Two distinct signaling pathways activated by activin A in glucose-responsive pancreatic beta-cell lines

In vitro differentiation potential of human haematopoietic CD34(+) cells towards pancreatic β-cells

Haematopoietic stem cells (HSCs) possess multipotent ability to differentiate into various types of cells on providing appropriate niche. In the present study, the differentiating potential of human HSCs into β-cells of islets of langerhans was explored. Human HSCs were apheretically isolated from a donor and cultured. Phenotypic characterization of CD34 glycoprotein in the growing monolayer HSCs was confirmed by immunocytochemistry and flow cytometry techniques. HSCs were induced by selection with beta cell differentiating medium (BDM), which consists of epidermal growth factor (EGF), fibroblast growth factor (FGF), transferrin, Triiodo-l-Tyronine, nicotinamide and activin A. Distinct morphological changes of differentiated cells were observed on staining with dithizone (DTZ) and expression of PDX1, insulin and synaptophysin was confirmed by immunocytochemistry. Quantitative real-time polymerase chain reaction (qRT-PCR) analysis revealed distinct expression of specific β-cell markers, pancreatic and duodenal homeobox-1 (PDX1), glucose transporter-2 (GLUT-2), synaptophysin (SYP) and insulin (INS) in these differentiated cells compared to HSCs. Further, these cells exhibited elevated expression of INS gene at 10 mM glucose upon inducing with different glucose concentrations. The prominent feature of the obtained β-cells was the presence of glucose sensors, which was determined by glucokinase activity and high glucokinase activity compared with CD34(+) stem cells. These findings illustrate the differentiation of CD34(+) HSCs into β-cells of islets of langerhans.

The Genetic Basis of the Polycystic Ovary Syndrome: A Literature Review Including Discussion of PPAR-gamma

Polycystic ovary syndrome (PCOS) is the most common endocrine disorder of the women of reproductive age. Familial clustering of PCOS has been consistently reported suggesting that genetic factors play a role in the development of the syndrome although PCOS cases do not exhibit a clear pattern of Mendelian inheritance. It is now well established that PCOS represents a complex trait similar to type-2 diabetes and obesity, and that both inherited and environmental factors contribute to the PCOS pathogenesis. A large number of functional candidate genes have been tested for association or linkage with PCOS phenotypes with more negative than positive findings. Lack of universally accepted diagnostic criteria, difficulties in the assignment of male phenotype, obscurity in the mode of inheritance, and particularly small sample size of the study populations appear to be major limitations for the genetic studies of PCOS. In the near future, utilizing the genome-wide scan approach and the HapMap project will provide a stronger potential for the genetic analysis of the syndrome.

Activin receptor-like kinase and the insulin gene

The biological responses of the transforming growth factor-β (TGF-β) superfamily, which includes Activins and Nodal, are induced by activation of a receptor complex and Smads. A type I receptor, which is a component of the complex, is known as an activin receptor-like kinase (ALK); currently seven ALKs (ALK1-ALK7) have been identified in humans. Activins signaling, which is mediated by ALK4 and 7 together with ActRIIA and IIB, plays a critical role in glucose-stimulated insulin secretion, development/neogenesis, and glucose homeostatic control of pancreatic endocrine cells; the insulin gene is regulated by these signaling pathways via ALK7, which is a receptor for Activins AB and B and Nodal. This review discusses signal transduction of ALKs in pancreatic endocrine cells and the role of ALKs in insulin gene regulation.

Role of follistatin in promoting adipogenesis in women

CONTEXT

Follistatin is a glycoprotein that binds and neutralizes biological activities of TGFbeta superfamily members including activin and myostatin. We previously identified by expression profiling that follistatin levels in white adipose tissue (WAT) were regulated by obesity.

OBJECTIVE

The objective of the study was to elucidate the role of follistatin in human WAT and obesity.

DESIGN

We measured secreted follistatin protein from WAT biopsies and fat cells in vitro. We also quantified follistatin mRNA expression in sc and visceral WAT and in WAT-fractionated cells and related it to obesity status, body region, and cellular origin. We investigated the effects of follistatin on adipocyte differentiation of progenitor cells in vitro.

PARTICIPANTS

Women (n = 66) with a wide variation in body mass index were recruited by advertisement and from a clinic for weight-reduction therapy.

