INTRODUCTION: In the avascular disc, cells are adapted to reduced oxygen supply. With ongoing disc degeneration, oxygen concentration can decrease to insufficient 1%. In contrast, unphysiological 21% oxygen might also harm nucleus pulposus (NP) cells. Aim of our study was to investigate the influence of variations in the oxygen concentration on gene expression of matrix proteins, catabolic enzymes and angiogenic factors.
METHODS: NP cells isolated from bovine (n=8) or human discs (n=8) were transferred to pellet culture for 4 weeks in chondrogenic medium at either 21% (group A) or 6% oxygen (group B). After 4 weeks, oxygen was reduced to 1% to investigate the influence of oxygen deprivation on gene expression of aggrecan, collagen I, II, Sox9, HIF1α, VEGF, FGF, PTN, MMP3, MMP13, and aggrecanase. Significant results were verified by immunohistology. Statistics: Wilcoxon, p<0.05.
RESULTS: A 4‐week adaption to 6% or 21% oxygen deminished gene expression of NP cells at 6% with stronger effects in bovine than human NP cells. In bovine cells a further reduction to 1% oxygen changed the expression of all target genes, with more pronounced effects in group A that was adapted to 21% oxygen: most genes were significantly reduced, but VEGF was significantly up‐regulated (2.4‐fold). In human NP cells angiogenic genes were significantly reduced. Immunohistology confirmed gene expression results.
DISCUSSION: In NP long‐term cultures the oxygen environment influenced NP cell sensitivity towards further oxygen reduction, whereas for most target genes expression was higher at atmospheric oxygen. NP cells adapted to low oxygen supply were less sensitive to further oxygen reduction. As hypoxia diminished matrix protein expression and up‐regulated VEGF expression, the balance in matrix turnover might shift towards a faster degradation thus facilitating ingrowth of blood vessels possibly regulated by VEGF (GENODISC: HEALTH‐F2‐2008‐201626).