Progesterone Receptor Regulation by 17β-Estradiol in Human Endometrial Carcinoma Grown in Nude Mice*

Regulation of progesterone receptor (PR) in human endometrial carcinoma was investigated in vivo in a multisite nude mouse tumor experimental system by estrogen administration and withdrawal. The cytosolic PR concentration was low in tumors grown in the absence of 17β-estradiol, but increased rapidly upon estrogen administration, reaching a maximal receptor concentration of 1.4–1.6 pmol/mg cytosol protein within 7 days. Protein blot analysis using a monoclonal antibody (hPRa 1) raised against PR from EnCa 101 showed no immunoreactivity in tumors grown in the absence of estrogen. Immunoreactive proteins of mol wt 116,000 and 81,000 were detectable 8 h after estrogen administration and increased in intensity as the cytosolic PR concentration increased. Interestingly, the protein of mol wt 116,000 was composed of mol wt isoforms and was detectable as a doublet 8 h after estrogen administration and finally as a triplet. The effect of estrogen withdrawal on EnCa 101 PR concentration and structure was determine...