Abstract
Degenerative disc disease is associated with increased expression of pro-inflammatory cytokines in the intervertebral disc (IVD). However, it is not completely clear how inflammation arises in the IVD and which cellular compartments are involved in this process. Recently, the endoplasmic reticulum (ER) has emerged as a possible modulator of inflammation in age-related disorders. In addition, ER stress has been associated with the microenvironment of degenerated IVDs. Therefore, the aim of this study was to analyze the effects of ER stress on inflammatory responses in degenerated human IVDs and associated molecular mechanisms. Gene expression of ER stress marker GRP78 and pro-inflammatory cytokines IL-6, IL-8, IL-1β, and TNF-α was analyzed in human surgical IVD samples (n = 51, Pfirrmann grade 2–5). The expression of GRP78 positively correlated with the degeneration grade in lumbar IVDs and IL-6, but not with IL-1β and TNF-α. Another set of human surgical IVD samples (n = 25) was used to prepare primary cell cultures. ER stress inducer thapsigargin (Tg, 100 and 500 nM) activated gene and protein expression of IL-6 and induced phosphorylation of p38 MAPK. Both inhibition of p38 MAPK by SB203580 (10 µM) and knockdown of ER stress effector CCAAT-enhancer-binding protein homologous protein (CHOP) reduced gene and protein expression of IL-6 in Tg-treated cells. Furthermore, the effects of an inflammatory microenvironment on ER stress were tested. TNF-α (5 and 10 ng/mL) did not activate ER stress, while IL-1β (5 and 10 ng/mL) activated gene and protein expression of GRP78, but did not influence [Ca2+]i flux and expression of CHOP, indicating that pro-inflammatory cytokines alone may not induce ER stress in vivo. This study showed that IL-6 release in the IVD can be initiated following ER stress and that ER stress mediates IL-6 release through p38 MAPK and CHOP. Therapeutic targeting of ER stress response may reduce the consequences of the harsh microenvironment in degenerated IVD.
Highlights
Low back pain (LBP) is the leading cause of disability, activity limitation, and lost productivity throughout the world today, with approximately 80% of all people suffering from back pain at least once in their life [1]
GRP78 expression in cervical and lumbar intervertebral disc (IVD) together showed a tendency to increase with the degeneration grade (p = 0.0593; Figure 1A) and association with the duration of the pain symptoms (p = 0.015, sub-acute vs chronic; Figure 1B), with high variability in the acute group
In lumbar IVDs, the expression of GRP78 significantly increased with increasing degeneration grade (2 vs 3 p = 0.0019, 2 vs 4 p = 0.0399, Figure 1 | The expression of GRP78 in human degenerated intervertebral discs (IVDs)
Summary
Low back pain (LBP) is the leading cause of disability, activity limitation, and lost productivity throughout the world today, with approximately 80% of all people suffering from back pain at least once in their life [1]. Degenerative disc disease (DDD), a progressive multifactorial disorder of intervertebral disc (IVD), often caused by inflammation and subsequent irritation of spinal nerves, is one of the main factors in the development of LBP [1]. Despite the significant socio-economic consequences associated with DDD, no therapy can presently stop or reverse the degenerative process [2, 3]. Success of novel biological therapies (e.g., stem cell implantation) is often hindered by the pre-existing harsh inflammatory microenvironment in the degenerated IVD [5, 6]. Mechanisms underlying the degenerative microenvironment in the IVD are not yet sufficiently understood
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