Hyperthermia enhances mapatumumab-induced apoptotic death through ubiquitin-mediated degradation of cellular FLIP(long) in human colon cancer cells.

Colorectal cancer is the third leading cause of cancer-related mortality in the world; the main cause of death of colorectal cancer is hepatic metastases, which can be treated with hyperthermia using isolated hepatic perfusion (IHP). In this study, we report that mild hyperthermia potently reduced cellular FLIP(long), (c-FLIP(L)), a major regulator of the death receptor (DR) pathway of apoptosis, thereby enhancing humanized anti-DR4 antibody mapatumumab (Mapa)-mediated mitochondria-independent apoptosis. We observed that overexpression of c-FLIP(L) in CX-1 cells abrogated the synergistic effect of Mapa and hyperthermia, whereas silencing of c-FLIP in CX-1 cells enhanced Mapa-induced apoptosis. Hyperthermia altered c-FLIP(L) protein stability without concomitant reductions in FLIP mRNA. Ubiquitination of c-FLIP(L) was increased by hyperthermia, and proteasome inhibitor MG132 prevented heat-induced downregulation of c-FLIP(L). These results suggest the involvement of the ubiquitin-proteasome system in this process. We also found lysine residue 195 (K195) to be essential for c-FLIP(L) ubiquitination and proteolysis, as mutant c-FLIP(L) lysine 195 arginine (arginine replacing lysine) was left virtually un-ubiquitinated and was refractory to hyperthermia-triggered degradation, and thus partially blocked the synergistic effect of Mapa and hyperthermia. Our observations reveal that hyperthermia transiently reduced c-FLIP(L) by proteolysis linked to K195 ubiquitination, which contributed to the synergistic effect between Mapa and hyperthermia. This study supports the application of hyperthermia combined with other regimens to treat colorectal hepatic metastases.

Cancer stem cells with genetic instability: the best vehicle with the best engine for cancer.

Our understanding of the role of stem cells in cancer development is evolving quickly. In the course of tumor expansion, a subpopulation of tumor cells with stem cell-like features has been noted. These cancer stem cells give rise to transit amplifying tumor cells, which comprise the majority of the tumor mass prior to terminal differentiation. Combining this finding with genetic instability, a well-known engine for cancer development and metastases, a new model emerges for cancer where normal stem cells and their cellular pathway acquire stochastic malignant abilities. In this model, when cancer stem cells self-renew, many genetic variants are produced. Just as microbes 'learn' to defeat antibiotics, genetically heterogeneous cancer stem cells may possibly acquire resistance to various chemotherapeutic approaches. Drug-resistant microorganisms selected by spontaneous mutation of bacterial DNA may not be so different than the drug-resistant and genetically instable cancer stem cells recurring after chemotherapeutic treatment. In this gloomy view of cancer, cancer stem cells with genetic instability can be considered as 'the best vehicle with the best engine', a formidable challenge for the future development of new anticancer therapies.

AML1-ETO expression is directly involved in the development of acute myeloid leukemia in the presence of additional mutations.

The t(8;21) is one of the most frequent chromosomal abnormalities associated with acute myeloid leukemia (AML). The translocation, which involves the AML1 gene on chromosome 21 and the ETO gene on chromosome 8, generates an AML1-ETO fusion transcription factor. To examine the effect of the AML1-ETO fusion protein on leukemogenesis, we made transgenic mice in which expression of AML1-ETO is under the control of the human MRP8 promoter (hMRP8-AML1-ETO). AML1-ETO is specifically expressed in myeloid cells, including common myeloid progenitors of hMRP8-AML1-ETO transgenic mice. The transgenic mice were healthy during their life spans, suggesting that AML1-ETO alone is not sufficient for leukemogenesis. However, after treatment of newborn hMRP8-AML1-ETO transgenic mice and their wild-type littermates with a strong DNA-alkylating mutagen, N-ethyl-N-nitrosourea, 55% of transgenic mice developed AML and the other 45% of transgenic mice and all of the wild-type littermates developed acute T lymphoblastic leukemia. Our results provide direct evidence that AML1-ETO is critical for causing myeloid leukemia, but one or more additional mutations are required for leukemogenesis. The hMRP8-AML1-ETO-transgenic mice provide an excellent model that can be used to isolate additional genetic events and to further understand the molecular pathogenesis of AML1-ETO-related leukemia.

