Uterine endometrium displays telomerase activity in a menstrual cycle-dependent manner, despite its somatic origin. This study was performed to elucidate the regulation of telomerase in human endometrium. Telomerase activity and human telomerase reverse transcriptase mRNA expression in proliferative endometrium were significantly stronger than those in secretory endometrium. Their expression was only detected in epithelial cells, although stromal cells also showed proliferation. The growth of epithelial cells decreased day by day in accordance with the decline of telomerase activity. Telomerase activity was significantly stronger in co-cultures of epithelial and stromal cells than in cultures of epithelial cells alone. Moreover, the telomerase activity of co-cultured cells was increased by estradiol or basic fibroblast growth factor, whereas that of epithelial cells cultured alone showed no change. Thus, human endometrium shows reversible telomerase activation during the menstrual cycle, unlike cancer tissues. Also, the telomerase activity of uterine endometrial epithelial cells might be modulated by paracrine effectors released from stromal cells, and not only by the direct action of sex steroids such as estradiol and progesterone.

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