POSITIVE INTERACTION BETWEEN 17-BETA-ESTRADIOL AND PARATHYROID-HORMONE IN NORMAL HUMAN OSTEOBLASTS CULTURED LONG-TERM IN THE PRESENCE OF DEXAMETHASONE

We previously developed two models of human osteoblasts with distinct differentiation stages using cells derived from iliac crest trabecular bone explants cultured long term in the presence (HOB+DEX) and absenc e (HOB-DEX) of 10 nM dexamethasone (DEX) (Wong et al., J Bone Miner Re s 1990;5:803). Using these models from 36 subjects aged 41-80 years, w e examined the effects of 17 beta-estradiol (E(2)) on cell proliferati on, osteocalcin (OC) production, alkaline phosphatase (ALP) and basal and parathyroid hormone (PTH)-stimulated adenylate cyclase activities, as well as the steady-state mRNA levels of ALP, collagen type I(COLL) , OC, and receptors for E(2) (ER) and PTH (PTHr). E(2) alone had no ef fect on [H-3]thymidine uptake in (HOB-DEX) cells but appeared to stimu late the uptake in (HOB+DEX) cells in a dose-dependent manner, with ma ximum effect at 10(-10)M (p<0.05). However, in the presence of 10(-6)m PTH, E(2) inhibited the uptake in (HOB-DEX) cells (ANOVA, KW=18.95, p <0.005) but stimulated the uptake in (HOB+DEX) cells (KW=13.52, p<0.02 5). E(2) decreased the amount of osteocalcin in culture media from bot h (HOB-DEX) and (HOB+DEX) cells (p<0.05). PTH alone or E(2), alone or in combination with 10(-9)M PTH, had no effect on ALP activity in (HOB -DEX) cells. In contrast, in (HOB+DEX) cells, E(2)+PTH but not E(2) al one, had biphasic effects on ALP activity, with maximum stimulation ob served at 10(-11) and 10(-10)m E(2), and a return to basal levels at 1 0(-9)M E(2). E(2) decreased basal adenylate cyclase activities in a do se-dependent manner in (HOB+DEX) but not (HOB-DEX) cells (KW=13.48, p< 0.05). In (HOB+DEX) cells, E(2) had biphasic effects on PTH-stimulated adenylate cyclase activity, with significant stimulation observed at 10(-10)M (p<0.05). While E(2) had no significant effect on osteoblasti c marker mRNA levels in (HOB-DEX) cells, it decreased osteocalcin and stimulated PTHr mRNA levels in (HOB+DEX) cells. Thus, in our human ost eoblastic cell models, estrogen regulated metabolic function largely i n the more differentiated cells, by modifying the effects of PTH.