Regulation of proliferation and cytokine expression of bone marrow fibroblasts: role of c-myb

Collagen loss and impaired wound healing is associated with c-Myb deficiency

Collagen type I serves as an abundant structural and signalling component of skin. It is also an established target gene of the transcription factor, c-Myb. When c-myb-/- embryos were examined it was observed that their skin was markedly thinner than normal. Importantly, immunohistochemical investigation showed complete absence of collagen type I. Although these homozygous knock-out embryos fail to develop beyond day 15, fibroblasts established from these embryos (mouse embryonic fibroblasts [MEFs]) show defective proliferative responses. Furthermore, in vitro scratch wound assays demonstrated that these c-myb-/- MEFs also exhibit slower closure than their wild-type counterparts. Embryonic lethality has meant that examination of the role of c-Myb in adult mouse skin has not been reported to date. However, in view of the abundance of collagen type I in normal skin, its role in skin integrity and the in vitro data showing proliferative and migration defects in c-myb-/- MEFs, we investigated the consequences of heterozygous c-myb loss in adult mice on the complex process of skin repair in response to injury. Our studies clearly demonstrate that heterozygous c-myb deficiency has a functional effect on wound repair, collagen type I levels and, in response to wounding, transforming growth factor-beta1 (an important collagen stimulating factor) induction expression is aberrantly high. Manipulation of c-Myb may therefore provide new therapeutic opportunities for improving wound repair while uncontrolled expression may underpin some fibrotic disorders.

Oncogenic mutations cause dramatic, qualitative changes in the transcriptional activity of c-Myb

The v-Myb oncoprotein encoded by Avian Myeloblastosis Virus is highly oncogenic, induces leukemias in chickens and mice and transforms immature hematopoietic cells in vitro. The v-Myb protein is a mutated and truncated version of c-Myb, a DNA-binding transcription factor expressed in many cell types that is essential for normal hematopoiesis. Previous studies suggested that two types of differences, DNA binding domain mutations and the deletion of a C-terminal negative regulatory domain were important for increasing the transforming activity of v-Myb. Here, we combined structure-function studies of the v-Myb and c-Myb proteins with unbiased microarray-based transcription assays to compare the transcriptional specificities of the two proteins. In human cells, the v-Myb and c-Myb proteins displayed strikingly different activities and regulated overlapping, but largely distinct sets of target genes. Each type of mutation that distinguished v-Myb from c-Myb, including the N- and C-terminal deletions, DNA binding domain changes and mutations in the transcriptional activation domain, affected different sets of target genes and contributed to the different activities of c-Myb and v-Myb. The results suggest that v-Myb is not just a de-repressed version of c-Myb. Instead, it is a distinct transcriptional regulator with a unique set of activities.

Defective stem cell factor expression in c-myb null fetal liver stroma

High levels of c-Myb are observed in immature precursor myeloid and lymphoid cells, while downregulation of c-myb accompanies terminal differentiation to a mature phenotype. This has established c-Myb as a crucial transcription factor for hematopoiesis. Further evidence for this is the embryonic death of the c-myb homozygous mutant mouse at ED15 due to defective fetal liver erythropoiesis. Cells from fetal liver of wild-type and c-myb-/- embryos were examined in detail for their hematopoietic potential and the capacity of the stroma to support wild-type hematopoiesis. The c-myb-/- fetal liver was shown to harbor sevenfold fewer spleen focus-forming cells and a similarly lower number of cells with long-term repopulating capacity (high proliferative potential cells). However, shorter term repopulating cells were not substantially reduced. c-myb-/- stromal cells were unable to support the proliferation of wild-type bone marrow lineage-negative cells. This was found to be partly due to a decrease in stem cell factor (SCF) expression while partial rescue of the stromal cell cultures was achieved through the addition of exogenous SCF. DNA binding studies for two sites within the SCF promoter demonstrated an in vitro interaction between the SCF promoter and c-Myb and transient transfection studies demonstrated that c-Myb could substantially transactivate the SCF promoter in HEK293 cells. These data explain why the c-myb-/- embryos are so impaired in their ability to establish hematopoiesis.

