Acidification changes affect the inflammasome in human nucleus pulposus cells
Brand et al. Journal of Inflammation
Acidification changes affect the inflammasome in human nucleus pulposus cells
Frank J. Brand III 1 2
Mahtab Forouzandeh 1 2
Harmanpreet Kaur 1 2
Francesco Travascio 2 3
Juan Pablo de Rivero Vaccari 0 1 2
0 Department of Neurological Surgery, Lois Pope LIFE Center , 1095 NW 14th Terrace, 3-25JJ, Miami, FL 33136-1060 , USA
1 Department of Neurological Surgery, The Miami Project to Cure Paralysis, Miller School of Medicine, University of Miami , Miami, FL 33136 , USA
2 Abbreviations: ALR, AIM-2-like receptor; ASC, Apoptosis-associated speck-like protein containing a caspase recruitment domain; ATP , Adenosine tri-phosphate; ECM, Extracellular matrix; HNPC, Human nucleus pulposus cells; IDD, Intervertebral disc degeneration; IL, Interleukin; IVD, Intervertebral disc; LDH, Lactose dehydrogenase; LPS, Lipopolysaccharide; MMP, Matrix metalloproteinases; NLR, NOD-like receptor; TLR, Toll-like receptor; TNF, Tumor necrosis factor; XIAP, X-linked inhibitor of apoptosis protein
3 Biomechanics Research Laboratory, Department of Industrial Engineering, University of Miami , Coral Gables, FL 33146 , USA
Background: Interleukin (IL)-1β is involved in the pathology of intervertebral disc degeneration. Under normal conditions, IL-1β is present in cells in an inactive form (pro-IL-1β). However, under pathological conditions, pro-IL-1β is turned into its active form (IL-1β) by the inflammasome, a multi-protein complex of the innate immune response that activates caspase-1. Under conditions of degeneration, the disc experiences an environment of increased acidification. However, the implications of acidification on the innate immune response remain poorly explored. Methods: Here we have studied how pH changes in human nucleus pulposus cells affect inflammasome activation by immunoblot analysis of protein lysates obtained from nucleus pulposus cells that were exposed to different pH levels in culture. Results: In this study, we have found that in nucleus pulposus cells, with increased acidification, there was a decrease in inflammasome activation consistent with lower levels of active IL-1β. However, this effect at a pH of 6.5, the lowest pH level tested, was abrogated when cells were treated with IL-1β. Conclusions: Taken together, these findings suggest that the inflammatory response through IL-1β experienced by the human disc is not initiated in nucleus pulposus cells when the stimulus is acidification.
Innate Immunity; Intervertebral disc degeneration; Caspase-1; Inflammasome; Inflammation
Background
Low back pain is a major pathological condition that
affects approximately 84 % of the population at some
point in their life [
1
]. Estimates indicate that in the
United States, 25 % of the population suffers from low
back or neck pain [
2
]. Accordingly, the societal costs
associated with low back and neck pain exceeds $100
billion per year in the United States alone [
3
].
Intervertebral disc (IVD) degeneration (IDD) is believed to be a
major contributor to back pain [
4–7
]. The etiology of
IDD has been linked to genetic factors, aging or
excessive manual labor [8]. All these factors contribute to
increase inflammation, which eventually leads to increased
degradation of aggrecan and collagen in the extracellular
matrix of the disc [
9
].
Historically most studies looking at inflammation have
focused on looking at professional immune cells as
neutrophils and macrophages. However, recent studies
indicate that cells that are not professionally considered as
immune cells do play an important role on inflammation.
This concept applies to cells as diverse as neurons
[
10–13
] sperm cells [
14, 15
] or keratinocytes [
16, 17
].
Importantly, targeting inflammation in these cells has
been shown to offer an important therapeutic
potential. Thus in the context of IDD, it is important to
understand the contribution of cells like NP or
annulus fibrosus in regards to the inflammatory response in
order to gain a better understanding of how
inflammation contributes to IDD.
The inflammasome is a multi-protein complex in which
caspase-1 is activated followed by processing of
prointerleukin (IL)-1β and pro-IL-18 into their respective
active forms. The inflammasome has been previously
shown to play a role in infections [
18–20
], metabolic
syndromes [
21, 22
], autoimmune diseases [
23–25
] and
injury [
11, 13, 26–28
]. A positive correlation between
the degree of IDD and inflammasome content in the
disc has also been previously reported [29].
The pro-inflammatory cytokines IL-1β and tumor
necrosis factor (TNF) are two key cytokines that are
involved in the pathology of IDD [
9, 30–32
] and
degradation of the IVD [
33, 34
]. IL-1β is present in the
cell in an inactive form as pro-IL-1β, and it relies on the
inflammasome for its maturation into active IL-1β [
35
].
The inflammasome is a multi-protein complex
comprised of a nod-like receptor (NLR) such as NLRP1 or
NLRP3, (...truncated)