Data from Sphingosine kinase and sphingosine-1-phosphate regulate epithelial cell architecture by the modulation of de novo sphingolipid synthesis

Sphingolipids regulate several aspects of cell behavior and it has been demonstrated that cells adjust their sphingolipid metabolism in response to metabolic needs. Particularly, sphingosine-1-phosphate (S1P), a final product of sphingolipid metabolism, is a potent bioactive lipid involved in the regulation of various cellular processes, including cell proliferation, cell migration, actin cytoskeletal reorganization and cell adhesion. In previous work in rat renal papillae, we showed that sphingosine kinase (SK) expression and S1P levels are developmentally regulated and control de novo sphingolipid synthesis. The aim of the present study was to evaluate the participation of SK/S1P pathway in the triggering of cell differentiation by external hypertonicity. We found that hypertonicity evoked a sharp decrease in SK expression, thus activating the de novo sphingolipid synthesis pathway. Furthermore, the inhibition of SK activity evoked a relaxation of cell-cell adherens junction (AJ) with accumulation of the AJ complex (E-cadherin/β-catenin/α-catenin) in the Golgi complex, preventing the acquisition of the differentiated cell phenotype. This phenotype alteration was a consequence of a sphingolipid misbalance with an increase in ceramide levels. Moreover, we found that SNAI1 and SNAI2 were located in the cell nucleus with impairment of cell differentiation induced by SK inhibition, a fact that is considered a biochemical marker of epithelial to mesenchymal transition. So, we suggest that the expression and activity of SK1, but not SK2, act as a control system, allowing epithelial cells to synchronize the various branches of sphingolipid metabolism for an adequate cell differentiation program.

鞘脂类（Sphingolipids）调控细胞行为的多个方面，已有研究证实细胞可根据代谢需求调整其鞘脂代谢过程。尤为关键的是，1-磷酸鞘氨醇（sphingosine-1-phosphate, S1P）作为鞘脂代谢的终产物，是一种强效生物活性脂质，参与调控包括细胞增殖、细胞迁移、肌动蛋白细胞骨架重排以及细胞黏附在内的多种细胞进程。在以往针对大鼠肾乳头的研究中，本团队证实鞘氨醇激酶（sphingosine kinase, SK）的表达及S1P水平受发育调控，并可调控鞘脂的从头合成途径。本研究旨在探讨SK/S1P通路在外界高渗刺激触发细胞分化过程中的参与作用。研究结果显示，高渗刺激可显著降低SK的表达，从而激活鞘脂的从头合成通路。此外，抑制SK活性可导致细胞间黏着连接（adherens junction, AJ）松弛，并使AJ复合物（E-钙粘蛋白/β-连环蛋白/α-连环蛋白）在高尔基体（Golgi complex）中蓄积，进而阻碍细胞分化表型的获得。这种表型改变源于鞘脂代谢失衡及神经酰胺水平升高。此外，本团队还发现，SK抑制诱导的细胞分化受损时，SNAI1与SNAI2会定位于细胞核内，这一现象被视为上皮间质转化（epithelial to mesenchymal transition）的生化标志物。据此，本研究认为SK1而非SK2的表达与活性可作为调控系统，使上皮细胞能够协同调控鞘脂代谢的各个分支，以完成正常的细胞分化程序。