The role of autophagy in canine osteosarcoma

Canine osteosarcoma (OS) is a primary tumour in the bone and is mesenchymal in origin. The common sites affected are the metaphyseal regions of long bones, particularly the front limbs of dogs. Osteosarcoma in dogs shares many biological and clinical resemblances in humans with molecular and genetic abnormalities. We have previously shown that autophagy plays a major role in promoting the development of osteosarcoma in humans, however, its role canine in osteosarcoma is unclear. Targeting autophagy as an anticancer therapy is complicated by its nature of dual role: activating autophagy can prevent apoptosis promoting cell survival or it could be a trigger leading to cell death. Drug resistance is thought to be the main cause and overcoming this phenomenon is a key step towards greater efficacy in OS therapy. The current study investigated the role of autophagy in canine OS and further analyzed the migration and metastatic progression of OS when compared to normal canine osteoblasts. The results from this study showed conflicting results at the gene expression level when examining levels of autophagy in canine OS biopsies or primary OS cells, however, at the protein level, OS cell lines treated with doxorubicin or carboplatin showed an increase in LC3 puncta and p62 clearance, detected by Immunofluorescence.

When considering migration, the use of chemotherapeutic drugs had a significant impact on inhibiting or slowing down the rate of cell migration through the wound scratch in primary canine osteoblasts but was significantly less effective in canine OS cells. Preliminary data from our group has shown that migratory body formation can be modulated by co-culturing osteosarcoma cells with bone-marrow-derived mesenchymal stem cells. We hypothesized that these migratory bodies are early sarcosphere or sarcosphere precursors that may contain tumour-initiating cells and could mimic the early stages of metastasis. Formation of migratory bodies was directly proportional to the number of cells plated, and these migratory bodies expressed increased levels of stem cell marker genes Nanog, STAT and Oct3/4 and key markers of autophagy (MAP1LC3, ATG5, ATG7 and SQSTM1), emphasizing the importance of autophagy in the induction of the metastatic process.

Chemoresistance is a major challenge in the treatment of both human and canine patients with metastatic OS. Our overall data suggest that autophagy indeed has a prominent role in canine OS, and may be involved in progression, metastatic potential, and chemotherapy resistance.