Probing glucocorticoid-dependent osteogenesis in rat and chick cells in vitro by specific blockade of osteoblastic differentiation with progesterone and RU38486

Glucocorticosteroid-induced spinal osteoporosis: scientific update on pathophysiology and treatment

Glucocorticosteroid-induced spinal osteoporosis (GIOP) is the most frequent of all secondary types of osteoporosis. The understanding of the pathophysiology of glucocorticoid (GC) induced bone loss is of crucial importance for appropriate treatment and prevention of debilitating fractures that occur predominantly in the spine. GIOP results from depressed bone formation due to lower activity and higher death rate of osteoblasts on the one hand, and from increase bone resorption due to prolonged lifespan of osteoclasts on the other. In addition, calcium/phosphate metabolism may be disturbed through GC effects on gut, kidney, parathyroid glands and gonads. Therefore, therapeutic agents aim at restoring balanced bone cell activity by directly decreasing apoptosis rate of osteoblasts (e.g., cyclical parathyroid hormone) or by increasing apoptosis rate of osteoclasts (e.g., bisphosphonates). Other therapeutical efforts aim at maintaining/restoring calcium/phosphate homeostasis: improving intestinal calcium absorption (using calcium supplementation, vitamin D and derivates) and avoiding increased urinary calcium loss (using thiazides) prevent or counteract a secondary hyperparthyroidism. Bisphosphonates, particularly the aminobisphosphonates risedronate and alendronate, have been shown to protect patients on GCs from (further) bone loss to reduce vertebral fracture risk. Calcitonin may be of interest in situation where bisphosphonates are contraindicated or not applicable and in cases where acute pain due to vertebral fracture has to be manage. The intermittent administration of 1-34-parathormone may be an appealing treatment alternative, based on its documented anabolic effects on bone resulting from the reduction of osteoblastic apoptosis. Calcium and vitamin D should be a systematic adjunctive measure to any drug treatment for GIOP. Based on currently available evidence, fluoride, androgens, estrogens (opposed or unopposed) cannot be recommended for the prevention and treatment of GIOP. However, substitution of gonadal hormones may be indicated if GC-induced hypogonadism is present and leads to clinical symptoms. Data using the SERM raloxifene to treat or prevent GIOP are lacking, as are data using the promising bone anabolic agent strontium ranelate. Kyphoplasty performed in appropriately selected osteoporotic patients with painful vertebral fractures is a promising addition to current medical treatment.

Progesterone- and dexamethasone-dependent osteoprogenitors in bone cell populations derived from rat vertebrae are different and distinct

Previous experiments have demonstrated that bone cell populations derived from explants of lumbar vertebral bone of adult female rats contain osteoprogenitors that require dexamethasone (Dex) or progesterone (Prog) to proliferate and differentiate into fully differentiated bone-forming osteoblasts. We now show that the Prog-dependent population cannot be detected in male rats after sexual maturation, but is present in prepubertal rats of both sexes and can be induced in adult male-derived populations by culturing the explants in medium containing 17beta-estradiol (10(-9)-10(-8) M). This suggested that the Prog- and Dex-dependent osteoprogenitors in adult female-derived populations were probably distinct populations and that the survival of the Prog-dependent osteoprogenitors and/or their ability to proliferate are dependent on the presence of estrogen. We then proceeded to prove this by using replica plating. When one of the paired colonies duplicated was cultured in medium containing Dex (10(-8) M) and the other in medium containing Prog (10(-5) M), 5.0% of duplicates formed bone in Prog only, 11.1% formed bone in Dex only, and 3.4% formed bone in both Prog and Dex. In all cases the size of the bone-forming colonies in Dex-treated cultures was larger than that in Prog-treated cultures, indicating that the effects of Dex on osteoprogenitor proliferation are greater than those of Prog. The results demonstrate the existence of three classes ofosteoprogenitors in adult female rat-derived bone cell populations: a class responding to Dex only, a class responding to Prog only, and a class responding to both Dex and Prog. The results also indicate that the effects of Prog are not mediated by Prog binding to the glucocorticoid receptor and imply that Prog plays an important role in maintaining bone mass through regulating the class of osteoprogenitors responsive to Prog.

