Functional alterations of type I insulin-like growth factor receptor in placenta of diabetic rats

Maternal Diabetes Impairs Insulin and IGF-1 Receptor Expression and Signaling in Human Placenta

BACKGROUND

Maternal high blood glucose during pregnancy increases the risk for both maternal and fetal adverse outcomes. The mechanisms underlying the regulator effects of hyperglycemia on placental development and growth have not been fully illustrated yet. The placenta expresses high amounts of both insulin receptor (IR) and insulin-like growth factor receptor (IGF-1R). It has been reported that the placenta of diabetic women has structural and functional alterations and the insulin/IGF system is likely to play a role in these changes. The aim of the present study was to measure the content of IR and IGF-1R and their phosphorylation in the placenta of women with type 1 diabetes mellitus (T1D) or with gestational diabetes mellitus (GDM) compared to women with normal glucose tolerance (NGT) during pregnancy.

METHODS

Placental tissues were obtained from 80 Caucasian women with a singleton pregnancy. In particular, we collected placenta samples from 20 T1D patients, 20 GDM patients and 40 NGT women during pregnancy. Clinical characteristics and anthropometric measures of all women as well as delivery and newborn characteristics were recorded. Patients were also subdivided on the basis of peripartum glycemia either ≥90 mg/dl or <90 mg/dl, regardless of the diagnosis.

RESULTS

In T1D patients, a higher rate of adverse outcomes was observed. Compared to the GDM women, the T1D group showed significantly higher average capillary blood glucose levels at the third trimester of pregnancy and at peripartum, and higher third-trimester HbA1c values. In both T1D and GDM women, HbA1c values during pregnancy correlated with glucose values in the peripartum period (R-squared 0.14, p=0.02). A positive correlation was observed between phosphorylation of placental IR and the glucose levels during the third trimester of GDM and T1D pregnancy (R-squared 0.21, p=0.003). In the placenta of T1D patients, IGF-1R phosphorylation and IR isoform A (IR-A) expression were significantly increased (p=0.006 and p=0.040, respectively), compared to the NGT women. Moreover, IGF-1R phosphorylation was significantly increased (p<0.0001) in the placenta of patients with peripartum glucose >90 mg/dl, while IR-A expression was increased in those with peripartum blood glucose higher than 120 mg/dl (p=0.046).

CONCLUSIONS

To the best of our knowledge, our study represents the first one in which an increased maternal blood glucose level during pregnancy is associated with an increased IGF-1R phosphorylation and IR-A expression in the placenta. Both these mechanisms can promote an excessive fetal growth.

Placental phenotype and the insulin-like growth factors: resource allocation to fetal growth

The placenta is the main determinant of fetal growth and development in utero. It supplies all the nutrients and oxygen required for fetal growth and secretes hormones that facilitate maternal allocation of nutrients to the fetus. Furthermore, the placenta responds to nutritional and metabolic signals in the mother by altering its structural and functional phenotype, which can lead to changes in maternal resource allocation to the fetus. The molecular mechanisms by which the placenta senses and responds to environmental cues are poorly understood. This review discusses the role of the insulin-like growth factors (IGFs) in controlling placental resource allocation to fetal growth, particularly in response to adverse gestational environments. In particular, it assesses the impact of the IGFs and their signalling machinery on placental morphogenesis, substrate transport and hormone secretion, primarily in the laboratory species, although it draws on data from human and other species where relevant. It also considers the role of the IGFs as environmental signals in linking resource availability to fetal growth through changes in the morphological and functional phenotype of the placenta. As altered fetal growth is associated with increased perinatal morbidity and mortality and a greater risk of developing adult-onset diseases in later life, understanding the role of IGFs during pregnancy in regulating placental resource allocation to fetal growth is important for identifying the mechanisms underlying the developmental programming of offspring phenotype by suboptimal intrauterine growth.

Diabetes-associated changes in the fetal insulin/insulin-like growth factor system are organ specific in rats

BACKGROUND

Diabetes in pregnancy affects fetal growth and development. The insulin/insulin-like growth factors (IGF) system comprising insulin, IGF, their receptors, and binding proteins, has been implicated in fetal growth regulation. This study tested the hypothesis that maternal diabetes alters the fetal insulin/IGF system in a tissue-specific manner.

