Accelerated bone resorption, due to dietary calcium deficiency, promotes breast cancer tumor growth in bone.

The skeleton is a major site of breast cancer metastases. High bone turnover increases risk of disease progression and death. However, there is no direct evidence that high bone turnover is causally associated with the establishment and progression of metastases. In this study, we investigate the effects of high bone turnover in a model of breast cancer growth in bone. Female nude mice commenced a diet containing normal (0.6%; 'Normal-Ca') or low (0.1%; 'Low-Ca') calcium content. Mice were concurrently treated with vehicle or osteoprotegerin (1 mg/kg/d s.c; n = 16 per group). Three days later (day 0), 50,000 Tx-SA cells (variant of MDA-MB-231 cells) were implanted by intratibial injection. On day 0, mice receiving Low-Ca had increased serum parathyroid hormone (PTH) and tartrate-resistant acid phosphatase 5b levels, indicating secondary hyperparathyroidism and high bone turnover, which was maintained until day 17. Osteoprotegerin increased serum PTH but profoundly reduced bone resorption. On day 17, in mice receiving Low-Ca alone, lytic lesion area, tumor area, and cancer cell proliferation increased by 43%, 24%, and 24%, respectively, compared with mice receiving Normal Ca (P < 0.01). Osteoprotegerin treatment completely inhibited lytic lesions, reduced tumor area, decreased cancer cell proliferation, and increased cancer cell apoptosis. Increased bone turnover, due to dietary calcium deficiency, promotes tumor growth in bone, independent of the action of PTH. Breast cancer patients frequently have low dietary calcium intake and high bone turnover. Treatment to correct calcium insufficiency and/or treatment with antiresorptive agents, such as osteoprotegerin, may be of benefit in the adjuvant as well as palliative setting.

Bone resorption increases tumour growth in a mouse model of osteosclerotic breast cancer metastasis.

Osteosclerotic metastases account for 20% of breast cancer metastases with the remainder osteolytic or mixed. In mouse models, osteolytic metastases are dependent on bone resorption for their growth. However, whether the growth of osteosclerotic bone metastases depends on osteoclast or osteoblast actions is uncertain. In this study, we investigate the effects of high and low bone resorption on tumour growth in a mouse model of osteosclerotic metastasis. We implanted human breast cancer, MCF-7, cells into the tibiae of mice. Low and high levels of bone resorption were induced by osteoprotegerin (OPG) treatment or calcium deficient diet respectively. We demonstrate that OPG treatment significantly reduces tumour area compared to vehicle (0.42 +/- 0.06 vs. 1.27 +/- 0.16 mm2, P < 0.01) in association with complete inhibition of osteoclast differentiation. In contrast, low calcium diet increases tumour area compared to normal diet (0.90 +/- 0.30 vs. 0.58 +/- 0.20 mm2, P < 0.05) in association with increased osteoclast numbers (84.44 +/- 5.18 vs. 71.11 +/- 3.56 per mm2 bone lesion area, P < 0.05). Osteoblast surfaces and new woven bone formation were similarly increased within the tumour boundaries in all treatment groups. Tumour growth in this model of osteosclerotic metastasis is dependent on ongoing bone resorption, as has been observed in osteolytic models. Bone resorption, rather than bone formation, apparently mediates this effect as osteoblast surfaces in the tumour mass were unchanged by treatments. Treatment of breast cancer patients through correction of calcium deficiency and/or with anti-resorptive agents such as OPG, may improve patient outcomes in the adjuvant as well as palliative settings.

Inhibition of bone resorption, rather than direct cytotoxicity, mediates the anti-tumour actions of ibandronate and osteoprotegerin in a murine model of breast cancer bone metastasis.

Inhibition of bone resorption either by bisphosphonate (BP) treatment or by blocking RANKL signalling with osteoprotegerin (OPG) has been shown to reduce tumour burden in bone and inhibit bone destruction in murine xenograft models of breast cancer. However, whether the anti-tumour effect of OPG or BP in bone is mediated by inhibition of bone resorption or by direct effects on tumour cells is uncertain. The current study is designed to investigate anti-tumour effects of OPG and ibandronate (IBN), dosed alone or in combination, on tumour growth to determine if there is experimental support for combination treatments and to provide evidence for the presence of direct anti-tumour effects. To this aim, 10 microl (5 x 10(6) cells/ml) of the bone-seeking MDA-MB-231 (Tx-SA) cell line was injected intra-tibially into nude mice. After 10 days, when the tumours were evident radiologically, mice were treated with vehicle, OPG (1 mg/kg/day), ibandronate (IBN) (160 microg/kg/day) or IBN and OPG at the same doses (IBN+OPG) for a week, and the effects of each treatment on lytic lesions, tumour cell growth, cell apoptosis and proliferation were measured by radiography, immunohistochemistry and histomorphometry. Compared to vehicle controls, in vivo treatment with OPG, IBN, or IBN+OPG, each prevented the expansion of osteolytic bone lesions (increase in lytic lesion area day 10 to day 17: OPG -3.2%, IBN 6.6%, IBN+OPG 3.6%, Vehicle 232.5%; p<0.01). Treatment with OPG, IBN or IBN+OPG each produced similar reductions in tumour area relative to vehicle-treated mice (OPG 52%, IBN 54%, IBNp and OPG 48%, p<0.01 vs. vehicle) OPG and IBN alone and in combination each produced a similar increase in cancer cell apoptosis (OPG 330%, IBN 342%, IBN and OPG 347%, p<0.01 vs. vehicle) and a decrease in cancer cell proliferation (OPG 59%, IBN 62%, IBN and OPG 58%, p<0.05 vs. vehicle). Our findings indicate that (i) combined treatment with OPG and a bisphosphonate is not significantly more effective than either agent alone; and that (ii) inhibition of bone resorption, rather than direct anti-tumour action, mediates the effects of these agents on tumour growth in this in vivo model.

Endogenous glucocorticoid signalling in osteoblasts is necessary to maintain normal bone structure in mice.

The role of endogenous glucocorticosteroids (GC) in bone development is ill-defined. Using the Col2.3-11betaHSD2 transgenic (tg) mouse model, we examined the effect of osteoblast-targeted disruption of intracellular GC signalling on bone growth and strength, and its modulation by factors such as age, gender and skeletal site. Tibiae and L3 vertebrae of 3 and 7-week-old, male and female wild type (WT) mice and their tg, age and sex matched littermates were analysed by micro-CT and mechanical testing. Data were analysed separately for 3 and 7-week-old mice by 2-way ANOVA using genotype (WT, tg), gender and their interactions as factors. Transgenic mice were characterised by lower bone volume, lower trabecular number and higher trabecular separation in tibial trabecular bone, as well as lower tibial cortical bone area and periosteal and endosteal perimeters. These changes resulted in a marked decrease in mechanical bone strength and stiffness in sexually mature, 7-week-old mice. In the tibia, the observed transgene effect was present in 3 and 7-week-old animals, indicating that the biological effect of disrupted GC signalling was independent of sexual maturity. This was not the case for the vertebral bones, where significant differences between tg and WT mice were seen in 7 but not in 3-week-old animals, suggesting that the effects of the transgene at this site may be modulated by age and/or changes in circulating sex hormone levels. Taken together, our results demonstrate that endogenous glucocorticoids may be required for normal bone growth but that their effect on bone structure and strength varies according to the skeletal site and sexual maturity of the animals.