Notochordal cell-derived conditioned medium protects human nucleus pulposus cells from stress-induced apoptosis

Abstract

Background context: Degenerative disc disease (DDD) remains without an effective therapy and presents a costly burden to society.

Purpose: Based upon prior reports concerning the effects of notochordal cell-conditioned medium (NCCM) on disc cells, we performed a proof of principle study to determine whether NCCM could reduce cytotoxic stress-induced apoptosis in human disc nucleus pulposus (NP) cells.

Study design/setting: This is an "in vitro" fundamental or basic science study.

Methods: Nucleus pulpous cells derived from 15 patients undergoing spinal surgery were treated with interleukin (IL)-1β and Fas ligand or etoposide in the presence of NCCM. We determined pro- or antiapoptotic events using activated caspase assays and determined genomic regulation of apoptosis using polymerase chain reaction arrays validated using Western blotting methods. We interrogated cellular apoptotic regulation using JC-1 dye and flow cytometry and performed enzyme-linked immunosorbent assays to evaluate NP inflammatory cytokine secretion.

Results: Notochordal cell-conditioned medium inhibits cytotoxic stress-induced caspase-9 and -3/7 activities and maintains the mitochondrial membrane potential in human NP cells, thereby suppressing the intrinsic apoptotic pathway. Gene expression analysis revealed the X-linked inhibitor of apoptosis protein as a key player responsible for evading etoposide-induced apoptosis in the presence of NCCM, and we verified these data using Western blotting. Enzyme-linked immunosorbent assay results revealed distinct differences in IL-6 and IL-8 secretions by NP cells in response to etoposide in the presence of NCCM.

Conclusions: Here we demonstrate for the first time that NCCM reduces cytotoxic stress-induced apoptosis in human NP cells. Soluble factors present in NCCM could be harnessed for the development of novel therapeutics for the treatment of DDD.

Keywords: Apoptosis; Cell proliferation; Cytokines; Degenerative disc disease; Human nucleus pulposus; Notochordal cell; X-linked inhibitor of apoptotic proteins.