Relationships between histomorphometric features of bone formation and bone cell characteristics in vitro in renal osteodystrophy

Changing bone patterns with progression of chronic kidney disease

It is commonly held that osteitis fibrosa and mixed uremic osteodystrophy are the predominant forms of renal osteodystrophy in patients with chronic kidney disease. Osteitis fibrosa is a high-turnover bone disease resulting mainly from secondary hyperparathyroidism, and mixed uremic osteodystrophy is in addition characterized by a mineralization defect most often attributed to vitamin D deficiency. However, there is ancient and more recent evidence that in early chronic kidney disease stages adynamic bone disease characterized by low bone turnover occurs first, at least in a significant proportion of patients. This could be due to the initial predominance of bone turnover-inhibitory conditions such as resistance to the action of parathyroid hormone (PTH), reduced calcitriol levels, sex hormone deficiency, diabetes, and, last but not least, uremic toxins leading to repression of osteocyte Wnt/β-catenin signaling and increased expression of Wnt antagonists such as sclerostin, Dickkopf-1, and sFRP4. The development of high-turnover bone disease would occur only later on, when serum PTH levels are able to overcome peripheral PTH resistance and the other inhibitory factors of bone formation. Whether FGF23 and Klotho play a direct role in the transition from low- to high-turnover bone disease or participate only indirectly via regulating PTH secretion remains to be seen.

Primary osteoblast-like cells from patients with end-stage kidney disease reflect gene expression, proliferation, and mineralization characteristics ex vivo

Osteocytes regulate bone turnover and mineralization in chronic kidney disease. As osteocytes are derived from osteoblasts, alterations in osteoblast function may regulate osteoblast maturation, osteocytic transition, bone turnover, and skeletal mineralization. Thus, primary osteoblast-like cells were cultured from bone chips obtained from 24 pediatric ESKD patients. RNA expression in cultured cells was compared with RNA expression in cells from healthy individuals, to RNA expression in the bone core itself, and to parameters of bone histomorphometry. Proliferation and mineralization rates of patient cells were compared with rates in healthy control cells. Associations were observed between bone osteoid accumulation, as assessed by bone histomorphometry, and bone core RNA expression of osterix, matrix gla protein, parathyroid hormone receptor 1, and RANKL. Gene expression of osteoblast markers was increased in cells from ESKD patients and signaling genes including Cyp24A1, Cyp27B1, VDR, and NHERF1 correlated between cells and bone cores. Cells from patients with high turnover renal osteodystrophy proliferated more rapidly and mineralized more slowly than did cells from healthy controls. Thus, primary osteoblasts obtained from patients with ESKD retain changes in gene expression ex vivo that are also observed in bone core specimens. Evaluation of these cells in vitro may provide further insights into the abnormal bone biology that persists, despite current therapies, in patients with ESKD.

Abnormalities of insulin-like growth factor-I signaling and impaired cell proliferation in osteoblasts from subjects with osteoporosis

IGF-I regulates bone acquisition and maintenance, even though the cellular targets and signaling pathways responsible for its action in human bone cells are poorly understood. Whether abnormalities in IGF-I action and signaling occur in human osteoblasts under conditions of net bone loss has not been determined. Herein we carried out a comparative analysis of IGF-I signaling in primary cultures of human osteoblasts from osteoporotic and control donors. In comparison with control cells, osteoporotic osteoblasts showed increased tyrosine phosphorylation of the IGF-I receptor in the basal state and blunted stimulation of receptor phosphorylation by IGF-I. Augmentation of basal IGF-I receptor phosphorylation was associated with coordinate increases in basal tyrosine phosphorylation of insulin receptor substrate (IRS)-2 and activation of Erk, which were also minimally responsive to IGF-I stimulation. By contrast, phosphorylation levels of IRS-1, Akt, and glycogen synthase kinase-3 were similar in the basal state in control and osteoporotic osteoblasts and showed marked increases after IGF-I stimulation in both cell populations, even though these responses were significantly lower in the osteoporotic osteoblasts. The IGF-I signaling abnormalities in osteoporotic osteoblasts were associated with reduced DNA synthesis both under basal conditions and after stimulation with IGF-I. Interestingly, treatment of the osteoporotic osteoblasts with the MAPK kinase inhibitor PD098059 reduced the elevated levels of Erk phosphorylation and increased basal DNA synthesis. Collectively, our data show that altered osteoblast proliferation in human osteoporosis may result from dysregulation of IGF-I receptor signaling, including constitutive activation of the IRS-2/Erk signaling pathway, which becomes unresponsive to IGF-I, and defective induction of the IRS-1/Akt signaling pathway.

