Distinct fatty acid signatures in infrapatellar fat pad and synovial fluid of patients with osteoarthritis versus rheumatoid arthritis

Anne-Mari Mustonen,Petri Lehenkari,Tommi Kääriäinen,Johanna Huhtakangas,Antti Joukainen,Tommi Paakkonen,Sanna Turunen,Heikki Kröger,Petteri Nieminen,Reijo Käkelä

doi:10.1186/s13075-019-1914-y

Abstract

BackgroundInfrapatellar fat pad (IFP) has recently emerged as a potential source of inflammation in knee arthropathies. It has been proposed to be one source of adipocytokines, fatty acids (FA), and FA-derived lipid mediators that could contribute to the pathophysiological processes in the knee joint. Alterations in synovial fluid (SF) lipid composition have been linked to both osteoarthritis (OA) and rheumatoid arthritis (RA). The aim of the present study was to compare the FA signatures in the IFP and SF of RA and OA patients.MethodsPairs of IFP and SF samples were collected from the same knees of RA (n = 10) and OA patients (n = 10) undergoing total joint replacement surgery. Control SF samples (n = 6) were harvested during diagnostic or therapeutic arthroscopic knee surgery unrelated to RA or OA. The FA composition in the total lipids of IFP and SF was determined by gas chromatography with flame ionization and mass spectrometric detection.ResultsArthropathies resulted in a significant reduction in the SF proportions of n-6 polyunsaturated FA (PUFA), more pronouncedly in OA than in RA. OA was also characterized with reduced percentages of 22:6n-3 and lower product/precursor ratios of n-3 PUFA. The proportions of total monounsaturated FA increased in both RA and OA SF. Regarding IFP, RA patients had lower proportions of 20:4n-6, total n-6 PUFA, and 22:6n-3, as well as lower product/precursor ratios of n-3 PUFA compared to OA patients. The average chain length of SF FA decreased in both diagnoses and the double bond index in OA.ConclusionsThe observed complex alterations in the FA signatures could have both contributed to but also limited the inflammatory processes and cartilage destruction in the RA and OA knees.

Highlights

Infrapatellar fat pad (IFP) has recently emerged as a potential source of inflammation in knee arthropathies
The main findings of the present study were as follows: (i) n-6 polyunsaturated FA (PUFA) reduced in proportion in rheumatoid arthritis (RA) synovial fluid (SF) and especially in OA SF compared to control knees, (ii) 22:6n-3 and product/precursor ratios of n-3 PUFA decreased in OA SF, (iii) Total average chain length (TACL) and Double bond index (DBI) reduced especially in OA SF, and (iv) total n-6 PUFA, 20:4n-6, 22:6n-3, and product/precursor ratios of n-3 PUFA were lower in RA IFP than in OA IFP
To sum up the present results, the major differences between the control knees and the RA and OA joints were an increase in monounsaturated FA (MUFA) and a simultaneous decrease in n-6 PUFA

Summary

Introduction

Infrapatellar fat pad (IFP) has recently emerged as a potential source of inflammation in knee arthropathies. It has been proposed to be one source of adipocytokines, fatty acids (FA), and FA-derived lipid mediators that could contribute to the pathophysiological processes in the knee joint. The potential inflammatory role of the infrapatellar fat pad (IFP) has recently become a topic of interest for osteoarthritis (OA) [1] and rheumatoid arthritis (RA) research [2]. IFP is an intracapsular but extrasynovial organ that does not directly interact with the articular cartilage [3]. It has, been proposed to be a source of fatty. Increased consumption of long-chain n-3 PUFA may have beneficial effects on pain alleviation and function of OA joints, the evidence is less convincing [19]

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