Inhibition of the antigen‐induced activation of rodent mast cells by putative Janus kinase 3 inhibitors WHI‐P131 and WHI‐P154 in a Janus kinase 3‐independent manner

1 We analyzed the effects of the Janus kinase 3 (Jak3)‐specific inhibitor WHI‐P131 (4‐(4′‐hydroxyphenyl)‐amino‐6,7‐dimethoxyquinazoline) and the Jak3/Syk inhibitor WHI‐P154 (4‐(3′‐bromo‐4′‐hydroxyphenyl)‐amino‐6,7‐dimethoxyquinazoline) on the antigen‐induced activation of mast cells. 2 In the rat mast cell line RBL‐2H3, both WHI‐P131 and WHI‐P154 inhibited the antigen‐induced degranulation and phosphorylation of p44/42 mitogen‐activated protein kinase (MAPK), p38 MAPK and c‐Jun N‐terminal kinase (JNK). 3 The phosphorylation of Gab2, Akt and Vav was also inhibited by WHI‐P131 and WHI‐P154, indicating that these inhibitors suppress the activation of phosphatidylinositol 3‐kinase (PI3K). 4 In bone marrow‐derived mast cells (BMMCs) from Jak3‐deficient (Jak3 −/− ) mice, degranulation and activation of MAPKs were induced by the antigen in almost the same extent as in BMMCs from wild‐type mice. In addition, the antigen‐induced degranulation and activation of MAPKs were inhibited by WHI‐P131 and WHI‐P154 in both groups of BMMCs, indicating that these compounds inhibit a certain step except for Jak3. 5 The antigen‐induced increase in the activity of Fyn, a probable tyrosine kinase of Gab2, was also inhibited by WHI‐P131 and WHI‐P154 in RBL‐2H3 cells. In BMMCs from Jak3 −/− mice, the antigen stimulation induced tyrosine phosphorylation of Fyn, which was inhibited by WHI‐P131, as well as in BMMCs from wild‐type mice and in RBL‐2H3 cells. 6 These findings suggest that Jak3 does not play a significant role in the antigen‐induced degranulation and phosphorylation of MAPKs, and that WHI‐P131 and WHI‐P154 inhibit the PI3K pathway by preventing the antigen‐induced activation of Fyn, thus inhibiting the antigen‐induced degranulation and phosphorylation of MAPKs in mast cells.British Journal of Pharmacology (2005) 145 , 818–828. doi: 10.1038/sj.bjp.0706240