A novel role for atypical MAPK kinase ERK3 in regulating breast cancer cell morphology and migration

ERK3 is an atypical Mitogen-activated protein kinases (MAPK6). Despite the fact that the Erk3 gene was originally identified in 1991, its function is still unknown. MK5 (MAP kinase- activated protein kinase 5) also called PRAK is the only known substrate for ERK3. Recently, it was found that group I p21 protein activated kinases (PAKs) are critical effectors of ERK3. PAKs link Rho family of GTPases to actin cytoskeletal dynamics and are known to be involved in the regulation of cell adhesion and migration. In this study we demonstrate that ERK3 protein levels are elevated as MDA-MB-231 breast cancer cells adhere to collagen I which is concomitant with changes in cellular morphology where cells become less well spread following nascent adhesion formation. During this early cellular adhesion event we observe that the cells retain protrusive activity whilst reducing overall cellular area. Interestingly exogenous expression of ERK3 delivers a comparable reduction in cell spread area, whilst depletion of ERK3 expression increases cell spread area. Importantly, we have detected a novel specific endogenous ERK3 localization at the cell periphery. Furthermore we find that ERK3 overexpressing cells exhibit a rounded morphology and increased cell migration speed. Surprisingly, exogenous expression of a kinase inactive mutant of ERK3 phenocopies ERK3 overexpression, suggesting a novel kinase independent function for ERK3. Taken together our data suggest that as cells initiate adhesion to matrix increasing levels of ERK3 at the cell periphery are required to orchestrate cell morphology changes which can then drive migratory behaviour.细胞外调节蛋白激酶3（Extracellular signal-regulated kinase 3，ERK3）属于非典型丝裂原活化蛋白激酶（Mitogen-activated protein kinase，MAPK6）。1991年Erk3基因首次被鉴定，但截至目前其具体功能仍未阐明。MK5（丝裂原活化蛋白激酶激活的蛋白激酶5，MAP kinase-activated protein kinase 5，又称PRAK）是目前已知的ERK3唯一底物。近期研究发现，I族p21激活激酶（p21-activated kinases，PAKs）是ERK3的关键效应因子。PAKs可将Rho家族GTP酶与肌动蛋白细胞骨架的动态调控过程相连接，已知其参与细胞黏附与迁移的调控。本研究证实，当MDA-MB-231乳腺癌细胞黏附于I型胶原蛋白时，ERK3蛋白水平显著升高，这一变化与细胞形态改变同步：细胞在形成初始黏附后，铺展程度明显降低。在这一早期细胞黏附事件中，我们观察到细胞仍维持突起活性，但整体细胞面积有所缩减。值得注意的是，外源过表达ERK3可使细胞铺展面积出现相似程度的降低，而敲低ERK3的表达则会增加细胞的铺展面积。尤为关键的是，我们在细胞外周检测到一种全新的特异性内源性ERK3定位模式。此外，过表达ERK3的细胞呈现出圆形形态，且细胞迁移速度加快。令人意外的是，外源表达激酶失活型ERK3突变体可重现ERK3过表达的表型，这提示ERK3存在一种不依赖于激酶活性的全新功能。综合以上研究结果，我们的数据表明：当细胞启动与细胞外基质的黏附过程时，细胞外周ERK3水平的升高是协调细胞形态变化所必需的，而这一过程可进一步驱动细胞的迁移行为。