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Background: We have previously demonstrated that estrogen could significantly enhance expression of apolipoprotein M (apoM), whereas the molecular basis of its mechanism is not fully elucidated yet. To further investigate the mechanism behind the estrogen induced up-regulation of apoM expression. Results: Our results demonstrated either free 17β-estradiol (E2) or membrane-impermeable bovine serum albumin-conjugated E2 (E2-BSA) could modulate human apoM gene expression via the estrogen receptor alpha (ER-α) pathway in the HepG2 cells. Moreover, experiments with the luciferase activity analysis of truncated apoM promoters could demonstrate that a regulatory region (from-1580 to -1575 bp (-GGTCA-)) upstream of the transcriptional start site of apoM gene was essential for the basal transcriptional activity that regulated by the ER-α. With the applications of an electrophoresis mobility shift assay and a chromatin immunoprecipitation assay, we could successfully identify a specific ER-α binding element in the apoM promoter region. Conculsion: In summary, the present study indicates that 17β-estradiol induced up-regulation of apoM in HepG2 cells is through an ER-α-dependent pathway involving ER-α binding element in the promoter of the apoM gene. [ABSTRACT FROM AUTHOR]