Serine palmitoyltransferase regulates de novo ceramide generation during etoposide-induced apoptosis

Detection of drug-induced apoptosis and necrosis in human cervical carcinoma cells using 1H NMR spectroscopy

Apoptosis and necrosis need to be differentiated in order to distinguish drug-induced cell death from spontaneous cell death due to hypoxia. The ability to differentiate between these two modes of cell death, especially at an early stage in the process, could have a significant impact on accessing the outcome of anticancer drug therapy in the clinic. Nuclear magnetic resonance spectroscopy was used to distinguish apoptosis from necrosis in human cervical carcinoma (HeLa) cells. Apoptosis was induced by treatment with the topoisomerase II inhibitor etoposide, whereas necrosis was induced by the use of ethacrynic acid or cytochalasin B. We found that the intensity of the methylene resonance increases significantly as early as 6 h after the onset of apoptosis, but that no such changes occur during necrosis. The spectral intensity ratio of the methylene to methyl resonances also shows a high correlation with the percentage of apoptotic cells in the sample (r2=0.965, P<0.003).

Mutations in SPTLC1, encoding serine palmitoyltransferase, long chain base subunit-1, cause hereditary sensory neuropathy type I

Hereditary sensory neuropathy type I (HSN1) is the most common hereditary disorder of peripheral sensory neurons. HSN1 is an autosomal dominant progressive degeneration of dorsal root ganglia and motor neurons with onset in the second or third decades. Initial symptoms are sensory loss in the feet followed by distal muscle wasting and weakness. Loss of pain sensation leads to chronic skin ulcers and distal amputations. The HSN1 locus has been mapped to chromosome 9q22.1-22.3 (refs. 3,4). Here we map the gene SPTLC1, encoding serine palmitoyltransferase, long chain base subunit-1, to this locus. Mutation screening revealed 3 different missense mutations resulting in changes to 2 amino acids in all affected members of 11 HSN1 families. We found two mutations to be located in exon 5 (C133Y and C133W) and one mutation to be located in exon 6 of SPTLC1 (V144D). All families showing definite or probable linkage to chromosome 9 had mutations in these two exons. These mutations are associated with increased de novo glucosyl ceramide synthesis in lymphoblast cell lines in affected individuals. Increased de novo ceramide synthesis triggers apoptosis and is associated with massive cell death during neural tube closure, raising the possibility that neural degeneration in HSN1 is due to ceramide-induced apoptotic cell death.

The AMP-activated protein kinase prevents ceramide synthesis de novo and apoptosis in astrocytes

Fatty acids induce apoptosis in primary astrocytes by enhancing ceramide synthesis de novo. The possible role of the AMP-activated protein kinase (AMPK) in the control of apoptosis was studied in this model. Long-term stimulation of AMPK with 5-aminoimidazole-4-carboxamide ribonucleoside (AICAR) prevented apoptosis. AICAR blunted fatty acid-mediated induction of serine palmitoyltransferase and ceramide synthesis de novo, without affecting fatty acid synthesis and oxidation. Prevention of ceramide accumulation by AICAR led to a concomitant blockade of the Raf-1/extracellular signal-regulated kinase cascade, which selectively mediates fatty acid-induced apoptosis. Data indicate that AMPK may protect cells from apoptosis induced by stress stimuli.

Glucosylceramide synthase does not attenuate the ceramide pool accumulating during apoptosis induced by CD95 or anti-cancer regimens

Ceramide (Cer) accumulating during the execution phase of apoptosis is generated from plasma membrane sphingomyelin (SM), which gains access to a sphingomyelinase due to phospholipid scrambling (Tepper, A. D., Ruurs, P., Wiedmer, T., Sims, P., Borst, J., and van Blitterswijk, W. J. (2000) J. Cell. Biol. 150, 155-164). To evaluate the functional significance of this Cer pool, we aimed to convert it to glucosylceramide (GlcCer), by constitutive overexpression of glucosylceramide synthase (GCS). Jurkat cells, retrovirally transduced with GCS cDNA, showed a 10-12-fold increase in GCS activity in vitro and a 7-fold elevated basal GlcCer level in vivo. However, Cer accumulating during apoptosis induced by ligation of the death receptor CD95, treatment with the anti-cancer drug etoposide, or exposure to gamma-radiation was not glycosylated by GCS. Likewise, Cer liberated at the plasma membrane by bacterial SMase was not converted by the enzyme. Thus, GCS, located at the Golgi, is topologically segregated from Cer produced in the plasma membrane. In contrast, de novo synthesized Cer as well as an exogenously supplied cell-permeable Cer analog were efficiently glycosylated, apparently due to different Cer topology and distinct physicochemical behavior of the synthetic Cer species, respectively. Exogenous cell-permeable Cer species, despite their conversion by GCS, effectively induced apoptosis. We also observed that GCS activity is down-regulated in cells undergoing apoptosis. In conclusion, GCS can convert de novo synthesized Cer but not SM-derived Cer, and, therefore, the ability of GCS overexpression to protect cells from possible detrimental effects of Cer accumulation is limited.

Antisense palmitoyl protein thioesterase 1 (PPT1) treatment inhibits PPT1 activity and increases cell death in LA-N-5 neuroblastoma cells

Infantile neuronal ceroid lipofuscinosis (INCL) is a childhood neurodegenerative disease caused by the selective death of cortical neurons and retinal degeneration, as the result of a palmitoyl protein thioesterase 1 (PPT1) deficiency. Recently, we showed that overexpression of PPT1 protects LA-N-5 human neuroblastoma cells against apoptotic death (Cho and Dawson [2000a] J. Neurochem. 74:1478-1488) and we now show that inhibition of PPT1 increases the susceptibility of these cells to apoptotic cell death. Transient transfection of LA-N-5 neuroblastoma cells with PPT1-FLAG resulted in a strong expression of PPT-FLAG-tagged protein as evidenced by Western blot analysis and immunofluorescence. Co-transfection of a reverse-oriented (antisense) PPT1 (AS-PPT1) decreased the expression of PPT-FLAG to almost zero, reduced PPT1 enzyme activity (as measured by an in vitro assay) and increased the susceptibility to apoptosis induced by C(2) ceramide. Similarly, inhibition of PPT1 with a synthetic inhibitor (AcG-palmitoyl diaminoproprionate-VKIKK) (DAP1) (100 microM) increased the susceptibility of the cells to apoptosis induced by either C(2)-ceramide or etoposide, a common chemotherapeutic agent used in the treatment of neuroblastoma. Cells stably overexpressing PPT1 were resistant to apoptosis induced by DAP1 suggesting that the inhibitor has a specific action and confirming that low levels of protein palmitoylation block the death pathway. Drugs that raise the level of protein palmitoylation are pro-apoptotic and PPT1 inhibition may enhance the killing efficacy of chemotherapeutic agents used to kill neuroblastoma-derived cells.