The purpose of this study was to investigate the fluctuation of expression and activation of mitogen-activated protein kinase in normal human endometrium throughout the menstrual cycle.

Thirty-three normal endometrial tissues were obtained from fertile women who had undergone hysterectomies for reasons other than endometrial disease. Extracellular signal-regulated kinase, -1, and -2 expression were studied by immunohistochemistry. Moreover, extracellular signal-regulated kinase activity was analyzed by gel kinase assay.

Western blotting analysis with anti–pan-extracellular signal–regulated kinase antibody mainly demonstrated an immunoreactive band of 42 kd that corresponded to extracellular signal–regulated kinase 2 in the endometrium. The expression of extracellular signal–regulated kinase 2 tended to increase in the secretory phase. Immunohistochemical analysis for extracellular signal–regulated kinase 1 in endometrial sections revealed a weak staining of glands and almost no staining of stromal cells. Immunohistochemical analysis for extracellular signal–regulated kinase 2 in endometrial sections revealed a distinct staining of glands in both proliferative and secretory phases and a weak staining of stromal cells. Although the intensity of staining for extracellular signal–regulated kinase 2 in stromal cells did not change during the secretory phase, in the glands the extracellular signal–regulated kinase 2 was highly stained in the mid-to-late secretory phase. In gel kinase assay revealed that extracellular signal–regulated kinase activity was increased significantly in the mid-to-late secretory phase.

Expression and activation of extracellular signal–regulated kinase in the human endometrium was increased particularly during the secretory phase. We suggest that fluctuation of extracellular signal–regulated kinase in the human endometrium may be induced by ovarian steroid hormones.