Characterization of the role of versican in modulating cell activities

Versican is a large aggregating chondroitin sulfate proteoglycan that is ubiquitously expressed in the matrix of a variety of tissues. Abnormal expression of versican has been found in developmental defects and malignancies. Versican actively interacts with many other matrix molecules, suggesting its close involvement in regulating cell activities. However, little has been known about the mechanism of versican actions. We, for the first time, identified the domain, the versican C-terminal CRD- and CBP-like sub-domains (the G3 domain lacing two epidermal growth factor (EGF)-like motifs, named G3ΔEGF) responsible for inhibition of endogenous versican secretion and binding in astrocytoma cells. This might be a potential feedback mechanism in versican regulation, which determines the level of versican in the matrix. Expression of G3ΔEGF in astrocytoma cells increased adhesion, reduced proliferation and migration, restored anchorage-dependent growth and survival, and reversed transformation. Further investigation revealed that versican bound to β1 integrin through its C-terminal domain. Presence of the G3 domain or G3ΔEGF in the cell cultures induced focal adhesion kinase phosphorylation, increased cell spreading and enhanced cell adhesion. This provided the first evidence that versican interacted with cell surface receptor and induced cell signaling. In addition, astrocytoma cells expressing G3ΔEGF demonstrated increased adhesion-induced association of integrin with EGF receptor (EGFR) but reduced EGFR activation. Consequently, the cells relied on exogenous matrix molecules and growth factors for growth and survival. In the absence of either matrix molecules or growth factors, the cells underwent apoptosis. Versican modulating EGFR activation was further confirmed in the over-expression of versican isoform V1. Stable expression of V1 in PC12 rat phaeochromocytoma cells remarkably up-regulated EGFR expression and produced enhanced and sustained EGFR and extracellular signal-regulated kinase (ERK)/mitogen-activated protein kinase (MAPK) tyrosine phosphorylation, which led to PC12 cell neuronal differentiation. The effects of versican V1 in promoting neurite outgrowth were also observed in primary hippocampal neurons and neural stem cells. Furthermore, versican V1 altered NGF-induced expression and current patterns of nicotinic acetylcholine receptor (nAChR) in PC12 cells. Our results have provided evidence on how versican regulates cell growth, differentiation, survival and apoptosis.