AP1 REGULATION OF PROLIFERATION AND INITIATION OF APOPTOSIS IN ERYTHROPOIETIN-DEPENDENT ERYTHROID-CELLS

The transcription factor AP1 has been implicated in the induction of a poptosis in cells in response to stress factors and growth factor with drawal. We report here that AP1 is necessary for the induction of apop tosis following hormone withdrawal in the erythropoietin (EPO)-depende nt erythroid cell line HCD57. AP1 DNA binding activity increased upon withdrawal of HCD57 cells from EPO. A dominant negative AP1 mutant ren dered these cells resistant to apoptosis induced by EPO withdrawal and blocked the downregulation of Bcl-X-L. JunB is a major binding protei n in the API complex observed upon EPO withdrawal; JunB but not c-Jun was present in the AP1 complex 3 h after EPO withdrawal in HCD57 cells , with a concurrent increase in junB message and protein. Furthermore, analysis of API DNA binding activity in an apoptosis-resistant subclo ne of HCD57 revealed a lack of induction in AP1 DNA binding activity a nd no change in junB mRNA levels upon EPO withdrawal. In addition, we determined that c-Jun and AP1 activities correlated with EPO-induced p roliferation and/or protection from apoptosis. AP1 DNA binding activit y increased over the first 3 h following EPO stimulation of HCD57 cell s, and suppression of AP1 activity partially inhibited EPO-induced pro liferation. c-Jun but not JunB was present in the AP1 complex 3 h afte r EPO addition. These results implicate AP1 in the regulation of proli feration and survival of erythroid cells and suggest that different AP I factors may play distinct roles in both triggering apoptosis (JunB) and protecting erythroid cells from apoptosis (c-Jun).