Tumor-associated karyotypic lesions coselected with in vitro macrophage differentiation

PU.1 is a suppressor of myeloid leukemia, inactivated in mice by gene deletion and mutation of its DNA binding domain

In most myeloid leukemias induced in mice by gamma-radiation, one copy of chromosome 2 has suffered a deletion. To search for a potential tumor suppressor gene in that region, we have delineated the deletions in a panel of these tumors. A commonly deleted region of 2 megabase pairs (Mbp) includes the gene encoding the PU.1 transcription factor, a powerful inducer of granulocytic/monocytic differentiation. Significantly, in 87% of these tumors the remaining PU.1 allele exhibited point mutations in the PU.1 DNA binding domain. Surprisingly, 86% of these mutations altered a single CpG, implicating deamination of deoxycytidine, a common mutational mechanism, as the origin of this lesion. The "hot spot" resides in the codon for a contact residue essential for DNA binding by PU.1. In keeping with a tumor suppressor role for PU.1, enforced expression of wild-type PU.1 in the promyelocytic leukemia cells inhibited their clonogenic growth, induced monocytic differentiation, and elicited apoptosis. The mutant PU.1 found in tumors retained only minimal growth suppressive function. The results suggest that PU.1 normally suppresses development of myeloid leukemia by promoting differentiation and that the combination of gene deletion and a point mutation that impairs its ability to bind DNA is particularly leukemogenic.

c-Rel regulates interleukin 12 p70 expression in CD8(+) dendritic cells by specifically inducing p35 gene transcription

Interleukin 12 (IL-12) is a 70-kD proinflammatory cytokine produced by antigen presenting cells that is essential for the induction of T helper type 1 development. It comprises 35-kD (p35) and 40-kD (p40) polypeptides encoded by separate genes that are induced by a range of stimuli that include lipopolysaccharide (LPS), DNA, and CD40 ligand. To date, the regulation of IL-12 expression at the transcriptional level has mainly been examined in macrophages and restricted almost exclusively to the p40 gene. Here we show that in CD8(+) dendritic cells, major producers of IL-12 p70, the Rel/nuclear factor (NF)-kappaB signaling pathway is necessary for the induction of IL-12 in response to microbial stimuli. In contrast to macrophages which require c-Rel for p40 transcription, in CD8(+) dendritic cells, the induced expression of p35 rather than p40 by inactivated Staphylococcus aureus, DNA, or LPS is c-Rel dependent and regulated directly by c-Rel complexes binding to the p35 promoter. This data establishes the IL-12 p35 gene as a new target of c-Rel and shows that the regulation of IL-12 p70 expression at the transcriptional level by Rel/NF-kappaB is controlled through both the p35 and p40 genes in a cell type-specific fashion.

Leukemic cells from murine myeloid leukemia display an intrinsic ability for autonomous proliferation

OBJECTIVE

Human acute myeloid leukemia (AML) cells can proliferate in vitro in the absence of added growth factors when cultured at high cell density. Autocrine growth factor production is a postulated mechanism of autonomous growth. We sought to examine this using murine AML cells.

MATERIALS AND METHODS

We have utilized a Moloney murine leukemia virus (M-MuLV) model of AML to investigate the nature of autonomous in vitro growth of myeloid leukemic cells.

RESULTS

Like human AML, M-MuLV-induced myeloid leukemic cells displayed autonomous growth in unstimulated high cell density cultures. However, replating of individual, primary, growth factor autonomous colonies of leukemic cells demonstrated the presence of clonogenic cells capable of autonomous growth when cultured at low cell density. In addition, there was heterogeneity in the progeny of these cells: both factor-dependent leukemic cells and cells autonomous of exogenous factor were observed.

CONCLUSION

We propose that clonogenic cells capable of autonomous growth at low cell density represent leukemic progenitors while the majority of leukemic cells derived from these "autonomous" leukemic cells are factor-dependent.