Antisense therapy in cancer

This review discusses laboratory and clinical studies of antisense oligodeoxynucleotides as potential treatments for haematological malignancies and solid tumours. Mechanisms of action, pharmacokinetics, toxicities and potential clinical applications of these agents are described.

Expression of c-myb and B-myb oncogenes on myelofibrotic marrow fibroblasts

The term IMF (Idiopathic Myelofibrosis) refers to a primary bone marrow disease in which the normal haematopoietic bone marrow cells are for unknown reasons replaced by connective tissue. The pathogenesis of the disease has not been clarified yet. We have speculated that the increment of proliferation of bone marrow fibroblasts in IMF may be the consequence of the over-expression of some oncogenes, leading or contributing to the fibrosis via a cell amplification. Thus, we investigated the possible role of the c-myb and B-myb genes in IMF and control bone marrow fibroblasts in different culture conditions to evaluate proliferation parameters in the absence or presence of serum. Using the reverse transcriptase polymerase chain reaction technique, we demonstrated that the kinetics of induction was similar for both c-myb and B-myb during the proliferation of normal bone marrow fibroblasts. When compared to normal controls, cultured IMF fibroblasts showed more elevated values of c-myb and B-myb RNA; furthermore, after a 72 hours stimulation with serum, c-myb and B-myb messages remained relatively high in myelofibrotic fibroblasts. Finally, after serum starvation, c-myb and to a lesser extent B-myb RNA levels remained unusually high in IMF fibroblasts, while under the same experimental conditions c-myb and B-myb messages became virtually undetectable in normal bone marrow fibroblasts. To our knowledge this work represents the first description of an abnormal behavior of these genes in IMF fibroblasts.

C-myb, but not B-myb, upregulates type I collagen gene expression in human fibroblasts

C-myb and B-myb belong to the myb family of transcription factors. We have shown previously that c-myb is deregulated in fibroblasts from systemic sclerosis (scleroderma) patients relative to normal fibroblasts. Scleroderma fibroblasts are known to express elevated levels of collagen genes and transforming growth factor beta is known to be a pro-fibrotic cytokine and to induce transcription of type I collagen genes. We have therefore investigated the role of c-myb and B-myb in the regulation of type I collagen genes in response to transforming growth factor beta in normal human fibroblasts. We show that, in these cells, transforming growth factor beta treatment induces c-myb as well as collagen alpha1(I) and alpha2(I) gene expression, but not B-myb. Furthermore we demonstrate by cotransfection assays that c-myb can upregulate alpha1(I) and alpha2(I) collagen promoters by 6-10-fold whereas B-myb is inactive. The activity of c-myb on both type I collagen promoters requires a functional c-myb DNA binding domain suggesting a direct interaction between c-myb and these promoters. Indeed c-myb is active also on a 500 bp fragment of the alpha2(I) collagen promoter and can bind to this fragment in electrophoretic mobility shift assays. Finally, we show that anti-c-myb anti-sense treatment reduces alpha1(I) and to a lesser extent alpha2(I) collagen gene expression. These data strongly suggest that c-myb, but not B-myb, plays a direct role in the upregulation of type I collagen gene expression in response to transforming growth factor beta.

Antisense modulation of the ICAM-1 phenotype of a model human bone marrow stromal cell line

Efficient stable gene transfer was achieved in a model human bone marrow stromal cell line, KM-102, using both Epstein-Barr virus and BK virus episomal expression vectors. Using this episomal expression system, effective overexpression and inhibition of ICAM-1 expression was achieved in stably transfected KM-102 cells by sense and antisense RNA gene transfer, respectively. Loss of surface ICAM-1 on antisense KM-102 transfectants did not significantly affect adhesion to LFA-1-bearing JY hematopoietic cells. However, KM-102 ICAM-1 overexpressors demonstrated enhanced binding (2.5-fold) to phorbol ester-treated, but not untreated, LFA-1-bearing JY cells. The increased binding could be blocked with anti-ICAM-1 antibodies. These findings suggest that while ICAM-1 is not required for basal adhesion between stromal and hematopoietic cells, stromal ICAM-1 may contribute to stromal:leukemic cellular interaction when bound to the phorbol ester-dependent high-avidity state of hematopoietic LFA-1.

