Gong J, Traganos F, Darzynkiewicz Z
Cancer Research Institute, New York Medical College, Valhalla, USA.
Cell Prolif. 1995 Jun;28(6):337-46. doi: 10.1111/j.1365-2184.1995.tb00075.x.
Complexes of cyclin-dependent kinases (cdk) and their partner cyclins drive the cell through the cell cycle, each such complex phosphorylating a distinct set of proteins at a particular check-point or phase of the cycle. Immunocytochemical detection of cyclins combined with measurement of cellular DNA content by flow cytometry makes it possible to relate expression of each of these proteins with the actual cell cycle position, without the necessity of cell synchronization. In the present study, we have investigated expression of E and D type cyclins in G1 cells and in cells entering S phase, in eight different human hematopoietic and solid tumour cell lines (two leukaemias, a lymphoma, three breast carcinomas, a colon carcinoma and a bladder transitional cell carcinoma) during their exponential phase of growth, as well as in normal mitogen stimulated lymphocytes. In all the cell types studied, the average level of D type cyclin expression was invariable throughout the cell cycle. A great intercellular variability, in particular of the G1 cell subpopulations, and the presence of a large fraction of G1, S and G2 + M cells that were cyclin D negative (20-40% in tumour cell lines and about 80% among lymphocytes), were other characteristic features of D type cyclin expression. In contrast to D type cyclins, the expression of cyclin E was discontinuous during the cycle, peaking at the time of cell entrance to S. Also, a well defined threshold in expression of cyclin E characterized cells that were entering S phase, and virtually no cyclin E negative cells were seen during the early portion of S phase. The data indicate that while cell entrance to S phase is unrelated to expression of D type cyclins (at the time of entrance), accumulation of cyclin E up to critical level is a prerequisite for initiation of DNA replication. The great intercellular variability in expression of D type cyclins and their invariant average level across the cell cycle suggest that these cyclins, in addition to their acknowledged function in promoting cell progression through mid- to late-G1 may have other role(s), related or unrelated to the cell cycle progression. The presence of a large number of D type cyclin negative cells in all phases of the cycle suggests that during exponential growth the cells may not express this protein and yet may traverse the cycle, including G1 phase.
细胞周期蛋白依赖性激酶（cdk）与其伴侣细胞周期蛋白形成的复合物驱动细胞通过细胞周期，每一种这样的复合物在细胞周期的特定检查点或阶段磷酸化一组不同的蛋白质。通过免疫细胞化学检测细胞周期蛋白，并结合流式细胞术测量细胞DNA含量，就可以将这些蛋白质中的每一种的表达与细胞的实际细胞周期位置联系起来，而无需细胞同步化。在本研究中，我们调查了八种不同的人类造血和实体瘤细胞系（两种白血病、一种淋巴瘤、三种乳腺癌、一种结肠癌和一种膀胱移行细胞癌）在指数生长期以及正常有丝分裂原刺激的淋巴细胞中，G1期细胞和进入S期的细胞中E型和D型细胞周期蛋白的表达情况。在所有研究的细胞类型中，D型细胞周期蛋白表达的平均水平在整个细胞周期中是不变的。D型细胞周期蛋白表达的另一个特征是细胞间存在很大的变异性，特别是G1细胞亚群，并且存在很大一部分G1、S和G2+M期细胞是细胞周期蛋白D阴性的（肿瘤细胞系中为20-40%，淋巴细胞中约为80%）。与D型细胞周期蛋白相反，细胞周期蛋白E的表达在周期中是不连续的，在细胞进入S期时达到峰值。此外，细胞周期蛋白E表达的一个明确阈值表征了进入S期的细胞，并且在S期早期几乎看不到细胞周期蛋白E阴性的细胞。数据表明，虽然细胞进入S期与D型细胞周期蛋白的表达（在进入时）无关，但细胞周期蛋白E积累到临界水平是启动DNA复制的先决条件。D型细胞周期蛋白表达的细胞间巨大变异性及其在整个细胞周期中不变的平均水平表明，这些细胞周期蛋白除了在促进细胞通过G1中期到后期进程中公认的功能外，可能还有其他与细胞周期进程相关或不相关的作用。在周期的所有阶段都存在大量D型细胞周期蛋白阴性细胞，这表明在指数生长期间，细胞可能不表达这种蛋白质，但仍可能经历包括G1期在内的整个周期。
