All-trans-retinoic acid inhibits Jun N-terminal kinase by increasing dual-specificity phosphatase activity

Jun N-terminal kinases (JNKs) are serine-threonine kinases that play a crit ical role in the regulation of cell growth and differentiation. We previous ly observed that JNK activity is suppressed by all-trans-retinoic acid (t-R A), a ligand for retinoic acid nuclear receptors (RARs), in normal human br onchial epithelial cells, which are growth inhibited by t-RA. In this study , Ne investigated the mechanism by which t-RA inhibits JNK and the possibil ity that this signaling event is blocked in non-small cell lung cancer (NSC LC) cells. Virtually ail NSCLC cell lines are resistant to the growth-inhib itory effects of t-RA, and a subset of them have a transcriptional defect s pecific to retinoid nuclear receptors. We found that in NSCLC cells express ing functional retinoid receptors, serum-induced JNK phosphorylation and ac tivity were inhibited by t-RA in a bimodal pattern, transiently within 30 m in and in a sustained fashion beginning at 12 h. Retinoid receptor transcri ptional activation was required for the late, but not the early, suppressio n of JNK activity. t-RA inhibited serum-induced JNK activity by blocking mi togen-activated protein (MAP) kinase kinase 4-induced signaling events. Thi s effect of t-RA was phosphatase dependent and involved an increase in the expression of the dual-specificity MAP kinase phosphatase 1 (MKP-1). t-RA d id not activate MKP-1 expression or inhibit JNK activity in a NSCLC cell li ne,vith retinoid receptors that are refractory to ligand-induced transcript ional activation. These findings provide the first evidence that t-RA suppr esses JNK activity by inhibiting JNK phosphorylation. Retinoid receptor tra nscriptional activation was necessary for the sustained inhibition of JNK a ctivity by t-RA, and this signaling event was disrupted in NSCLC cells with retinoid receptors that are refractory to ligand-induced transcriptional a ctivation.