Effects of stable transfection of human fetal osteoblast cells with estrogen receptor-alpha on regulation of gene expression by tibolone

Tibolone is a synthetic steroid which undergoes tissue selective metabolism into several metabolites having estrogenic, progestogenic or androgenic activities. The effects of 3α-hydroxy tibolone (Org 4094), 3β-hydroxy tibolone (Org 30126) and their sulfated metabolites were investigated on human fetal osteoblasts (hFOB). Tibolone had no effect on selected osteoblast marker proteins in estrogen-receptor negative hFOB cells. In contrast, 3α-hydroxy and 3β-hydroxy tibolone resulted in dose-dependent increases in alkaline phosphatase activity in estrogen receptor (ER) α-positive hFOB cells. The maximum increase for both metabolites was comparable to the effects of an optimal dose of 17β-estradiol, and occurred at 10 μM. At 20 μM, both metabolites increased mRNA levels for alkaline phosphatase and type 1 collagen and protein levels for osteocalcin. Sulfated metabolites of tibolone also increased alkaline phosphatase activity. The estrogen receptor antagonist ICI 182, 780 inhibited stimulation of alkaline phosphatase activity by sulfated and non-sulfated tibolone metabolites, but was more potent on the former. Taken together, these results suggest that stable transfection of ERα into hFOB cells confers regulation by 3α-hydroxy and 3β-hydroxy tibolone metabolites of osteoblast metabolism.