Human gliomas show extraordinary cellular and genomic heterogeneity while the heparan sulfate proteoglycan glypican-1 (GPC1) is nearly universally overexpressed in human gliomas but absent in normal glia. Cell membrane-associated heparan sulfate proteoglycans, including GPC1, can enhance the activity of many heparan sulfate-binding growth factors and cytokines to promote cell growth and development. Our previous studies using normal human astrocytes and U87-MG, a human glioma cell line carrying relatively low basal levels of GPC1, showed that transduction of GPC1 dramatically stimulates G1-S cell cycle progression and DNA replication, ultimately leading to DNA re-replication and DNA damage. This cell cycle effect of GPC1 appears to be at least partially attributable to the deregulation of G1/S checkpoint resulting from significant downregulation of pRb, p21Cip/p27Kip and CDH1 (FZR1) and significant upregulation of cyclin E, cyclin-dependent kinase 2 (CDK2) and Skp2 by GPC1 {Qiao, D., et al., Mol. Cell. Biol. 2013, 33(22):4408}. Here we further report that c-Myc acts as a key mediator for GPC1-mediated G1-S cell cycle deregulation. Both c-Myc protein and transactivation activity can be significantly induced by ectopic expression of GPC1 in a cell type-independent fashion. Inhibition of GPC1-induced c-Myc transactivation by the c-Myc/Max-specific inhibitor 10058-F4 almost completely abolishes GPC1-induced E2F activation and DNA re-replication. Consistent with these results, inhibition of Myc activity completely restores the protein levels of cyclin E, CDK2, pRb, p27Kip and Skp2 which were significantly altered by expression of GPC1. GPC1-induced reduction of p21Cip was partially restored upon inhibition of c-Myc. Expression of GPC1 also induced cell detachment and rounding, accompanied by a significant downregulation of the β1 and β3 integrin subunits. Inhibition of Myc effectively blocked GPC1 induction of both cell detachment/rounding and downregulation of β1 and β3 integrins. These results suggest a key role for c-Myc in GPC-1-mediated cellular effects and cell signaling. In addition to direct transcriptional activation of cyclin E, E2Fs and Skp2, c-Myc may also modulate specific protein degradation activities. The c-Myc-mediated transcriptional repression of p21Cip/p27Kip and of integrins may also contribute to GPC1-induced effects on the cell cycle and on cell morphology. Given the potent oncogenic activity of c-Myc and the fact that c-Myc is required for the maintenance of stemness of glioma stem cells, our data imply that GPC1 may play a significant role in both glioma tumorigenesis and growth. Further in vitro and in vivo studies are required to fully understand the role and mechanisms of both GPC1 and c-Myc in human gliomas.

Citation Format: Dianhua Qiao, Kristy Meyer, Andreas Friedl. c-Myc is a key mediator of glypican-1 (GPC1)-dependent deregulation of the cell cycle. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 1307. doi:10.1158/1538-7445.AM2014-1307