Glucose and scyllo-inositol impair phosphoinositide hydrolysis in the 10.5-day cultured rat conceptus: a role in dysmorphogenesis?

Hyperglycemia alters retinoic acid catabolism in embryos exposed to a maternal diabetic milieu

Pregestational diabetes is highly associated with increased risk of birth defects. We previously reported that the expression of Cyp26a1, the major catabolizing enzyme for controlling retinoic acid (RA) homeostasis, is significantly down-regulated in embryos of diabetic mice, thereby increasing the embryo's susceptibility to malformations caused by RA dysregulation. However, the underlying mechanism for the down-regulation of Cyp26a1 remains unclear. This study aimed to investigate whether elevated maternal blood glucose in the diabetic milieu is a critical factor for the altered Cyp26a1 expression. Streptozotozin-induced diabetic pregnant mice were treated with phlorizin (PHZ) to reduce blood glucose concentrations via induction of renal glucosuria. Embryonic Cyp26a1 expression level, RA catabolic activity and susceptibility to various RA-induced abnormalities were examined. To test the dose-dependent effect of glucose on Cyp26a1 level, early head-fold stage rat embryos of normal pregnancy were cultured in vitro with varying concentrations of D-glucose, followed by quantification of Cyp26a1 transcripts. We found that Cyp26a1 expression, which was down-regulated in diabetic pregnancy, could be normalized under reduced maternal blood glucose level, concomitant with an increase in RA catabolic activity in embryonic tissues. Such normalization could successfully reduce the susceptibility to different RA-induced malformations including caudal regression, cleft palate and renal malformations. The expression level of Cyp26a1 in the embryo was inversely correlated with D-glucose concentrations. Diabetic patients suffer from retinopathy, dermopathy, male infertility and increased cancer risk. Coincidentally, RA dysregulation is also associated with these health problems. Our results provided evidence that elevated glucose can down-regulate Cyp26a1 expression level and disturb RA homeostasis, shedding light on the possibility of affecting the health of diabetic patients via a similar mechanism.

A review of the role of inositols in conditions of insulin dysregulation and in uncomplicated and pathological pregnancy

Inositols, a group of 6-carbon polyols, are highly bioactive molecules derived from diet and endogenous synthesis. Inositols and their derivatives are involved in glucose and lipid metabolism and participate in insulin-signaling, with perturbations in inositol processing being associated with conditions involving insulin resistance, dysglycemia and dyslipidemia such as polycystic ovary syndrome and diabetes. Pregnancy is similarly characterized by substantial and complex changes in glycemic and lipidomic regulation as part of maternal adaptation and is also associated with physiological alterations in inositol processing. Disruptions in maternal adaptation are postulated to have a critical pathophysiological role in pregnancy complications such as gestational diabetes and pre-eclampsia. Inositol supplementation has shown promise as an intervention for the alleviation of symptoms in conditions of insulin resistance and for gestational diabetes prevention. However, the mechanisms behind these affects are not fully understood. In this review, we explore the role of inositols in conditions of insulin dysregulation and in pregnancy, and identify priority areas for research. We particularly examine the role and function of inositols within the maternal-placental-fetal axis in both uncomplicated and pathological pregnancies. We also discuss how inositols may mediate maternal-placental-fetal cross-talk, and regulate fetal growth and development, and suggest that inositols play a vital role in promoting healthy pregnancy.

Mechanisms of Congenital Malformations in Pregnancies with Pre-existing Diabetes

PURPOSE OF REVIEW

Fetuses of diabetic mothers are at increased risk for congenital malformations. Research in recent decades using animal and embryonic stem cell models has revealed many embryonic developmental processes that are disturbed by maternal diabetes. The aim of this review is to give clinicians a better understanding of the reasons for rigorous glycemic control in early pregnancy, and to provide background to guide future research.

RECENT FINDINGS

Mouse models of diabetic pregnancy have revealed mechanisms for altered expression of tissue-specific genes that lead to malformations that are more common in diabetic pregnancies, such as neural tube defects (NTDs) and congenital heart defects (CHDs), and how altered gene expression causes apoptosis that leads to malformations. Embryos express the glucose transporter, GLUT2, which confers susceptibility to malformation, due to high rates of glucose uptake during maternal hyperglycemia and subsequent oxidative stress; however, the teleological function of GLUT2 for mammalian embryos may be to transport the amino sugar glucosamine (GlcN) from maternal circulation to be used as substrate for glycosylation reactions and to promote embryo cell growth. Malformations in diabetic pregnancy may be not only due to excess glucose uptake but also due to insufficient GlcN uptake. Avoiding maternal hyperglycemia during early pregnancy should prevent excess glucose uptake via GLUT2 into embryo cells, and also permit sufficient GLUT2-mediated GlcN uptake.

