Identification of a pathway by which glucose regulates β-catenin signalling via the cAMP/protein kinase A pathway in β-cell models

Pancreatic β-cells are highly responsive to changes in glucose, but the mechanisms involved are only partially understood. There is increasing evidence that the β-catenin signalling pathway plays an important role in regulating β-cell function, but the mechanisms regulating β-catenin signalling in these cells is not well understood. In the present study we show that β-catenin levels and downstream signalling are regulated by changes in glucose levels in INS-1E and β-TC6-F7 β-cell models. We found a glucose-dependent increase in levels of β-catenin in the cytoplasm and nucleus of INS-1E cells. Expression of cyclin D1 also increased with glucose and required the presence of β-catenin. This was associated with an increase in phosphorylation of β-catenin on Ser⁵⁵², which is known to stabilize the molecule and increase its transcriptional activity. In a search for possible signalling intermediates we found forskolin and cell-permeable cAMP analogues recapitulated the glucose effects, suggesting a role for cAMP and PKA (cAMP-dependent protein kinase/protein kinase A) downstream of glucose. Furthermore, glucose caused sustained increases in cAMP. Two different inhibitors of adenylate cyclase and PKA signalling blocked the effects of glucose, whereas siRNA (small interfering RNA) knockdown of PKA blocked the effects of glucose on β-catenin signalling. Finally, reducing β-catenin levels with either siRNA or pyrvinium impaired glucose- and KCl-stimulated insulin secretion. Taken together the results of the present study define a pathway by which changes in glucose levels can regulate β-catenin using a mechanism which involves cAMP production and the activation of PKA. This identifies a pathway that may be important in glucose-dependent regulation of gene expression and insulin secretion in β-cells.