Weak evidence of thyrotropin receptors in primary cultures of human osteoblast-like cells

Extra-thyroidal thyrotropin (TSH) receptors (TSHRs) have been demonstrated in several tissues and cells, including human and rat osteosarcoma cell lines. We have explored whether human TSHR (hTSHRs) also are present in primary cultures of human osteoblast-like (hOB) cells. [(125) I]TSH binding was limited in hOB cells, but somewhat higher in UMR 106-01 cells and considerably higher in hTSHR-transfected CHO cells. In hOB cells, the basal intracellular cAMP levels increased 282% after stimulation with 10 U/L TSH. In the hTSHR-transfected CHO cells, the cAMP increase was 3030% in response to 10 U/L TSH and 1240% after 1 U/L TSH. Free cytoplasmic calcium did not change in response to TSH in hOB cells. HTSHR mRNA was detected in hOB cells from 3/4 bone by reverse transcriptase polymerase chain reaction RT-PCR and nucleotide sequencing HTSHR mRNA, but could not be demonstrated with the RNase protection technique in hOB cells from 5 different donors. In conclusion, even after the use of several methods, we have found only weak evidence for expression and presence of functionally active hTSHR in hOB cells. Given the low level of expression, specific binding and cAMP signaling, we suggest that it is unlikely that circulating TSH plays a physiological role for bone metabolism mediated through osteoblasts.

Extracellular calcium increases free cytoplasmic calcium and DNA synthesis in human osteoblasts

A high concentration of extracellular calcium (8 mM) induced an increase in free cytoplasmic calcium, a lower cyclic AMP level and increased DNA synthesis in primary cultures of human osteoblast-like cells. Inhibition of protein kinase C with bisindolylmaleimide I inhibited the stimulatory effect of extracellular calcium on DNA synthesis in human osteoblast-like cells, whereas inhibition of protein kinase A with Rp-cAMPs had no effect on DNA synthesis. This indicates that protein kinase C, possibly via increased free cytoplasmic calcium, mediates the effect of extracellular calcium on DNA synthesis in osteoblast-like cells rather than a relative decrease in cyclic AMP and protein kinase A activity. Furthermore, a low concentration (0.5 mM) of extracellular calcium decreased DNA synthesis. In conclusion, these data suggest that a high extracellular calcium level may be a coupling factor that recruits osteoblasts in the bone remodeling process, and that a low level of extracellular calcium may also regulate osteoblast function.

Rapid suppression of S-PTH by oral calcitriol and calcium in healthy premenopausal women

The influence of oral calcium +/- cholecalciferol or calcitriol on S-PTH and whole blood ionized calcium (B-Ca++) in the very short term has not been elucidated. B-Ca++ and S-PTH were measured after overnight fast every 5 or 15 min for 4-h in 7 healthy premenopausal women (30-45 years) in a crossover design where the subjects were studied on 4 different days. Study 1: (control), 125-ml tap water (also given in studies 2-4); study 2: 1000 mg calcium, as calcium carbonate; study 3: 1000 mg calcium and 400 IU (5 microg) cholecalciferol; and study 4: 0.5 microg calcitriol plus 1000 mg calcium. Calcium plus 1,25(OH)2 vitamin D3 induced a rapid and significant fall in S-PTH compared to the control period (p < 0.02). Calcium alone or calcium plus cholecalciferol did not change the PTH levels significantly compared to control (water). B-Ca++ increased significantly after calcium plus 1,25(OH)2 vitamin D3 (p<0.01) and calcium plus cholecalciferol (p<0.05) compared to the control period. The B-Ca++ elevation was significantly higher after calcium plus 1,25 vitamin D3 than after calcium plus cholecalciferol (p<0.05). In conclusion, oral calcitriol plus calcium causes a rapid elevation in B-Ca++ and suppression of the PTH secretion also in the short term (hours).

