Löwik C W, Olthof A A, van Leeuwen J P, van Zeeland J K, Herrmann-Erlee M P
Laboratory of Cell Biology and Histology, University of Leiden, The Netherlands.
Calcif Tissue Int. 1988 Jul;43(1):7-18. doi: 10.1007/BF02555162.
We investigated the role of cAMP and Ca2+ as mediators in parathyroid hormone (PTH)-induced ornithine decarboxylase (ODC) activity in primary cultures of chicken osteoblasts. We present evidence that the induction of ODC activity by PTH is most likely a receptor-mediated process and that cAMP is a mediator. However, using three different approaches we have strong indications that cAMP is not the exclusive mediator of PTH-induced ODC activity. First, when the dose-response curve of PTH-induced ODC activity is compared with that of PTH-stimulated cAMP production, the ED50 for cAMP production is about five times as high as that for the induction of ODC activity. Second, 1 mM 9-(tetrahydro-2-furanyl) adenine (SQ 22.536) almost completely inhibited PTH-stimulated cAMP production whereas there was only a small inhibitory effect on PTH-induced ODC activity. Third, some PTH fragments unable to stimulate cAMP production were still able to induce ODC activity. We therefore propose that apart from cAMP, an additional messenger, most likely Ca2+, must be present. Evidence for this concept are the observations that substances affecting extracellular and intracellular Ca2+ levels (EGTA, A23187, CoCl2, verapamil) or antagonizing calmodulin (Trifluoroperazin, Compound 48/80) also strongly affect PTH-induced ODC activity. These effects could not be explained by a positive interaction of Ca2+ with the hormone-stimulated cAMP system as 2 mM EGTA strongly enhanced PTH-stimulated cAMP production but at the same time completely inhibited PTH-induced ODC activity. A similar dissociation between hormone-induced cAMP production and induction of ODC activity was found with the Ca2+ -ionophore A23187 (10(-7) M) which significantly inhibited PTH-stimulated cAMP production but strongly enhanced PTH-induced ODC activity. Our results suggest that intracellular Ca2+, and possibly calmodulin, in addition to cAMP, are involved in PTH-induced ODC activity in chicken osteoblasts. Most probably Ca2+ is the initial messenger and cAMP acts in a coordinate pattern as a synarchic messenger making the induction of ODC activity by PTH more sensitive to Ca2+. Furthermore, the present findings are in agreement with our concept of the existence of two receptors or two receptor-sites for PTH on osteoblasts. One receptor is coupled to the production of cAMP and is presumably activated when the first two aminoacids of the NH2-terminus of the hormone are present and the other, suggested to be responsible for the increase in intracellular Ca2+, is thought to be activated by a region of the hormone sequence between amino acid 3 and 34.(ABSTRACT TRUNCATED AT 250 WORDS)
我们研究了环磷酸腺苷（cAMP）和钙离子（Ca2+）作为介质在甲状旁腺激素（PTH）诱导鸡成骨细胞原代培养物中鸟氨酸脱羧酶（ODC）活性过程中的作用。我们提供的证据表明，PTH诱导ODC活性很可能是一个受体介导的过程，且cAMP是一种介质。然而，通过三种不同的方法，我们有充分的迹象表明cAMP并非PTH诱导ODC活性的唯一介质。首先，当将PTH诱导ODC活性的剂量反应曲线与PTH刺激cAMP产生的剂量反应曲线进行比较时，cAMP产生的半数有效剂量（ED50）约为诱导ODC活性的ED50的五倍。其次，1 mM 9 -（四氢-2-呋喃基）腺嘌呤（SQ 22.536）几乎完全抑制了PTH刺激的cAMP产生，而对PTH诱导的ODC活性只有轻微的抑制作用。第三，一些无法刺激cAMP产生的PTH片段仍能够诱导ODC活性。因此，我们提出除了cAMP之外，必定还存在另一种信使，很可能是Ca2+。支持这一概念的证据是，观察到影响细胞外和细胞内Ca2+水平的物质（乙二醇双四乙酸（EGTA）、A23187、氯化钴、维拉帕米）或拮抗钙调蛋白的物质（三氟拉嗪、化合物48/80）也强烈影响PTH诱导的ODC活性。这些效应无法用Ca2+与激素刺激的cAMP系统的正向相互作用来解释，因为2 mM EGTA强烈增强了PTH刺激的cAMP产生，但同时完全抑制了PTH诱导的ODC活性。在Ca2+离子载体A23187（10(-7) M）的作用下，也发现了激素诱导的cAMP产生与ODC活性诱导之间类似的解离，它显著抑制了PTH刺激的cAMP产生，但强烈增强了PTH诱导的ODC活性。我们的结果表明，除了cAMP之外，细胞内Ca2+以及可能的钙调蛋白也参与了PTH诱导鸡成骨细胞中ODC活性的过程。很可能Ca2+是初始信使，而cAMP作为协同信使以协调的方式起作用，使得PTH诱导ODC活性对Ca2+更敏感。此外，目前的研究结果与我们关于成骨细胞上存在两种PTH受体或两个受体位点的概念一致。一种受体与cAMP的产生偶联，推测当激素NH2末端的前两个氨基酸存在时被激活，另一种受体被认为负责细胞内Ca2+的增加，据认为它被激素序列中氨基酸3至34之间的区域激活。（摘要截短至250字）
