T. John Martin

Physiology and Pharmacology of Bone

Calcium homeostasis bone remodelling and bone structure biology of osteoblast cytokines of bone hormonal factors which regulate bone resorption factors which regulate bone formation mineralization the pathogenesis of osteoporosis vitamin D metabolism bisphosphonates Pagets disease hyperparathyroid and hypoparathyroid bone disease skeletal responses to physical loading parathyroid hormone calcitonin and gene products parathyroid hormone-related protein pathophysiology of skeletal complications of cancer the proteins of bone bone morphogenetic proteins.

Inhibitory effects of parathyroid hormone on growth of osteogenic sarcoma cells

SummaryThe effects of the bone resorbing hormone, parathyroid hormone (PTH), on the growth of malignant osteoblastic cells have been examined. The malignant osteoblastic cells were a clonal line (UMR 106) derived from a transplantable rat osteogenic sarcoma. The predominant effect of PTH at doses above 10−10M was an inhibition of replication and DNA synthesis. Replication was decreased by PTH in both the presence or absence of serum and at various cell seeding densities. Both bovine PTH (1-84) and the synthetic hormone, human PTH (1-34), inhibited replication, but with bovine hormone being an order of magnitude more potent. The effects could be observed in as short a time as 6 hours with DNA synthesis and 24 hours with replication.

Unidirectional migration of osteosarcoma cells with osteoblast characteristics in response to products of bone resorption.

SummaryTo investigate the mechanisms by which bone-forming cells are attracted to areas of bone resorption during bone remodeling, we have usedin vitro methods to look for signals released by resorbing bone, which may be chemotactic for cultured bone cells. We have found that cultured rat osteosarcoma cells, which have characteristics associated with the osteoblastic phenotype, migrate in a unidirectional manner in response to a signal released by resorbing bones. These cells also migrated unidirectionally in response to Type I collagen, which comprises 95% of the bone matrix. This phenomenon of chemotaxis of bone-forming cells to sites of previous resorption may be an important component of the process of bone remodeling and the coupling of bone formation to bone resorption.

Cyclic AMP-dependent and -independent effects on tissue-type plasminogen activator activity in osteogenic sarcoma cells; evidence from phosphodiesterase inhibition and parathyroid hormone antagonists.

The plasminogen activator (PA) in clonal osteogenic sarcoma cells of rat origin (UMR 106-01 and UMR 106-06) and in osteoblast-rich rat calvarial cells has been characterized using specific antibodies to be tissue-type PA (tPA). An Mr value of 75,000 by SDS-polyacrylamide gel electrophoresis and fibrin autoradiography supports this characterization. There was also evidence for an Mr 105,000 component, which could be due to a proteinase-inhibitor complex. The mechanism of regulation of this tPA activity has been studied in the clonal osteogenic sarcoma cells. Parathyroid hormone (PTH) and prostaglandin E2, which increase cyclic AMP production in the sarcoma cells, also increased tPA activity. The sensitivity and magnitude of the tPA response to PTH and prostaglandin E2 were increased by simultaneous treatment with isobutylmethylxanthine (IBMX) at drug concentrations which had little effect themselves on tPA activity. In UMR 106-06 cells, which unlike UMR 106-01 cells show a cyclic AMP response to calcitonin, tPA activity was also increased in response to calcitonin, and the effect was enhanced by IBMX. 1,25-Dihydroxyvitamin D-3 also increased tPA activity in the cells, but this response was not modified by IBMX. Synthetic peptide antagonists of PTH-responsive adenylate cyclase, [34Tyr]-hPTH (3-34) amide and [34Tyr]-hPTH (5-34) amide, inhibited the PTH-induced increase in tPA activity over the same concentration range at which they inhibited cyclic AMP production, but the antagonist peptides had no effect on the tPA responses to prostaglandin E2, calcitonin or 1,25-dihydroxyvitamin D-3. These data indicate that cyclic AMP mediates the actions of PTH, prostaglandin E2 and calcitonin in increasing tPA activity in the clonal osteogenic sarcoma cells. 1,25-Dihydroxyvitamin D-3, on the other hand, increases tPA activity through a mechanism independent of cyclic AMP.

Comparison of the effects of amino-terminal synthetic parathyroid hormone-related peptide (PTHrP) of malignancy and parathyroid hormone on resorption of cultured fetal rat long bones

SummaryWe have compared the effects of of various synthetic amino-terminal forms of human parathyroid hormone-related peptide (PTHrP) of malignancy with synthetic parathyroid hormone (PTH) on the resorptive responses of fetal rat long bones in organ culture. PTH and PTHrP increased45Ca release at concentrations of 0.1–25 nM. PTHrP (1–40) and bovine PTH (1–34) were more potent than human PTH (1–34) and PTHrP (1–34). However, the slopes of the dose-response curves and the maximal resorptive effects were similar. There was a marked decrease in the potency of amino-terminal PTHrP peptides as the length was decreased. PTHrP (1–29) and PTHrP (1–25) were inactive at 120 nM. Further comparison of bPTH (1–34) and PTHrP (1–34) showed that both could induce bone resorption after a brief (6 hours) exposure and that the response to PTHrP (1–34) was qualitatively similar to that of bPTH (1–34) with respect to enhancement by ACTH and inhibition by calcitonin and glucocorticoids. Hydroxyurea and indomethacin did not block the resorptive response to either agonist. Cyclic AMP production in response to PTHrP (1–34) and (1–40) was similar to that for bPTH (1–34) in ROS 17/2.8 cells. The cyclic AMP (cAMP) response was much smaller in fetal rat long bones and calvariae, and bPTH was more potent than PTHrP. These studies confirm that PTHrP is quantitatively similar in its effects on bone resorption to PTH and are consistent with the two agents acting on the same receptor.