Calautti E, Cabodi S, Stein P L, Hatzfeld M, Kedersha N, Paolo Dotto G
Cutaneous Biology Research Center, Harvard Medical School and Massachusetts General Hospital, Charlestown, Massachusetts 02129, USA.
J Cell Biol. 1998 Jun 15;141(6):1449-65. doi: 10.1083/jcb.141.6.1449.
In their progression from the basal to upper differentiated layers of the epidermis, keratinocytes undergo significant structural changes, including establishment of close intercellular contacts. An important but so far unexplored question is how these early structural events are related to the biochemical pathways that trigger differentiation. We show here that beta-catenin, gamma-catenin/plakoglobin, and p120-Cas are all significantly tyrosine phosphorylated in primary mouse keratinocytes induced to differentiate by calcium, with a time course similar to that of cell junction formation. Together with these changes, there is an increased association of alpha-catenin and p120-Cas with E-cadherin, which is prevented by tyrosine kinase inhibition. Treatment of E-cadherin complexes with tyrosine-specific phosphatase reveals that the strength of alpha-catenin association is directly dependent on tyrosine phosphorylation. In parallel with the biochemical effects, tyrosine kinase inhibition suppresses formation of cell adhesive structures, and causes a significant reduction in adhesive strength of differentiating keratinocytes. The Fyn tyrosine kinase colocalizes with E-cadherin at the cell membrane in calcium-treated keratinocytes. Consistent with an involvement of this kinase, fyn-deficient keratinocytes have strongly decreased tyrosine phosphorylation levels of beta- and gamma-catenins and p120-Cas, and structural and functional abnormalities in cell adhesion similar to those caused by tyrosine kinase inhibitors. Whereas skin of fyn-/- mice appears normal, skin of mice with a disruption in both the fyn and src genes shows intrinsically reduced tyrosine phosphorylation of beta-catenin, strongly decreased p120-Cas levels, and important structural changes consistent with impaired keratinocyte cell adhesion. Thus, unlike what has been proposed for oncogene-transformed or mitogenically stimulated cells, in differentiating keratinocytes tyrosine phosphorylation plays a positive role in control of cell adhesion, and this regulatory function appears to be important both in vitro and in vivo.
在从表皮基底层向上分化层的进程中，角质形成细胞会经历显著的结构变化，包括建立紧密的细胞间接触。一个重要但迄今尚未探索的问题是，这些早期结构事件如何与触发分化的生化途径相关联。我们在此表明，在经钙诱导分化的原代小鼠角质形成细胞中，β-连环蛋白、γ-连环蛋白/桥粒斑蛋白和p120-Cas均显著发生酪氨酸磷酸化，其时间进程与细胞连接形成的时间进程相似。伴随着这些变化，α-连环蛋白和p120-Cas与E-钙黏蛋白的结合增加，而酪氨酸激酶抑制可阻止这种结合。用酪氨酸特异性磷酸酶处理E-钙黏蛋白复合物表明，α-连环蛋白结合的强度直接取决于酪氨酸磷酸化。与生化效应并行的是，酪氨酸激酶抑制会抑制细胞黏附结构的形成，并导致分化的角质形成细胞黏附强度显著降低。Fyn酪氨酸激酶在经钙处理的角质形成细胞的细胞膜上与E-钙黏蛋白共定位。与该激酶的参与一致，fyn缺陷的角质形成细胞中β-连环蛋白、γ-连环蛋白和p120-Cas的酪氨酸磷酸化水平大幅降低，并且细胞黏附存在结构和功能异常，类似于酪氨酸激酶抑制剂所导致的异常。虽然fyn-/-小鼠的皮肤看起来正常，但fyn和src基因均被破坏的小鼠皮肤显示β-连环蛋白的酪氨酸磷酸化内在降低、p120-Cas水平大幅下降以及与角质形成细胞黏附受损一致的重要结构变化。因此，与癌基因转化或有丝分裂原刺激的细胞中所提出的情况不同，在分化的角质形成细胞中，酪氨酸磷酸化在细胞黏附控制中起积极作用，并且这种调节功能在体外和体内似乎都很重要。
