Caspase 8L, a novel inhibitory isoform of caspase 8, is associated with undifferentiated neuroblastoma

Abstract

Caspase 8 is a key apoptotic factor in the receptor/ligand apoptosis-signaling cascade. Absent caspase 8 expression is shown to correlate with poor prognosis in neuroblastoma. Paradoxically, the caspase 8 gene can produce as plice variant and novel inhibitor of itself-caspase 8l. The presence of caspase 8 alone in tumors may not necessarily portend a good prognosis. We sought to determine whether caspase 8l is present in neuroblastoma and whether over-expression of this protein could inhibit caspase 8-dependent apoptosis. Six of 6 histologically undifferentiated and 2 of 5 differentiated neuroblastoma tumors expressed the caspase 8l isoform, whereas caspase 8l was absent in 3 of 3 ganglioneuromas. Seven human neuroblastoma cell lines were surveyed. Two of the 5 cell lines that expressed caspase 8 also expressed the caspase 8l isoform and both were of a less differentiated neuronal phenotype. Over-expression of caspase 8l in cell lines afforded protection against TRAIL, but not against etoposide induced apoptosis. Conversely, blockade of Caspase 8l in cells that express this splice variant made them more sensitive to apoptosis induced cell death. We demonstrate the caspase 8l isoform is present in neuroblastoma and appears to be associated with undifferentiated cell lines and tumors. Furthermore, it suppresses caspase 8-dependent apoptosis.

Caspase-8 as a therapeutic target in cancer

“…Although the functions of such isoforms are not yet precisely known, the DEDs of caspase-8 are required for recruitment to the death inducing signaling complex (DISC), and it has been reported that the these DED-only proteins can act as competitive inhibitors to caspase-8 DISC recruitment [14] [15], as observed for twin DED-proteins found in viruses such as molluscum contagiosum [16]. This possible function is intriguing, as it also suggests that for each procaspase-8 activation event, one would actually result in generation of a caspase-8 inhibitor (ie., the mature prodomains).…”

Evasion of Apoptosis as a Cellular Stress Response in Cancer

“…In addition, homo- or heterozygous genomic deletions were detected in neuroblastoma [33]. Alternative splicing of intron 8 of the caspase-8 gene resulting in the generation of caspase-8L, a catalytically inactive splice variant presents another mechanism of caspase-8 inactivation [34, 35]. Epigenetic silencing secondary to hypermethylation of regulatory sequences of the caspase-8 gene occurs in various tumors, for example, neuroblastoma, malignant brain tumors, Ewing tumor, retinoblastoma, rhabdomyosarcoma, or small lung cell carcinoma [33, 36–39].…”

The Enigmatic Roles of Caspases in Tumor Development

“…It was detected by western blotting in AML and ALL and suggested to protect their progenitors from regulatory apoptosis [81]. In a small panel of neuroblastomas, caspase-8L transcripts could be detected in six out of six undifferentiated tumors, but only in two out of five differentiated tumors [82]. Because most anti-caspase-8 antibodies will not discriminate between caspase-8 and -8L, it is currently unclear to which extent caspase-8L may have contributed to the caspase-8 expression detected in the immunohistochemical analyses of various cancer types.…”