Leptin induces production of eicosanoids and proinflammatory cytokines in human synovial fibroblasts

Background

Leptin is an adipocyte-derived hormone with an important role in regulation of feeding behavior, energy expenditure and, therefore, body weight via central nervous system mechanisms. Plasma leptin levels are reliably increased with increasing body mass index. Leptin receptors are widely distributed in peripheral tissues as well. Several different forms of leptin receptor have been identified, although the long isoform or Ob-Rb is the only isoform with clearly demonstrated signaling capability. Ob-Rb is related to the type I cytokine receptors and signals via janus-activated kinase-signal transducer and activator of transcription pathways; however, other second messenger systems may also be stimulated. A role for leptin as a modulator of the immune response and inflammatory processes has been proposed. Our group has demonstrated that leptin increases production of eicosanoids in rat macrophages. A role for leptin as an intermediary of obesity-related osteoarthritis (OA) has been proposed [1]. Leptin levels are elevated in synovial fluid of OA patients and correlate with the body mass index. Leptin and its receptor (Ob-Rb) are expressed in OA chondrocytes, and leptin treatment increases expression of transforming growth factor beta and insulin-like growth factor 1.

Aim

There is a paucity of information regarding the effect of leptin on human synovial cells, and we aimed to determine whether leptin altered production of eicosanoids and proinflammatory cytokines.

Methods and results

All experiments were performed in primary human synovial cells of patients with rheumatoid arthritis or OA collected at the time of surgery, and were repeated at least three times with cells from three different patients. We determined that the leptin receptor (Ob-Rb) is expressed by human synovial fibroblast-like cells using RT-PCR. Leptin receptor expression is unaffected by treatment with IL-1β (1 ng/ml). Treatment of human synovial fibroblasts with leptin (100 ng/ml) increased production of IL-6, IL-8, and prostaglandin (PG) (1 ng/ml) used E₂, although to a modest degree compared with IL-1β as a positive control. The mechanism of increased PGE₂ production involves several PG biosynthetic enzymes. We demonstrated that leptin increased phosphorylation of cPLA₂ by 15 min after treatment. Leptin also increased cyclooxygenase-2 protein levels. Microsomal prostaglandin E synthase 1 expression was not changed. We next evaluated the signaling pathways that may contribute to increased production of inflammatory mediators. Leptin (100 ng/ml) increased phosphorylation of p42/p44 ERK, p38, and JNK by 15–30 min after treatment, and phospho-MAPK levels were increased for 1–2 hours.

Conclusions

Taken together, these data suggest that leptin may affect production of proinflammatory mediators in synovial tissues and enhance inflammation in patients with arthritis.