Tamoxifen attenuates the effects of exogenous glucocorticoid on bone formation and growth in piglets

Tamoxifen (Tam) has been shown to inhibit dexamethasone (Dex)-mediated effects on bone formation in vitro. Our objective was to determine whether Tam would block Dex-induced osteopenia and growth inhibition in growing piglets. Four-day-old male Yorkshire piglets were adapted to a liquid formula diet (400 ml/kg x day) and randomized to one of four groups (n = 5/group): Dex (0.5 mg/kg x day), Tam (1 mg/kg x day), Dex plus Tam, or placebo control (vehicle only). Both drugs were administered by orogastric gavage twice daily for 12 days. At baseline and at the end of treatment, whole body bone mineral density (BMD) was determined by dual energy x-ray absorptiometry (Hologic QDR1000W). Plasma osteocalcin and PTH were measured on days 0 and 12, and urinary N-telopeptide was measured on day 12. Changes in axial length and daily weight were also measured. Delta whole body BMD was 29% lower (P < 0.05) in Dex alone treated piglets than in controls (0.033 vs. 0.047 g/cm2, respectively), whereas the maximum change in BMD in Dex plus Tam group (0.046 g/cm2) was similar to that in controls. Concurrent Tam administration reduced the Dex-induced deficit in weight gain by 56% (P < 0.05) and the deficit in axial length gain by 72% (P < 0.01). In Dex alone treated piglets, PTH was significantly elevated (7-fold), whereas osteocalcin and N-telopeptide were significantly reduced compared with control values. These effects were prevented by Tam. These data suggest that the suppression of growth and other changes in parameters of bone metabolism induced by glucocorticoids in vivo can be attenuated by Tam.

Cell sorting enriches osteogenic populations in rat bone marrow stromal cell cultures

The presence of multiple cell types in bone marrow stromal populations complicates interpretation of cytokine and hormone effects on the osteoprogenitors present, indicating a need for a method for purification of the osteoprogenitor population. Flow cytometric sorting of 7 day primary rat bone marrow stromal cell cultures was performed on the basis of alkaline phosphatase (AP) expression with an antibody against AP (RBM 211.13). The resultant AP(high), AP(low), or control cells were plated to determine osteoprogenitor, macrophage, and adipocyte distribution and frequency. Approximately 50% of osteoprogenitor/bone nodule-forming cells were lost during processing/sorting when compared with unsorted controls. Nevertheless, within the AP(high) fraction, the numbers of AP-positive colonies and osteoprogenitors (bone nodules) were significantly enriched compared with the unfractionated control; the increase in osteoprogenitor frequency ranged from approximately 2 to 100-fold. There were few assayable osteoprogenitors in either the AP(high) or AP(low) fractions in the absence of dexamethasone (dex), suggesting that RBM stroma contains largely dex-dependent osteoprogenitor populations, and that dex may regulate osteoprogenitors subsequent to the upregulation of AP. Osteoprogenitor/bone nodule numbers in either the AP(high) or AP(low) fraction did not follow a linear relationship with decreasing plating density. The AP(high) fraction of cells was depleted for adipocyte and macrophage colonies. In contrast, within the AP(low) fraction of cells, adipocyte and macrophage colonies were consistently enriched. We conclude that flow-cytometric sorting of RBM stromal populations according to high or low AP expression is an effective technique for enrichment of AP-positive colonies and osteoprogenitors/bone nodule-forming cells.

Tamoxifen attenuates glucocorticoid actions on bone formation in vitro

Tamoxifen is a synthetic estrogen analog which may regulate osteogenesis in vivo by virtue of its antiglucocorticoid properties. We have examined tamoxifen regulation of glucocorticoid-induced osteogenesis in two different in vitro bone systems: the chicken periosteal osteogenesis model (CPO) and rat bone marrow stromal cells (RBMC). Hormone uptake studies were conducted with the osteosarcoma cell line, ROS 17/2.8. In the CPO model, alkaline phosphatase (AP) activity and collagen synthesis were stimulated by the glucocorticoid dexamethasone (Dex; 0.1 microM). These Dex-mediated effects were inhibited by increasing concentrations of tamoxifen (10-100 microM). Similarly, in the RBMC model, Dex-dependent (0.01 microM Dex) mineralized tissue formation and AP activity were blocked by tamoxifen (0.1 microM). Although tamoxifen inhibited Dex-mediated increases of AP activity in ROS 17/2.8 cells, it did not inhibit uptake of 3H-Dex or of 3H-estrogen. Northern analyses showed that tamoxifen did not affect messenger RNAs (mRNAs) for AP. Tamoxifen did seem to reduce mRNA for collagen type I, but not bone sialoprotein, osteopontin, and osteocalcin. Dex-induced increases for all proteins mRNAs in the RBMC model were not reduced by tamoxifen. Similarly, tamoxifen had no effects on cellular proliferation. We conclude that tamoxifen has no direct effect on gene expression of bone-related proteins of osteoblastic cells. Further, in the ROS 17/2.8 cell line, the antiglucocorticoid properties of tamoxifen do not appear to be mediated through either Dex or estrogen receptors.

