We have previously demonstrated that enhanced survival rather than increased proliferation accounts for the accumulation of natural killer (NK) cells in large granular lymphocyte (LGL) leukemia patients. To further elucidate the mechanism by which NK survival is enhanced, we analyzed leukemic NK cells isolated either from patient peripheral blood or rat splenic cells for altered expression of members of the inhibitor of apoptosis protein (IAP) family, which act as suppressors of apoptosis in a variety of human solid tumors and hematological malignancies. We now report that the IAP, survivin, was highly expressed in NK cells. In contrast, survivin was barely detectable in NK cells from the blood of normal human donors or the splenic cells from normal rats. We next asked if the lipid-derived second messenger, ceramide, which selectively induces apoptosis in cancer cells would diminish survivin protein expression. Treatment of NKL, a human NK-LGL leukemia cell line, or RNK-16, a rat NK-LGL leukemia cell line, with the cell-permeable, short-chain C6-ceramide (C6) in a pegylated liposomal formulation, led to cell apoptosis and diminished survivin protein expression, in a time and dose dependent manner. We next extended these in vitro studies to an in vivo rat model of NK-LGL leukemia. Systemic i.v. delivery of liposomal ceramide displayed significant anti-leukemia activity in a syngeneic Fischer F344 rat NK-LGL leukemia model that exhibits clonal expansion of CD3-CD8a+ lymphocytes. Over a 6-week treatment period, a well-tolerated dose of 40 mg/kg liposomal-C6, three times a week, elicited a 3 to 10- fold reduction in the weight of various lymphoid and non-lymphoid organs, compared with liposomal formulations that did not contain ceramide (ghost). Untreated or ghost-treated leukemic rats presented with lymphocytosis (LGL counts between 2 × 1011 and 3.5 × 1011/L), anemia and thrombocytopenia. Furthermore, the percentage of NK LGL cells, defined as CD3-CD8a+ by flow cytometry, was drastically elevated in the spleen, lymph node, thymus, bone marrow, blood, liver and lung in these leukemic rats, compared with their normal counterparts. In contrast, leukemic rats treated with liposomal ceramide had undetectable LGL cells in the blood and normal counts of red blood cells and platelets. Additionally, the CD3-CD8a+ NK cells in spleen, thymus and liver were found to be remarkably decreased, whereas the NK cells in bone marrow, blood and lung were within normal range. Collectively, these results indicate that bioactive ceramide analogues can be incorporated into pegylated liposomal vehicles for anti-leukemic efficacy in a rat model of NK LGL leukemia, possibly via decreased survivin expression or signaling.
Disclosures: Kester:Tracon Pharmaceutical: Penn State Reseach Foundation has licensed the ceramide liposome to Tracon Pharmaceuticals, San Diago, CA for the development.