Synthetic human parathyroid hormone fragment stimulates prostaglandin E2 synthesis by chick calvariae

Human parathyroid hormone fragment stimulates the de novo synthesis of prostaglandin endoperoxide synthase in chick calvaria

The human parathyroid hormone N-terminal fragment [hPTH-(1–34)] increases the conversion of exogenous unsaturated fatty acids to prostaglandins (PGs) in calvarial homogenates. Enzyme activities were completely blocked by indomethacin (5 × 10⁻⁷ M), a PG synthase inhibitor, and actinomycin D (5 μM), an inhibitor of transcription, by binding to DNA. In addition, a potent inhibitor of protein synthesis, cycloheximide (10 μM), totally inhibited the stimulating effect of hPTH-(1–34) on prostaglandin endoperoxide synthase (PG synthase, EC 1.14.99.1). The stimulatory effect of hPTH-(1–34) on PG synthase was also reduced by the addition of stannous chloride. However, epidermal growth factor (EGF), platelet-derived activating factor (PDGF), and ionophore A23187 did not show the same stimulating effect as hPTH-(1–34) on PG synthase in calvaria. The results further demonstrated that PG synthase is a membrane-bound enzyme in chick calvaria. In this communication, evidence is presented that hPTH-(1–34) stimulates the de novo synthesis of PG synthase as demonstrated by the increased activity in calvarial homogenates and microsomes.

Morphological changes induced by prostaglandin E in cultured rat osteoblasts

Prostaglandin E (PGE)-induced morphological changes of osteoblasts and its possible mechanisms were investigated in cultured calvaria and isolated osteoblasts from long bone fragments of neonatal rats. The control osteoblasts, either on the calvaria or isolated from the long bone fragments, were flat, polygonal in shape, and arranged in a monolayer under scanning electron microscopy (SEM) or phase contrast microscopy. Treatment with 1 mumol/L of prostaglandin E2 (PGE2, 2 h) caused these bone cells to contract a soma, whereas 10 and 100 mumol/L PGE2 (2 h) caused 18%-30% of the bone cells to elongate and expose the undersurface. Incubation of the cultured osteoblasts with PGE2 at different time periods showed a bell-shaped pattern with the optimal response at 2 h of incubation. A similar reaction can be induced by treatment with prostaglandin E1 (PGE1) or dibutyryl cyclic adenosine monophosphate (DBcAMP) in combination with 3-isobutyl-1-methylxanthine (IBMX). Furthermore, we assessed the percentage of responsive isolated bone cells to investigate interactions with other agents. The morphological changes induced by PGEs were inhibited by H-8, a protein kinase inhibitor. On the other hand, elevated intracellular calcium enhanced the PGE-induced morphological changes. Fluorescence labeling showed that PGEs caused the breakdown of the actin microfilaments, but spared the microtubules and vimentin filaments in the isolated osteoblast-like cells. These results suggest that the morphological changes of osteoblasts induced by PGEs may be related to the intracellular cAMP and calcium levels.

Insulin-like growth factor-I mediates osteoclast-like cell formation stimulated by parathyroid hormone