Expression of GM-CSF receptor and "in vitro" effects of GM-CSF on human fibroblasts

In the present study the effects of Granulocyte-Macrophage Colony Stimulating Factor (GM-CSF) on fibroblast growth and activity have been studied. In this regard the AA have evaluated in primary cultures of human gengival normal fibroblasts (PG1 cells): a)-the expression of GM-CSF receptor (GM-CSFR) (alfa unit) on the cell surface; b)-the in vitro effects of different doses of GM-CSF on the GM-CSFR expression and on the proliferation and activity of fibroblasts. PG1 cells have been stimulated in vitro with different concentrations of GM-CSF (10, 50, 80, 100 and 150 ng/ml) using promonocytic cell line U937 as positive control for GM-CSFR expression. GM-CSFR was investigated by flow cytometry, with mouse monoclonal antibody (mAb) against the alfa chain of the human GM-CSFR and fluorescein-conjugated goat antimouse immunoglobulin G (IgG). At high GM-CSF concentration (80 ng/ml) the AA observed: 1)-A marked increase of GM-CSFR expression evaluated as fluorescence intensity (about three fold in respect to the controls); 2)-Maximal increase of PG1 cells proliferation. Moreover immunofluorescence on fibroblasts obtained from culture plates showed increased actin stress fibers and fibronectin production with low stimulation by GM-CSF, while higher concentration of this cytokine determined increased proliferation of cells, but a decreased formation of actine fibers and vinculin plaques. These results demonstrate: 1)-The presence of GM-CSFR on the surface of fibroblasts; 2)-The proliferation and the synthesis activity of these cells (in vitro) are modulated by different concentration of GM-CSF. We hypothesize that GM-CSF until 80 ng/ml can upregulate the expression of the receptor. Therefore, on the basis of previous findings of high serum levels of GM-CSF in course of scleroderma, a disease characterized by fibroblast hyperactivity, a possible role of this cytokine in the pathogenic process of this disease can be hypothesized.

Myb-dependent regulation of thrombospondin 2 expression. Role of mRNA stability

The nuclear transcription factor c-Myb, which is highly expressed in hematopoietic cells, has been shown to be functional in NIH 3T3 cells: cells that do not possess detectable levels of c-Myb. To identify endogenous target genes of c-Myb in fibroblasts, RNA isolated from NIH 3T3 cells stably transfected with a full-length or a dominant negative c-myb construct (GREMyb and GREMEn, respectively) was subjected to differential display analysis. 5'-Rapid amplification of cDNA ends of a selected band, sequencing, and a nucleotide homology search led to the identification of thrombospondin 2 (TSP 2) as the gene product repressed in GREMyb and induced in GREMEn cells. The pattern of TSP 2 expression during the cell cycle was consistent with c-myb-dependent regulation. The possibility that the identified transcript was TSP 1, a homologous product known to be repressed by v-Src, c-Jun, and v-Myc, was ruled out by using a TSP 2-specific DNA probe and by showing a distinct pattern of regulation of TSP 1 and TSP 2 expression. Nuclear run-on and TSP 2 promoter-reporter (chloramphenicol acetyltransferase) assays showed similar transcriptional levels in GREMyb and NIH 3T3 cells. However, mRNA stability studies showed a much shorter TSP 2 mRNA half-life in GREMyb compared with wild type NIH 3T3 cells, suggesting that c-myb affects TSP 2 expression via a post-transcriptional mechanism. The implications of a protooncogene-mediated suppression of TSP expression are discussed.

