Effect of heparin on bone formation in cultured fetal rat calvaria

[Long-term low-molecular-weight heparin therapy during pregnancy: is there a bone risk?]

Unfractionated heparin (UFH) and low-molecular-weight heparins (LMWH) are broadly used for the thromboprophylaxis during pregnancy. Long-term heparin therapy has raised concern about the risk of bone loss and osteoporotic fractures while pregnancy by itself favors osteoporosis. Experimental studies show a comparable effect of UFH and LMWH on bone formation while UFH has a more marked effect on bone resorption. Available clinical studies do not provide evidence of a difference between UFH and LMWH or among different LMWH on the bone risk. However, clinical studies show that bone loss observed with LMWH is not different from the physiological loss related to pregnancy, and osteoporotic fractures are associated with comorbidities or osteoporosis risk factors. LMWH represent the preferred alternative for thromboprophylaxis during pregnancy.

Recombinant human acidic fibroblast growth factor and fibrin carrier regenerates bone

Bone regeneration promoted by acidic recombinant human fibroblast growth factor (rhFGF-1), rabbit demineralized bone matrix (rDBM), and a fibrin (f) delivery system was measured in critical-sized defects in rabbits' radii. A unilateral segmental defect 20 mm in length was prepared in radii of 48 skeletally mature New Zealand White rabbits divided equally between 4- and 8-week cohorts. The temporal cohorts were divided equally among four treatment groups: rDBM, rDBM/f, rDBM/rhFGF-1/f, and rhFGF-1/f. Data for the fifth group, untreated critical-sized defects, were exploited from previous published reports from this laboratory. In response to experimental treatments, radiomorphometric and histomorphometric methods were used to derive quantitative outcome data that were tested by analysis of variance and post hoc multiple comparison tests (significance p </= 0.05). Radiomorphometric data (percentage of radiopacity of defect) were acquired at the day of the operation and every 2 weeks thereafter, whereas histomorphometric data (square millimeters of new bone formation) were determined at term. The objective for the study was to develop candidate bone regenerative therapies. Therefore, the hypotheses were that experimental treatments would promote bone formation within critical-sized defects and that one treatment would be superior to the rest. Testing hypotheses was achieved with quantitative methodology, and data were subjected to statistical models. Radiopacity at each 2-week period was greater in treated defects than in untreated critical-sized defects. The amount of radiopacity promoted by rDBM/f and rhFGF-1/f at 8 weeks was equivalent and was greater than antecedent times. Histomorphometric data analyses indicated that rDBM/f and rDBM evoked the same quantity of new bone formation at 4 weeks; by 8 weeks, all treatments except rDBM/f had more new bone within the critical-sized defects in comparison to untreated defects. That rDBM/f promoted less new bone than rDBM alone may suggest fibrin decreases bone formation, perhaps by impeding local solubility of endogenous and rDBM-containing signaling molecules. However, rhFGF-1/f promoted a significant and unexpected increase in bone formation response that could refute the previous notion. In conclusion, the combination of rDBM/rhFGF-1/f may represent a significant, new osteogenic therapeutic regimen. Additional assessments in higher order species must be accomplished to corroborate efficacy.

Effects of heparan-like polymers associated with growth factors on osteoblast proliferation and phenotype expression

Heparan-like polymers derived from dextran, named RGTA, were shown to stimulate bone repair in different bone defect models. Like heparin and heparan sulfates, RGTA potentiate in vitro the biological activities of heparin-binding growth factors (HBGFs), such as fibroblast growth factor (FGF), by stabilizing them against denaturations and by enhancing their binding with cellular receptors. RGTA were postulated to stimulate bone healing by interacting with HBGFs released in the wound site and, subsequently, by promoting the proliferation and/or differentiation of cells implicated in this process. We examined the effects of RGTA alone and associated with HBGFs on MC3T3-E1 osteoblastic cell proliferation and differentiation. RGTA inhibited cell proliferation, as measured by [3H]-thymidine incorporation into DNA. They enhanced the inhibition of DNA synthesis caused by transforming growth factor-beta (TGF-beta1) and bone morphogenetic protein-2 (BMP-2). RGTA alone increased the alkaline phosphatase and parathyroid hormone-responsive adenylate cyclase activities in MC3T3. RGTA enhanced the stimulation of the alkaline phosphatase activity induced by BMP-2 and decreased or suppressed the inhibition caused by TGF-beta1 and FGF-2. Furthermore, RGTA increased the response to parathyroid hormone stimulated by BMP-2. In conclusion, RGTA stimulate the expression of osteoblast phenotype features alone or in association with HBGFs. The ability to promote the differentiation of bone-forming cells is a potential explanation of the stimulating effect of RGTA on bone repair.