There have been several lines of evidence that parathyroid hormone (PTH) stimulates production of insulinlike growth factor I (IGF-I) in bone and that IGF-I stimulates osteoclast formation. Thus, the present study was performed to clarify the possible role of IGF-I in PTH-stimulated osteoclastlike cell formation and the role of PTH-responsive dual signal transduction systems (cyclic [c] AMP-dependent protein kinase [PKA] and calcium/protein kinase C [PKC]) in its mechanism. Treatment with anti-IGF-I antibody (1-10 micrograms/ml) partially but significantly blocked hPTH-(1-34)-stimulated osteoclastlike cell formation in unfractionated mouse bone cell cultures, although it did not affect osteoclastlike cell formation stimulated by 1,25-dihydroxyvitamin D3. Rp-cAMP5 (10(-4) M), a direct PKA inhibitor, as well as two types of PKC inhibitors, H-7 (10 microM) and staurosporine (3 nM), and dantrolene (10(-5) M), an inhibitor of calcium mobilization from intracellular calcium stores, all significantly blocked PTH-stimulated osteoclastlike cell formation. Anti-IGF-I antibody (3 micrograms/ml) significantly blocked osteoclastlike cell formation stimulated by 10(-4) M dbcAMP, 10(-4) M Sp-cAMPS, a direct PKA activator, and 10(-5) M forskolin in mouse bone cell cultures. Dibutyryl cAMP, forskolin, and hPTH-(1-34) significantly stimulated mRNA expression of both IGF-I and IGF-binding protein 5 (IGFBP-5) in these cultures, but neither 10(-7) M PMA, a PKC activator, nor 10(-7) M A23187 did. Moreover, anti-IGF-I antibody significantly blocked osteoclastlike cell formation stimulated by the conditioned medium from MC3T3-E1 cells pretreated with 10(-8) PTH-(1-34), which induced IGF-I and IGFBP-5 mRNA expression in these cells. In conclusion, the present study indicates that IGF-I mediates osteoclastlike cell formation stimulated by PTH and that the PKA pathway is involved in its mechanism. However, IGF-I does not seem to be the sole effector molecule to be active in this system.

Dietary lipids modulate bone prostaglandin E2 production, insulin-like growth factor-I concentration and formation rate in chicks

This study examined the effects of dietary fat on the fatty acid composition of liver and bone, and on the concentration of insulin-like growth factor-I (IGF-I) in liver and bone, as well as the relationship of these factors to bone metabolism. Day-old male broiler chicks were given a semipurified diet containing one of four lipid sources: soybean oil (SBO), butter+corn oil (BC), margarine+corn oil (MAC), or menhaden oil+corn oil (MEC) at 70 g/kg of the diet. At 21 and 42 d of age, chicks fed MEC had the highest concentration of (n-3) fatty acids [20:5(n-3), 22:5(n-3) and 22:6(n-3)] in polar and neutral lipids of cortical bone but the lowest amount of 20:4(n-6) in polar lipids. Diets containing t-18:1 fatty acids (MAC and BC) resulted in t18:1 accumulation in bone and liver. Bone IGF-I concentration increased from 21 to 42 d in chicks given the SBO and BC diets. Tibial periosteal bone formation rate (BFR) was higher in chicks given BC compared with those consuming SBO and MEC at 21 d. The higher BFR and concentrations of hexosamine in serum and IGF-I in cartilage, but lower 20:4(n-6) content in bone polar lipids in chicks given BC compared with those given SBO suggest that BC optimized bone formation by altering the production of bone growth factors. A second study confirmed that dietary butter fat lowered ex vivo prostaglandin E2 production and increased trabecular BFR in chick tibia. These studies showed that dietary fat altered BFR perhaps by controlling the production of local regulatory factors in bone.

Regulation of PG synthase by EGF and PDGF in human oral, breast, stomach, and fibrosarcoma cancer cell lines

Prostaglandins may inhibit or promote tumor cell replication, depending on the cell system that is investigated. In our laboratory, we have established and characterized four different specific human cancer cell lines. The objectives of this study were to examine and compare the prostaglandin endoperoxide synthase (PG synthase, EC 1.14.99.1) activity of these cell lines by measuring the conversion of arachidonate to 3H-PGE2 and 3H-PGF2 alpha. We found that the oral epidermal carcinoma cell line (OEC-M1) had a moderate degree of PG synthase activity. Enzyme activity could be partially blocked (statistically significant) by the addition of epidermal growth factor (EGF) at 20 ng/mL and almost completely inhibited by platelet-derived growth factor at (PDGF) 20 mU/mL. By contrast, we discovered that the human breast adenocarcinoma cell line (BC-M1) did not contain significant PG synthase, and enzyme activity could be significantly activated by the addition of epidermal growth factor at 20 ng/mL and platelet-derived growth factor at 20 mU/mL. We also found that the human stomach adenocarcinoma cell line (SCM-1) had a significant amount of PG synthase activity, and these PG synthase activities were not activated or inhibited by EGF at 20 ng/mL or PDGF at 20 mU/mL. Furthermore, the human fibrosarcoma (FS-M1) cell line also contained a moderate degree of PG synthase activity, which could be significantly inhibited by PDGF at 20 mU/mL but was not inhibited by EGF at 20 ng/mL. The results suggest that EGF and PDGF may be involved in the regulation of the PG synthase activities of human oral, breast, stomach, and fibrosarcoma cancer cells.