Altered mineral metabolism and bone mass in children during treatment for acute lymphoblastic leukemia

Children with acute lymphoblastic leukemia (ALL) often develop bone pain, abnormal gait, and unusual fractures while in remission and receiving continuing chemotherapy. A prospective longitudinal cohort study was undertaken of bone mass and biochemical mineral status in 40 consecutive children (27 male, 13 female, aged 0.3-17.0 years) receiving therapy on the Dana-Farber Cancer Institute protocol 87-01. Radiography, lumbar spine dual-photon absorptiometry, and biochemical measurements of mineral status were performed at diagnosis and at 6-month intervals throughout 24 months of chemotherapy. Eleven patients were not completely evaluated (4 deaths and 7 off study). Radiographic evidence of osteopenia was observed in 10, 64, and 76% at diagnosis, 12 and 24 months, respectively. Fractures occurred in 39% of children during treatment. Reduction in bone mineral content (BMC), as measured by Z scores, occurred in 64% of patients and was most severe in those greater than 11 years of age at diagnosis. Reduction in BMC during the first 6 months of therapy had a positive predictive value of 64%, while an increase in BMC had a negative predictive value of 82% for subsequent fracture. By 6 months of therapy, 31/37 (84%) children were hypomagnesemic, of whom 16 (52%) were hypermagnesuric. Plasma osteocalcin was subnormal at diagnosis in 29/40 (73%) but increased to normal by 6 months of treatment. Vitamin D status was normal throughout, but plasma 1,25-dihydroxyvitamin D remained subnormal in greater than 70% of children. Urinary cross-link N-telopeptide was normal at diagnosis and became elevated in 58% of children by the end of therapy. Suppressed bone mineralization is evident at diagnosis in a minority of children with ALL. Skeletal morbidity and a reduction in bone mineral mass become more prevalent during treatment, with increased bone resorption, perhaps mainly as a consequence of corticosteroid administration.

Effects of progesterone on serum levels of IGF-1 and on femur IGF-1 mRNA in ovariectomized rats

Local and systemic insulin-like growth factors (IGFs) may be involved in the regulation of bone formation by sex hormones. The present studies describe the in vivo effects of estradiol, progesterone, or both on IGF-1 mRNA abundance in bone, serum IGF-1 levels, and bone formation. Rats were sham-operated (SHAM) or ovariectomized (OVX) at 12 weeks of age and used a week later in three experiments. First, OVX rats were treated with vehicle, estradiol, and/or medroxyprogesterone (MPA) for 3 weeks, and bone formation was assessed in the tibial metaphysis. Second, OVX rats were treated in the same manner and serum IGF-1 levels measured. Third, OVX rats were treated with an injection of vehicle, estradiol, and/or progesterone, and 24 h later, levels of IGF-1 mRNA in the femur were analyzed. The mineralized surface, mineral opposition rate, and bone formation rate (BFR) were higher in OVX than in SHAM rats. The BFR was decreased in estrogen-treated but increased in MPA-treated rats compared with vehicle-treated OVX rats. Circulating levels of IGF-1 were higher in OVX than in SHAM rats but were not affected by sex hormones in a 3-week experiment, whereas these levels were not different among groups in a 24-h experiment. Northern analysis detected 7.5 and 0.8 kb IGF-1 mRNA transcripts. The abundance of IGF-1 mRNA was higher in OVX than in SHAM rats. IGF-1 transcripts 7.5 and 0.8 kb were decreased by 72 and 29%, respectively, in estrogen-treated and increased by 44 and 43%, respectively, in progesterone-treated rats compared with vehicle-treated OVX rats. We conclude that in the short term, estrogen lowers and progesterone raises bone IGF-1 mRNA and these changes are followed by coordinated changes in bone formation rate.