The "Other" Inositols and Their Phosphates: Synthesis, Biology, and Medicine (with Recent Advances in myo-Inositol Chemistry)

Cell signaling via inositol phosphates, in particular via the second messenger myo-inositol 1,4,5-trisphosphate, and phosphoinositides comprises a huge field of biology. Of the nine 1,2,3,4,5,6-cyclohexanehexol isomers, myo-inositol is pre-eminent, with "other" inositols (cis-, epi-, allo-, muco-, neo-, L-chiro-, D-chiro-, and scyllo-) and derivatives rarer or thought not to exist in nature. However, neo- and d-chiro-inositol hexakisphosphates were recently revealed in both terrestrial and aquatic ecosystems, thus highlighting the paucity of knowledge of the origins and potential biological functions of such stereoisomers, a prevalent group of environmental organic phosphates, and their parent inositols. Some "other" inositols are medically relevant, for example, scyllo-inositol (neurodegenerative diseases) and d-chiro-inositol (diabetes). It is timely to consider exploration of the roles and applications of the "other" isomers and their derivatives, likely by exploiting techniques now well developed for the myo series.

Can we prevent diabetic birth defects with micronutrients?

Congenital malformations are more common in infants of diabetic women than in children of non-diabetic women. The mechanisms behind diabetes-induced congenital anomalies are not known. Disturbed micronutrient metabolism, in concert with oxidative stress, has been suggested as a cause of diabetes-induced malformations by several studies. In experimental work, administration of inositol, arachidonic acid and several antioxidative compounds, as well as folic acid, to the embryo, has proven to attenuate the teratogenic effects of a diabetic environment. Future therapeutic efforts may include supplementation with antioxidants or micronutrients, such as folic acid, to the pregnant diabetic woman, although exact compounds and doses need to be determined.

Congenital anomalies in diabetic pregnancy

Congenital malformations are more common in infants of diabetic women than in children of non-diabetic women. The etiology, pathogenesis and prevention of the diabetes-induced malformations have spurred considerable clinical and basic research efforts. The ultimate aim of these studies has been to obtain an understanding of the teratogenic process, which may enable precise preventive therapeutic measures in diabetic pregnancies. The results of the clinical and basic studies support the view of an early gestational induction of the malformations in diabetic pregnancy by a teratogenic process of multifactorial etiology. There may be possible targets for new therapeutic efforts revealed by the research work. Thus, future additions to the therapeutic efforts may include supplementation with antioxidants and/or folic acid, although more research is needed to delineate the dosages and compounds to be used. As the research into genetic predisposition for the teratogenic induction of malformations by maternal diabetes starts to reveal new genes and gene products involved in the etiology of the malformations, a set of new targets for intervention may arise.

Mouse models of neural tube defects: investigating preventive mechanisms

Neural tube defects (NTD), including anencephaly and spina bifida, are a group of severe congenital abnormalities in which the future brain and/or spinal cord fail to close. In mice, NTD may result from genetic mutations or knockouts, or from exposure to teratogenic agents, several of which are known risk factors in humans. Among the many mouse NTD models that have been identified to date, a number have been tested for possible primary prevention of NTD by exogenous agents, such as folic acid. In genetic NTD models such as Cart1, splotch, Cited2, and crooked tail, and NTD induced by teratogens including valproic acid and fumonisins, the incidence of defects is reduced by maternal folic acid supplementation. These folate-responsive models provide an opportunity to investigate the possible mechanisms underlying prevention of NTD by folic acid in humans. In another group of mouse models, that includes curly tail, axial defects, and the Ephrin-A5 knockout, NTD are not preventable by folic acid, reflecting the situation in humans in which a subset of NTD appear resistant to folic acid therapy. In this group of mutants alternative preventive agents, including inositol and methionine, have been shown to be effective. Overall, the data from mouse models suggests that a broad-based in utero therapy may offer scope for prevention of a greater proportion of NTD than is currently possible.

Concise chemoenzymatic synthesis of epi-inositol

epi-Inositol was synthesized in six steps in 40% overall yield from a bacterial bromobenzene metabolite. The chemoenzymatic route involved toluene dioxygenase oxidation, substrate-directed catalytic osmylation, m-CPBA epoxidation, radical debromination, and Amberlite-catalized hydrolysis. The route described is amenable to scaleup and could allow access to cis-inositol, and deoxy derivatives of epi-inositol.

Induction of embryonic dysmorphogenesis by high glucose concentration, disturbed inositol metabolism, and inhibited protein kinase C activity

BACKGROUND

Exposure to a diabetic environment causes excess reactive oxygen species (ROS), decreased prostaglandin E(2) (PGE(2)) concentration, and increased embryonic maldevelopment. The aim of the present work was to study whether embryonic dysmorphogenesis is also dependent on alterations of inositol and associated intracellular metabolites.

METHODS

Day 9 rat embryos were cultured for 24 or 48 hr and evaluated for gene expression. Day 10 and day 11 embryos from normal and diabetic rats were also examined. RT-PCR was used to study embryonic gene expression of protein kinase C (PKC) and cytosolic phospholipase A(2) (cPLA(2)).

