Endothelial cell-mediated type I collagen gel contraction is regulated by hemin

The contraction of three-dimensional type I collagen gels is regarded as a model of contraction during wound healing and tissue remodeling. Because such a process could contribute to vessel narrowing, we hypothesized that endothelial cells may be able to mediate gel contraction. To demonstrate this, type I collagen was extracted from rat tail tendon and used to prepare collagen gels. Bovine arterial endothelial cells (BAECs) or human pulmonary artery endothelial cells (HPAECs) were then plated on the top of the gels in serum-tree Ham's F- 12 medium or 2% fetal calf serum-endothelium growth medium-2 (FCS-EGM₂), respectivety. After 48 hours of attachment, gels were released and floated in 0.2% FCS-Ham's F-12 medium (BAECs) or 2% FCS-EGM₂ (HPAECs). Gel size was measured with an image analyzer daily for 5 consecutive days. Gels were then digested with collagenase to quantify DNA and hydroxyproline. BAECs contracted the gels in a time-dependent manner over the 5 days. Contraction was dependent on cell density (gel size was 100% of initial size after 5 days with no cells vs 66.4% ± 0.5% with 0.9 x 10⁴ cells/cm² and 22.1% ± 0.3% with 7.5 x 10⁴ cells/cm²) and was inversely related to collagen concentration (gel size was 22.3% ± 0.05%, 46.4% ± 0.9%, 72.3% ± 0.4%, and 87.4% ± 0.3% of initial size for gels prepared with 0.5 mg/mL, 0.75 mg/mL, 1 mg/mL, and 2 mg/mL of collagen, respectively). Heroin (a precursor for CO) and cytochalasin D inhibited collagen gel contraction mediated by both bovine and human endothelial cells without changing cell number or hydroxyproline content. In contrast, prostaglandin E₂, an inhibitor, and transforming growth factor-β1, a stimulator of fibroblast-mediated gel contraction, had no effect on endothelial cell-mediated contraction. These findings demonstrate that endothelial cells are able to contract native type I collagen gels and that this process can be modulated by exogenous mediators. Such a capability may cause remodeling of subjacent matrix of endothelial cells and may contribute to vessel narrowing.