Studies on gender-specific disruption of bone tissue homeostasis by dioxins

Abstract

Dioxins are widespread environmental pollutants, known to cause immunosuppression, developmental and reproductive defects, as well as cancer. The toxic effects of dioxins are mediated by the aryl hydrocarbon receptor (AhR, also referred to as the dioxin receptor). Dioxins display endocrine disrupting properties and especially disturbances of the estrogen signaling system has been reported. Effects of dioxins on the estrogen system have been observed at several levels, e.g., increased metabolism of estrogen and interactions of the AhR with signaling of the estrogen receptors. As a result of the endocrine disrupting properties, dioxins might cause different responses in females and males. Bone is a dynamic tissue highly regulated by numerous factors, where estrogen is one of the key players. Bone loss is a well known effect of estrogen deficiency and can lead to osteoporosis, e.g., in post-menopausal women. A few studies have shown that dioxins interfere with bone tissue, however the mechanisms remain unknown. Moreover, no studies have been performed regarding potentially gender-related effects of dioxins in adult bone tissue.

Humans are continuously exposed to low levels of dioxins from early embryonic development throughout life. Therefore, we studied a transgenic mouse with a constitutively active AhR (CA-AhR). The bone phenotype of the female CA-AhR mice displayed loss of bone tissue, which was primarily due to an increased bone resorption. The bones in females also became softer, which might indicate an altered mineralization. The bones of CA-AhR males were on the other hand largely unaffected. However, male rats exposed to a single high dose of TCDD displayed alterations in the trabecular bone tissue already after five days exposure, indicating a responsiveness of bone tissue of either gender towards dioxins. Gender-specific responses were also observed in differentiation cultures of osteoclasts derived from bone marrow cells of transgenic mice. Consistent with the in vivo results, bone marrow cultures from CA-AhR females displayed an elevated osteoclast formation. In contrast, osteoclast differentiation was inhibited in cultures from the transgenic males. At variance to the CA-AhR females, osteoclast differentiation in cultures from female wild type mice exposed to dioxin, did not display an increased osteoclast differentiation, which might be due to differences in the mode of AhR activation. The CA-AhR represents a chronic exposure situation, mimicking a long term, low dose exposure, which the cells might have adapted to, while the TCDD exposure is more acute and possible inducing additional effects. Osteoclast differentiation in the cultures from the male wild type mice exposed to TCDD responded similarly to the osteoclasts derived from CA-AhR, i.e. inhibition of osteoclast formation.

The results in this thesis show that bone is a sensitive target for dioxin exposure, leading to rapid alterations in trabecular bone tissue and gene expression in bone cells. Moreover, females seem to be more severely affected by a continuously active AhR than males. Specifically, the resorption of bone mediated by osteoclasts was altered in a gender specific manner by chronic activation of the AhR. Thus, continuous exposure to low levels of AhR ligands, such as dioxins and related environmental toxins, might be one part of the explanation for the increased incidence of osteoporosis in aging women.