RESULTS

Embryos exposed to 30 mmol/L glucose (30G), 500 or 750 micromol/L of scyllo-inositol (500SI or 750SI) had higher malformation score than control embryos cultured in 10 mmol/L glucose (10G). Adding 1.6 mmol/L inositol to the 30G or 750SI culture medium partly corrected these embryos, and completely normalized 500SI embryonic development. Adding 0.5 mmol/L N-acetylcysteine (NAC) or 280 nmol/L PGE(2) protected, and failed to protect, the SI-exposed embryos, respectively. 10G embryos exposed to the PKC inhibitor GF-109203X displayed dose-dependent dysmorphogenesis. Addition of 1.6 mmol/L inositol or 0.5 mmol/L NAC to the PKC-inhibitor-exposed 10G embryos largely normalized the outcome, whereas PGE(2) again failed to protect embryonic development. 30G culture tended to decrease the expression of cPLA(2) after 24 hr in vitro. We also found decreased mRNA levels of cPLA(2) in offspring of diabetic rats on gestational day 10 and of PKC on day 11, as compared with normal offspring.

CONCLUSIONS

High glucose concentration causes dysmorphogenesis in embryos by an interaction of oxidative stress and inositol depletion.

Pathogenesis of diabetes-induced congenital malformations

The increased rate of fetal malformation in diabetic pregnancy represents both a clinical problem and a research challenge. In recent years, experimental and clinical studies have given insight into the teratological mechanisms and generated suggestions for improved future treatment regimens. The teratological role of disturbances in the metabolism of inositol, prostaglandins, and reactive oxygen species has been particularly highlighted, and the beneficial effect of dietary addition of inositol, arachidonic acid and antioxidants has been elucidated in experimental work. Changes in gene expression and induction of apoptosis in embryos exposed to a diabetic environment have been investigated and assigned roles in the teratogenic processes. The diabetic environment appears to simultaneously induce alterations in several interrelated teratological pathways. The complex pathogenesis of diabetic embryopathy has started to unravel, and future research efforts will utilize both clinical intervention studies and experimental work that aim to characterize the human applicability and the cell biological components of the discovered teratological mechanisms.

Congenital malformations in experimental diabetic pregnancy: aetiology and antioxidative treatment. Minireview based on a doctoral thesis

Diabetes mellitus in pregnancy causes congenital malformations in the offspring. The aim of this work was to characterize biochemical and morphologic anomalies in the conceptus of an animal model of diabetic pregnancy. In addition, a preventive treatment against diabetes-induced dysmorphogenesis was developed. Congenital cataract was often found in the offspring of diabetic rats. The fetal lenses had increased water accumulation, sorbitol concentration and aldose reductase activity compared to control lenses. The results suggest that the cataracts form via osmotic attraction of water due to sorbitol accumulation in the fetal lens. Another set of malformations, with possible neural crest cell origin, occurred frequently in offspring of diabetic rats. These included low set ears, micrognathia, hypoplasia of the thymus, thyroid and parathyroid glands, as well as anomalies of the heart and great vessels. Furthermore, diabetes caused intrauterine death and resorptions more frequently in the late part of gestation. When the pregnant diabetic rats were treated with the antioxidants butylated hydroxytoluene, vitamin E or vitamin C, the occurrence of gross malformations was reduced from approximately 25% to less than 8%, and late resorptions from 17% to 7%. This suggests that an abnormal handling of reactive oxygen species (ROS) is involved in diabetes-induced dysmorphogenesis in vivo. Indeed, an increased concentration of lipid peroxides, indicating damage caused by ROS, was found in fetuses of diabetes rats. In addition, embryos of diabetic rats had low concentrations of the antioxidant vitamin E compared to control embryos. These biochemical alterations were normalized by vitamin E treatment of the pregnant diabetic rats. The antioxidants are likely to have prevented ROS injury in the embryos of the diabetic rats, in particular in the neural crest cells, thereby normalizing embryonic development. These results provide a rationale for developing new anti-teratogenic treatments for pregnant women with diabetes mellitus.

The pathogenesis of diabetes-associated congenital malformations

Congenital malformations convey a major financial and social burden to society. Epidemiologic, clinical, and animal studies indicate that these malformations occur in early pregnancy, are influenced by an aberrant metabolic fuel milieu, and seem to result from a combination of more than one factor acting synchronously. Unfortunately, during the critical period of organogenesis, the pregnancy is hardly recognizable, making evaluation and study of relevant maternal embryonic parameters extremely difficult. Additionally, there are obvious limitations to human study for technical and ethical reasons. Animal experimentation, however, has demonstrated that these malformations can be produced in many vertebrates and are similar to those seen in humans. The mechanism for induction of dysmorphogenesis in experimental diabetic pregnancy has been shown to include generation of free oxygen radicals and are associated with alterations in the embryonic levels of arachidonic acid, prostaglandins, and myo-inositol. Most of the earlier experimental studies focused on defects at the level of the embryo excluding the extraembryonic membranes. Current investigations provide evidence that the yolk sac has an integral role in diabetic embryopathy. The experimental use of several different compounds, such as arachidonic acid, myo-inositol, and antioxidants, offers significant promise for the future in possibly serving as a pharmacologic prophylaxis against diabetic embryopathy.