Shepard John B, Jeong Jae-Wook, Maihle Nita J, O'Brien Sean, Dealy Caroline N
Arthritis Res Ther. 2013;15(3):R60. doi: 10.1186/ar4233.
Signals from the epidermal growth factor receptor (EGFR) have typically been considered to provide catabolic activities in articular cartilage, and accordingly have been suggested to have a causal role in osteoarthritis progression. The aim of this study was to determine in vivo roles for endogenous EGFR signal activation in articular cartilage.
Transgenic mice with conditional, limb-targeted deletion of the endogenous intracellular EGFR inhibitor Mig-6 were generated using CreLoxP (Mig-6-flox; Prx1Cre) recombination. Histology, histochemical staining and immunohistochemistry were used to confirm activation of EGFR signaling in the articular cartilage and joints, and to analyze phenotypic consequences of Mig-6 loss on articular cartilage morphology, proliferation, expression of progenitor cell markers, presence of chondrocyte hypertrophy and degradation of articular cartilage matrix.
The articular cartilage of Mig-6-conditional knockout (Mig-6-cko) mice was dramatically and significantly thicker than normal articular cartilage at 6 and 12 weeks of age. Mig-6-cko articular cartilage contained a population of chondrocytes in which EGFR signaling was activated, and which were three to four times more proliferative than normal Mig-6-flox articular chondrocytes. These cells expressed high levels of the master chondrogenic regulatory factor Sox9, as well as high levels of putative progenitor cell markers including superficial zone protein (SZP), growth and differentiation factor-5 (GDF-5) and Notch1. Expression levels were also high for activated β-catenin and the transforming growth factor beta (TGF-β) mediators phospho-Smad2/3 (pSmad2/3). Anabolic effects of EGFR activation in articular cartilage were followed by catabolic events, including matrix degradation, as determined by accumulation of aggrecan cleavage fragments, and onset of hypertrophy as determined by type × collagen expression. By 16 weeks of age, the articular cartilage of Mig-6-cko knees was no longer thickened and was degenerating.
These results demonstrate unexpected anabolic effects of EGFR signal activation in articular cartilage, and suggest the hypothesis that these effects may promote the expansion and/or activity of an endogenous EGFR-responsive cell population within the articular cartilage.
表皮生长因子受体（EGFR）发出的信号通常被认为在关节软骨中发挥分解代谢作用，因此有人提出其在骨关节炎进展中起因果作用。本研究的目的是确定内源性EGFR信号激活在关节软骨中的体内作用。
利用CreLoxP（Mig-6-flox；Prx1Cre）重组技术构建了条件性、肢体靶向敲除内源性细胞内EGFR抑制剂Mig-6的转基因小鼠。采用组织学、组织化学染色和免疫组织化学方法确认关节软骨和关节中EGFR信号的激活，并分析Mig-6缺失对关节软骨形态、增殖、祖细胞标志物表达、软骨细胞肥大的出现以及关节软骨基质降解的表型后果。
在6周龄和12周龄时，Mig-6条件性敲除（Mig-6-cko）小鼠的关节软骨比正常关节软骨显著增厚。Mig-6-cko关节软骨中有一群EGFR信号被激活的软骨细胞，其增殖能力比正常Mig-6-flox关节软骨细胞高三到四倍。这些细胞高水平表达主要的软骨生成调节因子Sox9，以及高水平的假定祖细胞标志物，包括浅表区蛋白（SZP）、生长分化因子-5（GDF-5）和Notch1。活化的β-连环蛋白和转化生长因子β（TGF-β）介质磷酸化Smad2/3（pSmad2/3）的表达水平也很高。EGFR激活在关节软骨中的合成代谢作用之后是分解代谢事件，包括由聚集蛋白聚糖裂解片段的积累所确定的基质降解，以及由Ⅹ型胶原表达所确定的肥大的开始。到16周龄时，Mig-6-cko膝关节的关节软骨不再增厚并开始退化。
这些结果证明了EGFR信号激活在关节软骨中具有意想不到的合成代谢作用，并提出了这样一种假说，即这些作用可能促进关节软骨内源性EGFR反应性细胞群体的扩增和/或活性。
