Cyclo-oxygenase products of arachidonic acid metabolism in rat osteoblasts in culture

The metabolism of arachidonic acid to its cyclo-oxygenase products was studied in monolayer cultures of osteoblast-rich rat calvarial cells and of clonal cell lines from a rat osteogenic sarcoma, enriched in the osteoblast phenotype. Prostanoids were measured by radioimmunoassay after extraction of media and fractionation by high pressure liquid chromatography. In both normal and malignant osteoblasts the major cyclooxygenase product was 6-oxo-prostaglandin F1 alpha, the hydration product of prostacyclin, with lesser amounts of prostaglandin E2 and prostaglandin F2 alpha. No significant thromboxane B2 was detected. Prostaglandins are thought to have a local role in the regulation of bone resorption. These results point to the possible importance of prostacyclin either in bone resorption or in some other local function, e.g., regulation of bone blood flow.

Selective hormonal activation of cyclic AMP-dependent protein kinase isoenzymes in normal and malignant osteoblasts

The pattern of cyclic AMP-dependent protein kinase isoenzyme response to acute hormonal activation has been studied in cultured cells derived from rat osteogenic sarcoma and osteoblast-rich cells grown from newborn rat calvaria. Using multiple small anion exchange columns and a batch elution technique, a rapid method of separating the isoenzymes of cyclic AMP-dependent protein kinase was developed and the acute activation by parathyroid hormone and prostaglandin E2 of each isoenzyme was studied. Activation was rapid, being detectable at 5 s, maximal at 15-30 s, and persisting for up to 6 h. Both hormones showed a dose-dependent activation of each isoenzyme in both cell types, but the patterns of response differed. Parathyroid hormone predominantly stimulated isoenzyme I in the clonal osteogenic sarcoma cells but showed equivalent activation of each isoenzyme in calvarial cells. Prostaglandin E2 also predominantly stimulated isoenzyme I in the malignant cells, whereas in the calvarial strain there was a major effect on isoenzyme II with almost no stimulation of isoenzyme I. Half-maximal stimulation of cyclic AMP-dependent protein kinase in the malignant cell strain was achieved for both hormones at concentrations an order of magnitude lower than those in the normal strain. These studies demonstrate selective activation of cyclic AMP-dependent protein kinase isoenzymes by hormones. Furthermore, the nature of the response differs between the normal and the corresponding neoplastic cell types for the same hormone stimulus.

Activity ratio measurements reflect intracellular activation of adenosine 3',5'-monophosphate-dependent protein kinase in osteoblasts

Parathyroid hormone, prostaglandin E2, and prostacyclin activate cAMP-dependent protein kinase in osteoblast-rich normal rat calvarial cells and in clonal rat osteogenic sarcoma cells of osteoblastic phenotype. The present study was undertaken to determine the activation of the enzyme in relation to cellular cAMP concentrations at increasing doses of the three hormones and also to test that the activity ratio measurement of the enzyme (ratio of the activity in the absence of cAMP to the activity in the presence of excess cAMP) was a true reflection of intracellular activation of the enzyme. With each hormone, using either normal or malignant osteoblasts, activation of the enzyme took place at hormone concentrations lower than those required to produce detectable changes in cAMP concentrations in the incubations. Stimulation of activity was abolished by addition of the heat-stable inhibitor of cAMP-dependent protein kinase, indicating that activation was of cAMP-dependent protein kinase alone. To demonstrate that protein kinase activation occurred intracellularly and not during sample preparation, charcoal was added at the time of cell disruption to absorb free cAMP. Under these conditions, no change was observed in the concentration of bovine parathyroid hormone required to cause activation of cAMP-dependent protein kinase. Finally, addition of purified cAMP-dependent protein kinase type I or type II to treated cells at the time of lysis did not result in significant activation of added isoenzyme, except at hormone concentrations sufficient to increase the total cAMP concentration of incubations. It is concluded that activity ratio measurement reflects the intracellular state of activation of cAMP-dependent protein kinase in the osteoblast-like cells treated by hormones and, furthermore, that only a fraction of the maximally generated cAMP is necessary for full enzyme activation.

Adenylate cyclase responsiveness of human insulinomas

Adenylate cyclase activity was assayed in a crude particulate fraction of one benign and one malignant human insulinoma. Adenylate cyclase of both tumours responded to 5'-guanylyl-imidodiphosphate, sodium fluoride, glucagon and prostaglandin E2, and in addition the adenylate cyclase of the benign tumour responded to isoprenaline. Glucose and prostaglandin I2 (prostacyclin) did not stimulate the adenylate cyclase in either tumour, although prostaglandin I2 stimulated insulin secretion in cultures of the benign tumour. The in vitro responsiveness of the adenylate cyclase to glucagon did not correlate closely with the effect of glucagon on insulin secretion in vivo.

