Cycling and early pregnant endometrium as a site of regulated expression of the vitamin D system

Abstract

In addition to its calciotropic function, the secosteroid 1,25-dihydroxyvitamin D3 (1,25(OH)2D3), has potent anti-proliferative/immunomodulatory effects on various tissues. Consistently, the enzyme that catalyzes the synthesis of 1,25(OH)2D3, 1α-hydroxylase (1α-OHase) and the vitamin D receptor have a widespread tissue distribution. Among site-specific functions, the hormone has been suggested to be involved in uterine physiology. However, molecular analysis of the vitamin D system in normal endometrium throughout the menstrual cycle as well as its regulation in the context of endometrial physiological and pathological events have received very limited attention. Thus, we have studied expression, localization and regulation of 1α-OHase in human cycling and early pregnant endometrium. The capacity for 1α-hydroxylation and the presence of vitamin D receptor in endometrial cells have also been evaluated. The functional significance of these findings has been tested by evaluating gene expression of the catabolic enzyme, vitamin D 24-hydroxylase, and of the adhesion protein, osteopontin. Finally, to verify any potential dysfunction of the vitamin D system in endometriosis, a reproductive disease characterized by immune-mediated anomalies, we have analyzed expression of 1α-OHase in both eutopic and ectopic endometrium of affected patients. Results obtained showed that the active form of the 1α-OHase gene was expressed in human endometrial stromal cells independent of the cycle phase but with a significant increase in early pregnant decidua. A similar profile was observed for the protein, which was abundantly expressed in the cytoplasm of both endometrial stroma and epithelial glands. Both cycling and early pregnant endometrial cells also expressed the vitamin D receptor. In the same cells, 1α-OHase mRNA levels were significantly stimulated by the pro-inflammatory cytokine interleukin (IL)-1β (50 and 500 pg/ml) while addition of the active form of the hormone could modulate both CYP24 and osteopontin gene expression. The 1α-OHase gene was also expressed in ectopic endometrium and its levels were increased in proliferative phase cultures derived from patients with endometriosis. Human cycling endometrium may be included among the extrarenal sites able to synthesize vitamin D. The IL-1β-mediated induction of 1α-OHase gene and the hormonal modulation of osteopontin support a role for the hormone in the immunological mechanisms underlying uterine function. Abnormalities of this system are present in endometriosis.