Scatter factor/hepatocyte growth factor stimulation of glioblastoma cell cycle progression through G₁ is c-Myc dependent and independent of p27 suppression, Cdk2 activation, or E2F1-dependent transcription

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 G₁/G₀ 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 G₁/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 G₁/G₀ arrest in the absence of SF/HGF. Transient overexpression of MadMyc, a dominant-negative chimera for c-Myc, caused G₁/G₀ arrest in logarithmically growing cells and blocked SF/HGF-mediated G₁/S transition. c-Myc did not exert its effects through p27 downregulation in these cell lines. SF/HGF induced E2Fl-dependent transcription, the inhibition of which did not block SF/HGF-induced cell cycle progression. We conclude that SF/HGF prevents G₁/G₀ arrest in glioma cell lines by a c-myc-dependent mechanism that is independent of p27, Cdk2, or E2F1.