Objective To investigate the effects of lipid-associated membrane proteins ( LAMPs) derived from Mycoplasma pneumoniae ( M.pneumoniae) strains on the expression of heme oxygenase 1 ( HO-1) in a human monocyte cell line (THP-1).Methods THP-1 cells were in vitro cultured with different concentrations of LAMPs for different times.The cytotoxicity of LAMPs to THP-1 cells was analyzed by using lactate dehydrogenase ( LDH) releasing test.The expression of HO-1 at protein and mRNA levels were de-tected by Western blot and real-time RT-PCR, respectively.The enzymatic activity of HO-1 protein was ex-amined by colorimetric assay.THP-1 cells stimulated with PBS and LPS were set up as the negative and pos-itive controls, respectively.Results A significantly enhanced LDH releasing rate was observed in THP-1 cells treated with 10 μg/ml of LAMPs.The expression of HO-1 at protein and mRNA levels in THP-1 cells were induced by LAMPs in a dose-dependent and time-dependent manner.The highest level of HO-1 protein was detected in THP-1 cells treated with 5.0 μg/ml of LAMPs.The transcriptional levels of HO-1 induced by LAMPs were significantly elevated at 3 h, peaked at 9 h and were decreased at 12 h.The expression of HO-1 protein in THP-1 cells was enhanced after 8 h of treatment with LAMPs and a significant decrease was observed at 20 h after reaching peaks at 12 h and 16 h.The activity of HO-1 protein was significantly en-hanced along with the increased expression of HO-1 protein.Conclusion The LAMPs derived from M.pneumoniae strains induced the expression of HO-1 at mRNA and protein levels.Moreover, the enzyme activity of HO-1 protein was enhanced in LAMPs treated THP-1 cells.