Abstract
During malignant invasion tumor cells establish contact with extracellular matrix proteins, including fibrillar collagen. In addition to providing a physical barrier against invasion, fibrillar collagen also restricts cell proliferation. It has been assumed that the growth regulatory activity of fibrillar collagen is the result of an indirect restrictive effect on cell spreading and cytoskeletal organization. Here we provide evidence for a direct inhibitory effect of fibrillar collagen on proliferation of human melanoma and fibrosarcoma cells that involves activation of the tyrosine kinase discoidin domain receptor 2 and is independent of effects on cell spreading. Cells plated in the presence of fibrillar collagen were growth arrested in the G0/G1 phase of the cell cycle. However treatment with the tyrosine kinase inhibitor genistein, down-regulation of discoidin domain receptor 2, or collagen deglycosylation that prevents discoidin domain receptor 2 activation allowed cells to enter the cell cycle in the presence of fibrillar collagen without a requirement for spreading and actin organization. Our data provide evidence for a novel direct mechanism by which cell contact with fibrillar collagen restricts proliferation.
Highlights
The extracellular matrix (ECM)2 is a complex mixture of structural and functional proteins that provides cell scaffolding and is critical in regulating cell survival, differentiation, migration, and proliferation [1]
Our experiments identify the discoidin domain receptor 2 (DDR2), a cell surface-associated tyrosine kinase receptor that binds to fibrillar collagen (FC), as providing a new mechanism responsible for the G0/G1 cell cycle arrest observed when tumor cells are in contact with FC
Whereas cells plated on monomeric-fibrillar collagen (MFC) spread and developed an organized cytoskeleton with bundles of actin filaments, cells plated on FC were rounded, developed long cytoplasmic “dendrite-like” processes, failed to spread, and showed no evidence of cytoskeletal organization (Fig. 1C)
Summary
The extracellular matrix (ECM) is a complex mixture of structural and functional proteins that provides cell scaffolding and is critical in regulating cell survival, differentiation, migration, and proliferation [1]. The second and growth factor-independent mechanism is interacting with integrins that through adhesion and spreading generate growth stimulatory signals [2, 3]. Most ECM proteins, such as fibronectin, laminin, vitronectin, and the proteoglycan hyaluronic acid, stimulate cell proliferation by their ability to promote cell spreading and induce formation of focal adhesions and activation of p125FAK (6 –10). Because cells cultured in the presence of FC typically fail to spread, it has been generally assumed that the growth inhibitory activity of FC is the result of an indirect inhibitory effect on cell spreading, cytoskeletal organization, and formation of focal adhesions [17]. We have explored whether FC could inhibit the proliferation of tumor cells by a direct mechanism that involves a receptor-mediated signal, independent of cell spreading and cytoskeletal organization. Our experiments identify the discoidin domain receptor 2 (DDR2), a cell surface-associated tyrosine kinase receptor that binds to FC, as providing a new mechanism responsible for the G0/G1 cell cycle arrest observed when tumor cells are in contact with FC
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