BCL-2 cooperates with promyelocytic leukemia retinoic acid receptor alpha chimeric protein (PMLRARalpha) to block neutrophil differentiation and initiate acute leukemia.

The promyelocytic leukemia retinoic acid receptor alpha (PMLRARalpha) chimeric protein is associated with acute promyelocytic leukemia (APL). PMLRARalpha transgenic mice develop leukemia only after several months, suggesting that PMLRARalpha does not by itself confer a fully malignant phenotype. Suppression of apoptosis can have a central role in tumorigenesis; therefore, we assessed whether BCL-2 influenced the ability of PMLRARalpha to initiate leukemia. Evaluation of preleukemic animals showed that whereas PMLRARalpha alone modestly altered neutrophil maturation, the combination of PMLRARalpha and BCL-2 caused a marked accumulation of immature myeloid cells in bone marrow. Leukemias developed more rapidly in mice coexpressing PMLRARalpha and BCL-2 than in mice expressing PMLRARalpha alone, and all mice expressing both transgenes succumbed to leukemia by 7 mo. Although both preleukemic, doubly transgenic mice and leukemic animals had abundant promyelocytes in the bone marrow, only leukemic mice exhibited thrombocytopenia and dissemination of immature cells. Recurrent gain of chromosomes 7, 8, 10, and 15 and recurrent loss of chromosome 2 were identified in the leukemias. These chromosomal changes may be responsible for the suppression of normal hematopoiesis and dissemination characteristic of the acute leukemias. Our results indicate that genetic changes that inhibit apoptosis can cooperate with PMLRARalpha to initiate APL.

Purified hematopoietic stem cells can differentiate into hepatocytes in vivo.

The characterization of hepatic progenitor cells is of great scientific and clinical interest. Here we report that intravenous injection of adult bone marrow cells in the FAH(-/-) mouse, an animal model of tyrosinemia type I, rescued the mouse and restored the biochemical function of its liver. Moreover, within bone marrow, only rigorously purified hematopoietic stem cells gave rise to donor-derived hematopoietic and hepatic regeneration. This result seems to contradict the conventional assumptions of the germ layer origins of tissues such as the liver, and raises the question of whether the cells of the hematopoietic stem cell phenotype are pluripotent hematopoietic cells that retain the ability to transdifferentiate, or whether they are more primitive multipotent cells.

Myeloid differentiation of FdCP1 cells is dependent on Stat5 processing.

The IL-3 family of cytokines transduces signals through Stat5 and regulates myeloid development. Previous studies have determined that a carboxy terminally truncated isoform of Stat5 is activated in immature myeloid cells. This isoform, which lacks a transcriptional activation domain, is generated by a protein-processing event. To determine whether Stat5 cleavage plays an important role in the growth and maturation of myeloid progenitors, the FdCP1 model of myeloid maturation was evaluated. FdCP1 cells are IL-3-dependent myeloid progenitors that differentiate into monocytes when cultured in granulocyte macrophage-colony-stimulating factor (GM-CSF). Consistent with their immature phenotype, when FdCP1 cells are cultured in IL-3 they exhibit robust protease activity and signal through truncated Stat5 isoforms. In contrast, maturation leads to a loss of protease activity and a switch to the expression to full-length Stat5 isoforms. Introduction of a noncleavable, full-length Stat5 mutant into undifferentiated FdCP1 cells leads to a partially differentiated phenotype and prevents further differentiation in response to GM-CSF. These results support our hypothesis that Stat5 processing is important for myeloid maturation. (Blood. 2000;96:1358-1365)

The PEBP2betaMYH11 fusion created by Inv(16)(p13;q22) in myeloid leukemia impairs neutrophil maturation and contributes to granulocytic dysplasia.

