Objective: Breast cancer (BC) is one of the most common malignant tumors among women. Gamma-aminobutyric acid (GABA) is abnormally expressed in various cancers, but its effect on BC remains unclear. This study aims to explore the expression changes of glutamic acid decarboxylase 1 (GAD1) in BC and its mechanism of promoting tumor occurrence and development by regulating GABA synthesis. Material and Methods: GAD1 expression and GABA levels in BC cells (MDA-MB-231) and normal breast epithelial cells (MCF-10A) were measured. GAD1 overexpression and knockdown cell lines were constructed to evaluate the changes in GABA levels and their effects on cell proliferation and invasion ability. Macrophage M2 polarization and protumor factor levels were analyzed by coculturing tumor cells with macrophages. Cell activity was detected by coculturing with CD8(+) T cells, and tumor immune responses were examined by immunofluorescence and cytokine detection. In vitro tube formation was used to simulate angiogenesis, and angiogenesis ability was detected. Western blot was used to analyze the expression of vascular endothelial cell markers, and calcium-tracer dyes were used to detect calcium ion (Ca2+) influx in endothelial cells. Results: GAD1 expression and GABA level in MDA-MB-231 cells were significantly higher than those in normal cells (P <0.01). GAD1 overexpression enhanced cell proliferation, invasion, and migration abilities and simultaneously promoted macrophage polarization to M2, releasing protumor factors interleukin-10 and transforming growth factor-beta and inhibiting the antitumor effect of M1-type macrophages (P <0.05). GAD1 inhibition had the opposite effect (P <0.01). GAD1 overexpression also suppressed the activity of CD8(+) T cells; decreased the expression of programmed death-1 in CD8(+)T cells and the levels of interferon-gamma, perforin, and granzyme B; weakened the immune response; and promoted tumor angiogenesis (P <0.05). The number of in vitro vascular lumen formation increased, the expression of vascular markers rose, and Ca2+ influx was enhanced (P <0.01). Reverse operation demonstrated that GABA promoted the initiation and advancement of BC by regulating the immune microenvironment and angiogenesis (P <0.05). Conclusion: GAD1 is highly expressed in BC. Increasing GABA synthesis promotes tumor cell proliferation and metastasis, regulates the immune microenvironment toward immunosuppression, and enhances tumor angiogenesis. This work reveals the crucial role of the GAD1-GABA pathway in BC and provides a potential therapeutic target for this disease.
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