Uht R M, Amos S, Martin P M, Riggan A E, Hussaini I M
Department of Pathology, School of Medicine, University of Virginia, Charlottesville, VA 22908, USA.
Oncogene. 2007 May 3;26(20):2885-93. doi: 10.1038/sj.onc.1210090. Epub 2006 Dec 4.
Glioblastoma multiforme (GBM) is the highest grade of astrocytoma. GBM pathogenesis has been linked to receptor tyrosine kinases and kinases further down signal-transduction pathways - in particular, members of the protein kinase C (PKC) family. The expression and activity of various PKC isoforms are increased in malignant astrocytomas, but not in non-neoplastic astrocytes. This suggests that PKC activity contributes to tumor progression. The level of PKC-eta expressed correlates with the degree of phorbol-12-myristate-13-acetate (PMA)-induced proliferation of two glioblastoma cell lines, U-1242 MG and U-251 MG. Normally, U-1242 cells do not express PKC-eta, and PMA inhibits their proliferation. Conversely, PMA increases proliferation of U-1242 cells that are stably transfected with PKC-eta (U-1242-PKC-eta). PMA treatment also stimulates proliferation of U-251 cells, which express PKC-eta. Here, we determined that extracellular signal-regulated kinase (ERK) and Elk-1 are downstream targets of PKC-eta. Elk-1-mediated transcriptional activity correlates with the PKC-eta-mediated mitogenic response. Pretreatment of U-1242-PKC-eta cells with inhibitors of PKC or MAPK/ERK kinase (MEK) (bisindolyl maleimide (BIM) or U0126, respectively) blocked both PMA-induced Elk-1 transcriptional activity and PMA-stimulated proliferation. An overexpressed dominant-negative PKC-eta reduced the mitogenic response in U-251 cells, as did reduction of Elk-1 by small interfering RNA. Taken together, these results strongly suggest that PKC-eta-mediated glioblastoma proliferation involves MEK/mitogen-activated protein (MAP) kinase phosphorylation, activation of ERK and subsequently of Elk-1. Elk-1 target genes involved in GBM proliferative responses have yet to be identified.
多形性胶质母细胞瘤（GBM）是星形细胞瘤的最高级别。GBM的发病机制与受体酪氨酸激酶以及信号转导通路下游的激酶有关，尤其是蛋白激酶C（PKC）家族的成员。各种PKC亚型的表达和活性在恶性星形细胞瘤中增加，但在非肿瘤性星形胶质细胞中则不然。这表明PKC活性有助于肿瘤进展。PKC-η的表达水平与佛波醇-12-肉豆蔻酸酯-13-乙酸酯（PMA）诱导的两种胶质母细胞瘤细胞系U-1242 MG和U-251 MG的增殖程度相关。正常情况下，U-1242细胞不表达PKC-η，PMA会抑制其增殖。相反，PMA会增加稳定转染PKC-η的U-1242细胞（U-1242-PKC-η）的增殖。PMA处理还会刺激表达PKC-η的U-251细胞增殖。在此，我们确定细胞外信号调节激酶（ERK）和Elk-1是PKC-η的下游靶点。Elk-1介导的转录活性与PKC-η介导的心因性反应相关。用PKC或丝裂原活化蛋白激酶/细胞外信号调节激酶（MEK）抑制剂（分别为双吲哚基马来酰亚胺（BIM）或U0126）预处理U-1242-PKC-η细胞，可阻断PMA诱导的Elk-1转录活性和PMA刺激的增殖。过表达的显性负性PKC-η降低了U-251细胞的心因性反应，小干扰RNA降低Elk-1的表达也有同样的效果。综上所述，这些结果强烈表明PKC-η介导的胶质母细胞瘤增殖涉及MEK/丝裂原活化蛋白（MAP）激酶磷酸化、ERK激活以及随后的Elk-1激活。参与GBM增殖反应的Elk-1靶基因尚未确定。