Chromosomal translocations involving the genes encoding the alpha and beta subunits of the Pebp2/Cbf transcription factor have been associated with human acute myeloid leukemia and the preleukemic condition, myelodysplasia. Inv(16)(p13;q22) fuses the gene encoding the beta subunit of Pebp2 to the MYH11 gene encoding a smooth muscle myosin heavy chain (Smmhc). To examine the effect of the inv(16)(p13;q22) on myelopoiesis, we used the hMRP8 promoter element to generate transgenic mice expressing the Pebp2betaSmmhc chimeric fusion protein in myeloid cells. Neutrophil maturation was impaired in PEBP2betaMYH11 transgenic mice. Although the transgenic mice had normal numbers of circulating neutrophils, their bone marrow contained increased numbers of immature neutrophilic cells, which exhibited abnormal characteristics. In addition, PEBP2betaMYH11 inhibited neutrophilic differentiation in colonies derived from hematopoietic progenitors. Coexpression of both PEBP2betaMYH11 and activated NRAS induced a more severe phenotype characterized by abnormal nuclear morphology indicative of granulocytic dysplasia. These results show that PEBP2betaMYH11 can impair neutrophil development and provide evidence that alterations of Pebp2 can contribute to the genesis of myelodysplasia.

Mice defective in two apoptosis pathways in the myeloid lineage develop acute myeloblastic leukemia.

Fas-deficient (Fas(lpr/lpr)) mice constitutively expressing Bcl-2 in myeloid cells by the hMRP8 promoter often develop a fatal disease analogous to human acute myeloblastic leukemia (AML-M2). Hematopoietic cells from leukemic Fas(lpr/lpr)hMRP8bcl-2 animals form clonogenic blast colonies in vitro and can transfer disease to wild-type mice. In vitro ligation of Fas on Fas+/+ hMRP8bcl-2 marrow cells depletes approximately 50% of myeloid progenitor activity, demonstrating that Bcl-2 can only partially block Fas-mediated death signals in myelomonocytic progenitors. In addition, Fas(lpr/lpr) marrow contains greatly increased numbers of myeloid colony-forming cells as compared to Fas+/+ controls. Taken together, these data suggest that Fas has a novel role in the regulation of myelopoiesis and that Fas may act as a tumor suppressor to control leukemogenic transformation in myeloid progenitor cells.

Enforced expression of Bcl-2 in monocytes rescues macrophages and partially reverses osteopetrosis in op/op mice.

Osteopetrotic (op/op) mice lack functional M-CSF and have depressed levels of macrophages and osteoclasts. We prepared transgenic mice (hMRP8bcl-2) that express human Bcl-2 in monocytes. In vitro hMRP8bcl-2 monocytes do not undergo apoptosis in the absence of serum and M-CSF, while op/op and wild-type monocytes die. These Bcl-2-expressing monocytes spontaneously undergo macrophage differentiation. In vivo, the op/op hMRP8bcl-2 mice show significant replenishment of tissue macrophages. Their long bone osteopetrosis is largely reversed, and extensive medullary hematopoiesis appears in the bone marrow. We propose that M-CSF augments monocyte survival, permitting them to respond to internal and external cues for their differentiation.

From stem cells to lymphocytes: biology and transplantation.

We review the development of the hematopoietic system, focusing on the transition from hematopoietic stem cells (HSCs) to T cells. This includes the isolation of HSCs, and recent progress in understanding their ontogeny, homing properties, and differentiation. HSC transplantation is reviewed, including the kinetics of reconstitution, engraftment across histocompatibility barriers, the facilitation of allogeneic engraftment, and the mechanisms of graft rejection. We describe progress in understanding T-cell development in the bone marrow and thymus as well as the establishment of lymph nodes. Finally, the role of bcl-2 in regulating homeostasis in the hematopoietic system is discussed.

