (circ_755) and exon-intron circRNA (EIcircRNA), hsa_circ_0077753 (circ_753), both linked to increased breast cancer risk. We examined their structural, molecular, and phenotypic roles in transient and stable knockdown S1 breast epithelial cells, using RT-qPCR, dual-luciferase, lumen formation and invasion assays, and extracellular vesicle (EV) analysis. Knockdown of cytoplasm-enriched circ_755 reduced Connexin-43 transcript and protein, whereas knockdown of nuclear-enriched circ_753 preserved GJA1. Both induced lumen disruption, β-catenin disorganization, increased invasion, and regulated onco-miR-182. More pronounced miR-182 upregulation and broader repression of tumor-suppressive targets (BCL2, FOXO1, RECK) were observed in circ_753 knockdown cells, alongside concurrent depletion of hsa_circ_0008932 (circ_8932), which is predicted to interact with miR-182. Similar reciprocal changes in circRNA/miR-182 were observed in released EVs. These data highlight circRNA-miR-182 networks with distinct effects on tumor initiation and potential value as breast cancer biomarkers.