Summary The mammary epithelium is organized as a bi-layer of luminal and basal/myoepithelial cells. During pregnancy the luminal compartment expands for milk production, while basal cells are thought to provide structural and contractile support. Here we reveal an unanticipated, pregnancy-specific role of basal epithelia as a central coordinator of lactogenesis. We demonstrate that genetic deletion of the transcription factor p63 (Trp63) gene exclusively within basal cells of the adult gland during pregnancy leads to dramatic defects in luminal cell proliferation and differentiation, resulting in lactation failure. This phenotype is explained by direct transcriptional activation of the EGF-family ligand gene Nrg1 by p63 selectively in basal cells, which is required for luminal ERBB4/STAT5A activation and consequent luminal progenitor cell maturation. Thus, paracrine basal-to-luminal cell signaling, controlled by p63 via NRG1, orchestrates the entire lactation program. Collectively, these findings redefine the paradigm for cellular interactions specifying the functional maturation of the mammary gland.

Background-Smooth muscle cells (SMCs) play a key role in intimal thickening in atherosclerosis and restenosis. The precise signaling pathways by which the proliferation of SMCs is regulated are largely unknown. The TR3 orphan receptor, the mitogen-induced nuclear orphan receptor (MINOR), and the nuclear receptor of T cells (NOT) are a subfamily of transcription factors belonging to the nuclear receptor superfamily and are induced in activated SMCs. In this study, we investigated the role of these transcription factors in SMC proliferation in atherogenesis. Methods and Results-Multiple human vascular specimens at distinct stages of atherosclerosis (lesion types II to V byAmerican Heart Association classification) derived from 14 different individuals were studied for expression of these transcription factors. We observed expression of TR3, MINOR, and NOT in neointimal SMCs, whereas no expression was detected in medial SMCs. Adenovirus-mediated expression of a dominant-negative variant of TR3, which suppresses the transcriptional activity of each subfamily member, increases DNA synthesis and decreases p27 Kip1 protein expression in cultured SMCs. We generated transgenic mice that express this dominant-negative variant or full-length TR3 under control of a vascular SMC-specific promoter. Carotid artery ligation of transgenic mice that express the dominant-negative variant of TR3 in arterial SMCs, compared with lesions formed in wild-type mice, results in a 3-fold increase in neointimal formation, whereas neointimal formation is inhibited 5-fold in transgenic mice expressing full-length TR3. Conclusions-Our results reveal that TR3 and possibly other members of this transcription factor subfamily inhibit vascular lesion formation. These transcription factors could serve as novel targets in the treatment of vascular disease.

Objective-Endothelial cells play a pivotal role in vascular homeostasis. In this study, we investigated the function of the nerve growth factor-induced protein-B (NGFI-B) subfamily of nuclear receptors comprising the TR3 orphan receptor (TR3), mitogen-induced nuclear orphan receptor (MINOR), and nuclear orphan receptor of T cells (NOT) in endothelial cells. Methods and Results-The mRNA expression of TR3, MINOR, and NOT in atherosclerotic lesions was assessed in human vascular specimens. Each of these factors is expressed in smooth muscle cells, as described before, and in subsets of endothelial cells, implicating that they might affect endothelial cell function. Adenoviral overexpression of TR3 in cultured endothelial cells resulted in decreased [ 3 H]thymidine incorporation, whereas a dominant-negative TR3 variant that inhibits the activity of endogenous TR3-like factors enhanced DNA synthesis. TR3 interfered with progression of the cell cycle by upregulating p27Kip1 and downregulating cyclin A, whereas expression levels of a number of other cell cycle-associated proteins remained unchanged.Conclusions-These findings demonstrate that TR3 is a modulator of endothelial cell proliferation and arrests endothelial cells in the G1 phase of the cell cycle by influencing cell cycle protein levels. We hypothesize involvement of TR3 in the maintenance of integrity of the vascular endothelium. The nerve growth factor-induced protein-B (NGFI-B) subfamily 6 of nuclear receptors (NR4A) belongs to the steroid/thyroid hormone superfamily of transcription factors and comprises TR3 orphan receptor (TR3), mitogen-induced nuclear orphan receptor (MINOR), and nuclear orphan receptor of T cells (NOT). 7 Like other members of the nuclear receptor superfamily, the NGFI-B-like factors contain a central DNA-binding domain, comprising two zinc fingers, that recognizes response elements in the promoters of specific target genes. The amino-terminal domain mediates transactivation, and the carboxy-terminal domain is involved in (hetero)dimerization and ligand binding. At present, the ligands for TR3, MINOR, and NOT are unknown, qualifying these transcription factors as orphan receptors. 8 Several lines of evidence indicate that TR3 and MINOR are involved in T-cell apoptosis. Antisense oligonucleotides directed against TR3 prevent apoptosis in cultured T cells and prostate and lung carcinoma cells. 9 -11 Furthermore, it has been shown that overexpression of TR3 or MINOR in developing T cells of transgenic mice results in massive apoptosis of thymocytes and reduced levels of peripheral T cells, whereas a dominantnegative variant of TR3 inhibits T-cell apoptosis. 12,13 Recently, it has been shown that in response to apoptotic stimuli, TR3 can translocate from the nucleus to the mitochondria to promote cytochrome c release and apoptosis. This proapoptotic effect of TR3 was shown to be independent of its transactivation activity. 14 We have recently shown that TR3 inhibits serumstimulated proliferation of vascular smooth muscle cells (SMCs). Mor...