A PMLRARalpha transgene initiates murine acute promyelocytic leukemia.

The malignant cells of acute promyelocytic leukemia (APL) contain a reciprocal chromosomal translocation that fuses the promyelocytic leukemia gene (PML) with the retinoic acid receptor alpha gene (RAR alpha). To test the hypothesis that the chimera PMLRAR alpha plays a role in leukemogenesis, we expressed a PMLRAR alpha cDNA in myeloid cells of transgenic mice. PMLRAR alpha transgenic mice exhibited impaired neutrophil maturation early in life, which progressed at a low frequency over the course of several months to overt APL. Both the preleukemic state and the leukemia could be transplanted to nontransgenic mice, and the transplanted preleukemia could progress to APL. The APL recapitulated features of the human disease, including a response to retinoic acid. Retinoic acid caused the leukemic cells to differentiate in vitro and in vivo, eliciting remissions of both the preleukemic state and APL in mice. Our results demonstrate that PMLRAR alpha impairs neutrophil differentiation and initiates the development of APL. The transgenic mice described here provide an apparently accurate model for human APL that includes clear evidence of tumor progression. The model should be useful for exploring the molecular pathogenesis of APL and the mechanisms of the therapeutic response to retinoic acid, as well as for preclinical studies of therapeutic regimens.

Myeloid differentiation is impaired in transgenic mice with targeted expression of a dominant negative form of retinoid X receptor beta.

To investigate the in vivo function of retinoid X receptor (RXR) on myelopoiesis, We generated transgenic (Tg) mice with targeted expression of a dominant negative form of RXR beta in myeloid cells. In these Tg mice the transgene is expected to suppress the function of hetero dimeric receptors composed of RXR and its counterparts, such as retinoic acid receptor. Out of 12 mice analysed, one Tg mouse exhibited a severe maturation arrest at the promyelocytic stage. Three other Tg mice showed a mild inhibition of myeloid differentiation, which was further augmented when mice were treated with 5-fluorouracil (5-FU). Furthermore, four Tg mice showed impaired myeloid differentiation in response to the treatment by 5-FU on granulocyte-colony stimulating factor (G-CSF), although they exhibited apparently normal myelopoiesis in the untreated state. The phenotype of Tg mice observed after G-CSF treatment correlated with the expression level of the transgene, although the correlation was not found in untreated mice. These results indicated that myeloid differentiation is perturbed in the Tg mice by the dominant negative effect of the transgenic RXR, indicating that RXR plays a role in myelopoiesis.

Flow cytometric identification of murine neutrophils and monocytes.

Flow cytometry and cell sorting have been employed in order to identify and purify murine neutrophils and monocytes. Using a combination of antibodies, we were able to distinguish between these two subsets of myeloid cells. The method described in this paper is simple to perform and can be applied to characterize myeloid cells from blood, spleen, bone marrow and after an induced inflammation.

Demonstration that Thy(lo) subsets of mouse bone marrow that express high levels of lineage markers are not significant hematopoietic progenitors.

We have been unable to reproduce experiments suggesting the existence of three lineage-restricted progenitor populations from mouse bone marrow. Thy1.1loMac-1+B220+ cells were reported to give rise to greatly expanded numbers of myeloid and lymphoid cells, while Thy1.1loMac-1+B220- and Thy1.1loMac-1-B220+ cells were reported to be highly proliferative myeloid and B-lineage-restricted progenitors, respectively. Both Mac-1+ cell types appear to be much less frequent than previously reported, and we observed no activity consistent with their characterization as committed progenitors of expanded numbers of cells. The original identification of these populations may have resulted from a failure to distinguish bonafide signals from autofluorescent background and nonspecific staining. The progenitor activities originally associated with these populations may have been due to hematopoietic stem cell contamination. This study shows that low levels of Mac-1 are expressed on cells with multipotent progenitor activity. Thy1.1loB220+Mac-1- cells can be purified from bone marrow, but in these experiments they do not give rise to detectable levels of progeny on injection into lethally irradiated mice. Thy1.1loB220+Mac-1- cells appear to be pro-B cells without significant proliferation potential in vivo. The finding that the described populations do not have the reported progenitor activities leaves the pathways of stem cell differentiation open to further study.

