1763-P: ATF6 Drives Glucose-Dependent Beta-Cell Proliferation by Increasing E2F1 Expression and Activity

Introduction and Objective: During elevated insulin demand, beta cells activate the Unfolded Protein Response (UPR); some also undergo proliferation. Activation of the ATF6 arm of the UPR induces beta cell proliferation, but only in high glucose. We aimed to identify the mechanism by which ATF6 increases proliferation, and how beta cells permit ATF6-driven replication in high glucose (HG: 15 mM) but not normal glucose (NG: 7.5 mM).

Methods: To acutely induce ATF6 activation, dispersed mouse and human islet cells were transduced with adenovirus expressing the N terminal domain of ATF6 fused to a destabilized DHFR; stabilizing DHFR with TMP activates ATF6. Beta cell proliferation was quantified by BrdU in insulin+ cells; mRNA or protein levels were estimated by qPCR or Western blot.

Results: ATF6 activation increased Cyclin E1 and Cyclin A2 but reduced Cyclin D2 levels. Surprisingly, ATF6 robustly induced E2F1 mRNA and protein in both NG and HG, even though ATF6 only increased proliferation in HG. E2F1 activity, measured by transcriptional targets Pola1, Mcm5, and Clspn, only increased in HG. pRb, a key inhibitor of E2F1, was phosphorylated (deactivated) only in HG, explaining low E2F1 activity in NG. Overexpressing active CDK4, which phosphorylates pRb, bypassed the requirement for HG and rescued ATF6-driven proliferation in NG in mouse beta cells. In human islet cells, overexpressing CDK6, but not CDK4, synergized with ATF6 activation to robustly increase beta cell proliferation.

Conclusion: Beta cells engage a safety mechanism to avoid inappropriate cell number increase by allowing stress-induced replication only during glucose excess. Here we establish E2F1 as a key ATF6 target driving proliferation and identify CDK4/6-pRb as a glucose sensor that permits ATF6-induced E2F1 to expand beta cell number only when more insulin is needed.