Bone is one of the most common sites of cancer metastasis in humans and is a significant source of morbidity and mortality. Bone metastases are considered incurable and result in pain, pathologic fracture, and decreased quality of life. Animal models of skeletal metastases are essential to improve the understanding of the molecular pathways of cancer metastasis and growth in bone and to develop new therapies to inhibit and prevent bone metastases. The ideal animal model should be clinically relevant, reproducible, and representative of human disease. Currently, an ideal model does not exist; however, understanding the strengths and weaknesses of the available models will lead to proper study design and successful cancer research. This review provides an overview of the current in vivo animal models used in the study of skeletal metastases or local tumor invasion into bone and focuses on mammary and prostate cancer, lymphoma, multiple myeloma, head and neck squamous cell carcinoma, and miscellaneous tumors that metastasize to bone.

BACKGROUND The gastrin-releasing peptide receptor (GRPr) is upregulated in early and late-stage human prostate cancer (PCa) and other solid tumors of the mammary gland, lung, head and neck, colon, uterus, ovary, and kidney. However, little is known about its role in prostate cancer. This study examined the effects of a heterologous GRPr agonist, bombesin (BBN), on growth, motility, morphology, gene expression, and tumor phenotype of an osteoblastic canine prostate cancer cell line (Ace-1) in vitro and in vivo. METHODS The Ace-1 cells were stably transfected with the human GRPr and tumor cells were grown in vitro and as subcutaneous and intratibial tumors in nude mice. The effect of BBN was measured on cell proliferation, cell migration, tumor growth (using bioluminescence), tumor cell morphology, bone tumor phenotype, and epithelial-mesenchymal transition (EMT) and metastasis gene expression (quantitative RT-PCR). GRPr mRNA expression was measured in primary canine prostate cancers and normal prostate glands. RESULTS Bombesin (BBN) increased tumor cell proliferation and migration in vitro and tumor growth and invasion in vivo. BBN upregulated epithelial-to-mesenchymal transition (EMT) markers (TWIST, SNAIL, and SLUG mRNA) and downregulated epithelial markers (E-cadherin and β-catenin mRNA), and modified tumor cell morphology to a spindle cell phenotype. Blockade of GRPr upregulated E-cadherin and downregulated VIMENTIN and SNAIL mRNA. BBN altered the in vivo tumor phenotype in bone from an osteoblastic to osteolytic phenotype. Primary canine prostate cancers had increased GRPr mRNA expression compared to normal prostates. CONCLUSION These data demonstrated that the GRPr is important in prostate cancer growth and progression and targeting GRPr may be a promising strategy for treatment of prostate cancer.

Feline mammary carcinoma (FMC) is similar to human breast cancer in the late age of onset, incidence, histopathologic features, biological behavior, and pattern of metastasis. Therefore, FMC has been proposed as a relevant model for aggressive human breast cancer. The goals of this study were to develop a nude mouse model of FMC tumor growth and metastasis and to measure the expression of genes responsible for lymphangiogenesis, angiogenesis, tumor progression, and lymph node metastasis in FMC tissues and cell lines. Two primary FMC tissues were injected subcutaneously, and 6 FMC cell lines were injected into 3 sites (subcutaneous, intratibial, and intracardiac) in nude mice. Tumors and metastases were monitored using bioluminescent imaging and characterized by gross necropsy, radiology, and histopathology. Molecular characterization of invasion and metastasis genes in FMC was conducted using quantitative real-time reverse transcription polymerase chain reaction in 6 primary FMC tissues, 2 subcutaneous FMC xenografts, and 6 FMC cell lines. The histologic appearance of the subcutaneous xenografts resembled the primary tumors. No metastasis was evident following subcutaneous injection of tumor tissues and cell lines, whereas lung, brain, liver, kidney, eye, and bone metastases were confirmed following intratibial and intracardiac injection of FMC cell lines. Finally, 15 genes were differentially expressed in the FMC tissues and cell lines. The highly expressed genes in all samples were PDGFA, PDGFB, PDGFC, FGF2, EGFR, ERBB2, ERBB3, VEGFD, VEGFR3, and MYOF. Three genes ( PDGFD, ANGPT2, and VEGFC) were confirmed to be of stromal origin. This investigation demonstrated the usefulness of nude mouse models of experimental FMC and identified molecular targets of FMC progression and metastasis.

Thyroid cancer is the most common endocrine malignancy in dogs. Dogs and humans are similar in the spontaneous development of thyroid cancer and metastasis to lungs; however, thyroid cancer has a higher incidence of metastasis in dogs. This study developed a preclinical nude mouse model of canine thyroid cancer using a canine thyroid adenocarcinoma cell line (CTAC) and measured the expression of important invasion and metastasis genes in spontaneous canine thyroid carcinomas and CTAC cells. CTAC cells were examined by electron microscopy. Short tandem repeat analysis was performed for both the original neoplasm and CTAC cells. CTAC cells were transduced with luciferase and injected subcutaneously and into the tail vein. Tumors and metastases were monitored using bioluminescent imaging and confirmed with gross necropsy and histopathology. Invasion and metastasis genes were characterized in 8 follicular thyroid carcinomas (FTCs), 4 C-cell thyroid carcinomas, 3 normal thyroids, and CTAC cells. CTAC cells grew well as xenografts in the subcutis, and they resembled the primary neoplasm. Metastasis to the kidney and lung occurred infrequently following subcutaneous and tail vein injection of CTAC cells. STR analysis confirmed that CTAC cells were derived from the original neoplasm and were of canine origin. Finally, 24 genes were differentially expressed in spontaneous canine thyroid carcinomas, CTAC, and normal thyroids. This study demonstrated the usefulness of a nude mouse model of experimental canine thyroid carcinoma and identified potential molecular targets of canine follicular and C-cell thyroid carcinoma.