Regulatory capacity of an androgen-specific enhancer of the mouse Slp gene in transgenic mice

In vitro and in vivo effects of a multimerized alphas 1-casein enhancer on whey acidic protein gene promoter activity

Experimental data obtained in previous works have led to postulate that enhancers increase the frequency of action of a linked promoter in a given cell and may have some insulating effects. The multimerized rabbit alpha s1-casein gene enhancer, the 6i multimer, was added upstream of the rabbit whey acidic protein gene (WAP) promoter (-6,300; +28 bp) fused to the firefly luciferase (luc) gene (6i WAP-luc construct). The 6i multimer increased reporter gene expression in mouse mammary HC11 cells. In transgenic mice, a very weak but significant increase was also observed. More noticeable, no silent lines were found when the 6i multimer was associated to the WAP-luc construct. This reflects the fact that the 6i multimer tends to prevent the silencing of the WAP-luc construct. After addition of the 5'HS4 insulator region from the chicken beta-globin locus upstream of the 6i multimer, similar luciferase levels were measured in 6i WAP-luc and 5'HS4 WAP-luc transgenic mice. Our present data and previous ones, which show that the 6i multimer has no insulating activity on a TK gene promoter construct indicate that the insulating activity of the 6i multimer is construct-dependent and not amplified by the 5'HS4 insulator.

The regulator of sex-limitation gene, rsl, enforces male-specific liver gene expression by negative regulation

Expression of a broad array of proteins is sexually dimorphic in rodent liver, dependent on sex-specific patterns of GH secretion. Mice carrying rsl (regulator of sex limitation) alleles, discovered as trans-acting loci affecting the mouse sex-limited protein (Slp) gene, reveal an additional axis in male-specific gene regulation. Slp expresses in adult males, but in rsl homozygous mice, Slp is also expressed in females. In this study, we examined congenic rsl strains to determine rsl's site of action, breadth of targets, and interaction with hormonal induction. We show that rsl affects Slp in liver, but not kidney, and that Rsl acts on a spectrum of male-specific liver genes, including mouse urinary proteins and a cytochrome P450 expressed predominantly by males, Cyp 2d-9, but does not act on the female-prominent P450, Cyp 2a-4. Slp expression in hypophysectomized or Tfm/Y rsl mice reveals that Rsl action is independent of GH or androgen signaling. Further, parabiosis of Rsl and rsl mice does not alter expression patterns, consistent with rsl action being liver intrinsic. Finally, Slp expression initiates earlier in rsl mice, suggesting that Rsl operates before, as well as independently of, hormonal induction. This characterization suggests Rsl functions to repress transcription of a set of genes that have in common their hormonal induction in male liver, and thus accentuates sexual dimorphism of liver gene expression.

Effect of the rabbit alphas1-casein gene distal enhancer on the expression of a reporter gene in vitro and in vivo

Several gene constructs containing the firefly luciferase gene and the herpes simplex virus thymidine kinase gene promoter (TK) were used to evaluate the transcriptional activity of the distal enhancer (-3442, -3285) of the rabbit alphas1-casein gene. Six copies of the enhancer (6i) were added upstream of the TK-luciferase construct in the presence or absence of the chicken beta-globin 5'HS4 insulator. The activity of the constructs was tested by transient transfection in CHO cells and in rabbit primary mammary cell cultured on plastic or on floating collagen. Constructs were also tested in stably transfected mouse mammary HC11 cells. In all cell types the multimerized alphas1-casein enhancer strongly stimulated luciferase gene expression in the presence of lactogenic hormones. It was also sensitive to the extracellular matrix in rabbit primary mammary cells. The constructs were used to generate transgenic mice. The 6i TK transgenic animals expressed the luciferase gene at very low levels irrespectively of the physiological state. No preferential expression in the mammary gland was observed. Addition of 5'HS4 insulator to the 6i TK construct did not prevent silencing in most of the transgenic lines. However, two lines expressed high luciferase levels specifically in the mammary gland. Our data suggest that 6i may confer, when insulated properly, a higher and mammary-specific expression to the TK promoter.

AML3/CBFalpha1 is required for androgen-specific activation of the enhancer of the mouse sex-limited protein (Slp) gene

A complex 120-base pair enhancer, derived from the mouse sex-limited protein (Slp) gene, is activated solely by the androgen receptor (AR) in specific tissues, although it contains a hormone response element recognized by several steroid receptors. The generation of this transcriptional specificity has been ascribed to the interactions of the receptor with tissue-specific nonreceptor factors bound to accessory sites within the enhancer. Protein-DNA interaction assays revealed two factors binding the 5' part of the enhancer that differ widely in abundance between cells showing AR-specific activation of the Slp element compared with those that also permit activation by glucocorticoid receptor (GR). The factor designated B formed a complex centered on the sequence TGTGGT, a core motif recognized by members of the AML/CBFalpha transcription factor family. This complex was competed by a high affinity binding site specific for AML/CBFalpha and was specifically supershifted by an antibody to AML3/CBFalpha1, placing factor B within the AML3/CBFalpha1 subclass. Interestingly, this factor was shown to bind to a second site in the 3' part of the enhancer, positioned between the two critical AR binding sites. Transfection studies revealed that AML1-ETO, a dominant-negative AML/CBFalpha construct, abrogated AR induction of the enhancer, but not of simple hormone response elements. Furthermore, overexpression of AML3/CBFalpha1 could rescue the AML1-ETO repression. Finally, glutathione S-transferase-AML/CBFalpha fusion proteins demonstrated direct interaction between AML/CBFalpha and steroid receptors. Although this interaction was equivalent between AML1/CBFalpha2 and AR or GR, AML3/CBFalpha1 showed stronger interaction with AR than with GR. These data demonstrate that AML3/CBFalpha1 is functionally required for hormonal induction of the Slp enhancer and that direct, preferential protein-protein interactions may contribute to AR-specific activation. These results demonstrate an intriguing role of AML3/CBFalpha1 in steroid- as well as tissue-specific activation of target genes.

Multiple receptor domains interact to permit, or restrict, androgen-specific gene activation

A critical problem within transcription factor families is how diverse regulatory programs are directed by highly related members. Androgen and glucocorticoid receptors (AR, GR) recognize a consensus DNA hormone response element (HRE), but they activate target genes with precise specificity, largely dependent on the promoter and cell context. We have assessed the role of different receptor domains in hormone-specific response by testing chimeras of AR and GR for their ability to activate the androgen-specific enhancer of the mouse sex-limited protein (Slp) gene. Although all of the mutant receptors activated simple HREs, only a few activated the androgen-specific element. One component shared by receptors functional on the AR-specific target was the AR DNA binding domain. Activation was not due to differential DNA affinity but rather to the AR DNA binding domain escaping suppression directed at the GR DNA binding domain in this enhancer context. A further mechanism increasing specific activation was cooperation of receptors at multiple and weak HREs, which was accentuated in the presence of both the AR N terminus and ligand binding domain. These domains together increased recognition of weak HREs, as demonstrated by in vitro DNase I footprinting and transactivation of mutant enhancers. Further, AR N-terminal subdomains reported to interact directly with the ligand binding domain relieved an inhibitory effect imposed by that domain. Therefore, functions intrinsic to AR augment steroid-specific gene activation, by evading negative regulation operating on the domains of other receptors and by enhancing cooperativity through intra- and inter-receptor domain interactions. These subtle distinctions in AR and GR behavior enforce transcriptional specificity established by the context of nonreceptor factors.