Pertussis toxin-sensitive activation of phospholipase A2 can be resolved from phosphoinositidase C in primary cultures of mouse osteoblasts using indomethacin

Recent work has established that various bone-resorbing hormones are able to activate phosphoinositide metabolism as well as eicosanoid production in osteoblast-like cells, although the relationship between these pathways is unclear. We used pertussis toxin and indomethacin to inhibit the stimulation of [3H]arachidonic acid release and [3H]phosphoinositide turnover caused by treating primary cultures of mouse osteoblasts with fetal calf serum. We found (1) that pertussis toxin and indomethacin each inhibited both pathways and (2) that although pertussis toxin inhibited [3H]arachidonic acid release to a greater extent than indomethacin, [3H]inositol phosphate accumulation was inhibited rather more effectively by indomethacin. These data suggest that whereas ligands in fetal calf serum activate [3H]arachidonic acid release largely directly via the action of a pertussis-sensitive G protein, activation of phosphoinositidase C is indirect, being substantially dependent upon eicosanoid production. These experiments suggest that serial activation of phospholipase A2 and phosphoinositidase C may occur in osteoblasts and that only the former enzyme is regulated by a pertussis toxin-sensitive G protein.

A model system for studying nutritional interventions on colon tumor growth: effects of marine oil

Lipid nutrition effects were evaluated on the growth of a transplantable colon tumor (CT-26) at various sites in the BALB/c mouse. CT-26 implanted into the back or flank of these mice grew well independent of the quality or quantity of fat in the diet. However, when implanted in the mid-portion of the descending colon, tumor growth was related to the level of dietary saturated (coconut oil) or n-6 unsaturated (safflower oil) fat in the diet. Similar findings were obtained when the tumor was utilized in a pulmonary colonization assay. Dietary marine oil (mainly EPA, and DHA n-3 polyunsaturated oils) was found to markedly impair the growth of CT-26 implanted in the bowel and lung, but not in the back. Thus, CT-26 exhibits nutrition responsiveness at certain sites, but not at others. This may help to explain contradictory findings concerning dietary lipids in certain studies. Inhibition of tumor growth by marine oils may afford preventive or chemotherapeutic implications as its mode of action unfolds. Histologic findings in bowel tumors from mice fed marine oil but not other oils revealed focal areas of necrosis. It is appreciated that arachidonate metabolism is competitively interfered with by EPA in both cyclooxygenase and lipoxygenase pathways. The possibility is raised that the metabolism of marine oils in this model system may generate lipid peroxidation products to a greater extent than n-6 lipids and in turn is associated with focal areas of necrosis. A model system of nutritionally non-responsive and nutritionally responsive sites for the post-promotional growth of a bowel tumor affords the opportunity to explore lipid effects with control and test tumors in hosts fed identical lipid nutriture.

Purinergic regulation of cytosolic calcium and phosphoinositide metabolism in rat osteoblast-like osteosarcoma cells