Decreased in vitro osteoblast proliferation and low turnover bone disease in nonuremic proteinuric patients

Patients with proteinuria, even those with normal glomerular filtration rate, often present abnormal bone histology. We evaluated bone histology and the in vitro proliferation of osteoblasts in samples obtained from 17 proteinuric patients with primary glomerulopathies. Histomorphometric analysis of bone biopsies was performed, and bone fragments were obtained for osteoblast culture, in which we evaluated cell proliferation. In comparison to controls, patients presented lower trabecular bone volume (20.9+/-14.5% vs 26.8+/-5.9%; P=0.0008); lower trabecular number (1.7+/-0.2/mm vs 2.0+/-0.3/mm; P=0.004); and greater trabecular separation (475.5+/-96.4 microm vs 368.3+/-86.2 microm, P=0.0002). We also found alterations in bone formation and resorption: lower osteoid volume (0.9+/-0.7% vs 2.0+/-1.4%; P=0.0022); lower osteoid thickness (6.4+/-2.8 microm vs 11.5+/-3.2 microm; P<0.0001); less mineralizing surface (4.6+/-3.1% vs 13.5+/-6.0%; P<0.0001); lower bone formation rate (0.03+/-0.04 microm(3)/microm(2)/day vs 0.09+/-0.05 microm(3)/microm(2)/day; P<0.0001); and greater osteoclast surface (0.35+/-0.6 vs 0.05+/-0.1%, P=0.0016). Mean in vitro osteoblast proliferation was lower in patients than in controls (910.2+/-437.1 vs 2261.0+/-1121.0 d.p.m./well, P=0.0016). Serum concentrations of 25-hydroxyvitamin-D(3) correlated negatively with proteinuria and positively with in vitro osteoblast proliferation. Our results demonstrate that nonuremic proteinuric glomerulonephritic patients present bone structure disorder, low bone formation and high bone resorption, as well as low osteoblast proliferation.

Diagnosis of renal osteodystrophy

The idiom renal osteodystrophy (ROD) represents a heterogeneous pattern of bone disturbances caused by chronic renal insufficiency and concomitant diseases. For the clinical decision of therapy it is most important to differentiate between high and low or adynamic turnover ROD because the therapeutically consequences of these two ends of the ROD spectrum are fundamentally different. Bone histology remains the gold standard for the exact classification of ROD. Serological markers of bone metabolism are not suited for the accurate nomenclature of ROD but are useful for the sequential follow up of ROD after a clear diagnosis has been made. Similarly, radiological diagnosis of ROD using dual energy X-ray absorptiometry (DEXA) or quantitative computer tomography scan (q-CT) is inaccurate and thus more suited for the routine follow up of established disease. Besides mineralization, bone strength and the rate of fractures are strongly determined by the architecture of the bone matrix. This information, however, is also only available on bone biopsy sections and cannot be estimated by non-invasive diagnostic methods. In summary, bone biopsy should be used more liberally for correct classification of bone disease. The sequential follow up and guidance of therapy success can be performed by non-invasive procedures such as biochemical bone marker determination in blood. X-ray imaging and densitometry is suitable only for sequential evaluation of osteoporosis.

