Department of Surgery, Washington University School of Medicine, S, Euclid Avenue, St, Louis, MO, USA.
J Exp Clin Cancer Res. 2012 May 2;31(1):41. doi: 10.1186/1756-9966-31-41.
Sigma-2 receptor ligands have been studied for treatment of pancreatic cancer because they are preferentially internalized by proliferating cells and induce apoptosis. This mechanism of apoptosis is poorly understood, with varying reports of caspase-3 dependence. We evaluated multiple sigma-2 receptor ligands in this study, each shown to decrease tumor burden in preclinical models of human pancreatic cancer.
Fluorescently labeled sigma-2 receptor ligands of two classes (derivatives of SW43 and PB282) localize to cell membrane components in Bxpc3 and Aspc1 pancreatic cancer cells and accumulate in lysosomes. We found that interactions in the lysosome are critical for cell death following sigma-2 ligand treatment because selective inhibition of a protective lysosomal membrane glycoprotein, LAMP1, with shRNA greatly reduced the viability of cells following treatment. Sigma-2 ligands induced lysosomal membrane permeabilization (LMP) and protease translocation triggering downstream effectors of apoptosis. Subsequently, cellular oxidative stress was greatly increased following treatment with SW43, and the hydrophilic antioxidant N-acetylcysteine (NAC) gave greater protection against this than a lipophilic antioxidant, α-tocopherol (α-toco). Conversely, PB282-mediated cytotoxicity relied less on cellular oxidation, even though α-toco did provide protection from this ligand. In addition, we found that caspase-3 induction was not as significantly inhibited by cathepsin inhibitors as by antioxidants. Both NAC and α-toco protected against caspase-3 induction following PB282 treatment, while only NAC offered protection following SW43 treatment. The caspase-3 inhibitor DEVD-FMK offered significant protection from PB282, but not SW43.
Sigma-2 ligand SW43 commits pancreatic cancer cells to death by a caspase-independent process involving LMP and oxidative stress which is protected from by NAC. PB282 however undergoes a caspase-dependent death following LMP protected by DEVD-FMK and α-toco, which is also known to stabilize the mitochondrial membrane during apoptotic stimuli. These differences in mechanism are likely dependent on the structural class of the compounds versus the inherent sigma-2 binding affinity. As resistance of pancreatic cancers to specific apoptotic stimuli from chemotherapy is better appreciated, and patient-tailored treatments become more available, ligands with high sigma-2 receptor affinity should be chosen based on sensitivities to apoptotic pathways.
西格玛-2 受体配体已被研究用于治疗胰腺癌，因为它们优先被增殖细胞内化，并诱导细胞凋亡。这种凋亡机制尚不清楚，不同的报告表明其依赖于半胱氨酸蛋白酶-3（caspase-3）。本研究评估了多种西格玛-2 受体配体，它们都在人类胰腺癌的临床前模型中显示出降低肿瘤负担的作用。
两种类型的荧光标记西格玛-2 受体配体（SW43 和 PB282 的衍生物）在 Bxpc3 和 Aspc1 胰腺癌细胞中定位于细胞膜成分，并在溶酶体中积累。我们发现，溶酶体中的相互作用对于西格玛-2 配体治疗后的细胞死亡至关重要，因为用 shRNA 选择性抑制保护性溶酶体膜糖蛋白 LAMP1，会大大降低细胞在治疗后的活力。西格玛-2 配体诱导溶酶体膜通透性（LMP）和蛋白酶易位，触发细胞凋亡的下游效应器。随后，SW43 处理后细胞的氧化应激大大增加，亲水性抗氧化剂 N-乙酰半胱氨酸（NAC）比脂溶性抗氧化剂 α-生育酚（α-toco）提供了更大的保护。相反，PB282 介导的细胞毒性依赖于细胞氧化作用的程度较小，尽管 α-toco 确实可以防止这种作用。此外，我们发现半胱氨酸蛋白酶抑制剂对 caspase-3 诱导的抑制作用不如抗氧化剂明显。NAC 和 α-toco 都可以防止 PB282 处理后 caspase-3 的诱导，而只有 NAC 可以在 SW43 处理后提供保护。半胱氨酸蛋白酶-3 抑制剂 DEVD-FMK 对 PB282 有显著的保护作用，但对 SW43 没有。
西格玛-2 配体 SW43 通过一种不依赖半胱氨酸蛋白酶的过程使胰腺癌细胞死亡，该过程涉及 LMP 和氧化应激，NAC 可以防止这种情况发生。然而，PB282 则通过 LMP 发生 caspase 依赖性死亡，DEVD-FMK 可以保护这种死亡，而 α-toco 也可以在凋亡刺激物中稳定线粒体膜。这些机制上的差异可能取决于化合物的结构类别与内在的西格玛-2 结合亲和力。随着人们更好地认识到胰腺癌对特定凋亡刺激的耐药性，以及患者定制化治疗的应用越来越广泛，应该根据对凋亡途径的敏感性来选择具有高西格玛-2 受体亲和力的配体。
