Darbinian-Sarkissian Nune, Czernik Marta, Peruzzi Francesca, Gordon Jennifer, Rappaport Jay, Reiss Krzysztof, Khalili Kamel, Amini Shohreh
Department of Neuroscience, Center for Neurovirology, Temple University School of Medicine, Philadelphia, Pennsylvania 19122, USA.
J Cell Physiol. 2006 Sep;208(3):506-15. doi: 10.1002/jcp.20675.
Examination of signal transduction pathways that modulate neuronal cell differentiation and protection against apoptosis has revealed a central role for the MAPK/Erk cascade. The activation of MAPK/Erk through the TrkA NGF signaling pathway is critical for growth and survival of neuronal cells. Here, we investigate the impact of HIV-1 Tat on the NGF-signaling pathway in SK-N-MC neuroblastoma cells. Expression of Tat decreased cell growth and induced apoptosis. Our results revealed dysregulation of various steps involved in the NGF pathway including suppression of MAPK, and inhibition of the promoter activity of Egr-1, a key pleiotropic mediator of the expression of genes involved in cell growth upon expression of Tat in SK-N-MC cells. Similarly, exposure of SK-N-MC to conditioned media derived from cells expressing Tat decreased phosphorylation of MAPK and reduced the level of Egr-1 protein expression in SK-N-MC cells. Furthermore, MAPK was able to phosphorylate Puralpha, a cellular protein that plays an important role in neuronal cell function and differentiation, and this was inhibited by Tat. The ability of Puralpha to interact with a GA/GC-rich sequence positioned upstream from the transcription start site of the Egr-1 promoter provided a rationale to examine Egr-1 expression. Expression of Tat decreased NGF-induced Egr-1 levels in SK-N-MC cells and reduced binding of Puralpha to the Egr-1 promoter. All of these observations support a model where the interplay between Tat and Puralpha dysregulates the NGF pathway including the MAPK/Erk network, resulting in reduced expression and activity of Egr-1 in neuronal cells.
对调节神经元细胞分化和抗凋亡信号转导途径的研究表明，MAPK/Erk级联发挥着核心作用。通过TrkA NGF信号通路激活MAPK/Erk对神经元细胞的生长和存活至关重要。在此，我们研究了HIV-1 Tat对SK-N-MC神经母细胞瘤细胞中NGF信号通路的影响。Tat的表达降低了细胞生长并诱导了凋亡。我们的结果显示，在SK-N-MC细胞中表达Tat时，NGF通路中多个步骤出现失调，包括MAPK的抑制以及Egr-1启动子活性的抑制，Egr-1是细胞生长相关基因表达的关键多效性介质。同样，将SK-N-MC细胞暴露于表达Tat的细胞所产生的条件培养基中，会降低MAPK的磷酸化水平，并降低SK-N-MC细胞中Egr-1蛋白的表达水平。此外，MAPK能够磷酸化Puralpha，一种在神经元细胞功能和分化中起重要作用的细胞蛋白，而这一过程受到Tat的抑制。Puralpha与Egr-1启动子转录起始位点上游富含GA/GC的序列相互作用的能力，为研究Egr-1的表达提供了理论依据。Tat的表达降低了SK-N-MC细胞中NGF诱导的Egr-1水平，并减少了Puralpha与Egr-1启动子的结合。所有这些观察结果支持一种模型，即Tat与Puralpha之间的相互作用使包括MAPK/Erk网络在内的NGF通路失调，导致神经元细胞中Egr-1的表达和活性降低。
