Targeting Acid Ceramidase and Bcl-2 in Acute Myeloid Leukemia

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PROJECT SUMMARY/ABSTRACT Acute myeloid leukemia (AML) is a heterogeneous disease characterized by uncontrolled proliferation of myeloid blasts resulting in bone marrow failure. The median age at diagnosis is 68 and the 5-year survival rate for individuals over 60 is 13%. Thus, there is an urgent unmet need for improved therapeutics for AML. Sphingolipids are an important class of amphipathic lipids that regulate proliferation, drug resistance, and apoptosis. The balance of pro-apoptotic ceramides and pro-survival sphingosine-1-phosphate (S1P) constitutes the bioactive core of sphingolipid signaling. Acid ceramidase (AC) is a lipid hydrolase that catabolizes ceramides and generates the critical substrate for S1P synthesis, sphingosine. We previously demonstrated that AML blasts rely on AC for survival. Transcriptomic analyses and AC activity assays showed AC gene expression and activity upregulated in primary AML samples. Genetic knockdown of AC with shRNA reduced cell viability and downregulated the pro-survival protein, Mcl-1, in human AML cell lines and patient samples. Pharmacological inhibition of AC with the ceramide analog, SACLAC, resulted in caspase-dependent apoptosis in multiple human AML cell lines and primary AML patient samples. Mass spectrometric sphingolipidomic profiling of SACLAC treated AML cell lines showed a reduction of S1P and a widespread increase in ceramide levels. Intriguingly, SACLAC treatment led to alternative splicing of Mcl-1 to its shorter pro-apoptotic isoform, Mcl-1S, and the ratio of Mcl-1S to full-length Mcl-1 determined cell survival. Importantly, full-length Mcl-1 overexpression or Mcl-1S knockdown attenuated SACLAC toxicity in AML. Venetoclax (ABT-199), a highly selective inhibitor of pro-survival Bcl-2, is approved for relapsed AML and is being tested in several clinical trials. However, responses to ABT- 199 as a single agent are short-lived and resistance develops through the upregulation of Mcl-1, a Bcl-2 homolog. Here, we propose dual targeting of AC and Bcl-2 in AML. Our central hypothesis is that targeting AC will synergize with ABT-199 and overcome ABT-199 resistance in AML. Since genetic and pharmacological inhibition of AC modulates Mcl-1, we hypothesize that targeting AC will synergize with ABT-199 to overcome ABT-199 resistance in AML through an Mcl-1-mediated mechanism (Aim 1). Because sphingolipid dysregulations are involved in cancer progression and drug resistance, we will characterize whether AML with acquired resistance to ABT-199 share a specific sphingolipid profile and whether certain sphingolipid species contribute to ABT-199 resistance (Aim 2). Overall, these experiments aim to identify a novel therapeutic approach for AML that combines AC inhibition and Bcl-2 family inhibitors in order to provide the rationale for further preclinical studies targeting AC and Bcl-2 family proteins in AML.