Rahmani Mohamed, Davis Eric Maynard, Crabtree Timothy Ryan, Habibi Joseph Reza, Nguyen Tri K, Dent Paul, Grant Steven
Department of Medicine, Division of Hematology/Oncology, MCV Station Box 230, Virginia Commonwealth University, Richmond, VA 23298, USA.
Mol Cell Biol. 2007 Aug;27(15):5499-513. doi: 10.1128/MCB.01080-06. Epub 2007 Jun 4.
Sorafenib is a multikinase inhibitor that induces apoptosis in human leukemia and other malignant cells. Recently, we demonstrated that sorafenib diminishes Mcl-1 protein expression by inhibiting translation through a MEK1/2-ERK1/2 signaling-independent mechanism and that this phenomenon plays a key functional role in sorafenib-mediated lethality. Here, we report that inducible expression of constitutively active MEK1 fails to protect cells from sorafenib-mediated lethality, indicating that sorafenib-induced cell death is unrelated to MEK1/2-ERK1/2 pathway inactivation. Notably, treatment with sorafenib induced endoplasmic reticulum (ER) stress in human leukemia cells (U937) manifested by immediate cytosolic-calcium mobilization, GADD153 and GADD34 protein induction, PKR-like ER kinase (PERK) and eukaryotic initiation factor 2alpha (eIF2alpha) phosphorylation, XBP1 splicing, and a general reduction in protein synthesis as assessed by [35S]methionine incorporation. These events were accompanied by pronounced generation of reactive oxygen species through a mechanism dependent upon cytosolic-calcium mobilization and a significant decline in GRP78/Bip protein levels. Interestingly, enforced expression of IRE1alpha markedly reduced sorafenib-mediated apoptosis, whereas knockdown of IRE1alpha or XBP1, disruption of PERK activity, or inhibition of eIF2alpha phosphorylation enhanced sorafenib-mediated lethality. Finally, downregulation of caspase-2 or caspase-4 by small interfering RNA significantly diminished apoptosis induced by sorafenib. Together, these findings demonstrate that ER stress represents a central component of a MEK1/2-ERK1/2-independent cell death program triggered by sorafenib.
索拉非尼是一种多激酶抑制剂，可诱导人白血病细胞和其他恶性细胞凋亡。最近，我们证明索拉非尼通过一种不依赖MEK1/2-ERK1/2信号传导的机制抑制翻译，从而降低Mcl-1蛋白表达，并且这一现象在索拉非尼介导的细胞致死性中起关键作用。在此，我们报告组成型活性MEK1的诱导表达不能保护细胞免受索拉非尼介导的致死性，这表明索拉非尼诱导的细胞死亡与MEK1/2-ERK1/2通路失活无关。值得注意的是，索拉非尼处理可诱导人白血病细胞（U937）发生内质网（ER）应激，表现为胞质钙立即动员、GADD153和GADD34蛋白诱导、PKR样内质网激酶（PERK）和真核起始因子2α（eIF2α）磷酸化、XBP1剪接以及通过[35S]甲硫氨酸掺入评估的蛋白质合成普遍减少。这些事件伴随着通过依赖于胞质钙动员的机制产生大量活性氧以及GRP78/Bip蛋白水平显著下降。有趣的是，IRE1α的强制表达显著降低了索拉非尼介导的细胞凋亡，而IRE1α或XBP1的敲低、PERK活性的破坏或eIF2α磷酸化的抑制则增强了索拉非尼介导的致死性。最后，小干扰RNA下调caspase-2或caspase-4可显著减少索拉非尼诱导的细胞凋亡。总之，这些发现表明内质网应激是索拉非尼触发的不依赖MEK1/2-ERK1/2的细胞死亡程序的核心组成部分。
