TAM family kinases are potential candidate targets for therapeutic intervention in chronic myeloid leukemia.

Background: Chronic Myeloid Leukemia (CML) is a hematologic disorder depicted by BCR::ABL1 translocation; a constitutively active tyrosine kinase (TK) and a hallmark of CML. Kinase domain mutations and the activation of alternative signaling pathways lead to drug resistance in CML. TAM family kinases, including TYRO3 receptor tyrosine kinase (TYRO3-RTK), AXL receptor tyrosine kinase (AXL-RTK), and MER receptor tyrosine kinase (MERTK), are often overexpressed in various types of cancers. AXL-RTK plays a significant role in the survival of leukemic stem cells (LSCs) and provides adaptive resistance to CML cells against Tyrosine kinase inhibitors (TKIs). However, the specific functions and mechanisms of two other receptors, TYRO3-RTK and MERTK, in both sensitive and resistant CML cells are not yet documented. Therefore, this study aimed to explore the expression patterns and roles of TAM family kinases in sensitive and resistant CML.

Methods and results: We have developed an Imatinib-Resistant CML model (K562-R) by administering an increasing dose of Imatinib over 4 months. Proliferation was evaluated through MTT assay. Apoptosis and expression analysis were conducted through the DNA fragmentation assay and RT-PCR, respectively. TYRO3-RTK, AXL-RTK, and MERTK were found to be 2, 7 and 25 folds higher in K562-R than K562-S cells respectively. Pharmacological targeting of TYRO3-RTK with LDC1267, AXL-RTK with R428, and MERTK with UNC2250 TAM inhibitors significantly interfered with the proliferation potential of both K562-S and K562-R cells. TAM kinase inhibitors also abridged colony formation in K562-S and K562-R cells. We have observed an additive antiproliferation effect by co-targeting TAM kinases with Imatinib in K562-S cells and a synergistic effect in K562-R cells. Mechanistically, apoptosis induction independent of p53, differential upregulation of cell cycle inhibitors, including p16, p21, and p27 in K562-S and K562-R cells were observed to be related with the proliferation inhibition. Furthermore, we showed that TAM family inhibitors interfere with the Wnt/β-catenin pathway by downregulation of downstream targets c-Myc, Axin2, its regulators, EYA3, and AXL-RTK in both K562-S and K562-R cells.

Conclusion: TAM family kinase inhibitors significantly reduce the proliferation and colony formation of K562-S and K562-R cells by inducing apoptosis, interfering with Wnt/β catenin pathway, and upregulating cell cycle inhibitors.