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Here to help
Long-range Induction
The tumour suppressor protein ~53 is known to act at least in part by transcriptional activation of target genes that, in turn, act to control a range of cellular processes, such as abrogation of cell-cycle progression, and induction of DNA repair and apoptosis. Cells that have undergone genotoxic stress have elevated levels of ~53 and consequently undergo one (or more) of the fates listed above. Buckbinder et al. have used a subtractive cloning approach to identify cDNAs enriched following induction
of the
effects
of ~53
BUCKBINDER, L. et al. (1995) growth inhibitor ICF-binding Nature 377, 646-649
protein
by ~53. In this screen, they identified a cDNA corresponding to the insulinlike growth factor binding protein 3 (ICF-BP3), which has been implicated as an inhibitory regulator of cell growth. They found that introns in the gene encoding ICF-BP3 contain consensus p53-binding sequences to which ~53 binds in vitro. More importantly, the authors demonstrate that these DNA sequences act as genuine p53 response elements in vivo, as introduction of these elements into a plasmid containing a heterologous promoter and the luciferase reporter
3 by ~53
gene confers p53-dependent inducibility on expression of luciferase. Significantly, expression of the gene for ICF-BP3 was found to be upregulated in a p53-dependent manner in mammalian cells treated with genotoxic agents. As IGF-BP3 has a high affinity for IGFs and thus interferes with growth factor signalling, these results suggest that the ability of ~53 to inhibit cell growth may not be limited simply to the cell itself suffering genotoxic stress, but may extend via a paracrine mechanism to neighbouring cells.
= ff-
‘Here to help’ Sequence
and
OQATE, S. A., TSAI, S. Y., TSAI, M-J. and O’MALLEY, B. W. (1995) characterization of a coactivator for the steroid hormone receptor Science 270, 1354-l 357
Nuclear receptorsthose for steroids, thyroid hormones and retinoids - are ligand-activated transcription factors. For several years it has been known that nuclear receptors compete with one another for one or more common factors involved in mediating transcriptional activation, the evidence for this being that activation of one type of receptor can inhibit induction of gene expression by another. This phenomenon is referred to as squelching, and equates to the titration of common components involved in the responses, as first shown for the progesterone and oestrogen receptors. The shared components are termed coactivators, and in this paper OAate et al. have used the yeast two-hybrid system to identify such coactivators for nuclear receptors. The C-terminal region of the progesterone receptor (P-R), which includes the ligand-binding domain, was used as bait, and a cDNA clone
54
selected on the basis of its ability to enhance transactivation by the P-R in the presence of progesterone. This clone contains a partial cDNA for a protein that the authors have called steroid receptor coactivator-1 (SRC-I), which shares no significant homology with any other protein in the data bases, and which is, therefore, distinct from any previously identified receptor-associated proteins. In HeLa cell transfection assays, SRC-1 enhanced the transactivation function of the five members of the nuclear receptor family tested, in all cases in a ligand-dependent manner. The authors also show that a P-Rantagonist blocked the coactivation function of SRC-1 at this receptor, consistent with in vitro results suggesting that SRC-1 is unable to bind to the antagonized receptor. Importantly, in the HeLa cell reporter assay, SRC-1 was not simply required for ligand-induced transcriptional trends
superfamily
activation by the above receptors, but, rather, served to enhance their transactivation potential. Another characteristic indicative of a coactivator function was the finding that expression of SRC-1 reversed the squelching of P-R transactivation caused by the oestrogen receptor. The cloning of the gene encoding SRC-1 has provided the clearest demonstration so far of a factor that enhances receptor-mediated transcriptional activation. It will now be important to define the regions of both SRC-1 and the receptors that take part in these interactions, and also to explore fully the range of transcriptional activators for which SRC-1 is a coactivator. The basis of the enhancing effect of SRC-1 is also of obvious interest, as is any possible relationship between this protein and other proposed steroid hormone receptor coactivators, which have yet to be cloned. in CELL BIOLOGY
(Vol.