Cultures of human osteoblastic cells from dialysis patients: influence of bone turnover rate on in vitro selection of interleukin-6 and osteoblastic cell makers

The factors contributing to renal osteodystrophy are still incompletely characterized. A variety of cytokines and growth factors appear to have ill-defined roles in this disease. Our aim is to compare osteoblastic cell growth and different osteoblastic markers in vitro with histomorphometric bone parameters and some serum bone-turnover markers in vivo in dialysis patients with either high- (HTBD) or low-turnover (LTBD) bone disease. Six patients were diagnosed to have LTBD, and another five patients, HTBD. Intact parathyroid hormone (PTH) and osteocalcin (OC) levels in serum were greater in patients with HTBD than in those with LTBD. Osteoblastic cells isolated from iliac crest biopsy specimens were grown in culture medium for different times up to 13 days. Osteoblastic cell growth (cell number and area under the cell growth curve) was greater in patients with HTBD than in those with LTBD. Static and dynamic bone formation parameters correlated with serum PTH levels. No correlation was found between PTH and osteoblastic cell proliferation. OC, C-terminal type I procollagen, and alkaline phosphatase osteoblastic secretion in vitro were similar in the HTBD and LTBD groups. However, interleukin-6 (IL-6) secretion was greater in cells isolated from patients with LTBD. Our results indicate that osteoblastic cell growth and osteoblastic IL-6 secretion are related to bone turnover in patients with osteodystrophy. Our findings support the hypothesis that factors other than PTH level might have an important role in affecting osteoblastic function in renal osteodystrophy.

Low bone turnover in patients with renal failure

Renal failure inevitably leads to metabolic bone disease. Low turnover disease or adynamic bone disease (ABD) is characterized by a low number of osteoblasts with normal or reduced numbers of osteoclasts. Mineralization proceeds at a normal rate, resulting in normal or decreased osteoid thickness. Recently, it became clear that the relative contribution of the various types of renal osteodystrophy (ROD) to the spectrum of the histologic picture in renal failure patients underwent profound changes during the last 25 years. At the moment, the exact physiopathological mechanisms behind ABD are not yet elucidated, and thus the reason(s) for its increasing prevalence remains poorly understood. A number of epidemiological and experimental data suggest a multifactorial pathophysiologic process, in which hypoparathyroidism and suppression of the osteoblast are the main actors. Compared to adynamic bone disease, osteomalacia has now become a much rarer disease (around 4%), at least in Western countries. On the other hand, recent studies indicate that this particular bone disease entity might still regularly occur in less developed countries. Osteomalacia originates from a direct effect on the mineralization process. With this type of renal bone disease, the effects of secondary hyperparathyroidism on bone are overridden by a number of metabolic abnormalities that finally result in a defective bone mineralization, as occurs, for instance, when the lag time between osteoid deposition and its mineralization is increased. The relationship between exogenous and endogenous vitamin D deficiency (mainly calcitriol) and the histologic finding of osteomalacia in uremic patients is well known. Recent data showed distinctly lowered 25-(OH) vitamin D3 levels in the presence of unaffected calcitriol concentrations in patients with osteomalacic lesions, as assessed radiologically by the presence of Looser's zones. Recently, we found that bone strontium levels were increased in patients with osteomalacia as compared to all other types of ROD. Strontium accumulation appeared to originate mainly from the use of strontium-contaminated dialysate, which resulted from the addition of strontium-containing acetate-based concentrates. Evidence for a causal role of the element in the development of a mineralization defect could be tested experimentally by adding strontium to drinking water in a chronic renal failure rat model.

A chimeric form of osteoprotegerin inhibits hypercalcemia and bone resorption induced by IL-1beta, TNF-alpha, PTH, PTHrP, and 1, 25(OH)2D3

Osteoprotegerin (OPG) is a secreted protein that inhibits osteoclast formation and activity and appears to be a critical regulator of bone mass and metabolism. In the current study, mice were challenged with various cytokines and hormones (interleukin-1beta, tumor necrosis factor-alpha, parathyroid hormone, parathyroid hormone-related protein, and 1alpha,25-dihydroxyvitamin D3) that are known to increase bone resorption and cause hypercalcemia and treated concurrently with either a recombinant chimeric Fc fusion form of human OPG, with enhanced biological activity (cOPG) (2.5 mg/kg/day) or vehicle. Mice receiving these bone-resorbing factors became hypercalcemic by day 3 after commencing treatment and had increased bone resorption as evidenced by elevated osteoclast numbers on day 5. Concurrent cOPG treatment prevented hypercalcemia (p < 0.05) and maintained osteoclast numbers in the normal range (p < 0.001). The demonstration that cOPG can inhibit bone resorption suggests that this molecule may be useful in the treatment of diseases including hyperparathyroidism, humoral hypercalcemia of malignancy, osteoporosis, and inflammatory bone disease, which are characterized, in part, by increases in osteoclastic bone resorption.

