Interactions between estrogen and mechanical strain effects on U2OS human osteosarcoma cells are not influenced by estrogen receptor type.

Estrogens (E) and mechanical strain (MS) exert direct effects on osteoblast activity, with good evidence of interactions between their respective effects. Osteoblasts express both forms of estrogen receptors (ER) ERalpha and ERbeta, and previous studies have suggested a specific role for each receptor. Therefore, our working hypothesis was that the interactions between E and MS on osteoblast activity vary depending on which ER is preferentially activated. Using human osteosarcoma cells U2OS stably transfected either with ERalpha or ERbeta, we evaluated the effects of cyclical cell loading on a F-3000 Flexercell Strain Unit (1.5% elongation, 10 min/day) in presence of estradiol (E2) 10(-8) M or not. The original U2OS cell line, which does not express ER, was characterized by low alkaline phosphatase (AP) activity. In both U2OS-ERalpha and U2OS-ERbeta cell lines, MS induced similar increases in AP activity and gene expression as measured by real-time quantitative RT-PCR, and a decrease in type I collagen gene expression. MS and E2 had a synergistic effect on AP activity as compared to each stimulus alone. No change in proliferation rate was observed. Neither proliferation nor differentiation of the original U2OS cell line was altered by strain or E2. In summary, our data showing differences in response to MS between the U2OS with no ER expression and the U2OS-ERalpha or -ERbeta cell lines provide additional evidence that ER plays a critical role in mechanotransduction. However, we were not able to demonstrate that interactions between E and MS were dependent on ER type in U2OS osteosarcoma cells.