MSCs are an attractive alternative candidate to conventional treatment for regenerative therapies [31]. They have been suggested as a new cell source for OA treatment due to their capability of differentiating in chondrocytes and the paracrine effects of secreted bioactive substances as well as their immunomodulatory effects [32]. CS is the major GAG component of native cartilage tissue which could provide cues to stimulate cells so as to proliferate, migrate, differentiate and produce the ECM compounds [33]. Moreover, CS has been shown to have anti-inflammatory effects reducing the concentration of pro-inflammatory cytokines such as TNF [34, 35] and IL-1β [36]. We hypothesized that the combination of ASCs with CS should be capable of enhancing the cartilage regeneration and diminishing the inflammation caused in OA. We evaluated the immunomodulatory effect of CS combined with ASCs in co-culture with inflamed chondrocytes. The expression of specific cartilage genes was also analysed.
Most previous studies which examined chondrocyte depletion during OA progression discovered that a variety of factors (including TNF) have been reported to induce a progressive cartilage joint degeneration in OA [37, 38]. In a proliferation assay we observed that the addition of inflammatory cytokine (TNF) slightly reduces the viability of chondrocytes, which is stable when CS is added. The addition of CS does not significantly affect the proliferation of any cell types since the cell viability remains very stable.
It has been reported that MSCs could be induced to express enhanced levels of IDO and PGE2. ASCs are known to constitutively produce PGE2 and this production significantly increases in co-cultures [39–41]. However, in our work PGE2 concentration remained at very low base levels in cells which did not receive an inflammatory stimulus. According to other references, CS had no effect on the basal PGE2 release [42]. The effect of TNF and IL-1, both in chondrocytes [43] and in MSCs [39] causes the increase of PGE2 expression. Although PGE2 is considered a pro-inflammatory cytokine, there exist different theories on the beneficial or detrimental effect which it produces on OA [44]. Ronca et al., 1998 [45], showed that the effects of CS in the treatment of OA are due to various mechanisms of action which decrease the concentration of prostaglandin PGE2 in the joint c[46, 47]. However, we observed that in TNF-stimulated cells the level of PGE2 production increased considerably under all conditions. The amount of PGE2 produced was significantly reduced, although it remained high compared to the non-stimulated controls under conditions of co-culture with CS and TNF in the culture medium. Currently, PGE2 seem to be involved in the up-regulation of the anti-inflammatory cytokine interleukin (IL)10 while reducing the secretion of TNF [48, 49]. We can therefore speculate that the inhibition of PGE2 production by CS could decrease the degradative effect in the OA deep zone cartilage. Lastly, CS is shown to inhibit the expression of enzymes involved in PGE2 synthesis, COX-2 and mPGES-1 [50].
As will be discussed later on, ASCs reduce levels of certain pro-inflammatory cytokines whose production is associated with PGE2. Therefore, CS and ASCs jointly manage to reduce OA processes through several different routes. Our results agree with those obtained by other authors. Cytokine IL-6 is responsible for pain in OA [50], being one of the main inducers of inflammation. MSCs reduced the levels of pro-inflammatory cytokines, including IL-6 [42]. With regard to PGE2 analysis, it is known that it accelerates expression of pain-associated molecules such as IL-6 e iNOS [50], but MSC-derived PGE2 always acts independently of IL-6 [40]. In our work, we observed that the expression of IL-6 significantly decreased in co-culture of ASCs and chondrocytes previously inflamed and with the addition of CS, in comparison with the inflamed chondrocytes without CS. Previous studies showed these effects in which MSCs and CS reduced the expression of pro-inflammatory cytokines, among which were IL-6 [51]. The combination of ASCs and CS resulted in a marked reduction in the expression of IL-6, although it was not as important as that produced solely by CS. Although it has been shown that IL-6 is one of the main interleukins which induce inflammation, its role is currently being debated due to evidence that this interleukin could have an anti-inflammatory role [52].
Contrary to what happened in the previous case for IL-6, the expression of iNOS underwent a greater reduction in the cultures of ASCs combined with CS as a treatment. Both CS and ASCs significantly reduced their expression, this reduction was more than 40 times lower with the use of CS and up to 50 times less when CS and ASCs were combined. When iNOS expression was examined in chondrocytes after stimulation with TNF, it was found that chondrocytes expressed extremely high iNOS levels, which agree with Charles et al., 1993 [53]. However, the expression level in ASCs was minimal, as predicted by Ren et al., 2009 [54, 55]. As in the previous cases, the treatment of chondrocytes with CS and ASCs dramatically reduced iNOS gene expression.
With regard to metalloproteinase, activated chondrocytes also produced MMP-1 and MMP-13. Increased expression of IL-6 is related to the production of enzymes from the MMPs group [48]. Nevertheless, their levels were reduced in inflamed chondrocytes treated with CS and ASCs, particularly MMP-13 production. Deletion of the MMP-13 gene attenuated articular cartilage degradation (it targets: type II collagen), and it has been shown that it is critical downstream target gene of TGF-β signaling during OA development [56]. It has recently been shown that global MMP-13 knockout could prevent articular cartilage erosion [57].
Both ASCs and inflamed chondrocytes showed high TNF expression, which decreased when treatment with CS occurred. Its levels may not have been as high as expected since PGE2 prevents proliferation of TNF [27]. This molecule together with IL-1β is considered to be a key inflammatory cytokine involved in the pathophysiological processes occurring in OA, and it affects blocking the chondrocytes synthesis of proteoglycan components, and Col2a1. Moreover, it is responsible for the increased production of iNOS and IL-6 [48].
TGF-β levels slightly increased when ASCs and chondrocytes were activated with TNF, although values were very close to those from the chondrocytes control group. In the presence of CS, levels were minimally reduced. Observations by Shen et al., 2014 [56]. have shown that TGF-β inhibition signaling in chondrocytes leads to chondrocyte terminal differentiation and development of OA, as this cytokine is responsible for stimulating the production of proteoglycans, Col2a1 and chondrogenesis. Other publications also confirm that the amount of TGF-β is low or even undetectable in patients with OA [48]. Lee et al., 2014 [41], have shown that the expression of IDO was induced in MSCs after tissue damage. Accordingly, in other tissues damaged or stimulated with TNF, the production of IDO increased [27]. This agrees with our results, which, after inducing cell inflammation, increased the expression of IDO, although it also increased with the addition of CS and ASCs. Our results showed that IDO is probably expressed as a natural protector against inflammation as shown by other authors [57].
During the more advanced stages of OA the amount of Col2a1 and ACAN decreases by denaturation [58, 59], which was influenced by the increase in the expression of MMPs. Levels of type II collagen and aggrecan increased significantly when CS was added to chondrocytes in inflamed co-cultures. This raises questions as to the connection of Col2a1 and ACAN to the increase of proteoglycans thanks to the CS, key components of ECM. CS is widely distributed in matrix where it forms an essential component of proteoglycans by covalent links with proteins [42], aiding the regenerative process. Our results confirm the boosting effect of CS on some gene expression of cartilage extracellular matrix (ACAN, Col2a1). Moreover, the expression of these genes was always higher when the ASCs were present compared to the inflamed chondrocytes without CS. There were no significate differences in SOX9 gene expression, a critical factor for chondrocyte differentiation that facilitates the expression Col2a1 [60].