c-Myb function in fibroblasts

The protooncogene c-myb is a nuclear transcription factor that shares significant sequence homology with two other myb family members, A-myb and B-myb. Recent studies have suggested that c-myb is involved in regulation of the cell cycle via control of intracellular calcium [Ca2+]i concentration. Given the limited cell type expression of the c-myb gene, we set out to investigate whether myb-dependent cell cycle regulation occurs in cells not known to express the c-myb protein. NIH 3T3 fibroblasts were stably transfected with an inducible c-myb dominant negative construct composed of a myb DNA binding domain linked to the Drosophila engrailed transcription suppresser (pGREMEn) and a full-length murine c-myb cDNA sequence. Induced expression of the dominant negative construct was associated with a G1 cell cycle arrest and a failure to increase late G1 intracellular calcium levels. Similar expression studies in mouse embryonic fibroblasts derived from the c-myb knockout mouse have demonstrated lower baseline [Ca2+]i levels than in normal mice fibroblasts that were not further lowered by MEn expression. We conclude that regulation of calcium homeostasis and cell cycle progression via myb-dependent transcription may play an important role in cells not possessing detectable levels of c-myb protein.

Potent and selective gene inhibition using antisense oligodeoxynucleotides

The development of antisense technology as a generally useful tool relies on the use of potent agents and the utilization of many controls in experiments. Here we describe our experience using oligodeoxynucleotides (ODNs) containing C-5 propynyl pyrimidine and phosphorothioate modifications as broadly applicable gene inhibition agents in cell culture. Methods include selection of antisense sequences, synthesis and purification of ODNs, choice of controls, delivery methods (microinjection, cationic lipid transfection, and electroporation), and analysis of gene inhibition.

c-myb proto-oncogene is expressed by quiescent scleroderma fibroblasts and, unlike B-myb gene, does not correlate with proliferation

Systemic sclerosis (scleroderma) is characterized by excessive deposition of extracellular matrix constituents. Although it has been proposed that tissue fibrosis is due to increased fibroblast synthesis of various collagen polypeptides, there is some experimental evidence that patients with systemic sclerosis have a defect in the control of fibroblast growth. The myb family of genes includes, among others, the c-myb proto-oncogene and the structurally related gene, B-myb, which are both implicated in the regulation of differentiation and/or proliferation of hematopoietic and nonhematopoietic cells. To elucidate the molecular basis responsible for scleroderma fibroblast proliferation, we therefore elected to investigate the expression of c-myb and B-myb genes in scleroderma and control cells. Using the reverse transcriptase polymerase chain reaction technique, we detected c-myb transcripts in scleroderma skin fibroblasts rendered quiescent by serum deprivation. Under the same experimental conditions, c-myb message was not found in normal skin fibroblasts, but, after serum stimulation, c-myb RNA was clearly evident from 3 to 72 h in both normal and pathologic cells. Treatment of these cells with c-myb antisense oligonucleotides caused downregulation of c-myb expression, and the inhibition of scleroderma fibroblast proliferation was 42%, whereas in normal fibroblasts the inhibition was weaker (22%). In contrast to c-myb, in normal and scleroderma fibroblasts the level of expression of B-myb correlated with cell proliferation assessed by cell count, and densitometric analysis showed that B-myb message was 1.5-5 times higher in most of pathologic cells studied. The antisense B-myb oligonucleotides had a weaker antiproliferative effect compared with antisense c-myb, inhibiting scleroderma and normal fibroblasts by 23% and 13%, respectively. These data suggest that the B-myb and c-myb genes may play a role in scleroderma fibroblast proliferation and function.

Regulation of hematopoietic cell proliferation and differentiation by the myb oncogene family of transcription factors

The myb family of genes include the virally encoded v-myb oncogene, its normal cellular equivalent c-myb and two related members called A-myb and B-myb. They are all transcription factors that recognize the same DNA sequence (PyAACG/TG) and are all involved in the regulation of proliferation and differentiation in different cell types, including hematopoietic cells. C-myb is most highly expressed in hematopoietic cells and its oncogenic activation leads to transformation of these cells. Several lines of evidence have demonstrated that c-myb regulates both the proliferation and differentiation of hematopoietic cells of different lineages. The mechanisms of action of c-myb and v-myb are becoming clearer, mostly through the study of the different genes that are regulated by these transcription factors and the cofactors with which c-myb and v-myb co-operate. More recently the biological and biochemical functions of the B-myb and A-myb gene products have been investigated. Evidence for the function of the different members of the myb family in relation to hematopoietic proliferation and differentiation is presented, and the different roles of the myb genes are discussed.

