Synergistic effect of glucose and prolactin on GLUT2 expression in cultured neonatal rat islets

Abstract

We studied the synergistic effect of glucose and prolactin (PRL) on insulin secretion and GLUT2 expression in cultured neonatal rat islets. After 7 days in culture, basal insulin secretion (2.8 mM glucose) was similar in control and PRL-treated islets (1.84 ± 0.06% and 2.08 ± 0.07% of the islet insulin content, respectively). At 5.6 and 22 mM glucose, insulin secretion was significantly higher in PRL-treated than in control islets, achieving 1.38 ± 0.15% and 3.09 ± 0.21% of the islet insulin content in control and 2.43 ± 0.16% and 4.31 ± 0.24% of the islet insulin content in PRL-treated islets, respectively. The expression of the glucose transporter GLUT2 in B-cell membranes was dosedependently increased by exposure of the islet to increasing glucose concentrations. This effect was potentiated in islets cultured for 7 days in the presence of 2 µg/ml PRL. At 5.6 and 10 mM glucose, the increase in GLUT2 expression in PRL-treated islets was 75% and 150% higher than that registered in the respective control. The data presented here indicate that insulin secretion, induced by different concentrations of glucose, correlates well with the expression of the B-cell-specific glucose transporter GLUT2 in pancreatic islets. Key wordsThe first step in glucose-induced insulin secretion is the entry of the sugar into the Bcells, which is mediated by the glucose transporter GLUT2, located on the B-cell plasma membrane. Alteration in the expression of GLUT2 has been implicated in the reduction of the secretory response to glucose (1,2). Since growth and differentiation of the endocrine pancreas are controlled by glucose and the somatolactogenic hormones, growth hormone (GH) and prolactin (PRL) (3), we determined the effect of glucose on insulin secretion and GLUT2 expression in neonatal rat islets cultured in the presence or absence of PRL.We have used islets from neonatal rats (2 to 48 h old) obtained by collagenase digestion and cultured for 7 days, as described previously (4). After 7 days, the culture medium was discarded and the islets were incubated for 90 min at 37 o C in a bicarbonatebuffered solution containing different concentrations of glucose. The supernatant was withdrawn for insulin measurements and the

Upregulation of the expression of tight and adherens junction-associated proteins during maturation of neonatal pancreatic islets in vitro

“…In agreement with previous reports (Boschero et al 1988, Crepaldi-Alves et al 1997, culture of neonatal rat islets induces significant increase in insulin release into the culture medium. Islets cultured for 3 days (0.46 ± 0.06 (n ¼ 17 wells; 8-10 islets per well) ng insulin/islet/h; P < 0.05) or 7 days (1.030 ± 0.17 (n ¼ 16 wells; 8-10 islets per well) ng insulin/islet/h; P < 0.001) showed an approximately 2-fold and 3-fold increase in insulin release compared to 24 h-cultured islets (0.26 ± 0.02 (n ¼ 17 wells; 8-10 islets per well) ng insulin/islet/h).…”

“…accordance with our work, numerous studies have shown that fetal and neonatal islets, when cultured for a certain period of time, display an increased insulin response to suprastimulatory concentrations of glucose as well as to other secretagogues (Shimizu et al 1983, Dudek et al 1984, Freinkel et al 1984, Masaki et al 1987, Boschero et al 1993, Yderstraede & Flindt-Egebak 1995, Collares-Buzato et al 2001. Such process of in vitro maturation is even enhanced by exposure to prolactin (Boschero et al 1993, Crepaldi-Alves et al 1997, CollaresBuzato et al 2001. It has been suggested that the gradual increase in insulin secretory capacity of islets and fetal pancreata in vitro can not only be explained by islet hyperplasia but involves the maturation of critical steps of stimulus-secretion coupling process in the B-cell (Dudek et al 1984, Freinkel et al 1984, Boschero et al 1993, Yderstraede & Flindt-Egebak 1995, CrepaldiAlves et al 1997.…”

“…Neonatal B-cells have an impaired insulin secretion compared to that of islets from adult rats (Boschero et al 1988(Boschero et al , 1993. The maintenance of neonatal islets in culture for prolonged periods results in a time-dependent improvement in the glucose-induced secretory response through a mechanism that is not completely understood (Boschero et al 1993, Crepaldi-Alves et al 1997, Collares-Buzato et al 2001. Thus, cultured neonatal islets provide a useful model for studying the cellular processes underlying the maturation of the pancreatic secretory machine.…”

Glucose Abnormalities Associated to Prolactin Secreting Pituitary Adenomas

“…In neonatal rat islets PRL treatment enhances islet insulin content and early insulin secretion (54–56), also increasing islet sensitivity to glucose (53). This effect may be partly explained by the increased β-cell and liver glucose transporter GLUT2 in membrane as seen in cultured neonatal rat islets (57, 58). In transgenic female mice with hyperprolactinemia, hyperandrogenism, and hyperprogesteronemia induced by overexpression of the human chorionic gonadotropin β-subunit, a 1-week treatment with cabergoline has been found to significantly reduce body weight and to improve dyslipidemia and insulin resistance up to complete normalization of triglycerides and insulin, besides PRL levels (61).…”