The relationship between cell transformation and p38 MAP kinase, a major mitogen-activated protein (MAP) kinase pathway converting signals of various extracellular stimuli into expression of specific target genes through activation of transcription factors, still remains unclear. The aim of the present study was to investigate the role of the p38 MAP kinase pathway in epidermal growth factor (EGF)-induced cell transformation in JB6 cells. Our data show that a dominant negative mutant of p38 MAP (DN-p38) kinase inhibits EGF-promoted JB6 Cl41 cell transformation and that SB202190, an inhibitor of p38 MAP kinase, also inhibits JB6 Cl41 cell transformation in a dose-dependent manner. Moreover, our results show that DN-p38 MAP kinase inhibits the phosphorylation of EGF-stimulated activating transcription factor-2 (ATF-2) and signal transducer and activator of transcription 1 (STAT1). Additionally, DN-p38 MAP kinase inhibits EGF-induced phosphorylation of c-Myc The mitogen-activated protein kinases (MAPKs), 1 including p38 kinase, c-Jun NH 2 -terminal kinases (JNKs), extracellular signal-regulated kinases (ERKs), and signal transduction cascades, are vital mediators of many cellular functions such as growth, development, proliferation, differentiation, malignant transformation, inflammation, and apoptosis (1-3). p38 MAP kinase is activated by cellular stresses including inflammatory cytokines, ultraviolet light, and growth factors, and the activated p38 MAP kinase has been shown to phosphorylate several transcription factors including ATF-2, STAT1, the Max/ Myc complex, and myocyte enhancer factor 2 (MEF2) (1, 4 -6). On the other hand, ERKs are predominantly activated by mitogenic stimuli, including mitogens and growth factors, and activated ERKs are involved in cell differentiation and development (2, 3). Although one major function of the p38 kinase and JNKs pathways is regulation of inflammation and apoptosis, in many cases the biological consequences of p38 kinase and JNK activation overlap with those of ERKs in mediation of cell growth and differentiation (2, 3, 6, 7).Previously, we studied ERKs regulation of epidermal growth factor (EGF)-induced cell transformation in promotion sensitive (P ϩ ) derivatives of the mouse epidermal JB6 cell line (8). Results showed that inhibition of ERKs appeared to be an important contributor to the tumor promotion-resistant phenotype in JB6 cells. A recent study showed that the p38 kinase and JNKs pathways cooperate to transactivate the vitamin D receptor through the c-Jun/activator protein-1 (AP-1) (9). This result indicates that a connection exists between p38 MAP kinase and AP-1. But AP-1 activation is involved in JB6 cell transformation promoted by EGF and the phorbol ester 12-Otetradecanoyl-phorbol-13-acetate (TPA) (10, 11). AP-1 is a dimeric complex consisting of proteins encoded by the jun and fos gene families, and the AP-1 binding region in these genes is referred to as the TPA response element (TRE). AP-1 induces the transcription of genes that are related to cell prol...