Activation of the γ-Aminobutyric Acid Receptor Type B Suppresses the Proliferation of Lung Adenocarcinoma Cells.

Abstract

Background/aim: The roles of γ-aminobutyric acid (GABA) and its receptors in lung cancer development and progression remain controversial. This study aimed to investigate the effects of activating GABA receptor type B (GABA-B receptor) using baclofen, a GABA-B receptor agonist, on the proliferation of lung adenocarcinoma cells and its underlying mechanisms.

Materials and methods: Differential expression of GABA-B receptors was analyzed using the online Gene Expression Profiling Interactive Analysis tool. Lung adenocarcinoma cell lines, A549 and PC-9, were cultured in RPMI-1640 medium and used for in vitro experiments. Effects of baclofen on cancer cell proliferation were determined using a 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay. Mechanisms of how baclofen inhibits cell proliferation were determined using flow cytometry and western blots.

Results: The expression of GABA-B1 receptor was down-regulated in lung adenocarcinoma compared with normal lung tissues, while GABA-B2 receptor expression was not different between cancer and normal samples. Baclofen significantly inhibited the proliferation of lung adenocarcinoma cells in a dose-dependent manner in both cell lines. Flow cytometry suggested that cancer cells were arrested at the G₁ phase of the cell cycle after being treated with baclofen. Thus, the expression of proteins of the G₁ cell cycle machinery, namely cyclin D1, cyclin E, cyclin-dependent kinase (CDK) 4, and CDK6, were significantly suppressed. The phosphorylation of CDK2 was also suppressed after baclofen treatment. Moreover, baclofen treatment increased phosphorylated glycogen synthase kinase 3 (GSK3) levels, suggesting inhibition of carcinogenic GSK3 signaling pathway in lung adenocarcinoma.

Conclusion: GABA-B1 receptors were down-regulated in lung adenocarcinoma cells. Non-specific activation of the GABA-B receptor by baclofen significantly inhibited lung adenocarcinoma cell proliferation by cell cycle arrest and suppressed the GSK3 signaling pathway.