3',5'-Cyclic adenosine monophosphate activation in osteoblastic cells: effects on parathyroid hormone-1 receptors and osteoblastic differentiation in vitro

PTH has anabolic and catabolic effects in bone through activation of the PTH-1 (PTH/PTHrP) receptor and the cAMP/protein kinase A pathway. The effects of agents that regulate cAMP in nontransformed osteoblasts in relation to cell differentiation have not been described. The purpose of this study was to determine the effects of PTH fragments with differing cAMP-stimulating activity, and nonPTH cAMP regulators on PTH-1 receptor expression and activity, and osteoblast differentiation in vitro using MC3T3-E1 and primary rat calvarial cells. PTH (1-34), but not PTH (53-84), (7-34), or PTHrP (107-139) treatment (24 h) resulted in down-regulation of steady-state messenger RNA for the PTH-1 receptor. Forskolin (a stimulator of cAMP accumulation) also down regulated the PTH-1 receptor, whereas 9-(tetrahydro-2-furyl) adenine (THFA) (an inhibitor of adenylyl cyclase) had no effect. Similarly, PTH (1-34) treatment for 48 h abolished PTHrP binding to cell surface receptors; however, neither the PTH analogs nor the cAMP regulating agents altered PTH binding or numbers of binding sites on osteoblastic cells. Basal levels of cAMP were reduced in cultured cells treated for 6 days with PTH (7-34) or THFA compared with controls. In contrast, PTH-stimulated cAMP levels were significantly increased in cultures treated with PTH (7-34) and THFA for 6 days during osteoblast differentiation and were decreased in cultures treated with PTH (1-34) and forskolin compared with controls. To evaluate effects of the cAMP pathway on osteoblast differentiation, cultures were treated continuously with PTH analogs and cAMP regulators during an 18-day differentiation regime, total RNA was isolated at multiple time points, and Northern blot analysis for osteocalcin (OCN) was performed. THFA and PTH (7-34)-treated cultures had increased OCN expression; whereas, PTH (1-34) and forskolin reduced OCN expression. Interestingly, PTH (7-34) and THFA-treated cultures had increased mineralized nodule formation, in contrast to PTH (1-34) and forskolin treatment, which reduced nodule formation. Similarly, calcium accumulation in cultures was significantly increased in the PTH (7-34) and THFA-treated cultures and reduced in the PTH (1-34) and forskolin-treated cultures. These data demonstrate that agents that increase cAMP down regulate PTH-1 receptor messenger RNA and inhibit osteoblast differentiation in vitro. Agents that reduce or block adenylyl cyclase or cAMP activity do not alter PTH-1 receptor expression or binding, but have striking effects on promoting osteoblast differentiation. We conclude that many effects of PTH on osteoblasts may be mimicked or antagonized by agents that alter cAMP activity and bypass the PTH-1 receptor.

Proto-oncogene c-fos is transcriptionally regulated by parathyroid hormone (PTH) and PTH-related protein in a cyclic adenosine monophosphate-dependent manner in osteoblastic cells

PTH and PTH-related protein (PTHrP) bind to the PTH-1 (PTH/PTHrP) receptor and produce anabolic and catabolic effects in bone. To investigate postreceptor mechanisms of action, MC3T3-E1 cells were induced to differentiate to optimize PTH-1 receptor expression, and differentiated MC3T3-E1 cells were treated with varying doses of PTH (1-34) for 1 h. Northern blot analysis revealed a dose-dependent stimulation of steady state c-fos messenger RNA (mRNA), with measurable expression at doses as low as 1 pM PTH. The time course of c-fos mRNA induction was rapid, with peak levels detected at 30-45 min. Increased steady state c-fos mRNA was due to increased transcription of the c-fos gene as demonstrated by nuclear run-on assays and was dependent on the temporal differentiation state of the MC3T3-E1 cells. Stimulation of c-fos mRNA was induced exclusively by N-terminal PTH and PTHrP (which is also responsible for cAMP activation), and did not occur with PTH (7-34), (53-84), or PTHrP (107-139). The effects of PTH (1-34) on c-fos stimulation were dependent on intracellular cAMP. Forskolin [a guanine-nucleotide-binding protein (G(alpha)) agonist] stimulated c-fos mRNA, whereas 9-(tetrahydro-2-furyl) adenine (THFA) (a cAMP antagonist), 1,9 dideoxyforskolin (a cAMP independent analog of forskolin), and phorbol 12-myristate 13-acetate (a protein kinase C activator) did not. Furthermore, THFA inhibited the ability of PTH (1-34) to stimulate c-fos mRNA in a time-dependent manner. These findings indicate that c-fos is transcriptionally regulated by PTH (1-34) in osteoblastic cells, and that cAMP is a mediator of PTH-stimulated c-fos induction. Several known bone-associated proteins contain DNA binding sites in their promoter regions that recognize c-fos in conjunction with c-jun (AP-1 sites). Consequently, the induction of c-fos by PTH (1-34) in osteoblastic cells may be a sensitive indicator of PTH effects in vitro and in vivo, and provide valuable information regarding mechanisms of PTH action in bone.

