Endothelial cells regulate ß-catenin activity in adrenocortical cells via secretion of basic fibroblast growth factor.

Endothelial cell-derived products influence the synthesis of aldosterone and cortisol in human adrenocortical cells by modulating proteins such as steroidogenic acute-regulatory (StAR) protein, steroidogenic factor (SF)-1 and CITED2. However, the potential endothelial cell-derived factors that mediate this effect are still unknown. The current study was perfomed to look into the control of ß-catenin activity by endothelial cell-derived factors and to identify a mechanism by which they affect ß-catenin activity in adrenocortical NCIH295R cells. Using reporter gene assays and Western blotting, we found that endothelial cell-conditioned medium (ECCM) led to nuclear translocation of ß-catenin and an increase in ß-catenin-dependent transcription that could be blocked by U0126, an inhibitor of the mitogen-activated protein kinase pathway. Furthermore, we found that a receptor tyrosin kinase (RTK) was involved in ECCM-induced ß-catenin-dependent transcription. Through selective inhibition of RTK using Su5402, it was shown that receptors responding to basic fibroblast growth factor (bFGF) mediate the action of ECCM. Adrenocortical cells treated with bFGF showed a significant greater level of bFGF mRNA. In addition, HUVECs secrete bFGF in a density-dependent manner. In conclusion, the data suggest that endothelial cells regulate ß-catenin activity in adrenocortical cells also via secretion of basic fibroblast growth factor.

Exendin-4 upregulates the expression of Wnt-4, a novel regulator of pancreatic ß-cell proliferation.

The Wnt-signaling pathway regulates ß-cell functions. It is not known how the expression of endogenous Wnt-signaling molecules is regulated in ß-cells. Therefore, we investigated the effect of antidiabetic drugs and glucose on the expression of Wnt-signaling molecules in ß-cells. Primary islets were isolated and cultured. The expression of Wnt-signaling molecules (Wnt-4, Wnt-10b, Frizzled-4, LRP5, TCF7L2) and TNFα was analyzed by semiquantitative PCR and Western blotting. Transient transfections were carried out and proliferation assays of INS-1 ß-cells performed using [(3)H]thymidine uptake and BrdU ELISA. Insulin secretion was quantified. A knockdown (siRNA) of Wnt-4 in ß-cells was carried out. Exendin-4 significantly increased the expression of Wnt-4 in ß-cells on the mRNA level (2.8-fold) and the protein level (3-fold) (P < 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 ß-cells. Wnt-4 had no effect on glucose-stimulated insulin secretion or insulin gene expression. Knocking down Wnt-4 decreased ß-cell proliferation to 45% of controls (P < 0.05). In addition, Wnt-4 and exendin-4 treatment decreased the expression of TNFaα mRNA in primary ß-cells. These data demonstrate that stimulation with exendin-4 increases the expression of Wnt-4 in ß-cells. Wnt-4 modulates canonical Wnt signaling and acts as regulator of ß-cell proliferation and inflammatory cytokine release. This suggests a novel mechanism through which GLP-1 can regulate ß-cell proliferation.