He Lu Mei, Sartori Daniel J, Teta Monica, Opare-Addo Lynn M, Rankin Matthew M, Long Simon Y, Diehl J Alan, Kushner Jake A
Division of Endocrinology and Diabetes, Children's Hospital of Philadelphia, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania 19104, USA.
Mol Endocrinol. 2009 Nov;23(11):1865-75. doi: 10.1210/me.2009-0057. Epub 2009 Jul 23.
The molecular determinants of beta-cell mass expansion remain poorly understood. Cyclin D2 is the major D-type cyclin expressed in beta-cells, essential for adult beta-cell growth. We hypothesized that cyclin D2 could be actively regulated in beta-cells, which could allow mitogenic stimuli to influence beta-cell expansion. Cyclin D2 protein was sharply increased after partial pancreatectomy, but cyclin D2 mRNA was unchanged, suggesting posttranscriptional regulatory mechanisms influence cyclin D2 expression in beta-cells. Consistent with this hypothesis, cyclin D2 protein stability is powerfully regulated in fibroblasts. Threonine 280 of cyclin D2 is phosphorylated, and this residue critically limits D2 stability. We derived transgenic (tg) mice with threonine 280 of cyclin D2 mutated to alanine (T280A) or wild-type cyclin D2 under the control of the insulin promoter. Cyclin D2 T280A protein was expressed at much higher levels than wild-type cyclin D2 protein in beta-cells, despite equivalent expression of tg mRNAs. Cyclin D2 T280A tg mice exhibited a constitutively nuclear cyclin D2 localization in beta-cells, and increased cyclin D2 stability in islets. Interestingly, threonine 280-mutant cyclin D2 tg mice had greatly reduced beta-cell apoptosis, with suppressed expression of proapoptotic genes. Suppressed beta-cell apoptosis in threonine 280-mutant cyclin D2 tg mice resulted in greatly increased beta-cell area in aged mice. Taken together, these data indicate that cyclin D2 is regulated by protein stability in pancreatic beta-cells, that signals that act upon threonine 280 limit cyclin D2 stability in beta-cells, and that threonine 280-mutant cyclin D2 overexpression prolongs beta-cell survival and augments beta-cell mass expansion.
β细胞质量扩张的分子决定因素仍知之甚少。细胞周期蛋白D2是在β细胞中表达的主要D型细胞周期蛋白，对成年β细胞生长至关重要。我们推测细胞周期蛋白D2在β细胞中可能受到积极调控，这可能使有丝分裂刺激影响β细胞扩张。部分胰腺切除术后，细胞周期蛋白D2蛋白急剧增加，但细胞周期蛋白D2 mRNA未发生变化，这表明转录后调控机制影响β细胞中细胞周期蛋白D2的表达。与该假设一致，细胞周期蛋白D2蛋白稳定性在成纤维细胞中受到有力调控。细胞周期蛋白D2的苏氨酸280被磷酸化，该残基严重限制D2稳定性。我们构建了在胰岛素启动子控制下，细胞周期蛋白D2苏氨酸280突变为丙氨酸（T280A）或野生型细胞周期蛋白D2的转基因（tg）小鼠。尽管tg mRNA表达相当，但β细胞中细胞周期蛋白D2 T280A蛋白的表达水平远高于野生型细胞周期蛋白D2蛋白。细胞周期蛋白D2 T280A tg小鼠在β细胞中表现出细胞周期蛋白D2持续定位于细胞核，且胰岛中细胞周期蛋白D2稳定性增加。有趣的是，苏氨酸280突变的细胞周期蛋白D2 tg小鼠的β细胞凋亡大幅减少，促凋亡基因的表达受到抑制。苏氨酸280突变的细胞周期蛋白D2 tg小鼠中β细胞凋亡受抑制导致老年小鼠的β细胞面积大幅增加。综上所述，这些数据表明细胞周期蛋白D2在胰腺β细胞中受蛋白稳定性调控，作用于苏氨酸280的信号限制β细胞中细胞周期蛋白D2的稳定性，且苏氨酸280突变的细胞周期蛋白D2过表达可延长β细胞存活并增强β细胞质量扩张。
