CREB-binding protein (CREBBP) is important for the cell-autonomous regulation of hematopoiesis, including the stem cell compartment. In the present study, we show that CREBBP plays an equally pivotal role in microenvironment-mediated regulation of hematopoiesis. We found that the BM microenvironment of Crebbp(+/-) mice was unable to properly maintain the immature stem cell and progenitor cell pools. Instead, it stimulates myeloid differentiation, which progresses into a myeloproliferation phenotype. Alterations in the BM microenvironment resulting from haploinsufficiency of Crebbp included a marked decrease in trabecular bone that was predominantly caused by increased osteoclastogenesis. Although CFU-fibroblast (CFU-F) and total osteoblast numbers were decreased, the bone formation rate was similar to that found in wild-type mice. At the molecular level, we found that the known hematopoietic modulators matrix metallopeptidase-9 (MMP9) and kit ligand (KITL) were decreased with heterozygous levels of Crebbp. Lastly, potentially important regulatory proteins, endothelial cell adhesion molecule 1 (ESAM1) and cadherin 5 (CDH5), were increased on Crebbp(+/-) endothelial cells. Our findings reveal that a full dose of Crebbp is essential in the BM microenvironment to maintain proper hematopoiesis and to prevent excessive myeloproliferation.

The physiologic roles of angiopoietin-like proteins (Angptls) in the hematopoietic system remain unknown. Here we show that hematopoietic stem cells (HSCs) in Angptl3-null mice are decreased in number and quiescence. HSCs transplanted into Angptl3-null recipient mice exhibited impaired repopulation. Bone marrow sinusoidal endothelial cells express high levels of Angptl3 and are adjacent to HSCs. Importantly, bone marrow stromal cells or endothelium deficient in Angptl3 have a significantly decreased ability to support the expansion of repopulating HSCs. Angptl3 represses the expression of the transcription factor Ikaros, whose unregulated overexpression diminishes the repopulation activity of HSCs. Angptl3, as an extrinsic factor, thus supports the stemness of HSCs in the bone marrow niche.

Bone marrow (BM) hematopoietic cells are selectively sensitive to polycyclic aromatic hydrocarbons (PAH) in vivo. 7,12-Dimethylbenz(a)anthracene (DMBA), but not benzo(a)pyrene (BP), depletes BM hematopoietic cells in C57BL/6 mice. This difference is due to a BP-selective aryl hydrocarbon receptor (AhR)-mediated recovery. Colony-forming unit assays show suppression of lymphoid progenitors by each PAH within 6 h but a subsequent recovery, exclusively after BP treatment. Suppression of myeloid progenitors (6 h) occurs only for DMBA. Each progenitor responded equally to DMBA and BP in congenic mice expressing the PAH-resistant AhR (AhR(d)). AhR, therefore, mediates this BP recovery in each progenitor type. These PAH suppressions depend on Cyp1b1-mediated metabolism. Paradoxically, few genes responded to DMBA, whereas 12 times more responded to BP. Progenitor suppression by DMBA, therefore, occurs with minimal effects on the general BM population. Standard AhR-mediated stimulations (Cyp1a1, Cyp1b1, Ahrr) were similar for each PAH and for the specific agonist 2,3,7,8-tetrachlorodibenzo-p-dioxin but were absent in AhR(d) mice. A group of 12 such AhR responses was sustained from 6 to 24 h. A second, larger set of BP responses (chemokines, cytokines, cyclooxygenase 2) differed in two respects; DMBA responses were low and BP responses declined extensively from 6 to 24 h. A third cluster exhibited BP-induced increases in protective genes (Nqo1, GST-mu) that appeared only after 12 h. Conversion of BP to quinones contributes oxidative signaling not seen with DMBA. We propose that genes in this second cluster, which share oxidative signaling and AhR activation, provide the AhR-dependent protection of hematopoietic progenitors seen for BP.