bcl-2 inhibits apoptosis of neutrophils but not their engulfment by macrophages.

Neutrophils, the most common inflammatory leukocytes, have the most limited life span of all blood cells. After they undergo apoptosis, they are recognized and engulfed by macrophages. bcl-2, a proto-oncogene rearranged and deregulated in B cell lymphomas bearing the t(14;18) translocation, is known to inhibit programmed death. bcl-2 expression is localized in early myeloid cells of the bone marrow but is absent in mature neutrophils. Transgenic mice that expressed bcl-2 in mature neutrophils showed that bcl-2 blocked neutrophil apoptosis. Despite this, homeostasis of neutrophil population is essentially unaffected. In fact, macrophage uptake of neutrophils expressing bcl-2 still occurred. This transgenic model indicates that the mechanism that triggers phagocytosis of aging neutrophils operates independently of the process of apoptosis regulated by bcl-2.

Mouse MRP8 and MRP14, two intracellular calcium-binding proteins associated with the development of the myeloid lineage.

MRP8 and MRP14 are two S100-like calcium-binding proteins of unknown function, associated with numbers of human inflammatory disorders. Both molecules have been described as L1 complex, cystic fibrosis antigen, or p8 and p14. We report here the cloning of mouse MRP8 and MRP14 and their pattern of expression during hematopoiesis. Mouse MRP8 and MRP14 proteins share 59% identity with their human counterparts, but they are more divergent than the other members of the S100 protein family. Mouse MRP proteins are coexpressed in fetal myeloid progenitors, where they are detected as early as day 11 of gestation. In fetal liver and yolk sac, MRP+ cell populations increased in number, in association with the development of the myeloid lineage. In adult mouse, we identified MRP8 and MRP14 proteins in immature myeloid cells of the bone marrow, myeloid cells in the splenic red pulp and marginal zone, in addition to monocytes and blood neutrophils. However, MRP expression is lost as cells terminally differentiate into tissue macrophages. In addition, using thioglycollate-induced peritoneal inflammatory exudates, we showed that MRP8 and MRP14 proteins are highly expressed in recruited neutrophils and monocytes.

Cloning and expression of two human genes encoding calcium-binding proteins that are regulated during myeloid differentiation.

The cellular mechanisms involved in chronic inflammatory processes are poorly understood. This is especially true for the role of macrophages, which figure prominently in the inflammatory response. Two proteins, MRP8 and MRP14, which are expressed in infiltrate macrophages during inflammatory reactions but not in normal tissue macrophages, have been characterized. Here we report that MRP8 and MRP14 mRNAs are specifically expressed in human cells of myeloid origin and that their expression is regulated during monocyte-macrophage and granulocyte differentiation. To initiate the analysis of cis-acting elements governing the tissue-specific expression of the MRP genes, we cloned the human genes encoding MRP8 and MRP14. Both genes contain three exons, are single copy, and have a strikingly similar organization. They belong to a novel subfamily of highly homologous calcium-binding proteins which includes S100 alpha, S100 beta, intestinal calcium-binding protein, P11, and calcyclin (2A9). A transient expression assay was devised to investigate the tissue-specific regulatory elements responsible for MRP gene expression after differentiation in leukemia HL60 cells. The results of this investigation demonstrated that the cis-acting elements responsible for MRP expression are present on the cloned DNA fragment containing the MRP gene loci.