Imatinib resistance and progression of CML to blast crisis: somatic hypermutation AIDing the way

Structure-Based Design of First-Generation Small Molecule Inhibitors Targeting the Catalytic Pockets of AID, APOBEC3A, and APOBEC3B

Activation-induced cytidine deaminase (AID) initiates antibody diversification by mutating immunoglobulin loci in B lymphocytes. AID and related APOBEC3 (A3) enzymes also induce genome-wide mutations and lesions implicated in tumorigenesis and tumor progression. The most prevalent mutation signatures across diverse tumor genomes are attributable to the mistargeted mutagenic activities of AID/A3s. Thus, inhibiting AID/A3s has been suggested to be of therapeutic benefit. We previously used a computational-biochemical approach to gain insight into the structure of AID’s catalytic pocket, which resulted in the discovery of a novel type of regulatory catalytic pocket closure that regulates AID/A3s that we termed the “Schrodinger’s CATalytic pocket”. Our findings were subsequently confirmed by direct structural studies. Here, we describe our search for small molecules that target the catalytic pocket of AID. We identified small molecules that inhibit purified AID, AID in cell extracts, and endogenous AID of lymphoma cells. Analogue expansion yielded derivatives with improved potencies. These were found to also inhibit A3A and A3B, the two most tumorigenic siblings of AID. Two compounds exhibit low micromolar IC₅₀ inhibition of AID and A3A, exhibiting the strongest potency for A3A. Docking suggests key interactions between their warheads and residues lining the catalytic pockets of AID, A3A, and A3B and between the tails and DNA-interacting residues on the surface proximal to the catalytic pocket opening. Accordingly, mutants of these residues decreased inhibition potency. The chemistry and abundance of key stabilizing interactions between the small molecules and residues within and immediately outside the catalytic pockets are promising for therapeutic development.

Nrf-2/Gst-α mediated imatinib resistance through rapid 4-HNE clearance

The advent of imatinib mesylate (IM) has dramatically improved the outcome of patients with chronic myeloid leukemia, but drug resistance, particularly in advanced stage of disease, portents eventual relapse and progression. To identify the candidate molecule responsible for resistance during IM treatment, an IM-resistant K562 cell line was generated by culturing in gradually increasing dose of IM. The expression of Nrf-2 and its downstream target, Gst-α, were significantly induced in these cells. GST-α, in turn, mediated cell survival by maintaining intracellular low level of 4-HNE. Inhibition of Nrf-2 effectively reduced the expression of Gst-α, resulting in accumulation of 4-HNE and elevated sensitiveness to IM. Moreover, in IM-sensitive K562 cells enforced Gst-α expression strikingly protected cells from the insult of IM. Finally, we also examined the levels of Nrf-2 in clinical bone morrow samples. Nrf-2 and Gst-α were more abundant in bone morrow of CML patients compared with that of healthy donors. In addition, Nrf-2 and Gst-α were further up-regulated in samples of patients with weak response to IM. In conclusion, our study shows that rapid clearance of 4-HNE by Nrf-2/GST may represents a novel molecular basis of IM resistance in CML.

H2AX phosphorylation regulated by p38 is involved in Bim expression and apoptosis in chronic myelogenous leukemia cells induced by imatinib

Increasing evidence suggests that histone H2AX plays a critical role in regulation of tumor cell apoptosis and acts as a novel human tumor suppressor protein. However, the action of H2AX in chronic myelogenous leukemia (CML) cells is unknown. The detailed mechanism and epigenetic regulation by H2AX remain elusive in cancer cells. Here, we report that H2AX was involved in apoptosis of CML cells. Overexpression of H2AX increased apoptotic sensitivity of CML cells (K562) induced by imatinib. However, overexpression of Ser139-mutated H2AX (blocking phosphorylation) decreased sensitivity of K562 cells to apoptosis. Similarly, knockdown of H2AX made K562 cells resistant to apoptotic induction. These results revealed that the function of H2AX involved in apoptosis is strictly related to its phosphorylation (Ser139). Our data further indicated that imatinib may stimulate mitogen-activated protein kinase (MAPK) family member p38, and H2AX phosphorylation followed a similar time course, suggesting a parallel response. H2AX phosphorylation can be blocked by p38 siRNA or its inhibitor. These data demonstrated that H2AX phosphorylation was regulated by p38 MAPK pathway in K562 cells. However, the p38 MAPK downstream, mitogen- and stress-activated protein kinase-1 and -2, which phosphorylated histone H3, were not required for H2AX phosphorylation during apoptosis. Finally, we provided epigenetic evidence that H2AX phosphorylation regulated apoptosis-related gene Bim expression. Blocking of H2AX phosphorylation inhibited Bim gene expression. Taken together, these data demonstrated that H2AX phosphorylation regulated by p38 is involved in Bim expression and apoptosis in CML cells induced by imatinib.

