The Regulation of Rasd1 Expression by Glucocorticoids and Prolactin Controls Peripartum Maternal Insulin Secretion

Abstract

The transition from gestation to lactation is characterized by a robust adaptation of maternal pancreatic β-cells. Consistent with the loss of β-cell mass, glucose-induced insulin secretion is down-regulated in the islets of early lactating dams. Extensive experimental evidence has demonstrated that the surge of prolactin is responsible for the morphofunctional remodeling of the maternal endocrine pancreas during pregnancy, but the precise molecular mechanisms by which this phenotype is rapidly reversed after delivery are not completely understood. This study investigated whether glucocorticoid-regulated expression of Rasd1/Dexras, a small inhibitory G protein, is involved in this physiological plasticity. Immunofluorescent staining demonstrated that Rasd1 is localized within pancreatic β-cells. Rasd1 expression in insulin-secreting cells was increased by dexamethasone and decreased by prolactin. In vivo data confirmed that Rasd1 expression is decreased in islets from pregnant rats and increased in islets from lactating mothers. Knockdown of Rasd1 abolished the inhibitory effects of dexamethasone on insulin secretion and the protein kinase A, protein kinase C, and ERK1/2 pathways. Chromatin immunoprecipitation experiments revealed that glucocorticoid receptor (GR) and signal transducer and activator of transcription 5b (STAT5b) cooperatively mediate glucocorticoid-induced Rasd1 expression in islets. Prolactin inhibited the stimulatory effect of GR/STAT5b complex on Rasd1 transcription. Overall, our data indicate that the stimulation of Rasd1 expression by glucocorticoid at the end of pregnancy reverses the increased insulin secretion that occurs during pregnancy. Prolactin negatively regulates this pathway by inhibiting GR/STAT5b transcriptional activity on the Rasd1 gene.

Argonaute2 Regulates the Pancreatic β-Cell Secretome

“…Of note, recent studies on Elavl4/HuD describe its role in regulating translation of the insulin mRNA, and transgenic overexpression in mice resulted in impaired insulin release (40). Furthermore, inhibition of Rasd1 was shown to result in increased insulin release in MIN6 cells (41). Taken together, these results indicate targets of miR-375 can either positively or negatively influence secretion, and therefore only after the identification of all targets can we reach an understanding of the complexity of its role in the ␤-cell.…”

Activators of G Protein Signaling Exhibit Broad Functionality and Define a Distinct Core Signaling Triad

“…Both AGS1 and Rhes have an extended carboxyl terminus as compared with Ras family proteins, and both proteins interact with G i /G o and regulate G protein signaling (Cismowski et al, , 2000Graham et al, 2002Graham et al, , 2004Takesono et al, 2002;Vargiu et al, 2004;Nguyen and Watts, 2005;Harrison and He, 2011). AGS1 has a range of functional roles, including inhibition of cell growth, N-methyl-D-aspartate receptor signaling, regulation of the circadian rhythm, and regulation of hormone secretion (Fang et al, 2000;Graham et al, 2001;Jaffrey et al, 2002;Takahashi et al, 2003;Cheng et al, 2004;Vaidyanathan et al, 2004;Lellis-Santos et al, 2012;McGrath et al, 2012;Chen et al, 2013;Harrison et al, 2013). Rhes …”

The Roles of Rasd1 small G proteins and leptin in the activation of TRPC4 transient receptor potential channels

“…Another study reported that dexamethasone increased the protein expression of Rasd1 in pancreatic b-cells and diminished insulin secretion. 35 These studies indicated that Rasd1 and TRPC4 could be common molecular candidates in controlling insulin secretion in b-cells (Fig. 4).…”

“…In pancreatic b-cell, dexamethasone decreased insulin secretion while mRNA level of Rasd1 was shown to be increased. 35 The upregulation of Rasd1 expression by glucocorticoids was due to glucocorticoid response element (GRE) in the 3'-flanking region of the human Rasd1 gene. 36 Rasd1 has all of the conserved domains of the Ras superfamily required for guanine nucleotide binding, hydrolysis, and effector interaction.…”

“…Also, inhibition of Rasd1 with small interfering RNA (siRNA) blunted dexamethasone-induced TRPC4 current in INS-1 cells, indicating that dexamethasone activates TRPC4 via Rasd1 in INS-1 cells. Though Rasd1 is functionally classified as a guanine nucleotide exchange factor (GEF) for Ga i proteins, Lellis-Santos et al 35 suggested that dexamethasone induced a rapid and sustained increase in Rasd1 mRNA expression in MIN6 cells (3.5-and fold15-compared with control after Figure 4. Schematic diagram for the signaling pathway involved in dexamethasone-induced TRPC4 channel activity.…”