Ultraviolet-induced junD activation and apoptosis in myeloblastic leukemia ML-1 cells

The exposure of mammalian cells to UV irradiation induces the expression of immediate early genes such as c-jun and c-fos and activates the transcription factors AP-1 and NF-kappaB. JunD is one of the three members of the Jun family and shares some functional characteristics with c-Jun. In the present study, we found that the exposure of myeloblastic leukemia ML-1 cells to UV light (UVC) caused a significant increase in junD mRNA expression within 5 min that persisted for a period of 3 h. The activation of protein kinase C (PKC) with 12-O-tetradecaoylphorbol-13-acetate (TPA) also induced increases in junD expression similar to those of UV irradiation. In addition, UV irradiation- and TPA-induced increases in junD expression were completely abolished by GF-109203X, a PKC-specific inhibitor. UV irradiation activated intracellular signaling pathways including extracellular regulated kinase-2 (Erk-2), c-Jun N-terminal kinases-1 (JNK-1), and p38. However, TPA-induced activation of PKC affected only Erk-2 activity, and GF-109203X (a PKC inhibitor) markedly suppressed UV-induced Erk-2 activation. To further investigate the effect of UV-induced Erk-2 activation on the expression of junD mRNA, cDNA encoding mitogen-activated protein kinase kinase (MEK1) was overexpressed in ML-1 cells. The overexpression of MEK1 enhanced substantially junD expression in response to UV or TPA. In contrast, the suppression of Erk activation with PD98059, a specific inhibitor of MEK1, inhibited UV- and TPA-induced junD mRNA expression, UV-induced increases in caspase-3 activities, and cell death. In addition, the overexpression of junD enhanced the UV irradiation-induced increases in caspase-3 activity and cell death. We conclude that UV irradiation-induced increases in junD expression in ML-1 cells are mediated through activation of the PKC-coupled Erk-2 signaling pathway and play an important role in ML-1 cell apoptosis.