Osteoarthritis (OA), the most common forms of arthritis, is a progressive degenerative joint disease that has a major impact on joint function and the patient's quality of life. This disorder is regulated by proinflammatory cytokines such as IL-1 and TNF-α that can activate a broad array of intracellular signal transduction mechanisms [
1]. Of the cytokine- activated pathways, MAP kinases are especially important because they regulate the production of several mediators of inflammation and cartilage damage [
2]. Members of the MAP kinase family phosphorylate a number of transcription factors including runt-related transcription factor-2 (RUNX-2), with subsequent activation of matrix metalloproteinases (MMPs) and inflammatory cytokine gene expressions [
3]. p38-MAPK, in particular, can regulate cytokine production through a variety of transcriptional and translational mechanisms [
4]. Furthermore, p38-MAPK participates in other inflammation-related events, such as neutrophil activation [
5], apoptosis [
6] and nitric oxide synthase induction [
7]. Inhibition of p38-MAPK with the commonly used pharmacological agent SB203580 reduces proinflammatory cytokine production in monocytes/macrophages, neutrophils, and T lymphocytes [
8]. In a rodent model of inflammatory arthritis, p38-MAPK inhibition suppressed the inflammation and bone destruction [
9]. TNF-α for instance, activates preferentially the MAPK-p38 isoform p38α in synovial macrophages and synovial fibroblasts (RASFs) in rheumatoid arthritis (RA) [
10]. In addition, p38α-MAPK is activated in human osteoclast precursor cells upon binding of RANKL, which indicates a possible role for p38α-MAPK isoform in bone destruction. Inhibition of p38-MAPK with small-molecule compounds has been successful for the treatment of experimental arthritis in different animal models [
11,
12]. Of note, inhibition of the p38α-MAPK isoform seems to be particularly effective with regard to cartilage and bone destruction, since a selective p38α-MAPK inhibitor reduced bone loss, numbers of osteoclasts and serum levels of cartilage breakdown metabolites in mice with CIA [
12]. The upstream kinases, MKK3 and MKK6 are important regulators of p38-MAPK and represent potential therapeutic targets to modulate cytokine production [
13]. Studies in MKK3 or MKK6 knockout mice demonstrate that both are essential for full p38-MAPK activation
in vivo [
14]. MKK3 selectively phosphorylates p38α-, γ-, and δ-MAPKs whereas MKK6 activates all four p38-MAPK isoforms (α, β, γ, and δ) [
15]. This suggests that substrate selectivity might contribute to the distinct functional profiles of MKK activation. It is well documented that activation of the runt-related transcription factor RUNX-2 is mediated by activated p38-MAPK as inhibition of p38-MAPK abrogates its activity and the expression of cartilage degrading enzymes in chondrocytes [
16]. These and other studies [
17] clearly show that inhibition of specific MAPKs or transcription factor may be an effective approach for the inhibition of joint destruction in arthritis.
Pomegranate fruit (
Punica granatum L) is revered through the ages for its medicinal properties. Pomegranate fruit extract (PFE) is a rich source of highly potent antioxidants and is widely used in several traditional medicinal systems for the treatment of inflammation and pain in arthritis and other diseases. Consistent with this notion, our published study is noteworthy where we showed for the first time that a standardized PFE preparation was non-toxic to human OA chondrocytes and prevented the IL-1β-induced cartilage breakdown by inhibiting the IL-1β-induced production of MMPs by blocking the activation of p38-MAPK and the transcription factor NF-κB in human OA chondrocytes [
18]. In other studies, we have shown that bioavailable metabolites of PFE inhibited COX-2 activity in human OA chondrocytes [
19], and that consumption of PFE suppressed inflammation and joint destruction in an animal model of inflammatory arthritis [
20].
Therefore, based on our published data and the studies showing that IL-1β activates the human chondrocytes and induces the expression of mediators that play a major role in cartilage degradation in OA, we determined whether PE inhibits the IL-1β-induced cartilage degradation by modulating the activation of relevant signal transduction pathways and transcription factors in human OA chondrocytes. Our results show that a standardized pomegranate extract (PE) selectively inhibited the IL-1β-induced activation of p38α-MAPK isoform and its upstream regulator MKK3 in primary human OA chondrocytes. In addition, IL-1β-induced DNA binding activity of RUNX-2 was also inhibited by PE in primary human OA chondrocytes and correlated with the inhibition of p38α-MAPK isoform.