METHODS

Wistar rats were rendered diabetic by neonatal administration of streptozotocin and mated with control rats. At day 21 of gestation, the weights of fetuses, placentas, and fetal organs (heart, lung, liver, stomach, intestine, and pancreas) were determined. Maternal and fetal plasma concentrations of insulin, IGF1, and IGF2 were measured by ELISA, and expression of IGF1, IGF2, IGF1R, IGF2R, IR, IGFBP1, BP2, and BP3 in placenta and fetal organs by qPCR.

RESULTS

The well-known increase in fetal growth in this model of mild diabetes is accompanied by elevated insulin and IGF1 levels and alterations of the insulin/IGF system in the fetus and the placenta. These alterations were organ and gene specific. The insulin/IGF system was generally upregulated, especially in the fetal heart, while it was downregulated in fetal lung.

CONCLUSION

In our model of mild diabetes, the effect of maternal diabetes on fetal weight and fetal insulin/IGF system expression is organ specific with highly sensitive organs such as lung and heart, and organs that were less affected, such as stomach.

Structural, functional and circulatory placental changes associated with impaired glucose metabolism

OBJECTIVE

To investigate associations between structural, functional and circulatory placental changes in pregnancies complicated by impaired glucose metabolism.

DESIGN

Umbilical artery (UA) blood flow resistance was measured by Doppler velocimetry in 21 gravidae with diabetes/impaired glucose tolerance (IGT) and 10 healthy gravidae. Umbilical and placental vessel segments were incubated for determination of prostacyclin and thromboxane synthesis, and tissues histologically examined. Non-parametric statistical tests at a two-tailed P<0.05 were used.

RESULTS

Placental lesions were more common in diabetes/IGT and, although not being an uniform finding, in general associated with a higher vascular synthesis of thromboxane and/or lower prostacyclin/thromboxane synthesis ratio. As an exception, ischemic villitis was associated with a higher ratio and higher UA flow resistance.

CONCLUSIONS

Placental lesions are associated with an altered vascular prostanoid synthesis in diabetes/IGT, but not until structural signs of ischemia develop is a rise of UA blood flow resistance detected.

Maternal-placental-fetal interactions in the endocrine regulation of fetal growth: role of somatotrophic axes

Inadequate growth in utero is associated not only with adverse fetal, perinatal, and neonatal outcomes, but also with an altered propensity for disease later in life. Conversely, fetal overgrowth is also associated with increased medical risks for both mother and fetus. The interaction between the fetal genome and the intrauterine environment determines in great part how fetal growth will progress. The placental, maternal, and fetal somatotrophic axes (growth hormone/insulin-like growth factor-1) play key roles in modulating this interaction. Experimental undernutrition in animal models has numerous effects over these axes and provides insight into understanding fetal growth and its abnormalities. This review addresses the contributions made by the placental, maternal, and fetal somatotrophic axes to the regulation of fetal growth.

D-glucose-induced dysmorphogenesis of embryonic kidney

An organ culture system was used to study the effect of D-glucose on embryonic kidneys, and to delineate the mechanism(s) relevant to their dysmorphogenesis. Metanephroi were cultured in the presence of 30 mM D-glucose. A notable reduction in the size and population of nephrons was observed. Ureteric bud branches were rudimentary and the acuteness of their tips, the site of nascent nephron formation, was lost. Metanephric mesenchyme was atrophic, had reduced cell replication, and contained numerous apoptotic cells. Competitive reverse transcriptase-PCR analyses and immunoprecipitation studies indicated a decrease in expression of heparan sulfate proteoglycan (perlecan). Status of activated protein-2 was evaluated since its binding motifs are present in the promoter region of the perlecan gene. Decreased binding activity of activated protein-2, related to its phosphorylation, was observed. D-glucose-treated explants also had reduced levels of cellular ATP. Exogenous administration of ATP restored the altered metanephric morphology and reduced [35S]sulfate-incorporated radioactivity associated with perlecan. The data suggest that D-glucose adversely affects the metanephrogenesis by perturbing various cellular phosphorylation events involved in the transcriptional and translational regulation of perlecan. Since perlecan modulates epithelial/mesenchymal interactions, its deficiency may have led to the metanephric dysmorphogenesis and consequential atrophy of the mesenchyme exhibiting accelerated apoptosis.

