Tennenbaum T, Li L, Belanger A J, De Luca L M, Yuspa S H
Laboratory of Cellular Carcinogenesis and Tumor Promotion, National Cancer Institute, National Institutes of Health, Bethesda, Maryland 20892, USA.
Cell Growth Differ. 1996 May;7(5):615-28.
In skin, the distribution of integrins is compartmentalized. Whereas the alpha 6 beta 4 integrin complex is polarized to the basal portion of proliferating cells in the basal layer juxtaposed to the basement membrane, alpha 3 beta 1 integrin receptors are localized on the cell surface surrounding basal and suprabasal cells, suggesting beta 1 integrins mediate both cell-matrix and cell-cell interactions. As initiation of maturation in skin is associated with the detachment of cells from the basement membrane, the early loss of alpha 6 beta 4, but not alpha 3 beta 1, integrin expression could be a determining factor in the transition from the proliferating to a differentiating keratinocyte. We have studied the regulation of adhesion potential and integrin expression during differentiation of mouse basal keratinocytes culture in 0.05 mM Ca2+ medium and induced to differentiate in 0.12 mM Ca2+ medium. Within 12-24 h after elevation of Ca2+, a selective loss of the alpha 6 beta 4 integrin complex is associated with the induction of the spinous cell marker keratin 1. This early differentiation phenotype coincides with loss of cell attachment mediated by alpha 6 beta 4 to laminins 1 and 5 but not a fibronectin or collagen IV. Selective loss of attachment to laminin is also detected in spinous cells isolated from newborn epidermis in vivo. The loss of alpha 6 and beta 4 protein expression is a consequence of transcriptional and posttranscriptional events, including reduction in mRNA transcripts, reduced synthesis of the alpha 6 protein, and enhanced processing of the alpha 6 and beta 4 chains as determined by Western blots and pulse-chase experiments in metabolically labeled keratinocytes. Selective processing of the beta 4 intracellular domain is detected before loss of beta 4 from the cell surface in basal keratinocytes, and this process is accelerated during differentiation. Whereas early keratinocyte maturation is linked to the selective loss of the alpha 6 beta 4 complex, loss of both beta 1 and beta 4 integrin mRNA and protein occurs as cells proceed to later stages in the differentiation program as induced by 0.5 mM Ca2+ or suspension culture. These conditions are characterized by accelerated expression of transglutaminase; reduced keratin 1 protein; loss of adhesion to fibronectin, laminin 1, laminin 5, and collagen IV; and rapid cell death. Contributing to the down-regulation of beta 1 integrins during terminal differentiation is a selective sensitivity of alpha 3 beta 1 but not alpha 6 beta 4 to down-regulation by transforming growth factors beta 1 and beta 2, factors that are also expressed differentially in normal skin. This study indicates that down-regulation of the alpha 6 beta 4 but not beta 1 integrins occurs during the initial steps of keratinocyte differentiation and is associated with detachment from the laminin matrix. Such changes could contribute an important signal to initiate the process of terminal keratinocyte differentiation.
在皮肤中，整合素的分布是分区化的。α6β4整合素复合物在基底层增殖细胞与基底膜相邻的基部区域呈极化分布，而α3β1整合素受体则定位在基底细胞和基底上层细胞周围的细胞表面，这表明β1整合素介导细胞与基质以及细胞与细胞之间的相互作用。由于皮肤成熟的起始与细胞从基底膜脱离相关，α6β4整合素表达的早期丧失而非α3β1整合素表达的丧失，可能是角质形成细胞从增殖状态转变为分化状态的一个决定性因素。我们研究了在0.05 mM Ca2+培养基中培养的小鼠基底角质形成细胞分化过程中黏附潜能和整合素表达的调控，并在0.12 mM Ca2+培养基中诱导其分化。在Ca2+浓度升高后的12 - 24小时内，α6β4整合素复合物的选择性丧失与棘状细胞标志物角蛋白1的诱导表达相关。这种早期分化表型与α6β4介导的细胞与层粘连蛋白1和5的黏附丧失相关，但与纤连蛋白或IV型胶原无关。在体内从新生表皮分离的棘状细胞中也检测到对层粘连蛋白黏附的选择性丧失。α6和β4蛋白表达的丧失是转录和转录后事件的结果，包括mRNA转录本减少、α6蛋白合成减少以及通过代谢标记角质形成细胞的蛋白质免疫印迹和脉冲追踪实验确定的α6和β4链加工增强。在基底角质形成细胞中β4从细胞表面丧失之前检测到β4胞内结构域的选择性加工，并且在分化过程中这个过程加速。虽然早期角质形成细胞成熟与α6β4复合物的选择性丧失相关，但当细胞在0.5 mM Ca2+或悬浮培养诱导下进入分化程序的后期阶段时，β1和β4整合素的mRNA和蛋白都会丧失。这些条件的特征是转谷氨酰胺酶表达加速；角蛋白1蛋白减少；对纤连蛋白、层粘连蛋白1、层粘连蛋白5和IV型胶原的黏附丧失；以及快速细胞死亡。在终末分化过程中导致β1整合素下调的一个因素是α3β1对转化生长因子β1和β2下调的选择性敏感性，而α6β4则不敏感，这些因子在正常皮肤中也有不同表达。这项研究表明，在角质形成细胞分化的初始步骤中发生的是α6β4而非β1整合素的下调，并且与从层粘连蛋白基质脱离相关。这种变化可能为启动角质形成细胞终末分化过程提供一个重要信号。
