Ceramide, a
tumor-suppressor
lipid, is generated by
sphingomyelin hydrolysis or by de novo synthesis when cells are activated by various stress stimuli as well as when
cancer cells are subjected to genotoxic
chemotherapy.
Ceramide may modulate apoptotic signaling pathways; however, its transcription-dependent effects remain unclear. Our data showed that
actinomycin D partially inhibited
ceramide-induced apoptosis. Using microarray analysis, we found that
ceramide up-regulated a tumor suppressor gene called
thioredoxin-interacting
protein (Txnip). Similarly, the chemotherapeutic agent
etoposide induced Txnip expression en route to apoptosis, which was blocked by inhibitors of
ceramide production. Txnip colocalized with
thioredoxin and reduced its activity, which caused dissociation of
thioredoxin from
apoptosis signal-regulating kinase 1 (ASK1). Cells expressing ASK1
siRNA were more resistant to
ceramide-induced apoptosis.
Ceramide caused ASK1-regulated
p38 mitogen-activated protein kinase (MAPK) and JNK activation, as well as activation of the endoplasmic reticulum (ER) stress cascade, and pharmacologic or
siRNA-mediated inhibition of
p38 MAPK or JNK partially reduced
ceramide-induced mitochondria-mediated apoptosis. Furthermore,
ceramide-induced ASK1, p38, and JNK phosphorylation and cell apoptosis were inhibited by Txnip
siRNA transfection. Taken together, we show that
ceramide exhibits a mechanism of transcriptional regulation involving up-regulation of Txnip expression, also induced by
etoposide, which results in ASK1 activation, ER stress, and p38 and JNK phosphorylation, all leading to apoptosis.