Circulating biochemical markers of bone remodeling in uremic patients

Chronic renal failure is often associated with bone disorders, including secondary hyperparathyroidism, aluminum-related low-turnover bone disease, osteomalacia, adynamic osteopathy, osteoporosis, and skeletal beta2-microglobulin amyloid deposits. In spite of the enormous progress made during the last few years in the search of noninvasive methods to assess bone metabolism, the distinction between high- and low-turnover bone diseases in these patients still frequently requires invasive and/or costly procedures such as bone biopsy after double tetracycline labeling, scintigraphic-scan studies, computed tomography, and densitometry. This review is focused on the diagnostic value of several new serum markers of bone metabolism, including bone-specific alkaline phosphatase (bAP), procollagen type I carboxy-terminal extension peptide (PICP), procollagen type I cross-linked carboxy-terminal telopeptide (ICTP), pyridinoline (PYD), osteocalcin, and tartrate-resistant acid phosphatase (TRAP) in patients with chronic renal failure. Most of the observations made by several groups converge to the conclusion that serum bAP is the most sensitive and specific marker to evaluate the degree of bone remodeling in uremic patients. Nonetheless, PYD and osteocalcin, in spite of their retention and accumulation in the serum of renal insufficient patients, are also excellent markers of bone turnover. The future generalized use of these markers, individually or in combination with other methods, will undoubtedly improve the diagnosis and the treatment of the complex renal osteodystrophy.

Influence of aluminum on the regulation of PTH- and 1,25(OH)2D3-dependent pathways in the rat osteosarcoma cell line ROS 17/2.8

The role of hormonal status in the development of aluminum (Al)-dependent renal osteodystrophy, which is characterized by reduced bone matrix deposition, still remains largely unknown. To address this question, we used the osteoblast-like osteosarcoma cell line ROS 17/2.8 to evaluate the role of Al on parathyroid hormone (PTH)- and 1,25-dihydroxyvitamin D3 (1,25(OH)2D3)-dependent activities in these cells. Al (1 microM) caused an inhibition of basal and 1,25(OH)2D3-induced alkaline phosphatase, but only at low doses (< 1 nM) of the steroid. Al partly inhibited basal osteocalcin (OC) secretion in ROS cells (p < 0.001), and the dose-dependent increase in 1,25(OH)2D3-induced OC release by these cells was also reduced by 1 microM Al at low concentrations of the steroid (< or = 1 nM), whereas high doses of 1,25(OH)2D3 (> or = 5 nM) totally prevented the inhibiting effects of Al. Al also had strong inhibitory actions on PTH-dependent cAMP production by ROS cells over the concentration range tested (0.5-50 nM). This inhibitory action of Al was also observed for PTH-related peptide- (PTHrp, 50 nM) but not for Isoproterenol-dependent (100 nM) cAMP formation. To evaluate more fully the mechanism of this inhibition of cAMP formation, we investigated the effect of Al on toxin-modulated, G protein-dependent regulation of cAMP formation and on the activation of adenylate cyclase by Forskolin. Cholera toxin (CT, 10 micrograms/ml), applied to cells for 4 h prior to PTH challenge, enhanced cAMP production about 2-fold above PTH alone (p < 0.001), a process that was further stimulated by Al. Pertussis toxin (PT, 1 microgram/ml, 4 h) did not modify basal PTH-dependent cAMP formation by ROS cells. However, PT treatment prevented the inhibitory effect of Al on cAMP formation by these cells (p < 0.025). The stimulation of adenylate cyclase by Forskolin (0.1 and 1 microM), which bypasses G protein regulation, was not modified by Al, indicating that Al does not affect adenylate cyclase directly. Northern blot analysis of PTH receptor mRNA levels showed that Al did not modify PTH receptor message in ROS cells. Likewise, Western blot analyses of G protein subunits showed that Al did not significantly alter Gs alpha subunit levels, in accordance with the results obtained for cAMP-dependent formation in response to CT. In contrast, Gi alpha-1 and Gi alpha-2 subunits were decreased by Al treatment, consistent with PT-restricted increases in cAMP formation in Al-treated ROS cells. Taken together, these results suggest that Al has multiple actions in osteoblast-like ROS cells. The effects of Al are modulated by hormonal control of the pathways investigated. Al affects 1,25(OH)2D3-regulated functions only when this steroid is low. Al has large inhibitory effects on PTH- and PTHrp-dependent cAMP formation. This last feature is related to the ability of Al to alter the G protein transducing pathway for PTH/PTHrp-dependent formation of cAMP since it does not affect adenylate cyclase activity directly and does not affect the PTH receptor message level. Thus, Al has stronger deleterious effects in osteoblast-like cells with an already compromised 1,25(OH)2D3 status and can modulate specifically PTH/PTHrp-mediated cAMP formation at the postreceptor level.

