Expression of stem cell factor receptor (c-kit) by the malignant mast cells from spontaneous canine mast cell tumours

Stem cell factor receptor (SCFR, c-kit), normally expressed on haematopoietic and mast cells, plays a regulatory role in cellular growth and differentiation. Dysregulated expression of SCFR may contribute to neoplastic transformation. We investigated expression of SCFR on malignant canine mast cells obtained directly from spontaneous canine mast cell neoplasms, in an attempt to determine whether these undifferentiated cells maintained expression of this growth-promoting cytokine receptor. Malignant mast cells (histological grade 2) from skin tumours or lymph node metastases were collected from canine patients, and SCFRs were detected by flow cytometric analysis of these cells. All of the tumours bound mouse and canine recombinant stem cell factor (SCF), indicating that the cells not only expressed SCFRs, but that the receptors possessed the functional property of ligand binding. Immunoglobulin Fc receptors for canine IgE were identified on these cells by flow cytometry, a further indication that the cells analysed were mast cells and retained some differentiated features. Immunohistochemical analysis of formalin-fixed, paraffin wax-embedded mast cell tumour biopsies confirmed expression of SCFRs by malignant cells from each tumour. The relative binding of SCF to suspensions of tumour cells, as assessed by flow cytometry, correlated with the intensity of immunolabelling for SCFR in sections of the same tumours, suggesting variability in SCFR expression between tumours. Agarose gel electrophoresis of the products of SCFR reverse transcription-polymerase chain reaction derived from each tumour had the molecular weight predicted for canine SCFR cDNA on the basis of the mouse and human counterparts. This further confirmed SCFR expression by malignant canine mast cells. Taken together, these results show that a membrane receptor capable of triggering cell growth is expressed by malignant canine mast cells, suggesting a role for this receptor in the aetiology of canine mast cell cancer. This relatively common malignancy of the dog would seem to present an opportunity for the investigation of the potential role of the SCF/SCFR pathway in the development of spontaneous malignancies of mast cells.