The Biological Role of the Female Sex Hormone Estrogen in the Periodontium

Abstract: Introduction: Several studies have addressed the association between changes in levels of the female sex hormone, estrogen, and changes in the parameters of periodontitis, but the mechanism behind estrogenic effects in the periodontium is poorly understood. There are two subtypes of estrogen receptors (ER), ERα and ERβ. The objectives of the present studies were to map periodontal ligament (PDL) cell ERsubtype expression patterns and to investigate their functional importance. This information is valuable for understanding the biological role of ERs in the periodontium. Methods: Human PDL cells were obtained from periodontal tissue explants from teeth that were extracted for orthodontic reasons. The progesterone receptor and ER-subtype expression patterns were studied using immunocytochemistry. The subcellular distribution of ERβ was determined by immunogold electron microscopy and confocal imaging using the mitochondria-selective probe MitoTracker® and ERβ immunostaining. Expression of the mitochondrial protein, cytochrome c oxidase subunit I, was investigated using Western blotting. DNA and collagen synthesis was determined by measuring the incorporation of [3H]thymidine and [3H]proline, respectively. Interleukin 6 (IL-6), monocyte chemoattractant protein-1 (MCP-1) and C-reactive protein (CRP) were analyzed using ELISA. Alkaline phosphatase activity was determined colorimetrically. Results and Discussion: Human PDL cells possessed immunoreactivity for ERβ but not ERα, suggesting that estrogenic effects in PDL cells are mediated via ERβ. PDL cells expressed no immunoreactivity for progesterone receptors, which implies that progesterone does not have a direct effect on PDL cell function. Confocal imaging and immunogold electron microscopy revealed that ERβ immunoreactivity was distributed not only in the nucleus but also in the mitochondria. Incubation with estrogen down-regulated expression of cytochrome c oxidase subunit I, indicating functional significance for mitochondrial ER. Physiological concentrations of estrogen had no effect on PDL cell collagen and DNA synthesis but enhanced DNA synthesis in human breast cancer MCF-7 cells, probably reflecting a cell-typespecific ER-subtype expression pattern. The bacterial endotoxin, LPS, had no effect on the physiological properties of PDL cells (demonstrated by alkaline phosphatase activity, and DNA and collagen synthesis). However, LPS enhanced inflammatory characteristics of PDL cells, such as enhanced IL-6 and MCP-1 protein production. The LPS-induced effect on PDL cells was not reversed by estrogen, suggesting that estrogen has no anti-inflammatory effect via this mechanism. The enhanced MCP-1 expression in response to LPS suggests that PDL cells contribute to the recruitment of leukocytes in periodontal inflammation.