Aberrant autophosphorylation of c-Kit receptor in canine mast cell tumor cell lines

Several studies indicated that KIT mutation could cause ligand-independent activation of c-Kit receptor in canine mast cell tumor (MCT). The objective of this study was to investigate mechanisms of c-Kit receptor activation in various canine MCT cell lines. Four cell lines, HRMC (derived from cutaneous MCT), VIMC1 (visceral MCT), CoMS1 (visceral MCT) and CMMC1 (cutaneous MCT), were cultured in stem cell factor (SCF, a ligand of c-Kit receptor)-free medium and subjected to analyses of KIT mutation, c-Kit receptor phosphorylation, SCF expression and the effects of SCF stimulation. In addition, the SCF/c-Kit receptor autocrine mechanism was verified in HRMC cells. HRMC cells expressed wild type c-Kit receptor. Both VIMC1 and CoMS1 cells had the same one amino acid (AA) substitution in the extracellular domain of c-Kit receptor. CMMC1 cells had at least three variants of c-Kit receptor such as one AA deletion in the extracellular domain (variant A), one AA substitution in the extracellular domain as well as an internal tandem duplication in the juxtamembrane domain (variant B), and a nonsense mutation (variant C). Both mature and immature forms of c-Kit receptor were observed and the c-Kit receptors were phosphorylated in all cell lines. While both mature and immature forms of c-Kit receptor were substantially phosphorylated in CMMC1 cells, the immature form was slightly phosphorylated in other cell lines. Phosphorylation of c-Kit receptor in HRMC, VIMC1 and CoMS1 cells were enhanced by SCF stimulation whereas no enhancement was observed in CMMC1 cells. There was no effect of SCF stimulation on proliferation of all the cell lines. SCF protein was detectable in only HRMC cells although mRNA expression of SCF was detected in all the cell lines. A tyrosine kinase inhibitor Dasatinib (internal inhibitor) inhibited c-Kit receptor phosphorylation in HRMC cells whereas anti-canine SCF antibody (external inhibitor) had no inhibitory effect. Thus there could be no external SCF/c-Kit receptor autocrine mechanism whereas there could be an internal autocrine mechanism within HRMC cells. The results indicated that consistent c-Kit receptor phosphorylation could be caused by the stimulation with autocrine SCF in HRMC cells while it could be caused by functional mutations of KIT in VIMC1, CoMS1 and CMMC1 cells. As the four canine MCT cell lines had various aberrations associated with c-Kit receptor phosphorylation, KIT mutation and SCF expression, such molecular biological diversity might reflect the different biological behavior in canine MCT.