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BACKGROUND P21-activated kinase 1 (Pak1) integrates various signaling pathways that are vital to cell survival and function. This study was performed to evaluate whether sex steroids may regulate the expression of Pak1 in endometrial cells as well as whether its expression is increased in the eutopic endometrium of women with endometriosis. METHODS Following in vitro estradiol (E2) and/or medroxyprogesterone acetate (MPA) treatment of Ishikawa cells and endometrial stromal cells (ESCs), Pak1 protein was analyzed utilizing western blot analysis and immunocytochemistry. Immunohistochemistry was performed to evaluate Pak1 immunoreactivity semiquantitatively in women with endometriosis and in controls. To assess the role of Pak1 on endometrial cell viability, crystal violet assay was performed following transfection of Ishikawa cells with Pak1 small interfering RNA (siRNA). RESULTS In vitro treatment with E2 plus MPA or MPA alone led to a significant decrease of Pak1 protein in Ishikawa cells and ESCs (both P P P P P CONCLUSIONS These findings suggest that Pak1 is down-regulated by progesterone during the secretory phase in normal endometrium and increased Pak1 activity during the secretory phase might lead to establishment of endometriosis. [ABSTRACT FROM AUTHOR]