Contribution of Stem Cell Factor Autocrine/Paracrine Mechanism to Aberrant Proliferation of Mast Cells

A) Background: Mast cells originated from canine mast cell tumors are useful tools to investigate KIT-dependent or -independent proliferation of mast cells. It is well known that gain of function mutations in the c-kit gene are seriously associated with neoplastic proliferation of mast cells in humans, rodents, and dogs. However, KIT-independent malignant proliferation of mast cells has been rarely explored. Since most patients of mast cell leukemia/sarcoma and more than 70% of patients with cutaneous mastocytosis preserves wild type KIT but not mutant KIT, we attempted to find out KIT-independent mechanisms that induce tumorigenic proliferation of mast cells.

B) Methods: We investigated a mechanism of tumorigenesis using a wild-type KIT-expressing canine mast cell line. Western blot analysis was conducted to detect KIT phosphorylation. Both RT-PCR and flow cytometry analysis were carried out to examine the stem cell factor (SCF) expression. Inhibitory effects of a SCF neutralizing antibody and RNA interference for SCF on the cell proliferation were also evaluated. SCF production in xenografts consisted of wild-type KIT-expressing tumor was identified.

C) Results: High expression of SCF was detected in the canine mast cell line with wild-type KIT used in this study. In the cells, KIT was spontaneously phosphorylated. Neutralization of SCF as well as SCF gene silencing inhibited the growth of the cells, suggesting SCF autocrine/paracrine action. Production of SCF was also observed in several mast cell lines originated from humans and rodents, which was enhanced after PMA/ionomycin stimulation. Ki-67-positive cells in the xenografts were markedly positive for SCF. Moreover, SCF was strongly detected in 3 of 5 samples isolated from canine mast cell tumors that express wild-type KIT.

D) Conclusion and discussion: These results indicate the broad contribution of SCF autocrine/paracrine to the neoplastic proliferation of wild-type KIT-expressing mast cells not only in dogs but also both humans and rodents. It suggests that targeting SCF production may become a novel strategy for treatment of mast cell malignancies.