Tafesse Fikadu G, Vacaru Ana M, Bosma Elleke F, Hermansson Martin, Jain Amrita, Hilderink Angelika, Somerharju Pentti, Holthuis Joost C M
Membrane Enzymology, Bijvoet Center and Institute of Biomembranes, Utrecht University, 3584 CH Utrecht, the Netherlands.
J Cell Sci. 2014 Jan 15;127(Pt 2):445-54. doi: 10.1242/jcs.138933. Epub 2013 Nov 20.
Cells synthesize ceramides in the endoplasmic reticulum (ER) as precursors for sphingolipids to form an impermeable plasma membrane. As ceramides are engaged in apoptotic pathways, cells would need to monitor their levels closely to avoid killing themselves during sphingolipid biosynthesis. How this is accomplished remains to be established. Here we identify SMSr (SAMD8), an ER-resident ceramide phosphoethanolamine (CPE) synthase, as a suppressor of ceramide-mediated cell death. Disruption of SMSr catalytic activity causes a rise in ER ceramides and their mislocalization to mitochondria, triggering a mitochondrial pathway of apoptosis. Blocking de novo ceramide synthesis, stimulating ceramide export from the ER or targeting a bacterial ceramidase to mitochondria rescues SMSr-deficient cells from apoptosis. We also show that SMSr-catalyzed CPE production, although essential, is not sufficient to suppress ceramide-induced cell death and that SMSr-mediated ceramide homeostasis requires the N-terminal sterile α-motif, or SAM domain, of the enzyme. These results define ER ceramides as bona fide transducers of mitochondrial apoptosis and indicate a primary role of SMSr in monitoring ER ceramide levels to prevent inappropriate cell death during sphingolipid biosynthesis.
细胞在内质网（ER）中合成神经酰胺作为鞘脂的前体，以形成不可渗透的质膜。由于神经酰胺参与凋亡途径，细胞需要密切监测其水平，以避免在鞘脂生物合成过程中自我杀伤。这一过程是如何实现的仍有待确定。在这里，我们鉴定出内质网驻留的神经酰胺磷酸乙醇胺（CPE）合酶SMSr（SAMD8）作为神经酰胺介导的细胞死亡的抑制剂。SMSr催化活性的破坏导致内质网神经酰胺水平升高及其向线粒体的错误定位，触发凋亡的线粒体途径。阻断从头合成神经酰胺、刺激神经酰胺从内质网输出或靶向细菌神经酰胺酶到线粒体可使SMSr缺陷细胞免于凋亡。我们还表明，SMSr催化的CPE产生虽然必不可少，但不足以抑制神经酰胺诱导的细胞死亡，并且SMSr介导的神经酰胺稳态需要该酶的N端无菌α基序或SAM结构域。这些结果将内质网神经酰胺定义为线粒体凋亡的真正转导因子，并表明SMSr在监测内质网神经酰胺水平以防止鞘脂生物合成过程中不适当的细胞死亡方面的主要作用。
