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The differentiated phenotype of chondrocyte is rapidly lost during in vitro culture by a process designated "dedifferentiation." In this study, we investigate the roles of protein kinase C (PKC) and extracellular signal-regulated protein kinase (ERK) in the maintenance of the differentiated chondrocyte phenotype. Chondrocytes isolated from rabbit articular cartilage underwent dedifferentiation upon serial monolayer culture with cessation of type II collagen expression and proteoglycan synthesis, which was reversed by culturing dedifferentiated cells in alginate gel. The expression pattern of PKC alpha was essentially the same as that of type II collagen during de- and redifferentiation, in that expression was decreased during dedifferentiation and increased during redifferentiation. In contrast to PKC alpha, ERK activity increased 15-fold during dedifferentiation. This enhanced activity was terminated during redifferentiation. Down-regulation of PKC alpha in passage 0 chondrocytes resulted in dedifferentiation. However, overexpression of PKC alpha did not affect type II collagen levels, suggesting that PKC alpha expression is not sufficient to maintain the differentiated phenotype. However, inhibition of ERK by PD98059 enhanced type II collagen expression and proteoglycan synthesis in passage 0 cells, retarded dedifferentiation during monolayer cultures, and reversed dedifferentiation caused by down-regulation of PKC. Unlike PKC-dependent ERK regulation of chondrogenesis, PKC and ERK independently modulated chondrocyte dedifferentiation, as confirmed by observations that PKC down-regulation and ERK inhibition did not alter ERK phosphorylation and PKC expression, respectively. In addition, expression of N-cadherin, alpha-catenin, and beta-catenin, which are oppositely regulated to type II collagen during phenotype alterations, were modulated by PKC and ERK during chondrogenesis but not dedifferentiation, supporting distinct mechanisms for the regulation of chondrocyte differentiation and maintenance of differentiated phenotype by these two protein kinases.

译文

软骨细胞的分化表型在体外培养过程中通过称为 “去分化” 的过程迅速丧失。在这项研究中,我们研究了蛋白激酶C (PKC) 和细胞外信号调节蛋白激酶 (ERK) 在维持分化的软骨细胞表型中的作用。从兔关节软骨分离的软骨细胞在连续单层培养后进行去分化,停止II型胶原表达和蛋白聚糖合成,这通过在藻酸盐凝胶中培养去分化细胞而被逆转。PKC α 在去分化和再分化过程中的表达模式与II型胶原的表达模式基本相同,因为在去分化过程中表达降低,而在再分化过程中表达增加。与PKC α 相反,去分化过程中ERK活性增加了15倍。这种增强的活性在再分化过程中终止。0代软骨细胞中PKC α 的下调导致去分化。然而,PKC α 的过表达并未影响II型胶原水平,这表明PKC α 的表达不足以维持分化的表型。然而,PD98059对ERK的抑制增强了第0代细胞中II型胶原的表达和蛋白聚糖的合成,延缓了单层培养过程中的去分化,并逆转了PKC下调引起的去分化。与PKC依赖性的ERK调节软骨形成不同,PKC和ERK独立调节软骨细胞去分化,这通过观察证实,PKC下调和ERK抑制分别不会改变ERK磷酸化和PKC表达。此外,在表型改变过程中与II型胶原相反调节的N-钙粘蛋白,α-连环蛋白和 β-连环蛋白的表达在软骨形成过程中受到PKC和ERK的调节,但没有去分化,支持这两种蛋白激酶调节软骨细胞分化和维持分化表型的独特机制。

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