RESULTS

WAT secreted follistatin in vitro. Follistatin mRNA levels in sc but not visceral WAT were decreased in obesity and restored to nonobese levels after weight reduction. Follistatin mRNA levels were high in the stroma-vascular fraction of WAT and low in adipocytes. Recombinant follistatin treatment promoted adipogenic differentiation of progenitor cells and neutralized the inhibitory action of myostatin on differentiation in vitro. Moreover, activin and myostatin signaling receptors were detected in WAT and adipocytes.

CONCLUSION

Follistatin is a new adipokine important for adipogenesis. Down-regulated WAT expression of follistatin in obesity may counteract adiposity but could, by inhibiting adipogenesis, contribute to hypertrophic obesity (large fat cells) and insulin resistance.

Activin receptor-like kinase 7 induces apoptosis of pancreatic beta cells and beta cell lines

AIMS/HYPOTHESIS

Activin receptor-like kinase 7 (ALK7), a member of the type I receptor serine/threonine kinases of the TGF-beta superfamily, was recently reported to regulate cell proliferation and apoptosis. We hypothesised that ALK7 may play a role in modulating pancreatic beta cell proliferation and/or apoptosis.

METHODS

We detected ALK7 expression in beta cells using RT-PCR, immunostaining and western blotting. Constitutively active, dominant negative or wild-type ALK7 was introduced into beta cells using adenoviral delivery. Proliferation was assessed using (3)H-thymidine incorporation and apoptosis was quantified using terminal deoxynucleotidyl transferase biotin-dUTP nick end labelling detection, DNA degradation analysis and caspase-3 assays.

RESULTS

Induction of constitutively active ALK7 in beta cells resulted in growth inhibition and enhanced apoptosis; no effect was seen with INS-1 cells expressing wild-type or dominant negative ALK7. Elevated glucose concentrations and fatty acid (palmitate) markedly increased expression levels of ALK7 transcripts and proteins in INS-1 and rat islets and increased beta cell apoptosis. Activation of ALK7 increased Smad2 phosphorylation, reduced protein kinase B (Akt) kinase activity and was associated with increased levels of the bioactive forms of caspase-3, whereas co-expression of constitutively active ALK7 with dominant negative Smad2 or constitutively active Akt significantly diminished ALK7-induced growth inhibition and apoptosis in INS-1 cells. Although overexpression of constitutively active Akt significantly reduced ALK7-induced growth inhibition and ALK7-enhanced beta cell apoptosis, ALK7-stimulated Smad2 phosphorylation was not affected.

CONCLUSIONS/INTERPRETATION

These results suggest that the pancreatic beta cell apoptosis induced by ALK7 activation occurs via the activation of two distinct downstream pathways: the suppression of Akt activation and the activation of the Smad2-caspase-3 cascade.

The molecular-genetic basis of functional hyperandrogenism and the polycystic ovary syndrome

The genetic mechanisms underlying functional hyperandrogenism and the polycystic ovary syndrome (PCOS) remain largely unknown. Given the large number of genetic variants found in association with these disorders, the emerging picture is that of a complex multigenic trait in which environmental influences play an important role in the expression of the hyperandrogenic phenotype. Among others, genomic variants in genes related to the regulation of androgen biosynthesis and function, insulin resistance, and the metabolic syndrome, and proinflammatory genotypes may be involved in the genetic predisposition to functional hyperandrogenism and PCOS. The elucidation of the molecular genetic basis of these disorders has been burdened by the heterogeneity in the diagnostic criteria used to define PCOS, the limited sample size of the studies conducted to date, and the lack of precision in the identification of ethnic and environmental factors that trigger the development of hyperandrogenic disorders. Progress in this area requires adequately sized multicenter collaborative studies after standardization of the diagnostic criteria used to classify hyperandrogenic patients, in whom modifying environmental factors such as ethnicity, diet, and lifestyle are identified with precision. In addition to classic molecular genetic techniques such as linkage analysis in the form of a whole-genome scan and large case-control studies, promising genomic and proteomic approaches will be paramount to our understanding of the pathogenesis of functional hyperandrogenism and PCOS, allowing a more precise prevention, diagnosis, and treatment of these prevalent disorders.

Follistatin and activin A serum concentrations in obese and non-obese patients with polycystic ovary syndrome

BACKGROUND

Activin promotes ovarian follicular development, inhibits androgen production and increases FSH and insulin secretion. Follistatin, an activin-binding protein, neutralizes activin bioactivity. Therefore, a decrease in the ratio of activin/follistatin might encourage characteristic features of polycystic ovary syndrome (PCOS). We investigated whether women with PCOS showed disordered follistatin and/or activin serum concentrations.

