Cross-talk between TLR4 and PPARγ pathways in the arachidonic acid-induced inflammatory response in pancreatic acini.

Arachidonic acid (AA) is generally associated with inflammation in different settings. We assess the molecular mechanisms involved in the inflammatory response exerted by AA on pancreatic acini as an approach to acute pancreatitis (AP). Celecoxib (COX-2 inhibitor), TAK-242 (TLR4 inhibitor) and 15d-PGJ2 (PPARγ agonist) were used to ascertain the signaling pathways. In addition, we examine the effects of TAK-242 and 15d-PGJ2 on AP induced in rats by bile-pancreatic duct obstruction (BPDO). To carry out in vitro studies, acini were isolated from pancreas of control rats. Generation of PGE2 and TXB2, activation of pro-inflammatory pathways (MAPKs, NF-κB, and JAK/STAT3) and overexpression of CCL2 and P-selectin was found in AA-treated acini. In addition, AA up-regulated TLR4 and down-regulated PPARγ expression. Celecoxib prevented the up-regulation of CCL2 and P-selectin but did not show any effect on the AA-mediated changes in TLR4 and PPARγ expression. TAK-242, reduced the generation of AA metabolites and repressed both the cascade of pro-inflammatory events which led to CCL2 and P-selectin overexpression as well as the AA-induced PPARγ down-regulation. Thus, TLR4 acts as upstream activating pro-inflammatory and inhibiting anti-inflammatory pathways. 15d-PGJ2 down-regulated TLR4 expression and hence prevented the synthesis of AA metabolites and the inflammatory response mediated by them. Reciprocal negative cross-talk between TLR4 and PPARγ pathways is evidenced. In vivo experiments showed that TAK-242 and 15d-PGJ2 treatments reduced the inflammatory response in BPDO-induced AP. We conclude that through TLR4-dependent mechanisms, AA up-regulated CCL2 and P-selectin in pancreatic acini, partly mediated by the generation of PGE2 and TXB2, which activated pro-inflammatory pathways, but also directly by down-regulating PPARγ expression with anti-inflammatory activity. In vitro and in vivo studies support the role of TLR4 in AP and the use of TLR4 inhibitors and PPARγ agonists in AP treatment.