G-protein signalling pathways and oestrogen: a role of balanced maintenance in osteoblasts

Oestrogen (E2) is an important regulator of bone cell function and alterations in oestrogen levels may cause abnormal bone metabolism in vivo. In this study we examined the long term effects of 17beta-oestradiol (17beta-E2) on G-proteins and the secondary signalling pathways of phospholipase C (PLC), cyclic adenosine monophosphate (cAMP), and 1,4,5-inositol triphosphate (IP3). Cells from neonatal mouse calvariae were cultured in phenol red-free RPMI 1640 medium supplemented with charcoal stripped foetal calf serum for 192 h with either oestrogen (10(-8) M), or oestrogen withdrawal after 48 h. Cultures were stimulated for the final 48 h with IL-6 (10(-10) M), or left unstimulated. Western blot analysis was undertaken on osteoblast membrane preparations obtained by 10 mM Tris-HCl, 0.1 mM EDTA pH 7.8 and centrifugation at 40,000 x g for 2 h. For cAMP study, cells were stimulated with IL-6 for either 15 min or 30 min. Intracellular cAMP was extracted from cells and measured by ELISA methodology. For the IP3 assay, cells were stimulated with IL-6 for 20 s and IP3 levels measured using radioimmunoassay. The blots revealed increased levels of Gialpha-, and Gqalpha-proteins with oestrogen withdrawal and IL-6 stimulation. This was in comparison to cells which were unstimulated, or stimulated with IL-6 with continuous 17beta-E2, or IL-6 alone. Gsalpha expression decreased with oestrogen withdrawal compared to the control. Limited amounts of Gialpha-, Gsalpha-, and Gqalpha-proteins were identified with continuous 17beta-E2. The levels of PLC isoforms PLCbeta1-2 were not affected by the differing oestrogen conditions. The cAMP production induced by IL-6 stimulation for 30 min and withdrawal of 17beta-E2 was lower and significantly different compared to the control study (P<0.05). Also IL-6 activation with continuous oestradiol increased cAMP levels and was significantly different from the control cells (P<0.01). However, 17beta-E2 had no effect on the formation of intracellular IP3, although IL-6 significantly lowered IP3 levels in all the groups compared to the control (P<0.01). These results suggest that oestrogen modulates the signal transduction pathways of G-protein molecules, and the secondary pathways of cAMP in mouse osteoblast-like cells.