Chronic myelogenous leukemia (CML) is a hematopoietic disorder originating from p210BCR/ABL-transformed stem cells, which begins as indolent chronic phase (CP) but progresses into fatal blast crisis (BC). To investigate molecular mechanism(s) underlying disease evolution, CML-exhibiting p210BCR/ABL transgenic mice were crossed with BXH2 mice that transmit a replication-competent retrovirus. Whereas nontransgenic mice in the BXH2 background exclusively developed acute myeloid leukemia, p210BCR/ABL transgenic littermates developed nonmyeloid leukemias, in which inverse polymerase chain reaction detected 2 common viral integration sites (CISs). Interestingly, one CIS was transgene's own promoter, which up-regulated p210BCR/ABL expression. The other was the 5' noncoding region of a transcription factor, Zfp423, which induced aberrant Zfp423 expression. The cooperative activities of Zfp423 and p210BCR/ABL were demonstrated as follows: (1) introduction of Zfp423 in p210BCR/ABL transgenic bone marrow (BM) cells increased colony-forming ability, (2) suppression of ZNF423 (human homologue of Zfp423) in ZNF423-expressing, p210BCR/ABL-positive hematopoietic cells retarded cell growth, (3) mice that received a transplant of BM cells transduced with Zfp423 and p210BCR/ABL developed acute leukemia, and (4) expression of ZNF423 was found in human BCR/ABL-positive cell lines and CML BC samples. These results demonstrate that enhanced expression of p210BCR/ABL and deregulated expression of Zfp423/ZNF423 contribute to CML BC.

Some cases of pre-B cell acute lymphoblastic leukemia (pre-B-ALL) are caused by the Philadelphia (Ph) chromosome-encoded BCR-ABL oncogene, and these tend to have a poor prognosis. Inhibitors of the PI3K/AKT pathway reduce BCR-ABL-mediated transformation in vitro; however, the specific PI3K isoforms involved are poorly defined. Using a murine model of Ph+ pre-B-ALL, we found that deletion of both Pik3r1 and Pik3r2, genes encoding class IA PI3K regulatory isoforms, severely impaired transformation. BCR-ABL-dependent pre/pro-B cell lines could be established at low frequency from progenitors that lacked these genes, but the cells were smaller, proliferated more slowly, and failed to cause leukemia in vivo. These cell lines displayed nearly undetectable PI3K signaling function and were resistant to the PI3K inhibitor wortmannin. However, they maintained activation of mammalian target of rapamycin (mTOR) and were more sensitive to rapamycin. Treatment with rapamycin caused feedback activation of AKT in WT cell lines but not PI3K-deficient lines. A dual inhibitor of PI3K and mTOR, PI-103, was more effective than rapamycin at suppressing proliferation of mouse pre-B-ALL and human CD19+CD34+)Ph+ ALL leukemia cells treated with the ABL kinase inhibitor imatinib. Our findings provide mechanistic insights into PI3K dependency in oncogenic networks and provide a rationale for targeting class IA PI3K, alone or together with mTOR, in the treatment of Ph+ ALL.

The T-cell protein tyrosine phosphatase (TC-PTP) is a negative regulator of the Jak/Stat cytokine signaling pathway. Our study shows that the absence of TC-PTP leads to an early bone marrow B-cell deficiency characterized by hindered transition from the pre-B cell to immature B-cell stage. This phenotype is intrinsic to the B cells but most importantly due to bone marrow stroma abnormalities. We found that bone marrow stromal cells from TC-PTP(-/-) mice have the unique property of secreting 232-890 pg/mL IFN-gamma. These high levels of IFN-gamma result in 2-fold reduction in mitotic index on IL-7 stimulation of TC-PTP(-/-) pre-B cells and lower responsiveness of IL-7 receptor downstream Jak/Stat signaling molecules. Moreover, we noted constitutive phosphorylation of Stat1 in those pre-B cells and demonstrated that this was due to soluble IFN-gamma secreted by TC-PTP(-/-) bone marrow stromal cells. Interestingly, culturing murine early pre-B leukemic cells within a TC-PTP-deficient bone marrow stroma environment leads to a 40% increase in apoptosis in these malignant cells. Our results unraveled a new role for TC-PTP in normal B lymphopoiesis and suggest that modulation of bone marrow microenvironment is a potential therapeutic approach for selected B-cell leukemia.