MB-02 cells undergo fetal to adult globin gene switching in response to erythropoietin

Bb1-3, a transgenic hybrid cell line with erythroid and megakaryocytic differentiation potential that expresses high levels of human gamma-globin and human beta-globin

We have characterized a murine hybrid cell line, Bb1-3, generated by the fusion of mouse primary erythroblasts with MEL cells. It proliferated in serum-free medium and displayed a low level of spontaneous erythroid and megakaryocyte differentiation. Terminal erythroid differentiation could be induced with HMBA and DMSO and was enhanced by serum. Treatment with phorbol esters resulted in a high proportion of megakaryocytes and the expression of megakaryocytic specific lineage markers. Bb1-3 cells contain a human beta-globin transgene that was expressed at levels of 20-50% of the endogenous mouse globin genes. Initially, expression was largely limited to the beta-globin gene but after adaptation to serum free growth, equal expression of both the human gamma- and human beta-globin genes was observed. This cell line provides further evidence that the differentiation potential of mouse erythroleukaemia cells is not restricted to the erythroid lineage and should be useful to study the mechanisms underlying both developmental globin gene regulation and the terminal differentiation of bipotential erythroid/megakaryocytic progenitor cells.

AP-1/C-jun and C-myc Regulation During Megakaryocytic Differentiation of a Human Bi-potential Growth-factor-dependent Cell Line

Terminal megakaryocytic development is characterized by nuclear poly ploidization, appearance of specific granules, and enhanced expression of membrane platelet glycoproteins. We utilized a human GM-CSF-dependent cell line, MB-02, which is capable of under going megakaryocytic differentiation, to examine the molecular events underlying this process. The responses of MB-02 to the protein kinase C (PKC) agonist, 12-O-tetradecanoyl-phorbol-13-acetate (TPA) were examined. GM-CSF dependent proliferation of MB-02, as measured by (3)H-thymidine uptake, was greater than 95% inhibited by TPA (16 nM), but was not affected by the inactive stereoisomer, 4-α-phorbol-12,13-didecanoate (4-αPDD). Transient exposure of cells to GM-CSF after growth factor deprivation led to rapid, high-level expression of normal-sized c-myc mRN A transcripts above baseline. C-myc expression was turned off by TPA (16 nM) stimulation of cells within 2-4 h. This TPA-mediated effect likely occurred at the transcriptional level since the half life of c-myc mRN A induced by GM-CSF was less than 30 min. Treatment of cells with TPA was associated with induction of c-jun and junB mRN A within 1-4 h. The protein products of these transcription factors are known to be part of the transcription factor complex Activator protein 1 (AP-1). Indeed, our data prove a rapid induction of AP-1 protein after TPA stimulation, as shown by mobility shift assays. In addition, TPA treatment resulted in expression of platelet surface glycoprotein IIb/IIIa complex (gpIIb/IIIa). These studies suggest a link between PKC stimulation by TPA and AP-1 activation with downregulation of c-myc transcription on a molecular level. At the cellular level, PKC activation was related to the acquisition of several features of the megakaryocyte development programme associated with the switch from cell proliferation to maturation.

Alternative splicing of the HOX 2.2 homeobox gene in human hematopoietic cells and murine embryonic and adult tissues

The HOX 2.2 homeobox gene is expressed in human hematopoietic cell lines with erythroid features (W.-F. Shen, et al, Proc. Natl. Acad. Sci. 86, 8536-8540, 1989). Both human and murine Hox 2.2 genes contain a single 1 kb intron which interrupts the sequence encoding the proposed homeobox protein. Four human erythroleukemia cell lines express the spliced, homeobox-coding transcript as the major form of message, and variable low amounts of unspliced HOX 2.2 mRNAs. Murine embryonic tissues and adult kidney and uterus contain approximately equal amounts of transcripts containing this intron and mRNAs from which the intron has been excised. The spliced transcript encodes a 224 amino acid homeobox protein, while the unspliced transcript would potentially encode a 140 residue protein containing the same N-terminal sequence but lacking the homeodomain.