p53 suppresses the activation of the Bcl-2 promoter by the Brn-3a POU family transcription factor.
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The Brn-3a POU family transcription factor has been shown to strongly activate expression of the Bcl-2 proto-oncogene and thereby protect neuronal cells from programmed cell death (apoptosis). This activation of the Bcl-2 promoter by Brn-3a is strongly inhibited by the p53 anti-oncogene protein. This inhibitory effect of p53 on Brn-3a-mediated transactivation is observed with nonoverlapping gene fragments containing either the Bcl-2 p1 or p2 promoters but is not observed with other Brn-3a-activated promoters such as in the gene encoding alpha-internexin or with an isolated Brn-3a binding site from the Bcl-2 promoter linked to a heterologous promoter. In contrast, p53 mutants, which are incapable of binding to DNA, do not affect Brn-3a-mediated activation of the Bcl-2 p1 and p2 promoters. Moreover, Brn-3a and p53 have been shown to bind to adjacent sites in the p2 promoter and to directly interact with one another, both in vitro and in vivo, with this interaction being mediated by the POU domain of Brn-3a and the DNA binding domain of p53. The significance of these effects is discussed in terms of the antagonistic effects of Bcl-2 and p53 on the rate of apoptosis and the overexpression of Brn-3a in specific tumor cell types. (+info)
POU domain factor Brn-3b is essential for retinal ganglion cell differentiation and survival but not for initial cell fate specification.
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While the mammalian retina is well understood at the anatomical and physiological levels, little is known about the mechanisms that give rise to the retina's highly ordered pattern or its diverse neuronal cell types. Previous investigations have shown that gene disruption of the POU-IV class transcription factor Brn-3b (Brn-3.2) resulted in the loss of most retinal ganglion cells in retinas of postnatal mice. Here, we used lacZ and human placental alkaline phosphatase genes knocked into the brn-3b locus to follow the fate of brn-3b-mutant cells in the developing retina. We found that Brn-3b was not required for the initial commitment of retinal ganglion cell fate or for the migration of ganglion cells to the ganglion cell layer. However, Brn-3b was essential for the normal differentiation of retinal ganglion cells; without it, the cells underwent enhanced apoptosis. Retinal ganglion cells lacking brn-3b extended processes at the appropriate time in development, but these processes were disorganized, resulting in a thinner optic nerve. Explanted retinas from brn-3b-null embryos also extended processes when cultured in vitro, but the processes were shorter and less bundled than in wild-type retinas. Ultrastructural and marker analyses showed that the processes of mutant ganglion cells had dendritic rather than axonal features, suggesting that mutant cells formed dendrites in place of axons. These results suggest that Brn-3b regulates the activity of genes whose products play essential roles in the formation of retinal ganglion cell axons. (+info)
POU domain factor Brn-3a controls the differentiation and survival of trigeminal neurons by regulating Trk receptor expression.
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Mice lacking the POU domain-containing transcription factor Brn-3a have several neuronal deficits. In the present paper, we show that Brn-3a plays two distinct roles during development of the trigeminal ganglion. In this ganglion, neurons expressing the neurotrophin receptors, TrkB and TrkC, are born between E9.5 and E11.5. In the absence of Brn-3a, very few neurons ever express TrkC, but TrkB-expressing neurons are present at E12.5 in elevated numbers, suggesting that Brn-3a may be a constituent of a regulatory circuit determining which Trk receptor is expressed by these early-born neurons. Most neurons expressing the neurotrophin receptor TrkA are generated between E11.5 and E13.5 in this ganglion and their initial generation is not prevented by absence of Brn-3a. However, after E12. 5, absence of Brn-3a results in a progressive loss in neuronal TrkA and TrkB expression, which leads to a massive wave of apoptosis that peaks at E15.5. Despite complete absence of the Trk receptors at E17. 5 and P0, approximately 30% of the normal complement of neurons survive to birth in Brn-3a mutants. Approximately 70% of these express the GDNF receptor subunit, c-ret; many can be sustained by GDNF, but not by NGF in culture. Thus, the vast majority of surviving neurons are probably sustained in vivo by trophic factor(s) whose receptors are not regulated by Brn-3a. In conclusion, our data indicate the specific functions of Brn-3a in controlling the survival and differentiation of trigeminal neurons by regulating expression of each of the three Trk receptors. (+info)
Autoregulatory sequences are revealed by complex stability screening of the mouse brn-3.0 locus.
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The POU-IV or Brn-3 class of transcription factors exhibit conserved structure, DNA-binding properties, and expression in specific subclasses of neurons across widely diverged species. In the mouse CNS, Brn-3.0 expression characterizes specific neurons from neurogenesis through the life of the cell. This irreversible activation of expression suggests positive autoregulation. To search for cis-acting elements that could mediate autoregulation we used a novel method, complex stability screening, which we applied to rapidly identify functional Brn-3.0 recognition sites within a large genomic region encompassing the mouse brn-3.0 locus. This method is based on the observation that the kinetic stability of Brn-3.0 complexes with specific DNA sequences, as measured by their dissociation half-lives, is highly correlated with the ability of those sequences to mediate transcriptional activation by Brn-3.0. The principal Brn-3.0 autoregulatory region lies approximately 5 kb upstream from the Brn-3.0 transcription start site and contains multiple Brn-3.0-binding sites that strongly resemble the optimal binding site for this protein class. This region also mediates transactivation by the closely related protein Brn-3.2, suggesting a regulatory cascade of POU proteins in specific neurons in which Brn-3.2 expression precedes Brn-3.0. (+info)
The Brn-3a transcription factor plays a critical role in regulating human papilloma virus gene expression and determining the growth characteristics of cervical cancer cells.
