Botanical Alkyl Hydroquinone Derivative HQ17(3) Induces Calcium-Associated Mitochondrial Damage and Mitophagy to Exert Ctotoxicity to Philadelphila Chromosome(+) ALL Cells

Introduction: Acute lymphoblastic leukemias (ALLs) harboring t(9;22)(Ph⁺-ALL) are very high risk (VHR) ALL displaying poor clinical outcome irrespective of intensive chemotherapies plus tyrosine kinase inhibitor (TKI) treatment. HQ17(3)[10'(Z),13'(E),15'(E)-heptadecatrienyl hydroquinone] isolated from sap of the lacquer tree showed rapid (within 24hrs) and potent cytotoxic effect at micromolar concentration on several ALL cell lines, including Imatinib-refractory Ph⁺-ALL SUP-B15 cells, but spared normal PB leukocytes, and showed nontoxic in experimental rats after 28-day injection. Therefore HQ17(3) presents as a potential anti-leukemic agents and provide a platform for exploring anti-leukemic adjuvants. Our previous study showed HQ17(3)-induced rapid cell demise, characterized by oxidative stress, mitochondrial membrane potential disturbance, loss of membrane integrity, and nuclear DNA fragmentation. HQ17(3)-induced cell death is a caspase-independent program, and is different from the RIP1-mediated controlled necroptosis since both pan-caspase inhibitor and RIP-1 inhihitor failed to protect SUP-B15 cells from death. The ER stress markers (chaperon Grp78 and phosphorylated-eIF2α) were up-regulated as early as 5hrs after HQ17(3) treatment. Here we aim to illustrate the characters of the HQ17(3)-induced non-classical death on Ph⁺-SUP-B15 cells, focus on ER stress-associated mitochondrial Ca²⁺ homeostasis.

Methods: Cell death and changes of mitochondria in response to HQ17(3) w/wo inhibitors were analyzed. Cells were stained by Annexin V/PI and analyzed by flow cytometry for cell death. Mitochondria mass, mitochondrial Ca²⁺ accumulation was detected by fluorescent Mitotracker Green and Rhod-2 probes, respectively. Mitochondrial superoxide was measured by Mitosox stain. Western blot analysis was used to analyze MFN1/2, OPA1 (mitochondrial markers). Nuclear accumulation of apoptosis inducing factor (AIF), co-localization of mitochondrial COX-IV and LC3-II (mitophagy) were revealed by immunofluorescence stain and confocal microscopy.

Results: We showed mitochondrial Ca²⁺ accumulation at the early time when ER stress occurred (Fig 1), accompanied by mitochondrial superoxide elevation, followed by loss of mitochondrial membrane potential (MMP) and nuclear translocation of apoptosis-inducing factor (AIF). HQ17(3) treatment lead to decreased mitochondrial proteins MFN1/2 and OPA1, while Mitotracker Green stain showed significant loss of mitochondrial mass preceded cell death, indicating damaged mitochondria underwent fission followed by mitophagy. Immunofluorescence stain showed evidence of mitophagy (COX IV and LC3B co-localization). Calpain-1 inhibitor PD150606 blocked AIF nuclear translocation but only slightly reduced the HQ17(3)-induced cell death (Fig 2). Further, Ca²⁺ chelator Bapta-AM prevented mitochondrial superoxide production, MMP loss, mitophagy (Fig 3), and rescued cell death (Fig 1) more effectively.

Conclusion: In Ph⁺-ALL SUP-B15 cells, HQ17(3) induce ER stress by yet-defined mechanism, this mobilizes Ca²⁺ to mitochondria and acts in multi-facet: a) results in AIF cleavage and translocation to mediate nuclear chromatin fragmentation, b) Ca²⁺-overload leads to oxidative stress and perturbs mitochondria integrity, c) damaged mitochondria trigger extensive mitophagy and cell death ensues. Therefore, agents that help elicit similar intricate effector network associated with ER/mitochondria stress will have potential to be adjuvants in aiding control of the Ph⁺ VHR-ALL cells refractory to conventional chemotherapies and TKI regime.

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