Role of ABL family kinases in cancer: from leukaemia to solid tumours

Abstract

The Abelson (ABL) family of nonreceptor tyrosine kinases, ABL1 and ABL2, transduces diverse extracellular signals to protein networks that control proliferation, survival, migration and invasion. ABL1 was first identified as an oncogene required for the development of leukaemias initiated by retroviruses or chromosome translocations. The demonstration that small-molecule ABL kinase inhibitors could effectively treat chronic myeloid leukaemia opened the door to the era of targeted cancer therapies. Recent reports have uncovered roles for ABL kinases in solid tumours. Enhanced ABL expression and activation in some solid tumours, together with altered cell polarity, invasion or growth induced by activated ABL kinases, suggest that drugs targeting these kinases may be useful for treating selected solid tumours.

Figure 1. Modular domain structure of ABL family kinases

A. Schematic representation of the modular domains of the ABL kinases. Alternative splicing of ABL1 and ABL2 produces several isoforms, including 1a isoforms (solid line) and the 1b isoforms (jagged line); the later are targeted for N-terminal myristoylation. The ABL N-termini are comprised of the Src homology 3 (SH3), SH2 and SH1 domains. The ABL C-termini contain a conserved F-actin-binding domain. ABL1 has a G-actin binding domain whereas ABL2 has a second internal F-actin binding domain. ABL1 has three nuclear localization signal (NLS) motifs and one nuclear export signal (NES) in its C-terminus. Both ABL1 and ABL2 have conserved PXXP motifs to mediate protein-protein interactions. Phosphorylation (P) of ABL1 at Y412 within the activation loop (ABL2 Y439) and Y245 in the SH2-kinase domain linker (ABL2 Y272) stabilizes the active conformation. B. Representation of auto-inhibited (closed) and active (open) ABL kinases to depict the role of intramolecular interactions in the regulation of ABL kinase activity. The SH3 domain binds to the linker sequence connecting the SH2 and kinase domains, while the SH2 domain interacts with C-terminal lobe of the kinase (SH1) domain forming a SH3-SH2 clamp structure. The myristoylated residue in the N-terminus of the 1b ABL isoforms binds to a hydrophobic pocket within the C-lobe of the kinase domain, stabilizing the auto-inhibited conformation. The configuration and interactions of the ABL C-terminal sequences are not included in the model. C. Specificity of selected ABL TKIs: imatinib (Gleevec, STI571; Novartis), nilotinib (Tasigna, AMN107; Novartis), dasatinib (Sprycel, BMS-354825; Bristol-Myers Squibb), ponatinib (Iclusig, AP24534; Ariad Pharmaceuticals) and GNF-2, GNF-5 (allosteric inhibitors in preclinical studies). The kinase selectivity profiles for imatinib, nilotinib, and dasatinib were generated based on the binding of cellular kinases to inhibitors immobilized on solid support matrices (reviewed previously in ). Ponatinib-sensitive kinases were identified by in vitro kinase assays; shown are targets with IC₅₀ values of less than 20 nM. Kinases sensitive to GNF-2 and GNF-5 were identified by in vitro kinase assays^{, ,} .

Figure 2. ABL activation by chromosome translocations in leukemia

Schematic representation of ABL1, and the various ABL1 and ABL2 fusion proteins that arise as a consequence of chromosome translocations in various types of human leukemia. The BCR-ABL1 and ETV6-ABL1 fusions are associated with several types of leukemia including chronic myeloid leukemia (CML), B-cell acute lymphoblastic leukemia (B-ALL), T-cell ALL (T-ALL) and acute myeloid leukemia (AML). The ETV6-ABL2 fusion protein is present in rare cases of T-ALL and AML. The NUP214-ABL1 and EML1-ABL1 fusions are primarily linked to T-ALL, while the RCSD1-ABL1, SFPQ-ABL1, ZMIZ1-ABL1, FOXP1-ABL1 and SNX2-ABL1 fusions are associated with B-ALL^, . The coiled-coil (CC) and other motifs in the fusion partner promote oligomerization of the resulting chimeric ABL kinases. The BCR-ABL1, NUP214-ABL1, ETV6-ABL1, ETV6-ABL2, EML1-ABL1, and ZMIZ1-ABL1 fusion proteins retain the SH3, SH2 and SH1 domains of the ABL kinase. The RCSD1-ABL1, SFPQ-ABL1, FOXP1-ABL1 and SNX2-ABL1 fusions lack the SH3 but retain part of or the entire SH2 domain. The v-Abl1 oncoprotein fuses Gag sequences of Abelson murine leukemia virus to sequences upstream of the Abl1 SH2 domain ^, . The v-Abl1 oncoprotein induces murine lymphosarcoma and acute pre-B cell leukemia^, (HELP = hydrophobic EMAP-like protein domain, RRM = RNA recognition motif)

Figure 3. Active ABL kinases regulate epithelial cell polarity through β1-integrin

A. Epithelial cells expressing control vector or constitutively active forms of the ABL kinases were grown in collagen gels and stained for the apical polarity marker gp135, the adherens junction marker E-cadherin and the nuclear DAPI stain. Active ABL2, and to a lesser extent ABL1, promote polarity inversion. All pictures were taken at 40× magnification. Scale bar: 25 μm. B. Model for the inversion of epithelial cell polarity by active ABL kinase. Activation of ABL downstream of RTKs, chemokine receptors, oxidative stress and other signals leads to disruption of β1-integrin signaling and localization by regulation of the Rap1 GTPase via ABL2-mediated phosphorylation of CRK and disruption of the CRK-C3G (RAPGEF1) complex. Active ABL2 also impairs Rac1-mediated laminin assembly in epithelial cysts.

Figure 4. ABL kinases regulate cancer cell invasion

Activation of ABL kinases downstream of hyperactive RTKs, chemokine receptors and Src kinases, or in response to oxidative stress promotes cancer cell invasion by direct or indirect activation of actin regulatory proteins such as cortactin, N-WASP, Abl interactor 1 (Abi1), WAVE, CrkL, HEF1/NEDD9, and the Rac GTPase^{, , -,} . ABL kinases regulate MT1-MMP localization and activity. ABL2 interacts with MT1-MMP promoting its phosphorylation, and ECM degradation. One or both ABL kinases are required for cancer cell invasion depending on the cellular context^, . ABL kinases also regulate cancer cell invasion by modulating expression of several MMPs and other genes involved in EGFR-mediated invasion^, . ABL1 regulates MMP1 expression through a STAT3-dependent pathway, and ABL2 regulates the expression of MMP1, MMP3, and MT1-MMP through yet to be identified transcription factors. Solid lines indicate direct links between ABL and target proteins; dotted lines indicate indirect links.