Secondary osteoporosis

Generalized osteoporosis currently represents a heterogeneous group of conditions with many different causes and pathogenetic mechanisms, that often are variably associated. The term "secondary" is applied to all patients with osteoporosis in whom the identifiable causal factors are other than menopause and aging. In this heterogeneous group of conditions, produced by many different pathogenetic mechanisms, a negative bone balance may be variably associated with low, normal or increased bone remodeling states. A consistent group of secondary osteoporosis is related to endocrinological or iatrogenic causes. Exogenous hypercortisolism may be considered an important risk factor for secondary osteoporosis in the community, and probably glucocorticoid-induced osteoporosis is the most common type of secondary osteoporosis. Supraphysiological doses of corticosteroids cause two abnormalities in bone metabolism: a relative increase in bone resorption, and a relative reduction in bone formation. Bone loss, mostly of trabecular bone, with its resultant fractures is the most incapacitating consequence of osteoporosis. The estimated incidence of fractures in patients prescribed corticosteroid is 30% to 50%. Osteoporosis is considered one of the potentially serious side effects of heparin therapy. The occurrence of heparin-induced osteoporosis appeared to be strictly related to the length of treatment (over 4-5 months), and the dosage (15,000 U or more daily), but the pathogenesis is poorly understood. It has been suggested that heparin could cause an increase in bone resorption by increasing the number of differentiated osteoclasts, and by enhancing the activity of individual osteoclasts. Hyperthyroidism is frequently associated with loss of trabecular and cortical bone; the enhanced bone turnover that develops in thyrotoxicosis is characterized by an increase in the number of osteoclasts and resorption sites, and an increase in the ratio of resorptive to formative bone surfaces, with the net result of bone loss. Despite these findings, the occurrence of pathological fractures in patients with hyperthyroidism is relatively low, and probably due to the fact that deficiencies in bone mass may be reversed by treatment of the thyroid disease. Most, but not all, studies on insulin-dependent diabetes mellitus (IDDM) report an association with osteopenia. In IDDM, the extent of bone loss is usually slight, which helps explain the discrepancy between the frequency of decreased bone mineral density, and the frequency of osteoporotic fractures in long-standing diabetes. Contradictory results have been obtained in non-insulin-dependent diabetes mellitus (NIDDM) patients. Increased rates of bone loss at the radius and lumbar spine were demonstrated either in patients with two-thirds gastric resection and Billroth II reconstruction, or in those with one-third resection and Billroth I anastomosis, and the metabolic bone disease following gastrectomy may consist also of osteomalacia or mixed pattern of osteoporosis-osteomalacia, with secondary hyperparathyroidism. Miscellaneous causes of secondary osteoporosis are also immobilization, pregnancy and lactation, and alcohol abuse.

The effects of fibroblast growth factor-2 on human neonatal calvaria osteoblastic cells are differentiation stage specific

