-Author name in bold denotes the presenting author
-Asterisk * with author name denotes a Non-ASH member
Clinically Relevant Abstract denotes an abstract that is clinically relevant.

PhD Trainee denotes that this is a recommended PHD Trainee Session.

Ticketed Session denotes that this is a ticketed session.

712 Inhibition of PAK6-Mediated Survival and Cell Cycle Controls Selectively Targets Therapy-Resistant CML Stem/Progenitor Cells

Program: Oral and Poster Abstracts
Type: Oral
Session: 631. CML: Biology and Pathophysiology, excluding Therapy: Mechanisms of Resistance and Progression in CML
Hematology Disease Topics & Pathways:
HSCs, apoptosis, Biological, CML, Diseases, cell regulation, Combinations, Therapies, DNA damage, Biological Processes, Technology and Procedures, gene therapy, Cell Lineage, gene editing, Clinically relevant, Myeloid Malignancies, flow cytometry, stem cells, metabolomics, RNA sequencing, senescence, signal transduction
Monday, December 7, 2020: 2:15 PM

Andrew Wu, BSc1, Min Chen, PhD2*, Ryan Yen, BSc1 and Xiaoyan Jiang, MD, PhD3

1Terry Fox Laboratory, British Columbia Cancer Research Centre, Vancouver, BC, Canada
2Terry Fox Laboratory, British Columbia Cancer, Vancouver, BC, Canada
3Terry Fox Laboratory, BC Cancer Research Centre, Vancouver, BC, Canada

The resistance of chronic myeloid leukemia (CML) leukemic stem cells (LSC) to ABL tyrosine kinase inhibitor (TKI) monotherapy remains a challenge in curing CML. We have recently identified miR-185 as a useful biomarker to predict therapy response in treatment-naïve CML stem/progenitor cells. We also demonstrated that restored miR-185 expression in LSCs impaired survival, sensitizing them to TKIs in vitro and in preclinical patient-derived xenotransplantation models, indicating that miR-185 is a critical regulator mediating TKI response/resistance of CML stem/progenitor cells. PAK6, a serine/threonine-protein kinase, was uncovered as a target gene of miR-185 by RNA-seq and was found to be upregulated in CD34+ TKI-nonresponder cells vs. TKI-responders, but its biological functions in CML are largely unknown. To investigate the biological effects of inhibiting PAK6 activity in TKI-resistant cells, we tested a pre-clinically validated pan-PAK inhibitor (PF-3758309) in vitro. PF-3758309 significantly reduced the growth of IM-resistant cell lines, including K562-resistant cells, BV173 blast cells (IC50 25-70 nM) and CD34+ TKI-nonresponder cells, as assessed by viability and CFC assays, and increased their apoptosis; these effects were significantly enhanced by TKIs (~2-fold, P<0.05). PF-3758309 alone, or in combination with a TKI, did not have obvious inhibitory effects on CD34+ normal bone marrow. These results were further confirmed in IM-resistant cells using a lentiviral knockdown system that specifically inhibits PAK6. Interestingly, PF-3758309 alone, or in combination with a TKI, greatly reduced mitochondrial activity in CD34+ TKI-nonresponder cells, as shown in functional assessments with MitoTracker, a dye that accumulates in active mitochondria, the site of OXPHOS; this was not seen with TKI alone (P<0.002). Similarly, CellROX analysis confirmed a reduction in ROS levels upon PF-3758309 treatment, or a combination of PF-3758309 with TKI, in these cells. In addtion, MDM2, a critical negative regulator of the p53 tumor suppressor, was identified as one of substrates of PAK6, by PhosphoSitePlus analysis. Its expression was found to be correlated with PF-3758309 treatment in CML cells based on CellMinerDB univariate analyses using gene-small-molecule association data from the CTRPv2 database. Indeed, Western blot analysis showed that PAK6 knockdown in K562 and IM-resistant cells led to a reduction in MDM2 protein levels. Furthermore, MDM2 downregulation by PAK6 inhibition corresponded to an increase in p21 levels, suggesting a mechanism of MDM2-mediated p21 regulation independent of p53, as these cells are p53-null. Most interestingly, PAK6 knockdown in IM-resistant cells leads to G2/M phase accumulation and increased senescence levels (2-fold, P<0.05), detected by senescence-associated β-galactosidase staining. PAK6 knockdown induced senescence was further supported by observations of enlarged cell size (p<0.05) and increased granulation, as well as changes in senescence-associated protein markers, including p21, p27, MMP-3 and the DNA damage marker pH2Ax, by Western blot analysis. Hence, our findings indicate that dual targeting of miR-185-PAK6-mediated survival, cell cycle and metabolic pathways, along with BCR-ABL, selectively eradicates drug-resistant CML stem/progenitors. Specifically, PAK6 plays roles in MDM2/p21-mediated apoptosis, senescence and cell cycle controls, offering a valuable therapeutic strategy for improved treatment and care.

Disclosures: No relevant conflicts of interest to declare.

<< Previous Abstract | Next Abstract
*signifies non-member of ASH