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Inhibition of KIT-signaling is a major molecular target for gastrointestinal stromal tumor (GIST) therapy, and imatinib mesylate (IM) is known as the most effective first-line treatment option for patients with advanced, unresectable, and/or metastatic GISTs. We show here for the first time that the inhibition of KIT-signaling in GISTs induces profound changes in the cellular secretome, leading to the release of multiple chemokines, including FGF-2. IM increased migration, invasion, and colony formation of IM-resistant GISTs in an FGF2-dependent manner, whereas the use of blocking anti-FGF2 antibodies or BGJ398, a selective FGFR inhibitor, abolished these effects, thus suggesting that the activation of FGF2-mediated signaling could serve as a compensatory mechanism of KIT-signaling inhibited in GISTs. Conversely, FGF-2 rescued the growth of IM-naive GISTs treated by IM and protected them from IM-induced apoptosis, consistent with the possible involvement of FGF-2 in tumor response to IM-based therapy. Indeed, increased FGF-2 levels in serum and tumor specimens were found in IM-treated mice bearing IM-resistant GIST xenografts, whereas BGJ398 used in combination with IM effectively inhibited their growth. Similarly, increased FGF-2 expression in tumor specimens from IM-treated patients revealed the activation of FGF2-signaling in GISTs in vivo. Collectively, the continuation of IM-based therapy for IM-resistant GISTs might facilitate disease progression by promoting the malignant behavior of tumors in an FGF2-dependent manner. This provides a rationale to evaluate the effectiveness of the inhibitors of FGF-signaling for IM-resistant GISTs.

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