2,3,7,8-Tetrachlorodibenzo-p-dioxin stimulates proliferation of HAPI microglia by affecting the Akt/GSK-3β/cyclin D1 signaling pathway

Abstract

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is an environmental toxin that induces apoptosis of neurons and a pro-inflammatory response in microglial cells. First, we found that TCDD induced proliferation of HAPI microglial cells in a dose- and time-dependent manner. Flow cytometry analysis showed that this proliferation by TCDD was due to mainly enhancing the G1 to S phase transition. Next, it was found that TCDD treatment led to up-regulation of cyclin D1, which induces cell cycle progression from G1 to S phase, in a time-dependent manner. As for molecular mechanism, we revealed that TCDD was capable of inducing Akt phosphorylation and activation, resulting in phosphorylation and inactivation of glycogen synthase kinase-3β (GSK-3β). Inactivated GSK-3β attenuated proteasomal degradation of cyclin D1 by reducing Thr(286)-phosphorylated cyclin D1 levels. Moreover, inactivated GSK-3β increased cyclin D1 gene transcription by increasing its transcription factor β-catenin in the nucleus. Further, blockage of phosphoinositide 3-kinase/Akt kinase with their specific inhibitors, LY294002 and Akt 1/2 kinase inhibitor, significantly reduced TCDD-enhanced proliferation of HAPI microglial cells. In conclusion, TCDD stimulates proliferation of HAPI microglial cells by affecting the Akt/GSK-3β/cyclin D1 signaling pathway.

Keywords: 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD); 2,3,7,8-tetrachlorodibenzo-p-dioxin; 5-ethynyl-2′-deoxyuridine; AhR; Akt; CNS; Cyclin D1; ELISA; ERK; EdU; GAPDH; GSK-3β; Glycogen synthase kinase-3β (GSK-3β); IL-1β; Microglia; PBS; PI; PI3K; Proliferation; RNS; ROS; RT-PCR; TCDD; TNF-α; aryl hydrocarbon receptor; central nervous system; enzyme-linked immunosorbent assay; extracellular signal-regulated kinase; glyceraldehyde-3-phosphate dehydrogenase; glycogen synthase kinase-3β; interleukin-1 beta; phosphate-buffered saline; phosphoinositide 3-kinase; propidium iodide; reactive nitrogen species; reactive oxide species; reverse transcription-polymerase chain reaction; tumor necrosis factor-alpha..