Effects of parathyroid hormone on the accumulation of cyclic AMP in bone of vitamin D-deficient rats

Short carboxyl terminal parathyroid hormone peptides modulate human parathyroid hormone signaling in mouse osteoblasts

BACKGROUND

Novel human parathyroid hormone (hPTH) peptides of unknown biological activity have recently been identified in the serum of subjects with normal renal function, chronic renal failure, and end-stage renal disease through the application of liquid chromatography-high resolution mass spectrometry.

PURPOSE

of experiments: To determine the bioactivity of these peptides, we synthesized hPTH28-84, hPTH38-84, and hPTH45-84 peptides by solid phase peptide synthesis and tested their bioactivity in MC3T3-E1 mouse osteoblasts, either individually or together with the native hormone, hPTH1-84, by assessing the accumulation of 3´,5´-cyclic adenosine monophosphate (cAMP) and the induction of alkaline phosphatase activity.

RESULTS

Increasing doses of hPTH1-84 (1-100 nM) increased the accumulation of cAMP and alkaline phosphatase activity in osteoblasts. hPTH28-84, hPTH38-84, and hPTH45-84 in concentrations of 1-100 nM were biologically inert. Surprisingly, 100 nM hPTH38-84 and hPTH45-84 increased the accumulation of cAMP in osteoblasts treated with increasing amounts of hPTH1-84. Human PTH28-84 had no effects on cAMP activity alone or in combination with hPTH1-84. Conversely, 100 nM hPTH38-84, hPTH45-84, and hPTH28-84 blocked the activation of alkaline phosphatase activity by hPTH1-84.

CONCLUSIONS

The data show that the short carboxyl-terminal hPTH peptides, hPTH38-84 and hPTH45-84, increase the amount of cellular cAMP generated in cultured osteoblasts in response to treatment with full-length hPTH1-84 when compared to full-length hPTH1-84 alone. Human PTH28-84 had no effect on cAMP activity alone or in combination with hPTH1-84. Human PTH28-84, hPTH38-84 and hPTH45-84 reduced the effects of hPTH1-84 in osteoblasts with respect to the induction of alkaline phosphatase activity compared to hPTH1-84 alone. Short carboxyl peptides of human PTH are biologically inert but when administered together with full-length hPTH1-84 modulate the bioactivity of hPTH1-84 in osteoblasts.

Effect of 1,25-dihydroxyvitamin D3 on cyclic AMP responses to hormones in clonal osteogenic sarcoma cells

The effect of 1,25-dihydroxyvitamin D3 on adenylate cyclase responsiveness was studied in the clonal osteogenic sarcoma cell line, UMR 106-06, which responds to several bone active hormones. 1,25-dihydroxyvitamin D3 treatment had no consistent effect on basal formation of cyclic AMP in intact cells, but the responses to parathyroid hormone, isoproterenol, prostaglandin E2, salmon calcitonin and the plant diterpene, forskolin, were all attenuated, by up to 90%. The effect of 1,25-dihydroxyvitamin D3 was dose-dependent, with half-maximal effectiveness at 0.1 nM, and required 48 h treatment of cells before it became apparent. The relative potencies of other vitamin D3 compounds correlated closely with their relative affinities for the 1,25-dihydroxyvitamin D3 receptor and their biological activities in other systems. 1,25-dihydroxyvitamin D3 treatment had no effect on the kinetics of labelled calcitonin binding to UMR 106-06 cells. Furthermore, the fact that such a range of hormones was affected made a receptor mediated mechanism unlikely. Nucleotide stimulatory (Ns) unit activity was assayed after 1,25-dihydroxyvitamin D3 treatment and found to be unchanged. Islet activating protein, an inhibitor of nucleotide inhibitory unit (Ni) activity, failed to modify the 1,25-dihydroxyvitamin D3 effect. Thus the effect of 1,25-dihydroxyvitamin D3 appeared to be exerted beyond hormone receptor and nucleotide regulatory components of the adenylate cyclase complex. It is concluded that 1,25-dihydroxyvitamin D3 attenuates adenylate cyclase response to hormones by a direct or indirect action on the catalytic component of adenylate cyclase.

