We report a mammalian-based promoter chromosomal array system developed for single-cell studies of transcription-factor function. Designed after the prolactin promoter-enhancer, it allows for the direct visualization of estrogen receptor α (ERα) and/or Pit-1 interactions at a physiologically regulated transcription locus. ERα- and ligand-dependent cofactor recruitment, large-scale chromatin modifications and transcriptional activity identified a distinct fingerprint of responses for each condition. Ligand-dependent transcription (more than threefold activation compared with vehicle, or complete repression by mRNA fluorescent in situ hybridization) at the array correlated with its state of condensation, which was assayed using a novel high throughput microscopy approach. In support of the nuclear receptor hit-and-run model, photobleaching studies provided direct evidence of very transient ER-array interactions, and revealed ligand-dependent changes in koff. ERα-truncation mutants indicated that helix-12 and interactions with co-regulators influenced both large-scale chromatin modeling and photobleaching recovery times. These data also showed that the ERα DNA-binding domain was insufficient for array targeting. Collectively, quantitative observations from this physiologically relevant biosensor suggest stochastic-based dynamics influence gene regulation at the promoter level.

We measured soluble epoxide hydrolase (sEH) renal gene expression in prehypertensive (4 to 5 weeks old) spontaneously hypertensive rats of the Heidelberg SP substrain (SHR [Heid]) and when blood pressure levels entered the hypertensive plateau (17 to 18 weeks old) and compared expression with matched Wistar-Kyoto (WKY [Heid]) rats. Less expression of the gene encoding sEH (EPHX2) was observed in SHR (Heid) than in WKY (Heid). Analysis of sEH protein abundance showed a similar difference. However, no correlation between sEH abundance and blood pressure was observed in the F(2) progeny of a parental strain cross. Measurement of protein abundance in SHR and WKY obtained from Charles River confirmed a recent report that abundance of sEH was greater in SHR (CRiv) than WKY (CRiv) strains. Polymorphisms were detected in EPHX2. Resequencing revealed that 2 alleles of EPHX2 exist in these 4 rat strains, differing by 4 single nucleotide polymorphisms, of which 3 produce nonsynonymous amino acid substitutions. The ancestral allele was shared by SHR (Heid) and WKY (CRiv), and the variant allele was shared by WKY (Heid) and SHR (CRiv). Activity of sEH was greater in animals carrying the variant allele. However, inheritance of this allele was not correlated with blood pressure in the F(2) progeny of a cross between SHR (Heid) and WKY (Heid). These data indicate that sequence variation determining functional alterations in EPHX2 is not likely to contribute to blood pressure levels in SHR.

The steroid receptor coactivator 3 gene (SRC-3) (AIB1/ACTR/pCIP/RAC3/TRAM1) is a p160 family transcription coactivator and a known oncogene. Despite its importance, the functional regulation of SRC-3 remains poorly understood within a cellular context. Using a novel combination of live-cell, high-throughput, and fluorescent microscopy, we report SRC-3 to be a nucleocytoplasmic shuttling protein whose intracellular mobility, solubility, and cellular localization are regulated by phosphorylation and estrogen receptor ␣ (ER␣) interactions. We show that both chemical inhibition and small interfering RNA reduction of the mitogenactivated protein kinase/extracellular signal-regulated kinase 1/2 (MEK1/2) pathway induce a cytoplasmic shift in SRC-3 localization, whereas stimulation by epidermal growth factor signaling enhances its nuclear localization by inducing phosphorylation at T24, S857, and S860, known participants in the phosphocode that regulates SRC-3 activity. Accordingly, the cytoplasmic localization of a nonphosphorylatable SRC-3 mutant further supported these results. In the presence of ER␣, U0126 also dramatically reduces (i) ligand-dependent colocalization of SRC-3 and ER␣, (ii) the formation of ER-SRC-3 complexes in cell lysates, and (iii) SRC-3 targeting to a visible, ER␣-occupied and -regulated prolactin promoter array. Taken together, these results indicate that phosphorylation coordinates SRC-3 coactivator function by linking the probabilistic formation of transient nuclear receptor-coactivator complexes with its molecular dynamics and cellular compartmentalization. Technically and conceptually, these findings have a new and broad impact upon evaluating mechanisms of action of gene regulators at a cellular system level.

Estrogen receptor-α (ER) transcription function is regulated in a ligand-dependent (e.g., estradiol, E2) or ligand-independent (e.g., growth factors) manner. Our laboratory seeks to understand these two modes of action. Using a cell line that contains a visible prolactin enhancer/promoter array (PRL-HeLa) regulated by ER, we analyzed ER response to E2 and EGF by quantifying image-based results. Data show differential recruitment of GFP-ER to the array, with the AF1 domain playing a vital role in EGF-mediated responsiveness. Temporal analyses of large-scale chromatin dynamics, and accumulation of array-localized reporter mRNA over 24 hours showed that the EGF response consists of a single pulse of reporter mRNA accumulation concomitant with transient increase in array decondensation. Estradiol induced a novel cyclical pattern of mRNA accumulation with a sustained increase in array decondensation. Collectively, our work shows that there is a stimuli-specific pattern of large-scale chromatin modification and transcript levels by ER.

In the print and online versions of this paper, the middle initial for Tanmay P. Lele was incorrect. The correct author list is shown above.