Secretory endometrium synthesizes placental protein 14

Saline extracts of human nonpregnant endometrium were found to contain placental protein 14 (PP14). The tissue PP14 content was highest in the late secretory phase (median, 7.7 mg/g protein; n = 14), whereas proliferative endometrium (n = 8) was either PP14 negative or showed a low PP14 content (median, 0.15 mg/g protein). By immunoperoxidase staining, PP14 was localized in the glandular epithelial cells of endometrium. In tissue culture, secretory endometrium released more PP14 than proliferative endometrium, and cycloheximide markedly decreased this release. Synthesis of PP14 by secretory endometrium was demonstrated by incorporation of [35S]methionine into immunoprecipitable PP14. These results show that PP14 is synthesized and secreted by the nonpregnant endometrium.