Regulation of Aicda expression and AID activity

Activation-induced cytidine deaminase (AID) is expressed in a B cell differentiation stage-specific fashion and is essential for immunoglobulin (Ig) gene class switch DNA recombination (CSR) and somatic hypermutation (SHM). CSR and SHM play a central role in the maturation of antibody and autoantibody responses. AID displays a mutagenic activity by catalyzing targeted deamination of deoxycytidine (dC) residues in DNA resulting in dU:dG mismatches, which are processed into point-mutations in SHM or double-strand breaks (DSBs) in CSR. Although AID specifically targets the Ig gene loci (IgH, Igκ and Igλ), it can also home into a wide array of non-Ig genes in B-and non-B-cell backgrounds. Aberrant expression of AID is associated with multiple diseases such as allergy, inflammation, autoimmunity and cancer. In autoimmune systemic lupus erythematosus, dysregulated AID expression underpins increased CSR, SHM and autoantibody production. As a potent mutator, AID is under stringent transcriptional, post-transcriptional and post-translational regulation. AID is also regulated in its targeting and enzymatic function. In resting naïve or memory B cells, AID transcripts and protein are undetectable. These, however, are readily and significantly up-regulated in B cells induced to undergo CSR and/or SHM. Transcription factors, such as HoxC4 and NF-κB, which are up-regulated in a B cell lineage-and/or differentiation stage-specific manner, regulate the induction of AID. HoxC4 induces AID expression by directly binding to the AID gene promoter through an evolutionarily conserved 5'-ATTT-3' motif. HoxC4 is induced by the same stimuli that induce AID and CSR. It is further up-regulated by estrogen through three estrogen responsive elements in its promoter region. The targeting of AID to switch (S) regions is mediated by 14-3-3 adaptor proteins, which specifically bind to 5'-AGCT-3' repeats that are exist at high frequency in S region cores. Like HoxC4, 14-3-3 adaptors are induced by the same stimuli that induce AID. These include "primary" inducing stimuli, that is, those that play a major role in inducing AID, i.e., engagement of CD40 by CD154, engagement of Toll-like receptors (TLRs) by microbial-associated molecular patterns (MAMPs) and cross-linking of the BCR, as synergized by "secondary" inducing stimuli, that is, those that synergize for AID induction and specify CSR to different isotypes, i.e., switch-directing cytokines IL-4, TGF-β or IFN-γ. In this review, we focus on the multi-levels regulation of AID expression and activity. We also discuss the dysregulation or misexpression of AID in autoimmunity and tumorigenesis.

Differential cytogenetic profile in advanced chronic myeloid leukemia with sequential lymphoblastic and myeloblastic blast crisis

Frequency of additional chromosomal abnormalities in chronic myeloid leukemia (CML) is estimated to be 7% in chronic phase and increases to 40–70% in advanced disease. Progression of CML from chronic phase to accelerated phase or blast crisis is often associated with secondary chromosomal aberrations. We report an exceptional case of CML as debut in lymphoblastic blast crisis and a subsequent progression in myeloblastic blast crisis with rare cytogenetic abnormalities.

Imatinib induces H2AX phosphorylation and apoptosis in chronic myelogenous leukemia cells in vitro via caspase-3/Mst1 pathway

AIM

Histone H2AX is a novel tumor suppressor and its phosphorylation at the C terminus (Ser139 and Tyr142) is required for tumor cell apoptosis. The aim of the present study was to elucidate the mechanisms underlying imatinib-induced C-terminal phosphorylation of H2AX in chronic myelogenous leukemia cells in vitro.

METHODS

BCR-ABL-positive K562 cells were used. Microscopy, Western blotting and flow cytometry were used to study the signaling pathways that regulate imatinib-induced H2AX phosphorylation and the apoptotic mechanisms.

RESULTS

Treatment of K562 cells with imatinib (1-8 μmol/L) induced phosphorylation of H2AX at Ser139 and Tyr142 in time- and dose-dependent manners. In contrast, imatinib at the same concentrations did not affect H2AX acetylation at Lys 5, and the acetylated H2AX maintained a higher level in the cells. Meanwhile, imatinib (1-8 μmol/L) activated caspase-3 and its downstream mammalian STE20-like kinase 1 (Mst1), and induced apoptosis of K562 cells. The caspase-3 inhibitor Z-VAD (40 μmol/L) reduced imatinib-induced H2AX phosphorylation at Ser139 and Tyr142 and blocked imatinib-induced apoptosis of K562 cells. Imatinib (4 μmol/L) induced expression of Williams-Beuren syndrome transcription factor (WSTF), but not wild-type p53-induced phosphatase 1 (Wip1) in K562 cells.

CONCLUSION

The caspase-3/Mst1 pathway is required for H2AX C-terminal phosphorylation at Ser139 and Tyr142 and subsequent apoptosis in Bcr-Abl-positive K562 cells induced by imatinib.

Targeted blockage of signal transducer and activator of transcription 5 signaling pathway with decoy oligodeoxynucleotides suppresses leukemic K562 cell growth

The protein signal transducer and activator of transcription 5 (STAT5) of the JAK/STAT pathway is constitutively activated because of its phosphorylation by tyrosine kinase activity of fusion protein BCR-ABL in chronic myelogenous leukemia (CML) cells. This study investigated the potential therapeutic effect of STAT5 decoy oligodeoxynucleotides (ODN) using leukemia K562 cells as a model. Our results showed that transfection of 21-mer-long STAT5 decoy ODN into K562 cells effectively inhibited cell proliferation and induced cell apoptosis. Further, STAT5 decoy ODN downregulated STAT5 targets bcl-xL, cyclinD1, and c-myc at both mRNA and protein levels in a sequence-specific manner. Collectively, these data demonstrate the therapeutic effect of blocking the STAT5 signal pathway by cis-element decoy for cancer characterized by constitutive STAT5 activation. Thus, our study provides support for STAT5 as a potential target downstream of BCR-ABL for CML treatment and helps establish the concept of targeting STAT5 by decoy ODN as a novel therapy approach for imatinib-resistant CML.