Kumar Pawan, Coltas Ila K, Kumar Bhavna, Chepeha Douglas B, Bradford Carol R, Polverini Peter J
Department of Biologic and Materials Sciences, University of Michigan School of Dentistry, 1011 North University Avenue, Ann Arbor, MI 48109, USA.
Cancer Res. 2007 Feb 1;67(3):1193-202. doi: 10.1158/0008-5472.CAN-06-2265.
The Bcl-2 oncoprotein is a potent inhibitor of apoptosis and is overexpressed in a wide variety of malignancies. Until recently, it was generally accepted that Bcl-2 primarily mediates its antiapoptotic function by regulating cytochrome c release from mitochondria. However, more recent studies have shown that Bcl-2 is present on several intracellular membranes and mitochondria may not be the only site where Bcl-2 exercises its survival function. In this study, we investigated if Bcl-2 can protect endothelial cells against gamma-radiation by a cytochrome c-independent signaling pathway. Human dermal microvascular endothelial cells (HDMEC), when exposed to gamma-radiation, exhibited a time-dependent activation of caspase-3 that was associated with increased cytochrome c release from mitochondria. Bcl-2 expression in endothelial cells (HDMEC-Bcl-2) significantly inhibited irradiation-induced caspase-3 activation. However, Bcl-2-mediated inhibition of caspase-3 was significantly reversed by inhibition of the Raf-mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (ERK) kinase (MEK)-ERK pathway. Interestingly, caspase-3 activation in HDMEC-Bcl-2 cells was not associated with cytochrome c release. We also observed that endothelial cell Bcl-2 expression significantly increased the expression of survivin and murine double minute-2 (Mdm2) via the Raf-MEK-ERK pathway. Endothelial cells expressing Bcl-2 also inhibited gamma-radiation-induced activation of p38 MAPK and p53 accumulation. Inhibition of p53 accumulation in HDMEC-Bcl-2 could be due to the enhanced expression of Mdm2 in these cells. Taken together, these results show three mechanisms by which Bcl-2 may mediate endothelial cell cytoprotection independently of cytochrome c release: (a) increased survivin expression, (b) inhibition of p53 accumulation, and (c) inhibition of p38 MAPK.
Bcl-2癌蛋白是一种有效的细胞凋亡抑制剂，在多种恶性肿瘤中均有过表达。直到最近，人们普遍认为Bcl-2主要通过调节细胞色素c从线粒体的释放来介导其抗凋亡功能。然而，最近的研究表明，Bcl-2存在于多个细胞内膜上，线粒体可能不是Bcl-2发挥其生存功能的唯一部位。在本研究中，我们调查了Bcl-2是否能通过一条不依赖细胞色素c的信号通路保护内皮细胞免受γ射线辐射。人真皮微血管内皮细胞（HDMEC）在受到γ射线辐射时，会出现半胱天冬酶-3的时间依赖性激活，这与线粒体中细胞色素c释放增加有关。内皮细胞（HDMEC-Bcl-2）中的Bcl-2表达显著抑制了辐射诱导的半胱天冬酶-3激活。然而，通过抑制Raf-丝裂原活化蛋白激酶（MAPK）/细胞外信号调节激酶（ERK）激酶（MEK）-ERK通路，Bcl-2介导的对半胱天冬酶-3的抑制作用被显著逆转。有趣的是，HDMEC-Bcl-2细胞中的半胱天冬酶-3激活与细胞色素c释放无关。我们还观察到，内皮细胞Bcl-2表达通过Raf-MEK-ERK通路显著增加了生存素和小鼠双微体-2（Mdm2）的表达。表达Bcl-2的内皮细胞也抑制了γ射线辐射诱导的p38 MAPK激活和p53积累。HDMEC-Bcl-2中p53积累的抑制可能是由于这些细胞中Mdm2表达的增强。综上所述，这些结果显示了Bcl-2可能独立于细胞色素c释放介导内皮细胞细胞保护作用的三种机制：（a）增加生存素表达，（b）抑制p53积累，以及（c）抑制p38 MAPK。
