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Objectives Progesterone treatment can effectively increase levels of circulating endothelial progenitor cells ( EPCs) and improve neurological functional outcome in a traumatic brain injury ( TBI) rat model. However, the mechanisms of progesterone's effects on EPC viability remain elusive. The CXCL12/ CXCR4 ( CXC chemokine ligand 12/ CXC chemokine receptor 4) signalling pathway regulates cell proliferation; we hypothesize that it mediates progesterone-induced EPC viability. Materials and methods EPCs were isolated from bone marrow-derived mononuclear cells ( BM- MNCs) and treated with progesterone (5, 10 and 100 n m). MTS assay was used to investigate EPC viability. Protein expression was examined by Western blotting, ELISA assay and flow cytometry. Cell membrane and cytoplasm proteins were extracted with membrane and cytoplasm protein extraction kits. CXCR4 antagonist ( AMD3100) and phosphatidylinositol 3-kinases ( PI3K) antagonist ( LY294002) were used to characterize underlying mechanisms. Results Progesterone-induced EPC viability was time- and dose-dependent. Administration of progesterone facilitated EPC viability and increased expression of CXCL12 and phosphorylated Akt (also known as protein kinase B, pAkt) activity ( P < 0.05). Progesterone did not regulate CXCR4 protein expression in cultured EPC membranes or cytoplasm. However, progesterone-induced EPC viability was significantly attenuated by AMD3100 or LY294002. Inhibition of the signalling pathway with AMD3100 and LY294002 subsequently reduced progesterone-induced CXCL12/ CXCR4/ PI3K/ pAkt signalling activity. Conclusions The CXCL12/ CXCR4/ PI3K/ pAkt signalling pathway increased progesterone-induced EPC viability. [ABSTRACT FROM AUTHOR]