Changes in gene expression
Curcuma is not only an ancient spice, but also a traditional remedy that has been used in Indian and Chinese medicine to treat indigestion and many other medical issues. Since the 1970s, the anti-inflammatory compounds called curcuminoids were discovered in the spice, with one (and probably the most important component) being curucmin[
22]. Because of its anti-inflammatory properties, curcuma and its components (especially curcumin) have been investigated in osteoarthritis and rheumatoid arthritis during the past one to two decades, while only one paper has been published on the effects of curcumin on intervertebral disc cells so far[
23].
Our results clearly show that the different curcuma extracts influenced cellular behavior in a different manner. While the curcuma EtOH extract (which contained only little amounts of curcumin, as shown by HPLC/MS) had no effect and was thus considered to be not suitable for further investigations, the curcuma DMSO extract as well as the DMSO-soluble compound curcumin (which was shown to be present in the DMSO extract by HPLC/MS) reduced levels of some disc-specific, major proinflammatory cytokines and matrix degrading enzymes in our
in vitro experiments. We were able to demonstrate that the observed effect was not due to the biologic activity of the solvents DMSO and EtOH (Additional file
1: Figure
3 and Additional file
2: Figure
4), although the anti-inflammatory properties of DMSO have most recently been described in human intestinal cells[
24].
Specifically, we could demonstrate a reduction in gene expression of IL-6, MMP1, MMP3 and MMP13 when treating IL-1β prestimulated cells with the curcuma DMSO extract. Additionally, IL-1β and IL-8 were reduced by curcumin treatment after 6 hours. As effects were comparable between the curcuma DMSO extract and curcumin and as curcumin was detected at high concentrations in the DMSO extract by HPLC/MS, we hypothesize that the major bioactive substance in curcuma DMSO extracts acting on human intervertebral disc cells could be curcumin. Due to the beneficial effects of curcumin, this natural compound may be of benefit for patients with inflammation-related back pain, with the potential mode of application being intradiscal injection. Albeit curcumin is well known for its low bioavailability in case of oral consumption, in vivo concentrations after injections should not be a limiting factor.
The observed gene expression results are similar to effects that were observed when treating other cell types with curcumin, e.g. leading to a reduction in IL-1β[
25‐
28], IL-6[
25,
28‐
30], IL-8[
25,
31], MMP1[
32], MMP3[
26,
32,
33] and MMP13[
32,
34]. For IL-6, we observed a slight increase at the lowest concentrations (1.5 fold), but a decrease at higher concentrations. This may be due to biphasic effects of curcumin that are based on its dual function to either scavenge or produce reactive oxygen species[
35]. However, the biphasic nature of curcumin cannot explain that higher concentrations of curcumin strongly stimulated expression of TNF-α in human intervertebral disc cells (both, without pretreatment as well as after IL-1β prestimulation), which is different from what is described in the literature[
25,
28,
36]. Based on the current study we do not know whether this effect would also occur on the protein level and
in vivo. Therefore, further studies are thus required to demonstrate safety and usefulness of curcumin in human application.
So far, only one study investigated the effect of curcumin on human intervertebral disc cells[
23]. This study tested curcumin for its effects on matrix protein gene expression, but not on the expression of proinflammatory cytokines or matrix degrading enzymes. Results of Yu
et al.’s study indicated that curcumin is also able to attenuate an IL-1 induced inhibition of SOX9 and collagen–II expression at 20 μg/ml (= 54.3 μM), which is higher than the concentrations used in the present study and which was shown to be a damaging concentration for other (disc-related) cell types (e.g. C-28/I2 = a chondrocyte cell line)[
37]. Furthermore, it has to be noted that both, Yu’s as well as our study were performed in a 2D culture system, which can cause certain phenotypic changes of disc cells and may thus possibly influence cellular behavior to the tested treatment.