Ceramide-dependent PP2A regulation of TNFα-induced IL-8 production in respiratory epithelial cells

Cornell TT, Hinkovska-Galcheva V, Sun L, Cai Q, Hershenson MB, Vanway S, Shanley TP. Ceramide-dependent PP2A regulation of TNF alpha-induced IL-8 production in respiratory epithelial cells. Am J Physiol Lung Cell Mol Physiol 296: L849-L856, 2009. First published March 13, 2009; doi:10.1152/ajplung.90516.2008.-IL-8 is a key mediator in the pathophysiology of acute lung injury. TNF alpha stimulates IL-8 production in respiratory epithelial cells by activating both the NF-kappa B and MAP kinase pathways. The precise mechanism by which these pathways are downregulated to terminate IL-8 production remains unclear. We studied the regulatory role of the serine/threonine phosphatase, PP2A, on the signaling pathways involved in IL-8 production from respiratory epithelial cells. Inhibition of PP2A using okadaic acid or gene knockdown using siRNA resulted in an augmentation of TNF alpha-induced IL-8 production. We also found that PP2A inhibition resulted in prolonged activation of JNK, p38, and ERK resulting in both increased transcriptional activation of the IL-8 promoter and posttranscriptional stabilization of IL-8 mRNA. Because TNF alpha had been shown to activate ceramide accumulation, and separate studies had linked ceramide with activation of PP2A, we hypothesized the pathway of TNF alpha-inducing ceramide to activate PP2A comprised an endogenous regulatory pathway. Inhibition of the immediate sphingomyelinase-dependent pathway as well as the de novo synthesis pathway of ceramide production reduced serine/threonine phosphatase activity and augmented IL-8 production. These data suggest that ceramide plays a role in activating PP2A to terminate ongoing IL-8 production. In summary, our data suggest that in respiratory epithelium, TNF alpha induces ceramide accumulation, resulting in subsequent activation of PP2A, which targets those kinases responsible for transcriptional activation of IL-8.