We have shown that ATP increases cytosolic Ca2+ in UMR-106 cells through P2-purinergic receptor stimulation (Calcif Tissue Int 45:251-254). This response was further characterized using cells loaded with indo-1/AM or prelabeled with [3H]inositol. ATP elicited a rapid transient increase in Ca2+ from 148 to 540 nM, followed by a biphasic decline (first rapid and then slower) to basal within 1 minute and then a late slow rise to 200 nM by 4 minutes. ADP also elicited a rapid transient increase, but this was followed by a second smaller transient and a later, slow increase above basal Ca2+. These transient increases in Ca2+ induced by ATP and ADP were dose dependent, detected at 10(-6)M ATP and 10(-7)M ADP, and saturated at 10(-4)M with both nucleotides. The maximum increase in Ca2+ was 20% greater with ATP than ADP. EGTA chelation of extracellular Ca2+ abolished the biphasicity of the ATP-induced Ca2+ transient, the second ADP-induced transient, and all late slower increases in Ca2+. Desmethoxyverapamil pretreatment attenuated the biphasicity of the ATP-induced transient and the second peak elicited by ADP. Elevated extracellular Ca2+ (5 mM) prevented the return to the basal level that normally follows the ATP-induced Ca2+ transient and amplified the sustained increase in Ca2+ but had little effect on the response to ADP. IP3 and IP4 increased rapidly after addition of ATP, with I(1,4,5)P3 increasing before I(1,3,4)P3. These data indicate that P2-purinergic stimulation of UMR-106 cells causes three consecutive responses in cytosolic Ca2+: (1) a transient increase due to IP3-mediated mobilization of intracellular Ca2+; (2) a transient increase due in part to influx, probably associated with a Ca2+ channel; and (3) a later sustained increase that requires extracellular calcium.

Homeostatic control of bone structure: an application of feedback theory

The regulation of bone mass and structure in the weight-bearing skeleton is governed to a great extent by the mechanical demands placed upon the bone tissue. The apparent biological goal is the maintenance of a minimum adequate structure, in which the margin of safety between normal mechanical demands and fracture is balanced by the cost of excessive bone mass on mobility. Frost has developed two powerful postulates concerning bone adaptation: (a) there exist threshold levels of mechanical strain, above or below which bone adaptation is turned on, and (b) the set point for normal bone structure can be modulated by hormones. A model was developed, using Frost's postulates and simple feedback theory, that describes the interaction between biochemical influences and mechanical influences on bone structure. The model predicts that biochemical agents that influence bone structure independently of the mechanical feedback system (e.g., calcitonin) are capable of only limited anabolic effects on bone mass because their influences conflict with mechanical influences. However, biochemical agents that influence bone structure by changing the set point of the mechanical feedback system (e.g., estrogen) will provide lasting changes in bone structure. Age-related changes occur within the effector and transduction components of the mechanical feedback system that tend to make it sluggish. These changes may lead to increased bone fragility because the system is no longer capable of maintaining adequate bone structure.

H(+)-stimulated release of prostaglandin E2 and cyclic adenosine 3',5'-monophosphoric acid and their relationship to bone resorption in neonatal mouse calvaria cultures

The addition of protons to the medium of neonatal mouse calvaria cultures stimulated bone resorption and release of calcium into the medium. In addition, added protons significantly increased the release of prostaglandin E2 (PGE2) and cyclic adenosine 3',5'-monophosphoric acid (cAMP) from the bones. Indomethacin significantly inhibited the release of calcium, PGE2 and cAMP from proton-treated cultures. The positive control, parathyroid hormone (PTH)-treated cultures, also gave rise to bone resorption and calcium release into the medium. However, unlike the addition of protons, the addition of PTH did not stimulate PGE2 release nor did indomethacin inhibit calcium release from PTH-treated cultures. In addition, indomethacin only slightly inhibited cAMP release from PTH-treated cultures, as compared to the marked inhibition by indomethacin of cAMP release from proton-treated cultures. These findings indicate that bone resorption due to added protons is dependent on both PGE2 and cAMP production, whereas bone resorption due to PTH only involves cAMP production.

Parathyroid hormone, but not prostaglandin E2, changes the shape of osteoblasts maintained on bone in vitro

Parietal bones from 2-week-old rats were dissected free from the sutural regions, dura mater, and periosteum, leaving the surface covered with osteoblasts and some osteoclasts. Prostaglandin (PG) production by these "stripped" bones under basal conditions and after exposure to parathyroid hormone (PTH) was measured by radioimmunoassay of the culture medium (minimum essential medium with or without added 10% heat-inactivated fetal calf serum). Cultured specimens were examined by scanning electron microscopy for changes in osteoblast length, orientation, ruffling, and overlap. As demonstrated previously, PTH caused the osteoblasts to elongate, align, and show fewer ruffles compared to controls. PTH increased PG synthesis by the stripped bones. Indomethacin inhibited PG formation but did not affect the osteoblast shape change. PGE2, indomethacin, or both drugs together had no discernible effect on any morphologic features. These findings indicate that PGE2 does not change osteoblast shape and that the cell shape change with PTH is not mediated by endogenous prostanoids.