Fibroblast growth factors (FGFs) appear to play an important role in human cranial osteogenesis. We therefore investigated the effects of recombinant human FGF-2 (rhFGF-2) on human calvaria (HC) osteoblastic cells. Immunocytochemical analysis showed that confluent HC cells express both FGF receptors -1 and -2. In short-term culture, rhFGF-2 (0.1-100 ng/ml, 2-5 days) increased HC cell growth and decreased alkaline phosphatase (ALP) activity and type I collagen (ColI) synthesis, as evaluated by P1CP levels. When HC cells were induced to differentiate in long-term culture in the presence of 50 microg/ml ascorbic acid and 3 mM phosphate, HC cells initially proliferated, then ALP activity and ColI synthesis decreased and calcium content in the extracellular matrix increased. Continuous treatment with rhFGF-2 (50 ng/ml) for 1-28 days, or a transient rhFGF-2 treatment for 1-7 days, slightly increased DNA synthesis at 7 days, whereas a late treatment for 8-28 days had no effect on cell growth. The continuous and transient treatments with rhFGF-2 decreased ALP activity, ColI synthesis, and matrix mineralization. This was associated with a transient fall in osteocalcin (OC) production at 7 days. In contrast, the late rhFGF-2 treatment for 8-28 days only slightly inhibited ALP activity and increased matrix mineralization. In addition, both continuous and late treatments with rhFGF-2 increased OC production in more mature cells at 3-4 weeks of culture. We also found that the early and late treatments with rhFGF-2 had opposite effects on transforming growth factor beta2 production in proliferating cells and more mature cells. The results show that rhFGF-2 slightly stimulates cell growth and reduces the expression of osteoblast markers in less mature cells, whereas it induces OC production and matrix mineralization in more mature cells, indicating that the effects of FGF-2 are differentiation stage specific and that FGF-2 may modulate HC osteogenesis by acting at distinct stages of cell maturation.

Effects of short-term, high dose, heparin therapy on biochemical markers of bone metabolism

To evaluate the effects of the short-term, high-dose sodium heparin therapy on biochemical markers of bone metabolism, we studied 20 patients (11 males and 9 females) with pulmonary embolism, treated with sodium heparin (daily dose range: 40,000-45,000 I.U. by continuous i.v. infusion). Heparin therapy lasted 5-7 days, after which patients received warfarin over 12 months. Eleven patients (6 males and 5 females) with ischaemic stroke, treated with i.v. glycerol and pentoxifilline, were used as controls. Before and after therapy serum and urinary markers of bone metabolism were evaluated; in 12 heparin-treated pts., the parameters were also evaluated 4 months after discontinuation of warfarin therapy. After heparin therapy a significant reduction vs. basal value was observed in levels of serum osteocalcin (ng/ml;mean + SEM): 3.32 & 0.19 vs. 2.05 + 0.21; p < 0.001. In the 12 patients evaluated 4 months after discontinuation of warfarin therapy, serum osteocalcin levels returned to basal value: 3.41 + 0.12 ng/ml (p:n.s.). No significant changes of the examined parameters were observed in controls. In conclusion, our data seem to indicate an effect of i.v. short-term heparin therapy on bone metabolism. This effect seems to be characterized by an inhibition of osteoblast function as suggested by the reduction of serum osteocalcin levels.

Effects of heparin on osteoclast activity

The effect of heparin on osteoclastic bone resorption was studied in vitro using the disaggregated osteoclast resorption assay. Bone resorption was assessed by counting the resorption lacunae on bone slices by light microscopy. Low concentrations of heparin (5 micrograms/ml) increased bone resorption by isolated chick and rat osteoclasts. Among other glycosaminoglycans tested at 5 micrograms/ml, only dextran sulfate showed a small but significant stimulation of resorption. Chondroitin sulfates A, B, and C were without effect at 25 and 100 micrograms/ml, whereas resorption was increased by 100 micrograms/ml of heparan sulfate. With chick osteoclasts, which could be maintained in serum-free conditions, a stimulatory effect of heparin was found both in the presence of 5% fetal calf serum and in serum-free media containing insulin, transferrin, and selenium. The magnitude of the heparin-induced increase in resorption was similar in the presence or absence of serum. The stimulation of resorption was associated with an increase in the number of osteoclasts on bone slices. Pretreatment of the bone slices with heparin also enhanced resorption. In time course experiments, 5 micrograms/ml of heparin caused a doubling of chick osteoclast activity index (number of resorption pits per number of osteoclasts) at 12 and 24 h. In 24 h cultures, treatment with 10 micrograms/ml of the arginine-rich basic protein, protamine, 1 microgram/ml of the immunosuppressant, cyclosporine A, or 5 micrograms/ml of the cysteine-proteinase inhibitor, leupeptin, negated the heparin effect on bone resorption. Leupeptin also inhibited basal resorption.(ABSTRACT TRUNCATED AT 250 WORDS)

