Hsi L C, Eling T E
National Institute of Environmental Health Sciences, Laboratory of Molecular Carcinogenesis, Eicosanoid Biochemistry Section, Research Triangle Park, NC 27709, USA.
Prostaglandins Leukot Essent Fatty Acids. 1998 Apr;58(4):271-81. doi: 10.1016/s0952-3278(98)90036-9.
Lipid metabolism can play an important role in the development and progression of human cancers. We have used Syrian hamster embryo (SHE) fibroblasts as a model system to study how lipid metabolites can alter cell proliferation and apoptosis. For example, the linoleic acid metabolite 13(S)-HpODE enhances EGF-dependent growth by inhibiting de-phosphorylation of the EGFR which leads to activation of the MAP kinase pathway. In contrast, the arachidonic acid metabolite, PGE2, inhibits EGF-dependent mitogenesis and the expression of the proto-oncogenes c-myc, c-jun, and jun-B. In this study, we have investigated the mechanism by which PGE2 attenuates these responses by studying the EGF signaling cascade in SHE cells. PGE2 pretreatment caused a concentration-dependent decrease in EGF-dependent phosphorylation of MAP kinase and a corresponding inhibition of EGF-stimulated MAP kinase activity. Pretreatment of the SHE cells with PGE2 had little effect on the magnitude of EGF-dependent receptor auto-phosphorylation and the phosphorylation of GAP suggesting a down-stream target. Treatment of cells with forskolin and EGF causes similar inhibition of MAP kinase phosphorylation as observed with PGE2 and EGF. Since PGE2 elevates cAMP in these cells, it may act by altering cAMP accumulation. Raf-1 activity can be inhibited by a cAMP-dependent process. Raf-1 activity, measured by phosphorylation of Mek-1, was attenuated by the addition of PGE2. To determine if inhibition of Raf-1 activity causes inhibition of the MAP kinase pathway, cells were concomitantly incubated with PGE2 and EGF. Inhibition of MAP kinase phosphorylation was observed. From these data, we propose that in SHE cells PGE2 increases cAMP levels, which in turn causes inhibition of Raf-1 activity. The MAP kinase pathway is thus downregulated which decreases mitogenesis and proto-oncogene expression. This study demonstrates that an arachidonic acid metabolite can modulate phosphorylation and activity of key signal transduction proteins in a growth factor mitogenic pathway.
脂质代谢在人类癌症的发生和发展过程中可能发挥重要作用。我们已将叙利亚仓鼠胚胎（SHE）成纤维细胞用作模型系统，以研究脂质代谢产物如何改变细胞增殖和凋亡。例如，亚油酸代谢产物13(S)-HpODE通过抑制表皮生长因子受体（EGFR）的去磷酸化来增强表皮生长因子（EGF）依赖性生长，这会导致丝裂原活化蛋白激酶（MAP激酶）途径的激活。相反，花生四烯酸代谢产物前列腺素E2（PGE2）抑制EGF依赖性有丝分裂以及原癌基因c-myc、c-jun和jun-B的表达。在本研究中，我们通过研究SHE细胞中的EGF信号级联反应，探究了PGE2减弱这些反应的机制。PGE2预处理导致EGF依赖性MAP激酶磷酸化呈浓度依赖性降低，并相应抑制了EGF刺激的MAP激酶活性。用PGE2预处理SHE细胞对EGF依赖性受体自身磷酸化的程度以及GAP的磷酸化影响很小，提示存在一个下游靶点。用毛喉素和EGF处理细胞会导致与PGE2和EGF处理时观察到的类似的MAP激酶磷酸化抑制。由于PGE2会升高这些细胞中的环磷酸腺苷（cAMP）水平，它可能通过改变cAMP积累来发挥作用。Raf-1活性可被一个依赖cAMP的过程所抑制。通过Mek-1的磷酸化来测定的Raf-1活性，会因添加PGE2而减弱。为了确定Raf-1活性的抑制是否会导致MAP激酶途径的抑制，细胞同时与PGE2和EGF一起孵育。观察到MAP激酶磷酸化受到抑制。根据这些数据，我们提出在SHE细胞中PGE2会增加cAMP水平，这进而导致Raf-1活性受到抑制。MAP激酶途径因此被下调，从而减少有丝分裂和原癌基因表达。本研究表明，一种花生四烯酸代谢产物可以调节生长因子有丝分裂途径中关键信号转导蛋白的磷酸化和活性。
