Lumican suppresses cell proliferation and aids Fas-Fas ligand mediated apoptosis: implications in the cornea.
ABSTRACT Lumican, an extracellular matrix (ECM) keratan sulfate proteoglycan, binds fibrillar collagen and limits collagen fibril diameter in the cornea, skin and tendon. Lumican-deficient mice (Lum(-/-)) develop abnormally thick collagen fibrils, translucent corneas and fragilities of the skin and the tendon. In addition to modulating interstitial ECM structure, here we hypothesized that lumican regulates proliferation and apoptosis of cells residing in the interstitium. Corneal and embryonic fibroblasts from the Lum(-/-) mouse show increased growth in culture. Lum(-/-) mouse embryonic fibroblasts (MEF), compared to their wild type counterparts, display increased rates of proliferation and decreased apoptosis. Ectopic expression of lumican in Lum(-/-) MEF or exogenous recombinant lumican in the culture medium reduces proliferation to rates seen in the Lum(+/+) MEF. We further investigated the implications of lumican's proliferation and apoptosis regulatory role in the cornea where lumican is a major component of the stromal matrix. Stromal keratocytes undergo proliferation and apoptosis during corneal maturation and in the healing of injured cornea. The Lum(-/-) mouse shows increased proliferation and decreased apoptosis of stromal keratocytes during postnatal corneal maturation at the 10-day age. Apoptosis is also significantly down regulated in Lum(-/-) vis-à-vis Lum(+/+) mice during stromal wound healing in the adult 6-week age. Lumican appears to regulate these cellular functions by modulating specific cell growth and apoptosis mediators. Thus, Lum(-/-) MEF have decreased p21(WAF1/CIP1), a universal inhibitor of cyclin-dependent kinases and a consequent increase in cyclins A, D1 and E. Furthermore, the tumor suppressor p53, an upstream regulator of p21 is down regulated in the MEF and the cornea of Lum(-/-) mice. The evidence suggests regulation of p21 by lumican in a p53-dependent way. The MEF and the cornea of Lum(-/-) mice also show a dramatic decrease in Fas (CD95). The Lum(-/-) MEF fail to induce Fas upon treatment with Fas ligand. Fas-Fas ligand interaction is an initiating event in apoptosis and its disruption in lumican-deficiency may partly explain the observed decrease in apoptosis. Fas-Fas ligand mediated apoptosis is critical for maintaining the immune privileged status of the cornea, which implies a new and exciting role for lumican in the cornea.
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ABSTRACT: Ectopic expression of decorin induces profound cytostatic effects in transformed cells with diverse histogenetic backgrounds. The mechanism of action has only recently begun to be elucidated. Exogenous decorin activates the epidermal growth factor (EGF) receptor, thereby triggering a signaling cascade that leads to phosphorylation of mitogen-activated protein (MAP) kinase, induction of p21, and growth suppression. In this study we demonstrate a direct interaction of decorin with the EGF receptor. Binding of decorin induces dimerization of the EGF receptor and rapid and sustained phosphorylation of MAP kinase in squamous carcinoma cells. In a cell-free system, decorin induces autophosphorylation of purified EGF receptor by activating the receptor tyrosine kinase and can also act as a substrate for the EGF receptor kinase itself. Using radioligand binding assays we show that both immobilized and soluble decorin bind to the EGF receptor ectodomain or to purified EGF receptor. The binding is mediated by the protein core and has relatively low affinity (Kd approximately 87 nM). Thus, decorin should be considered as a novel biological ligand for the EGF receptor, an interaction that could regulate cell growth during remodeling and cancer growth.Journal of Biological Chemistry 03/1999; 274(8):4489-92. · 4.65 Impact Factor
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ABSTRACT: It is well-known that after trauma to the ocular surface such as that seen in refractive surgeries like photorefractive keratectomy (PRK; laser corneal surface ablation), there is a "dropout" of underlying stromal keratocytes. Recently it was proposed by Wilson et al. that this loss of keratocytes may be the result of apoptosis. This process can be initiated by several cellular factors that result in release of endonucleases, DNA segmentation, and eventual cellular destruction. One protein, bcl-2, has been shown to have a profound role in its inhibition. The goal of this study was to evaluate the mechanisms and level of apoptosis in the cornea after PRK. Rabbits were anesthetized systemically with ketamine/xylazine. The corneal epithelium was removed by using phototherapeutic keratectomy (PTK, 100 pulses). PRK was then performed (-9.0 D, 5.0 mm optical zone). Rabbits were killed at 4 days and 4 weeks, and the corneas were prepared for light and electron microscopy, as well as for apoptosis evaluation. The apoptosis assay demonstrated that (a) the normal cornea exhibited a limited level of apoptosis, primarily in the superficial epithelium, very little in the basal epithelium, with none in the keratocytes and endothelium; (b) the entire epithelial layer was found to be apoptotic 4 days and 4 weeks after PRK; (c) an elevated level of apoptosis was detected in both keratocytes and endothelial cells after PRK at the same time points. The immunohistochemical staining showed that (a) the normal cornea had a low level of bcl-2 protein, exclusively in the superficial epithelium; (b) bcl-2 was induced in the basal epithelial cells and anterior keratocytes under the wound bed 4 days after PRK and increased 4 weeks after PRK. These findings suggest that apoptosis plays an important role in the corneal wound-healing process, which appears to be mediated in part by bcl-2. Bcl-2 induction may protect corneal epithelial cells from apoptosis after trauma such as PRK.Cornea 04/1997; 16(2):200-8. · 1.75 Impact Factor
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ABSTRACT: Collagen fibrillogenesis is finely regulated during development of tissue-specific extracellular matrices. The role(s) of a leucine-rich repeat protein subfamily in the regulation of fibrillogenesis during tendon development were defined. Lumican-, fibromodulin-, and double-deficient mice demonstrated disruptions in fibrillogenesis. With development, the amount of lumican decreases to barely detectable levels while fibromodulin increases significantly, and these changing patterns may regulate this process. Electron microscopic analysis demonstrated structural abnormalities in the fibrils and alterations in the progression through different assembly steps. In lumican-deficient tendons, alterations were observed early and the mature tendon was nearly normal. Fibromodulin-deficient tendons were comparable with the lumican-null in early developmental periods and acquired a severe phenotype by maturation. The double-deficient mice had a phenotype that was additive early and comparable with the fibromodulin-deficient mice at maturation. Therefore, lumican and fibromodulin both influence initial assembly of intermediates and the entry into fibril growth, while fibromodulin facilitates the progression through growth steps leading to mature fibrils. The observed increased ratio of fibromodulin to lumican and a competition for the same binding site could mediate these transitions. These studies indicate that lumican and fibromodulin have different developmental stage and leucine-rich repeat protein specific functions in the regulation of fibrillogenesis.The Journal of Cell Biology 12/2000; 151(4):779-88. · 10.82 Impact Factor