Abstract
A dimer of the basic region leucine zipper proteins c‐Jun and c‐Fos constitutes the classical activator protein‐1 (AP‐1) transcription factor. c‐Jun is thought to play essential roles in many important cellular pathways, including the control of proliferation and cell death. To investigate the roles of c‐Jun and c‐Fos concentrations in the regulation of neuronal cell death, we generated conditional alleles by fusing c‐Jun and c‐Fos to the ligand binding domain of the murine estrogen receptor (ER), with the aim of controlling the biological activities of c‐Jun and c‐Fos by the synthetic ligand 4‐hydroxytamoxifen (4OHT). Transient transfection experiments revealed an increase in AP‐1 activity following transfection of an expression vector encoding a c‐Jun/estrogen receptor fusion protein (c‐JunER) and stimulation with 4OHT. In contrast, a c‐Fos/estrogen receptor fusion protein (c‐FosER) was only weakly active in HT22 immortalized hippocampal cells and in PC12 pheochromocytoma cells. Highest levels of AP‐1 activity were obtained by cotransfection of c‐FosER and c‐JunER and stimulation with 4OHT. Using retroviral gene transfer, we generated HT22 and PC12 cells expressing either c‐JunER or c‐FosER. The AP‐1 activity was moderately increased in 4OHT‐treated HT22 and PC12 cells expressing c‐JunER, whereas no biological activity was observed in cells expressing c‐FosER. We tested the influcence of 4OHT‐activated c‐JunER or c‐FosER upon cell survival and cell death by quantification of mitochondrial reduction capacity of 3‐(4,5‐dimethylthiazol‐2‐yl)‐2,5‐diphenyltetrazolium bromide to formazan dye crystals. We did not observe any 4OHT‐dependent decrease in cell survival in cells expressing c‐JunER or c‐FosER. Likewise, the number of pycnic nuclei did not increase in HT22 or PC12 cells expressing c‐JunER or c‐FosER. We conclude that an increase in the c‐Jun concentration is not sufficient to trigger neuronal cell death. We propose that it is not the concentration of c‐Jun that is critical for cell survival but rather the concentration of active, i.e., phosphorylated c‐Jun. © 2002 Wiley‐Liss, Inc.