An Evaluation of the Cytotoxic Effect of the Natural Product Aaptamine Against t(4;11) Leukemias

The marine environment has been shown to be a rich source of pharmacologically-active secondary metabolites. Three marine- compounds have FDA approval for cancer indications. Aaptamine is a sponge-derived alkaloid that exhibits multiple pharmacological activities including proapoptotic/antiproliferative effects on leukemia cell lines. The effect of the aaptamine class has not been previously studied for high risk leukemias with mixed-lineage leukemia (MLL) gene rearrangements. Using the CellTiter-Glo cell viability assay we evaluated the cytotoxic effect of aaptamine against a panel of leukemia cell lines. We observed that cell lines containing t(4;11) are the most sensitive to aaptamine. Translocation (4;11) is associated with mixed-lineage leukemia and responsible for a very aggressive and refractory pediatric leukemia. Specifically, infants less than one year with t(4;11) have poor survival rates (≈ 19%) and new therapies are urgently needed. Interestingly other MLL cell lines that contain t(9;11) are comparatively less susceptible to aaptamine-mediated cytotoxicity. Jurkat cells overexpressing MLL-AF4 fusion protein are also more sensitive to aaptamine-induced cytotoxicity than wild type or MLL-AF9 overexpressing Jurkat cells indicating the specificity of aaptamine for t(4;11). To further confirm the specificity we conducted a flow based apoptosis assay and observed that aaptamine induces significant apoptosis and necrosis in RS4;11 and MV4;11 cell lines starting at 10µM but not in the t(9;11) containing THP1 cell line. We also found that aaptamine treatment induced G0/G1 arrest specifically in t(4;11) containing cell lines but not in THP1. Additionally we observed that aaptamine did not induce any resistance to the sensitive cell lines after 27 days of chronic exposure. Importantly the compound was well tolerated by healthy activated PBMCs and mice at high concentrations.

In order to decipher the mechanism of specificity, we conducted a global proteomic study with treated and untreated RS4;11 and THP1 cell lines. Our proteomic data revealed a significant upregulation of p21 and p27 in aaptamine treated RS4;11 cells but not in THP1. In agreement with the proteomic data, we observed a dose-dependent upregulation of p21 and p27 in both protein and mRNA levels in RS4;11 and MV4;11 cells but not in resistant THP1 cells. Using p21 and p27 promoter-driven luciferase reporter constructs, we observed a significant upregulation of luminescence signal in the RS4;11 cell line at much lower concentration of aaptamine (1µM) whereas the THP1 cell line required 50µM of aaptamine for significant increase in luminescence signal.

Cyclin-dependent kinase regulates the G1/S cell cycle transition by phosphorylating retinoblastoma protein (RB). Upregulation of cyclin-dependent kinase inhibitors, such as p27 and p21, promote RB hypophosphorylation and induce G0/G1 arrest. To confirm that this molecular mechanism is responsible for aaptamine induced G0/G1 arrest, we investigated the effect of aaptamine on Rb phosphorylation. We observed a dose dependent downregulation of Rb phosphorylation by aaptamine in sensitive cell lines and predicted it as a major cause of cell cycle arrest. Previous studies have shown that translocation (4;11) is associated with p27 upregulation; thus we hypothesize by further upregulating p27, aaptamine may induce G0/G1 arrest specifically in t(4;11) containing cell lines. To validate the efficacy of aaptamine in vivo, we xenografted 10 NSG mice with 1 million luciferase expressing RS4;11 cells. Four days after leukemia induction we treated half of the mice with subcutaneous injection of aaptamine (100mg/kg, daily) and the other half received vehicle treatment. Bioluminescence imaging (BLI) data revealed a significantly lower disease (p< 0.03) burden in the aaptamine treated group compared to vehicle treated group after 2 weeks. These findings are being confirmed in patient samples. Additional aaptamine analogs are being designed and will be evaluated for improved therapeutic efficacy.

Together our in vitro and in vivo findings suggest that by inducing p21 and p27 aaptamine can induce cell cycle arrest and eventually apoptosis specifically in leukemia cells that contain t(4;11) with relatively low toxicity . Therefore the aaptamine class of drug may provide additional therapeutic options for t(4;11) containing high-risk MLL leukemia patients.