Objective:To investigate the effects of SINC, a novel secreted protein of Chlamydia psittaci, on the apoptosis of host cells and the regulatory role of mitogen-activated protein kinase/extracellular signal-regulated kinase (MAPK/ERK) signaling pathway in it. Methods:HeLa cells were treated with recombinant SINC. The expression of Bax and Bcl-2 at protein level and the phosphorylation of ERK1/2 were analyzed by Western blot. Hoechst 33258 staining was used to detect the apoptosis of HeLa cells after SINC stimulation. Moreover, HeLa cells were pretreated with MEK1/2 inhibitor U0126 (50 μmol/L), and then stimulated with different concentrations of SINC for different time. Flow cytometry was used to detect the changes in cell apoptosis rates and Western blot was performed to detect the expression of Bax and Bcl-2 at protein level.Results:Treating HeLa cells with 10 μg/ml of SINC for 18 h resulted in down-regulated Bax and up-regulated Bcl-2 at protein level. Besides, the ratio of Bax/Bcl-2 was the lowest and a significant increase in the ratio of phosphorylated ERK1/2 (p-ERK1/2) to ERK1/2 was observed. Hoechst 33258 staining showed that the number of apoptotic bodies decreased significantly after stimulating HeLa cells with 5, 10 and 15 μg/ml of SINC. In the presence of MEK1/2 inhibitor U0126, the expression of Bcl-2 at protein level was down-regulated, while the expression of cleaved PARP was significantly up-regulated. Flow cytometry showed a significantly enhanced apoptosis of HeLa cells.Conclusions:SINC can inhibit the apoptosis of HeLa cells through activating the MAPK/ERK signaling pathway.