Myb: an old oncoprotein with new roles

Over the last decade, the c-myb gene and its protein product, Myb, have undergone extensive examination and manipulation in hemopoietic tissues. Although it is rarely disputed that, as a transcription factor, Myb regulates cell cycling, proliferation and differentiation, identification of genes directly controlled by Myb has been surprisingly difficult. More recently, genes with promoter regions that contain Myb recognition sequences have been identified, but a direct proliferative response to Myb via these 'target genes' has yet to be demonstrated. Mutagenesis studies have defined domains of the protein which influence its transcriptional activity and transforming potential; however how the molecule interacts with itself and with other cellular factors is only beginning to be understood. A broader examination of c-myb expression in normal and malignant tissues suggests an analogous role for Myb in proliferation, differentiation and transformation of non-hemopoietic tissues.

Antisense inhibition of butyrylcholinesterase gene expression predicts adverse hematopoietic consequences to cholinesterase inhibitors

1. To investigate the possibility that cholinesterase inhibitors may cause adverse hematopoietic effects, we employed antisense oligodeoxynucleotides selectively inhibiting butyrylcholinesterase gene expression (AS-BCHE). Complementary sense (S) oligonucleotides served as controls. 2. In primary bone marrow cell cultures grown with interleukin 3 (IL-3), AS-BCHE but not S-BCHE reduced growth of megakaryocyte colony-forming units (CFU-MK) in a dose-dependent manner at the micromolar range. 3. In cultures grown with IL-3, transferrin, and erythropoietin (Epo), cell counts increased up to twofold, yet colony counts (CFU-GEMM) remained unchanged under AS-BCHE treatment. 4. Electrophoretic measurements of DNA ladder as an apoptotic index revealed that the above oligonucleotide effects were not due to nonspecific induction of programmed cell death. 5. Differential cell counts demonstrated increased myeloidogenesis and reduced levels of early megakaryocytes in CFU-GEMM under AS-BCHE, suggesting requirement of the BuChE protein for megakaryopoiesis. 6. In vivo injection of AS-BCHE reduced BCHE mRNA levels in both young and mature megakaryocytes for as long as 20 days, as shown by in situ hybridization. 7. Ex vivo growth of primary bone marrow cells revealed a twofold reduction in CFU-MK colonies grown from the AS-BCHE- but not the S-BCHE-injected mice, 15 days posttreatment. 8. These findings demonstrate that deficient butyrylcholinesterase expression, and hence interference with this enzyme's activity through treatment with or exposure to cholinesterase inhibitors, may cause hematopoietic differences in treated patients.

Regulation of the expression of the hematopoietic stem cell antigen CD34: role of c-myb

The CD34 antigen defines a subset of hematopoietic progenitor cells with self-renewal capacity and the ability to reconstitute hematopoiesis in irradiated primates and marrow-ablated humans, but its function remains unknown. The c-myb protooncogene plays a fundamental role in hematopoiesis, most likely via its transcriptional regulator function. We report that c-myb protein transactivates the CD34 promoter via specific interaction with multiple Myb binding sites in the 5' flanking region of the gene and induces expression of the endogenous CD34 mRNA in rodent fibroblasts. Also, constitutive expression of c-myb in CD34-negative human glioblastoma cells induces expression of CD34 mRNA and synthesis of the surface membrane antigen. These data directly demonstrate that c-myb regulates the expression of the hematopoietic stem cell antigen CD34 and raise the possibility that c-myb regulates hematopoiesis inducing a cascade of differentiation-related events.