1,25-dihydroxyvitamin D3-mediated transforming growth factor-beta release is impaired in cultured osteoblasts from patients with multiple pituitary hormone deficiencies

To evaluate the osteoblastic function in patients with multiple pituitary hormone deficiencies (M-PHD) and with isolated growth hormone deficiency (I-GHD), bone cells were cultured and the effects of 10(-8) M 1,25-dihydroxyvitamin D3 (1,25[OH]2D3) on parameters of cell proliferation, osteoblastic differentiation, and local paracrine regulation were measured. Three days of 1,25(OH)2D3 treatment increased alkaline phosphatase activity and osteocalcin release but inhibited [3H]thymidine incorporation in all cell cultures from patients as well as from controls. In addition, 1,25(OH)2D3 increased the release of both total and active transforming growth factor-beta (TGF-beta) in bone cells from controls by, respectively, 4.9- and 3.2-fold and in bone cells from I-GHD by 5.1- and 1.5-fold, respectively. However, in bone cells from M-PHD, the stimulation of total TGF-beta release was significantly lower (1.3-fold) than in control and I-GHD cells, and active TGF-beta release was not stimulated at all. One year of supplementation with human growth hormone did not improve this deficient TGF-beta release in bone cells from M-PHD. We conclude that cultured bone cells from I-GHD and M-PHD show a normal response to 1,25(OH)2D3 regarding cell proliferation and osteoblastic differentiation, which implicates a normal 1,25(OH)2D3-receptor function. In cells from controls and I-GHD, 1,25(OH)2D3 enhanced both total and active TGF-beta release. However, bone cells from M-PHD showed a deficient TGF-beta response to 1,25(OH)2D3. These results suggest that the regulation of TGF-beta production is a major paracrine factor involved in hypopituitarism.

The renal bone diseases in children treated with dialysis

Renal osteodystrophy represents a spectrum from high- to low-turnover bone lesions. The specific pattern, however, may change during selected therapeutic interventions. As in the past, osteitis fibrosa remains the most frequent histologic lesion in pediatric patients on dialysis, although recently the prevalence of low-turnover bone lesions without aluminum toxicity has been increasing in the pediatric population. This may be a consequence of aggressive calcitriol and calcium therapy. The different factors involved in the development of secondary hyperparathyroidism include hyperphosphatemia, hypocalcemia, altered vitamin D synthesis, impairments in parathyroid hormone (PTH) secretion and metabolism, and, recently, possible downregulation of renal PTH/PTH-rP messenger RNA receptor. New developments in molecular biology have demonstrated the relationship between vitamin D and PTH. The use of high-dose pulse intravenous, intraperitoneal, and oral calcitriol therapy has significantly decreased serum PTH levels and retarded the progression of osteitis fibrosa. These therapeutic interventions, however, may have led to the development of adynamic bone lesions. The impact of adynamic bone lesions in the young and growing skeleton remains to be determined.