METHODS

The study group included 24 obese and 20 non-obese (body mass index vertical line and <27 kg/m2 respectively) clomiphene-failure PCOS patients. The control group included 16 obese and 46 non-obese patients with normal ovulatory cycles. Blood samples were obtained from the patients on day 3-5 of a progesterone-induced or spontaneous cycle and were assayed for LH, FSH, testosterone, 17-hydroxy-progesterone, androstenedione, follistatin, activin A, fasting glucose and insulin.

RESULTS

Follistatin concentrations were comparable between obese and non-obese PCOS patients (mean +/- SE; 1171 +/- 103 and 1045 +/- 159 pg/ml respectively) and significantly higher than their respective controls (628 +/- 61 and 592 +/- 49 pg/ml, P < 0.0001 and P < 0.02 respectively). Activin A concentrations were comparable between the four groups (590 +/- 35, 513 +/- 74, 661 +/- 87 and 595 +/- 43 pg/ml in obese and non-obese PCOS and controls respectively). Stepwise regression analyses for relationships between follistatin or activin A levels and all other variables indicated that follistatin was significantly and independently positively affected by PCOS (P < 0.0001), age (P < 0.02), androstenedione (P < 0.03) and weight (P < 0.05). Activin A was significantly and independently negatively affected by PCOS (P < 0.003) and FSH (P < 0.03), and positively affected by weight (P < 0.009) and androstenedione (P < 0.02).

CONCLUSIONS

Serum follistatin is increased in PCOS patients, regardless of obesity. PCOS is the most significant variable that relates to high follistatin and low activin A serum concentration. A high follistatin/activin ratio could well contribute to the pathophysiology of PCOS.

Activin A stimulates insulin secretion in cultured human pancreatic islets

Activin A is a dimeric glycoprotein showing a high sequence homology with transforming growth factor-beta (TGF-beta) and playing autocrine/paracrine actions in reproductive tissues. However, since the synthesis of activin is ubiquitous it may have a role in regulating cell growth and differentiation in several tissues. Previous studies showed that activin A is expressed by insulin-positive B cells of human pancreatic islets, and women with gestational diabetes have higher serum activin A levels than healthy pregnant women at the same gestational age. The present study aimed to evaluate the effect of activin A on insulin secretion from cultured human pancreatic islets. With this purpose human pancreatic islets were incubated with varying concentrations of activin A (0.1 to 10.0 nM). In absence of glucose, activin A did not modify insulin secretion at the different concentrations used. In absence of activin A, 8.3 mM and 16.7 mM glucose significantly increased insulin secretion, with a dose-dependent pattern. In presence of a non stimulatory concentration of glucose (3.3 mM), activin A significantly increased insulin secretion starting from low concentration (0.1 nM). Furthermore, the addition of activin A to 8.3 mM and 16.7 mM glucose induced an additional effect of the dose-dependent glucose-mediated insulin secretion (p<0.001). The present data could support a role for activin A in human endocrine pancreas in modulating insulin response to different glucose concentrations.

Thirty-seven candidate genes for polycystic ovary syndrome: strongest evidence for linkage is with follistatin

Polycystic ovary syndrome (PCOS) is a common endocrine disorder of women, characterized by hyperandrogenism and chronic anovulation. It is a leading cause of female infertility and is associated with polycystic ovaries, hirsutism, obesity, and insulin resistance. We tested a carefully chosen collection of 37 candidate genes for linkage and association with PCOS or hyperandrogenemia in data from 150 families. The strongest evidence for linkage was with the follistatin gene, for which affected sisters showed increased identity by descent (72%; chi(2) = 12.97; nominal P = 3.2 x 10(-4)). After correction for multiple testing (33 tests), the follistatin findings were still highly significant (P(c) = 0.01). Although the linkage results for CYP11A were also nominally significant (P = 0.02), they were no longer significant after correction. In 11 candidate gene regions, at least one allele showed nominally significant evidence for population association with PCOS in the transmission/disequilibrium test (chi(2) >/= 3.84; nominal P < 0.05). The strongest effect in the transmission/disequilibrium test was observed in the INSR region (D19S884; allele 5; chi(2) = 8.53) but was not significant after correction. Our study shows how a systematic screen of candidate genes can provide strong evidence for genetic linkage in complex diseases and can identify those genes that should have high (or low) priority for further study.