It is not known how the expression of endogenous Wnt-signaling molecules is regulated in -cells. Therefore, we investigated the effect of antidiabetic drugs and glucose on the expression of Wnt-signaling molecules in -cells. Primary islets were isolated and cultured. The expression of Wnt-signaling molecules (Wnt-4, Wnt-10b, Frizzled-4, LRP5, TCF7L2) and TNF␣ was analyzed by semiquantitative PCR and Western blotting. Transient transfections were carried out and proliferation assays of INS-1 -cells performed using [3 H]thymidine uptake and BrdU ELISA. Insulin secretion was quantified. A knockdown (siRNA) of Wnt-4 in -cells was carried out. Exendin-4 significantly increased the expression of Wnt-4 in -cells on the mRNA level (2.8-fold) and the protein level (3-fold) (P Ͻ 0.001). The effect was dose dependent, with strongest stimulation at 10 nM, and it was maintained after long-term stimulation over 4 wk. Addition of exd-(9 -39), a GLP-1 receptor antagonist, abolished the effect of exendin-4. Treatment with glucose, insulin, or other antidiabetic drugs had no effect on the expression of any of the examined Wnt-signaling molecules. Functionally, Wnt-4 antagonized the activation of canonical Wnt-signaling in -cells. Wnt-4 had no effect on glucosestimulated insulin secretion or insulin gene expression. Knocking down Wnt-4 decreased -cell proliferation to 45% of controls (P Ͻ 0.05). In addition, Wnt-4 and exendin-4 treatment decreased the expression of TNFa␣ mRNA in primary -cells. These data demonstrate that stimulation with exendin-4 increases the expression of Wnt-4 in -cells. Wnt-4 modulates canonical Wnt signaling and acts as regulator of -cell proliferation and inflammatory cytokine release. This suggests a novel mechanism through which GLP-1 can regulate -cell proliferation.glucagon-like peptide-1; diabetes RECENT RESULTS FROM INDEPENDENT GROUPS identified the Wntsignaling pathway as a regulator of -cell proliferation and insulin secretion (3,22,27,28). Interestingly, the Wnt-signaling pathway and the incretin system interact at different stages. On one hand, glucagon-like peptide-1 (GLP-1) receptor agonists activate the canonical Wnt-signaling pathway in -cells and increase -cell proliferation, at least partly, through canonical Wnt-signaling (12). On the other hand, canonical Wnt signaling regulates the expression of GLP-1 in intestinal L cells (16,30).The canonical Wnt-signaling pathway consists of extracellular ligands (Wnts), cell membrane receptors [Frizzled and LDL receptor-related protein (LRP)5/6], and intracellular signaling molecules. In the inactivated state, -catenin is constitutively phosphorylated by GSK-3, which leads to the ubiquitin-mediated degradation of -catenin in the cytoplasm. Ligand binding leads to the inhibition of GSK-3 and stabilization of -catenin. -Catenin is the central player in the canonical Wnt-signaling pathway; as a transcriptional coactivator it translocates to the nucleus and binds to transcription factors of the T cell-specific transcription factor (TC...
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