Increased GABAergic activity inhibits α-fetoprotein mRNA expression and the proliferative activity of the HepG₂ human hepatocellular carcinoma cell line

Background/Aims: Gamma aminobutyric acid (GABA) is a potent inhibitory neurotransmitter with growth regulatory properties. Recent data indicate that increased GABAergic activity inhibits hepatocyte proliferation in regenerating livers. In the present study, we aimed to investigate whether GABA inhibits the growth of malignant hepatocytes. Methods: Increasing concentrations of muscimol (0.05-50 μM), a specific GABA(A) receptor agonist, were added to HepG2 human hepatocellular carcinoma cells and alpha- fetoprotein (AFP) and albumin mRNA expression were determined for varying periods of time (maximum 24 h) thereafter. Cell proliferation was also documented after 48 h of exposure to muscimol. Results: Muscimol significantly (p<0.0001) decreased AFP mRNA expression (maximum decrease: 65% below baseline values) without affecting albumin mRNA expression. However, the effect on AFP mRNA was transient (maximum duration: 3-6 h) and not associated with changes in cell proliferation. Because preliminary data indicate that GABA(A) receptor activity is markedly downregulated in malignant hepatocytes, transfection studies were performed wherein HepG2 cells were cotransfected with GABA(A) receptor β₂ and γ₂ subunit genes in a pCDM8 expression vector or vector alone followed by re-exposure to either muscimol (5 μM) or saline. In this series of experiments, in addition to AFP mRNA inhibition being as extensive and more prolonged (maximum duration: 6-12 h) in muscimol-treated, GABA(A) receptor-transfected cells, proliferative activity was also significantly inhibited when compared to saline-treated GABA(A) receptor-transfected controls (p<0.01) and muscimol-treated cells transfected with vector alone (p<0.005). Conclusion: The results of this study indicate that increased GABAergic activity inhibits AFP mRNA expression and cell proliferation in this malignant hepatocyte cell line.