Erythropoietin activation of AP1 (Fos/Jun)

Erythropoietin (Epo) is the principal natural inducer of erythroid differentiation. The mechanisms by which signals generated at the Epo receptor (Epo-R) are transmitted to the nucleus are being explored. We now report that Epo strongly increases the activity of the transcription factor AP1 in both transformed and normal erythroid cells. Using antibodies to Fos and Jun, we have found that the Epo-induced AP1 heterodimer is composed primarily of authentic Fos and Jun proteins. Blocking protein kinase C (PKC) activity with H7 completely prevented the increase in AP1 activity in response to Epo. Importantly, the increase in AP1 activity was not due to increased expression of either c-fos or c-jun, as evidenced by the steady-state mRNA levels of both genes. Our results suggest that Epo may induce AP1 activity via a co- or posttranslational mechanism, presumably through modification of the Fos and/or Jun proteins.