Evidence for coordinated regulation of osteoblast function by 1,25-dihydroxyvitamin D3 and parathyroid hormone.
From several animal studies and clinical observations it became evident that at target tissue level 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3) and parathyroid hormone (PTH) must act in an interrelated manner. In the present study we examined the interaction between 1,25-(OH)2D3 and PTH in the target cell of these hormones in bone, the osteoblast. In addition we studied the role of PTH-activated signal pathways. The three osteoblastic cell lines UMR 106, ROS 17/2.8 and MG-63 were used as model systems. In UMR 106 cells 1,25-(OH)2D3 and PTH caused a synergistic up-regulation of the vitamin D receptor (VDR) which was accompanied by a synergistic induction of VDR mRNA expression whereas in both ROS 17/2.8 and MG-63 cells no interaction was observed. In UMR 106 cells the effect of PTH on homologous up-regulation of VDR could be mimicked by the cAMP agonist forskolin and by dibutyrylic-cAMP. Phorbol ester activation of protein kinase C reduced basal as well as 1,25-(OH)2D3-induced up-regulation of VDR. 1,25-(OH)2D3 induced 24-hydroxylase activity in UMR 106 and MG 63 cells and, in contrast to VDR regulation, in both cell lines PTH and 1,25-(OH)2D3 synergistically induce 24-hydroxylase activity. Similar to VDR regulation the effect of PTH was mimicked by activation of cAMP production whereas protein kinase C activation reduced the induction by 1,25-(OH)2D3. Finally, we examined the interaction with respect to osteocalcin synthesis. In ROS 17/2.8 and MG-63 cells 1,25-(OH)2D3 stimulated osteocalcin production. In ROS 17/2.8 cells PTH as well as stimulation of cAMP production by forskolin enhanced 1,25-(OH)2D3-induced osteocalcin production whereas, as we have shown previously, activation of protein kinase C does not change 1,25-(OH)2D3-stimulated osteocalcin production. In MG-63 cells neither PTH nor forskolin significantly changed 1,25-(OH)2D3 induction of osteocalcin synthesis. From the present study it can be concluded that indeed at target cell level 1,25-(OH)2D3 and PTH act in a coordinated manner. On basis of the potentiation of 1,25-(OH)2D3 action by PTH in osteoblasts together with the previously reported inhibition of PTH-stimulated cAMP production by 1,25-(OH)2D3 we postulate a negative feedback-loop at target cell level. The activation of the cAMP pathway results in an enhancement of the 1,25-(OH)2D3 action whereas the protein kinase C pathway attenuates the 1,25-(OH)2D3 action. Finally, the present study provides a basis for the indications from in vivo observations about an interrelated action of 1,25-(OH)2D3 and PTH at the target cell. More generally it demonstrates on the basis of analyses of endogenous cellular responses evidence for an interplay between receptor-activated pathways of peptide and steroid hormones.
van Leeuwen
J P
JP
Department of Internal Medicine III, Erasmus University Medical School, Rotterdam, The Netherlands. vanleeuwen@inw3.fgg.eur.nl
Birkenhäger
J C
JC
van den Bemd
G C
GC
Pols
H A
HA
D001498:3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethyl-5-nitro-4-(2-(trifluoromethyl)phenyl)-, Methyl ester / Q000494:pharmacology; D000818:Animals; D003994:Bucladesine / Q000494:pharmacology; D002117:Calcitriol / Q000494:pharmacology*; D002118:Calcium / Q000502:physiology; D002120:Calcium Channel Agonists / Q000494:pharmacology; D002460:Cell Line; D005576:Colforsin / Q000494:pharmacology; D000242:Cyclic AMP / Q000819:agonists / Q000096:biosynthesis; D003577:Cytochrome P-450 Enzyme System; D004357:Drug Synergism; D004789:Enzyme Activation; D006801:Humans; D010006:Osteoblasts / Q000187:drug effects* / Q000378:metabolism / Q000502:physiology*; D015675:Osteocalcin / Q000096:biosynthesis; D010281:Parathyroid Hormone / Q000494:pharmacology*; D011493:Protein Kinase C / Q000378:metabolism; D012333:RNA, Messenger / Q000096:biosynthesis; D051381:Rats; D018167:Receptors, Calcitriol / Q000096:biosynthesis / Q000235:genetics; D015398:Signal Transduction / Q000187:drug effects; D013250:Steroid Hydroxylases / Q000378:metabolism; D013755:Tetradecanoylphorbol Acetate / Q000494:pharmacology; D015854:Up-Regulation; D065668:Vitamin D3 24-Hydroxylase
D002120:Calcium Channel Agonists; D010281:Parathyroid Hormone; D012333:RNA, Messenger; D018167:Receptors, Calcitriol; D015675:Osteocalcin; D005576:Colforsin; D003994:Bucladesine; D001498:3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethyl-5-nitro-4-(2-(trifluoromethyl)phenyl)-, Methyl ester; D003577:Cytochrome P-450 Enzyme System; D000242:Cyclic AMP; D013250:Steroid Hydroxylases; D065668:Vitamin D3 24-Hydroxylase; D011493:Protein Kinase C; D002117:Calcitriol; D013755:Tetradecanoylphorbol Acetate; D002118:Calcium