The human bone morphogenetic protein (BMP) family consists of over 20 growth factor proteins that are involved in bone formation and developmental processes. BMPs are extracellular signalling molecules that are able to regulate various cellular functions, proliferation, differentiation, apoptosis and migration. For the last 10 years, these powerful cytokines have increasingly been studied in several cancers, and aberrant expression patterns of BMPs have been reported. Functional studies have suggested that BMPs are involved in both cancer promotion and inhibition. The role these signalling molecules play in breast cancer is only starting to emerge: thus far, studies have been even contradictory. Different BMP ligands have been shown to decrease as well as increase cancer cell growth and migration. Furthermore, they are involved in bone metastases, which are a common feature in breast cancer. In this sense, BMPs resemble a closely related protein transforming growth factor b, which possesses a bidirectional role in cancer cell regulation. In this review, we focus on the current knowledge of BMP expression, functional roles and involvement in bone metastasis in breast cancer.

Background. Breast cancer is the most common cancer in women worldwide. Bone morphogenetic proteins (BMPs), members of the transforming growth factor β superfamily, are known to regulate cell proliferation, differentiation and motility, and have also been shown to be involved in cancer pathogenesis, also in breast cancer. We have previously demonstrated that BMP4 is able to consistently reduce breast cancer cell proliferation through G1 cell cycle arrest and to simultaneously induce migration and invasion in a subset of breast cancer cell lines. Similarly, our clinical data revealed a correlation between elevated BMP4 expression in primary breast tumors and reduced proliferation as well as increased risk of recurrence. The growth inhibitory effects of BMP4 have also been demonstrated in vivo but its possible metastasis promoting functions are less well characterized. Here we set out to investigate this topic using a xenograft mouse model. Methods. MDA-MB-231 breast cancer cells were transduced with a luciferase-expressing vector to allow monitoring of the metastasis formation using bioluminescence imaging. Cells (2 × 105) were injected into the mice intracardially and BMP4 (100 ng/g, 10 animals) or vehicle control (11 animals) was administered through tail vein three times a week. After seven weeks, the mice were sacrificed and metastases collected for histological analyses. Results. The overall amount of metastases was similar in both groups (13 in BMP4-treatment group vs. 12 in control group). There was a slight but non-significant trend of metastases developing earlier in the BMP4 group compared to controls. Most of the metastases occurred in bone and adrenal glands. There were somewhat more metastases in bone in the BMP4-treated mice (10 vs. 7) and more adrenal gland metastases in vehicle-treated animals (5 vs. 1). To assess the contribution of BMP4 to the characteristics of the metastases, the tumors were stained for pSMAD1/5/9 (BMP signaling activation), Ki67 (proliferation), MECA32 (blood vessels), mesenchymal marker vimentin, α-SMA (cancer-associated fibroblasts) and basal markers K5 and K14. No major dissimilarities were observed between the BMP4 and vehicle tumor groups in the staining patterns. Interestingly, the osteoclast marker Tartrate-resistant acid phosphatase (TRAP) was expressed in both groups in the cancer cells whereas Toluidine Blue staining revealed that the bone morphology was not detrimentally affected by BMP4 treatment. Conclusions. Despite its ability to enhance breast cancer cell migration and invasion in vitro, BMP4 does not seem to have a dramatic impact on in vivo metastasis formation, although a small acceleration in appearance of the metastases was observed. However, the limitations of the xenograft model do not allow us to exclude the possible long-term effects of BMP4 that might be more applicable to human situation. Citation Format: Minna Ampuja, Emma L. Alarmo, Philip Owens, Riikka Havunen, Agnes E. Gorska, Harold L. Moses, Anne Kallioniemi. The impact of BMP4 on breast cancer metastasis in an in vivo xenograft mouse model. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 629.