Effect of colchicine and calcitonin on calcemic response to metabolic acidosis

To examine the mechanism responsible for enhanced calcium mobilization from bone in metabolic acidosis, we evaluated the effect of colchicine and calcitonin, two blockers of cell-mediated bone resorption, on the calcemic response to acute metabolic acidosis in thyroparathyroidectomized rats. Metabolic acidosis lasting 16 hr and induced by the feeding of NH4Cl led to a significant rise in serum calcium of 1.2 to 1.9 mg/dl. The administration of colchicine or calcitonin led to a decrement in serum calcium of 1.1 +/- 0.2 (P less than 0.01) and 0.7 +/- 0.2 mg/dl (P less than 0.05), respectively. Cyclic AMP levels in calvaria from rats with metabolic acidosis and from control rats were not different. These data suggest that mobilization of calcium from bone which occurs in metabolic acidosis is due, in part, to increased bone resorption, which is mediated by a cAMP-independent mechanism.

Renal parathyroid hormone receptors in the chick: downregulation in secondary hyperparathyroid animal models

We characterized the binding of 125I-[Nle8, Nle18, Tyr34]parathyroid hormone-(1-34) amide [125I-nlPTH-(1-34)] to renal plasma membranes prepared from chicks to determine the effects of secondary hyperparathyroid states on renal PTH receptors. This radioligand exhibited specific binding to membranes with high affinity (Kd, 2-3 X 10(-9) M). Agonists or competitive antagonists of PTH were effective in competing for binding sites labeled with 125I-nlPTH-(1-34), whereas an inactive fragment of PTH, salmon calcitonin, and bovine growth hormone did not compete with the radioligand for renal PTH receptors. Newly hatched chicks raised on control diet with adequate vitamin D and calcium or diets deficient in either vitamin D or calcium were used to study the regulation of renal PTH receptors in experimental models of secondary hyperparathyroidism. We found that both experimental diets resulted in marked hypocalcemia and progressive loss of renal cyclic AMP responsiveness to PTH in vitro. Associated with this refractoriness to the hormone was a marked reduction in PTH receptors in membranes from both vitamin D-deficient and calcium-deficient chick kidney. No change in the affinity of the PTH receptors was found. Vitamin D3, in a single dose of 250 micrograms, partially restored serum calcium of vitamin D-deficient birds toward normal by 72 h and also partly restored renal cyclic AMP responsiveness to PTH and the PTH receptor number toward control values. We conclude that renal refractoriness to PTH observed in experimentally hyperparathyroid animals models is due to a marked loss of plasma membrane receptor sites for PTH without an apparent change in the affinity of the receptors for the hormone.

Demonstration of a lack of change in serum 1 alpha,25-dihydroxyvitamin D in response to parathyroid extract in pseudohypoparathyroidism

Studies were carried out to compare the effects of parathyroid extract (PTE) on serum and urinary calcium (Ca) and phosphorus (P), serum 25-hydroxyvitamin D (25-OHD), serum 24,25-dihydroxyvitamin D (24,25(OH)2D), serum 1 alpha,25-dihydroxyvitamin D (1 alpha,25(OH)2D), and urinary cyclic AMP in two normal subjects, two patients with hypoparathyroidism (HP) and six patients with pseudohypoparathyroidism (PHP), some of whom were on suboptimal treatment with vitamin D. Two of the patients with PHP were studied while on long-term treatment with 1 alpha,25-(OH)2D3. Before PTE, serum 1 alpha, 25(OH)2D was at the lower limit of normal in one patient and was abnormally low in the other five patients. None of these individuals was on treatment with 1 alpha,25(OH)2D3. Serum 25-OHD and 24,25(OH)2D were either increased or at the upper limit of normal in the patients given vitamin D and were normal in the other patients. PTE lowered the serum P and increased the serum 1 alpha,25(OH)2D, serum and urinary Ca, urinary P, and urinary cyclic AMP in the normal subjects and patients with HP. In individual studies, changes in serum 1 alpha,25(OH)2D and serum Ca occurred in parallel before, during, and after PTE. In contrast, PTE had very little effect in the patients with PHP. Whereas there were highly significant positive correlations between serum 1 alpha,25(OH)2D in each of the normal subjects and patients with HP, there were significant correlations in only one of the patients with PHP. An increase in serum Ca in response to PTE was observed in one of the two patients with PHP who were on long-term treatment with 1 alpha,25(OH)2D3. In these individuals, PTE produced only slight increases in serum 1 alpha,25(OH)2D. Serum 25-OHD and 24,25(OH)2D were not changed by PTE in any of the subjects or patients. The results provide evidence that hypocalcemia in HP and PHP arises in part from low circulating 1 alpha,25-(OH)2D, and indicate that the lack of change in serum 1 alpha,25(OH)2D with PTE in patients with PHP is related to impaired renal adenylate cyclase and phosphaturic responses. These and previous results support the idea that diminished renal production of 1 alpha,25(OH)2D, because of a defect in the parathyroid hormone-responsive adenylate cyclase system, may be a contributing factor in the pathogenesis of the abnormal calcium metabolism in PHP.

