Background: Two main pathways of Fas-mediated apoptosis have been
identified: the Type-I (death-inducing signaling complex) pathway and the
Type-II (mitochondrial) pathway. While apoptotic cell death has been
implicated in lumbar degenerative disc disease, we are not aware of any
studies in which surgically removed discs from live humans have been examined
to determine which of the two pathways is involved in the apoptosis of disc
cells. As an initial step in the development of therapies to inhibit
inappropriate or premature apoptosis of disc cells, our objective was to
determine which pathway is involved.
Methods: We examined thirty-two samples of herniated lumbar disc
tissue with use of immunohistochemical staining and Western blot analysis to
determine the presence of several proteins, including caspase-8 (associated
with the Type-I pathway); BID (BH3 interacting domain death agonist),
cytochrome-c, and caspase-9 (associated with the Type-II pathway); and
caspase-3 (an executioner of apoptosis). The TUNEL (terminal deoxynucleotidyl
transferase [TDT]-mediated dUTP nick end labeling) assay was performed to
confirm the occurrence of apoptosis of the disc cells.
Results: The proteins associated with the Type-II pathway (BID,
cytochrome-c, and caspase-9) stained positively in all samples. Although the
protein associated with the Type-I pathway (caspase-8) was not detected on
immunohistochemical analysis, a small amount of caspase-8 was detected on
Western blot analysis. However, the expression of Type-II proteins was still
higher than the expression of caspase-8 on Western blot analysis. The
expression of caspase-3 was identified in all samples with immunohistochemical
and Western blot analysis. TUNEL-positive disc cells were identified in all
samples.
Conclusions: The results of the present study suggest that human
disc cells are Type-II cells, which undergo apoptotic cell death through
mitochondrial involvement.
Clinical Relevance: Future therapeutic modalities to inhibit
inappropriate or premature apoptotic cell death in degenerating discs should
be directed to the mitochondrial pathway.