Interleukin-1beta upregulates PTHrP-mRNA expression and protein production and decreases TGF-beta in normal human osteoblast-like cells

Osteoblasts are regulated by complex interactions among systemic hormones, cytokines, and local growth factors. Bone resorption, at the level of the basic multicellular unit, is initiated by stimulation of osteoblast activity. The stimulatory effect of interleukin-1beta (IL-1beta) on bone resorption has not been fully clarified. We have therefore studied the influence of IL-1beta on the local production and secretion of parathyroid hormone-related protein (PTHrP) and transforming growth factor-beta (TGF-beta) from normal human osteoblast-like cells (hOB cells). Using a quantitative PCR-assay following reverse transcription of RNA, in situ hybridization, and a two-site immunofluorometric assay for PTHrP, we demonstrate that IL-1beta in a dose- and time-dependent manner increases PTHrP-mRNA expression and PTHrP-protein secretion. In addition, IL-1beta decreased the TGF-beta protein concentration in conditioned medium. Our results suggest that the actions of IL-1beta on bone may be mediated by novel mechanisms involving both local increase of PTHrP, a potent stimulator of bone resorption, and a decrease of TGF-beta, an important anabolic and coupling factor for bone turnover.

International comparison of hip fracture rates in 1988-89

A comparison of hip fracture rates among nine countries (Canada, Chile, Finland, Hong Kong, Scotland, Sweden, Switzerland, the United States and Venezuela) was made using national hospital discharge data for the same time interval. The rates increased by age and were higher for females than males in all nine countries. When based on overall discharge rates, the incidence of hip fracture appeared high in three European countries (Finland, Scotland and Sweden) relative to the other countries. However, when transfer cases were removed and adjustments made for differences in case definition, the risk of hip fracture for both men and women was much similar among the four European and two North American countries, but higher than in Hong Kong. Rates of fracture were lowest in Venezuela and Chile, varying from three to 11 times less than for residents of the other seven countries. Although there are limitations in using hospital discharge data as a measure of incidence, the wide variation in the risk of hip fracture across the nine countries appears real but differences between North American and north European countries may not be as great as previously reported. Such cross-national comparisons may help clarify different etiologic hypotheses.

Glucose turnover in hyperthyroid patients with normal glucose tolerance

To determine the diabetogenic effect(s) of thyroid hormones, we simultaneously measured glucose turnover in six hyperthyroid patients and six normal subjects. All had normal fasting blood glucose concentration and oral glucose tolerance test values. We determined hepatic total glucose output (HTGO) and total glucose phosphorylation with [2-3H]glucose and hepatic glucose production (HGP) and irreversible glucose uptake using [6-3H]glucose. The difference between the two turnover rates indicates the extent of hepatic glucose cycling (glucose in equilibrium glucose-6-phosphate). Measurements were made both in the postabsorptive steady state and during a 2-h glucose infusion (11.1 mumol/kg.min). The postabsorptive HTGO and total glucose phosphorylation were increased in the hyperthyroid patients [13.5 +/- 0.8 (+/- SE) vs. 11.3 +/- 0.4 mumol/kg.min; P less than 0.05]. HGP and irreversible glucose uptake also were slightly but not significantly higher. During the glucose infusion, HTGO and HGP were less suppressed in the hyperthyroid patients than in the normal subjects, while the increments in peripheral glucose uptake were normal. In hyperthyroidism, glucose cycling was increased both postabsorptively (2.35 +/- 0.27 vs. 1.17 +/- 0.25 mumol/kg.min; P less than 0.025) and during glucose infusion (2.57 +/- 0.34 vs. 1.31 +/- 0.35 mumol/kg.min; P less than 0.05). We conclude that increases in HTGO and HGP are important features of hyperthyroidism, especially during glucose infusion. The increase in GC indicates increased activities of both glucokinase and glucose-6-phosphatase. The diabetogenic effect of hyperthyroidism, as revealed most markedly by [2-3H]glucose, could be accounted for by augmented glucose production, possibly due to increased glucose-6-phosphatase activity.