Differential synthesis of beta-major and beta-minor globin proteins in murine erythroleukemia cells is regulated at the transcriptional level

Activation of beta-major globin gene transcription is associated with recruitment of NF-E2 to the beta-globin LCR and gene promoter

The mouse beta-globin gene locus control region (LCR), located upstream of the beta-globin gene cluster, is essential for the activated transcription of genes in the cluster. The LCR contains multiple binding sites for transactivators, including Maf-recognition elements (MAREs). However, little is known about the specific proteins that bind to these sites or the time at which they bind during erythroid differentiation. We have performed chromatin immunoprecipitation experiments to determine the recruitment of the erythroid-specific transactivator p45 NF-E2/MafK (p18 NF-E2) heterodimer and small Maf proteins to various regions in the globin gene locus before and after the induction of murine erythroleukemia (MEL) cell differentiation. We report that, before induction, the LCR is occupied by small Maf proteins, and, on erythroid maturation, the NF-E2 complex is recruited to the LCR and the active globin promoters, even though the promoters do not contain MAREs. This differentiation-coupled recruitment of NF-E2 complex correlates with a greater than 100-fold increase in beta-major globin transcription, but is not associated with a significant change in locus-wide histone H3 acetylation. These findings suggest that the beta-globin gene locus exists in a constitutively open chromatin conformation before terminal differentiation, and we speculate that recruitment of NF-E2 complex to the LCR and active promoters may be a rate-limiting step in the activation of beta-globin gene expression.

Effects of hemin on erythropoiesis

It is clear that in vitro hemin increases the number of blood BFU-E derived colonies from normal donors. This occurs with sickle donors as well, despite the increased levels of hemin in vivo in these patients. The effect of hemin on relative gamma globin synthesis is inconsistent, however. In a few cases, delayed addition of hemin led to increased gamma globin synthesis. In time course studies of cultures from normal donors, hemin added on day 0 shifted the day of peak colony number from 13-14 to 16-20 days. The temporal decline in gamma globin synthesis was not altered. In cultures from sickle donors we found that the time for maximal colony number was later than in normals, occurring at 16-20 days even without hemin, and was not further delayed by hemin. The relative proportion of gamma globin synthesis was higher on day 14 in the sickle than the normal cultures, and the temporal decline was somewhat slowed in the sickle cultures by hemin. The elevated gamma synthesis and the later time for peak colony growth in the sickle cultures suggest that the erythroid progenitors in the blood of the sickle patients are less mature than those from normal individuals. There are several possible explanations for the detection of increased numbers of colonies in cultures containing hemin. Hemin may delay the final maturation of erythroblasts within erythroid colonies, thus shifting the time of maximal growth. It may also increase the extent of final maturation, leading to more complete hemoglobinization of the erythroblasts within the colonies, and thus increasing the number of colonies that are eventually recognized as erythroid.(ABSTRACT TRUNCATED AT 250 WORDS)