Walter Kevin A, Hossain Mir Ahamed, Luddy Carey, Goel Nidhi, Reznik Thomas E, Laterra John
Department of Neurosurgery, The Johns Hopkins University School of Medicine, Baltimore, Maryland 21205, USA.
Mol Cell Biol. 2002 Apr;22(8):2703-15. doi: 10.1128/MCB.22.8.2703-2715.2002.
Scatter factor/hepatocyte growth factor (SF/HGF) expression has been linked to malignant progression in glial neoplasms. Using two glioma cell lines, U373MG and SNB-19, we have demonstrated that SF/HGF stimulation allows cells to escape G(1)/G(0) arrest induced by contact inhibition or serum withdrawal. SF/HGF induced effects on two mechanisms of cell cycle regulation: suppression of the cyclin-dependent kinase inhibitor p27 and induction of the transcription factor c-Myc. Regulation of p27 by SF/HGF was posttranslational and is associated with p27 nuclear export. Transient transfections of U373MG and SNB-19 with wild-type p27 and a degradation-resistant p27T187A mutant were insufficient to induce cell cycle arrest, and SF/HGF downregulation of p27 was not necessary for cell cycle reentry. Analysis of Cdk2 kinase activity and p27 binding to cyclin E complexes in the presence of exogenous wild-type p27 or p27T187A demonstrated that Cdk2 activity was not necessary for SF/HGF-mediated G(1)/S transition. Similarly, overexpression of dominant-negative forms of Cdk2 did not block SF/HGF-triggered cell cycle progression. In contrast, SF/HGF transcriptionally upregulated c-Myc, and overexpression of c-Myc was able to prevent G(1)/G(0) arrest in the absence of SF/HGF. Transient overexpression of MadMyc, a dominant-negative chimera for c-Myc, caused G(1)/G(0) arrest in logarithmically growing cells and blocked SF/HGF-mediated G(1)/S transition. c-Myc did not exert its effects through p27 downregulation in these cell lines. SF/HGF induced E2F1-dependent transcription, the inhibition of which did not block SF/HGF-induced cell cycle progression. We conclude that SF/HGF prevents G(1)/G(0) arrest in glioma cell lines by a c-myc-dependent mechanism that is independent of p27, Cdk2, or E2F1.
分散因子/肝细胞生长因子（SF/HGF）的表达与神经胶质瘤的恶性进展有关。利用两种胶质瘤细胞系U373MG和SNB-19，我们证明了SF/HGF刺激可使细胞逃避接触抑制或血清剥夺诱导的G(1)/G(0)期阻滞。SF/HGF对细胞周期调控的两种机制产生影响：抑制细胞周期蛋白依赖性激酶抑制剂p27以及诱导转录因子c-Myc。SF/HGF对p27的调控发生在翻译后水平，且与p27的核输出有关。用野生型p27和抗降解的p27T187A突变体对U373MG和SNB-19进行瞬时转染不足以诱导细胞周期阻滞，并且SF/HGF对p27的下调对于细胞重新进入细胞周期并非必需。在存在外源性野生型p27或p27T187A的情况下，对Cdk2激酶活性以及p27与细胞周期蛋白E复合物结合的分析表明，Cdk2活性对于SF/HGF介导的G(1)/S期转变并非必需。同样，Cdk2显性负性形式的过表达并未阻断SF/HGF触发的细胞周期进程。相反，SF/HGF转录上调c-Myc，并且在不存在SF/HGF的情况下，c-Myc的过表达能够防止G(1)/G(0)期阻滞。MadMyc（一种c-Myc的显性负性嵌合体）的瞬时过表达在对数生长期细胞中引起G(1)/G(0)期阻滞，并阻断SF/HGF介导的G(1)/S期转变。在这些细胞系中，c-Myc并非通过下调p27发挥其作用。SF/HGF诱导E2F1依赖性转录，对其抑制并未阻断SF/HGF诱导的细胞周期进程。我们得出结论，SF/HGF通过一种不依赖于p27、Cdk2或E2F1的c-myc依赖性机制来防止胶质瘤细胞系中的G(1)/G(0)期阻滞。
