Minella Alex C, Loeb Keith R, Knecht Andrea, Welcker Markus, Varnum-Finney Barbara J, Bernstein Irwin D, Roberts James M, Clurman Bruce E
Division of Clinical Research, Fred Hutchinson Cancer Research Center, Seattle, WA 98109, USA.
Genes Dev. 2008 Jun 15;22(12):1677-89. doi: 10.1101/gad.1650208.
Phosphorylations within N- and C-terminal degrons independently control the binding of cyclin E to the SCF(Fbw7) and thus its ubiquitination and proteasomal degradation. We have now determined the physiologic significance of cyclin E degradation by this pathway. We describe the construction of a knockin mouse in which both degrons were mutated by threonine to alanine substitutions (cyclin E(T74A T393A)) and report that ablation of both degrons abolished regulation of cyclin E by Fbw7. The cyclin E(T74A T393A) mutation disrupted cyclin E periodicity and caused cyclin E to continuously accumulate as cells reentered the cell cycle from quiescence. In vivo, the cyclin E(T74A T393A) mutation greatly increased cyclin E activity and caused proliferative anomalies. Cyclin E(T74A T393A) mice exhibited abnormal erythropoiesis characterized by a large expansion of abnormally proliferating progenitors, impaired differentiation, dysplasia, and anemia. This syndrome recapitulates many features of early stage human refractory anemia/myelodysplastic syndrome, including ineffective erythropoiesis. Epithelial cells also proliferated abnormally in cyclin E knockin mice, and the cyclin E(T74A T393A) mutation delayed mammary gland involution, implicating cyclin E degradation in this anti-mitogenic response. Hyperproliferative mammary epithelia contained increased apoptotic cells, suggesting that apoptosis contributes to tissue homeostasis in the setting of cyclin E deregulation. Overall these data show the critical role of both degrons in regulating cyclin E activity and reveal that complete loss of Fbw7-mediated cyclin E degradation causes spontaneous and cell type-specific proliferative anomalies.
N 端和 C 端降解结构域内的磷酸化分别独立控制细胞周期蛋白 E 与 SCF（Fbw7）的结合，进而控制其泛素化和蛋白酶体降解。我们现已确定了该途径介导的细胞周期蛋白 E 降解的生理意义。我们描述了一种敲入小鼠的构建，其中两个降解结构域的苏氨酸均被丙氨酸替代而发生突变（细胞周期蛋白 E（T74A T393A）），并报告两个降解结构域的缺失消除了 Fbw7 对细胞周期蛋白 E 的调控。细胞周期蛋白 E（T74A T393A）突变破坏了细胞周期蛋白 E 的周期性，导致细胞从静止状态重新进入细胞周期时细胞周期蛋白 E 持续积累。在体内，细胞周期蛋白 E（T74A T393A）突变极大地增加了细胞周期蛋白 E 的活性并导致增殖异常。细胞周期蛋白 E（T74A T393A）小鼠表现出异常的红细胞生成，其特征为异常增殖的祖细胞大量扩增、分化受损、发育异常和贫血。该综合征概括了人类早期难治性贫血/骨髓增生异常综合征的许多特征，包括无效的红细胞生成。在细胞周期蛋白 E 敲入小鼠中，上皮细胞也出现异常增殖，并且细胞周期蛋白 E（T74A T393A）突变延迟了乳腺退化，这表明细胞周期蛋白 E 降解参与了这种抗有丝分裂反应。过度增殖的乳腺上皮细胞中凋亡细胞增加，这表明在细胞周期蛋白 E 失调的情况下，凋亡有助于维持组织稳态。总体而言，这些数据表明两个降解结构域在调节细胞周期蛋白 E 活性方面的关键作用，并揭示 Fbw7 介导细胞周期蛋白 E 降解的完全丧失会导致自发的、细胞类型特异性的增殖异常。