PTH/PTHrP receptor is temporally regulated during osteoblast differentiation and is associated with collagen synthesis

The temporal sequence of PTH/PTHrP receptor mRNA, binding, biologic activity, and its dependence on matrix synthesis was determined using MC3T3-E1 preosteoblast-like cells and primary rat calvarial cells in vitro. Osteoblastic cells were induced to differentiate and form mineralized nodules with the addition of ascorbic acid and beta-glycerophosphate, and samples were collected from 0-26 days of culture. DNA levels as determined by fluorometric analysis increased 12- and 17-fold during the collection period for both MC3T3-E1 and primary calvarial cells respectively. Steady state mRNA levels for the PTH/PTHrP receptor as determined by northern blot analysis, were initially low for both cell types, peaked at day 4 and 5 for MC3T3-E1 and primary calvarial cells respectively, and declined thereafter. Competition binding curves were performed during differentiation using 125I-PTHrP. The numbers of receptors per microgram DNA were greatest at days 3 and 5 for MC3T3-E1 and primary calvarial cells respectively. The biologic activity of the receptor was evaluated by stimulating the cells with 10 nM PTHrP and determining cAMP levels via a binding protein assay. The PTHrP-stimulated cAMP levels increased 5-fold to peak values at day 5 for MC3T3-E1! cells and 6-fold to peak values at day 4 for the primary calvarial cells. Ascorbic acid was required for maximal development of a PTH-dependent cAMP response since ascorbic acid-treated MC3T3-E1 cells had twice the PTH-stimulated cAMP levels as non-treated cells. When the collagen synthesis inhibitor 3,4-dehydroproline was administered to MC3T3-E1 cultures prior to differentiation, there was a subsequent diminution of the PTH/PTHrP receptor mRNA gene expression and numbers of receptors per cell; however, if administered after the initiation of matrix synthesis there was no reduction in PTH/PTHrP receptor mRNA. These findings indicate that the PTH/PTHrP receptor is associated temporally at the level of mRNA, protein, and biologic activity, with a differentiating, matrix-producing osteoblastic cell in vitro.

Effects of differentiation and transforming growth factor beta 1 on PTH/PTHrP receptor mRNA levels in MC3T3-E1 cells

TGF beta has opposing effects on osteoblasts which are thought to be differentiation stage dependent; however, little is known concerning the effects of TGF beta on osteoblastic characteristics at different stages of maturation. The purpose of this study was to characterize the pattern of mRNA expression for the PTH/PTHrP receptor during normal osteoblastic differentiation in vitro, and evaluate the effects of TGF beta 1 on PTH/PTHrP receptor and osteocalcin (OCN) steady-state mRNA at different stages of osteoblastic differentiation. MC3T3-E1 preosteoblasts were plated at low density and induced to differentiate with ascorbic acid and beta-glycerophosphate. The first group served as a vehicle control and the remaining five groups received a single 48 h TGF beta 1 (3.0 ng/ml)-pulse staggered on a weekly basis for 30 days. Cell cultures were harvested weekly and evaluated for: steady-state PTH/PTHrP receptor and OCN mRNA levels via northern analysis, calcium and phosphorous levels, bone nodules via Von Kossa staining, alkaline phosphatase enzyme levels, and hydroxyproline levels. Group 1 (control) samples followed a normal pattern of proliferation, extracellular matrix deposition, and mineralization. PTH/PTHrP receptor and OCN mRNA expression increased 8-fold and 10-fold respectively, over the collection periods. When TGF beta 1 was administered during the first 48 h period (group 2) while cells were rapidly proliferating, there was a persistent inhibition of PTH/PTHrP receptor expression and a striking reduction in OCN mRNA expression at all time points. There was also a down-regulation of PTH/PTHrP receptor and OCN expression when TGF beta 1 was administered later during osteoblast differentiation (groups 3-6); however, these effects were not persistent. In addition there was a total lack of bone nodule formation in group two cultures, whereas groups 3-6 had increasing bone nodule formation because the TGF beta 1 was administered later in the culture period. These studies indicate that expression of the PTH/PTHrP receptor increases with osteoblastic differentiation and suggest that TGF beta 1 inhibits osteoblastic maturation with more persistent effects found in less differentiated osteoblastic cells.

Transforming growth factor-beta1 regulates steady-state PTH/PTHrP receptor mRNA levels and PTHrP binding in ROS 17/2.8 osteosarcoma cells

The effect of transforming growth factor beta1 (TGF-beta1) on the expression of mRNA for the parathyroid hormone receptor and binding of iodinated parathyroid hormone-related protein in ROS 17/2.8 osteosarcoma cells was evaluated. TGF-beta1 stimulated a 2-7-fold increase in steady state mRNA levels for the parathyroid hormone receptor at a maximal dose of 5 ng/ml, with increased levels of expression at 6 h of TGF-beta1-incubation, and peak levels at 8-24 h. Receptor binding studies revealed a significant increase in PTHrP-specific binding with TGF-beta1 doses as low as 0.5 ng/ml and a 55% increase in numbers of receptors with no alteration in binding affinity with 5.0 ng/ml TGF-beta1. Time course studies indicated that receptor binding was increased at 24 h with peak levels reached at 48 h of treatment. PTH-stimulated cAMP levels were significantly increased in ROS 17/2.8 cells treated with TGF-beta1 (0.5 ng/ml) for 48 h. These data indicate that TGF-beta1 upregulates steady-state mRNA, ligand binding and PTH/PTHrP receptor signaling in rat osteosarcoma cells. The effects of TGF-beta1 on bone may be attributed in part to regulation of the PTH/PTHrP receptor at the molecular level.