In vitro study of osteoblastic cells from patients with idiopathic osteoporosis and comparison with cells from non-osteoporotic controls

We have examined bone cells derived from iliac crest trabecular explants of 30 patients with idiopathic osteoporosis and 45 control subjects in order to determine whether intrinsic abnormalities in osteoblast function may contribute to the decreased bone formation observed in this disease. Bone cells isolated from all subjects expressed several in vitro characteristics of the osteoblast phenotype including adenylate cyclase responsiveness to parathyroid hormone (PTH) and prostaglandin E1 (PGE1), basal and 1,25(OH)2D3-stimulated alkaline phosphatase activity and osteocalcin production. Results were compared amongst three subject groups; young controls less than 40 years old, older controls over 40 years old, and osteoporotics. Osteoporotic cells were found in general to be fully active in vitro. There were no differences between osteoporotic and control cells in their basal levels of adenylate cyclase, or alkaline phosphatase, in their growth rates, or cell morphology. The cyclic AMP (cAMP) response to PTH was significantly lower in osteoporotic cells (71%, p < 0.01) and older control cells (64%, p < 0.005) relative to the response in cells from younger controls, suggesting that the decreased responsiveness in osteoporotic cells was due to subject age rather than the osteoporotic state. At the same time, the cAMP responses to PGE1 and cholera toxin were similar in cells from all three subject groups. The response to forskolin was reduced to about 40% in osteoporotic cells compared with controls, but this was not mirrored by similar differences in the responses to PTH, PGE1 or cholera toxin, suggesting that the availability of catalytic subunits is not rate-limiting in these cells. 1,25(OH)2D3-stimulated osteocalcin production was 220% higher in osteoporotics than in older controls, but the numbers tested were small and the difference did not reach significance. The one significant abnormality we observed in osteoporotic cells was in alkaline phosphatase activity: 1,25(OH)2D3-stimulated alkaline phosphatase activity was twofold higher in osteoporotics than in younger (p < 0.05), older (p < 0.05) and pooled controls (p < 0.025). The significance of this finding is unknown, but we postulate that it may reflect an intrinsic abnormality in osteoblast function in patients with idiopathic osteoporosis.

Skeletal unloading in rat decreases proliferation of rat bone and marrow-derived osteoblastic cells

The effects of skeletal unloading on osteoblastic cells were evaluated in tail-suspended rats. Hindlimb elevation for 14 days induced osteopenia, decreased histomorphometric indexes of bone formation in tibial metaphysis, and reduced plasma osteocalcin and alkaline phosphatase (ALP) levels compared with controls. The in vitro proliferation of osteoblastic cells isolated from the endosteal bone surface of suspended tibias was decreased by 42 and 31% at 2 and 4 days of culture, respectively, compared with controls, as shown by [3H]thymidine labeling and cell number. The proliferation of ALP-positive marrow stromal cells was also decreased by 20-24% at 1 and 2 days of culture. However, ALP activity in bone-derived cells and marrow stromal cells was not different in unloaded and control rats, and the number of bone cells synthesizing osteocalcin, osteonectin, and type I or type III collagen was identical in the two groups. The results indicate that the inhibition of bone formation induced by skeletal unloading is related to a decreased proliferation of putative osteoblast precursor cells present along the endosteal bone surface and in the marrow stroma.

