Progesterone-associated endometrial protein--a constitutive marker of human erythroid precursors

Progesterone-associated endometrial protein (PAEP/PP14) is a 28-kD glycoprotein with sequence homology to beta-lactoglobulins containing a retinol-binding motif. PAEP/PP14 is present at nanomolar concentrations in human serum. It is produced by secretory and decidualized endometrium in women and by seminal vesicle epithelium in men. We report here that PAEP mRNA is constitutively expressed in normal hematopoietic tissue. Western immunoblotting of bone marrow cells with rabbit antibodies to PAEP gave a band at 28 kD, and immunocytochemical staining with monoclonal antibodies localized PAEP into the cytoplasm of erythroid precursors representing different stages of the normoblast series. PAEP was not detected in mature red blood cells, platelets, or in cells of the myeloid lineage. Untreated K562 leukemia cells did not contain PAEP, whereas treatment of the cells with tetradecanoylphorbol acetate (TPA) induced strong expression of PAEP mRNA and synthesis of the intact protein that was found both in the cytoplasm of the differentiating cells and in the supernatant of TPA-treated cultures. These findings add a new member to the growing family of genes that are constitutively expressed both in the reproductive tract and in the hematopoietic system.