Vij Neeraj, Roberts Luke, Joyce Sarah, Chakravarti Shukti
Departments of Medicine, The Johns Hopkins University School of Medicine, 720 Rutland Avenue, Ross 935, Baltimore, MD 21205, USA.
Exp Eye Res. 2004 May;78(5):957-71. doi: 10.1016/j.exer.2003.12.006.
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.
角膜蛋白聚糖是一种细胞外基质（ECM）硫酸角质素蛋白聚糖，可结合纤维状胶原蛋白并限制角膜、皮肤和肌腱中胶原纤维的直径。缺乏角膜蛋白聚糖的小鼠（Lum(-/-)）会形成异常粗大的胶原纤维、半透明角膜以及皮肤和肌腱的脆弱性。除了调节间质ECM结构外，我们在此假设角膜蛋白聚糖可调节间质中细胞的增殖和凋亡。来自Lum(-/-)小鼠的角膜和成纤维细胞在培养中显示出生长增加。与野生型对照相比，Lum(-/-)小鼠胚胎成纤维细胞（MEF）显示出增殖速率增加和凋亡减少。在Lum(-/-) MEF中异位表达角膜蛋白聚糖或在培养基中添加外源性重组角膜蛋白聚糖可将增殖降低至Lum(+/+) MEF中的水平。我们进一步研究了角膜蛋白聚糖在角膜中增殖和凋亡调节作用的意义，角膜蛋白聚糖是基质的主要成分。在角膜成熟过程以及受伤角膜愈合过程中，基质角膜细胞会发生增殖和凋亡。在出生后10天大的Lum(-/-)小鼠角膜成熟过程中，基质角膜细胞显示出增殖增加和凋亡减少。在6周龄成年小鼠的基质伤口愈合过程中，与Lum(+/+)小鼠相比，Lum(-/-)小鼠的凋亡也明显下调。角膜蛋白聚糖似乎通过调节特定的细胞生长和凋亡介质来调节这些细胞功能。因此Lum(-/-) MEF中细胞周期蛋白依赖性激酶的通用抑制剂p21(WAF1/CIP1)减少，从而导致细胞周期蛋白A、D1和E增加。此外，肿瘤抑制因子p53是p21的上游调节因子，在Lum(-/-)小鼠的MEF和角膜中表达下调。证据表明角膜蛋白聚糖以p53依赖性方式调节p21。Lum(-/-)小鼠的MEF和角膜中Fas（CD95）也显著减少。用Fas配体处理后，Lum(-/-) MEF无法诱导Fas。Fas-Fas配体相互作用是凋亡的起始事件，其在角膜蛋白聚糖缺乏中的破坏可能部分解释了观察到的凋亡减少。Fas-Fas配体介导的凋亡对于维持角膜的免疫赦免状态至关重要，这意味着角膜蛋白聚糖在角膜中具有新的、令人兴奋的作用。
