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...

The endothelium releases factors stimulating the adrenal cortex. It is also known that endothelin-1 (ET-1) promotes generation of cortisol and aldosterone, and proliferation of adrenocortical cells. The aim of the study was to find out whether the effect of the endothelium on adrenocortical cells is dominated by the action of ET-1. The effects of endothelial cell-conditioned medium (ECCM), obtained during growth of human umbilical cord vein endothelial cells, on aldosterone and cortisol release by cells of the adrenocortical cancer cell-line NCI-H295R and the promoter activity of steroidogenic acute-regulatory protein (StAR) were studied. The effect of ECCM on proliferation of human primary normal adrenocortical and NCI-H295R cells was also investigated. Concentration-dependent increases in cortisol release that reached 192.7 ± 62.8 in percent of basal secretion, in aldosterone release that reached 188.2 ± 52.3 in percent of basal secretion, and in proliferation after stimulation with ECCM at concentrations of 10-50% were found. ECCM significantly activated the StAR promoter 3-fold in NCI-H295R cells if the ECCM was not pretreated with pronase. These effects of the endothelium were not reversed after co-incubation with endothelin receptor antagonists and could not be mimicked by incubation with endothelin-1. In conclusion, the cultured endothelial cells secrete a protein that stimulates steroidogenesis in adrenal cells and their growth. It was also shown that the ET-1 does not mediate the effect of ECCM on the NCI-H295R cell line.

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