Elke Schönherr

Abstract: Decorin, a small multifunctional proteoglycan, has been shown to be causally involved in the formation of capillary‐like structures and a decrease in apoptosis. Here we investigated signal transduction pathways mediating effects of decorin on endothelial cells (ECs). Addition of decorin led to a fourfold increase in phosphorylation of Akt/protein kinase B on Thr307 and a l.4‐fold increase on Ser473 after 10 min, but this phosphorylation could not be blocked by preincubation with Ly29400 (10 μM). Six hours after the addition of decorin, the synthesis of p21 and p27, two inhibitors of cyclin‐dependent kinases, started and increased up to 18 h, while synthesis of cyclin A peaked at 12 h and decreased after 24 h below base level. Induction of dominan‐negative Akt by a replication‐deficient adenovirus blocked p21 and cyclin A synthesis, but had no effect on p27. Dominant‐negative Akt also blocked the antiapoptotic effect of decorin on ECs, but induction of dominant‐positive Akt could not rescue the cells from apoptosis. Thus, the matrix proteoglycan decorin is a signaling molecule in ECs that affects cell survival by Akt‐dependent and ‐independent pathways.

EA.hy 926 cells, a derivative of human umbilical vein endothelial cells, in the presence of fibroblasts show the phenomena of angiogenesis, express the proteoglycan decorin and escape apoptosis, when they are maintained in collagen lattices, while fibroblast-free cultures do not show these changes. Virus-mediated decorin expression can substitute for the presence of fibroblasts. Since the expression of matrix metalloproteinases (MMPs) is an essential step in the formation of capillaries, several MMPs and tissue inhibitors of metalloproteinases (TIMPs) were investigated. MMP-1, MMP-2, MMP-9, and the cell-associated MMP-14 were augmented on the protein level in the presence of fibroblasts. No effect was seen with respect to MMP-3, TIMP-1, and TIMP-2. Semiquantitative RT-PCRs of endothelial cells in co-culture revealed a 7-, 19-, and 11-fold increase for mRNAs of MMP-1, MMP-2, and MMP-14 after six days, respectively. Virus-mediated decorin expression also was accompanied by an up-regulation of these MMPs. The expression of MMP-1 mRNAs increased 5-fold after 2 days and gradually declined thereafter. In contrast, MMP-2 and MMP-14 showed a 7-fold and a 14-fold increase on day two which returned to basal levels within 24 h, indicating that the expression of MMP-1 is differentially regulated from MMP-2 and MMP-14. In spite of the upregulation of the proteases, an enhanced degradation of decorin was not observed. These results indicate that the expression of decorin is a sufficient signal in EA.hy 926 cells for a finely tuned induction of selected MMPs which are involved in angiogenesis whereas the up-regulation of MMPs does not lead to the degradation of the responsible proteoglycan.

The extracellular matrix (ECM) as well as soluble mediators like cytokines can influence the behavior of cells in very distinct as well as cooperative ways. One group of ECM molecules which shows an especially broad cooperativety with cytokines and growth factors are the proteoglycans. Proteoglycans can interact with their core proteins as well as their glycosaminoglycan chains with cytokines. These interactions can modify the binding of cytokines to their cell surface receptors or they can lead to the storage of the soluble factors in the matrix. Proteoglycans themselves may even have cytokine activity. In this review we describe different proteoglycans and their interactions and relationships with cytokines and we discuss in more detail the extracellular regulation of the activity of transforming growth factor‐β (TGF‐β) by proteoglycans and other ECM molecules. In the third part the interaction of heparan sulfate chains with fibroblast growth factor‐2 (FGF‐2, basic FGF) as a prototype example for the interaction of heparin‐binding cytokines with heparan sulfate proteoglycans is presented to illustrate the different levels of mutual dependence of the cytokine network and the ECM.