6) February
1996
The tumour suppressor protein ~53 is known to act at least in part by transcriptional activation of target genes that, in turn, act to control a range of cellular processes, such as abrogation of cell-cycle progression, and induction of DNA repair and apoptosis. Cells that have undergone genotoxic stress have elevated levels of ~53 and consequently undergo one (or more) of the fates listed above. Buckbinder et al. have used a subtractive cloning approach to identify cDNAs enriched following induction
of the
effects
of ~53
BUCKBINDER, L. et al. (1995) growth inhibitor ICF-binding Nature 377, 646-649
protein
by ~53. In this screen, they identified a cDNA corresponding to the insulinlike growth factor binding protein 3 (ICF-BP3), which has been implicated as an inhibitory regulator of cell growth. They found that introns in the gene encoding ICF-BP3 contain consensus p53-binding sequences to which ~53 binds in vitro. More importantly, the authors demonstrate that these DNA sequences act as genuine p53 response elements in vivo, as introduction of these elements into a plasmid containing a heterologous promoter and the luciferase reporter
3 by ~53
gene confers p53-dependent inducibility on expression of luciferase. Significantly, expression of the gene for ICF-BP3 was found to be upregulated in a p53-dependent manner in mammalian cells treated with genotoxic agents. As IGF-BP3 has a high affinity for IGFs and thus interferes with growth factor signalling, these results suggest that the ability of ~53 to inhibit cell growth may not be limited simply to the cell itself suffering genotoxic stress, but may extend via a paracrine mechanism to neighbouring cells.
= ff-
‘Here to help’ Sequence
and
OQATE, S. A., TSAI, S. Y., TSAI, M-J. and O’MALLEY, B. W. (1995) characterization of a coactivator for the steroid hormone receptor Science 270, 1354-l 357
Nuclear receptorsthose for steroids, thyroid hormones and retinoids - are ligand-activated transcription factors. For several years it has been known that nuclear receptors compete with one another for one or more common factors involved in mediating transcriptional activation, the evidence for this being that activation of one type of receptor can inhibit induction of gene expression by another. This phenomenon is referred to as squelching, and equates to the titration of common components involved in the responses, as first shown for the progesterone and oestrogen receptors. The shared components are termed coactivators, and in this paper OAate et al. have used the yeast two-hybrid system to identify such coactivators for nuclear receptors. The C-terminal region of the progesterone receptor (P-R), which includes the ligand-binding domain, was used as bait, and a cDNA clone
54
selected on the basis of its ability to enhance transactivation by the P-R in the presence of progesterone. This clone contains a partial cDNA for a protein that the authors have called steroid receptor coactivator-1 (SRC-I), which shares no significant homology with any other protein in the data bases, and which is, therefore, distinct from any previously identified receptor-associated proteins. In HeLa cell transfection assays, SRC-1 enhanced the transactivation function of the five members of the nuclear receptor family tested, in all cases in a ligand-dependent manner. The authors also show that a P-Rantagonist blocked the coactivation function of SRC-1 at this receptor, consistent with in vitro results suggesting that SRC-1 is unable to bind to the antagonized receptor. Importantly, in the HeLa cell reporter assay, SRC-1 was not simply required for ligand-induced transcriptional trends
superfamily
activation by the above receptors, but, rather, served to enhance their transactivation potential. Another characteristic indicative of a coactivator function was the finding that expression of SRC-1 reversed the squelching of P-R transactivation caused by the oestrogen receptor. The cloning of the gene encoding SRC-1 has provided the clearest demonstration so far of a factor that enhances receptor-mediated transcriptional activation. It will now be important to define the regions of both SRC-1 and the receptors that take part in these interactions, and also to explore fully the range of transcriptional activators for which SRC-1 is a coactivator. The basis of the enhancing effect of SRC-1 is also of obvious interest, as is any possible relationship between this protein and other proposed steroid hormone receptor coactivators, which have yet to be cloned. in CELL BIOLOGY
(Vol.
6) February
1996