Increased proliferation of osteoblast precursor cells in estrogen-deficient rats

We have evaluated the in vivo and in vitro changes in osteoblast characteristics induced by estrogen deficiency and 17 beta-estradiol (E2) treatment in ovariectomized (OVX) rats. Estrogen deficiency induced osteopenia and increased bone turnover, as evidenced by bone histomorphometry at 1, 3, and 6 mo postovariectomy. Bone surface osteoblastic cells (OB) isolated from tibias of OVX rats, OVX rats treated with E2 (10 micrograms/kg body wt), and sham rats showed no difference in alkaline phosphatase activity and osteocalcin production in vitro. In contrast the proliferation rate of OB cells was higher in OVX rats compared with sham rats at all time points post-surgery, as shown by [3H]thymidine incorporation and cell number. The proliferation rate of alkaline phosphatase-positive marrow cells was also higher in OVX rats compared with sham rats. E2 treatment of OVX rats corrected histologic indexes of bone resorption and formation and normalized OB cell proliferation. induced by estrogen deficiency in OVX rats is related to an increased proliferation of osteoblast precursor cells present in the marrow stroma and along the endosteal bone surface.

Stimulation of bone formation in osteoporosis patients treated with fluoride associated with increased DNA synthesis by osteoblastic cells in vitro

In this study we evaluated whether the fluoride-induced increased bone formation in osteoporosis is mediated by stimulation of bone cell proliferation and/or differentiation. We analyzed the kinetics of DNA synthesis and the phenotypic features of osteoblastic cells isolated from the trabecular bone surface in relationship to histomorphometric indices of bone formation evaluated on the same bone biopsy in 12 osteoporotic patients treated with fluoride. Osteoblastic cells isolated from patients with a higher than normal bone formation rate, increased mean wall thickness of trabecular bone packets, and high trabecular bone volume after fluoride therapy displayed a higher than normal rate of DNA synthesis in vitro. The peak of [3H]thymidine incorporation into DNA, the maximal DNA synthesis, and the area under the growth curve of osteoblastic cells isolated from these patients were higher than the values in normal bone cells obtained from age-matched controls. By contrast, in vitro parameters of osteoblastic cell proliferation were not different from normal in fluoride-treated osteoporosis patients in whom bone formation was not increased, although the duration of treatment and bone fluoride content were not different. Parameters of bone cell proliferation in vitro were increased in correlation with the mean wall thickness, and the latter correlated with the trabecular bone volume, indicating that the augmentation of bone formation and bone volume induced by fluoride was paralleled by an increased proliferation of osteoblastic cells. Basal osteocalcin production (corrected for cell protein) and alkaline phosphatase activity in vitro were comparable, and the response to 1,25-dihydroxyvitamin D3 (10 nmol/liter, 48 h) was not different in normal osteoblastic cells and in cells from fluoride-treated osteoporosis patients whether they had high or normal bone formation. The results show that the fluoride-induced increased bone formation in osteoporotic patients is associated with an increased in vitro proliferative capacity of osteoblastic cells lining the trabecular bone surface, whereas parameters of osteoblast differentiation are not affected. The data also suggest that induction of a higher than normal bone cell proliferation is prerequisite for the stimulation of bone formation by fluoride.

Decreased DNA synthesis by cultured osteoblastic cells in eugonadal osteoporotic men with defective bone formation

To determine the osteoblastic dysfunction that may be involved in the pathophysiology of osteoporosis in men we have compared histomorphometric indices of bone formation with in vitro characteristics of osteoblastic cells isolated from the trabecular bone surface in 23 untreated men with eugonadal osteoporosis. In most patients (n = 14), trabecular bone loss resulted from decreased bone formation evidenced by a lower than normal osteoblast surface, double tetracycline labeled surface, bone formation rate, and mean wall thickness. In these patients, DNA synthesis by cultured osteoblastic cells was altered. The peak of [3H]thymidine incorporation into DNA, the maximal DNA synthesis, and the area under the curve of cell proliferation were lower than the values in normal bone cells from age-matched controls. Parameters of bone cell growth were decreased in correlation with the extent of actively bone forming surfaces. By contrast, in patients (n = 9) with normal histomorphometric indices of bone formation, bone cell proliferation in vitro was not different from normal. Parameters of osteoblastic differentiation in vitro such as osteocalcin production and alkaline phosphatase activity were normal in the two groups of patients. This study shows that the trabecular bone loss resulting from defective bone formation in eugonadal osteoporotic men is associated with a lower than normal proliferative capacity of osteoblastic cells lining the trabecular bone surface.