Low Molecular Weight Hyaluronan Activates Cytosolic Phospholipase A2α and Eicosanoid Production in Monocytes and Macrophages

Hyaluronan (HA) is the major glycosaminoglycan in the extracellular matrix. During inflammation, there is an increased breakdown of HA, resulting in the accumulation of low molecular weight (LMW) HA and activation of monocytes and macrophages. Eicosanoids, derived from the cytosolic phospholipase A(2) group IVA (cPLA(2)) activation, are potent lipid mediators also attributed to acute and chronic inflammation. The aim of this study was to determine the effect of LMW HA on cPLA(2) activation, arachidonic acid (AA) release, and subsequent eicosanoid production and to examine the receptors and downstream mechanisms involved in these processes in monocytes and differently polarized macrophages. LMW HA was a potent stimulant of AA release in a time- and dose-dependent manner, induced cPLA(2), ERK1/2, p38, and JNK phosphorylation, as well as activated COX2 expression and prostaglandin (PG) E-2 production in primary human monocytes, murine RAW 264.7, and wild-type bone marrow-derived macrophages. Specific cPLA(2) inhibitor blocked HA-induced AA release and PGE(2) production in all of these cells. Using CD44, TLR4, TLR2, MYD88, RHAMM or STAB2 siRNA-transfected macrophages and monocytes, we found that AA release, cPLA(2), ERK1/2, p38, and JNK phosphorylation, COX2 expression, and PGE(2) production were activated by LMW HA through a TLR4/MYD88 pathway. Likewise, PGE(2) production and COX2 expression were blocked in Tlr4(-/-) and Myd88(-/-) mice, but not in Cd44(-/-) mice, after LMW HA stimulation. Moreover, we demonstrated that LMW HA activated the M1 macrophage phenotype with the unique cPLA(2)/COX2(high) and COX1/ALOX15/ALOX5/LTA4H(low) gene and PGE(2)/PGD(2)/15-HETEhigh and LXA(4)(low) eicosanoid profile. These findings reveal a novel link between HA-mediated inflammation and lipid metabolism.