Cell Cycle Regulators in Pancreatic Development and Disease

糖尿病患者产生胰岛素的胰岛细胞被破坏、严重衰竭或功能受损。因此，替代功能性β细胞团将促进糖尿病的临床管理。我们之前已经证明了Cdk4在调节β细胞质量中的重要性。cdk4缺陷小鼠表现为β细胞发育不全并发展为糖尿病，而表达活性Cdk4R24C激酶的小鼠则表现为β细胞增生。虽然β细胞复制似乎是导致β细胞质量增加的主要机制，但大量证据也支持胰腺导管上皮在生成新β细胞方面的贡献。我们得出结论，Cdk4不仅促进β细胞复制，还促进导管上皮中β细胞祖细胞的激活。此外，我们发现Cdk4通过招募静止细胞进入细胞周期来控制β细胞质量。我们发现Cdk4及其下游转录因子E2F1在胰腺二次转化之前和期间调节胰腺发育。由于间质发育受损和Pdx1+胰腺祖细胞数量限制，Cdk4缺失导致胚胎胰腺大小减小。有趣的是，活化Cdk4R24C激酶的表达导致Nkx2.2+和Nkx6.1+细胞增加，Ngn3+内分泌前体细胞的数量和增殖增加，导致细胞系扩增。此外，我们发现E2F1结合并激活Ngn3启动子，从而以依赖cdk4的方式调节胚胎胰腺中Ngn3的表达水平。这些结果表明，Cdk4通过其调节胰腺祖细胞和内分泌前体的增殖潜能的能力，是早期胰腺发育的重要调节因子。

Insulin-producing pancreatic islet beta cells are destroyed, severely depleted or functionally impaired in diabetes. Therefore, replacing functional beta cell mass would advance clinical diabetes management. We have previously demonstrated the importance of Cdk4 in regulating beta cell mass. Cdk4-deficient mice display beta cell hypoplasia and develop diabetes, whereas beta cell hyperplasia is observed in mice expressing an active Cdk4R24C kinase. While beta cell replication appears to be the primary mechanism responsible for beta cell mass increase, considerable evidence also supports a contribution from the pancreatic ductal epithelium in generation of new beta cells. We conclude that Cdk4 not only promotes beta cell replication, but also facilitates the activation of beta cell progenitors in the ductal epithelium. In addition, we show that Cdk4 controls beta cell mass by recruiting quiescent cells to enter the cell cycle. We show that Cdk4 and its downstream transcription factor E2F1 regulate pancreas development prior to and during the secondary transition. Deficiency of Cdk4 results in reduced embryonic pancreas size due to impaired mesenchyme development and limitation of the number of Pdx1+ pancreatic progenitor cells. Interestingly, expression of activated Cdk4R24C kinase leads to increased Nkx2.2+ and Nkx6.1+ cells and a rise in the number and proliferation of Ngn3+ endocrine precursor cells resulting in expansion of the cell lineage. Further, we show that E2F1 binds and activates the Ngn3 promoter thereby modulating Ngn3 expression levels in the embryonic pancreas in a Cdk4-dependent manner. These results identify Cdk4 as an important regulator of early pancreas development by virtue of its ability to modulate the proliferation potential of pancreatic progenitors and endocrine precursors. We have begun a systematic analyses of other cell cycle regulators in pancreas development and function. We have recently uncovered an important role for CDK2 in beta cell function and post-natal beta cell proliferation. Cdk2 loss results in loss of beta cell function. Further, age associated reduction in beta cell mass is seen upon Cdk2 deficiency. Also, we observe that metabolic stress further accentuates the defecst due to loss of Cdk2 by further deterioration of beta cell mass and function. We are presently investigating the mechanisms that underlie these findings. We are also studying the interaction of endocrine pancreas and other organs.