Most breast cancer patients die when tumor cells spread to other organs (metastasis). The most common site of breast cancer metastasis is the bone, which occurs in approximately 80% of patients with advanced disease. One of the hallmarks of breast cancer metastasis to bone is bone loss due to the activation of bone-resorbing cells, which leads to release of tumor-promoting factors from the bone matrix; however, the mechanisms behind this are not well understood. Bone metastasis has a significant impact on the quality of life for the majority of breast cancer patients, causing fractures, extreme pain, and hypercalcemia; detection of bone metastases often shifts the prognosis of the disease from treatable to incurable.

Our lab has discovered that up to 20% of breast tumors overexpress three genes in the Macrophage Stimulating Protein (MSP) pathway: MSP, its receptor RON, and the protease that cleaves and activates MSP, MT-SP1. The overexpression of these three genes is an independent prognostic factor for poor outcome. These patients were shown to have significantly shorter metastasis-free survival as well as a shorter overall survival. In addition, the breast cancers of these patients also demonstrate a preference for bone metastasis, suggesting a role for the MSP signaling pathway in this process. Work from our lab also showed a causative role for MSP in metastasis: overexpression of MSP promotes spontaneous metastasis to bone in a mouse model. The receptor for MSP, RON, is expressed on osteoclasts, and when MSP is secreted from tumor cells, it can directly activate these osteoclasts in vitro. Together, these data strongly suggest that MSP may have an important function in metastasis of breast cancer in both humans and mice, and our experimental systems provide a unique opportunity to better understand this process. Our data has led to the hypothesis that crosstalk between tumor cells and bone cells, via MSP, promotes metastasis of breast tumors to bone. We propose that MSP is a critical signal that tumor cells send to bone cells, causing them to initiate bone resorption and create an optimal environment for the tumor to grow.

We propose to address our hypothesis with several specific aims. First, we will test the requirement for the MSP pathway in bone metastasis and determine whether inhibiting this pathway (either genetically or with drugs) prevents or reduces bone metastasis. To do these experiments, we will utilize a human breast cancer cell line that naturally overexpresses the three genes in the MSP signaling pathway, and thus mimics the tumors we studied in our clinical analyses mentioned above. We have found that this cell line forms tumors in mouse mammary glands, which spontaneously metastasize to bone at a high frequency. To our knowledge, this is the first human breast cancer cell line that has ever been shown to spontaneously metastasize to bone, providing us with an ideal model in which to study this process. We will test the efficacy of drugs that block the MSP pathway in preventing metastasis of human breast cancer to bone. We will also utilize RON knockout mice in order to determine the genetic requirement for RON in tumor cells and/or bone cells during bone metastasis.

Second, we will determine the mechanisms by which MSP activates bone cells, thus making a hospitable environment for tumor growth. We will study the interactions of tumor cells and bone cells using relatively simple in vitro (three-dimensional tumor-bone co-cultures) and in vivo (intratibial injection) models for the initial experiments, and then we will validate our findings in the spontaneous metastasis model. Specifically, we will determine whether MSP directly activates RON on bone osteoclasts, and whether this initiates bone resorption. We will also determine if MSP expression causes activation of the TGF-beta pathway, which has previously been implicated in bone resorption.

Lastly, we will use genome-wide gene expression analysis to determine which genes are activated in bone cells in response to MSP, and determine their role in bone metastasis as well as tumor growth in the bone. The results generated from our research should shed light on why breast cancer shows preferential metastasis to bone and the role of the MSP signaling pathway in this process. An improved understanding of the mechanisms underlying bone metastasis in breast cancer can then be used to design and implement effective therapeutic approaches and should consequently have a significant impact on the lives of breast cancer patients.