Decorin, a prototype member of the growing family of the small leucine-rich proteoglycans (SLRP's), plays significant roles in tissue development and assembly, as well as playing both direct and indirect signaling roles. This review will concentrate on decorin's function in collagen fibrillogenesis as determined through the study of mice with a disrupted decorin gene. The fragile skin and abnormal tendon phenotypes initially observed were found to be due to fundamental alterations in collagen fibers, highlighting the crucial role of proteoglycans in general and SLRP's in particular in collagen fibrillogenesis. The altered fibril formation within tissues in turn leads to observable and quantifiable changes at the organismal level. Research into certain fibrotic processes with concomitant upregulation or reduction of decorin makes interesting comparisons with the collagen malformations seen in Dcn(-/-) mice. Overall, decorin is shown to be a vital player in maintaining skin and tendon integrity at the molecular level, among other functions.

Endorepellin, the COOH-terminal domain of the heparan sulfate proteoglycan perlecan, inhibits several aspects of angiogenesis. We provide evidence for a novel biological axis that links a soluble fragment of perlecan protein core to the major cell surface receptor for collagen I, α2β1 integrin, and provide an initial investigation of the intracellular signaling events that lead to endorepellin antiangiogenic activity. The interaction between endorepellin and α2β1 integrin triggers a unique signaling pathway that causes an increase in the second messenger cAMP; activation of two proximal kinases, protein kinase A and focal adhesion kinase; transient activation of p38 mitogen-activated protein kinase and heat shock protein 27, followed by a rapid down-regulation of the latter two proteins; and ultimately disassembly of actin stress fibers and focal adhesions. The end result is a profound block of endothelial cell migration and angiogenesis. Because perlecan is present in both endothelial and smooth muscle cell basement membranes, proteolytic activity during the initial stages of angiogenesis could liberate antiangiogenic fragments from blood vessels' walls, including endorepellin.

The small leucine-rich proteoglycan decorin interacts with the epidermal growth factor receptor (EGFR) and triggers a signaling cascade that leads to elevation of endogenous p21 and growth suppression. We demonstrate that decorin causes a sustained down-regulation of the EGFR. Upon stable expression of decorin, the EGFR number is reduced by ϳ40%, without changes in EGFR expression. However, EGFR phosphorylation is nearly completely abolished. Concurrently, decorin attenuates the EGFR-mediated mobilization of intracellular calcium and blocks the growth of tumor xenografts by down-regulating the EGFR kinase in vivo. Thus, decorin acts as an autocrine and paracrine regulator of tumor growth and could be utilized as an effective anticancer agent.

Decorin, a small leucine-rich proteoglycan, is a key regulator of tumor growth by acting as an antagonist of the epidermal growth factor receptor (EGFR) tyrosine kinase. To search for cell surface receptors interacting with decorin, we generated a decorin/alkaline phosphatase chimeric protein and used it to screen a cDNA library by expression cloning. We identified two strongly reactive clones that encoded either the full-length EGFR or its ectodomain. A physiologically relevant interaction between decorin and EGFR was confirmed in the yeast two-hybrid system and further validated by experiments using EGF/EGFR interaction and transient cell transfection assays. Using a panel of deletion mutants, decorin binding was mapped to a narrow region of the EGFR within its ligand-binding L2 domain. Moreover, the central leucine-rich repeat 6 of decorin was required for interaction with the EGFR. Site-directed mutagenesis of the EGFR L2 domain showed that a cluster of residues, His 394 -Ile 402 , was essential for both decorin and EGF binding. In contrast, K465, previously shown to be cross-linked to epidermal growth factor (EGF), was required for EGF but not for decorin binding. Thus, decorin binds to a discrete region of the EGFR, partially overlapping with but distinct from the EGF-binding domain. These findings could lead to the generation of protein mimetics capable of suppressing EGFR function.Decorin, a prototype member of an expanding family of small leucine-rich proteoglycans (1), plays pivotal roles in modulating matrix assembly (2-5) and cell proliferation (6 -8). Most of the biological functions of decorin are mediated by the protein core's unique organization of 10 tandem leucine-rich repeats (LRR) 1 which fold into an arch-shaped structure (9) whose concave surface is well suited to bind both globular and nonglobular proteins (2, 10, 11) as well as metal ions (12). Decorin expression is markedly suppressed in most transformed cells derived from primary malignant tumors (13-15) or in cells transformed by oncogenes such as vSrc (16), vJun (17), and ATF3 (18). On the contrary, decorin expression is markedly up-regulated during quiescence (19,20), and its levels can reach ϳ40-fold in post-confluent fibroblasts (21). We have previously shown that decorin expression is enhanced around invasive carcinomas (22) and have proposed that decorin might represent a natural antagonist to the growing cancer cells (1). This working hypothesis is corroborated by the established effects of decorin on growth factor-mediated tumor progression (23, 4) and by its profound cytostatic effects on a wide variety of tumor cell lines (13,14,24,25). Lack of decorin is permissive for tumor development insofar as bitransgenic mice, lacking both decorin and the tumor suppressor p53, develop an accelerated lymphoma tumorigenesis (26). These earlier reports have been supported by the recent observation that decorin gene expression is differentially down-regulated in hepatocellular (27) and ovarian (28) carcinomas vis á vis their normal counter...