Isoforms of jun kinase are differentially expressed and activated in humanmonocyte/macrophage (THP-1) cells

Ten isoforms of c-jun N-terminal kinase (JNK) have been described that aris e by differential mRNA splicing of three genes. In that the relative expres sion and function of these different JNK proteins in human monocytic cells is not known, we have examined the JNK isoforms in THP-1 monocyte/macrophag e cells. Differentiation of THP-1 cells by exposure to 10(-8) M PMA for 42- 48 It enhances cellular responses to LPS, including enhanced activation of total JNK activity and increased phosphorylation of p54 JNK as well as p46 JNK. Examination of JNK proteins on Western blots reveals a predominance of p46 JNK1 and p54 JNK2 proteins. Clearing of lysates by immunoprecipitation of JNK1(99% effective) removes 46% of the JNK enzymatic activity (p < 0.01 ), whereas clearing of JNK1 plus JNK2 (70% effective) depletes the sample o f 72% of the JNK activity (p < 0.01). Further analysis, undertaken with rea l-time RT-PCR, revealed that 98% of the JNK messages code for three isoform s: JNK1 beta1, JNK2 alpha1, and JNK2 alpha2. The p54 JNK that is phosphoryl ated in LPS-stimulated, PMA-differentiated THP-1 cells is most likely JNK2a 2 because 97% of the p54 JNK-encoding messages code for JNK2 alpha2. By ana logous reasoning, the p46 JNKs that are not heavily phosphorylated, but acc ount for approximately half of the N-terminal e-jun kinase enzymatic activi ty, are most likely either JNK1 beta1 or JNK2 alpha1 because they account f or 98% of the messages that can code for 46kDa JNKs.