Abstract
Kit is a receptor-type tyrosine kinase found on the plasma membrane. It can transform mast cells through activating mutations. Here, we show that a mutant Kit from neoplastic mast cells from mice, Kit(D814Y), is permanently active and allows cells to proliferate autonomously. It does so by activating two signalling pathways from different intracellular compartments. Mutant Kit from the cell surface accumulates on endolysosomes through clathrin-mediated endocytosis, which requires Kit’s kinase activity. Kit(D814Y) is constitutively associated with phosphatidylinositol 3-kinase, but the complex activates Akt only on the cytoplasmic surface of endolysosomes. It resists destruction because it is under-ubiquitinated. Kit(D814Y) also appears in the endoplasmic reticulum soon after biosynthesis, and there, can activate STAT5 aberrantly. These mechanisms of oncogenic signalling are also seen in rat and human mast cell leukemia cells. Thus, oncogenic Kit signalling occurs from different intracellular compartments, and the mutation acts by altering Kit trafficking as well as activation.
Highlights
Kit is a receptor-type tyrosine kinase found on the plasma membrane
Immunoprecipitation assays confirmed that Kit(wt) in R cells and pt[18] cells[28] was activated in a ligand-dependent manner, whereas Kit(D814Y) was phosphorylated and associated with the PI3K p85 subunit without stem cell factor (SCF) (Fig. 1e; see Fig. 3i), and was permanently active
When clathrin-mediated endocytosis (CME) was blocked, Kit(D814Y) became substantially ubiquitinated (Fig. 3j), presumably as it trafficked via non-clathrin endocytosis (NCE). These results suggest that, in RCM cells, most Kit(D814Y) undergoes CME depending on its kinase activity, and accumulates on the cytoplasmic surface of endolysosomes, but not in a fully ubiquitinated state
Summary
Kit is a receptor-type tyrosine kinase found on the plasma membrane. It can transform mast cells through activating mutations. Kit binds to other cytoplasmic enzymes containing Src hohomogy-2 (SH2) domain, and this complex phosphorylates other proteins[3,4,5,6] This process activates the phosphatidylinositol 3-kinase-Akt (PI3K-Akt) pathway, the Ras-Raf-Erk cascade and Src kinases, which regulate cell growth, gene expression and cytoskeletal structures[7,8,9,10]. Cancer-causing mutants of RTKs, such as Met(D1246N) and Flt3-internal tandem duplication (Flt3-ITD), can cause incorrect signalling not just from the PM, and from intracellular compartments, because their activated kinase domain is always exposed[22,23,24,25,26] These mutations change the trafficking and degradation of the receptors, because they change glycosylation and ubiquitination, and receptors accumulate in the wrong compartments[22,23,24,25,26,27]. Signalling by oncogenic RTKs on intracellular compartments has been implicated in their transforming ability, but the mechanism of signalling by mutant Kit is not fully understood
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