As solid tumor types, breast and prostate cancer are rivalled only by lung cancer in their propensity to metastasize to bone in the later stages of disease. At advanced stages of disease, approximately 80% of breast and 90% of prostate cancer patients will present with bone metastases. Bone metastases are often a painful conclusion to the lives of these patients, resulting in bone pain, hypercalcemia, pathological fractures and spinal cord compression. The culmination of these comorbidities considerably reduces a patient’s quality of life and prolonged survival. Hormone depletion is used as a first line of treatment in the majority of cases, negatively regulating bone health due to increased bone resorption by osteoclasts and decreased bone formation by osteoblasts. Not only is bone integrity undermined, but this action of increased bone turnover is beneficial for the colonization of metastasizing cells which co-opt and enhance the same mechanisms to establish and maintain their own growth. This is termed ‘the vicious cycle’ of osteolytic bone metastasis.
Current research approaches aim to identify bone-targeted therapies which not only inhibit tumor growth but concurrently protect bone. In this study, Dickkopf-1 (DKK-1), mechanistic target of rapamycin (mTOR) and p38 mitogen-activated kinases (p38 MAPK) are presented as novel targets. Pro-tumor roles have been described for all and clinical trials are currently investigating their efficacy in different cancer types. In normal bone biology DKK-1 is an inhibitor of the canonical Wnt signaling pathway which promotes osteoblastogenesis while mTOR signaling is a promoter of osteoclastogenesis. P38 MAPK inhibitors have been shown to regulate DKK-1 expression and bone destruction in preclinical models of multiple myeloma. The aims of this current study were to 1) investigate the role of DKK-1 in the biology of osteotropic breast cancer, 2) to assess the potential bone protective effects of mTOR inhibition by everolimus in the context of osteotropic cancers and 3) to test the hypothesis that p38 MAPK is a regulator of DKK-1 expression in prostate cancer, potentially supporting an osteolytic phenotype by impairing osteoblastogenesis.
In aim 1, analysis of a breast cancer tissue microarray demonstrated that DKK-1 expression was elevated in advanced and invasive tumor stages. Strikingly, positive DKK-1 expression correlated with a significantly reduced survival rate only in estrogen receptor-negative (ER-) breast cancer patients compared to patients with tumors which were negative for DKK-1 expression. In MDA-MB-231 breast cancer xenograft models, neutralization of secreted DKK-1 by treating mice with the monoclonal DKK-1 antibody BHQ880 or knocking out the expression of DKK-1 in MDA-MB-231 cells using CRISPR-Cas9 mediated gene editing, resulted in reduced tumor growth and burden by ≥ 50% (p < 0.05). In aim 2, the mTOR inhibitor everolimus is presented as an anti-tumor and bone-protective agent. The anti-tumor effects of everolimus were confirmed in two subcutaneous tumor models and a model of breast cancer bone metastasis, were tumor burden in the bone was reduced by 45.4% (p < 0.01). Bone loss induced by a hormone-deprived environment in ovariectomized mice was prevented with everolimus treatment as was bone destruction in the metastasis model. In more detail, it could be shown that everolimus maintained osteoblast function while specifically inhibiting osteoclast function. In aim 3, p38 MAPK is presented as a regulator of DKK-1 in prostate cancer. While the activation of p38 MAPK upregulated DKK-1, inhibition of p38 MAPK using small molecule inhibitors and siRNAs inhibited DKK-1 expression. Furthermore, assessment of different p38 MAPK isoforms revealed MAPK11 as the most effective regulator of DKK-1 and inhibition of DKK-1 by interfering with p38 MAPK signaling was sufficient to prevent the inhibitory effects of prostate cancer-derived DKK-1 on osteoblastogenesis in vitro.
This study has assessed multiple targets and their concurrent roles in cancer and bone cell biology. Specifically, DKK-1 has been proven to be a tumor promoter in ER- breast cancer and can be targeted therapeutically to inhibit tumor growth. MTOR inhibition by everolimus has been shown to be an effective mono-therapy in ER- breast cancer, inhibiting the growth of subcutaneous tumor and bone metastases and preventing bone loss induced by estrogen ablation. This further supports its use in postmenopausal women with breast cancer who are predisposed to developing osteoporosis and bone metastases. It also supports the use of everolimus in hormone receptor-negative or triple receptor-negative breast cancer, for which it has not yet been approved. A clear link has been made between p38 MAPK signaling and DKK-1 expression in prostate cancer and its consequent regulation of osteoblastogenesis. A future focus on the inhibition of a specific MAPK isoform, MAPK11 in particular, may help in translating these encouraging in vitro results into promising pre-clinical trials in vivo.
As a whole, these investigations provide a foundation for further research and could be valuable for the design of future clinical trials, leading to improvements in the treatment and prognosis of osteolytic bone metastases.
| Identifer | oai:union.ndltd.org:DRESDEN/oai:qucosa:de:qucosa:29528 |
| Date | 31 August 2017 |
| Creators | Browne, Andrew |
| Contributors | Hofbauer, Lorenz, Bornhäuser, Martin, Technische Universität Dresden |
| Source Sets | Hochschulschriftenserver (HSSS) der SLUB Dresden |
| Language | English |
| Detected Language | English |
| Type | doc-type:doctoralThesis, info:eu-repo/semantics/doctoralThesis, doc-type:Text |
| Rights | info:eu-repo/semantics/openAccess |
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