Sean W Limesand

PMID: 10518781;Abstract:

Ovine placental lactogen (oPL), a member of the growth hormone/prolactin gene family, is produced by chorionic binucleate cells at the maternal-fetal interface, and is thought to modulate metabolic processes and enhance fetal growth. We have determined that the oPL gene contains five exons and four introns, and the transcriptional start site was mapped 91 bp 5' of the initiation codon (AUG). An additional 4.5 kb of 5'-flanking sequence was sequenced and used for transient transfection analysis in human (BeWo) and rat (Rcho-1) choriocarcinoma cell lines to examine trophoblast cell-specific activity. Trophoblast cell-specific transactivation of the reporter gene was conferred by the proximal 1.1 kb of oPL gene 5'-flanking sequence. Transfection of deletion constructs derived from the 1.1 kb of 5'-flanking sequence resulted in varying profiles of transactivation between the two choriocarcinoma cell lines, but maximal activation in both cell lines resided within the proximal 383 bp of oPL gene 5'-flanking sequence. DNase I protection analysis using ovine chorionic binucleate cell nuclear protein, identified 19 footprints within the 1.1-kb sequence, six of which are located within the 383-bp region. Electrophoretic mobility-shift assays and mutational analysis identified two functional GATA (-67, -102) sequences as transactivators of the oPL gene. However, a previously undefined element (GAGGAG) residing at -338 and -283 is required for full transactivation, and mutation of either significantly reduces reporter activity. In addition, an AP-2 site (-58) and an E-box (-163) were identified and may coordinate oPL transactivation. Transcriptional regulation of human and rodent PL genes has been previously characterized, and our results indicate that tissue-specific regulation of oPL expression may result from cis-acting elements in common with human and rat genes expressed within the placenta. However, our data indicate that regulation of oPL also results from novel cis-acting elements.

Fetal insulin secretion is inhibited by acute hypoxemia. The relationship between prolonged hypoxemia and insulin secretion, however, is less well defined. To test the hypothesis that prolonged fetal hypoxemia impairs insulin secretion, studies were performed in sheep fetuses that were bled to anemic conditions for 9 ± 0 days (anemic, n = 19) and compared with control fetuses (n = 15). Arterial hematocrit and oxygen content were 34% and 52% lower, respectively, in anemic vs. control fetuses (P 0.0001). Plasma glucose concentrations were 21% higher in the anemic group (P 0.05). Plasma norepinephrine and cortisol concentrations increased 70% in the anemic group (P 0.05). Glucose-, arginine-, and leucine-stimulated insulin secretion all were lower (P 0.05) in anemic fetuses. No differences in pancreatic islet size or β-cell mass were found. In vitro, isolated islets from anemic fetuses secreted insulin in response to glucose and leucine as well as control fetal islets. These findings indicate a functional islet defect in anemic fetuses, which likely involves direct effects of low oxygen and/or increased norepinephrine on insulin release. In pregnancies complicated by chronic fetal hypoxemia, increasing fetal oxygen concentrations may improve insulin secretion.

PMID: 18056789;PMCID: PMC3857025;Abstract:

Hepatic glucose production is normally activated at birth but has been observed in response to experimental hypoglycemia in fetal sheep. The cellular basis for this process remains unknown. We determined the impact of 2 wk of fetal hypoglycemia during late gestation on enzymes responsible for hepatic gluconeogenesis, focusing on the insulin-signaling pathway, transcription factors, and coactivators that regulate gluconeogenesis. Hepatic phosphoenolpyruvate carboxykinase and glucose-6-phosphatase mRNA increased 12-fold and 7-fold, respectively, following chronic hypoglycemia with no change in hepatic glycogen. Chronic hypoglycemia decreased fetal plasma insulin with no change in glucagon but increased plasma cortisol 3.5-fold. Peroxisome proliferator-activated receptor-γ coactivator-1α mRNA and phosphorylation of cAMP response element binding protein at Ser¹³³ were both increased, with no change in Akt, forkhead transcription factor FoxO1, hepatocyte nuclear factor-4α, or CCAAT enhancer binding protein-β. These results demonstrate that chronic fetal hypoglycemia triggers signals that can activate gluconeogenesis in the fetal liver. Copyright © 2008 the American Physiological Society.

PMID: 16339204;Abstract:

We determined in vivo and in vitro pancreatic islet insulin secretion and glucose metabolism in fetuses with intrauterine growth restriction (IUGR) caused by chronic placental insufficiency to identify functional deficits in the fetal pancreas that might be caused by nutrient restriction. Plasma insulin concentrations in theIUGRfetuses were 69% lower at baseline and 76% lower after glucose-stimulated insulin secretion (GSIS). Similar deficits were observed with arginine-stimulated insulin secretion. Fetal islets, immunopositive for insulin and glucagon, secreted insulin in response to increasing glucose and KCl concentrations. Insulin release as a fraction of total insulin content was greater in glucose-stimulated IUGR islets, but the mass of insulin released per IUGR islet was lower because of their 82% lower insulin content. A deficiency in islet glucose metabolism was found in the rate of islet glucose oxidation at maximal stimulatory glucose concentrations (11 mmol/liter). Thus, pancreatic islets from nutritionally deprived IUGR fetuses caused by chronic placental insufficiency have impaired insulin secretion caused by reduced glucose-stimulated glucose oxidation rates, insulin biosynthesis, and insulin content. This impaired GSIS occurs despite an increased fractional rate of insulin release that results from a greater proportion of releasable insulin as a result of lower insulin stores. Because this animal model recapitulates the human pathology of chronic placental insufficiency and IUGR, the β-cell GSIS dysfunction in this model might indicate mechanisms that are developmentally adaptive for fetal survival but in later life might predispose offspring to adult-onset diabetes that has been previously associated with IUGR. Copyright © 2006 by The Endocrine Society.