Growth promoting peptides in diabetic and non-diabetic pregnancy: interactions with trophoblastic receptors and serum carrier proteins

Infantile macrosomia in diabetic pregnancy (DP) is commonly attributed to fetal hyperinsulinism. However, insulin-like growth factors in the mother and the fetus, their binding proteins and their placental receptors may also play roles in the process of fetal overgrowth. We measured levels of maternal and cord serum IGF-I, IGF-II, C-peptide, IGFBP-1, IGFBP-2 and IGFBP-3 in 8 White Class B insulin dependent DP and 8 non-diabetic pregnancies (NP). These results were correlated with the concentration and affinity of placental trophoblastic membrane receptors (TR) for insulin (IN), IGF-I and IGF-II as well as with infant and placenta weights and maternal body mass indices. Significant respective differences between the diabetic and non-diabetic groups were found in mean infant weight, 4248 +/- 114 vs 3555 +/- 119 g (p < 0.001), placental weight 765 +/- 51 vs 575 +/- 24 g (p < 0.01), maternal body mass index 32.8 +/- 3.8 vs 21.3 +/- 1.2 (p < 0.02), cord serum IGF-I 136.8 +/- 6.6 vs 85.9 +/- 5.7 ng/ml (p < 0.01), cord serum C-peptide 18.7 +/- 3.5 vs 9.0 +/- 1.7 ng/ml (p < 0.025), cord serum IGFBP-1 21.9 +/- 4.7 vs 133.2 +/- 43.2 ng/ml (p < 0.025), cord serum IGFBP-2 672.0 +/- 76 vs 1206 +/- 220 ng/ml (p < 0.05) and cord serum IGFBP-3 11.5 +/- 1.0 vs 5.6 +/- 0.6 ng/ml (p < 0.001). No significant differences were found between DP and NP with respect to cord serum IGF-II, maternal serum IGF-I, IGF-II, C-peptide, IGFBP-1, IGFBP-2 and IGFBP-3, and the concentration and affinity of TR for IN, IGF-I and IGF-II. Analysis of variance revealed an interaction between infant weight and the weight of the placenta (p < 0.01), cord IGF-I (p < 0.02), cord C-peptide (p < 0.01) and cord IGFBP-3 (p < 0.01). Regression analysis revealed significant correlations of cord IGF-I with cord values of IGFBP-2 (r = -0.52, p = 0.04) and IGFBP-3 (r = 0.66, p < 0.005). Maternal serum IGF-I significantly correlated only with maternal IGFBP-3 (r = 0.65, p < 0.01). These results suggest that increased fetal production of insulin and IGF-I may contribute to the development of infantile macrosomia in DP. Concomitant changes in fetal production of IGFBPs, particularly IGFBP-2 and IGFBP-3, may modulate the action of insulin and IGFs. The lack of change in number or binding affinity of placental trophoblastic receptors for insulin, IGF-I and IGF-II tends to exclude a significant regulatory role of these receptors in the production of fetal macrosomia.

Glucose-induced stimulation of human insulin-receptor mRNA and tyrosine kinase activity in cultured cells

The effects of high glucose on insulin-receptor tyrosine kinase activity and gene expression were investigated in 3T3-HIR cells. Cells incubated for 48 h in the presence of 25 mM glucose showed a 5-fold increase in the amount of insulin receptors per cell, receptor autophosphorylation and phosphorylation of the exogenous substrate poly(Glu/Tyr) compared with cells grown in the absence of glucose but in the presence of 25 mM fructose. These effects were associated with a 4-fold stimulation in steady-state levels of insulin-receptor mRNA. Significant cellular glucose utilization and lactate production were observed in the presence of high glucose in the culture medium, indicating a functional glycolytic pathway in glucose-treated cells, but not in cells treated with fructose. Such a differential response to hexoses favours the hypothesis of a carbohydrate regulation via a glycolytic intermediate. This was further supported by a similar glucose-induced increase in mRNA levels of the enzyme glyceraldehyde-3-phosphate dehydrogenase. To test the hypothesis that the stimulatory effect of glucose on amount of insulin receptors and phosphorylation state could result from post-transcriptional modifications, cells exposed to glucose were incubated with actinomycin D, a potent inhibitor of gene transcription. In cells challenged with high glucose plus inhibitor, insulin-receptor mRNA half-life was increased from 1 to 3 h, indicating that posttranscriptional mechanisms are involved in these processes of glucose regulation. Inhibition of protein synthesis by cycloheximide induced an overexpression of insulin-receptor mRNA levels in the presence of glucose, suggesting that labile repressor protein(s) could be implicated in the effects of glucose. We conclude that (1) long-term culture with high glucose increases the amount of insulin receptors and their tyrosine kinase activity and (2) the glucose-induced increase in insulin-receptor mRNA levels can be accounted for, at least in part, by posttranscriptional events.