Bone Abstracts

Background: Breast cancer clinical trials have shown an enhanced anti-tumour activity of bone-targeted agents in postmenopausal patients. We have reported that zoledronic acid (ZOL) decreases serum levels of the tumour promoter follistatin in postmenopausal women and also inhibits expression of follistatin by breast tumour cells both in vitro and in vivo. We hypothesised that inhibin-A (InA) and ZOL may be altering bone levels of follistatin and its bound tumour suppressor protein activin, to enhance the anti-tumour effects of ZOL in postmenopausal women.

Objectives: To evaluate the effects of InA+ZOL on the bone microenvironment, including the levels of follistatin/activin as well as breast tumour cell homing, using an in vivo model of post-menopausal bone.

Methods: Twelve-week old BALB/c nude mice were ovariectomised (OVX)/sham-OVX and implanted with sub-cutaneous osmotic pumps delivering 10/60/120 ng/day InA/saline for 4 weeks. ZOL (100 μg/kg, IP) was injected weekly. DiD-labeled MDA-MB-231 cells were injected IC after 4 weeks. Serum was processed to ELISA for InA, TRAP and P1NP. Calvaria were crushed and supernatant processed to ELISA for activin and follistatin. Hind legs were fixed in 4%PFA and analysed using μCT+TRAP stain or frozen and processed for detection of DiD-labelled tumour cells by 2-photon microscopy.

Results: One hundred and twenty ng/day InA increased serum levels (mean InA (pg/ml); OVX-control=29.1, OVX-InA=68.3 P=0.0079) and prevented OVX-induced bone loss (mean BV/TV (%); OVX-control=4.4, OVX-InA=7.8 P=0.0079). InA did not affect numbers of Ob/Oc but, in OVX animals, increased TRAP (mean TRAP (U/I); OVX-control=2.3, OVX-InA 2.7, P=0.008) and decreased calvaria activin (mean activin (pg/ml); OVX-control=65.2, OVX-InA=37.2, P=0.01) but did not affect the number of DiD positive events/mm³. ZOL decreased serum follisatin in OVX animals (mean follistatin (pg/ml); OVX-control=67.8, OVX-ZOL=48, P=0.036).

Conclusions: In OVX animals, InA alters bone activin levels but does not affect tumour homing to bone. ZOL decreases bone follistatin levels in vivo, which may explain the serum effects seen in postmenopausal women after ZOL treatment. Combined effects of InA and ZOL on tumour growth in bone require investigation.