We have shown that estrogens and calcitriol, the hormonally active form of vitamin D, increase the concentration of intracellular calcium ([Ca 2؉ ] i ) within 5 s by mobilizing calcium from the endoplasmic reticulum and the formation of inositol 1,4,5-trisphosphate and diacylglycerol. Because the activation of effectors as phospholipase C (PLC) coupled to G-proteins is the early event in the signal transduction pathway leading to the inositol 1,4,5-trisphosphate formation and to [Ca 2؉ ] i increase, we described different PLC isoforms (1, 2, ␥1, and ␥2, but not 4) in female rat osteoblasts using Western immunoblotting. The data showed that phospholipase C  was involved in the mobilization of Ca 2؉ from the endoplasmic reticulum of Fura-2-loaded confluent osteoblasts by calcitriol and 17 estradiol, and PLC ␥ was ineffective. The data also showed that only a PLC 1 linked to a Pertussis toxin-insensitive G-protein and a PLC 2 coupled to a Pertussis toxin-sensitive G-protein are involved in the effects of calcitriol and 17 estradiol on the mobilization of Ca 2؉ from intracellular Ca 2؉ stores. In conclusion, these results may be an important step toward understanding membrane effects of these steroids and may be an additional argument in favor of membrane receptors to steroid hormones.An increase in the turnover of inositol lipids in response to receptor is one of the most important molecular mechanisms used by cells for transmembrane signaling. The initial event is the hydrolysis of phosphatidylinositol 4,5-bisphosphate, a reaction catalyzed by a phosphoinositide-specific phospholipase C (PLC), 1 which generates two intracellular second messengers, inositol 1,4,5-trisphosphate and 1,2-diacylglycerol (1-4). Inositol 1,4,5-trisphosphate binds to specific receptors on the endoplasmic reticulum (5) and mobilizes intracellular calcium, whereas diacylglycerol activates protein kinase C (6), which results in increased phosphorylation of cellular proteins.Molecular cloning has revealed at least three major families of PLC, , ␥, and ␦ (7-9). Each of these families occurs in a number of isoforms. The enzymes are classified on the basis of their size and their immunological and structural similarities. The PLC isoforms have two highly conserved domains, X and Y, which form the active site of the protein. PLC ␦ and PLC ␥ proteins differ from PLC  in that they have shorter C-terminal extensions past the end of the Y domain (9). This diversity among the PLC isoforms also extends to distinct mechanisms of regulation and function for the three PLC families. PLC ␥ is regulated via the phosphorylation of tyrosine residues between the X and Y domains by receptor tyrosine kinases (10 -12). PLC  enzymes, of which there are four isoforms, PLC 1-4, are regulated via heterotrimeric G-proteins in response to an agonist binding to a receptor (13-15). The way in which PLC ␥ is regulated is not yet known, but enzyme activity is not affected by either the G-protein subunits or by receptor tyrosine kinases (16).The activat...
Only phospholipase C-1 is involved in the mobilization of Ca2؉ from the endoplasmic reticulum of Fura-2-loaded confluent OHS-4 cells and the formation of inositol 1,4,5-trisphosphate by CGRP; PLC-␥ have no effect. Activation of PLC-1 by CGRP involves the G␣ q/11 subunit, which is insensitive to pertussis toxin, but not G␥ subunits. We therefore believe that CGRP causes the activation of two separate G-proteins.
Abstract17-estradiol and 1,25-dihydroxyvitamin D 3 (calcitriol) rapidly increase (Ͻ 5 sec) the concentration of intracellular calcium by mobilizing Ca 2ϩ from the endoplasmic reticulum and forming inositol 1,4,5-trisphosphate (InsP 3 ) and diacylglycerol. Calcitriol increases InsP 3 formation via activation of phospholipase C (PLC)-1 linked to a pertussis toxin (PTX)-insensitive G-protein, and estradiol via activation of PLC-2 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 (G␣ q/11 , G␣s, G␣i, G and G␥) in female rat osteoblasts using Western immunoblotting. The action of calcitriol on InsP 3 formation and Ca 2ϩ mobilization in Fura-2-loaded confluent osteoblasts involved G␣ q/11 . The membrane effects of estradiol involved G␥; subunits, and principally G subunits, but not ␣-subunits. These results may provide additional evidence for membrane receptors of steroid hormones. Since PLC-1 is the target effector of G␣ q/11 , whereas PLC-2 is only activated by ␥ subunits, this specificity may help to generate membrane receptor-specific responses in vivo.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.