Requirement of lipocalin 2 for hematopoietic and solid tumor malignancies
Introduction
The lipocalin 2 gene encodes a secreted glycoprotein involved in iron binding, stabilizing metalloproteinase MMP-9, and induction of apoptosis. Our previous studies first showed that soft agar clones of a CML cell line (K562 cells) caused atrophy of spleen and marrow cells and a wasting syndrome in some of the leukemic mice, leading to early death (Lin et al., 2001). In later studies, we found that Bcr-Abl+ mouse hematopoietic cells and agar clones of CML cell line K562 secrete lipocalin 2, and that conditioned medium from these cells induces apoptosis in target bone marrow cells (Lin et al., 2005, Leng et al., 2008). We also found that reducing lipocalin 2 expression in mouse Bcr-Abl+ cells by either anti-sense or siRNA knockdown strongly reduces over-growth of BCR-ABL+ mouse myeloid 32D cells in marrow and spleen of NOD/scid mice (Lin et al., 2005).
To explore further the role of lipocalin 2 in BCR-ABL induced leukemia, we used the mouse transplant model, which causes a lethal myeloid leukemia involving enlarged spleens, liver and marrow and blood invasion in recipient mice receiving retroviral infected BCR-ABL+ bone marrow cells. Consistent with our previous findings (Lin et al., 2005), we found that donor marrow cells expressing BCR-ABL but lacking lipocalin 2 (24p3) (obtained from 24p3 null mice) did not cause leukemia or any disease in non-irradiated wild type recipient mice.
Cultured bone marrow cells expressing BCR-ABL from wild-type mice as expected caused solid tumors in nude mice following s.c. injection of marrow cells whereas BCR-ABL+ bone marrow cells from lipocalin 2 null mice did not cause tumors, indicating that lipocalin 2 expression is also required for tissue invasion and solid tumor formation. These findings demonstrate that lipocalin 2 has two critical functions in tumor formation: one involving over-growth in marrow and spleen tissues in leukemia and the other involving invasion of tissues needed for solid tumor formation and metastasis.
Section snippets
Materials and methods
The methods, laboratory procedures, animal experiments were described in papers by Lin et al., 2001, Lin et al., 2005 and Leng et al. (2008).
Mouse transplant leukemia studies: lipocalin 2 is required for leukemia
We examined the role of lipocalin 2 in the mouse transplant model. This model is considered to be the best model for mimicking chronic myeloid leukemia. Our findings show that Bcr-Abl+ marrow cells from lipocalin 2 knock-out mice were completely ineffective for causing leukemia under conditions of transplant in which the recipient marrow and spleen tissue were intact (no preconditioning with irradiation) (Fig. 1a and b). However, if mice were irradiated before transplant, marrow cells from 24p3
Summary
Our published studies indicate that lipocalin 2 is a critical factor for invasion of hematopoietic tissues by chronic myelogenous leukemia cells. Lipocalin 2 is secreted by Bcr-Abl+ chronic myeloid leukemia cells causing apoptosis in normal hematopoietic cells but not in Bcr-Abl+ cells. Secreted lipocalin 2 forms a complex with metalloproteinase MMP-9, and is reported to stabilize its activity, which implies that lipocalin 2 may have a role in solid tumor formation. Our studies indicate that
Acknowledgments
This research was supported in part by a grant from NIH CA49639.
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