Abstract
Estradiol-17β (E2) and the Foxo1 transcription factor have each been implicated in
the regulation of β-cell proliferation. Interaction between Foxo1and estrogen receptor
alpha (ERα), effecting cell cycle, has been demonstrated in breast cancer cells, but
has not been studied thus far in β-cells. Using human islets and the INS1-E β-cell
line, this study investigated the contribution of Foxo1 to E2-mediated β-cell replication.
Foxo1 expression was knocked down in INS1-E cells using siRNA and Foxo1 activity was
inhibited in human islets with a specific Foxo1 inhibitor (AS1842856). Cells were
treated with E2 and the ERα agonist PPT and evaluated for proliferation by 3[H]-thymidine incorporation and for transcriptional activity through the estrogen
response element by the luciferase assay. As Foxo1 activity is regulated by post-translational
modifications, the effect of E2 on phosphorylation was also assessed. In INS1-E cells,
knock down of Foxo1 abrogated the proliferative response to E2 and PPT. In human islets,
inhibition of Foxo1 abrogated E2-mediated proliferation and attenuated the response
to PPT. Foxo1 knock down and inhibition reduced activity through the estrogen response
element by 25% (p<0.05) and 50% (p<0.01) respectively, in INS1-E cells. E2 increased
Foxo1 phosphorylation in a time dependent manner in INS1-E and human islets (p<0.01,
p<0.05, respectively). These findings suggest that Foxo1 is involved in E2-mediated
proliferation in INS1-E cells and human islets. This may have implications vis-à-vis
variations in circulating endogenous E2 concentrations in diabetes.
Key words
estrogen - diabetes - replication