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The Brn-3a POU family transcription factor has previously been shown to activate the human papilloma virus type 16 (HPV-16) promoter driving the expression of the E6- and E7-transforming proteins. Moreover, Brn-3a is overexpressed approximately 300-fold in cervical biopsies from women with cervical intra-epithelial neoplasia type 3 (CIN3) compared with normal cervical material. To test the role of Brn-3a in cervical neoplasia we have manipulated its expression in cervical carcinoma-derived cell lines with or without endogenous HPV genes. In HPV-expressing cells, reduction in Brn-3a expression specifically reduces HPV gene expression, growth rate, saturation density and anchorage-independent growth, whereas these effects are not observed when Brn-3a expression is reduced in cervical cells lacking HPV genomes. Together with our previous observations, these findings indicate a critical role for Brn-3a in regulating HPV gene expression and thereby in controlling the growth/transformation of cervical cells. (+info)
The Brn-3b POU family transcription factor represses expression of the BRCA-1 anti-oncogene in breast cancer cells.
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The BRCA-1 tumour supressor gene was identified on the basis of mutations which occur in familial breast cancer indicating that its inactivation can cause this disease. Although BRCA-1 does not appear to be mutated in sporadic breast cancer, its expression has been shown to be reduced in tumour material from such cases. We show here that mammary tumours which have reduced levels of BRCA-1 expression show enhanced expression of the Brn-3b POU family transcription factor at both the mRNA and protein levels. This elevated expression of Brn-3b is not found in normal mammary cells, benign tumours or in malignant tumour samples which do not exhibit reduced levels of BRCA-1. In contrast, no correlation was noted between BRCA-1 and expression of the related factor Brn-3a. Moreover, Brn-3b but not Brn-3a can strongly repress the BRCA-1 promoter approximately 20-fold in mammary tumour cells. To our knowledge, this is the first report of a transcription factor which regulates BRCA-1 expression. Thus, Brn-3b may play an important role in regulating expression of BRCA-1 in mammary tumours with enhanced expression of Brn-3b resulting in reduced BRCA-1 expression and thereby being potentially important in tumour development. (+info)
All Brn3 genes can promote retinal ganglion cell differentiation in the chick.
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Targeted gene disruption studies in the mouse have demonstrated crucial roles for the Brn3 POU domain transcription factor genes, Brn3a, Brn3b, Brn3c (now called Pou4f1, Pou4f2, Pou4f3, respectively) in sensorineural development and survival. During mouse retinogenesis, the Brn3b gene is expressed in a large set of postmitotic ganglion cell precursors and is required for their early and terminal differentiation. In contrast, the Brn3a and Brn3c genes, which are expressed later in ganglion cells, appear to be dispensable for ganglion cell development. To understand the mechanism that causes the functional differences of Brn3 genes in retinal development, we employed a gain-of-function approach in the chick embryo. We find that Brn3b(l) and Brn3b(s), the two isoforms encoded by the Brn3b gene, as well as Brn3a and Brn3c all have similar DNA-binding and transactivating activities. We further find that the POU domain is minimally required for these activities. Consequently, we show that all these Brn3 proteins have a similar ability to promote development of ganglion cells when ectopically expressed in retinal progenitors. During chick retinogenesis, cBrn3c instead of cBrn3b exhibits a spatial and temporal expression pattern characteristic of ganglion cell genesis and its misexpression can also increase ganglion cell production. Based on these data, we propose that all Brn3 factors are capable of promoting retinal ganglion cell development, and that this potential may be limited by the order of expression in vivo. (+info)
The BRN-3A transcription factor protects sensory but not sympathetic neurons from programmed cell death/apoptosis.
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Inactivation of the gene encoding the POU domain transcription factor BRN-3A results in the absence of specific neurons in knockout mice. Here we demonstrate for the first time a direct effect of BRN-3A on the survival of neuronal cells. Specifically, overexpression of BRN-3A in cultured trigeminal ganglion or dorsal root ganglion sensory neurons enhanced their survival following the withdrawal of nerve growth factor. Moreover, reduction of BRN-3A levels impaired the survival of these neurons. The survival of sympathetic neurons was not affected by either approach. Similarly, overexpression of BRN-3A activated the endogenous Bcl-2 gene in trigeminal neurons, but not in sympathetic neurons. The protective effect of BRN-3A on trigeminal neuron survival following nerve growth factor withdrawal significantly increased during embryonic development. In contrast, overexpression of the related factor BRN-3B enhanced survival of trigeminal neurons only at an early stage of embryonic development. Thus, BRN-3A (and in some circumstances, BRN-3B) can promote the survival of nerve growth factor-dependent sensory but not sympathetic neurons, allowing it to play a direct role in the survival of some (but not all) neuronal populations in the developing and adult nervous systems. (+info)