Frost J A, Alberts A S, Sontag E, Guan K, Mumby M C, Feramisco J R
Department of Medicine, University of California School of Medicine, La Jolla 92093-0636.
Mol Cell Biol. 1994 Sep;14(9):6244-52. doi: 10.1128/mcb.14.9.6244-6252.1994.
The simian virus 40 small tumor antigen (small t) specifically interacts with protein phosphatase type 2A (PP2A) in vivo and alters its catalytic activity in vitro. Among the substrates for PP2A in vitro are the activated forms of MEK and ERK kinases. Dephosphorylation of the activating phosphorylation sites on MEK and ERKs by PP2A in vitro results in a decrease in their respective kinase activities. Recently, it has been shown that overexpression of small t in CV-1 cells results in an inhibition of PP2A activity toward MEK and ERK2 and a constitutive upregulation of MEK and ERK2 activity. Previously, we have observed that overexpression of either ERK1, MEK1, or a constitutively active truncated form of c-Raf-1 (BXB) is insufficient to activate AP-1 in REF52 fibroblasts. We therefore examined whether overexpression of small t either alone or in conjunction with ERK1, MEK1, or BXB could activate AP-1. We found that coexpression of small t and either ERK1, MEK1, or BXB resulted in an increase in AP-1 activity, whereas expression of either small t or any of the kinases alone did not have any effect. Similarly, coexpression of small t and ERK1 activated serum response element-regulated promoters. Coexpression of kinase-deficient mutants of ERK1 and ERK2 inhibited the activation of AP-1 caused by expression of small t and either MEK1 or BXB. Coexpression of an interfering MEK, which inhibited AP-1 activation by small t and BXB, did not inhibit the activation of AP-1 caused by small t and ERK1. In contrast to REF52 cells, we observed that overexpression of either small or ERK1 alone in CV-1 cells was sufficient to stimulate AP-1 activity and that this stimulation was not enhanced by expression of small t and ERK1 together. These results show that the effects of small t on immediate-early gene expression depend on the cell type examined and suggest that the mitogen-activated protein kinase activation pathway is distinctly regulated in different cell types.
猿猴病毒40小肿瘤抗原（小t）在体内与2A型蛋白磷酸酶（PP2A）特异性相互作用，并在体外改变其催化活性。PP2A在体外的底物包括MEK和ERK激酶的活化形式。PP2A在体外使MEK和ERK上的活化磷酸化位点去磷酸化，导致它们各自的激酶活性降低。最近研究表明，小t在CV-1细胞中的过表达导致PP2A对MEK和ERK2的活性受到抑制，以及MEK和ERK2活性的组成性上调。此前，我们观察到ERK1、MEK1或组成型活性截短形式的c-Raf-1（BXB）的过表达不足以激活REF52成纤维细胞中的AP-1。因此，我们研究了小t单独或与ERK1、MEK1或BXB共同过表达是否能激活AP-1。我们发现小t与ERK1、MEK1或BXB的共表达导致AP-1活性增加，而单独表达小t或任何一种激酶均无任何影响。同样，小t与ERK1的共表达激活了血清反应元件调控的启动子。ERK1和ERK2的激酶缺陷型突变体的共表达抑制了由小t与MEK1或BXB表达引起的AP-1激活。抑制小t和BXB引起的AP-1激活的干扰性MEK的共表达，并未抑制小t和ERK1引起的AP-1激活。与REF52细胞相反，我们观察到在CV-1细胞中单独过表达小t或ERK1足以刺激AP-1活性，并且小t和ERK1共同表达并未增强这种刺激。这些结果表明，小t对早期即刻基因表达的影响取决于所检测的细胞类型，并提示丝裂原活化蛋白激酶激活途径在不同细胞类型中受到明显调控。
