The Marine Product Elatol Is a Novel Inhibitor of Mitochondrial Translation Triggering the Integrated Stress Response and Apoptosis in Leukemia and Lymphoma Cells

Background: Leukemia stem cells (LSCs) show unique, targetable dependence on activity of mitochondrial ribosomes (mitoribosomes). In addition, aggressive B-cell lymphomas may have gene expression signatures of increased dependence on oxidative phosphorylation (OXPHOS) and show sensitivity to mitoribosome inhibitors such as the FDA approved antibiotic tigecycline. Discovery and optimization of new mitoribosome inhibitors, however, is relatively unexplored in drug development for hematologic malignancies despite demonstrated therapeutic potential. Here, we assess the marine natural product elatol as a novel mitochondrial translation inhibitor.

Methods and Results: We have previously reported that elatol, the major compound from the red alga Laurencia microcladia, is effective against several non-Hodgkin lymphomas and primary chronic lymphoid leukemia (CLL) cells. In vitro studies showed elatol inhibits eIF4A1 helicase activity, suppressing cytoplasmic cap-dependent translation initiation. Further assessments using 35S-methionine incorporation in newly synthesized polypeptides in HEK293T cells with or without single-digit micromolar concentrations of elatol for short time periods revealed strong attenuation of mitochondrial protein synthesis with no effect on mitochondrial transcription. Based on previously published work highlighting the effectiveness of tigecycline in leukemia and our data supporting elatol cytotoxicity in hematologic malignancies, we assessed elatol cytotoxicity and mitochondrial translation inhibition in several leukemia and lymphoma cell lines in comparison to tigecycline. All but one line decreased viability by 50% or more (compared to vehicle control) after elatol incubation with EC50 values near 1 μM. This potency was 10-40x higher than for tigecycline in side-by-side comparisons across several lines when compared at 72h. The compound cooperated to activate a heightened integrated stress response (ISR) through DELE1 cleavage by the mitochondrial stress-activated protease OMA1. This in turn suppressed glycolytic capacity, resulting in adenosine triphosphate (ATP) depletion and subsequent cell death. We found elatol does not affect integrity of small and large mitoribosome subunits through sedimentation property analysis using sucrose gradients. Although the specific target on the mitochondrial translation apparatus remains elusive, we found that its mechanism of action differs from that of chloramphenicol, which inhibits translation elongation.

Conclusions: We have performed proof-of-concept studies using HEK293T and HeLa cell lines, isolated mitochondria from HEK293T, and leukemia and lymphoma cell lines to reveal that elatol is a potent inhibitor of mitochondrial protein synthesis at concentrations that do not affect cytoplasmic protein synthesis and that this mechanism differs from chloramphenicol. Elatol triggers the OMA1DELE1ATF4 mitochondrial stress pathway and apoptosis of leukemia and lymphoma cells. Tigecycline's compelling preclinical data in combination with kinase inhibitors informed design of a pending clinical trial (NCT02883036). Elatol's greatly improved potency provides a potential starting point for further optimization of this paradigm.