BMP7 influences proliferation, migration, and invasion of breast cancer cells
Introduction
Bone morphogenetic protein 7 (BMP7) is a member of the bone morphogenetic protein (BMP) family of signaling molecules. BMPs are closely related to transforming growth factor β (TGFβ), and currently over 20 BMP family members have been identified in humans [1]. These proteins share structural similarity and a cysteine knot with each other and with TGFβ family members [2]. The major pathway leading to BMP-induced gene responses consists of cell surface serine/threonine kinase receptors and the Smad family of transcription factors [3]. There are three type I receptors (BMPR1A, BMPR1B, ACVR1) and three type II receptors (BMPRII, ACVR2A, AVCR2B) that can specifically bind BMP ligands [4]. BMP binding activates these membrane receptors, which in turn activate the cytosolic Smad proteins. Through a versatile system of Smad coregulators, different BMP responses are launched [5]. BMP signaling is regulated in an intricate manner both inside and outside of the cell [3], [6]. BMPs are also known to activate and cross-talk with other important cellular signaling pathways, such as the MAP kinase pathways [7], [8].
BMP7 has an established role as a potent bone inducer [9], [10] and is essential during development, particularly in the formation of the skeleton, kidney, and eye [11]. In normal adult tissues, BMP7 is expressed in renal cells and has been linked to diverse renal injuries [12]. BMP7 has also been implicated in various types of cancer. Most of these studies have focused on prostate cancer, and clearly reflect the current paradoxical state of BMP research by demonstrating that BMP stimulation can lead to diverse, even contradictory, phenotypes depending on the ligand and the exact cell type [1]. BMP7 has been reported to protect prostate cancer cells from apoptosis [13], inhibit proliferation [14], [15], induce epithelial-mesenchymal transdifferentiation [14], induce migration and invasion [16], or reduce invasion and motility [17]. In thyroid carcinoma, myeloma, and colon cancer, BMP7 reduced cell proliferation [18], [19], [20]. BMP7 overexpression has been detected in colorectal cancer [21] and moreover, in prostate cancer bone metastasis and melanoma metastases [22], [23], [24] indicating metastasis-specific expression. Based on these data, BMP7 appears to have an impact on cancer cell behavior.
The number of studies examining BMPs in breast cancer is still very limited. Our group and others have shown that BMP7 is widely expressed and at elevated levels in breast tumors as compared to normal mammary gland [25], [26], [27]. We also recently demonstrated that BMP7 expression in primary breast tumors leads to accelerated bone metastasis formation and is, in fact, an independent prognostic indicator for early bone metastasis [28]. The cancer-specific expression pattern and, more importantly, the link to early bone metastasis formation suggest that BMP7 is likely to influence breast cancer cell behavior. Here, we both inhibited BMP7 expression in breast cancer cells with high endogenous expression using RNA interference (RNAi) and treated non-expressing breast cancer cells with BMP7 ligand to establish the contribution of BMP7 to the breast cancer cell phenotype.
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Cell lines
Three breast cancer cell lines with high BMP7 expression levels (BT-474, MCF7, SK-BR-3) [25] were used in the RNAi studies. Five cell lines with either no detectable or low levels of BMP7 expression (MDA-MB-231, MDA-MB-361, HCC1954, T-47D, ZR-75-30) [25] were used in the ligand treatment assays. All cell lines were obtained from the American Type Culture Collection (ATCC, Manassas, VA) and maintained under the recommended conditions.
Real time quantitative RT-PCR (qRT-PCR)
The BMP7 mRNA levels were measured by qRT-PCR using the
Quantitation of BMP7 expression by real time qRT-PCR
In our previous work, BMP7 expression levels were determined in a large panel of breast cancer cell lines using standard semiquantitative RT-PCR [25]. Based on this data, we selected eight cell lines and confirmed BMP7 expression levels using qRT-PCR to obtain more accurate measurements. As expected, the highest expression levels were observed for BT-474, MCF7, and SK-BR-3 cells (Fig. 1). In HCC1954 and MDA-MB-361 cells, BMP7 expression was very low, and virtually no BMP7 mRNA was detected in
Discussion
Bone morphogenetic protein 7 is a pleiotropic signaling molecule that has a well established role in development. It belongs to the family of bone morphogenetic proteins that have also been studied with regard to their possible contribution to cancer pathogenesis. This field is currently rather controversial since there is great diversity in the proposed functions of various BMPs, depending on the BMP ligand and cancer type studied. Similar diverse outcomes have also been observed with another
Conflict of interest statement
The authors declare that they have no conflict of interest.
Acknowledgements
The authors greatly appreciate the skillful assistance of Ms. Kati Rouhento. This study was supported in part by the Academy of Finland, the Finnish Cancer Organizations, the Sigrid Juselius Foundation, and the Medical Research Fund of Tampere University Hospital.
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