Major signal transduction pathways activated by the mutant KIT D816V receptor. The mutant receptor is represented under its monomeric form and at the cell membrane, although it remains unclear if it is present mainly in the cytoplasm or at the cell membrane in malignant mast cells, and if it is active under monomeric and/or dimeric form. Note the prominent role of STAT5 and AKT signaling in the proliferation induced by constitutive activation of the KIT receptor. It has been recently postulated that STAT5 can be directly activated by the mutant KIT receptor in the absence of JAK proteins (blue curved arrow). Moreover, STAT1 and STAT3 homo- and heterodimers were shown to be activated by mutant KIT and these can interact on higher order chromatin structures with STAT5 complexes so called STAT-Oligomers. STAT5 is efficiently dimerized and translocated to the nucleus upon full phosphorylation, which requires both tyrosine and serine phosphorylation as indicated by small orange circle or asterix. The cytoplasmic retention of pSTAT5 via the GAB2 scaffold protein was demonstrated to control PI3K-AKT signaling and this mechanism is important for survival of neoplastic mast cells, but it also slows down the transcription through nuclear pSTAT5. Furthermore, the lipid phosphatase PTEN (3′-inositol phosphatase and tensin homolog) further regulates negatively AKT signaling by dephosphorylation of PIP3. Other prominent signaling pathways triggering transcription factor changes in the nucleus such as forkhead family member phosphorylation which then leaves the nucleus upon AKT phosphorylation or RAS-RAF-MAPK signaling and their influence on transcription factor activation by phosphorylation events such as Serum response factor (SRF), AP-1 (Jun/Fos/ATF-1 homo- or heterodimers) is also illustrated. Moreover, unphosphorylated STAT5a was recently shown to interact with heterochromatin protein 1 (HP1α) and this mechanism protects DNA from noxias through keeping the densely packed chromatin, but pSTAT5 opens up chromatin most likely by its ability to induce oligomers via the N-terminus (black circles labeled with N and a dimerizer itself) and loop formation on DNA. This culminates into STAT5 target gene induction and most prominent genes known to be under control of STAT5 signaling in MC are exemplified, which leads to survival and growth.

Effects of BCR-ABL1 (p210) functional domains on downstream signaling effectors. BCR-ABL1 signaling leads to enhanced proliferation, reduced apoptotic potential, and altered cell adhesion. Contributions from both BCR and ABL domains on downstream signaling are illustrated. OLI: oligomerization domain; S/T kinase: serine/threonine kinase; DH: Dbl homology; PH: Pleckstrin homology; SH2 or SH3: Src homology 2/3; TK kinase: tyrosine kinase; DBD: DNA binding domain; ABD: actin binding domain. Note the central place occupied by both AKT and STAT5 (red circle) in the signal transduction pathways leading to altered apoptosis and enhanced proliferation of CML cells.

General structure of the PKB/AKT and STAT5a/b proteins. (A). AKT. All the PKB isoforms possess the kinase domain in the central region of the molecule. The PH (pleckstrin homology) domain acts as a phosphoinositide-binding module. The hydrophobic motif (HM) is located at the carboxyl-terminus adjacent to the kinase domain. Phosphorylation sites in the activation loop of the kinase domain and the hydrophobic motif are indicated. Length of the protein and positions of the phosphorylation sites may vary depending on the isoform. (B). Domain structure of the STAT5a/b isoforms. The N-terminal domain (ND), which prevents autoactivation and is the docking domain to cytokine receptors is known to interact with nuclear hormone receptors such as the glucocorticoid receptor (GR; similarly progesterone (PR), estrogen (ER), androgen (AR) and mineralocorticoid receptor (MR) might follow similar rules of docking to the STAT5 N-terminus, which is postulated due to close homology to the GR, but experimentally largely not precisely mapped). Moreover, the N-terminus of STAT5 is a dimerization domain that can execute higher order chromatin structures by STAT oligomerization and co-factor recruitment, where particular interaction with a histone methylase involved in many cancers called EZH2 is of significance. The coiled-coil dimerization domain (CCD) was reported to be involved in repression together with the extreme C-terminus and here SMRT co-repressor interaction was mapped. Moreover, it is crucial for transrepression in context of a RARalpha fusion oncoprotein found in AML patients and Nmi-mediated histone acetyl transferase activity through p300/CBP interaction was also linked to this domain. Moreover, p300/CBP are also bound by the C-terminus, which is also called transactivation and protein activity domain since tyrosine phosphatases recognize the end domain around the critical pY. The C-terminus binds also other proteins which were mapped and it is a domain that also undergoes acetylation, sumoylation and serine phosphorylation plus splicing or proteolytic processing. The SH2 domain (SD) is important for stable parallel dimer formation and efficient nuclear transport and DNA binding complex formation and the laboratory of Patrick Gunning used this domain for specific STAT inhibitors successfully.11 The DNA binding domain (DBD) is in the middle and separated by a linker domain (LD) from the SH2 domain. Apart from critical tyrosine phosphorylation, also Serine phosphorylation (shown in red) is important for myeloid transformation with regard to STAT5a and STAT5b proteins. Of further importance are findings of gain of function mutations in STAT5b found in patients with large granulocytic leukemia (N642H or Y665F) both present in the SH2 domain. Finally, gain of function mutations like the STAT5a/b S710/S715F oncogenic variants were described elsewhere47 and originate from retroviral screens to make IL-3-dependent myeloid cells factor independent.

Activation of growth factor receptor protein tyrosine kinases results in autophosphorylation on tyrosine residues. PI3K is recruited to the membrane by directly binding to phosphotyrosine consensus residues of growth factor receptors or adaptors through one or both SH2 domains in the adaptor subunit. This leads to allosteric activation of the catalytic subunit. Activation results in production of the second messenger phosphatidylinositol (PtdIns)-3,4,5-trisphosphate (PIP3). The lipid product of PI3K, PIP3, recruits a subset of signaling proteins with pleckstrin homology (PH) domains to the membrane, including PDK1 and AKT. PTEN (a PI 3,4,5-P3 phosphatase) negatively regulates the PI3K/AKT pathway. Once activated, AKT mediates the activation and inhibition of several targets, resulting in cellular survival, growth and proliferation through various mechanisms.