Estelita S. Ong

PPARγ Is Required for Placental, Cardiac, and Adipose Tissue Development

The nuclear hormone receptor PPAR gamma promotes adipogenesis and macrophage differentiation and is a primary pharmacological target in the treatment of type II diabetes. Here, we show that PPAR gamma gene knockout results in two independent lethal phases. Initially, PPAR gamma deficiency interferes with terminal differentiation of the trophoblast and placental vascularization, leading to severe myocardial thinning and death by E10.0. Supplementing PPAR gamma null embryos with wild-type placentas via aggregation with tetraploid embryos corrects the cardiac defect, implicating a previously unrecognized dependence of the developing heart on a functional placenta. A tetraploid-rescued mutant surviving to term exhibited another lethal combination of pathologies, including lipodystrophy and multiple hemorrhages. These findings both confirm and expand the current known spectrum of physiological functions regulated by PPAR gamma.

Adipose-specific peroxisome proliferator-activated receptor gamma knockout causes insulin resistance in fat and liver but not in muscle.

Syndrome X, typified by obesity, insulin resistance (IR), dyslipidemia, and other metabolic abnormalities, is responsive to antidiabetic thiazolidinediones (TZDs). Peroxisome proliferator-activated receptor (PPAR) γ, a target of TZDs, is expressed abundantly in adipocytes, suggesting an important role for this tissue in the etiology and treatment of IR. Targeted deletion of PPARγ in adipose tissue resulted in marked adipocyte hypocellularity and hypertrophy, elevated levels of plasma free fatty acids and triglyceride, and decreased levels of plasma leptin and ACRP30. In addition, increased hepatic glucogenesis and IR were observed. Despite these defects, blood glucose, glucose and insulin tolerance, and insulin-stimulated muscle glucose uptake were all comparable to those of control mice. However, targeted mice were significantly more susceptible to high-fat diet-induced steatosis, hyperinsulinemia, and IR. Surprisingly, TZD treatment effectively reversed liver IR, whereas it failed to lower plasma free fatty acids. These results suggest that syndrome X may be comprised of separable PPARγ-dependent components whose origins and therapeutic sites may reside in distinct tissues.

A direct repeat in the cellular retinol-binding protein type II gene confers differential regulation by RXR and RAR

The vitamin A derivative retinoic acid exerts its effects on transcription through two distinct classes of nuclear receptors, the retinoic acid receptor (RAR) and the retinoid X receptor (RXR). We provide evidence that expression of the gene for cellular retinol-binding protein type II (CRBPII), a key protein in the intestinal absorption of vitamin A, is dramatically up-regulated by retinoic acid in the presence of RXR but not RAR. This regulation is conferred through a specific cis element in the CRBPII promoter that contains five nearly perfect tandem repeats of the sequence AGGTCA spaced by a single nucleotide. The discovery of this new RX response element provides a means for distinguishing between the two retinoid receptor systems and suggests that an RXR-mediated pathway exists for modulating vitamin A metabolism.

Effects of peroxisome proliferator-activated receptor δ on placentation, adiposity, and colorectal cancer

Targeting of the nuclear prostaglandin receptor peroxisome proliferator-activated receptor δ (PPARδ) by homologous recombination results in placental defects and frequent (>90%) midgestation lethality. Surviving PPARδ−/− mice exhibit a striking reduction in adiposity relative to wild-type levels. This effect is not reproduced in mice harboring an adipose tissue-specific deletion of PPARδ, and thus likely reflects peripheral PPARδ functions in systemic lipid metabolism. Finally, we observe that PPARδ is dispensable for polyp formation in the intestine and colon of APCmin mice, inconsistent with its recently proposed role in the establishment of colorectal tumors. Together, these observations reveal specific roles for PPARδ in embryo development and adipocyte physiology, but not cancer.

Peroxisome Proliferator-Activated Receptor γ Controls Muc1 Transcription in Trophoblasts

ABSTRACT The nuclear receptor peroxisome proliferator-activated receptor γ (PPARγ) is essential for placental development. Here, we show that the mucin gene Muc1 is a PPARγ target, whose expression is lost in PPARγ null placentas. During differentiation of trophoblast stem cells, PPARγ is strongly induced, and Muc1 expression is upregulated by the PPARγ agonist rosiglitazone. Muc1 promoter is activated strongly and specifically by liganded PPARγ but not PPARα or PPARδ. A PPAR binding site (DR1) in the proximal Muc1 promoter acts as a basal silencer in the absence of PPARγ, and its cooperation with a composite upstream enhancer element is both necessary and sufficient for PPARγ-dependent induction of Muc1. In the placenta, MUC1 protein is localized exclusively to the apical surface of the labyrinthine trophoblast around maternal blood sinuses, resembling its luminal localization on secretory epithelia. Last, variably penetrant maternal blood sinus dilation in Muc1-deficient placentas suggests that Muc1 regulation by PPARγ contributes to normal placental development but also that the essential functions of PPARγ in the organ are mediated by other targets.

Developmental and Hormonal Regulation of Neuroendocrine Gene Transcription

Publisher Summary The quantitative regulation of gene transcription during development and by hormones appears to require the interaction of specific rate-limiting transcriptional factors that bind to structurally distinct genomic sequences. The precise molecular mechanisms by which the binding of these trans-acting factors increases the rate of accurate transcriptional initiation remain unknown; however, it is clear that the regulatory cis-active regions can represent either simple or complex elements. In the case of heritable patterns of neuroendocrine expression, a combinatorial reaction of several sequences and their cognate transcription factors appears requisite for cell-specific gene transcription. The multi-factorial nature of this regulation provides the possibility for a more restricted pattern of gene expression than that exhibited by the cognate trans-acting transcription factors. In the case of the rat prolactin gene, three to five discrete factors may be required for the upstream enhancer function. Additional important cell-specific enhancers appear to be present in proximity to the promoter. By contrast, several regulatory cis-active elements, such as those that transfer hormone regulation, appear to bind a unique transcription factor, which may be sufficient for activation of gene transcription.