In human mast cells, derived from CD34 ؉ peripheral blood cells, we observed that Kit ligand (KL) failed to induce degranulation but acted in synergy with antigen to markedly enhance degranulation, levels of cytokine gene transcripts, and production of cytokines. Further examination revealed that antigen and KL activated common and unique signaling pathways to account for these varied responses. KL, unlike antigen, failed to activate protein kinase C but activated phospholipase C␥ and calcium mobilization and augmented these signals as well as degranulation when added together with antigen. Both KL and antigen induced signals that are associated with cytokine production, namely phosphorylation of the mitogen-activated protein kinases, phosphatidylinositol 3-kinase-dependent phosphorylation of protein kinase B (also known as Akt), and phosphorylation of nuclear factor B (NFB). However, only KL stimulated phosphorylation of signal transducer and activator of transcription 5 (STAT5) and STAT6, whereas antigen weakly stimulated the protein kinase C-dependent induction and phosphorylation of c-Jun and associated activating protein-1 (AP-1) components, an action that was markedly potentiated by costimulation with KL. Interestingly, most signals were down-regulated on continuous exposure to KL but were reactivated along with cytokine gene transcription on addition of antigen. The findings, in total, indicated that a combination of Fc⑀RI and Kit-mediated signals and transcriptional processes were required for optimal physiologic responses of human mast cells to

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Nitric oxide-donating aspirin (NO-aspirin), representing a new concept in the development of more efficacious nonsteroidal anti-inflammatory drugs, consists of traditional aspirin bearing -ONO 2 , which releases NO. Conventional aspirin prevents human colon cancer, but its toxicity precludes its application as a chemopreventive agent. NO-aspirin seems safer and in cultured cancer cells it is Ͼ1000-fold more potent than aspirin. To determine the mechanism by which NO-aspirin inhibits cell growth, we studied its effect on mitogen-activated protein kinase (MAPK) signaling in HT-29 human colon cancer cells. NO-aspirin stimulated the phosphorylation of extracellular signal-regulated kinase 1/2 and Akt only marginally. The greatest increases in phosphorylation were seen in cJun NH 2 -terminal kinase (JNK) and p38 MAP kinases, which were observed as early as 5 min and after 1 h of treatment, averaged more than 10-fold over control. The activation of JNK and p38 was accompanied by large increases in the phosphorylation of the downstream transcription factors cJun and activating transcription factor 2 (ATF-2). We used specific MAPK inhibitors, small interfering (siRNA) gene silencing methods, and dominant-negative cJun to determine the relevance of these phosphorylation events to the ability of NO-aspirin to inhibit colon cancer cell growth. Only the dual inhibitor of p38 and JNK and the use of combined siRNA silencing of p38 and cJun abrogated the ability of NO-aspirin to block cell growth. Our data indicate that NO-aspirin is dependent on both the p38 and the JNK MAP kinase pathways for its ability to inhibit the growth of colon cancer cells.