Identification of Basophils as Source of Hepatocyte Growth Factor (HGF) In CML: a Potential Trigger of Disease Acceleration.

Abstract 1202

Chronic myeloid leukemia (CML) is a myeloproliferative disorder in which BCR/ABL leads to enhanced survival of leukemic cells. Several different angiogenic molecules, including vascular endothelial growth factor (VEGF) and hepatocyte growth factor (HGF), have been implicated in the pathogenesis of CML. Enhanced production of these pro-angiogenic molecules in CML may be associated with disease acceleration. However, little is known so far about the exact origin of these growth factors and about mechanisms underlying their production and secretion in CML cells. In the current study, we analyzed the cellular distribution of HGF and its receptor c-MET in CML cells, and explored the mechanism of expression of HGF in BCR/ABL-transformed cells. As assessed by immunostaining of bone marrow sections and isolated blood and bone marrow samples, the HGF protein was found to be expressed in a subset of leukemic cells in all patients tested. In addition, CML cells were found to express HGF mRNA and c-MET mRNA. In consecutive experiments, we were able to show that basophils are the primary source of HGF in CML. In particular, highly enriched sorted CD203c+ CML basophils were found to express substantial amounts of HGF mRNA as well as the HGF protein. Correspondingly, leukemic cell samples obtained from patients with accelerated phase CML were found to contain higher HGF mRNA levels compared to cells obtained from patients in chronic phase or blast phase CML. Finally, HGF mRNA and the HGF protein were detectable in the basophil-committed CML cell line KU812, but not in the more immature Ph+ cell line K562. We next asked whether expression of HGF in CML cells depends on BCR/ABL. To address this question, Ba/F3 cells with doxycycline-inducible expression of BCR/ABL were employed. However, BCR/ABL failed to induce expression of HGF mRNA or the HGF protein in Ba/F3 cells. Correspondingly, the BCR/ABL-blocker imatinib was found to inhibit expression of VEGF mRNA, but did not inhibit HGF mRNA expression in KU812 cells. Next, we examined the expression of c-MET in CML cells. c-MET mRNA was found to be expressed in KU812 and K562 cells, in highly enriched CD34+/CD38- CML stem cells, and less abundantly in more mature CD34+/CD38+ CML cells and CML basophils. The c-MET inhibitor MSC-2156119J-15 (Merck-Serono Darmstadt, Germany) was found to counteract growth of primary CML cells, K562 cells, and KU812 cells with comparable IC₅₀ values (0.5-1.0 μM). In summary, our data suggest that HGF is a BCR/ABL-independent basophil-derived mediator in CML. Basophils and basophil-derived mediators may play a more active role in CML-acceleration as has been considered previously. Whether targeting of HGF or/and c-MET is an effective approach to block acceleration in CML remains to be elucidated.