cis-9,trans-11-Conjugated linoleic acid down-regulates phorbol ester-induced NF-κB activation and subsequent COX-2 expression in hairless mouse skin by targeting IκB kinase and PI3K-Akt

Abstract

Conjugated linoleic acid (CLA) has been reported to inhibit mouse skin carcinogenesis, particularly in the promotion stage, but underlying molecular mechanisms remain poorly understood. Since persistent induction of cyclooxygenase-2 (COX-2) is frequently implicated in carcinogenesis, we investigated the effect of cis-9,trans-11-CLA (9Z,11E-CLA) on the tumor promoter-induced COX-2 expression in HR-1 hairless mouse skin in vivo. Topical application of 9Z,11E-CLA caused significant inhibition of COX-2 expression at 6 h induced by 10 nmol 12-O-tetradecanoylphorbol-13-acetate (TPA) in HR-1 mouse skin. Since NF-kappa B is known to regulate COX-2 gene expression, we determined the effect of 9Z,11E-CLA on TPA-induced activation of this transcription factor. Treatment of mouse skin with 9Z,11E-CLA reduced TPA-induced DNA binding as well as nuclear translocation of NF-kappa B by blocking phosphorylation and subsequent degradation of I kappa B alpha. In addition, 9Z,11E-CLA attenuated TPA-induced phosphorylation of extracellular signal-regulated protein kinase, p38 mitogen-activated protein kinase and Akt. To further elucidate the molecular mechanism underlying the inactivation of NF-kappa B by 9Z,11E-CLA, we investigated its effect on TPA-induced activation of I kappa B kinase (IKK), an upstream kinase that regulates NF-kappa B via phosphorylation and degradation of I kappa B alpha. 9Z,11E-CLA treatment down-regulated phosphorylation and catalytic activities of IKK alpha/beta in TPA-treated mouse skin. Co-treatment of mouse skin with the IKK beta-specific inhibitor SC-514 (1 mu mol) attenuated TPA-induced activation of Akt and NF-kappa B, and also the expression of COX-2 in hairless mouse skin. Taken together, 9Z,11E-CLA inhibits NF-kappa B driven-COX-2 expression by blocking the IKK and PI3K-Akt signaling in TPA-treated hairless mouse skin in vivo, which may account for its previously reported anti-tumor promoting effects.