Is Osteopontin a Good Marker for Bone Metastasis in Canine Mammary Gland Tumor and Prostate Cancer?

SIMPLE SUMMARY: Bone metastasis can develop from several tumor subtypes, and the presence of bone metastasis confers a negative prognosis to patients. In dogs, a small group of tumors, including urogenital and mammary gland tumors (MGTs), is known to develop bone metastases. The mechanisms involved in the development of bone metastases in dogs are unknown, and few studies have been published on this subject. Osteopontin (OPN) is a glycoprotein involved in tumor progression, angiogenesis, and metastasis. Several studies have implicated OPN overexpression in a higher incidence of bone metastases. In addition, OPN overexpression has been shown to be correlated with increased bone resorption in patients with cancer. Although OPN expression has been shown to be important in several cancer subtypes, in Veterinary Medicine, no previous studies had investigated the role of OPN in patients with bone metastasis. Therefore, our study aimed to evaluate OPN levels immunohistochemically and associate them with the detection of bone metastasis in canine mammary tumors and prostate cancer. ABSTRACT: Osteopontin (OPN) is a protein synthesized by a large number of cells, and its overexpression has been associated with the development and prognosis of cancer. OPN overexpression has been claimed to be a marker for the development of bone metastasis in human cancers, but no prior research has investigated the association between OPN expression and the metastasis of canine mammary gland tumors (MGTs) and prostate cancer (PC). Therefore, we investigated OPN expression in MGTs and PC samples from 50 canine patients with or without metastasis (bone vs. other sites). Higher OPN expression was detected in primary tumor samples from animals with bone metastasis than in those without bone involvement (p = 0.0321). In MGT samples, a significantly lower survival rate was observed in patients with higher OPN expression (p = 0.0171). In animals with PC, there was a strong trend toward lower survival in animals with positive OPN expression; however, this trend was not statistically significant (p = 0.0779). From these findings, it can be concluded that OPN may be a promising target for future MGTs and PC studies because of its role in enhancing cell invasion and metastasis.