Pharmacodynamics, clinical indications, and adverse effects of heparin

Heparins are a heterogenous group of naturally occurring glycosaminoglycans characterized by anticoagulant activity and a wide range of molecular weights (low molecular weight or fractionated heparins evolving within the past two decades). Cofactors for endogenous inhibitors of coagulation (antithrombin III and heparin cofactor II), heparin administration results in a hypocoagulable state. Various platelet activities, including inhibition of activity induced by platelet-derived growth factors on vascular smooth muscle, also have been noted. Divorced of anticoagulant nature, novel applications may include a role in atherosclerosis prevention, acceleration of collateral coronary as well as peripheral circulation (i.e., angiogenesis), and continued (chronic) post-myocardial infarction therapy. Established indications include treatment of various thrombotic diseases, unstable angina, and thrombosis chemoprophylaxis in medical/surgical patients. The antithrombotic potential of the heparins is used also in thrombosis management related to extracorporeal circulatory assistance or dialysis devices. Heparin's therapeutic potential in the postphlebitic syndrome as well as in acute treatment of myocardial infarction (primarily and adjunctively with various thrombolytic agents) continues to undergo evaluation; however, early data review shows favorable trends for its inclusion in situations that favor thrombus generation (e.g., anterior myocardial infarction). Although associated with thrombocytopenia or hypertransaminasemia, the heparins are relatively well tolerated. In a small subset of patients, a severe thrombocytopenia may ensue, which generally resolves on medication withdrawal. As this class of glycosaminoglycans becomes better characterized, new indications may emerge for both native and the newer fractionated heparins.

Effects of acid and basic fibroblast growth factor and heparin on resorption of cultured fetal rat long bones

We tested acid and basic fibroblast growth factor (aFGF and bFGF), members of the heparin binding FGF family, for their ability to stimulate bone resorption as measured by the release of previously incorporated 45Ca from cultured fetal rat long bones in the presence and absence of heparin. Purified low-molecular-weight heparin (LMW heparin) at 5-125 micrograms/ml had no direct stimulatory effect. There was little effect from aFGF (10(-11)-10(-8) M) alone, but increased resorption was observed in the presence of LMW heparin. With bFGF, increased bone resorption was observed at 10(-9) M but not at 10(-8) M. The stimulatory effects of aFGF and bFGF in the presence of LMW heparin were not blocked by the addition of indomethacin (10(-6) M), which blocks prostaglandin production, or hydroxyurea (10(-3) M), which blocks DNA synthesis. However, pretreatment with aphidicolin (3 x 10(-5) M), a potent inhibitor of DNA synthesis, blocked the effect of acid FGF and diminished the effect of bFGF. These results indicate that both aFGF and bFGF can stimulate bone resorption by a prostaglandin-independent mechanism, particularly in the presence of heparin. The activation of FGF-mediated bone resorption by heparin could play a role in producing the osteoporosis that has been described with heparin therapy and mastocytosis.

Structural determinants of the capacity of heparin to inhibit collagen synthesis in 21-day fetal rat calvariae

Earlier work from our laboratory demonstrated that heparin inhibits type I collagen and DNA synthesis in fetal rat calvariae in vitro. In this paper we have analyzed the structural features of heparin that determine its inhibitory effect on collagen synthesis. These experiments were performed using unmodified heparins and low-molecular-weight heparins from different manufacturers, nonheparin glycosaminoglycans, desulfated heparins, anticoagulant and nonanticoagulant heparin, and chemically defined heparin oligosaccharides. Low-molecular-weight heparin (Mr 3700-5100) inhibited collagen synthesis, but oligosaccharides (disaccharides to decasaccharide, Mr 665-3000) did not. The glycosaminoglycans chondroitin sulfate B, heparan sulfate, and hyaluronic acid did not alter collagen synthesis but dextran sulfate was as inhibitory as unmodified heparin. Nonanticoagulant as well as anticoagulant low-molecular-weight heparin fractions inhibited collagen synthesis. Modification of heparin by total desulfation, O-desulfation, or N-desulfation and re-N-acetylation resulted in the loss of inhibitory property, suggesting that the degree of sulfation contributed to heparin's inhibitory effect. Low-molecular-weight heparins from different manufacturers were just as inhibitory as native heparin on collagen synthesis. We therefore conclude that low-molecular-weight heparin compounds offer no protection against heparin-induced osteoporosis. Our findings also suggest that the size and sulfation of a heparin-derived oligosaccharide contribute to its ability to inhibit collagen synthesis in bone.