Receptor tyrosine kinase Axl is required for resistance of leukemic cells to FLT3-targeted therapy in acute myeloid leukemia

Abstract

In acute myeloid leukemia (AML), about 25-30% of patients harbor a constitutively active receptor tyrosine kinase (RTK) FLT3 encoded by a FLT3 allele harboring internal tandem duplication (FLT3-ITD) mutation. The presence of FLT3-ITD correlates with poor prognosis in AML and it makes FLT3 an attractive therapeutic target in AML. Unfortunately, to date small-molecule inhibitors of FLT3 have resulted in only partial and transient clinical responses with residual leukemic blasts resistant to FLT3 inhibitors detected in blood or bone marrow. In this study, we investigated whether the RTK Axl is responsible for resistance of FLT3-ITD(+) AML cells to PKC412 and AC220, FLT3 inhibitors currently under clinical trials for FLT3-ITD(+) AML patients. Upon treatment with PKC412 or AC220, phosphorylation of Axl was significantly enhanced in the FLT3-ITD(+) MV4-11 AML cell line and in primary blasts from a FLT3-ITD(+) AML patient. Consistently, a PKC412-resistant AML cell line and PKC412-resistant primary blasts from FLT3-ITD(+) AML patients had significantly higher levels of constitutively phosphorylated Axl and total Axl when compared with a PKC412-sensitive AML cell line and PKC412-sensitive primary blasts from FLT3-ITD(+) AML patients. We also found that resistance of AML cells against the FLT3 inhibitor PKC412 and AC220 was substantially diminished by the inhibition of Axl via a small-molecule inhibitor TP-0903, a soluble receptor Axl fusion protein Axl-Fc or knockdown of Axl gene expression by shRNA. Collectively, our study suggests that Axl is required for resistance of FLT3-ITD(+) AML cells against the FLT3 inhibitor PKC412 and AC220, and that inhibition of Axl activation may overcome resistance to FLT3-targeted therapy in FLT3-ITD(+) AML.

The MERTK/FLT3 inhibitor MRX-2843 overcomes resistance-conferring FLT3 mutations in acute myeloid leukemia

“…However, another member of the TAM family, AXL, has been implicated in acquired resistance to a variety of TKIs and in multiple tumor types. Notably, in AML cell lines, AXL knockdown via shRNA restored sensitivity to the FLT3 inhibitor midostaurin (32). Given the known role of MERTK in promoting survival and resistance to traditional chemotherapies and its close relationship to AXL, it is likely that MERTK also has a role in mediating resistance to targeted therapies, and thus dual inhibition of MERTK and FLT3 may decrease the development of resistance relative to inhibition of FLT3 alone.…”

TAMing resistance to multi-targeted kinase inhibitors through Axl and Met inhibition

“…Interestingly, the upregulation of Axl and Met signaling pathways seems in fact independent of the contextual differences portraying the biologic progression of the diverse malignancies studied and the pharmacodynamic specificity of each inhibitor to include, for instance, epidermal growth factor pathway inhibitors in non-small cell lung 11 and colorectal cancers, 12 lapatinib in Her-2-positive breast cancer, 13 FLT-3 inhibitors in acute myeloid leukemia, 14 imatinib in chronic myeloid leukemia 15 and many others.…”

Targeting the TAM Receptors in Leukemia

“…Overexpression of AXL and its role in resistance to targeted and cytotoxic therapies have been shown in several different tumor types, including imatinib-resistant CML and gastrointestinal stromal tumors [80], AML with acquired resistance to FLT3 TKIs [81], cisplatin-resistant ovarian cancer [82], HER-2 positive breast cancer resistant to lapatinib [83], and rhabdomyosarcoma resistant to IGF1R inhibition [84]. While AXL has been implicated in resistance to therapy in a variety of leukemia subtypes and therapeutic settings, the mechanisms of resistance vary.…”

“…In addition, treatment of a FLT3-ITD AML cell line and patient sample with the FLT3 inhibitors PKC412 or quizartinib resulted in increased activation of AXL and treatment with PKC412 increased signaling through pathways downstream of AXL, including ERK1/2, AKT, and STAT5 [81]. AXL activation was inhibited by MEK/ERK and PI3K targeted inhibitors, indicating that the activation of AXL is mediated mainly through these pathways.…”

“…Interestingly, when PKC412-resistant AML cell lines were treated with the AXL inhibitor TP-0903 they were resensitized to FLT3 targeted therapy both in vitro and in vivo. The in vitro results were replicated using sAXL and shRNA-mediated AXL knockdown [81]. …”