Targeting HMG-CoA Reductase in Acute Lymphoblastic Leukemia (ALL): Statins Inhibit Proliferation and Induce Apoptosis in ALL Cells

Abstract

HMG-CoA reductase inhibitors (statins) are commonly used, FDA-approved agents for the treatment of hypercholesterolemia. In addition to reducing serum cholesterol, statins have been shown to have anti-inflammatory properties and anti-proliferative effects on a number of cell types, including lymphocytes. Many of these effects occur because inhibition of HMG-CoA reductase results in depletion of a number of important cellular intermediates required for signal transduction through the AKT, Ras, and Erk pathways. Statins have been demonstrated to have anti-proliferative effects in a number of malignancies, including acute myeloid leukemia using preclinical models; however, there are no published reports evaluating the efficacy of these agents against acute lymphoblastic leukemia. Large population studies have also suggested that statins may reduce the incidence of a number of cancers, furthering interest in the use of these agents in malignancies, especially since the dose-limiting toxicities (hepatitis and rhabdomyolysis) do not overlap with the majority of commonly used cytotoxic agents. We hypothesized that ALL cells are dependent on the function of HMG-CoA reductase for survival. We tested this hypothesis by studying the efficacy of statins against ALL cells. We studied the effects of 5 different compounds (mevastatin, pravastatin, fluvastatin, lovastatin, and simvastatin) against 7 ALL cell lines (4 human and 3 murine). We found that the more potent 2nd generation statins (fluvastatin, lovastatin, and simvastatin) had marked effects on ALL cells, whereas the less potent first generation statins (mevastatin and pravastatin) had less significant effects. We found that fluvastatin, lovastatin, and simvastatin inhibited proliferation of all 7 ALL cell lines using MTT assay (p <0.05). We also found these three statins induced apoptosis, leading to profound cell death in all 7 cell lines (p<0.05) as assessed by flow cytometry for Annenix-V staining and 7-AAD. IC50 dosing for the three agents ranged between 500nM and 5uM depending on the cell line, levels easily obtainable in humans. The most likely explanation for this remarkable, single-agent effect on ALL is that ALL cells are indeed dependent on cholesterol biosynthesis. Other potential explanations include the possibility that ALL cells are dependant on one of the signal transduction pathways affected by targeting HMG-CoA reductase, or the formal possibility of an off-target effect of statins separate from the inhibition of cholesterol biosynthesis. To determine if the effects of the statins were due to a direct effect on HMG-CoA reductase, we treated cells with melavonolactone, the product of conversion of HMG-CoA by HMGCoA reductase. We found that the addition of melavonolactone completely reversed the effects of all statins even at high doses (>10uM) in all cell lines, making the explanation of an off-target, non-HMG-CoA reductase-based mechanism unlikely. Ongoing work includes the testing of statins in NOD/SCID xenograft models of primary human ALL models, assessing the effects of statins on AKT, Ras, and Erk in ALL cells, and assessing the combination of statins with cytotoxic agents in ALL. In conclusion, we found that ALL cells are dependent of HMG-CoA reductase for survival. Since statins are safe and well-tolerated, this class of agents should be further explored in patients with ALL.