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

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.