Study On The Role Of Ligand-dependent Stem Cell Factor/KIT Signaling In The Proliferation Of Gastrointestinal Stromal Tumor

Gastrointestinal stromal tumors (GIST) are the most common mesenchymal neoplasm of the gastrointestinal tract characteristed with the extensive expression of KIT protein. The autophosphorylation of KIT protein resulting from gain-of-function mutations of c-kit gene is the most important pathogenesis mechanism of GIST. The activating KIT permits the phosphorylation of the receptor tyrosine kinases perpetuating the receptor-initiated signal and causing activation of the downstream effectors. However, it is unknown if there is another signaling pathway activating KIT receptor in GIST. Imatinib is a small molecule tyrosine kinase inhibitor and effective against GIST. Although imatinib has greatly improved the quality of life and survival of patients with advanced GIST, most patients are not cured due to imatinib resistance. Stem cell factor (SCF) is the ligand of KIT receptor which binding with KIT results in phosphorylation of KIT receptors and several signaling substrates known to promote cell growth and survival. Activating of SCF/KIT signaling was discovered involving in the proliferation of several malignant tumors. However, nothing is known if activating of SCF/KIT pathway is another mechanism of KIT activating in GIST. Furthermore, the contribution of SCF/KIT pathway to Imatinib resistance in GIST is not known. In this study, we detected the expression of SCF in GIST and discovered the role of SCF/KIT signaling pathway in proliferation of GIST.First, we detected the expression of SCF in 68 GIST cases by immunohistochemistry and western blott. We found 52 cases expressing SCF (52/68,76%) and of these,51 cases showed co-expression of SCF and KIT (51/68,75%). The number of mistosis and Ki-67 index were significantly higer in SCF-positive cases. These results indicated that proliferation of GIST cells can be caused not only by the gain-of-function mutations of c-kit, but also by the autocrine mechanism of SCF/KIT system. Second, we found that there was no correlation between SCF expression and the mutation of c-kit in GIST suggesting that the SCF/KIT system is an independent pathway activating KIT in GIST. Following that, to verify whether SCF could promote GIST cells proliferation, we built up GIST primary cultures in vitro. We observed the GIST cells proliferated significantly stimulated by SCF. Furthermore, the KIT protein in GIST cells presented high level of phosphorylation after SCF stimulating. These results indicated that ligand-dependent pathway play a key role in proliferation of GIST cells. Last and importantly, we detected SCF expression in GIST cells treated with imatinib for 72h and found the higer level of SCF expression. We also checked KIT phosphorylation in GIST cells treated with imatinb for 90min prior to stimulation with SCF. The result verified imatinib could not inhibit the ligand-dependent KIT activation.These results suggest the essential role of SCF/KIT signaling in the proliferation of GIST and the resistance of imatinib for this pathway. Thus, we believed that the inhibition of not only ligand-independent but also ligand-dependent KIT signaling pathways, the proliferation of GIST cells could be inhibited effectively. This study may help to discover new mechanism of proliferation of GIST cells and provide another potential way for GIST targeted therapy.