Activation of adenosine 3',5'-monophosphate-dependent protein kinase in normal and malignant bone cells by parathyroid hormone, prostaglandin E2, and prostacyclin

Hormonal activation of cAMP-dependent protein kinase has been studied in cultured cells derived from a rat osteogenic sarcoma and in osteoblast-rich cells grown from newborn rat calvaria. Both cell strains contain adenylate cyclase activities which respond to parathyroid hormone (PTH) and a variety of prostanoids. PTH, prostaglandin E2 (PGE2), and prostacyclin (PGI2) were all capable of activating cAMP-dependent protein kinase(s) in suspensions of the two cell types. Activation was very rapid in all cases, being detectable at 10 sec and maximal between 30-60 sec. Using saturating concentrations of hormones, the protein kinase activity ratio remained elevated (between 0.6-0.9) for up to 35 min after the start of PGE2 stimulation, but declined toward basal activity ratio 5-10 min after stimulation with PTH or PGI2. Each of the hormones caused a dose-dependent increase in activation of cAMP-dependent protein kinase in both cell types. Half-maximal activation of the enzyme occurred at 2 X 10(-9) M bovine PTH for calvarial cells, at 10(-8) M bPTH for osteogenic sarcoma cells, and at 2-4 X 10(-8) M PGE2 and 1-3 X 10(-7) M PGI2 for both cell types. Maximal activation of protein kinase occurred before maximal cAMP accumulated, implying that only a fraction of cAMP is biologically significant. These two cell strains provide a useful means of analyzing postreceptor events in the hormonal regulation of bone cells.

Functional properties of hormonally responsive cultured normal and malignant rat osteoblastic cells

Certain metabolic properties of hormonally responsive osteogenic sarcoma cells derived from a transplantable rat tumor have been compared with those of related normal rat bone cells. All studies were carried out on cells grown in monolayer culture. Normal rat bone cells derived by repeated collagenase/trypsin digestion of newborn rat calvaria. Bone cells selected for comparison were thought to be osteoblast-like, as judged by enrichment of alkaline phosphatase and adenylate cyclase responsiveness to parathyroid hormone and prostaglandin E2. The adenylate cyclases of the two cell strains were similarly stimulated by a range of prostanoids and their metabolites and analogs. Morphology showed the two cell strains to be similar; the only obvious difference was a multilayering of cells in the sarcoma cultures, while the normal cultures showed abundant extracellular fibril formation which was not seen in the tumor cells. Investigation of the cAMP-dependent protein kinase isoenzymes showed the presence of two forms in both cell types, one eluting at a low salt concentration and the other at a high salt concentration. There was approximately twice the amount of the first isoenzyme compared to the second isoenzyme. The results indicate the usefulness of the two cell strains to elucidate further the molecular mechanisms of action of parathyroid hormone and prostaglandins.

Insulin inhibition of protein degradation in cell monolayers

Protein degradation has been measured in confluent monolayers of eleven lines of contact-inhibited cells and ten transformed lines as the rate of release of trichloroacetic acid-soluble radioactivity after prelabeling cell protein with [3H]leucine. Insulin, at contrations from 10(-12) M to 10(-6) M, has been added at the beginning of the 4-hour degradation period to detect selective effects of this hormone as an inhibitor of the inducible proteolysis occurring in serum-free medium. In addition insulin binding measurements have been performed on selected cell lines in an attempt to relate receptor properties to insulin action. Substantial effects of insulin are found in most cells with a selective inhibition at low insulin concentrations noted in several of the transformed lines. The difference in insulin sensitivity is not entirely definitive because temperature-sensitive transformation mutants of NRK cells are not more sensitive to insulin at a temperature where they show the transformed phenotype. Although insulin receptors on different cell lines have similar binding properties, two of the hepatomas used, H35 and MH1C1, show inhibition of protein degradation at insulin concentrations where receptor occupancy is extremely low. Calvarial osteoblast-like cells have a high rate of protein degradation which can be reduced by growth factors but not by insulin. The lack of an insulin response is a consequence of poor insulin binding to the cells. Insulin binds to the osteogenic sarcoma cells in substantial amounts. However, its normal action to inhibit the induced proteolysis is restricted because with these cells no increase of proteolysis occurs in serum-free medium. Generally higher rates of protein degradation are observed in the contact-inhibited lines than the transformed cells. We suggest that this difference may provide a selective growth advantage to transformed cells.

Premature induction of glucokinase in the neonatal rat by thyroid hormone

1. It was shown that the development of liver glucokinase in the rat coincided with a peak in the levels of circulating thyroid hormone at about the 16th postnatal day. 2. Administration of thyroid inhibitors blocked the development of the enzyme and administration of thyroid hormone restored activity to normal levels. 3. Glucokinase could be induced prematurely as early as the 2nd postnatal day by the administration of thyroid hormone followed by daily injection of glucose (10 mg/g body weight). 4. Glucocorticoids and corticotropin failed to induce glucokinase activity prematurely. 5. The postnatal increase in circulating thyroid hormone levels together with increased intake of carbohydrate at weaning may be the normal physiological stimulus for induction of this enzyme.