Restoration of axial and appendicular bone volumes by h-PTH(1-34) in parathyroidectomized and osteopenic rats

Effects of h-PTH(1-34) were histomorphometrically evaluated in both cancellous and cortical bone tissues of axial and appendicular skeletons of parathyroidectomized and osteopenic rats. Osteopenia was induced by hemicordotomy immobilization and by estrogen depletion as a result of ovariectomy. The rats received intramuscular injections of 50 units (15 micrograms) of h-PTH(1-34) six times weekly during weeks 11 through 18 of the experiment. Significant restoration of cancellous and cortical bone volumes was observed in axial and appendicular skeletons of animals treated with h-PTH(1-34). The stimulatory effects of h-PTH(1-34) on osteoid surface, independent of eroded surface, in cancellous and cortical bone tissues, were clearly observed histomorphometrically. Mineralizing surface, mineral apposition rate and bone formation rate in cancellous bone tissues were markedly increased by h-PTH(1-34). It should be noted that h-PTH(1-34) increased trabecular thickness, but did not increase trabecular number. In conclusion, h-PTH(1-34) stimulated bone formation independent of bone resorption, and restored cancellous and cortical bone volumes in parathyroidectomized and osteopenic rats.

Neurotransmitter regulation of cytosolic calcium in osteoblast-like bone cells

The nervous system may play a role in regulation of bone metabolism. The effects of norepinephrine(NE), vasoactive intestinal peptide(VIP), and ATP on cytosolic Ca2+ were assessed in a rat osteoblast-like osteosarcoma cell line (UMR-106) responsive to PTH. All three transmitters transiently increased Ca2+, with ATP much greater than PTH greater than NE = VIP, and then caused sustained increases in Ca2+. The ATP-induced transient resulted from mobilization of intracellular Ca2+ store, while NE and VIP-induced transients also involved influx of Ca2+. Later sustained increases by all agonists were dependent upon extracellular Ca2+. Release of intracellular Ca2+ by ATP was associated with a marked increase in IP3 but without a significant change in cAMP. NE, VIP, and ATP, through regulation of Ca2+ metabolism, may be involved in various osteoporotic conditions.

Parathyroid hormone and prostaglandin E2 stimulate both inositol phosphates and cyclic AMP accumulation in mouse osteoblast cultures

Parathyroid hormone (PTH) and prostaglandin E2 (PGE2) are physiological agonists which stimulate bone cells to resorb bone, a process by which the mineralized extracellular bone matrix is dissolved. Bone resorption has a key role in the maintenance of plasma calcium levels. It has been established that both PTH and PGE2 activate adenylate cyclase in osteoblasts, but it is apparent that (1) the two agents have qualitatively different effects on osteoblasts, and (2) the generation of cyclic AMP cannot account for all the effects of PTH on bone cell metabolism. Others have demonstrated that PTH and PGE2 may also elevate intracellular calcium levels, but the mechanism by which this is achieved has not been fully defined. Here we have investigated the effects of PTH on neonatal mouse osteoblasts in culture and shown that physiological concentrations of the hormone (50 nM) caused a small increase (22%) in total inositol phosphates accumulation, with a larger increase (40%) in inositol trisphosphate. We found that this activation occurred at lower concentration than was necessary to activate adenylate cyclase. PGE2 was a more effective activator of inositol phosphates accumulation than PTH, causing up to 300% increase in the total inositol phosphates after 30 min. Both PTH and PGE2 stimulated cyclic AMP accumulation, but the activation of adenylate cyclase by forskolin did not enhance inositol phosphates production. We conclude that both PTH and PGE2 stimulate phosphoinositide turnover in mouse osteoblasts and suggest that this mechanism may contribute to their elevation of intracellular calcium in bone cells.