G?q/11 and G?? proteins and membrane signaling of calcitriol and estradiol

Abstract

17β-estradiol and 1,25-dihydroxyvitamin D3(calcitriol) rapidly increase (< 5 sec) the concentration of intracellular calcium by mobilizing Ca2+ from the endoplasmic reticulum and forming inositol 1,4,5-trisphosphate (InsP3) and diacylglycerol. Calcitriol increases InsP3 formation via activation of phospholipase C (PLC)-β1 linked to a pertussis toxin (PTX)-insensitive G-protein, and estradiol via activation of PLC-β2 linked to a PTX-sensitive G-protein. Since PLC are effectors of different subunits of various G-proteins, we looked for and identified several G-subunits (Gαq/11, Gαs, Gαi, Gβ and Gγ) in female rat osteoblasts using Western immunoblotting. The action of calcitriol on InsP3 formation and Ca2+ mobilization in Fura-2-loaded confluent osteoblasts involved Gαq/11. The membrane effects of estradiol involved Gβγ; subunits, and principally Gβ subunits, but not α-subunits. These results may provide additional evidence for membrane receptors of steroid hormones. Since PLC-β1 is the target effector of Gαq/11, whereas PLC-β2 is only activated by βγ subunits, this specificity may help to generate membrane receptor-specific responses in vivo. J. Cell. Biochem. 75:138–146, 1999. © 1999 Wiley-Liss, Inc.