INOSITOL TRISPHOSPHATE RECEPTOR GENE-EXPRESSION AND HORMONAL-REGULATION IN OSTEOBLAST-LIKE CELL-LINES AND PRIMARY OSTEOBLASTIC CELL-CULTURES

The inositol trisphosphate receptor (IP3R) is an intracellular calcium channel that mediates the cellular actions of a wide variety of hormo nes, growth factors, and cytokines, In osteoblastic cell cultures, man y bone resorbing hormones increase phosphoinositide turnover, inositol trisphosphate production, mobilization of intracellular calcium, and the secretion of osteoclast recruitment and activating factors, In thi s study, the effects of 17 beta-estradiol, 1,25-dihydroxyvitamin D-3 ( 1,25(OH)(2)D-3), phorbol ester, and serum on IP3R mRNA levels were eva luated in osteogenic-osteosarcoma cells and in primary osteoblastic cu ltures derived from neonatal rat calvaria. Type-specific reverse trans cription polymerase chain reaction (RT-PCR) indicated that all cell ty pes evaluated (G-292, U-2OS, Saos-2, MC3T3-E1, UMR-106, and calvarial osteoblastic cells) express IP3R mRNA type I; G-292, U-2OS, MC3T3-E1, and calvarial osteoblastic cells also express type II IP3R mRNA; and U MR-106 and the calvarial osteoblastic cells express type III IP3R mRNA . Northern blot and RT-PCR analyses of human G-292 osteosarcoma cells and rat calvarial osteoblastic cells showed that phorbol ester and ser um increase IP3R mRNA levels, whereas 17 beta-estradiol and 1,25(OH)(2 )D-3 decrease these levels. In G-292 cells, the effect of 17 beta-estr adiol was not due to accelerated IP3R mRNA degradation and required co ntinued protein synthesis, The results show that multiple IP3R types a re expressed in osteoblasts and osteoblastic osteosarcoma cells and th at this expression is regulated by 17 beta-estradiol and other osteopo rotic and antiosteoporotic hormones, These findings indicate that horm onal control of IP3R expression may be relevant in the chronic regulat ion of osteoblast secretory activity.