Renal parathyroid hormone-dependent adenylate cyclase activity after repletion of vitamin D-deficient rats with vitamin D-2

Rats fed a diet deficient in both vitamin D and Ca2+ exhibited a greater depression of the renal parathyroid hormone (PTH)-dependent adenylate cyclase than was observed in rats fed diets deficient in either vitamin D or calcium. Total serum Ca2+ was decreased from a control level of 11.2 mg/dl to 8.5 mg/dl in rats fed the diet deficient in calcium alone, and to 5.4 mg/dl in rats fed the diet deficient in vitamin D. Serum calcium was decreased further to 4.3 mg/dl in rats fed the diet deficient in both vitamin D and Ca2+. Serum immuno-reactive PTH was significantly elevated over control levels when rats were fed the test diets; however, there were no significant differences between the elevated levels in the three experimental groups. Repletion of rats deficient in vitamin D only with a single oral dose of 3200 I.U. vitamin D-2 resulted in restoration of serum calcium to normal levels, a return of serum PTH to the control state, and an associated increase in PTH-dependent adenylate cyclase activity to the control level by 72 h. Repletion of rats deficient in both vitamin D and Ca2+ with the same dose of vitamin D-2 raised serum Ca2+ to 7.2 mg/dl by 72 h, but did not cause a reduction in circulating PTH, nor did it result in any significant improvement in the responsiveness of the membrane adenylate cyclase to PTH. These results suggest that elevated PTH is a factor in the down regulation of the PTH-dependent adenylate cyclase, but do not rule out a role for calcium as a regulatory factor.

The actions of parathyroid hormone on bone: relation to bone remodeling and turnover, calcium homeostasis, and metabolic bone diseases. II. PTH and bone cells: bone turnover and plasma calcium regulation

Kinetic and morphologic studies in patients with parathyroid disease, and a wide variety of studies in experimental animals indicate that one major effect of PTH is to increase the proliferation of osteoprogenitor cells into osteoclasts and so to increase bone turnover. PTH stimulates bone cells by increasing cell membrane permeability to calcium and consequently increasing calcium influx and by activating membrane-bound adenyl-cyclase. It is likely that the former event precedes the latter and that calcium is the second messenger and cyclic AMP the third messenger. PTH increases the production by bone cells of lactate, citric and carbonic acids, lysosomal enzymes, collagenase, and hyaluronic acid, some or all of which are concerned in the mechanism of bone resorption. With the exception of lactate which probably comes mainly from osteocytes, the increase in metabolic activity is largely due to the increase in the number of osteoclasts. There is also ultrastructural, biochemical, and biophysical evidence that PTH stimulates existing osteoclasts, but this most likely represents the transformation of inactive cells into an active state, and is a transient and nonsustainable effect. As yet, there is no evidence that either increased osteoprogenitor cell proliferation or increased osteoclast activity is mediated by adenyl-cyclase activation. PTH also acts on the deep osteocyte to cause rapid mobilization of calcium from the zone of hypomineralized metabolically active perilacunar bone. This effect is mediated by adenyl-cyclase activation and is preceded by a slight fall in plasma calcium probably due to the movement of calcium into bone cells. The function of this rapid hypercalcemic response to PTH is correct errors in the prevailing steady-state level of plasma calcium...

Failure of 1, 25 dihydroxycholecalciferol (1, 25-(OH) 2-D3) to modify cyclic AMP levels in parathyroid hormone-treated and untreated bone cells

Treatment with 1, 25-(OH) 2-D3 (1 ng/ml-25 ng/ml) for periods ranging from 2.5 min. to 60 min. did not alter cyclic AMP levels in bone cells isolated from periosteum-free rat calvaria, or in cells isolated from rat periosteal tissues. 1, 25-(OH) 2-D3 failed to modify the acute increases in cyclic AMP elicited by PTH (10 ng/ml-1 ug/ml). Two separate 1, 25-(OH) 2-D3 preparations, biologically active in other systems, were ineffective under a wide variety of experimental conditions. These results suggest that 1, 25-(OH) 2-D3 is not an acute modulator of cyclic AMP metabolism in PTH-treated and untreated bone cells.