Collagen type 1 retards tube formation by human microvascular endothelial cells in a fibrin matrix

Angiogenesis, or the formation of new microvessels, is often encountered in pathological situations. A fibrinous exudate can often act as a temporary matrix for the ingrowth of these new microvessels. This matrix consists mainly of fibrin, but is mingled with other plasma components and interstitial collagen fibres. In vitro, capillary-like tube formation can be mimicked by exposing human microvascular endothelial cells (hMVECs), seeded on top of a three-dimensional fibrin matrix, to an angiogenic growth factor (e.g. fibroblast growth factor (FGF)-2) and the cytokine tumour necrosis factor (TNF)-alpha. Plasmin activity is required in this process. We investigated whether the angiogenic potential of hMVECs was altered by the presence of collagen. The addition of type I collagen to fibrin matrices dose-dependently inhibited tube-formation. Tube-formation in these fibrin/collagen matrices by hMVECs required matrix metalloprotease (MMP) activity, as well as plasmin activity. On a pure collagen type I matrix, hMVECs were not able to form tube-like structures in the matrix but formed sprouts. This sprouting required MMP activity and was, in contrast to the tube-like structures in a fibrin matrix, not influenced by hypoxia. These data indicate that the interaction between endothelial cells and different matrix components is of importance for the angiogenic potential of these cells.