Role of Acid Ceramidase in Regulating Survival and Drug Resistance in Acute Myeloid Leukemia
Open Access
- Author:
- Tan, Su-fern
- Graduate Program:
- Molecular Medicine
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- October 16, 2013
- Committee Members:
- Andrew Paul Loughran, Dissertation Advisor/Co-Advisor
"Thomas P Loughran, Jr", Committee Chair/Co-Chair
Charles H Lang, Committee Chair/Co-Chair
Hong Gang Wang, Committee Member
Jin Ming Yang, Committee Member
David F Claxton, Special Member - Keywords:
- AML
Acid Ceramidase
Mcl-1
P-glycoprotein
NF-kB
sphingolipids - Abstract:
- Acute myeloid leukemia (AML) is a heterogeneous disease that affects the differentiation of myeloid precursors. In normal hematopoiesis, hematopoietic stem cells committed to the myeloid lineage form platelets, white cells and red cells. AML patients have the characteristic feature of accumulated immature myeloid precursors known as blasts. Prognosis for AML patients depends on several risk factors, including cytogenetic abnormalities, age, prior exposure to chemotherapy, and pre-existing hematological disorder such as myelodysplastic syndrome (MDS). The five-year survival rate for AML patients remains low and drug resistance is also a major problem for relapsed patients. Hence, AML patients require better treatments. Sphingolipids are a group of sphingoid-based lipids that maintain cellular integrity and mediate signal transduction and gene regulation. Two sphingolipids are important in regulating cell survival: ceramide and sphingosine-1-phosphate (S1P). The ceramide and S1P balance is known as the “sphingolipid rheostat”, which modulates cell survival. Sphingolipids are dysregulated in many types of cancer, including AML. Acid ceramidase (AC) is a lysosomal enzyme that catalyzes ceramide breakdown into sphingosine and free fatty acids. Sphingosine is then phosphorylated by sphingosine kinases to form S1P. AC is crucial for embryonic development but is found in high levels in several different types of cancer, including prostate, colon and breast cancer. In this study, we investigated the role of AC in AML blast survival and drug resistance. We show that AC expression and activity is elevated in AML patients. Corresponding to elevated AC activity is the elevation of S1P levels also detected in patient samples. Targeting AC using ceramide analog LCL204 and AC shRNA decreased viability and myeloid cell leukemia differentiation protein-1(Mcl-1) expression, a B-cell lymphoma 2 (Bcl-2) family protein essential for AML survival. AC inhibition also decreased the multidrug resistance protein (MDR1)/P-glycoprotein (P-gp) and sensitized cells to AML chemotherapeutic drugs. Furthermore, AC overexpression in AML cell line HL-60 increased S1P and decreased total ceramide levels. We also found that AC overexpression increased NF-κB activation, a transcription factor responsible for many survival pathways including the regulation of Mcl-1 and P-gp transcription. We hypothesized that AC overexpression increases NF-κB activation that then serves as a positive feedback loop back to AC. We demonstrate that NF-κB inhibition decreased AC, Mcl-1 and P-gp expression, supporting the positive feedback loop hypothesis. Pharmacological inhibition of AC in a murine AML model also significantly increased the survival of leukemic mice. Taken together, we show that AC is important for blast survival and drug resistance in AML. Targeting AC changes the sphingolipid rheostat to increase ceramide and decrease S1P levels, thereby leading to cell death. We also demonstrate for the first time that AC regulates Mcl-1 and P-gp expression, potentially through the NF- κB pathway. Collectively, the results support AC as a promising and effective chemotherapeutic target in AML.