Canine hemangiosarcomas have been ascribed to an endothelial origin based on histologic appearance; however, recent findings suggest that these tumors may arise instead from hematopoietic progenitor cells. To clarify this ontogenetic dilemma, we used genome-wide expression profiling of primary hemangiosarcomas and identified three distinct tumor subtypes associated with angiogenesis (group 1), inflammation (group 2), and adipogenesis (group 3). Based on these findings, we hypothesized that a common progenitor may differentiate into the three tumor subtypes observed in our gene profiling experiment. To investigate this possibility, we cultured hemangiosarcoma cell lines under normal and sphere-forming culture conditions to enrich for tumor cell progenitors. Cells from sphere-forming cultures displayed a robust self-renewal capacity and exhibited genotypic, phenotypic, and functional properties consistent with each of the three molecular subtypes seen in primary tumors, including expression of endothelial progenitor cell (CD133 and CD34) and endothelial cell (CD105, CD146, and αvβ3 integrin) markers, expression of early hematopoietic (CD133, CD117, and CD34) and myeloid (CD115 and CD14) differentiation markers in parallel with increased phagocytic capacity, and acquisition of adipogenic potential. Collectively, these results suggest that canine hemangiosarcomas arise from multipotent progenitors that differentiate into distinct subtypes. Improved understanding of the mechanisms that determine the molecular and phenotypic differentiation of tumor cells in vivo could change paradigms regarding the origin and progression of endothelial sarcomas.

Targeted toxins have the potential to overcome intrinsic or acquired resistance of cancer cells to conventional cytotoxic agents. Here, we hypothesized that EGFuPA-toxin, a bispecific ligand - targeted toxin (BLT) consisting of a deimmunized Pseudomonas exotoxin (PE) conjugated to epidermal growth factor and urokinase, would efficiently target and kill cells derived from canine hemangiosarcoma (HSA), a highly chemotherapy resistant tumor, as well as cultured hemangiospheres, used as a surrogate for cancer stem cells (CSC). EGFuPA-toxin showed cytotoxicity in four HSA cell lines (Emma, Frog, DD-1, and SB) at a concentration of ≤100 nM, and the cytotoxicity was dependent on specific ligand-receptor interactions. Monospecific targeted toxins also killed these chemoresistant cells; in this case, a “threshold” level of EGFR expression appeared to be required to make cells sensitive to the monospecific EGF-toxin, but not to the monospecific uPA-toxin. The IC50 of CSCs was higher by approximately two orders of magnitude compared to non-CSCs, but these cells were still sensitive to EGFuPA-toxin at nanomolar (i.e., pharmacologically relevant) concentrations, and when targeted by EGFuPA-toxin, resulted in death of the entire cell population. Taken together, our results support the use of these toxins to treat chemoresistant tumors such as sarcomas, including those that conform to the cancer stem cell model. Our results also support the use of companion animals with cancer for further translational development of these cytotoxic molecules.

BackgroundHuman angiosarcoma and canine hemangiosarcoma are thought to arise from vascular tissue or vascular forming cells based upon their histological appearance. However, recent evidence indicates a hematopoietic or angioblastic cell of origin for these tumors. In support of this idea, we previously identified an endothelial-myeloid progenitor cell population with high expression of endothelial cell markers and the myeloid cell marker, colony stimulating factor 1 receptor (CSF-1R). Here, we further characterized these cells to better understand how their cellular characteristics may impact current therapeutic applications.MethodsWe performed cell enrichment studies from canine hemangiosarcoma and human angiosarcoma cell lines to generate cell populations with high or low CSF-1R expression. We then utilized flow cytometry, side population and cell viability assays, and fluorescence based approaches to elucidate drug resistance mechanisms and to determine the expression of hematopoietic and endothelial progenitor cell markers.ResultsWe demonstrated that cells with high CSF-1R expression enriched from hemangiosarcoma and angiosarcoma cell lines are more drug resistant than cells with little or no CSF-1R expression. We determined that the increased drug resistance may be due to increased ABC transporter expression in hemangiosarcoma and increased drug sequestration within cellular lysosomes in both hemangiosarcoma and angiosarcoma.ConclusionsWe identified drug sequestration within cellular lysosomes as a shared drug resistance mechanism in human and canine vascular sarcomas marked by high CSF-1R expression. Taken together, our results demonstrate that studies in highly prevalent canine hemangiosarcoma may be especially relevant to understanding and addressing drug resistance mechanisms in both the canine and human forms of this disease.

Canine hemangiosarcoma is a rapidly progressive disease that is poorly responsive to conventional chemotherapy. Despite numerous attempts to advance treatment options and improve outcomes, drug resistance remains a hurdle to successful therapy. To address this problem, we used recently characterized progenitor cell populations derived from canine hemangiosarcoma cell lines and grown as non-adherent spheres to identify potential drug resistance mechanisms as well as drug-resistant cell populations. Cells from sphere-forming cultures displayed enhanced resistance to chemotherapy drugs, expansion of dye-excluding side populations and altered ATP-binding cassette (ABC) transporter expression. Invasion studies demonstrated variability between cell lines as well as between sphere and monolayer cell populations. Collectively, our results suggest that sphere cell populations contain distinct subpopulations of drug-resistant cells that utilize multiple mechanisms to evade cytotoxic drugs. Our approach represents a new tool for the study of drug resistance in hemangiosarcoma, which could alter approaches for treating this disease.

We evaluated the effect of Aurora kinase inhibitors AZD1152 and VX680 on canine osteosarcoma cell lines. Cytotoxicity was seen in all four cell lines; however, half-maximal inhibitory concentrations were significantly higher than in human and canine leukemia cells. AZD1152 reduced Aurora kinase B phosphorylation, indicating resistance was not due to failure of target recognition. Efflux mediated by ABCB1 and ABCG2 transporters is one known mechanism of resistance against these drugs and verapamil enhanced AZD1152-induced apoptosis; however, these transporters were only expressed by a small percentage of cells in each line and the effects of verapamil were modest, suggesting other mechanisms contribute to resistance. Our results indicate canine osteosarcoma cells are resistant to Aurora kinase inhibitors and suggest that these compounds are unlikely to be useful as single agents for this disease. Further investigation of these resistance mechanisms and the potential utility of Aurora kinase inhibitors in multi-agent protocols is warranted.