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The nucleosomal response refers to the rapid phosphorylation of histone H3 on serine 10 and HMG-14 on serine 6 that occurs concomitantly with immediate-early (IE) gene induction in response to a wide variety of stimuli. Using antibodies against the phosphorylated residues, we show that H3 and HMG-14 phosphorylation is mediated via different MAP kinase (MAPK) cascades, depending on the stimulus. The nucleosomal response elicited by TPA is ERK-dependent, whereas that elicited by anisomycin is p38 MAPK-dependent. In intact cells, the nucleosomal response can be selectively inhibited using the protein kinase inhibitor H89. MAPK activation and phosphorylation of transcription factors are largely unaffected by H89, whereas induction of IE genes is inhibited and its characteristics markedly altered. MSK1 is considered the most likely kinase to mediate this response because (i) it is activated by both ERK and p38 MAPKs; (ii) it is an extremely efficient kinase for HMG-14 and H3, utilizing the physiologically relevant sites; and (iii) its activity towards H3/HMG-14 is uniquely sensitive to H89 inhibition. Thus, the nucleosomal response is an invariable consequence of ERK and p38 but not JNK/SAPK activation, and MSK1 potentially provides a link to complete the circuit between cell surface and nucleosome.

The ERK, JNK/SAPK and p38/RK MAP kinase subtypes (reviewed in [1]) are differentially activated in mammalian cells by various stimuli, which elicit induction of immediate-early (IE) genes, such as c-fos and c-jun (reviewed in [1-3]), as well as phosphorylation of histone H3 [4] and HMG-14 [5]. Anisomycin and UV radiation have been suggested to induce c-fos and c-jun transcription via JNK/SAPK-mediated phosphorylation of TCF (ternary complex factor), for c-fos induction [6-8], and c-Jun and/or ATF-2 for c-jun induction [9-11] [12,13]. We report here that anisomycin and ultraviolet radiation (UV) activate MAP kinase kinase-6 (MKK6) [14,15], p38/RK [16] [17,18] and MAPKAP kinase-2 (MAPKAP K-2) [17-19]. By using the p38/RK inhibitor SB 203580 [20,21], we show that activation of p38/RK and/or its downstream effectors are essential for anisomycin- and UV-stimulated c-fos/c-jun induction and histone H3/HMG-14 phosphorylation, whereas JNK/SAPK activation and phosphorylation of c-Jun and ATF-2 are insufficient for these responses.