Coordinate Induction of the Three Neurofilament Genes by the Brn-3a Transcription Factor

Abstract

The POU domain transcription factor Brn-3a is able to stimulate neurite outgrowth when overexpressed in the neuronal ND7 cell line, whereas the closely related Brn-3b factor does not have this effect. We show that Brn-3a overexpression also enhances the expression of the three neurofilament genes at both the mRNA and protein levels, whereas Brn-3b overexpression has no effect. In addition Brn-3a activates the three neurofilament gene promoters in co-transfection assays in both neuronal and non-neuronal cells. As observed for enhanced neurite outgrowth, the stimulation of neurofilament gene expression and activation of the neurofilament gene promoters is observed with the isolated POU domain of Brn-3a. A single amino acid change in the POU homeodomain of Brn-3a to the equivalent amino acid in Brn-3b abolishes its ability to activate the neurofilament promoters, whereas the reciprocal change converts Brn-3b to an activator of these promoters.The POU (Pit-Oct-Unc) family of transcription factors (for reviews see Refs. 1 and 2) was originally defined on the basis of a common 150 -160-amino acid DNA binding domain held in common between the mammalian transcription factors Pit-1, Oct-1, and Oct-2 and the regulatory protein Unc-86 which plays a key role in the development of neuronal cell types, particularly sensory neurons in the nematode (3, 4). A number of subsequent studies (see for example Ref. 5) have defined a large number of other POU family transcription factors in a variety of organisms that have been classified into subgroups according to their relationships to one another (for review see Ref.2). Among the mammalian POU factors identified in this way, the Brn-3 factors show the closest homology to the nematode protein Unc-86 and together with the Drosophila factors I-POU and tI-POU (6, 7) these factors constitute the POU IV subfamily within the POU family (2). Indeed it has been proposed (8) that the Brn-3 factors constitute the mammalian homologues of .Unlike the situation in the nematode, however, three distinct Brn-3 factors have been identified in mammals that show close homology in the DNA binding POU domain but are considerably less homologous outside it and are encoded by three different genes (9, 10). These factors are Brn-3a (also known as Brn-3 or Brn-3.0) (5, 11, 12), Brn-3b (also known as Brn-3.2) (12, 13), and Brn-3c (also known as Brn-3.1) (11,14). Each of these three factors is expressed in distinct but overlapping sets of neurons in the developing and adult nervous system (5, 11, 13-15) with Brn-3a for example defining the earliest postmitotic neurons to appear in the developing central nervous system (15). This suggests that, like Unc-86, the mammalian Brn-3 factors may play a critical role in the development of specific neuronal cells within the nervous system with each factor having distinct but possibly related roles. In agreement with this, recent experiments (8, 16) have shown that inactivation of the Brn-3b gene results in considerable loss of retinal neurons, whereas mice in which the...

Activation of CDK4 Gene Expression in Human Breast Cancer Cells by the Brn-3b POU Family Transcription Factor

“…In contrast, the only other known target for activation by Brn-3b, the nicotinic acetylcholine receptor α2 subunit gene is not activated by the isolated POU domain of Brn-3b. 26 This is similar to observations with the related Brn-3a factor, where some genes are activated by the isolated POU domain alone [23][24][25] whereas others require additional N-terminal sequences. 35,36 Whatever the case, the data presented here identify CDK4 as a target for transcriptional activation by Brn-3b.…”

“…Thus, although Brn-3b can activate the promoter of the nicotinic acetylcholine receptor α2 subunit gene. 26 Brn-3b represses the expression of a variety of genes involved in neuronal differentiation including SNAP-25 24 and the neurofilaments 25 and thereby represses neuronal differentiation itself. 27 Our initial studies in breast cancer cells, identified the BRCA-1 gene as a target for transcriptional repression by Brn-3b.…”

“…21,22 A construct containing the isolated POU domain of Brn-3b was also able to activate the CDK4 promoter, indicating that, as in the related Brn-3a factor, [23][24][25] the POU domain can act as a transcriptional activation domain, as well as a DNA binding domain.…”

Brn-3a Transcription Factor Blocks p53-mediated Activation of Proapoptotic Target Genes Noxa and Bax in Vitro and in Vivo to Determine Cell Fate

“…Thus, the overexpression of Brn-3a in the neuronal cell line ND7 increases survival on the removal of neurotrophic factors compared with controls (9). This protective effect of high Brn-3a levels is also observed in cultured DRG and TG neurons (10), whereas a reduction of Brn-3a expression using an antisense approach results in apoptosis of these neurons even in the presence of neurotrophic factors (11).…”

The Brn-3b POU family transcription factor represses expression of the BRCA-1 anti-oncogene in breast cancer cells

“…Thus, Brn-3b can activate the promoters of the neuronal nicotinic acetyl choline receptor a2 gene (Milton et al, 1995) and the synapsin 1 gene whereas it represses the activity of a number of other neuronal promoters such as those derived from the genes encoding SNAP-25 (Morris et al, 1997) ainternexin (Budhram-Mahadeo et al, 1995) and the neuro®laments (Smith et al, 1997) and also inhibits Figure 8 Luciferase reporter assay of MCF7 cells co-transfected with a promoter/reporter construct containing the full length BRCA-1 promoter together with expression vector lacking any insert (V) or the same vector expressing either Brn-3a, Brn-3b or p53. Values have been equalized relative to the activity obtained in the co-transfection with the reporter construct and empty expression vector (set at 100%) and are the mean of ®ve determinations whose standard deviation is shown by the bars their activation by the closely related Brn-3a factor Budhram-Mahadeo et al, 1995;Smith et al, 1997). In contrast, Brn-3b has no e ect on the promoter of the Bcl-2 proto-oncogene, although this promoter is also activated by Brn-3a (Smith et al, 1998).…”