Department of Biological Sciences, University of South Carolina, Columbia, SC 29208,USA.
Gene. 2013 Jul 15;524(1):15-21. doi: 10.1016/j.gene.2013.03.124. Epub 2013 Apr 15.
Interferons (IFNs) are a family of cytokines that exhibit antiviral, antiproliferative, and immunomodulatory properties. PKR (protein kinase, RNA activated) is of central importance in mediating the antiproliferative actions of IFNs. Our research has established that PKR inhibits vascular smooth muscle cell (VSMC) proliferation by regulating G1 to S transition. Many cardiovascular diseases result from complications of atherosclerosis, a chronic and progressive inflammatory condition often characterized by excessive proliferation of VSMC. Thus, an effective method for inhibiting VSMC proliferation is likely to arrest atherosclerosis and restenosis at early stages. Our research establishes that PKR activation in VSMC leads to a G1 arrest brought about by an inhibition of cyclin-dependent kinase 2 (Cdk2) activity by p27(kip1). In quiescent VSMC, p27(kip1) levels are high and when stimulated by serum/growth factors, p27(kip1) levels drop by destabilization of the protein. Under conditions that lead to activation of PKR, there is a marked inhibition of p27(kip1) down-regulation due to increased stability of p27(kip1) protein. In order to understand the mechanism of heparin-induced stabilization of p27(kip1) in VSMC, we examined the involvement of the Signal Transducer and Activator of Transcription-1 (STAT1), which is an important player in mediating antiproliferative effects of IFNs. Our results demonstrate that PKR overexpression in VSMC leads to an increase in p27(kip1) protein levels and this increase requires the catalytic activity of PKR. PKR activation induced by antiproliferative agent heparin leads to phosphorylation of STAT1 on serine 727, which is essential for the cell cycle block. STAT1 null VSMCs are largely defective in heparin-induced cell cycle arrest and in PKR null cells the STAT1 phosphorylation in response to heparin was absent. These results establish that heparin causes STAT1 phosphorylation on serine 727 via activation of PKR and that this event is required for the G1 arrest in VSMC.
干扰素（IFNs）是一类细胞因子，具有抗病毒、抗增殖和免疫调节特性。PKR（RNA 激活的蛋白激酶）在介导 IFNs 的抗增殖作用中具有核心重要性。我们的研究已经确定，PKR 通过调节 G1 到 S 期转换来抑制血管平滑肌细胞（VSMC）的增殖。许多心血管疾病是由动脉粥样硬化的并发症引起的，动脉粥样硬化是一种慢性和进行性炎症状态，通常表现为 VSMC 的过度增殖。因此，一种有效的抑制 VSMC 增殖的方法可能会在早期阶段阻止动脉粥样硬化和再狭窄。我们的研究确定，PKR 在 VSMC 中的激活导致 G1 期停滞，这是由 cyclin-dependent kinase 2（Cdk2）活性被 p27（kip1）抑制引起的。在静止的 VSMC 中，p27（kip1）水平较高，当受到血清/生长因子刺激时，p27（kip1）水平通过蛋白不稳定而下降。在导致 PKR 激活的条件下，由于 p27（kip1）蛋白稳定性增加，p27（kip1）下调受到明显抑制。为了了解肝素诱导 VSMC 中 p27（kip1）稳定的机制，我们研究了信号转导和转录激活因子 1（STAT1）的参与，STAT1 是介导 IFNs 抗增殖作用的重要参与者。我们的结果表明，PKR 在 VSMC 中的过表达导致 p27（kip1）蛋白水平增加，这种增加需要 PKR 的催化活性。抗增殖剂肝素诱导的 PKR 激活导致 STAT1 丝氨酸 727 磷酸化，这对于细胞周期阻滞是必不可少的。STAT1 缺失的 VSMC 在肝素诱导的细胞周期阻滞中存在很大缺陷，而在 PKR 缺失的细胞中，肝素引起的 STAT1 磷酸化缺失。这些结果表明，肝素通过激活 PKR 导致 STAT1 丝氨酸 727 磷酸化，并且该事件是 VSMC 中 G1 期阻滞所必需的。
