Rao L, Perez D, White E
Center for Advanced Biotechnology and Medicine, Cancer Institute of New Jersey, Piscataway 08854, USA.
J Cell Biol. 1996 Dec;135(6 Pt 1):1441-55. doi: 10.1083/jcb.135.6.1441.
Expression of the adenovirus E1A oncogene stimulates both cell proliferation and p53-dependent apoptosis in rodent cells. p53 implements apoptosis in all or in part through transcriptional activation of bax, the product of which promotes cell death. The adenovirus E1B 19K product is homologous in sequence and in function to Bcl-2, both of which bind to and inhibit the activity of Bax and thereby suppress apoptosis. The E1B 19K protein also interacts with the nuclear lamins, but the role of this interaction in the regulation of apoptosis is not known. Lamins are, however, substrates for members of the interleukin-1 beta-converting enzyme (ICE) family of cysteine proteases that are activated during apoptosis and function downstream of Bcl-2 in the cell death pathway. lamins are degraded during E1A-induced p53-dependent apoptosis. Lamin A and C are cleaved into 47- and 37-kD fragments, respectively, and the site of proteolysis is mapped to a conserved aspartic acid residue at position 230. The cleavage of lamins during apoptosis is consistent with the activation of an ICE-related cysteine protease down-stream of p53. No lamin protease activity was detected in cells expressing the E1B 19K protein, indicating that 19K functions upstream of protease activation in inhibiting apoptosis. Substitution of the aspartic acid at the cleavage site produced a mutant lamin protein that was resistant to proteolysis both in vitro and in vivo. Expression of uncleavable mutant lamin A or B attenuated apoptosis, delaying cell death and the associated DNA fragmentation by 12 h. Mutant lamin expressing cells failed to show the signs of chromatin condensation and nuclear shrinkage typical of cell death by apoptosis. Instead, the nuclear envelope collapsed and the nuclear lamina remained intact. However, the late stage of apoptosis was morphologically unaltered and formation of apoptotic bodies was evident. Thus, lamin breakdown by proteolytic degradation facilitates the nuclear events of apoptosis perhaps by facilitating nuclear breakdown.
腺病毒E1A癌基因的表达在啮齿动物细胞中既能刺激细胞增殖，又能引发p53依赖的细胞凋亡。p53全部或部分地通过bax的转录激活来实现细胞凋亡，bax的产物可促进细胞死亡。腺病毒E1B 19K产物在序列和功能上与Bcl-2同源，二者都能结合并抑制Bax的活性，从而抑制细胞凋亡。E1B 19K蛋白还与核纤层蛋白相互作用，但这种相互作用在细胞凋亡调控中的作用尚不清楚。然而，核纤层蛋白是半胱氨酸蛋白酶白细胞介素-1β转换酶（ICE）家族成员的底物，这些蛋白酶在细胞凋亡过程中被激活，并在细胞死亡途径中位于Bcl-2的下游发挥作用。在E1A诱导的p53依赖的细胞凋亡过程中，核纤层蛋白会被降解。核纤层蛋白A和C分别被切割成47kD和37kD的片段，蛋白水解位点定位于第230位保守的天冬氨酸残基。细胞凋亡过程中核纤层蛋白被切割，这与p53下游ICE相关半胱氨酸蛋白酶的激活是一致的。在表达E1B 19K蛋白的细胞中未检测到核纤层蛋白酶活性，这表明19K在抑制细胞凋亡中在蛋白酶激活的上游发挥作用。切割位点处天冬氨酸的替换产生了一种突变型核纤层蛋白，该蛋白在体外和体内均对蛋白水解具有抗性。不可切割的突变型核纤层蛋白A或B的表达减弱了细胞凋亡过程，将细胞死亡及相关的DNA片段化延迟了12小时。表达突变型核纤层蛋白的细胞未表现出细胞凋亡典型的染色质凝聚和核收缩迹象。相反，核膜塌陷，核纤层保持完整。然而，细胞凋亡的后期形态未改变，凋亡小体的形成很明显。因此，蛋白水解降解导致的核纤层蛋白分解可能通过促进核解体来推动细胞凋亡的核事件。
