Research Communications of the 24th ECVIM-CA Congress

No conflicts of interest reported. In canines mastocytomas are among the most frequently diagnosed neoplasms of the skin. High grade mastocytomas (grade III, Patnaik classification) are characterized by an uncontrolled growth of neoplastic mast cells (MC) and a poor prognosis. Recently, the KIT-targeting tyrosine kinase inhibitors masitinib and toceranib have been approved for the treatment of canine MC tumors. These drugs are able to induce responses in mastocytoma patients. However, in many patients, relapses are seen. Therefore, research is focusing on new drug targets. Recently, the transcription factor STAT5 has been reported to play an important role in the proliferation and survival of human neoplastic MC. The aim of the present study was to evaluate the JAK2-STAT5 pathway in canine mastocytomas. To address this aim, the canine mastocytoma cell lines C2 and NI-1 as well as inhibitors directed against JAK2 or STAT5 were employed. As assessed by immunocytochemistry, C2 cells and NI-1 cells were found to express pSTAT5 in their cytoplasm and nuclei. Intracellular expression of pSTAT5 was confirmed by flow cytometry. Interestingly, C2 cells were found to express higher levels of pSTAT5 compared to NI-1 cells. Next, we treated C2 cells and NI-1 cells with various concentrations of the STAT5 inhibitors piceatannol and pimozide and the JAK2 inhibitors AZD1480 and TG101348. As assessed by 3 H-thymidine uptake, all 4 compounds were found to inhibit the proliferation of canine MC in a dose-dependent manner. Drug effects were found to vary in different cell lines, with the following rank-order of potency (IC 50 values): TG101348: 0.1-0.75 lM; pimozide: 0.1-2.5 lM; AZD1480: 1-2 lM; piceatannol: 5-50 lM. To further explore the mechanism of drug-induced inhibition of proliferation, we examined cell cycle progression and apoptosis in drug-exposed cells. Whereas all 4 drugs tested induced only moderate cell cycle arrests in the G1 phase, these drugs were found to induce substantial apoptosis in C2 cells and NI-1 cells as evidenced by microscopy and Annexin-V/PI staining. Together, our data show that JAK2-and STAT5-targeting drugs exert anti-proliferative and apoptosis-inducing effects in canine mastocytoma cells suggesting that this signaling pathway may be a promising new therapeutic target in canine mastocytomas. The clinical relevance of this observation remains to be determined.