p38 MAP kinase mediates endotoxin-induced expression of cyclooxygenase-2 in enterocytes

Background: Necrotizing enterocolitis (NEC) occurs only after bacterial colonization of the intestine, suggesting that bacterial products, including lipopolysaccharide (endotoxin,) interact with enterocytes in the pathogenesis of this disease. Inflammatory molecules such as cyclooxygenase-2 (COX-2) are important mediators of the septic response leading to NEC. We therefore hypothesized that endotoxin activates production of COX-2 in enterocytes and explored the relative contributions of known mitogen-activated protein kinases (MAPK) pathways in this process.

Methods: IEC-6 enterocytes were treated with 5 microg/mL endotoxin, or various stresses, or media alone, and COX-2 protein levels were assayed by immunoblots with anti-COX-2 antibodies. Activation of MAPK was examined by immunoblots with phospho-MAPK antibodies. MAPK activity was blocked by treatment with pharmacologic inhibitors or transfection with dominant-negative MAPK constructs.

Results: Endotoxin treatment caused increased expression of the COX-2 protein 24 hours after treatment. This was preceded by rapid and transient activation of the 3 major MAPKs: extracellular-regulated kinase (ERK), c-Jun N-terminal kinase (JNK), and p38. SB203580, a specific inhibitor of p38, but not U0126 (ERK inhibitor) or SP600125 (JNK inhibitor), blocked endotoxin-induced accumulation of COX-2 protein. This response was also blocked by expression of dominant-negative p38 but not by the dominant-negative ERK construct. Genotoxic stress that activated p38 but not ERK was an effective inducer of COX-2, whereas stresses that activated both p38 and ERK were not effective. ERK inhibition by U1026 enhanced endotoxin-induced production of COX-2, consistent with negative regulation of COX-2 by ERK. These data point to p38 as the MAPK that mediates endotoxin-induced production of COX-2 in enterocytes.

Conclusions: Endotoxin may be capable of inducing the production of COX-2 in enterocytes via the p38 MAPK pathway, which may be relevant to the development of NEC.