OBJECTIVE: To explore the hypoxic regulation of sumoylation pathways and cell 
viability in nucleus pulposus (NP) and annulus fibrosus (AF) cells.
DESIGN: Expression of small ubiquitin-like modifier (SUMO) molecules, SUMO E1 
activating enzymes SAE1 and SAE2, SUMO E2 conjugating enzyme UBC9, and 
de-sumoylation enzyme sentrin/SUMO-specific proteases (SENP)1 was 
immunolocalized in rat intervertebral disc (IVD) cells. NP and AF cells were 
cultured in hypoxia and cell viability was evaluated by quantifying cell 
proliferation, cellular senescence, apoptosis, and cell cycle distribution. 
Hypoxic regulation of sumoylation pathways was studied by analyzing the 
transcription and expression of SUMO molecules and sumoylation enzymes. Loss of 
function study using SENP1 siRNA was performed to investigate the regulatory 
role of sumoylation on the function of hypoxia inducible factor 1α (HIF-1α) and 
the hypoxic tolerance of IVD cells.
RESULTS: Sumoylation pathways were expressed in IVD cells and localized 
predominantly in nuclei. Both NP and AF cells maintained viability under hypoxia 
and upregulated the expression of SENP1. In NP cells hypoxia transiently 
increased the expression of SUMO-1, SUMO-2/3, SAE2, and UBC9, whereas SUMO-1 was 
elevated while SUMO-2/3, SAE1, SAE2, and UBC9 were reduced by low oxygen 
tensions in AF cells. Although downregulation of SENP1 decreased the 
transcriptional activity of HIF-1α, the viability of disc cells showed no 
significant loss under hypoxia.
CONCLUSIONS: NP and AF cells equally tolerate oxygen deficiency, but differently 
regulate the sumoylation pathways under hypoxia. The distinct sumoylation 
dynamics may help extend our understanding of the cell-specific regulation of 
the molecular basis that promotes cell survival in the hypoxic IVD.