The Receptor Tyrosine Kinase RON Is Constitutively Activated in Acute Myeloid Leukemia and May Represent a New Therapeutic Target.

Abstract 994

Poster Board I-16

Recepteur d'Origine Nantais (RON) is a receptor tyrosine kinase (RTK) expressed mostly on epithelial cells but also in normal hematopoietic stem cells and macrophages. RON is closely related to c-Met in terms of homology and function. Its oncogenic properties have been documented in solid tumors leading to the clinical development of small molecules inhibitors. However, the role of RON in haematological malignancies and specifically in AML, has received little attention. Interestingly, a truncated form of RON lacking most of the RON receptor extracellular domain but retaining the whole transmembrane and intracellular domains has been described in the leukemic cell line KG1 (Bardella C et al, Cancer Res 64, 5154–5161, 2004). Since RTK and TK play a crucial role in leukemogenesis, we have assessed the expression and the role of RON in both AML cell lines and patient samples.

The expression of both full length (fl) and short form (sf) of RON was assessed in 71 AML samples by RT-PCR. fl-RON and sf-RON were found in 48/71 pts (68%) and 26/71 (37%) respectively, whereas both forms were undetectable in 17 samples (24%). RON expression was further confirmed at the protein level by western blot analysis in 20 samples. Conversely, c-Met was not expressed at the protein level in 10 cases tested. In leukemic cell lines, fl-RON and sf-RON were expressed both at the mRNA and protein levels in KG1 and KG1a but not in HL60 and U937. Immunoprecipitation analysis showed that fl-RON and sf-RON were constitutively phosphorylated on tyrosine in KG1/KG1a cells. This phosphorylation was fully inhibited by the dual c-Met/RON inhibitor, SU-11274. Cell signaling induced by RON has also been explored. Specific down regulation by si-RNA to RON induced a significant decrease of Lyn phosphorylation. Conversely, AKT phosphorylation was not influenced by RON down regulation. We then assessed the activity of SU-11274 on the proliferation and survival of KG1 (RON+) and HL60 (RON-). The proliferation of KG1 (IC50=3.5μM) but not HL60 (IC50 not reached at 10μM) was strongly inhibited in a time and dose dependant manner. This inhibition of proliferation was mostly due to apoptosis induction. Accordingly using clonogenic and cytotoxic assays, we show that only RON positive samples from AML patients responded to SU-11274 (n=8). Moreover, specific down regulation of RON by si-RNA inhibited the clonogenic properties of KG1 cells.

Altogether, these data demonstrate that the tyrosine kinase RON is aberrantly deregulated in AML cells, control cell proliferation and could represent a new target for the treatment of AML.