van Leeuwen J P, Birkenhäger J C, van den Bemd G C, Pols H A
Department of Internal Medicine III, Erasmus University Medical School, Rotterdam, The Netherlands.
Biochim Biophys Acta. 1996 Jun 5;1312(1):54-62.
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.
多项动物研究和临床观察表明，在靶组织水平上，1,25 - 二羟基维生素D3（1,25-(OH)2D3）和甲状旁腺激素（PTH）必须以相互关联的方式发挥作用。在本研究中，我们检测了1,25-(OH)2D3和PTH在这些激素于骨骼中的靶细胞——成骨细胞中的相互作用。此外，我们还研究了PTH激活的信号通路的作用。使用三种成骨细胞系UMR 106、ROS 17/2.8和MG - 63作为模型系统。在UMR 106细胞中，1,25-(OH)2D3和PTH协同上调维生素D受体（VDR），同时伴随VDR mRNA表达的协同诱导，而在ROS 17/2.8和MG - 63细胞中均未观察到相互作用。在UMR 106细胞中，PTH对VDR同源上调的作用可被cAMP激动剂福斯可林和二丁酰环磷腺苷模仿。蛋白激酶C的佛波酯激活降低了基础以及1,25-(OH)2D3诱导的VDR上调。1,25-(OH)2D3在UMR 106和MG 63细胞中诱导24 - 羟化酶活性，与VDR调节不同的是，在这两种细胞系中PTH和1,25-(OH)2D3协同诱导24 - 羟化酶活性。与VDR调节相似，PTH的作用可通过激活cAMP产生来模仿，而蛋白激酶C激活则降低了1,25-(OH)2D3的诱导作用。最后，我们检测了在骨钙素合成方面的相互作用。在ROS 17/2.8和MG - 63细胞中，1,25-(OH)2D3刺激骨钙素的产生。在ROS 17/2.8细胞中，PTH以及福斯可林对cAMP产生的刺激增强了1,25-(OH)2D3诱导的骨钙素产生，而正如我们之前所表明的，蛋白激酶C的激活并不改变1,25-(OH)2D3刺激的骨钙素产生。在MG - 63细胞中，PTH和福斯可林均未显著改变1,25-(OH)2D3对骨钙素合成的诱导作用。从本研究可以得出结论，实际上在靶细胞水平上，1,25-(OH)2D3和PTH以协调的方式发挥作用。基于PTH在成骨细胞中对1,25-(OH)2D3作用的增强以及先前报道的1,25-(OH)2D3对PTH刺激的cAMP产生的抑制，我们推测在靶细胞水平存在一个负反馈环。cAMP途径的激活导致1,25-(OH)2D3作用增强，而蛋白激酶C途径减弱1,25-(OH)2D3的作用。最后，本研究为体内观察到的1,25-(OH)2D3和PTH在靶细胞上的相互关联作用提供了依据。更普遍地说，它基于对内源性细胞反应的分析，证明了肽类激素和甾体激素的受体激活途径之间存在相互作用。
