Stimulation of pancreatic β-cell replication by incretins involves transcriptional induction of cyclin D1 via multiple signalling pathways

Abstract

The incretin hormones, glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic peptide (GIP), have been suggested to act as -cell growth factors and may therefore be of critical importance for the maintenance of a proper -cell mass. We have investigated the molecular mechanism of incretin-induced -cell replication in primary monolayer cultures of newborn rat islet cells. GLP-1, GIP and the long-acting GLP-1 derivative, liraglutide, increased -cell replication 50-80% at 10-100 nM upon a 24 h stimulus, whereas glucagon at a similar concentration had no significant effect. The stimulatory effect of GLP-1 and GIP was efficiently mimicked by the adenylate cyclase activator, forskolin, at 10 nM (90% increase) and was additive (170-250% increase) with the growth response to human growth hormone (hGH), indicating the use of distinct intracellular signalling pathways leading to mitosis by incretins and cytokines, respectively. The response to both GLP-1 and GIP was completely blocked by the protein kinase A (PKA) inhibitor, H89. In addition, the phosphoinositol 3-kinase (PI3K) inhibitor wortmannin and the mitogen-activated protein kinase kinase (MEK) inhibitor PD98059, both inhibited GLP-1-and GIP-stimulated proliferation. The p38 mitogen-activated protein kinase (MAPK) inhibitor, SB203580, had no inhibitory effect on either GLP-1 or GIP stimulated proliferation. Cyclin Ds act as molecular switches for the G0/G1-S phase transition in many cell types and we have previously demonstrated hGH-induced cyclin D2 expression in the insulinoma cell line, INS-1. GLP-1 time-dependently induced the cyclin D1 mRNA and protein levels in INS-1E, whereas the cyclin D2 levels were unaffected. However, minor effect of GLP-1 stimulation was observed on the cyclin D3 mRNA levels. Transient transfection of a cyclin D1 promoter-luciferase reporter construct into islet monolayer cells or INS-1 cells revealed approximately a 2-3 fold increase of transcriptional activity in response to GLP-1 and GIP, and a 4-7 fold increase in response to forskolin. However, treatment of either cell type with hGH had no effect on cyclin D1 promoter activity. The stimulation of the cyclin D1 promoter by GLP-1 was inhibited by H89, wortmannin, and PD98059. We conclude that incretin-induced -cell replication is dependent on cAMP/PKA, p42 MAPK and PI3K activities, which may involve transcriptional induction of cyclin D1. GLP-1, GIP and liraglutide may have the potential to increase -cell replication in humans which would have significant impact on long-term diabetes treatment.

The Akt/FoxO1/p27 pathway mediates the proliferative action of liraglutide in β cells

“…Nevertheless, the underlying mechanisms have not yet been fully understood and await future investigation. It has been recognized that liraglutide notably increases the mitotic index at a maximal stimulatory concentration of 10-100 nM, while 0.1-1.0 nM, liraglutide does not show significant increases in β-cell replication (19). The present study investigated the proproliferative action of liraglutide at final concentrations of 10 and 100 nM and explored the potential mechanisms.…”

Mechanisms of action of glucagon-like peptide 1 in the pancreas

“…Members of E2F family of transcription factors are the effectors that control the G1/S transition and in particular transgenic mice lacking E2F1 display defective insulin secretion in response to a glucose challenge due to inadequate β cell mass and a disregulation in PDX-1 (Fajas et al, 2004). In two separate recent publications GLP-1R activation has been shown to regulate Cyclin D expression in models of β cell growth (Friedrichsen et al, 2006;Kim et al, 2006). Surprisingly both reports demonstrated that GLP-1 or Ex-4 treatment in the INS-1 insulinoma cell line caused significant increases in Cyclin D1 mRNA expression but had little effect on Cyclin D2 expression.…”

“…Friedrichsen and colleagues investigated GLP-1-induced proliferation in monolayers of freshly isolated neonatal rat islets as well as INS-1 cells. Using pharmacological inhibition they showed that this process was PKA-, PI3 kinase-and MEK/ERK-dependent (Friedrichsen et al, 2006). They examined Cyclin D1 expression in GLP-1 (100 nM) treated INS-1 cells at 6 and 12 hr and found it to be increased 100 and 37% respectively above basal levels using qRT-PCR analysis.…”

Glucagon-like Peptide-1 Activation of TCF7L2-dependent Wnt Signaling Enhances Pancreatic Beta Cell Proliferation

“…GLP-1 is reported to stimulate ERK1/2 (p42/44 MAPK) via cAMP and PKA (48) and to activate beta cell replication in an ERK1/2-dependent manner (49). Involvement of p38 MAPK and an atypical protein kinase C isoform, PKC, was demonstrated in GLP-1-induced replication of INS-1 cells (9).…”