Role of Lipocalin 24p3 in Apoptosis and Leukemia.

Programmed cell death or Apoptosis, is a critical aspect of normal physiology as well as the genesis and treatment of cancer. Certain apoptotic pathways are transcriptionally regulated; in these cases, apoptosis is induced by the transcriptional activation of genes encoding proapoptotic proteins. We originally identified lipocalin 24p3 as the gene undergoing maximum transcriptional stimulation following induction of apoptosis by cytokine-deprivation of interleukin 3 (IL-3) dependent cells. 24p3 is a member of the lipocalin family of carrier proteins – a group of small-secreted molecules that bind and transport low-molecular weight ligands. By delivering this cargo via cell-surface receptors they are known to influence many responses. 24p3 is a secreted lipocalin, which we have found induces apoptosis when added to a variety of lymphoid cells. These and other results revealed a model in which IL-3 deprivation activates 24p3 transcription, leading to synthesis and secretion of 24p3, which induces apoptosis through an autocrine/paracrine pathway. We have isolated the 24p3 cell surface receptor (24p3R) and found that 24p3 induces apoptosis through a novel pathway culminating in a decrease in intracellular iron levels (a biological iron chelator). Interestingly, iron chelators inhibit cellular proliferation and induce apoptosis, and are under active investigation as chemotherapeutic agents. The basis by which decreased intracellular iron induces apoptosis is not well understood. We performed expression-profiling experiments to identify differentially regulated genes in 24p3 and as a control in Deferoxamine (DFO), a synthetic iron chelator, treated cells. Our preliminary results suggest that 24p3 activates the expression of a novel gene, ING-2 (inhibitor of growth-2). ING-2 prevents cell growth by inducing cell cycle arrest at the G2/M phase. In contrast, the synthetic iron chelator, DFO activates the expression of NDRG1 (n-Myc downstream-regulated gene 1), which induces cell cycle arrest at G0/G1 phase. These results suggest that 24p3 induces cell death by activating regulators of the cell cycle. Finally, we have also found that the oncogene BCR-ABL counteracts the 24p3 proapoptotic pathway by misregulating expression of 24p3 and 24p3R. To study the contribution of 24p3 apoptotic pathway in the progression of CML, we have performed CML modeling experiments in mice. BCR-ABL transformed 24p3 deficient bone marrow cells failed to induce myeloproliferative disease in recipients upon transplantation. However, wild-type bone marrow cells when transduced with BCR-ABL oncogene readily induced CML-like disease in transplanted mice. Therefore, the secretion of 24p3 by BCR-ABL transformed cells facilitates the progression of CML. We have also demonstrated that 24p3 plays an important role in Gleevec-induced cell death in BCR-ABL transformed cells. These studies have therapeutic implications for Gleevec resistant CML.