SCF/Kit Signaling Enhances Fgfr2b-dependent Expansion of Salivary Gland Stem/Progenitor Cells

Irradiation damage to adult submandibular glands (SMGs) causes irreversible loss of gland function. Transplanting Kit+ SMG stem/progenitor cells post-irradiation restored normal saliva production in mice. In addition, fibroblast growth factor 10  (Fgf10) signaling, via its receptor Fgfr2b, is essential for survival and proliferation of SMG progenitor cells. We previously found that stem cell factor (SCF), which signals via its receptor Kit, increases Kit+K14+Sox10+ SMG progenitors by upregulating transcription factors downstream of Fgf10/Fgfr2b. Our current work investigates the signaling mechanism involved in the self-renewal of these SMG progenitor cells. Objectives: Determine how Kit and Fgfr2b signaling pathways interact to enhance downstream gene expression. Methods: Using biochemical pulldown assays with recombinant Fgfr2b-Fc, Kit, and their ligands we examined a potential direct receptor interaction. We measured Kit phosphorylation in SMG epithelia stimulated with SCF and/or Fgf10. We used chemical inhibitors of Kit and Fgfr2b to investigate whether they can transactivate each other. Finally, we investigated the link between Kit and Fgfr2b signaling pathways by analyzing the temporal activation of MAPK and PI3K pathways downstream of receptor stimulation. Results: Our in vitro data suggests a direct interaction between Kit and Fgfr2b in the absence of ligands, and that addition of ligands for either receptor dissociates the complex. With Fgf10 stimulation, Fgfr2b signaling increases Kit phosphorylation, suggesting it may transactivate the c-Kit receptor in the SMG. Finally, SCF combined with Fgf10 increases and sustains MAPK/PI3K signaling in SMG epithelia, which increases gene expression of transcription factors involved in self-renewal. Conclusion: Our data suggests that the Kit receptor is transactivated by Fgf10/Fgfr2b leading to increased and sustained MAPK/PI3K signaling. Ultimately, this increases the expression of transcription factors involved in the expansion of Kit+K14+Sox10+ stem/progenitor cells. Taken together, our data indicate that both SCF/Kit and Fgf10/Fgfr2b are required for expansion of SMG epithelial progenitor cells.