Department of Medicine, University of Miami Miller School of Medicine, Miami, Florida, United States of America.
PLoS One. 2010 Feb 18;5(2):e9295. doi: 10.1371/journal.pone.0009295.
The MAPK/ERK1/2 serine kinases are primary mediators of the Ras mitogenic signaling pathway. Phosphorylation by MEK activates MAPK/ERK in the cytoplasm, and phospho-ERK is thought to enter the nucleus readily to modulate transcription.
Here, however, we observe that in primary cultures of breast and ovarian epithelial cells, phosphorylation and activation of ERK1/2 are disassociated from nuclear translocalization and transcription of downstream targets, such as c-Fos, suggesting that nuclear translocation is limited in primary cells. Accordingly, in import assays in vitro, primary cells showed a lower import activity for ERK1/2 than cancer cells, in which activated MAPK readily translocated into the nucleus and activated c-Fos expression. Primary cells express lower levels of nuclear pore complex proteins and the nuclear transport factors, importin B1 and importin 7, which may explain the limiting ERK1/2 import found in primary cells. Additionally, reduction in expression of nucleoporin 153 by siRNA targeting reduced ERK1/2 nuclear activity in cancer cells.
ERK1/2 activation is dissociated from nuclear entry, which is a rate limiting step in primary cells and in vivo, and the restriction of nuclear entry is disrupted in transformed cells by the increased expression of nuclear pores and/or nuclear transport factors.
丝裂原活化蛋白激酶/细胞外信号调节激酶 1/2（MAPK/ERK1/2）丝氨酸激酶是 Ras 有丝分裂信号通路的主要介质。MEK 的磷酸化使 MAPK/ERK 在细胞质中被激活，磷酸化的 ERK 被认为可以轻易进入细胞核来调节转录。
然而，在这里，我们观察到在乳腺和卵巢上皮细胞的原代培养物中，ERK1/2 的磷酸化和激活与核转位和下游靶标（如 c-Fos）的转录分离，这表明核转位在原代细胞中受到限制。相应地，在体外导入实验中，与癌细胞相比，原代细胞的 ERK1/2 导入活性较低，在癌细胞中，激活的 MAPK 很容易进入细胞核并激活 c-Fos 表达。原代细胞表达较低水平的核孔复合体蛋白和核转运因子，如 importin B1 和 importin 7，这可能解释了原代细胞中发现的 ERK1/2 导入受限。此外，通过靶向核孔蛋白 153 的 siRNA 降低其表达会降低癌细胞中 ERK1/2 的核活性。
ERK1/2 的激活与核进入分离，这是原代细胞和体内的限速步骤，而核进入的限制在转化细胞中被核孔和/或核转运因子的表达增加所破坏。
