FGF2-Containing Exosomes Secreted from Bone Marrow Stromal Cells Protect Leukemia Cells from Tyrosine Kinase Inhibitors

Introduction. Mutational activation of kinases is a frequent event in leukemia, however resistance to kinase inhibitors remains a clinical dilemma. There is considerable evidence that proteins expressed by the bone marrow microenvironment protect leukemia cells from the effects of therapy. We previously reported that fibroblast growth factor 2 (FGF2) from bone marrow stroma protected chronic myeloid leukemia (CML) cells in a paracrine fashion. FGF2 expression was significantly increased in the marrow stroma of resistant CML patients without kinase domain mutations and resistance could be overcome with concomitant inhibition of FGFR (Traer et al. Blood 2014). Furthermore, resistant patients with increased marrow FGF2 expression had decreased FGF2 after FGFR inhibition, suggesting FGF2 also acts as an autocrine growth factor for stroma. Recently we have found a similar increase in marrow FGF2 in acute myeloid leukemia with FLT3 internal tandem duplication (FLT3+ AML), suggesting a more general mechanism of resistance (submitted). Since FGF2-mediated resistance appears to be conserved, we investigated FGF2 paracrine protection and autocrine stimulation in more detail.

Results. FGF2 is expressed by stromal cells and plays an active role in hematopoiesis, however FGF2 does not have a signal peptide and thus its mechanism of secretion remains controversial. We used the related human stromal cell lines HS-5 and HS-27 to investigate FGF2 secretion (HS-5 strongly expresses FGF2 whereas HS-27 does not). FGF2 was found by Western blot to be enriched in the extracellular vesicle (ECV) pellet after centrifugation at 100,000g. These findings were confirmed by Luminex multiplex cytokine assay, where FGF2 was found to be uniquely enriched in ECVs. In order to further purify the ECV fraction, we performed a sucrose step-gradient fractionation and Western blot analysis. FGF2 was enriched in the exosome fraction, along with exosomal markers CD9 and tsg-101, whereas extracellular matrix proteins and apoptotic bodies localized to different fractions. Exosomes also conferred to K562 CML cell lines and MOLM14 FLT3+ AML cell lines treated with BCR-ABL and FLT3 inhibitors, respectively. To evaluate if FGF2 was contained within exosomes, we treated HS-5 exosomes with proteinase-K to digest proteins outside of the lipid membrane and found that FGF2 is present both inside and outside of exosomes. Exosomes were labeled with a fluorescent dye (DiI) and incubated with K562 and MOLM cells. Microscopy demonstrated uptake of exosomes into the leukemia cells. Protection by FGF2-containing exosomes could be partially abrogated by PD173074, a selective FGFR inhibitor, suggesting that protection by exosomes is not mediated entirely by FGF2, and other components of exosomes such as miRNAs and other proteins confer protection as well.

To investigate FGF2 autocrine stimulation of marrow stroma, HS-5 and HS-27 cells were treated with PD173074. Growth of HS-5 cells was attenuated by inhibition of FGFR, whereas HS-27 cells were relatively unaffected. Treatment with PD173074 also caused distinctive changes in the morphology of HS-5. We then investigated the effects of FGFR inhibitor on FGF2 and exosome secretion. Although the intracellular FGF2 was unchanged by PD173074, the amount of secreted exosomes was decreased, as measured by FGF2, CD9 and tsg-101 by Western blot. This reduction in secreted exosomes was confirmed by NanoSight analysis, where increasing concentrations of PD173074 led to a dose-dependent decrease in secreted vesicles (Figure 1) indicating that exosome secretion is regulated by FGFR activation

Conclusion. FGF2 signaling is a conserved mechanism of resistance to targeted therapy in CML, FLT3+ AML and other malignancies. FGFR inhibition by PD173074 leads to 1) reduced autocrine expansion of FGF2-expressing stroma, 2) decreased secretion of FGF2-containing exosomes, and 3) attenuation of the exosome-mediated protection of leukemia cells. Our findings suggest that exosomes are important purveyors of protective signaling to leukemic blasts in leukemia microenvironment, and that FGFR-inhibition may be a clinically relevant option to modulate the marrow stroma and overcome microenvironment-mediated resistance.

Figure 1.