Introduction: Pomegranate has been revered throughout history for its medicinal properties. p38-MAPK is a major signal-transducing pathway in osteoarthritis (OA) and its activation by interleukin-1β (IL-1β) plays a critical role in the expression and production of several mediators of cartilage catabolism in OA. In this study we determined the effect of a standardized pomegranate extract (PE) on the IL-1β-induced activation of MKK3/6, p38-MAPK isoforms and the activation of transcription factor RUNX-2 in primary human OA chondrocytes.
Methods: Human chondrocytes were derived from OA cartilage by enzymatic digestion, treated with PE and then stimulated with IL-1β. Gene expression of p38-MAPK isoforms was measured by RT-PCR. Western immunoblotting was used to analyze the activation of MAPKs. Immunoprecipitation was used to determine the activation of p38-MAPK isoforms. DNA binding activity of RUNX-2 was determined using a highly sensitive and specific ELISA. Pharmacological studies to elucidate the involved pathways were executed using transfection with siRNAs.
Results: Human OA chondrocytes expressed p38-MAPK isoforms p38α, -γ and -δ, but not p38β. IL-1β enhances the phosphorylation of the p38α-MAPK and p38γ-MAPK isoforms but not of p38δ-MAPK isoform in human OA chondrocytes. Activation of p38-MAPK in human OA chondrocytes was preferentially mediated via activation of MKK3. In addition, we also demonstrate that polyphenol rich PE inhibited the IL-1β-induced activation of MKK3, p38α-MAPK isoform and DNA binding activity of the transcription factor RUNX-2.
Conclusions: Our results provide an important insight into the molecular basis of the reported cartilage protective and arthritis inhibitory effects of pomegranate extract. These novel pharmacological actions of PE on IL-1β stimulated human OA chondrocytes impart a new suggestion that PE or PE-derived compounds may be developed as MKK and p38-MAPK inhibitors for the treatment of OA and other degenerative/inflammatory diseases.